A Novel Oxidation–Reduction Route for the Morphology-Controlled Synthesis of Manganese Oxide Nanocoating as Highly Effective Material for Pseudocapacitors

Abstract

In recent years, pseudocapacitors have been receiving much attention as low-cost and safe energy storage technology for emerging applications in flexible and safe devices. However, creating high-energy-density electrode materials is now the main limit for high-performance pseudocapacitors. In this work, we propose a novel reduction route for the synthesis of uniform MnO₂ nanocoating with porous morphology on nickel foam via the SILD method as electrode material for high-effective pseudocapacitors. The obtained nanocoatings were characterized by SEM, TEM, EDX, XRD, XPS, and electrochemical techniques. Comparisons of MnO₂ coatings were conducted to obtain the reduction and oxidative routes of synthesis. The influence of the oxidation–reduction reaction type on the structures, morphologies, and capacity performance of manganese oxide was investigated. The results show that the nanocoatings synthesized via the reduction route were formed of amorphous uniform ultra-thick coating MnO₂ with a porous morphology of “nanoflakes.” Due to the unique morphology and uniform coating of nanosized manganese oxide, electrodes based on this process have shown a high specific capacity (1490 F/g at 1 A/g) and excellent cycling stability (97% capacity retention after 1000 charge–discharge cycles).

1. Introduction

In recent years, the need for the development and creation of new efficient energy sources, which determine the prospects for the development of industrial plants, electric transport, space, aircraft, etc., has increased. Such power sources are subject to a number of requirements: high values of specific energy and capacity, stability of characteristics at multiple charge–discharge cycles, long service life, high operating voltage, and power. In addition, these devices must be compact, environmentally sustainable, environmentally friendly, and inexpensive to produce [1,2].

Currently, among traditional power sources, the most widely used are lithium-ion batteries whose high values of specific energy (80–200 W·h/kg) and power allow for widespread practical use [3]. However, lithium-ion batteries have many disadvantages, namely the high cost of lithium raw materials caused by the constant increase in the production rates of such power sources and the problem of ensuring their safe disposal, a relatively large mass, and a long charge time (which is critical for their use in electric vehicles and portable devices), as well as the problems of overheating and the risk of explosion [4].

Consequently, there has been a growing interest in the development of alternative types of power sources, such as supercapacitors, known for their widespread use and low cost, as well as their increased operational safety compared to lithium-ion batteries. Of particular interest are hybrid batteries, which combine two mechanisms of energy storage, namely a double electric layer at the electrode–electrolyte interface and charge transfer in the electrolyte. The pseudocapacitor energy storage devices based on capacity and supercapacitor electrodes offer a promising way to construct devices with the merits of both secondary batteries and supercapacitors [5,6,7]. As electrodes for pseudocapacitors, carbon materials or electrodes based on transition metal oxides (Ni, Co, Mn, and Fe-based) are employed as pseudocapacitor electrodes, while aqueous alkaline solutions are used as electrolytes [8,9]. Thus, the synthesis of electroactive nanomaterials and nanocoatings based on transition metal oxides is the key point for the development of electrode materials for highly efficient energy storage devices in partially flexible energy storage devices [10,11,12].

From this standpoint, manganese oxides are ideal candidates for pseudocapacitive electrode materials owing to their multiple valence states enabling rich redox reactions as well as their excellent resistance in alkaline media [13,14,15,16]. Additionally, manganese oxides are more abundant and environmentally friendly than other transition metal oxides. However, the achievement of high specific capacitance, power density, and cyclic stability is limited by the structure and morphology of oxides, which play a key role in ensuring high electronic conductivity and a large number of active centers on the surface, as well as improving ion diffusion at the electrode–electrolyte interface. The solution to this problem may consist of obtaining ultrathin nanocoatings with a two-dimensional graphene-like morphology, providing a set of unique physical and chemical properties [13]. Currently, the main problem in the development of electrode materials for pseudocapacitors is the complexity of obtaining uniform ultrathin nanofilms and nanocoatings of oxides using existing methods of synthesis in a way that would significantly reduce the degradation of the electroactive material and close to the values of a theoretical specific capacity [17].

