Nanofluids and Ionic Fluids as Liquid Electrodes: An Overview on Their Properties and Potential Applications

Abstract

The current review work focuses on recent developments in the exploration of electroactive nanofluids, ionanofluids, and ionic liquids acting as liquid electrodes. The nanofluids used for this purpose are composed of organic or aqueous electrolytes as base fluids with the addition of nanoparticles in pure, oxidized, or hybrid forms. On the other hand, the ionic liquids are formed by adding a solution, which can be an acid, a base, or a salt, in water. The electrochemical properties, such as electrical conductivity and capacitance, of these innovative fluids are discussed thoroughly, along with their influencing factors, such as the nature and concentration of the included nanoparticles, the type of base fluids, and the operating temperature. Moreover, this overview summarizes the fundamental applications of electroactive nanofluids, ionanofluids, and ionic liquids in various possible flow-cell configurations and electrolysis methods, along with the associated feasibility factors. Additionally, this survey of scientific papers on the matter enabled the listing and evaluation of general aspects related to the usage of electroactive nanofluids, ionanofluids, and ionic liquids. Finally, it addresses the main problems associated with such types of fluids and outlines the primary prospects for further research and utilization of electroactive nanofluids, ionanofluids, and ionic liquids in diverse scientific and technological fields.

1. Introduction

Due to rapid advances in solid-state physics research and the ever-increasing exploration of nanomaterials, the utilization of nanostructures such as nanoparticles in various scientific explorations has become increasingly common. These nanoparticles often possess unique properties and may exhibit different characteristics from the original materials that generated them. When added to a base fluid, these nanoparticles form nanofluids, which are colloidal mixtures primarily known for enhancing the thermal conductivity of fluids. Nanofluids are extensively investigated and applied in diverse fields, ranging from microfluidics to renewable energies [1]. Certain types of nanoparticles dispersed in a base fluid can exhibit superior electrical properties when subjected to an external electric field. In this case, this especial class of nanofluids is known as electroactive nanofluids. Generally, metallic and metallic oxide nanoparticles such as copper, silver, gold, and iron oxides, among others, are the most commonly employed nanoparticles for preparing electroactive nanofluids due to their improved electrical conductivity. In addition to electroactive nanofluids, which involve the incorporation of nanoparticles with electrical properties into a base fluid, there is the possibility of using ionic solutions that can be also sensitized under the action of an electric field. These solutions are known as ionic liquids and are composed of ionic solutes that react with water and form new ions, usually originating from salts, acids, or bases. By taking the ionic liquids as base fluids and adding nanostructures, such as nanoparticles, one can prepare what are known as ionanofluids or ionic nanofluids. Figure 1 provides an overview of various possible solutions that involve the incorporation of nanostructures, nanoparticles, salts, acids, and bases into a base fluid or ionic liquid to function as liquid electrodes.

In energy storage and generation, electrochemical equipment such as fuel cells, batteries, and solar cells, as well as the electrolyte acting as a charge-transfer medium, play a crucial role in determining the energy release rate. To function properly, the electrolyte should be electrochemically stable and resistant to electrochemical reduction and oxidation processes. This requires a large electrochemical window to prevent the degradation within the working potential range. The electrochemical stability of the electrolytes is pivotal for averting decomposition when used in the working voltage range. Moreover, the electrolyte should exhibit improved insulation and ionic conductivity. Rechargeable batteries with enhanced heat storage capacity and charge/discharge cycling stability have seen a significant increase in global demand in recent decades. They offer lower inherent investment costs and are more environmentally friendly than disposable batteries. However, repeated charge-and-discharge cycles can lead to the growth of metal dendrites in the electrolyte, potentially bridging the anode and cathode and causing short-circuits and fires [2]. Traditional electrolytes are often composed of salts dissolved in organic or aqueous solvents, which typically have high dielectric constants. Polymer electrolytes have gained prominence, as they are safer than the traditional electrolytes containing flammable organic solvents. However, many polymers with high dielectric constants tend to have limited ion transport characteristics. Ionic liquids are being explored as liquid electrolytes due to their increased ionic conductivity and wider electrochemical windows [3]. Additionally, the use of nanoparticle dispersions in ionic liquids is being extensively investigated [4]. One representative example involves including 0.5% wt. of gold nanoparticles in [Emim][EtSO₄] ionic liquid, which enhances the electrolytic capacitance by 190% and improves ionic conductivity. These improvements can be interpreted based on the attraction of the [EtSO₄]ˉ anions to the surface of the gold nanoparticles, which alters the charge distribution and ionic structure [5]. Additionally, 0.08% wt. of copper nanoparticles encased in a carbon shell (designated herein by Cu@C) was added to a [Bmim][PF₆] ionic liquid for use as a liquid electrolyte in dye-sensitized solar cells. The resulting ionanofluid exhibited a 35% higher diffusion coefficient and a 65% higher electrical conductivity than the ionic liquid itself [6]. It was found that interactions between the Cu@C nanoparticles and the [Bmim]⁺ cations enhanced the electrical conductivity. The dispersion of Cu@C nanoparticles also contributed to an increase in the ionic bonding energy between the [Bmim]⁺ cations and the [PF₆]ˉ anions, which, in turn, enhanced the self-diffusion coefficient of the [Bmim]⁺ cations and reduced the viscosity of the ionic liquid [7]. Additionally, multi-walled carbon nanotubes dispersed in ionic liquid hybrids, when employed as liquid electrolytes in dye-sensitized solar cells, also exhibited increased thermal stability [8]. Similarly, the incorporation of 0.1% wt. of multi-walled carbon nanotubes incorporated in [Pmim]I ionic liquid improved the thermal stability and reduced the viscosity of the fluid. This observation was attributed to hydrogen bonding between the [Pmim]⁺ cations and the carboxylic groups of the carbon nanotubes, resulting in reduced coulombic attractions between the [Pmim]⁺ cations and the Iˉ anions [9]. In addition to enhancing electrochemical performance, the chemical stability of the electrolytes was improved by the creation of electric double layers in the ionic liquids, preventing the corrosion of the nanoparticles. Furthermore, silver nanoparticles dispersed in imidazolium dicyanamide-based ionic liquids were used as liquid electrolytes in a plasmonically enhanced thin dye-sensitized solar cell, demonstrating improved stability for the ionanofluid [10]. In experiments without the use of ionic liquids, a volatile acetonitrile electrolyte with redox compounds corroded the silver nanoparticles, causing damage to and deterioration of the device, thus reducing its efficiency [11]. Moreover, membranes are being employed for the separation of condensable and non-condensable gases. To achieve a high level of separation, the membrane should exhibit improved selectivity, permeability, stability, and cost effectiveness. Ionic liquids are preferred for gas separation methods due to their low vapor pressure, which helps to prevent the contamination of the gas stream and extends the working temperature range compared to that of plain absorption solvents.

2. Electrochemical Properties

2.1. Electrical Conductivity Models and Correlations

The electrical conductivity of nanofluids can be estimated using different models that consider the shape, size, and volumetric concentration of the dispersed nanoparticles. Two such models are the Maxwell model [12] and the Bruggeman model [13]. The Maxwell model is primarily suitable for low concentrations of spherical nanoparticles and provides an approximation of the electrical conductivity of a nanofluid by considering the electrical conductivity of both the nanoparticles and the base fluid. It is expressed by Equation (1):

$$\frac{{\mathsf{\sigma }}_{\mathrm{nf}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}=1+\frac{3\left(\left(\frac{{\mathsf{\sigma }}_{\mathrm{np}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}\right)-1\right)\mathsf{\phi }}{\left(\frac{{\mathsf{\sigma }}_{\mathrm{np}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}\right)+2\left(\left(\frac{{\mathsf{\sigma }}_{\mathrm{np}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}\right)-1\right)\mathsf{\phi }}$$

(1)

where σ_nf, σ_np, and σ_bf are respectively the electrical conductivities of the nanofluid, the nanoparticles, and the base fluid, and φ is the volumetric concentration of the nanoparticles. The Bruggeman model is given by Equation (2):

$$1-\mathsf{\phi }=\frac{{\mathsf{\sigma }}_{\mathrm{np}}-{\mathsf{\sigma }}_{\mathrm{nf}}}{{\mathsf{\sigma }}_{\mathrm{np}}-{\mathsf{\sigma }}_{\mathrm{bf}}}{\left(\frac{{\mathsf{\sigma }}_{\mathrm{bf}}}{{\mathsf{\sigma }}_{\mathrm{nf}}}\right)}^{1/3}$$

(2)

Both the Maxwell and Bruggeman models can be used to estimate electrical conductivity, but there is no consensus within the research community regarding their applicability range. Moreover, the researchers Zyla et al. [14] conducted a study where they synthesized nanodiamonds dispersed in ethylene glycol nanofluids and investigated their electrical conductivity. The results showed a non-linear change in electrical conductivity with an increase in the volumetric concentration of the nanoparticles. To describe this trend, the authors introduced an empirical correlation given by Equation (3):

$$\frac{{\mathsf{\sigma }}_{\mathrm{nf}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}=1+3734\mathsf{\phi }-25.65{\mathsf{\phi }}^2$$

(3)

The authors attributed the increase in the electrical conductivity of the nanofluids to the formation of an electric double layer and the creation of conducting pathways by the nanoparticles dispersed in the ethylene glycol. Additionally, the researchers Zyla and Fal [15] investigated aluminum nitride nanoparticles dispersed in ethylene glycol nanofluids and observed improvements in electrical conductivity as the concentration of nanoparticles increased. The research team also put forward an empirical correlation expressed in Equation (4):

$$\frac{{\mathsf{\sigma }}_{\mathrm{nf}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}=1+6950.56\mathsf{\phi }$$

(4)

Moreover, the research community has reached a consensus on that the Maxwell model does not typically provide a good match for experimental results. The observed increase is electrical conductivity is often significantly higher, sometimes reaching up to 600 times that of the base fluid itself, compared to what the Maxwell model predicts. In the case of silica nanoparticles dispersed in an ethylene glycol base fluid at 25 °C, it was noted that there was a linear rise in the electrical conductivity with an increasing concentration of nanoparticles. As a result, a new empirical correlation has been proposed, and it is represented by Equation (5) [16]:

$$\frac{{\mathsf{\sigma }}_{\mathrm{nf}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}=1+21.03\mathsf{\phi }$$

(5)

Furthermore, the authors Zyla and Fal [16] determined the electrical conductivity of silicon nitride nanoparticles dispersed in ethylene glycol. They observed inconsistencies in the predictions of the Maxwell model and as a result introduced a polynomial regression described by Equation (6) because of the observed inconsistency with the Maxwell law:

$$\frac{{\mathsf{\sigma }}_{\mathrm{nf}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}=1+\mathrm{78,609}\mathsf{\phi }-\mathrm{294,573}{\mathsf{\phi }}^2$$

(6)

The researchers Zyla et al. [17] explained significant improvements in electrical conductivity as a result of the combined influence of factors such as the nature and concentration of the nanoparticles and the type of base fluid. According to the authors, all these factors contributed to the creation of a pronounced electric double layer in the vicinity of the suspended nanoparticles and the formation of electrical conduction pathways. In a similar vein, the researchers Shen et al. [18] enhanced the Maxwell model taking into account the electrical conductivity derived from electrophoretic mobility and Brownian motion. They proposed Equation (7):

$$\mathsf{\sigma }={\mathsf{\sigma }}_{\mathrm{M}}+{\mathsf{\sigma }}_{\mathrm{E}}+{\mathsf{\sigma }}_{\mathrm{B}}$$

(7)

where σ_M is the electrical conductivity given by the Maxwell model, σ_E is the electrical conductivity due to the electrophoretic mobility of the nanoparticles, and σ_B is the electric conductivity caused by the Brownian motion. The electrical conductivity due to the electrophoretic mobility of the nanoparticles is given by Equation (8):

$${\mathsf{\sigma }}_{\mathrm{E}}=\frac{2{\mathsf{\phi }\mathsf{\epsilon }}_{\mathrm{r}}^2{\mathsf{\epsilon }}_0^2{\mathrm{U}}_0^2}{{\mathsf{\eta }\mathrm{r}}^2}$$

