Development of Micro-Nano Structured Electrodes for Enhanced Reactivity: Improving Efficiency Through Nano-Bubble Generation

Abstract

This study focuses on developing high-performance electrodes by applying micro/nano structures to aluminum mesh electrodes and evaluating their electrochemical performance through the electroflotation process. First, the most suitable electrode material for electroflotation was selected, followed by the application of micro-nano structures to analyze bubble generation and size distribution in comparison to conventional electrodes. The bubble generation rate and size were used to predict electroflotation efficiency, which was then validated through experiments. The developed electrodes demonstrated a ninefold reduction in purification time compared to traditional electrodes and achieved higher wastewater treatment efficiency than spontaneous flotation. This research highlights the potential of micro-nano structured electrodes to enhance electroflotation processes and offers valuable insights for industrial applications.

1. Introduction

Electrode technology is a key component in electrochemical reactions and is widely utilized across various industrial sectors. The study of electrodes began in the early 19th century with the invention of batteries [1], and over time, electrodes have become an essential component in energy storage and conversion devices. In particular, with the rapid development of green energy transition technologies in the 21st century, the importance of electrodes has become even more pronounced. Today, electrodes play a critical role not only in energy storage systems such as batteries [2,3,4,5,6,7,8,9,10], fuel cells [11,12,13], and supercapacitors [14,15,16], but also in hydrogen production through water electrolysis [17,18,19,20,21]. Both battery-based energy storage systems and water electrolysis technology are crucial in driving the transition to sustainable energy, highlighting the increasing importance of electrode technology in these fields.

Electrode technology is being researched with a focus on maximizing reaction efficiency and stability to improve performance in various applications. To optimize the reaction efficiency of electrodes, various factors must be considered, including the reaction environment [22,23,24], electrode materials [25,26,27,28], electrolytes [29,30,31], power supply conditions [32,33,34], and electrode modification [5,6,7,8,9,10,35,36]. Among these, electrode modification is currently dependent on techniques such as additive manufacturing [5,6,7] and electrodeposition [8,9,10], which face limitations in terms of practicality. While these methods have shown meaningful results in research settings, they may encounter challenges when scaled for industrial applications due to issues such as reduced adhesion between the electrode substrate and additives, leading to delamination during use. This is particu-larly true when the substrate material or electrolyte composition is not adequately considered, which can exacerbate detachment under operational conditions. Additionally, these techniques often fail to adequately control surface morphology and material uniformity, potentially compromising long-term durability.

In contrast, micro- and nano-structured modifications applied directly to the electrode surface, as proposed in this study, avoid these adhesion issues by structuring the original substrate material itself [37,38,39,40,41]. This eliminates concerns about delamination, resulting in enhanced durability and stability over extended use. Optimizing the material structure in this way allows for simultaneous improvement in electrochemical performance and industrial practicality.

In this study, we aim to develop electrodes by applying micro- and nano-structured modifications to various metal electrodes using chemical processes. These methods have demonstrated high productivity and durability in previous research [37,38,39,40,41], and we plan to evaluate their electrochemical performance using the electroflotation (EF) process.

EF, as illustrated in Figure 1, is an emerging wastewater treatment technology that has garnered attention in recent years for its numerous advantages over traditional methods, such as dissolved air flotation (DAF) [42,43,44]. EF effectively separates solids from liquids by utilizing gases produced during water electrolysis, offering significant benefits in terms of cost efficiency, stability, and environmental protection. The fundamental efficiency of EF depends on the amount and size of gas bubbles generated per unit electrode area. Smaller and more abundant bubbles improve separation efficiency by continuously facilitating efficient contact between the electrode and the liquid, which is closely related to reactivity [35,36,45,46]. In this study, we will select industrial-grade metals suitable for bubble generation, apply micro- and nano-structures to these metals, and meticulously examine bubble size and quantity during the EF process to develop electrodes with enhanced reactivity.

This study focuses on analyzing the effects of electrode morphology and micro-nano structures on the EF process, investigating the mechanisms of bubble generation and size distribution, and evaluating the reactivity and overall flotation performance of the developed electrodes. The findings of this research are expected to provide valuable insights into the design and fabrication of high-performance electrodes for EF applications, contributing to the improvement of system performance where electrodes are utilized.