In this respect, the SILD (Successive Ionic Layer Deposition) [18] or SILAR (Successive Ionic Layer Adsorption and Reaction) methods can be prospectively applied [19]. The SILD method, compared with other popular coating techniques, has unique traits and advantages. For example, it allows for the deposition of coatings on various surfaces without dimensional restrictions and for their thickness to be readily modified across a wide range by controlling the concentration of the precursor, the number of depositions, and so on [19]. Furthermore, using various synthetic routes, the deposited coatings can be tuned to exhibit preferential morphologies and crystallographic orientations. The ability to obtain coatings on the substrate surface without the use of additional binding materials is key for the synthesis of highly efficient electrode materials. Previously, we have successfully obtained electroactive nanocoatings based on transition metal oxides and hydroxides with different structures and morphologies using the SILD technique [20,21,22,23,24].

Currently, most studies use an oxidative synthesis route to obtain manganese oxide coatings via the SILAR method [25]. MnO₂ is synthesized using two main methods, namely Mn²⁺ oxidation and a redox reaction of Mn²⁺ and MnO⁴⁻. In the first method, MnO₂ is generated by treatment oxidants, such as H₂O₂, K₂S₂O₈, NaClO, etc. In the second case, MnO₂ can also be prepared by the reaction of MnO₄⁻ with Mn²⁺ under alkaline conditions. However, these methods are characterized by a long reaction time and require a specifically controlled pH, complicating the reaction conditions as well as introducing alkali metal impurities to the film. Previously, the effectiveness of the use of a reduction synthesis route for the production of manganese oxide was shown. For example, paper [26] describes the production of MnO₂ nanoflowers by reducing potassium permanganate with hydrogen peroxide in an acidic medium. In the process of such a synthesis, peroxide is decomposed into non-toxic H₂O and O₂. The in situ evolution of oxygen gas bubbles also plays an important role in the formation of the unique morphology. However, previously, such a synthesis route has not been used to obtain electroactive MnO₂-based nanocoatings via the SILD (SILAR) method.

In this work, we compare the oxidation and reduction routes for the synthesis of manganese oxide ultrathin nanocoatings via the SILD method. Manganese oxide nanocoatings were deposited from aqueous solutions of potassium permanganate under “soft conditions” via oxidation and reduction synthesis routes. For the first time, we used the reduction route of the SILD method for the synthesis of MnO₂ nanocoatings and explored their morphology, structure, and electrochemical properties as pseudocapacitor electrodes.

2. Materials and Methods

2.1. Materials

The nanolayers of manganese oxide were fabricated using high-purity analytical grade reagents: manganese acetate tetrahydrate (Mn(OAc)₂·4H₂O), potassium permanganate (KMnO₄), hydrogen peroxide (H₂O₂), hydrochloric acid (HCl), and potassium hydroxide (KOH). Nickel foam (NF) (110 PPI; size 20 × 30 mm) was used as a substrate.

2.2. Synthesis of the Nanolayers of Manganese Oxide via SILD Synthesis

In the first stage, the NF substrate was cleaned in ethanol and distilled water by sonication for 10 min to remove the impurities and then treated with a 3 M HCl solution for 10 min to remove the oxidation layer. The manganese nanolayers were fabricated by the SILD technique.

In the first case, aqueous solutions of salts Mn(OAc)₂ (0.01 M) were used as a precursor for the synthesis, and an aqueous solution of KMnO₄ (0.01 M) (MO1) was used as an oxidizer. In the second case, aqueous solutions of KMnO₄ (0.01 M) were used as precursors for the synthesis, and an aqueous solution of H₂O₂ (3%) was used as a reductant (MO2). The precleaned NF substrates were first dipped in the solutions of the first reagent for 30 s and then washed from excess reagent in distilled water for 15 s. In the following step, the substrates were dipped into a solution of oxidizer or reductant for 30 s and rewashed in water (Figure 1). After repeating 50 SILD cycles, an insoluble coating of the synthesized product was formed on the surface. Finally, the coated films were annealed at 150 °C under an argon atmosphere for 3 h.

2.3. Characterization

The morphology and structure of the synthesized films were investigated by using scanning electron microscopy SEM (Zeiss Merlin microscope, Zeiss, Jena, Germany), transmission electron microscopy (TEM) (Zeiss Libra 200 microscope, Zeiss, Jena, Germany), and X-ray diffraction analysis (XRD) (Rigaku Smartlab 3, Rigaku, Tokyo, Japan, diffractometer with CuKα radiation). Qualitative and quantitative analysis of the samples was performed using the XPS (ESCALAB 250Xi electron spectrometer, ThermoFisher Scientific, Waltham, MA, USA) and EDX (Oxford INCA energy 350, Oxford Instruments, Abingdon, UK) methods.