(8)

where ε_r, ε₀, U₀, η, and r are, respectively, the dielectric constant of the base fluid, the vacuum dielectric constant, the zeta potential of the nanoparticles, the viscosity of the nanofluid, and the radius of the nanoparticles. The electrical conductivity derived from Brownian motion is given by Equation (9):

$${\mathsf{\sigma }}_{\mathrm{B}}=\frac{3\mathsf{\phi }{ϵ}_{\mathrm{r}}{ϵ}_0{\mathrm{U}}_0\left(\frac{\mathrm{RT}}{\mathrm{L}}·\frac{1}{3\mathsf{\pi }\mathsf{\eta }}\right)}{{\mathrm{r}}^{3/2}}$$

(9)

where R, T, and L are, respectively, the thermodynamic constant, temperature, and Avogadro constant. Furthermore, Coelho et al. [19] investigated the electrical conductivity of copper oxide aqueous nanofluids. They synthesized various nanofluids with concentrations up to 2% vol. and conducted experiments at temperatures ranging from 25 °C to 75 °C. Their measurements were carried out using the electrical conductivity meter EC-Meter GLP 31 from CRISON^®. The results they obtained confirmed that the electrical conductivity increased with higher operating temperatures and higher amounts of copper oxide. The electrical conductivity increase was attributed by the researchers to the improved electrical conductivity of the incorporated nanoparticles. The findings were consistent with the relationship described by the Hill equation, as expressed in Equation (10) [20]:

$$\mathsf{\sigma }=\frac{{\mathsf{\phi }}_{\mathrm{CuO}}^{\mathrm{n}}{\mathrm{K}}_0}{1+{ \mathsf{\phi }}_{\mathrm{CuO}}^{\mathrm{n}}{\mathrm{K}}_0}$$

(10)

where K₀ and n are two fitting parameters employed to describe the degree of cooperation in different kinetic processes. In addition, Anu and Hemalatha [21] conducted an examination of the electrical conductivity in undoped and zinc-doped cobalt ferrite aqueous nanofluids. They compared their findings with those of the Maxwell [12] and Shen [18] models. The comparison revealed that the Maxwell model was not the most appropriate for their experimental data, as it consistently underestimated results. Furthermore, Ganguly et al. [22] reported a noticeable electrical conductivity increase in alumina nanofluids with rising temperature and volumetric concentration of alumina. The increase in electrical conductivity was directly proportional to both temperature and concentration, as described by Equation (11):

$$\frac{{\mathsf{\sigma }}_{\mathrm{nf}-{\mathsf{\sigma }}_{\mathrm{bf}}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}=3679.04\mathsf{\phi }+1.085799\mathrm{T}-43.6384$$

(11)

Furthermore, the researchers Minea and Luciu [23] also inferred the electrical conductivity of alumina aqueous nanofluids and reported a trend like the one found by the authors Ganguly et al. [22], but proposed a new empirical correlation, which is given by Equation (12):

$$\begin{gathered} {\mathsf{\sigma }}_{\mathrm{nf}}=176.69+588.41\mathsf{\phi }-13.64\mathrm{t}-86.31{\mathsf{\phi }}^2+0.36{\mathrm{t}}^2+1.07\mathrm{t}\mathsf{\phi }+11.06{\mathsf{\phi }}^3 \\ -0.003{\mathrm{t}}^3+0.18{\mathrm{t}}^2\mathsf{\phi }+1.01{\mathrm{t}\mathsf{\phi }}^2 \end{gathered}$$

(12)

where t is the temperature (°C) and φ is the volumetric concentration of the nanoparticles. The researchers reported a remarkable increase of approximately 380% in the electrical conductivity at a concentration of 4% vol. They also observed a linear increase in the electrical conductivity as the temperature increased. Furthermore, Sundar et al. [24] conducted experiments using a nanodiamond–nickel nanocomposite aqueous nanofluid and observed a remarkable enhancement in the electrical conductivity compared to that seen with pure water, reaching an increase of approximately 1340% at 24 °C. Notably, there were disparities between the experimental results and the predictions of the Maxwell and Bruggeman models. Additionally, the researchers Chereches and Minea [25] analyzed the electrical conductivity of aqueous mono and hybrid nanofluids containing silica and titania nanoparticles. They proposed empirical correlations that took into account the operating temperature and volumetric concentration. For silica nanofluids, they proposed the correlation expressed in Equation (13):

$${\mathsf{\sigma }}_{\mathrm{nf}}=-103.47+315.14\mathsf{\phi }+17.23{\mathsf{\phi }}^2+4.45\mathrm{T}$$

(13)

In the case of the titania aqueous nanofluids, the researchers proposed the correlation given by Equation (14):

$${\mathsf{\sigma }}_{\mathrm{nf}}=491.56+104.67\mathsf{\phi }+71.37{\mathsf{\phi }}^2+4.19\mathrm{T}$$

(14)

Moreover, Islam et al. [26] conducted an investigation on the electrical conductivity of titanium oxide nanoparticles suspended in a nanofluid comprising a mixture of ethylene glycol and water, with concentrations ranging from 0.05% vol. to 0.5% vol. The results obtained did not align with the predictions of the Maxwell model. Additionally, the authors observed that electrical conductivity increased as the temperature and nanoparticle concentration rose, attributing this trend to the features of the electric double layer. Furthermore, the researchers Islam and Shabani [27] measured the electrical conductivity of a nanofluid consisting of titanium oxide nanoparticles dispersed in a 50:50 vol. mixture of water and ethylene glycol. They found that the electrical conductivity increased with higher temperatures and greater nanoparticle concentrations and proposed the following empirical correlation expressed in Equation (15):

$$\frac{{\mathsf{\sigma }}_{\mathrm{nf}}}{{\mathsf{\sigma }}_{\mathrm{bf}}}=11.214+2.626\ln \mathsf{\phi }+0.2371\mathrm{lnT}$$

(15)

2.2. Electrical Conductivity Measurement

As will be mentioned throughout this review, electroactive nanofluids and ionic liquids offer a significant advantage in the development of innovative solutions in fields such as engineering and materials science. In this context, the remarkable electrical conductivity of these fluids has been the focus of extensive research. Therefore, in this section, an overview of the operating principles of the commercially available equipment used to measure the electrical conductivity of liquids is provided. Additionally, the underlying mechanisms of these processes will be addressed. There are some devices designed for measuring the electrical conductivity of the liquids, and they come in both portable and benchtop versions, often at relatively affordable prices. The instruments differ from each other regarding their sensitivity, and can offer measurements on the order of µS·cm⁻¹ and in different working temperature ranges. According to findings [28], there are three different available technologies in the market for conductivity meters: (I) two-pole conductivity cells, (II) four-pole conductivity cells, and (III) inductive conductivity measuring cells. To facilitate the understanding of the method of operation of these devices, two-pole conductivity cell equipment and its underlying principles will be briefly described in the following lines. As electrical conductivity is defined as a measure of the ability of the solutions to conduct or carry an electrical charge, implicitly, the transport of electricity through the solution requires the presence of charged particles. Regarding this, it is important to note that the technique is incapable of differentiating between the different types of ions; that is, electrical conductivity is a non-specific sum parameter over all ionic species in a solution, whether they come from of salts, acids, bases, or even organic substances [28]. The scheme represented in Figure 2 shows two electrodes, a cathode and an anode, which are immersed in an electrolytic solution and connected to a power supply. When a potential difference is applied between the electrodes within the solution, it creates an electric field, and the ions in the solution move toward the opposite electrode, carrying an electric current. However, this movement of ions can lead to an accumulation of charges near the surface of the electrodes, which reduces the strength of the electric field and can change the measured value of the conductivity of the liquid. This phenomenon is known as polarization. To solve this problem, alternating current (AC) is used, as it will prevent the complete migration of ions, since the polarity of the electrodes is reversed with each cycle of AC, reversing the direction of the ion flow.