2. Experimental Section

2.1. Material Selection

To select the optimal electrode material for enhancing EF performance, industrial aluminum (5052, >99.5% purity), titanium (6 A l4 V, <90% purity), and copper plate (C11,000, >99.9% purity) were processed into 50 mm × 10 mm × 1 mm specimens for use in the experiments. A 3.5% electrolyte solution was prepared using sodium chloride (NaCl, 99.5%), also sourced from SAMCHUN Chemical, Pohang, Korea. The metal plate of each candidate material was connected to the anode, while a platinum plate was attached to the cathode. The electrodes were then immersed in the prepared NaCl solution and subjected to a constant voltage of 10 V. The bubble generation rate was recorded and compared for each electrode material, with each experiment repeated three times to calculate the average and standard deviation for each material.

2.2. Electrode Modification

2.2.1. Surface Treatment

For the electrode modification experiments, industrial aluminum plates and aluminum mesh electrodes of the same dimensions were utilized. The aluminum plates were supplied by Almarket, Daegu, Korea, while the aluminum mesh was sourced from TWP Inc., Berkeley, CA, USA. The mesh had an opening size of 0.28 mm, a wire diameter of 0.23 mm, and a density of 50 pores per inch (ppi). Both the aluminum plate and mesh electrodes underwent a two-step process consisting of surface etching and crystallization to introduce micro-nano structures on the material surfaces. Initially, the electrodes were etched in a 1 M hydrochloric acid (HCl, 35.0~37%) solution, sourced from SAMCHUN Chemical, Korea, at room temperature for 30 s. Following the etching process, the electrodes were immersed in a solution of 1 M sodium hydroxide (NaOH, bead, 98.0%), also sourced from SAMCHUN Chemical, Korea, at 80 °C for 10 min to facilitate crystallization. The electrodes were then dried in an 80 °C oven for 60 min.

2.2.2. Characterization of Electrode Surface Morphology and Bubble Behavior

The surface morphology of the fabricated electrodes, including the micro-nano structures, was analyzed using a high-resolution Field Emission-Scanning Electron Microscope (FE-SEM; JM-7401F, JEOL, Tokyo, Japan) to confirm the successful formation of these structures on the electrode surface. The bubble behavior for both pristine and micro-nano structured electrodes was visually observed using a high-speed camera (Y7, IDT, Passadena, CA, USA) operating at 5000 frames per second. The size and distribution of the generated fine bubbles were measured using Nano Tracking Analysis (NTA), conducted using the NanoSight NS300 instrument from Malvern Panalytical (Malvern, UK). The total volume of bubbles produced was measured by recording the amount of gas generated at the anode for 5 min, and then calculating the bubble production rate by dividing the total volume by time (in seconds). This method was used for each electrode to obtain bubble production rates.

2.3. Evaluation of EF Performance

Oil-in-water emulsions were prepared to evaluate the flotation performance of the micro-nano structured mesh electrodes for various oil types, including gasoline, diesel, naphtha (Petroleum, 95.0%), n-hexane (C₆H₁₄, >96.0%), and toluene (C₆H₅CH₃, 99.5%). Gasoline and diesel were supplied by HD Hyundai Oilbank, Pohang, Korea, while the other oil types were sourced from SAMCHUN Chemical, Korea. The oils were mixed with a 3.5% sodium chloride (NaCl) solution at a volume ratio of 1:9, and the mixtures were homogenized at 5000 rpm for 10 min to ensure a uniform distribution of oil droplets in the solution.

The EF experiments were performed using the modified electrodes in a custom-built electrolytic cell. Two conditions were compared to assess the degree of oil flotation and residual oil content in the solution: spontaneous flotation (Figure 2a) and 10 min electrolysis at 100 V in a 3.5% NaCl solution (Figure 2b). The EF process was conducted using both pristine and micro-nano structured mesh electrodes to determine the impact of surface structures on the flotation performance.

After the EF process, the residual oil concentrations in the solution were measured using Total Organic Carbon (TOC) analysis with a Total Organic Carbon analyzer (TOC-L, Shimadzu, Japan). The results were compared to evaluate the efficiency of the micro-nano structured mesh electrodes in removing various oil types from the water.

3. Results and Discussion

3.1. Material Selection

EF, based on water electrolysis, is governed by two dominant reactions: the dissolution of metal at the anode due to oxidation and the generation of hydrogen gas at the cathode from the electrolysis of water [47]. These reactions can be described using Equations (1) and (2), respectively:

$$\mathrm{M}\left(s\right)\to {\mathrm{M}}^{n+}\left(aq\right)+n{e}^{-}$$

(1)

$$n{\mathrm{H}}_2\mathrm{O}+n{e}^{-}\to {\displaystyle \frac{1}{2}}n{\mathrm{H}}_2\left(g\right)+n{\mathrm{O}\mathrm{H}}^{-}$$

(2)

$${\mathrm{M}}^{n+}\left(aq\right)+n{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{M}{\left(\mathrm{O}\mathrm{H}\right)}_n\left(s\right)$$

(3)

During EF, metals dissolve into the solution, forming divalent or trivalent cations depending on the solution pH and potential. Simultaneously, the generated hydroxide ions (${\mathrm{O}\mathrm{H}}^{-}$) react with dissolved metal cations to form metal hydroxides, which precipitate as described in Equation (3). These precipitates can aggregate with other impurities, positively contributing to water purification [48].