2.4. Electrochemical Measurement

The electrochemical experiment was carried out using an Elins P45-X potentiostat with a three-electrode electrochemical cell in 1 M of KOH aqueous electrolyte solution. The working electrode was obtained through the SILD deposition of electroactive nanocoatings on an NF. An Ag/AgCl electrode and platinum foil were used as the reference and counter electrodes, respectively. The electrochemical characterization of the electrode was carried out using cyclic voltammetry (CVA), galvanostatic charge–discharge (GCD), and the EIS techniques. The specific capacitance C (F/g) at different current densities can be calculated via Equation (1):

$$\mathrm{C}=\frac{\mathrm{Idt}}{\mathrm{dVm}}$$

(1)

where I (mA) is the galvanostatic current, dV (mv) is the potential window, dt (h) is the discharge time of a cycle, and m (g) is the mass of the active material in the film electrode. The mass of the electroactive material of the working electrode was measured using a microbalance. The electrochemical impedance spectra (EIS) were measured at a voltage amplitude of 5 mV in a frequency range between 10 kHz and 0.1 Hz. The electroactive mass of manganese oxide (2.4 mg) for the working electrode was measured using a JOANLAB FA1204 microbalance.

3. Results and Discussion

Morphology and crystal structure were characterized via SEM, TEM, and XRD techniques. As can be seen (Figure 2a,b), the synthesized nanocoating of MO1 formed an agglomerate flower-like shape consisting of ultrathin (thickness 5–8 nm) “nanosheets” with porous morphologies. By contrast, the micrograph of the MO2 (Figure 2c,d) shows that the surface of the nickel foam is uniformly covered with a homogeneous ultrathin film of the synthesized compound with a “nanoflakes” morphology, whose thickness and dimensions are smaller than those of the MO1. As it follows from the obtained TEM images, both coatings representing ultrathin, two-dimensional formations have an amorphous structure without distinctive lattice fringes.

The XRD patterns (Figure 3a) of the MO1 and MO2 samples show broad featureless peaks close to the crystal structure of birnessite MnO₂ (amcsd #0004947). The obtained data may indicate a low crystallinity and an amorphous nature, which is consistent with the data obtained from TEM. The poor diffraction peaks observed are a distinctive feature of birnessite (MnO₂) nanocrystals with a nanosheets/nanoflakes morphology, as can be seen in a number of recent works [9,10,11,12]. The reasons for the weak peaks are the synthesis at room temperature and subsequent moderate heat treatment (150 °C).

The XPS analysis verified that the chemical compounds and oxidation states of the elements in the as-synthesized nanocoatings were identical for both samples. The two peaks (Figure 4a) at the binding energies of 642.1 eV and 653.8 eV can be assigned to Mn 2p_3/2 and Mn 2p_1/2, respectively, and can also be attributed to the Mn⁴⁺ oxidation state [27]. The peak of O 1s (Figure 4b) at 529.4 eV can correspond to metal–oxygen bonding in the oxide and the M–O bonding of the metal oxide, while the wide peak at 531.8 eV can correspond to OH bonding in the adsorbed H₂O [27]. The EDX quantitative and qualitative analysis of the MO1 and MO2 samples demonstrated the presence of Mn, O, K (no more than 1%), and Ni (from the substrate) atoms in the synthesized nanocoatings, which agrees well with the XPS data.

Based on the analysis of the results obtained via SEM, TEM, XRD, EDX, and XPS, we can assume that the obtained nanocoating is formed by MnO₂·nH₂O with a different morphology and amorphous structure. Comparing the morphology of manganese oxides obtained by oxidation and reduction, a significant difference can be observed, which confirms that the synthetic method plays an important role in controlling the morphology of nanocoatings.