2.3. Electrochemical Properties

As commented before, the electrical conductivity of a material refers to its ability to conduct electric current. Although metals are the main protagonists in this regard, ionic solutions can also be electrical conductors. Ionic solutions can be composed of different ions, either singly or in combination, that come from various sources, such as salts, acids, or bases. When an ionic compound dissolves in a liquid medium, it breaks down into positively charged ions (cations) and negatively charged ions (anions). When an electric field is applied to this solution, these free ions can conduct an electric current. Furthermore, the higher the concentration of ions, the greater the electrical conductivity. Apart from the concentration, the mobility of these charge carriers in the solution also affects the overall electrical conductivity. Certain types of ions are more likely to move through solution than others due to factors such as size, charge, and solvent interactions. Similarly, with ionic liquids, the electroactive nanofluids are designed to acquire unique electrical and electrochemical properties. Electroactive nanofluids are colloidal suspensions comprising nanoparticles dispersed in a base fluid, typically composed of conducting or semiconducting materials. These nanoparticles can be functionalized directly with electrically active groups or coated with conductive materials to increase their electrical conductivity. In the cases where the colloidal mixture is subjected to a potential difference or electric field, the nanoparticles can undergo changes in their properties or behavior, which can be interesting to adjust in several applications, such as energy conversion, sensors, actuators, and electronic equipment. Moreover, the authors Ezekwem and Dare [29] prepared nanofluids at concentrations from 0.5% vol. to 5% vol. by dispersing silicon carbide nanoparticles in water and ethylene glycol through a two-step method. The stability of the nanofluids over time was inferred through zeta potential and UV-VIS spectra analysis. It was verified that the silicon carbide–water had better stability than the silicon carbide–ethylene glycol nanofluids, and it was confirmed that both types of nanofluids showed an electrical conductivity increase with the increasing concentration of silicon carbide nanoparticles. Indeed, there were observed increases of 148- and 58-fold in the electrical conductivity of the silicon carbide–water and silicon carbide–ethylene glycol at 5% vol. nanofluids, respectively, compared to that of the base fluids. Furthermore, the authors Giwa et al. [30] studied the impact of the operating temperature and hybridized nanoparticle mass ratio on the electrical conductivity of aqueous alumina–multi-walled carbon nanotube hybrid nanofluids at 0.1% vol. At 55 °C, the nanofluids showed maximum electrical conductivity increases of approximately 443% and 288% for the nanofluids with 90:10 and 20:80 of mass ratios, respectively, compared to that attained with distilled water as the base fluid. The increase in operating temperature resulted in a reduction in the dynamic viscosity of the hybrid nanofluids, while a higher particle mass ratio significantly enhanced their electrical conductivity. Specifically, an increase in the particle mass ratio of the carbon nanotubes led to viscosity reductions of nearly 6.2% for the nanofluid with a 90:10 mass ratio and of 7.08% for the nanofluid with a 20:80 mass ratio. On the other hand, a decrease in the particle mass ratio led to a decrease in the electrical conductivity by around 155% and 160.5% at temperatures of 15 °C and 55 °C, respectively. Additionally, Guo et al. [31] conducted a study on the electrical conductivity of silica nanofluids with base fluids consisting of mixtures of water and ethylene glycol. They used a two-step method to produce the nanofluids with a concentration of 0.3% wt. The results confirmed that the electrical conductivity decreased as the ethylene glycol fraction in the base fluid mixture increased. Also, at a fixed ethylene glycol fraction, the electrical conductivity increased with increasing operating temperature. Furthermore, the researchers Iglesias et al. [32] determined the electrical conductivity and relative permittivity of alumina dispersed in Milli-Q and Milli-Ro water nanofluids at different temperatures ranging from 25 °C to 75 °C and various concentrations up to 2% vol. They observed that the relative permittivity decreased with rising temperature and increased with higher alumina content, being little affected by the purity of the water base fluid. The electrical conductivity enhancement was greater when the alumina nanoparticles were dispersed in the Milli-Q water with higher purity than in the lower-purity Milli-Ro water. Furthermore, the researchers observed that the electrical conductivity of the nanofluids increased with a higher volumetric concentration of alumina and with temperature. Moreover, the enhancements in the electrical conductivity were more pronounced with increased alumina concentration at a fixed temperature, but they decreased with higher temperatures at a fixed alumina concentration. In addition, they found that the presence of impurities in the lower-grade water only had a minor impact on the electrical conductivity in the less concentrated alumina solutions of up to 1% vol. within a temperature range from 25 °C to 35 °C. However, at higher concentrations and temperature values, the impurities had a significant effect on the electrical conductivity. Particularly, these impurities led to a reduction of approximately one quarter in the electrical conductivity compared to what was achieved when using the higher-grade water. Moreover, Ponmani et al. [33] determined the thermal and electrical conductivities of various nanofluids, including aqueous zinc oxide nanofluids with the addition of 5% wt. of polyvinylpyrrolidone dispersant, zinc oxide in polyethylene glycol, and copper oxide in polyethylene glycol with the addition of 5% wt. of polyvinylpyrrolidone. The nanofluids were prepared at concentrations of 0.1% wt., 0.3% wt., and 0.5% wt. and the thermal and electrical conductivities were measured using KD-2 Pro^® and PC 700 Eutech^® meters. The results indicated that the greatest improvements in the thermal and electrical conductivities were observed in the zinc oxide–polyvinylpyrrolidone–water nanofluid at different concentrations, followed by the copper oxide–polyvinylpyrrolidone–polyethylene glycol nanofluid. Additionally, Khdher et al. [34] conducted an investigation on the electrical conductivity of nanofluids with alumina nanoparticles dispersed in bio-glycol with a temperature range between 30 °C and 80 °C and volumetric concentrations ranging from 0.1% vol. to 1% vol. The nanofluids were synthesized using a two-step method without the addition of surfactants. They found that the nanofluids exhibited a maximum thermo-electric conductivity of 9.5 at 30 °C with 0.5% vol. of alumina nanoparticles, while a minimum value of 3.5 was observed with 0.7% vol. at 80 °C. Furthermore, Ijam et al. [35] determined the electrical conductivity, among other properties, of graphene oxide nanosheets dispersed in a mixture of deionized water and ethylene glycol as the base fluid. The authors found an impressive electrical conductivity enhancement in the nanofluid of around 1664% at a concentration of 0.1% wt. and at a temperature of 25 °C. Additionally, Minea and Luciu [23] investigated the impact of the temperature and concentration of alumina nanoparticles on the electrical conductivity of aqueous nanofluids. Their findings indicated that the electrical conductivity of the nanofluids increased nearly linearly with a higher alumina concentration. Notably, at a temperature of 25 °C, there was a remarkable enhancement of almost 380% in the electrical conductivity of the nanofluids at a concentration of 4% vol. Also, the authors Sarojini et al. [36] determined the electrical conductivity of aqueous nanofluids including alumina, copper, and copper oxide nanoparticles at various concentrations and with the addition of SDS surfactant. The research team reported that the incorporation of the SDS surfactant increased the stability and diminished the electrical conductivity of the nanofluids. However, a reduction in the size of the alumina nanoparticles led to an increase in the electrical conductivity of the nanofluids. This effect was due to the increased surface area and electrophoretic mobility of the nanoparticles. Also, there was not any appreciable enhancement reported in the electrical conductivity of the nanofluids with increasing operating temperature. Furthermore, the researchers Fal et al. [37] studied permittivity and electric conductivity at a broad frequency range between 0.01 Hz and 10 MHz and temperature values ranging from 0 °C to 60 °C for dispersions of titanium oxide nanoparticles in ethylene glycol at different concentrations. The obtained results showed that frequency had great impact on permittivity for values less than 10 kHz. The authors also found a 400% increase in electrical conductivity at 20% wt. and 25 °C. Also, the researchers Hadadian et al. [38] studied the electrical conductivity of graphene oxide aqueous nanofluids. They observed that this property increased with rising temperature and with an increase in the concentration of the nanoparticles. The maximum augmentation in the electrical conductivity was around 25.7% at 0.0006% wt., being appreciably higher than that of other carbon nanomaterials, metals, or metal oxides. Moreover, Bagheli et al. [39] prepared nanofluids with magnetite nanoparticles dispersed in water. They employed an ultrasonication process and added a surfactant. The electrical conductivity of these nanofluids was assessed at different concentrations and temperatures. The results revealed a substantial increase in the electrical conductivity, with a maximum enhancement of 360% observed at a concentration of 0.5% vol. and a temperature of 60 °C in comparison to the electrical conductivity of the deionized water base fluid. Moreover, White et al. [40] determined the electrical conductivity of nanofluids containing zinc oxide dispersed in propylene glycol nanofluids. Their findings showed that the electrical conductivity increased with higher nanoparticle concentration. They also found that, at a constant concentration, the electrical conductivity increased with a decrease in the size of the zinc oxide nanoparticles. Furthermore, the authors Ramalingam et al. [41] determined the electrical and chemical stability of copper sulfide aqueous nanofluids. The authors observed that the maximum enhancement in the electrical conductivity of the nanofluids was of nearly 73 µS·cm⁻¹, which was measured at 55 °C and 0.0038% wt. Furthermore, the ionanofluids were demonstrated to be more beneficial fluids for electrical conductivity enhancement than traditional nanofluids and ionic liquids. In essence, electrical conductivity is influenced by the free ion or electrons; hence, the availability of nanoparticles and ions provided by the ionic liquids contributed to the improvement in the electrical conductivity of the ionanofluids. Moreover, the researchers Rodriguez-Palmeiro et al. [42] studied the electrical conductivity of silver iodide nanoparticles at various concentrations dispersed in [P_6,6,6,14][Cl] ionic liquid. The maximum electrical conductivity enhancement was found at 20% wt., but beyond this concentration value, the electrical conductivity was found to be reduced. Furthermore, the researchers Alizadeh et al. [43] determined the electrical conductivity of ionanofluids composed of [Bmim][PF₆] and germanium nanoparticles and verified that the electrical conductivity of the ionanofluids increased with growing temperature and concentration of the nanoparticles. The electrical conductivity peak was registered around 25 mS/cm at 30 °C. Also, the researchers Chereches and Minea [44] determined the electrical conductivity of ionanofluids composed of alumina nanoparticles dispersed in a mixture of [C₂mim][CH₃SO₃] ionic liquid and water. The researchers found a remarkable electrical conductivity increase of approximately 300% in comparison with that of the base fluid. Moreover, the authors confirmed also that the electrical conductivity depended on the concentration and temperature of the nanoparticles. It could be observed that increases in the concentration of the nanoparticles reduced the electrical conductivity of the ionanofluids, which was attributed by the authors to the viscosity change due to the increasing impact of the dispersed nanoparticles within the ionic liquid. Also, consistent results were reported when the aqueous environment of [Mmim][DMP] contributed to the enhancement of the electrical conductivity in the ionanofluids [45]. The researchers He and Alexandridis [46] reviewed the published scientific articles on ionanofluids, mainly concerning their electrochemical application in fuel cells, batteries, sensors, and photovoltaic solar cells. The authors also highlighted that the ionanofluids exhibited a remarkable proton-transfer capability and electrochemical stability. They noted that the presence of nanoparticles dispersed in the ionic liquids enhanced the performance of the electrolyte by increasing the diffusion coefficient, ultimately leading to higher electrical conductivity. Additionally, the authors Hamm et al. [6] found that the incorporation of gold nanoparticles into an [Emim][EtSO₄] ionic liquid increased the capacitance by nearly 190% and reduced the resistance of the electrolyte by around 70%, resulting in a significant increase in the electrical conductivity. Furthermore, the authors Deb et al. [47] examined the electrical conductivity of ionanofluids with zinc sulfide dispersed in [Deim][NTf₂] at different temperature values and concentrations of nanoparticles. The researchers verified that the electrical conductivity of the ionanofluids increased with an increasing amount of added nanoparticles up to a certain limit, after which it decreased drastically. An inverse trend was also observed with the variations in operating temperature. In addition, the researchers Dubal et al. [48] proposed stable hybrid electroactive nanofluids combining capacitive and faradaic energy storage capabilities. These hybrids were composed of reduced graphene oxide and polyoxometalates for electrochemical energy storage purposes. Two hybrid materials were synthesized. One was composed of reduced graphene oxide phosphomolybdate, and the other was composed of reduced graphene oxide–phosphotungstate. These materials were dispersed in 1 M H₂SO₄ aqueous electrolyte. The nanofluids were tested in an in-house developed flow cell under static and continuous flowing conditions. The polyoxometalate-based electroactive nanofluids, at 0.025% wt., showed high specific capacitances of 273 F·g⁻¹ for the reduced graphene oxide–phosphotungstate nanofluid and 305 F·g⁻¹ for the reduced graphene oxide–phosphomolybdate nanofluid. Both nanofluids exhibited improved cycling stability of nearly 95 % and a Coulombic efficiency between 77% and 79% after 2000 cycles. The tested nanofluids effectively behaved as liquid electrodes with enhanced properties, indicating their potential application in energy storage in the innovative flow cell sector.

3. Ionic Liquids

A surprising number of devices used daily by everyone, from domestic to industrial levels, employ various types of fluids for heat transfer, distribution, or storage. However, analyzing the thermal performance of these fluids is indispensable for ensuring the operability, reducing the investment costs, maintaining safety levels, and minimizing environmental impacts. Within this context, certain properties of these fluids become prominent, including the thermal and electrical conductivities, specific heat, viscosity, density, and boiling and flash points, among others. According to the authors Oster et al. [49], one of these options is the use of ionic liquids. Ionic liquids are compounds formed from molten organic salts at room temperature, comprising organic cations and a significant number of charge-delocalized inorganic and organic anions, with a melting point below 100 °C [49,50,51]. Regarding inorganic salts, the bulk asymmetry of the cation/anion in ionic liquids leads to the shielding of intermolecular forces and prevents the aggregation of ions into the low-energy crystalline state. Due to their improved thermophysical properties, according with the authors Asleshirin et al. and Ribeiro et al. [52,53], in recent years, the ionic liquids have influenced the performance of equipment such as reactors, distillation columns, heat exchangers, and units for physical–chemical processing and reactions. The ionic liquids have negligible vapor pressure, improved thermal and electrochemical stabilities, broad solubility, and affinity for several chemical compounds. All these properties can be also adjusted through the coupling of cations and anions via controlling the van der Waals interactions and the Lewis acidity or basicity, among other ion functionalities. Based on the combinations of cations and anions and their enhanced biological, physical, chemical, and electrochemical characteristics, including miscibility in aqueous solvents, ionic conductivity, and acidity or basicity degree, ionic liquids can be classified into several categories. However, in general, ionic liquids can be categorized into three fundamental types as protic, aprotic, and zwitterionic fluids. Figure 3 provides an overview of the main types of ionic liquids.

Furthermore, it should be noted that not all ionic liquids remain in a liquid state below 100 °C, and some of them only exhibit a relatively low glass-transition temperature and slow crystallization kinetics. In a comprehensive study conducted by the researchers Chen et al. [54], it was demonstrated that ionic liquids are highly suitable for applications in electrochemical systems aimed at enhancing the hydrogen evolution reaction. The authors emphasize that these liquids possess a unique combination of specific features, including superior electrical conductivity, low vapor pressure, and high electrochemical stability, stemming from a large variety of available functional groups. These attributes imbue them with superior characteristics compared to those of traditional liquids. An electrochemical stability window of approximately 4.5 V and an electrical conductivity range between 0.1 mS·cm⁻¹ and 18 mS·cm⁻¹ have been found for the commonly used ionic liquids [55]. However, the notable viscosity characteristics of the ionic liquids may impede their ionic conductivity. Furthermore, electrolytes with a significant volumetric concentration of submicron-sized particles in an insoluble second phase have been developed, exhibiting an improved ionic conductivity compared to that obtained with a single-phase electrolyte [56]. Through these interactions, composite ionic liquids and nanoparticles have been found to enhance the electrochemical performance of an electrolyte, offering enhanced electrical conductivity, electrochemical and thermal stabilities, and diffusion coefficients that enable these hybrid fluids to function effectively as liquid electrolytes in cells and batteries. In this scope, various combinations of nanoparticles and ionic liquids have been employed, with nanoparticle dispersions in ionic liquids and ionic liquid-grafted nanoparticles being the most frequently explored approaches. An example of this is the addition of 0.5% wt. of gold nanoparticles to [Emim][EtSO₄] ionic liquid, resulting in a 190% increase in the capacitance of this electrolyte and a significant enhancement in the ionic conductivity. The observed electrochemical advantages were attributed to the interaction energy between the ionic constituents and the nanoparticles, stemming from the attraction of the [EtSO₄] anions to the surface of the gold nanoparticles, which led to alterations in the charge distribution and in ionic structure. Furthermore, ionic liquid-functionalized nanoparticles, also known as NIMs, have been incorporated into electrolytes to enhance their electrochemical stability. For instance, the brush-like structure of the ionic liquid tethered to the nanoparticles facilitated the suspension of silica nanoparticles in a propylene carbonate/LiTFSI electrolyte by providing steric and electric double layer interactions [57]. The nanoparticles created a robust and convoluted porous mesh, preventing the formation of dendrites that could bridge the anode and cathode, potentially causing short-circuits [58]. Ionic liquid-functionalized nanoparticles have also been evaluated for their potential use as liquid electrolytes in lithium-ion batteries because of their surface functionality, high dispersion degree, and salt dissolution capability [59]. By incorporating 10% of 1-methyl-3-propylimidazolium bis(trifluoromethanesulfone)imide ionic liquid-tethered silica nanoparticles into a conventional propylene carbonate–NaTFSI electrolyte, a rechargeability exceeding 20 cycles of a sodium–carbon dioxide–oxygen cell at 5 V of potential was achieved without detecting electrolyte decomposition [60]. Figure 4 illustrates the main challenges and benefits of ionic liquids, while Figure 5 provides an overview of the primary applications of these liquids.

4. Electroactive Nanofluids

Researchers Jehhef et al. [61] conducted a study on the electrical conductivity of nanofluids containing alumina, zirconia, and copper oxide at varying concentrations of nanoparticles and fluid temperatures. The results indicated that the addition of nanoparticles led to an increase in the electrical conductivity of the deionized water base fluid, though this effect was less pronounced at high temperatures. Among the nanofluids, the copper oxide nanofluid exhibited the most significant enhancement, with increases in electrical conductivity of about 80%, 75%, and 50%, respectively, at a concentration of 0.1% vol. and a temperature of 50 °C, compared with that of the water alone. Additionally, authors Chereches and Minea [25] conducted an experimental study to measure the electrical conductivity of both simple and hybrid nanofluids containing alumina, titania, and silica in aqueous solutions. The nanofluid concentration ranged between 1 and 3% vol., and the testing covered temperatures from 20 °C to 60 °C. The results demonstrated that the electrical conductivity of both single and hybrid nanofluids increased linearly with the operating temperature and the volumetric concentration of nanoparticles. The achieved electrical conductivity values were nearly 60 times greater than those obtained with the base fluid alone. In the case of silica nanofluids, the increase in the electrical conductivity ranged from 14-fold to 40-fold, while for titania nanofluids, the electrical conductivity increased between 30 times and 58 times. Among the hybrid nanofluids, the most significant increase was observed in the alumina–titania nanofluid, the electrical conductivity of which was 43 to 57 times higher than that of the base fluid. Additionally, authors Almeida et al. [62] conducted an experimental analysis of a graphene–transformer oil nanofluid at concentrations of 0.01%, 0.03%, and 0.05% vol. They concluded that the electrical conductivity of the nanofluid was up to almost 3.8 times higher than that of the transformer oil at a concentration of 0.05% vol and a temperature of 90 °C. The researchers argued that the addition of nanoparticles reduced the specific resistance, thus enhancing the breakdown voltage and consequently increasing the electrical conductivity of the nanofluid. Furthermore, researchers Ezekwem and Dare [63] carried out experimental measurements of the thermal and electrical conductivities of aluminum nitride dispersed in ethylene glycol nanofluids at concentrations ranging from 0.5% to 5% vol. The results indicated that both thermal and electrical conductivities increased with higher concentrations. The maximum increase in the thermal conductivity of up to 25% was achieved at 5% vol. of nanoparticles, while a significant increase in the electrical conductivity was observed when 0.5% vol. of aluminum nitride nanoparticles was added to the ethylene glycol base fluid. In the latter case, the electrical conductivity increased approximately 520 times at 28 °C compared to that of the base fluid. Additionally, authors Sen et al. [64] developed nanofluids containing battery–active anode nanoparticles, which underwent electrochemical charge–discharge processes through nanoparticle–electrode impact events. The authors achieved this by grafting organic molecules onto the surface of hematite nanoparticles, resulting in electroactive nanofluids with a solid phase content of 70% wt. in alkaline aqueous electrolytes. Additionally, the electrochemical testing of both pure hematite and surface-modified hematite in the form of solid electrodes exhibited a peak reversible discharge capacity of around 240 mA·h·g⁻¹ for both electrodes. However, the electrochemical evaluation of the nanofluids revealed a poorer discharge performance for the modified materials, with a capacity of 176 mA·h·g⁻¹ and 52% CE, compared to the pristine material with a capacity of 335 mA·h·g⁻¹ and 100% CE. This reduction in electrochemical activity was observed only in the fluidized form and not in the solid electrodes, which used highly conductive carbon additives. This suggests that the electrical conductivity of the nanoparticles and their surface coating had a strong influence on the PEI charge–discharge process. The surface-coated nanomaterials also effectively mitigated the risk of nanoparticle agglomeration, which was significant during the electrochemical cycling of the pure hematite. Furthermore, researchers Dubal and Gomez-Romero [65] developed electroactive nanofluids composed of reduced graphene oxide dispersed in an aqueous sulfuric acid solution. It was found that these electroactive nanofluids exhibited capacitive energy storage capabilities consistent with those of solid electrode supercapacitors, reaching 169 F·g⁻¹, and operated at faster rates from 1 mV·s⁻¹ to 10 V·s⁻¹. The concentrations of the reduced graphene oxide ranged from 0.025% wt. to 0.4% wt., and all the tested nanofluids exhibited viscosity levels similar to that of water, making them suitable for industrial flow cells. Consequently, it was confirmed that the prepared reduced graphene oxide nanofluid effectively served as a liquid electrode for energy storage and innovative flow cells. Moreover, authors Brahimi et al. [66] proposed a magnetic nanofluidic electrolyte for redox flow batteries, consisting of a dispersion of magnetic modified multi-walled carbon nanotubes in the positive electrolyte of a polysulfide–iodide redox flow battery with weight fractions below 0.3 g·L⁻¹. The electrochemical performance of the magnetic nanofluidic electrolyte was investigated through cyclic voltammetry at various fractions of carbon nanotubes with a carbon felt electrode. The authors employed a polysulfide–iodide redox flow battery to assess the impact of the electroactive nanofluid on battery performance using different flow velocities and current densities through galvanostatic charge–discharge, electrochemical impedance spectroscopy, and polarization tests. The research team observed reductions in ohmic resistance, charge-transfer resistance, and mass-transfer resistance when using the magnetic nanofluid compared to those achieved with the electrolyte without the inclusion of the carbon nanotubes. Adding magnetic carbon nanotubes to the electrolyte at a concentration of 0.3 g·L⁻¹ resulted in an enhanced performance of the polysulfide–iodide redox flow battery, with a 45% increase in peak power density. Moreover, at a current density of 20 mA·cm⁻², an energy efficiency of nearly 80% was achieved. The electroactive magnetic nanofluids also exhibited a Coulombic efficiency of around 100% and demonstrated good cycling performance after 200 cycles. After 50 cycles, at a current density of 30 mA·cm⁻², the energy efficiency of the battery using the magnetic nanofluid remained 10% higher than that achieved without the use of the magnetic carbon nanotubes. Furthermore, authors Myures and Suresh [67] formulated an electrolyte using nitrogen-doped reduced graphene oxide as an electrocatalyst for the positive half-cell in a vanadium redox flow battery. The electroactive nanofluids were prepared using an improved Hummers’ method, followed by inert atmosphere thermal annealing. The electrochemical kinetics of the positive redox pair VO₂⁺/VO²⁺ significantly improved when using the nanofluidic electrolyte, and this improvement was directly proportional to the concentration of the nitrogen-doped reduced graphene oxide. Specifically, the nanofluidic electrolyte at a concentration of 0.1% wt. exhibited enhanced electrochemical reversibility, a 35% increase in the peak current density, a 23% reduction (107 mV) in the overpotential, and a 58% decrease in the charge-transfer resistance compared to the base vanadium electrolyte. The improvement in the peak current and reduction in overpotential evolved with the increasing concentrations of nitrogen-doped reduced graphene oxide in the vanadium electrolyte. This enhanced peak current mitigated losses from polarization and increased the charge and mass transfer capability of the electrolyte. The improved electrochemical performance can be attributed to the electrochemical surface area, enhanced electrical conductivity, improved hydrophilic properties, and the presence of numerous defect sites in the developed nitrogen-doped reduced graphene oxide nanofluid. Consequently, the nanofluidic electrolyte, synthesized through a straightforward chemical process and subsequent pyrolysis technique, was confirmed as a highly suitable electrocatalyst nanofluid for the positive half-cell reaction in a vanadium redox flow battery. Additionally, authors Ajeeb and Murshed [68] conducted a study on the characteristics of silicon carbide and boron nitride nanoparticles dispersed in a mixture of ethylene glycol and distilled water nanofluids. The concentration of these nanofluids ranged from 0.01% vol. to 0.05% vol. The authors observed Newtonian behavior in the nanofluids. The viscosity and density increased by 5.2% and 0.3%, respectively, at a concentration of 0.05% vol. for the silicon carbide and boron nitride nanofluids compared to those of the base fluid mixture. Furthermore, the electrical conductivity increased with the growing concentration of nanoparticles, reaching a maximum 3.2-fold enhancement for the silicon carbide nanofluids and a 2.8-fold enhancement for the boron nitride nanofluids at a concentration of 0.05% vol. compared to the base fluid.

5. Ionanofluids

Ionanofluids are mixtures of ionic liquids and nanoparticles. These nanoparticles can be originated from metallic, inorganic, organic, or polymeric materials dispersed in an ionic liquid [69,70]. The term “ionanofluids” was introduced by Nieto de Castro et al. in 2009 [71,72]. Additionally, ionanofluids offer the advantage of being tunable in terms of their properties, allowing modifications to meet the specific requirements of various applications. This versatility makes ionanofluids a highly suitable choice for the development of more efficient and sustainable technologies. Furthermore, researchers Oster et al. [49] conducted a study to characterize the thermophysical properties of various nanofluids, including thermal capacity, thermal conductivity, thermal stability, density, and viscosity. These properties were experimentally determined for the selected nanofluids as a function of temperature, up to 363.15 K, and then compared with theoretical models. The chosen ionic liquids included a trihexyl(tetradecyl)phosphonium cation paired with acetate, butanoate, hexanoate, octanoate, or decanoate anions. To prepare the ionanofluids, nanoparticles like carbon nanotubes, graphite, boron nitride, and mesoporous carbon were incorporated with weight concentration of up to 3% wt. The researchers primarily concluded that the addition of nanoparticles to the ionic liquids had a positive effect on improving their thermal capacity, thermal conductivity, and density. However, this addition of nanoparticles led to a significant increase in the viscosity of the ionanofluids, ultimately offsetting the improvements. Other significant findings revealed that both temperature and the relationship between the type of ionic liquid and the nanoparticle concentration influenced heat-transfer enhancement. Finally, comparing the experimental results obtained by the researchers with the theoretical models for density and thermal conductivity demonstrated the models’ capability to reliably predict physical properties. On the other hand, authors Joseph et al. [73] evaluated the redox characteristics of an iron oxide ionanofluid prepared from FeSO₄·7H₂O and 1-butyl-4-methylpyridinium chloride. The authors explained that the contact of the ionic liquids with the surface of the iron oxide nanoparticles enhanced the efficiency of the redox charge transfer. Furthermore, an electroactive ionanofluid with elevated dielectric constants demonstrated the ability to undergo a charge–discharge process, even after 100 cycles, with a superiority of over 94%. Additionally, researchers Bakthavatchalam et al. [74] synthesized an ionanofluid composed of an ionic liquid with the incorporation of 2D MXene nanostructures for solar thermal applications. The authors studied the optical properties of the MXene–diethylene glycol nanofluid incorporated in the 1-ethyl-3-methyl imidazolium octyl sulfate [Emim][OSO₄] ionic liquid at MXene concentration values ranging from 0.1% wt. to 0.4% wt. At wavelength values from 240 nm to 790 nm, the presence of the ionic liquid, its stability, and amount of MXene in the dispersion significantly contributed to the absorbance capability of the MXene nanofluid. Furthermore, an increase in the amount of MXene led to additional absorbance peaks, promoting light absorption. High absorption peaks were observed, indicating improved stability in light absorbance capacity within the specified wavelength range. Additionally, a high extinction coefficient was obtained for the ionanofluid with a lower concentration of MXene. The electrical conductivity was enhanced with the inclusion of MXene nanoparticles, with a peak reaching 571 µS/cm at 0.4% wt. of MXene. Moreover, authors Deb and Bhattacharya [75] prepared hexafluoropropylene copolymer P(VDF-HFP) active polymer membranes by entrapping various amounts of pyrrolidinium ionic liquid in an ionanofluid. The authors observed that the ionanofluid improved the electroactive phase nucleation of P(VDF-HFP) and eliminated the crystallinity of the membranes. It was also found that the ionanofluid improved the electrolyte uptake capability and promoted the generation of channels with good ionic conductivity within the membranes. A gel–polymer electrolyte incorporating 50% wt. ionanofluid exhibited the highest ionic conductivity of 2.3 × 10⁻³ S·cm⁻¹ at 25 °C, along with improved compatibility with the lithium electrode, a significant lithium-ion transference number, and a superior electrochemical window of nearly 5.3 V. A LiFePO₄–lithium battery with an ionanofluid gel–polymer electrolyte demonstrated an improved C-rate behavior and cycling stability with a discharge capacity of nearly 156 mAhg⁻¹ and 116 mAhg⁻¹ at C/5 and 2C rates, respectively, and a capacity retention of over 95% after 50 cycles at a C/5 rate.