The formation of synthesized nanocoatings can be presented in two schemes. In the first step of the oxidation route, Mn_aq²⁺ cations are adsorbed on the surface of a substrate, and then the excess reagent is removed by washing it with water. In the second step, when the substrate is immersed in an oxidizer solution, the Mn²⁺ cations are oxidized, likely only partially, to Mn⁴⁺, resulting in a redox reaction with KMnO₄. As a result, an insoluble amorphous oxide is formed. Another route of synthesis is the reduction of potassium permanganate by hydrogen peroxide in a neutral medium. The MnO₄⁻ anions are adsorbed on the substrate of nickel foam, and when immersed in a solution of H₂O₂, they are reduced to MnO₂.

The electrochemical properties of the NF electrode based on manganese oxide nanocoatings were characterized using the CVA, GCD, and EIS methods. Figure 5a shows a comparison of the CVA curves of the MO1 and MO2 electrodes. As can be seen from the figure, the area of the MO2 CVA curve is larger than the area of the MO1 CVA curve, which indicates a greater specific capacity of this electrode. Further study was focused on the MO2 obtained using the reduction synthesis route. Figure 5b presents the CVA curves of the MO2 electrodes at different scan rates (5, 10, and 20 mV/s) within a potential window from 0.2 V to 0.55 V (vs. Ag/AgCl). On the CVA curves, one pair of wide redox peaks is observed over the entire range, suggesting a pseudocapacitive behavior of this material. Figure 5c shows the galvanostatic charge/discharge curves of the MO2 electrode at different current densities (1, 2, and 5 A/g), indicating a typical pseudocapacitance, which likely originates from the fast redox reaction of Mn³⁺ → Mn⁴⁺ and Mn⁴⁺ → Mn³⁺ on the electrode/electrolyte interface. The specific capacitance of the MO2 electrode, calculated by Equation (1), is 1490 F/g at a current density of 1 A/g.

Figure 5d shows the cycling stability of the MO2 electrode at a current density of 5 A/g in 1 M KOH electrolyte. A small increase in the capacity is observed during the first 150 charge–discharge cycles, after which it begins to decrease. After 1000 charge–discharge cycles, the initial capacitance decreased by only 3%, suggesting excellent electrochemical stability. XRD was performed for the MO2 electrodes before (as-synthesized sample) and after long-term analysis (sample after cyclic tests). The results are presented in Figure 3b. No noticeable changes were found in the X-ray diffraction patterns, indicating the high stability of the obtained MnO₂-based electrode material even after 1000 charge–discharge cycles. The high electrochemical performance of the sample obtained via the reduction route in comparison to the one obtained by the oxidation route can be explained by the unique porous “nanoflakes” morphology and uniform coating of MnO₂. This type of morphology ensures high electric conductivity and a large number of active sites, as well as improved ion diffusion on the electrode surface. The amorphous nature of manganese oxide is likely to benefit ionic charge transport.

The Nyquist plot of the MO1 and MO2 electrodes (Figure 6) displays a linear part in the low-frequency region and an absence of a semicircle in the high-frequency region, indicating a very low transfer resistance and fast electrochemical reactions. The electrode resistance was calculated from the Nyquist plot and is equal to 1.8 Ω for MO1 and 1.5 MO2, which proves low resistance and, as a consequence, high capacity for MO2.

The results indicate that the nanocoating of the MnO₂-synthesized reduction route via the SILD method has excellent potential for application as electroactive materials in pseudocapacitors. Thus, the proposed route of synthesis for uniform nanocoatings using the SILD method allowed for the obtainment of the synthesized materials on a substrate with an intricate morphology, and, in the future, they could be used in flexible energy storage devices. In our opinion, this route opens new possibilities for the fabrication of high-performance electroactive nanomaterials. The synthesis of the nanostructures of other TMOs via the SILD method will be the focus of forthcoming publications.

4. Conclusions

In this study, a MnO₂-based electroactive nanocoating was successfully deposited on nickel foam through reduction routes via the SILD technique. The influence of oxidation–reduction reaction type on the structures, morphologies, and capacity performance of manganese oxide was investigated. The manganese oxide synthesized via the reduction route of the SILD formed an ultrathin uniform nanocoating with an amorphous structure and showed good capacity performance. These electrodes have shown a high specific capacity (1490 F/g at 1 A/g) and excellent cycling stability (97% capacity retention after 1000 charge–discharge cycles) as pseudocapacitors in alkaline media. Therefore, this route opens new possibilities for the fabrication of high-performance MnO₂ nanocoatings, which are highly promising as capacitive materials for flexible energy storage devices in the future.