6. Electrical-Conductivity-Influencing Factors

The electrical conductivity of electroactive nanofluids and ionanofluids is influenced by several factors. The primary influencing factors are summarized in Figure 6.

6.1. Temperature

In general, it can be concluded that an increase in operating temperature leads to a linear improvement in the electrical conductivity of nanofluids, which is an expected trend commonly observed in colloidal suspensions. Most of the published works on this research topic have noted that the effect of the temperature is not as significant as the one resulting from changes in concentration, and the increase in the electrical conductivity follows a linear pattern. For instance, in an experimental study conducted by researchers Nurdin and Satrianada [76], the electrical conductivity of maghemite aqueous nanofluids was determined at various volumetric concentrations of the nanoparticles and operating temperatures. The electrical conductivity measurements were made using a four-cell conductivity electrode meter, and it was confirmed that this property linearly increased with higher concentrations of nanoparticles and elevated temperature values. The maximum enhancements in the electrical conductivity of the maghemite nanofluids, induced by changes in the concentration of nanoparticles and temperature, were approximately 160.5% at 2.5% vol. and 60 °C. Increasing the concentration of maghemite significantly improved the electrical conductivity of the nanofluids, while temperature variations had a less pronounced effect on increasing the electrical conductivity. The authors concluded that the greater enhancement in the electrical conductivity of maghemite nanofluids was mainly attributed to changes in nanoparticle concentration rather than the temperature effect. An increase of approximately 4.7% in electrical conductivity was recorded at 30 °C and 2.5% vol., whereas a nearly 22.6% increase in electrical conductivity was observed at the same concentration of 2.5% vol. and 60 °C. The enhancement in the electrical conductivity of the nanofluids due to temperature increases followed a first-order relationship, which was weaker than the concentration effect. These results were consistent with those reported in other studies on nanofluids [22,23,36]. However, the results did not align with those estimated by the Maxwell and Bruggeman models, as these models did not consider the impact of temperature changes. Nonetheless, there are some exceptions in the published literature. For instance, researchers Shirazi et al. [77] noted that the operating temperature does indeed affect the electrical conductivity of nanofluids. Nevertheless, since there is only scattered data available on this subject, no clear pattern or trend has been established. Authors Akilu et al. [78] reported an Arrhenius-type equation to describe their results, suggesting a temperature-dependent relationship. In addition, Sarojini et al. [34] offered an interpretation for the limited increase in the electrical conductivity with temperature. They highlighted that the aggregation of nanoparticles is a time-dependent phenomenon, and the time required for aggregation significantly decreases with increasing temperature. As a result, the non-uniform dispersion of nanoparticles can reduce the electrical conductivity of the nanofluids when temperature rises. Furthermore, even though there are many studies available on the effect of temperature on the electrical conductivity of nanofluids, a significant drawback is the scarcity, or even absence, of correlations that specifically address the impact of increasing temperature. Many of the published correlations that consider temperature as a relevant factor combine both the concentration and temperature effects into a single equation, making it challenging to determine the individual contributions of each factor to the electrical conductivity of nanofluids.

6.2. Concentration

The majority of the published results have shown that nanofluids exhibit an enhancement in the electrical conductivity with increasing nanoparticle concentration. However, it has been observed that this enhancement can be followed by a reduction when the concentration reaches a certain limit. Researchers such as Zawrah et al. [79] have interpreted this reduction in electrical conductivity as primarily resulting from a reduction in the diameter, induced by an increase in the specific surface area and number of nanoparticles. As the number of particles increases, the available electric charges for creating the electric double layer become insufficient, and the electrostatic attraction force among alumina nanoparticles turns into a repulsion force. Additionally, authors like Chereches et al. [80] investigated the thermophysical properties of ionanofluids incorporating alumina nanoparticles in ionic liquid1-ethyl-3-methylimidazolium methane sulfonate [C₂mim][CH₃SO₃], with concentrations ranging from 0.05% wt. to 10% wt. Their results, obtained at temperatures between 10 °C and 60 °C, indicated that the viscosity of the nanofluids increased with higher weight fraction of nanoparticles, with an increase of up to 250% for the ionanofluid containing 10% wt. of alumina. On the other hand, electrical conductivity decreased with increasing nanoparticle content but increased linearly with rising temperature. Generally, the increase in the electrical conductivity of the nanofluids with increasing nanoparticle concentration can be explained by various underlying mechanisms. These mechanisms include the processes involved in the formation of the electrical double layer, interactions between this layer and the nanoparticles, and enhanced electrophoretic mobility of the nanoparticles. Furthermore, an increase in nanoparticle concentration provides more electrical conducting pathways within the nanofluids, leading to an overall enhancement in the electrical conductivity. However, most researchers [81,82] have explained the increase in the electrical conductivity as a result of the electric double layer formed due to charge accumulation or separation occurring at the interface of the electrode, in this case, and the nanoparticles immersed in an electrolyte solution. The electric double layer consists of two parallel layers of charge adjacent to the nanoparticles. The first layer is composed of a positive or negative surface charge, containing adsorbed ions due to the existing chemical interfaces. The second layer consists of ions attracted to the surface by the Coulombic force, electrically screening the first layer. This second layer is composed of ions moving freely within the base fluid due to electric attraction. Additionally, researchers Sarojini et al. [34] conducted a study on the electrical conductivity of nanofluids containing alumina, copper, and copper oxide nanoparticles with various sizes and concentrations dispersed in both water and ethylene glycol. The results they obtained demonstrated an almost linear increase in the electrical conductivity of both water- and ethylene glycol-based nanofluids with the increasing volumetric concentration of alumina nanoparticles. It was also observed that the addition of a surfactant improved the stability of nanofluids but also led to a decrease in their electrical conductivity by increasing viscosity. The reduction in the size of the nanoparticles resulted in a higher electrical conductivity in aqueous nanofluids, which can be attributed to the specific surface area and electrophoretic mobility of the nanoparticles. The authors noted only a minor increase in the electrical conductivity of water-based nanofluids with rising operating temperature. Furthermore, they investigated the formation of the electrical double layer by adding various molar concentrations of hydrochloric acid to the alumina aqueous nanofluids. They found that the electrical conductivity increased in nanofluids with low electrolyte concentrations, while a decrease was observed in nanofluids with high electrolyte concentrations due to decreased surface conductance. They compared their experimental results with the model proposed by O’Brien [83], which considers the electrical conductivity based on the zeta potential and electrokinetic radius of the suspended nanoparticles. Additionally, Glover et al. [84] determined the electrical conductivity of single-wall carbon nanotubes in 50:50 mixtures of ethylene glycol and water, with concentrations ranging from 0.05% wt. to 0.5% wt. Their results indicated that as the concentration of carbon nanotubes increased from the minimum to the maximum of 0.5% wt. the corresponding electrical conductivity increased nearly 13-fold.

6.3. Type of Base Fluid

The type of base fluid and its polarity can have a significant impact on the electrical conductivity of nanofluids. Water is a polar fluid, while oils are non-polar liquids, and ethylene glycol is a symmetrical polar molecule with internal dipoles. Ethylene glycol contains polar O-H groups, possessing both non-polar and polar components. Similarly, bioglycol has a wide range of polarity. When these glycols are mixed with water, the resulting mixture can be considered polar. Therefore, the enhancement of the electrical conductivity in nanofluids can be influenced by factors such as ionic concentration, thermophysical properties, addition of surfactants, and the polarity of the base fluid. The polarity of the base fluid promotes the formation of electric charges on the surface of the dispersed nanoparticles. In cases with a fixed concentration of nanoparticles, the base fluid has a strong influence on the electrical conductivity of nanofluids. This is primarily due to the creation of the electric double layer and the synergistic effect between the suspended nanoparticles and the base fluid. The maximum electrical conductivity is typically achieved with a mixture of bio-glycol and water as the base fluid, while the minimum electrical conductivity values are observed with ethylene glycol as the base fluid. For example, water, with an average electrical conductivity of 5.5 µS/cm [85], exhibits electrical conductivity approximately 10 times that of ethylene glycol, whereas bio-glycol can reach an electrical conductivity of 45 µS/cm [86].

6.4. Addition of Surfactants

The influence of surfactants on the electrical conductivity of nanofluids is an area that has received limited investigation, and as a result, there are no definitive conclusions on the matter. However, a study conducted by Shoghl et al. [87] involved various types of nanoparticles dispersed in both water and water with the addition of an SDS surfactant at concentrations of 0.01% wt. and 0.02% wt. The researchers observed that the electrical conductivity of water increased with the addition of SDS, and this increase was proportional to the concentration of the surfactant. For nanofluids that include SDS in their base fluid, the influence of the surfactant was directly related to the nature of the added nanoparticles. Specifically, for nanofluids containing alumina, titanium oxide, zinc oxide, copper oxide, and magnesium oxide, the incorporation of nanoparticles led to an increase in the electrical conductivity of both the base fluid and the base fluid with surfactant. However, similar trends were not observed for multi-walled carbon nanotube nanofluids, whether or not surfactant was included. The research team that conducted the study compared their findings regarding carbon nanotubes nanofluids and the interpretation of the ionic conduction mechanism with those of Glover et al. [84], who studied nanofluids containing up to 0.2% wt. carbon nanotubes dispersed in a 50:50 mixture of ethylene glycol and water as the base fluid. They observed a significant linear increase in electrical conductivity, up to 13-fold, which was attributed to the functionality and ionic conductivity of the carbon nanotubes. However, the functionalized nanotubes led to a reduction in electrical conductivity compared to non-functionalized nanotubes, as they disrupted the conjugated bonds in the nanotube system. Additionally, another study by Sarojini et al. [34] evaluated the effect of the surfactant SDS at concentrations of 0.1 mM and 0.5 mM and compared the results to those without the surfactant. The researchers found that the enhancement in the electrical conductivity due to the inclusion of SDS was more significant at concentrations up to 0.3% and less significant at concentrations higher than 0.3%. However, in contrast, a study by Bagheli et al. [39] reported low electrical conductivity values with the addition of a tetra methyl ammonium hydroxide surfactant.

7. Applications

In this section, some of the applications of electroactive nanofluids, ionanofluids, and ionic fluids will be explored. These fluids have played a fundamental role in the transformation of multiple industrial areas and the advancement of cutting-edge technologies. Figure 7 summarizes the main applications of electroactive nanofluids, ionanofluids, and ionic fluids.

7.1. Fuel Cells

In a recently published study, the researchers Esfe and Afrand [88] showed that the use of nanofluids to ameliorate the heat transfer rate and diminish the dimensions of the thermal management systems of electrochemical fuel cells has been a prominent research topic for the scientific community. Moreover, the researchers Zakaria et al. [89] inferred the thermos-electric performance of a proton exchange membrane (PEM) fuel cell operating with alumina nanofluids. From data obtained for the thermos-electrical ratio and advantage ratio, the authors inferred the feasibility of using nanofluids considering the thermal and electrical performance, such as improvements in the fuel cell performance, heat transfer capability, and working fluid flow. What drew more attention from the researchers was that including only small concentrations of nanoparticles, for instance, 0.1% vol. in the nanofluids, was enough to improve the heat-transfer rate, entailing only a moderate decrease in the electrical power. In fact, and as discussed by Esfe and Afrand [88], the thermal management in the cell must be performed in such a way as to keep its operating temperature within the appropriate range, and alternatives must be sought to distribute this temperature uniformly at different points in the cell. Such temperature gradients should be seriously considered since they have the greatest influence on fuel cell efficiency, performance, and lifespan. So far, little research has been conducted to find new ways to improve fuel-cell cooling, and studies on cell cooling using nanofluids are rather scarce [89,90,91,92,93,94]. Also, concerning the exploration of the nanofluids to cool PEM fuel cells, one of the main limitations derives from the lack of stability over time of the nanofluids, as emphasized by the authors Islam at al. [91]. Figure 8 presents a schematic representation of a proton-exchange membrane cell.

7.2. Batteries

In the review published by the researchers Chen et al. [94], safety issues that have limited the design and implementation of the lithium-ion batteries were highlighted. The authors stated that the incorporation of small quantities of additives or charges to the traditional fluidic electrolytes could significantly avert misuse without compromising the electrochemical performance. The cathode, electrolyte, separator, and anode are the main components of a LIB. According to the researchers, as it facilitates the ion transfer between the electrodes, the electrolyte is the primary agent in a battery system. In these circumstances, the electroactive nanofluids and ionic liquids may serve as substitutes for the conventional electrolytes for improving the efficiency and safety of the lithium-ion batteries. Regarding the use of ionic liquids, the researchers Rana et al. [95] presented the latest advancements and challenges related to these liquids when they are employed as electrolytes in lithium-ion batteries. The authors reported that ionic liquids have emerged as more promising electrolytes compared to the highly flammable volatile organic electrolytes. Since the electrolyte is the primary component of a cell responsible for the ion transport, ionic liquids, owing to their unique properties, would not compromise the cyclability and safety of the batteries. Nonetheless, the researchers emphasized that ionic liquids do not appear to be entirely free from causing environmental concerns, particularly in terms of toxicity and degradability. Another problem highlighted by the authors Ortiz et al. [96] concerns the room-temperature use of the ionic liquids, as the practical application of ionic liquid electrolytes faces challenges such as their very high cost and disappointing performance, characterized by low rate capacity and low cycle performance. Also, the researchers Rana et al. [95] explained that a cold environment could decrease the ion mobility, resulting in a substantial rise in the internal resistance. Despite this, the research has been conducted to propose solutions for the exploration of lithium-ion batteries in low-temperature applications, as demonstrated in the works carried out by the authors Li et al. [97] and Jaguemont et al. [98]. Figure 9 presents a schematic representation of a typical lithium-ion battery.

7.3. Redox Flow Batteries

The researchers Rueda-Garcia et al. [99] used the battery material LiFePO₄ and supercapacitor graphene nanoparticles in water electrolytes to infer the electrochemical activity of the nanofluids. The electroactive nanofluids were produced by dispersing the mentioned nanoparticles in an aqueous Li₂SO₄ electrolyte, which were stabilized with the aid of diaminobenzoic acid. A cyclic voltammetry technique was employed to evaluate the electrochemical performance in cells having three electrodes, and charge and discharge experiments of the LiFePO₄/reduced graphene oxide (positive) against reduced graphene oxide (negative) electroactive nanofluids were carried out. A charge-transfer percolation effect caused by the incorporation of reduced graphene oxide was proposed by the authors as being the fundamental underlying parameter for the ameliorated behavior of the LiFePO₄ and reduced graphene oxide nanofluids. The polar molecule LiFePO₄ interacted with the reduced graphene oxide by π–π forces, which were similar to a solvating effect. This interaction guaranteed the stability of the reduced graphene oxide in the water electrolyte. In addition, the steric effect of diaminobenzoic acid functional groups precluded the restacking of the reduced graphene oxide layers, which remained separated in the dispersion. The results demonstrated the strong impact of adding reduced graphene oxide and diaminobenzoic acid to a nanofluid with LiFePO₄, enabling the percolation of the charge-transfer between the collector and the LiFePO₄ uncoated nanosheets. These were employed as an electroactive nanoparticulate phase and demonstrated a redox performance improvement in the nanofluid in comparison with the cases using a solid electrode. It is noteworthy that the developed nanofluid did not affect the electrochemical signal of the LiFePO₄ through the improvement by the reduced graphene oxide of the redox process of the diaminobenzoic acid. Regarding this aspect, it can be concluded that the comparatively fast and reversible electrochemical features of the LiFePO₄ precluded the irreversible oxidation of the diaminobenzoic acid, particularly at fast rates. This ameliorated performance should be considered for the effective charge transfer from actual redox flow batteries using reduced graphene oxide in the nanofluid. Furthermore, the researchers Aberoumand et al. [100] inferred the impact of the usage of an electrolyte-based nanofluid on the efficiency of redox flow batteries comprising vanadium. In this sense, the authors investigated the rheological characteristics and the electrical and thermal conductivities of an electroactive nanofluid of graphene oxide nanoflakes in vanadium electrolyte solution at distinct fractions and temperature values. The researchers confirmed that the ideal weight concentration of the nanoparticles in the electrolyte nanofluid was 0.05% wt. with increased thermal and electrical conductivities, improved stability, and minimized viscosity enhancement of only 12% with respect to the electrolyte solution. The viscosity of the nanofluid at 0.1% wt. exhibited an increase of approximately 20%, which is very high for the nanofluid to be employed in a redox flow battery. In addition, the peak increments in the thermal and electrical conductivities were of 4% and 12%, respectively, which can be a benefit to flow battery performance concerning the electrochemical activity on the surface of the electrode and the thermal balance of the redox-flow battery. Also, the researchers verified that the nanofluid electrical conductivity was enhanced by enhancing the concentration of nanoparticles and the temperature. Nevertheless, at 0.1% wt., a slight reduction in the electrical conductivity occurred, attributed mainly to the decreased stability, which led to a poorly structured electric double layer and enhanced viscosity, which reduced the ion mobility. Furthermore, a rheological study indicated a transition from Newtonian to non-Newtonian behavior of the electrolyte nanofluid for weight fractions greater than 0.05% wt. The authors also proposed an empirical model giving reasonable estimated values for the viscosity of the graphene oxide–vanadium electrolyte nanofluids valid for the considered temperature and concentration ranges. Figure 10 shows a schematic representation of a typical redox-flow battery.

7.4. Electrochemical Flow Capacitors

Electrochemical flow capacitors occupy a prominent place in the research and technological areas of energy storage. The process potentiality, ease, and scalability for energy storage inherent to electrochemical flow capacitors make them very suitable for power grid energy storage purposes. The present models for electrochemical flow capacitors employ activated carbon and other carbon-derived materials to be used as slurries, often exhibiting poor electrochemical performance. To overcome this limitation, the inclusion of graphene in the slurries is a very suitable alternative, given that this material is a nano-sized carbon form and exhibits a high specific surface area, being also an excellent choice for energy storage purposes. Additionally, the researchers Sasi et al. [101] evaluated the efficiency improvement of an electrochemical flow capacitor with a graphene nanoplatelet–ionic electrolyte slurry for energy storage under static and dynamic conditions. The slurry-coated thin-film layers were cost-effective, with comparatively low toxicity and an electrode capacitance greater than 300 F·g⁻¹. The electrochemical flow capacitor exhibited a capacitance of 64.5 F at 2 V and 5 mL of graphene slurry, corresponding to 14.3 W·h·L⁻¹ of energy density, whereas it exhibited a capacitance of 2.3 F with 1.6 V and 0.422 W·h·L⁻¹ of energy density using a graphite slurry. When the slurry containing graphene nanoplatelets was employed in the full electrochemical flow capacitor, it provided 1.08 F of capacitance, with 2 V and 24 mL of slurry. Furthermore, the combined usage of the graphene nanoplatelet slurry and the ionic electrolyte demonstrated an ameliorated efficiency of the electrochemical flow capacitor, attaining 6 W·h·kg⁻¹ of energy density in comparison to that attained by a coarse slurry of graphite with 0.75 F of capacitance, 1.6 V, and 2.2 W·h·kg⁻¹ of energy density.

7.5. Dye-Sensitized Solar Cells

The authors Lee et al. [102] demonstrated that a 0.08% wt. dispersion of copper nanoparticles encapsulated within a carbon shell in [Bmim][PF₆] was a very suitable electrolyte to be employed in dye-sensitized solar cells. The ionanofluid exhibited a 35% enhancement in the diffusion coefficient and a 65% increase in the electrical conductivity with respect to those of the ionic liquid itself. The increased electrical conductivity of the ionic liquid electrolyte was interpreted by the authors as being the result of the interactions between the encapsulated copper nanoparticles and the [Bmim]⁺ cations. The addition of the nanoparticles to the ionic liquid caused a decrease in the ionic bonding energy between the ionic liquid [Bmim]⁺ cations and [PF₆]⁻ anions conducive to a self-diffusion coefficient increase for the cations and a viscosity reduction in the ionic liquid. Additionally, multi-walled carbon nanotubes dispersed in an ionic liquid were evaluated by the researchers Neo and Ouyang [103] as a nanofluidic electrolyte to be employed in dye-sensitized solar cells. Incorporating 0.1% wt. of the nanotubes into 1-propyl-3-methylimidazolium iodide [Pmim]I ionic liquid provided hydrogen bond formation between the [Pmim]⁺ cation and the carbon nanotube carboxylic groups and diminished the Coulombic attraction between the cations and the anions I⁻, hence reducing the viscosity of the ionic liquid and enhanced the thermal stability of the electrolyte. In addition to the thermophysical and electrochemical improvements in the ionic liquid electrolytes via the incorporation of the nanoparticles, the ionic fluids could enhance the chemical stability of the electrolytes by creating ionic double layers to protect the added nanoparticles against potential corrosion. Furthermore, the authors Adhyaksa et al. [11] studied a plasmonically enhanced thin dye-sensitized solar cell operating with an electrolyte composed of silver nanoparticles in an imidazolium–dicyanamide ionic liquid. The authors reported that the solar cell exhibited improved stability, given that in the absence of the ionic fluid, the acetonitrile electrolyte that contained redox compounds, which corroded the silver nanoparticles, would degrade the device stability over time, thus deteriorating the performance of the solar cell. Nonetheless, the non-volatile ionic liquid electrolyte protected the surface of the silver nanoparticles from the contact with the redox compounds, creating an ionic double-layer in the vicinity of the silver nanoparticles surface, thus reducing the corrosive potential. Figure 11 presents a functioning scheme of a dye-sensitized solar cell.

7.6. Electrochemical Sensors

Electrochemical sensors can detect biological and chemical compounds and corresponding fractions by the conversion of a catalytic/binding event to a signal proportional to the concentration of the analyte [104]. Electrochemical sensors are being developed to obtain an improved sensitivity, rapid response, and better selectivity. Nonetheless, when dealing with biosensors, and because of the effect of the protein structure, the transport of electrons between the enzymatic active center and the electrodes is usually obstructed, hindering the electron transfer [105]. To overcome this limitation, biocompatible nanomaterials are being employed to immobilize the proteins on the surface of the electrode [105]. Metallic and oxide nanoparticles, among others, serving as electron transfer media have been applied to develop perfected electrochemical sensors [106]. The inclusion of the composites of ionic liquids with nanoparticles into the biosensors considerably improved the sensitivity of the electrochemical sensor while maintaining its superior catalytic activity and electrochemical stability. In addition, ionanofluids can be applied for the coating of the electrode. In this respect, the researchers Li et al. [105] dispersed multi-walled carbon nanotubes into a [Bmim][BF₄]-stabilized gold nanoparticle suspension for glucose oxidase immobilization. The immobilized glucose oxidase-coated carbon electrode possessed good reproducibility and stability. The carbon nanotubes offered enhanced surface area, electrical conductivity, and mechanical strength, while the gold nanoparticles promoted the electron exchange and electron transfer of the glucose oxidase. In turn, the ionic liquid provided the required electrostatic interactions for the suspended multi-walled carbon nanotube and gold nanoparticle stabilization. Furthermore, the authors Franzoi et al. [107] proposed a biosensor composed of gold or silver nanoparticles suspended in [Bmim][PF₆] and laccase immobilized on chitosan, which provided a considerably enhanced and accurate sensitivity for the electrochemical tracing of luteolin with 0.054 μM and 0.028 μM of detection limits employing the silver-[Bmim][PF₆] and gold-[Bmim][PF₆] biosensors, respectively. The suspension of the nanoparticles in the ionic fluid provided an adequate environment for the laccase, enabling the transport of the electrons between the immobilized laccase and the biosensor surface, resulting in an improved analysis capability. Furthermore, the same authors [108] verified that the combined usage of gold nanoparticles and [Bmim][PF₆] ionic fluids provided a 14% increase in the response of the biosensor to the luteolin solution. Via covalent bonding with the ionic fluids, the suspension of the ionic fluid-functionalized nanomaterials can be ameliorated because of the ionic liquid supramolecular structure. The enhanced electrochemical behavior of these nanomaterials augments their applicability in electrochemical sensors, presenting a wider linear range and decreased limit of detection. For instance, the researchers Wang et al. [108] incorporated functionalized graphene oxide nanosheets loaded with nanoparticles of gold into a 1-allyl-3-methylimidazolium chloride electrode. The developed electrochemical sensor possessed improved sensitivity, a wider range from 1 × 10⁻¹⁰ to 1 × 10⁻⁶ M for Sudan I and a limit of detection of 5×10⁻¹¹ M. The inclusion of the nanoparticles into the ionic liquid led to an extended surface area, which promoted the adsorption of greater amounts of Sudan I into the electrode. In addition, the authors Shang et al. [109] proposed 1-(3- aminopropyl)-3-methylimidazolium bromide-functionalized graphene nanoribbons built with palladium–silver hybrid hollow nanoparticles fused into the surface of the electrodes of a nifedipine electrochemical sensor. The developed sensor had good stability, reproducibility, and selectivity within 10 nM to 4000 nM of nifedipine and a 4 nM detection limit. It is noteworthy that the ionic fluids and nanoparticle composites can also be synthesized in the form of layer-by-layer assemblies and films, gathering combined benefits for the improvement of the efficiency of the electrochemical sensors. In this sense, the researchers Xiang et al. [110] proposed a biosensor based on a film composed of an ionic fluid, multi-walled carbon nanotubes, and gold nanoparticles. The authors found that the proposed sensor could play the role of a modifier to conduct the electron transfer of the redox proteins, and it possessed enhanced sensitivity to hydrogen peroxide. Also, the inclusion of catalytic nanoparticles could reduce the overpotentials of the electrochemical reactions and provide the redox reactions reversibility to produce highly sensitive electrochemical sensors. In this sense, the researchers Zhu et al. [111] prepared a layer-by-layer nanostructure composed of imidazolium ionic fluid-functionalized graphene nanosheets and citrate-stabilized platinum nanoparticles with negative charge. The nanostructure prevented the pH dependence of the properties of the ionic fluid on the carbon nanotubes and enabled an increased number of electrostatic interactions between the functionalized graphene and other components with opposite charge. The multi-layered film presented enhanced the thermophysical and electronic characteristics of the graphene together with superior electrical conductivity to the imidazolium ionic fluid and exhibited high electrocatalytic activity for oxygen reduction that could be adjusted by the number of film layers. Moreover, the researchers Bagheri et al. [112] proposed a potentiometric sensor comprising the ionic fluid 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide [BMP][NTf₂], titanium oxide nanoparticles, and multi-walled carbon nanotubes for the trace determination of thallium (I) (Tl(I)). The sensor possessed a fast response and increased dynamic operating range, which were attributed to the enhanced ionic conductivity, dielectric constant, and thermal and electrochemical stabilities of the mentioned ionic fluid. Also, the incorporation of the carbon nanotubes considerably increased the electrical conductivity of the system. Apart from this, the extended surface area and comparatively good electrical conductivity of the titanium oxide nanoparticles also ameliorated the response of the electrode.

7.7. Metal Electrodeposition

The ionic liquids can be a promising alternative to the aqueous electrolytes commonly employed in metal electrodeposition. Ionic liquids possess a wider electrochemical window than that of water, expanding the potential of electrodeposition. Nevertheless, the mass transport in ionic fluids is somewhat slow, and this greatly impacts the reaction rate at the interface between the solid and liquid, hindering the use of ionic fluids in high-efficiency electrodeposition procedures. Nevertheless, ionic liquids are preferred over common solvents when a distinct reactivity, extended electrochemical window, and smaller vapor pressure are needed [113]. Furthermore, common solvents and ionic liquids have much different mass-transfer properties. For example, the dynamic viscosity range is typically approximately 1 cp, which is two orders of magnitude lower than that of most ionic liquids [114,115]. The increased viscosity also impacts on the coefficient of diffusion. In fact, the coefficients of diffusion of the ionic liquids are inferior to those of water electrolytes. This greatly impacts situations where the rate is regulated by interface mass transport processes like absorption, heterogeneous catalysis, and electrochemistry. A limited current for mass transport can lead to the production of incoherent coatings, forming dendrites with poor adhesion and hindered functional characteristics. The use of convection-forced mass transport can limit these phenomena, but regulating the convection process requires a profound knowledge of the structure and the evolution of the convection boundary layers in the electrolytes. Figure 12 presents a schematic representation of a metal electrodeposition process assisted by ionic liquids.

In recent years, the search for new coatings presenting improved application suitability has paved the way for the usage of ionic liquids acting as electrolytic alternatives. Apart from possessing enhanced conductive characteristics, the growing interest in such solvents derives also from their environmentally appealing low toxicity. Additionally, the capability of ionic liquid electrolytes to dissolve compounds with less soluble metals makes the ionic liquids very promising substitutes for the common environmentally harmful solvents. The low-viscosity ionic liquids are preferred in metal electrodeposition studies because of their synthesis ease and electrochemical stability under oxidative and reducing scenarios [116]. Nonetheless, ionic liquids may possess a high viscosity, which may reduce the mass transfer capability, resulting in a mass transfer process much slower than that with aqueous electrolytes [117]. However, this limitation can be mitigated with the use of the ionic liquids in the form of a mixture of eutectic solvents. For instance, the choline chloride has a viscosity of 14.2 mm²/s. In certain works, the mixtures are composed of urea and/or ethylene glycol [118,119] to decrease the overall viscosity and achieve a global melting point that is lower than that of the pure components by enhancing the mass-transfer rate [120,121]. Among the metals, aluminum [122], nickel [123], and zinc [124] are the most frequently employed in electrodeposition with the aid of ionic liquids, mainly because of their protective features and fast mass transport during the process. Furthermore, there are preferable metal substrates, including alloys such as copper [125] and iron [126], that perform effective co-deposition and fast nucleation. Another relevant application for ionic liquids is the dissolving of salts, noble metals, and rare earth oxides and carrying out the deposition of these elements. The deposition procedure separates the metals from a complex mixture, making the usage of the ionic liquids of great relevance. The oxidizing capability of light metals like sodium, lithium, and magnesium can be better regulated by using ionic liquids in battery catalytic reactions, promoting the electro-adsorption process under conditions with less stability [127]. Consequently, such works emphasize the capability of ionic fluids to substitute traditional organic solvents. In this direction, this review addresses the use of ionic liquids in metal and alloy electrodeposition processes in recent years. The lower consumption of energy and uniform and smooth coating induced great interest in lithium-ion batteries research and technological areas. In this direction, the authors Yang et al. [128] examined the [EMIm]Cl ionic liquid as a liquid electrolyte for the electrodeposition of aluminum on nickel substrates. The researchers obtained a deposit that improved the corrosion resistance of the nickel and was very suitable to be applied to anodes in lithium-ion batteries. A uniform and smooth film was also formed at 20 mA/cm² of current density onto a substrate made of uranium, considerably augmenting the resistance against corrosion. Another finding was related to the magnetohydrodynamic effect of AlCl₃–[EMIm]Cl in decreasing the electrochemical reduction activation energy. Also, it augmented the electrical double layer capacitance and provided enhanced aluminum mass transfer [129]. The researchers He et al. [130] examined the combined use of lithium salt with bis(trifluoromethanesulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI) for lithium deposition onto copper employing a solid electrolyte interphase under low overpotentials. With an increment in the concentration of FSI, the maximum current also rose, and the maximum time was reduced. Also, a decrease in the nucleation overpotentials occurred with enhancing FSI concentration, verifying the interfacial effect of the SEI and lower interfacial energy with the TFSI:FSI 0:1 ratio. Also, the authors Hou et al. [131] inferred the effect of organic imidazolium cations on cycle durability and switching velocity according to the adsorption energy difference. A high adsorption energy promotes stronger adhesion between organic cations and silver surfaces. Such intense adsorption would possibly avert the aggregation process during the nucleation of the silver nanoparticles, resulting in a more compact and dense silver deposit. The combined effect of the improved cycling stability and rapid switching yielded tunable windows based on the silver reversible deposition and ionic liquid electrolytes, turning the REM devices into a very promising substitute for conventional smart-response materials. Furthermore, the authors Yang et al. [132] proposed an aqueous biphasic extraction using [EMIBr]. The suitability of the phosphate salts was investigated for gold extraction from alkaline solutions of aurocyanide. Gold recovery depended on the ionic liquid using an electrodeposition of −1.2 V at 25 °C. A gold deposition rate over 92% in the ionic liquid solution was observed without the formation of crystallites or impurities. It was discussed that in the deposition of gold, the ionic liquids themselves did not promote the production of films. Hence, other compounds were required to generate favorable interfaces that promoted the deposition. Conversely, a choline chloride and ethylene glycol eutectic mixture in a 1:2 molar ratio led to a gold deposition efficiency between 30% and 70%. Moreover, the authors Chen et al. [133] studied five ionic liquids of the imidazolium group: [C₁₂PIm]Br, [C₁₄PIm]Br, [C₁₆PIm]Br, [C₁₄PImNH₂]Br, and [C₁₄BIm]Br. After platinum extraction, the ionic liquids were evaluated for electrodeposition on a copper plate. The obtained results demonstrated that the most efficient electrolyte in the deposition of platinum was the [C₁₄PImNH₂]Br because of its hydrophobic character and extended alkyl chain in the imidazolium cation. Moreover, in an acidic solution, the protonation of amino groups produced –NH₃⁺ radicals, which played the role of providing extra binding sites for platinum and ameliorated the efficiency of deposition when the concentration of [C₁₄PImNH₂]Br increased. Additionally, the authors Shao et al. [134] obtained a recovery of 90% of palladium in an extraction system with the ionic liquid [C₈bet]Br/[C₄mim][NTf₂] at 75 °C, demonstrating that ionic liquids with different hydrophobicities enable the electrodeposition process at temperature values above room temperature. The authors also assessed palladium stripping using 1.2 M thiourea in 0.5 M hydrochloride acid, attaining a rate of recovery superior to 91% even after three cycles of the extraction–electrodeposition–stripping procedure. The correlation between palladium and ionic liquids was the most prominent in the group of platinum. Furthermore, the researchers Polzler and Whitehead et al. [135] studied zinc electrodeposition from a choline chloride and ethylene glycol mixture with a 1:2 molar ratio onto a substrate made of carbon. The authors compared the voltametric behavior of the zinc using an electrolyte free of choline containing only the ZnCl₂ salt and sodium ethoxide or ethylene glycol. The authors confirmed the choline chloride action and verified an unexpected cyclic voltammetry behavior, indicating that the choline ions obstructed the surface of the electrode during the forward cathodic potential sweep, as the zinc was deposited during the backward scan. Additionally, the researchers Wang et al. [136] achieved an efficient electrodeposition of zinc on copper in bis((trifluoromethyl)sulfonyl)imide anion [TFSI]⁻ ionic liquids using 1-n-butyl-1-methylpyrrolidinium[BMP] as the cation. In the process, the zinc species were incorporated by the anodic dissolution of the zinc or, alternatively, by the addition of Zn(TFSI)₂ or ZnCl₂. The ZnCl₂ exhibited improved solubility in the [BMP][TFSI] because of the enhanced interactions between the TFSI⁻ and Cl⁻ anions, resulting in the production of TFSI- coordinated Zn(II) species and partially Cl^- coordinated zinc species, which corresponded to the generation of two zinc species redox couples in the ZnCl₂ solution. Nonetheless, the deposited zinc with the Zn(TFSI)₂ and [BMP][TFSI] exhibited superior cathodic current efficiencies to those of the zinc deposited with the ZnCl₂ and [BMP][TFSI] ionic liquid. These lower efficiencies were attributed by the authors to the fact that the liberated Cl⁻ anions produced a non-reducible complex by coordinating with the Zn(II) species in ZnCl₂ redox couples, which were partially Cl⁻ coordinated Zn(II) species. Moreover, the researchers Steichen et al. [137] investigated hybrid zinc deposition using zinc(II)-containing liquid metal salts with N-alkylimidazole ligands and bis(trifluoromethylsulfonyl)imide Tf₂N⁻ anions. Nickel–zinc hybrid deposits were obtained using metal ionic liquids like [1-n-butyl-1-methylpyrrolidinium] bis((trifluoromethyl)sulfonyl)imide [TFSI][BMP]. Using these ionic liquids and exploring the advanced plating techniques like the pulsed, compositionally modulated multilayer, electromagnetic plating was shown to appreciably increase the zinc alloy and zinc deposition rates with the intended composition and thickness. Nonetheless, high current densities greater than −200 mA cm⁻² were needed to produce the deposit of zinc.

7.8. Thermo-Electrochemical Cells

The heat-to-electricity conversion rate of ionic fluids and small molecule electrolytes is mainly determined by the Soret effect of the ions in an electrolyte [138]. Particularly, under a temperature difference, the ions migrate and accumulate between the hot side and cold side of the cell. Consequently, the migration of ions in the electrolyte strongly impacts the heat conversion rate. If there is no difference in the mobility of anions and cations in the electrolyte, it is expected that both cations and anions arrive at the same time at the cold electrode, and under these conditions no thermopower or thermovoltage is generated for the thermo-electrochemical cells. Consequently, to achieve the high thermopower required for large thermovoltage generation, the fundamental demand for thermo-electrochemical cells should be the considerable difference in mobility between the anions and cations in the electrolyte. The cations and anions accumulate at the hot and cold electrodes, respectively, conducting too much increased thermopower and thermovoltage for the thermo-electrochemical cells. Also, the migration of the ions in the electrolyte can be controlled by controlling the interactions between the ions and their nearby regions according to the Soret effect. Furthermore, the authors Abraham et al. [139] demonstrated that thermos-electrochemical cells employing ionic liquid electrolytes showed to be very promising for applications at high temperatures over 100 °C and in thermal energy harvesting purposes. The I⁻/I₃⁻ dispersed in [C2mim][BF4] electrolytes attained power densities up to 29 mW/m² in unoptimized cells working with the hot side at a temperature of 130 °C. Also, the researchers obtained an adjusted thermos-electric figure of merit to evaluate the characteristics of the electrolytes, and this comparison required the determination of the thermal conductivity of the electrolyte, the diffusivity of the redox couple, and the Seebeck coefficient of the electrolyte. Additionally, the authors Cheng et al. [140] regulated the ion–dipole interactions between an ionic liquid and a gel matrix and obtained a very large thermopower of 26.1 mVK⁻¹. The thermopower can also be increased by regulating the small-molecule electrolyte interactions with their neighbors. For instance, the fixation of Na⁺ cations in a polyvinyl alcohol hydrogel matrix by coordinating interaction with hydroxyl groups was conducive to the migration of OH^- anions in the polyvinyl alcohol matrix, and a large thermopower value of 19.69 mV K1 was attained [141]. Finally, the researchers Li et al. [142] using the same NaOH small-molecule electrolyte achieved a thermopower of 24 mVK⁻¹ through the restriction of the migration of OH^- with the oxidized cellulose membrane. Figure 13 shows the functional scheme of a typical thermo-electrochemical cell.

7.9. Carbon Dioxide Capture

Carbon dioxide separations can be performed with the aid of ionic fluids [143]. Poly(ionic liquids) with an adequate pairing of anions and cations were already found to be capable of separating CO₂ through electrostatic complexation or repulsion, reversibly [144]. Though the ionic liquids ameliorate the selectivity for CO₂-CH₂ and CO₂-N₂ mixtures, these fluids are not the ideal candidates for large-scale applications given the intrinsic low gas permeability. Consequently, the membranes are composed of ionic liquids and hybrid nanoparticles to achieve an improved gas separation efficiency. An example of an often-used separation membrane is the coating of dispersions of copper nanoparticles in [Omim][BF₄] onto a polysulfone microporous membrane [145]. In addition, the gas permeance was increased and the carbon dioxide selectivity increased when the polarized surface of the nanoparticles was employed in an imidazolium ionic fluid [146]. The steric hindrance from the long alkyl chains gave comparatively free imidazolium cations, which interacted with the carbon dioxide molecules and strengthened their separation. A strong interaction was observed between copper nanoparticles and the ionic fluid that acted as a stabilizer and polarizer. The copper nanoparticles with positive charge conducted a reversible interaction with the carbon dioxide molecules, whose quadrupole moment was greater than that for the nitrogen and methane, improving the separation process [147]. Similarly, the authors Hudiono et al. [148] produced a membrane composed of copper nanoparticles dispersed in the ionic fluid [Bmim][NO₃] that promoted carbon dioxide transport selectively while maintaining constant methane and nitrogen permeances. Furthermore, the researchers Willa et al. [149] explored the dispersion of zeolite particles in the imidazolium-based ionic liquid [Emim][NTf₂] for producing a mixed-matrix membrane for gas separation applications. The inclusion in the system of the ionic fluid augmented the carbon dioxide–methane and carbon dioxide–nitrogen selectivity. The forces of adhesion between the ionic fluid and the zeolite particles surface were enhanced, given that the ionic fluid played the role of interfacial material between the dispersed particles, and the corresponding enhancement in the free volume of the membrane improved the gas permeability. Additionally, a film composed of poly[(p-vinylbenzyl)trimethylammonium hexafluorophosphate] P[VBTMA][PF₆] and La₂O₂CO₃ nanoparticles at concentrations up to 100% wt. was explored in poly(ionic liquid)-based carbon dioxide sensors [150]. The chemical reactivity of the poly(ionic liquid) P[VBTMA][PF₆] and the La₂O₂CO₃ could be transduced into an electrical signal by electrostatic interactions. The obtained increased electrically conductive channels at the interface between the poly(ionic liquid) and the nanoparticles increased the electrical conductivity with 150 ppm to 2400 ppm of carbon dioxide. Figure 14 presents the scheme of the carbon dioxide capture with a nanofluid process.

8. Recommendations for Further Research Studies

The recommendations for further research studies can be summarized in the following points:

• It is recommended to conduct more research studies on the electrochemical evaluation of the nanofluids with high concentrations of nanoparticles as liquid electrolytes and/or electrofuels to be applied in redox-flow batteries and on further improvement of the surface coating to optimize the electrochemical effectiveness.
• Functionalized nanotubes would reduce the electrical conductivity of nanofluids compared with that attained with non-functionalized prime nanotubes, given that they cause the rupture of the conjugated bond of the nanotubes. Nonetheless, such underlying mechanisms should be further addressed in additional research works on the topic.
• The addition of surfactants and additives may have a considerable impact on the electrochemical properties of the nanofluids, which should be further investigated and sustained by coordinated experimental works.
• Future research work should be focused on retrieving an empirical correlation considering the factors needed for determining the electrical conductivity of hybrid nanofluids in a more accurate way.
• Further research should be performed to infer the long-term electrochemical cycling performance of surface-modified nanomaterials dispersed in ionic liquids, stability of the surface graft, and overall efficiency in redox-flow cells in experiments with diverse concentrations of nanoparticles, ionic conductivities, and flow rates.
• Innovative electrochemical flow capacitors should be further investigated through experimental works using slurries with different nanomaterials, including the incorporation of materials with enhanced faradaic capacitance to be applied in better-performing flow electrochemical devices. Also, more works on the subject are most welcome.
• Regarding redox-flow batteries, the possibility of employing nano-sized pastes with high amounts of nanoparticles dispersed in liquids, thus combining the synergetic effect of the low viscosity of the nanofluids and good flowing features with the higher energy density of the pastes, should be further addressed.
• Further systematic molecular dynamics simulations are recommended for better understand ion mobility and the interactions at the interface between the ionic fluid electrolyte and the solid electrode.
• The development of ionic liquid precursors exhibiting synthesis ease, like, for instance, imidazolium bio-ionic liquids and porous ionic liquids, still needs further advances to obtain a durable super-capacitive activity derived from the charging mechanism of the electric double layer.
• Further attempts should be carried out to integrate the ionic liquid electrolyte and electrode components as individual equipment for manufacturing all-in-one ionic liquid-based capacitor systems.
• Most of the electrodeposition studies involving ionic liquids carried out so far are done at laboratory scale, and the literature findings about practical plating systems are rather scarce. Hence, further works on the scalability of the deposition processes are needed, along with more studies of the lifespan of metallic alloy coatings in the aerospace technological field. Also, there should be proposed ameliorated and innovative testing protocols to accurately assess the quality and lifespan of the coatings in laboratory environments mimicking real-life service conditions.
• Further research is needed on the suitability of the liquid electrodes (e.g., electroactive nanofluids) to be used in reserve batteries [151], in which one of the components is stored in an inactive form and gets activated when called upon. Also, the work performed by the researchers Parekh et al. [152] deserves special attention since it deals with the concept of lithium-ion reserve batteries, and particularly with the in-situ lithiation of lithium-ion free vanadium pentoxide cathode with graphitic anode.

9. Conclusions

The main concluding remarks of this overview work are presented in the following point:

• Ionic liquids, ionanofluids, and electroactive nanofluids are very suitable fluids to be applied in electrochemical processes like the ones described in this overview, given that they all possess improved electrochemical properties like enhanced electrical conductivity and capacitance.
• The electrical conductivity of the nanofluids can be influenced by the concentration and morphology of the nanoparticles and operating temperature, among other factors. A temperature increase leads to enhancements in the electrical conductivity mainly due to the Brownian motion increase. Also, a greater amount of solid phase enhances the nanofluid electrical conductivity.
• Most of the research found enhancements in the electrical conductivity of nanofluids derived from increases in the operating temperature and fraction of the nanoparticles. The main rationale behind the electrical conductivity growth at higher concentrations was the clustering of the nanoparticles. At high values of temperature, when the collision rate of the nanoparticles is enhanced owing to the greater kinetic energy, the electrical conductivity will also rise.
• The proposed empirical models and correlations to predict the electrical conductivity of the nanofluids are mostly functions of the temperature and concentration of the nanoparticles. The models are applicable only in a narrow range of concentrations and working temperatures. The temperature and concentration increases exhibited restrictions for increasing the electrical conductivity to a certain limit, beyond which an electrical conductivity decrease occurred. Also, the Maxwell model often could not predict the deviation in electrical conductivity when the nanoparticles were dispersed into the base fluid.
• It was verified that there is usually no considerable impact of the operating temperature on the electrical conductivity of nanofluids at temperatures between 30 °C and 60 °C. Such evidence demonstrates that the underlying mechanisms of electrical conductivity increase may be different from those of the nanofluid thermal conductivity increases.
• Ionic liquids have been gaining attention in automotive and aerospace research and technological areas for the electrodeposition of metals and alloys. Furthermore, the electrochemical stability of a great part of the ionic liquids under real plating scenarios at different current densities and temperatures still requires a better knowledge.
• The cation/anion designability of ionic liquids entails complex van der Walls interactions, electrostatic attractions, and hydrogen-bonding forces, allowing the exploration of ionic liquids in electrolyte and electrode synthesis for supercapacitors. Also, through controlling the pH, alkyl chain substituents, and other factors between the ions, the solid−liquid phase transitions of the ionic liquids are observed and adapt the integrated requirements in only one device.
• Poly(ionic liquid)s in a polymeric state are very promising alternatives to produce task-specific carbonaceous membranes in portable devices for gas separation.
• In the case of inert electrodes, the interfacial ion packing or double-layer capacitance can be increased through an increase in the pore compatibility between the electrode structures and the electrolyte ions of the ionic liquid or, alternatively, by blending the ratios of the ionic liquid eutectic mixtures. When dealing with pseudocapacitive electrodes, the energy storage capability can be strongly enhanced by protic ionic liquid electrolytes and redox-active mediators based on pseudocapacitive hybrid energy storage processes. Furthermore, porous ionic matrices are emerging to mitigate the environmental hazard derived from liquid leakage and corrosion and to be used in compact energy storage devices and systems.
• One of the fundamental concerns related with large-scale metal electrodeposition employing ionic liquids is to attain reproducible deposition with the intended throwing power, which is the ratio between the thickness of the deposit at high current density to thickness of the deposit at low current density of around one.