If improper electrode materials are selected or inappropriate electrolyte concentrations and voltage/current conditions are applied, unwanted side reactions may occur. For instance, gas evolution from the electrolyte can happen at the anode, or metal precipitation can occur at the cathode. In some cases, the actual oxidation rate at the anode does not match predictions based on Faraday’s law. This discrepancy can be attributed to the local pH increase caused by continuous ${\mathrm{O}\mathrm{H}}^{-}$ production or, at sufficiently high anode potentials, the occurrence of the oxygen evolution reaction, as represented by Equation (4) [43,49,50]:

$$2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}+4e^{-}$$

(4)

These reactions, including gas evolution and metal hydroxide precipitation, can be visually represented in Figure 1. Based on this theoretical understanding, an experiment was conducted to determine the most suitable electrode material for enhancing EF performance by maximizing bubble generation.

Since a higher bubble generation rate increases the likelihood of collisions between bubbles and oil droplets, leading to improved separation efficiency, it was crucial to select a material with a high bubble generation rate. To this end, we observed and compared the bubble generation rates for aluminum, titanium, and copper electrodes, as shown in Figure 3. The results indicated that aluminum electrodes exhibited the highest bubble generation rate of 19.2 mm³/s, surpassing both titanium and copper electrodes (Figure 3d). This outcome aligns with the known ionization tendencies of the metals (Al > Ti > Cu) under constant voltage conditions [51]. Based on these observations, aluminum was selected as the most suitable electrode material for further studies aimed at optimizing EF performance.

3.2. Evaluation of Bubble Generation for Various Types of Electrodes

When reactions occur at the electrode, gas bubbles are generated at the electrode surface, as represented by Equations (2) and (4). The generated bubbles are subject to buoyancy force ($F_b$) and surface tension ($F_{\sigma }$) at the electrode, and the buoyancy force can be expressed using Equations (5) and (6) (Figure 4a,b) [52]:

$$F_b={\displaystyle \frac{4}{3}}\pi \left({\rho }_{liquid}-{\rho }_{gas}\right)g{r}^3$$

(5)

$$F_{\sigma }=p\sigma sin\theta$$

(6)

where ${\rho }_{liquid}$ is the liquid density of the electrolyte, ${\rho }_{gas}$ is the gas density, g is gravitational acceleration, and r is the radius of the bubble. p is the length of the contact line and σ is the dynamic surface tension between the bubble and the electrode surface.

When surface tension exceeds buoyancy, larger bubbles are formed, and the buoyancy force increases. Once the buoyancy force surpasses surface tension, the bubbles detach from the electrode surface into the solution. The larger bubble size observed with the titanium electrode (Figure 3) is attributed to the higher surface tension (σ) of titanium (Ti > Cu > Al) compared to other materials [53,54,55].

According to Equation (6), on a pristine surface, where the contact angle θ is 90°, the maximum cohesive force acts on the bubbles, resulting in the formation of larger bubbles. When bubbles form on the electrode surface, no reaction occurs in the area covered by the bubbles, which negatively affects electrode efficiency. Therefore, the generation of large bubbles can hinder the electrode’s overall performance. In contrast, when micro-nano structures are present on the electrode surface, the value of θ changes, reducing the cohesive force and promoting the consistent formation of smaller bubbles. The continuous release of fine bubbles ensures that the electrolyte infiltrates the electrode surface, allowing for further reactions and enhancing the overall efficiency. Additionally, the use of micro-nano structures increases the surface area of the electrode, providing more active sites for reactions compared to a pristine surface (Figure 4c,d).

Based on this theoretical framework, we investigated the effect of surface structures on bubble size and distribution by fabricating four types of aluminum electrodes: Plate-Pristine (PP), Plate-Micro/Nano (PMN), Mesh-Pristine (MP), and Mesh-Micro/Nano (MMN). Mesh electrodes were chosen because they offer a simple method to increase surface area. The surface morphology of the fabricated electrodes was examined using Scanning Electron Microscopy (SEM) to confirm the successful formation of the micro/nano structures (Figure 5). In particular, the interfacial bonds between the micro/nano structures and substrates consist of hydrogen bonding with the hydroxyl groups, which leads to a condensation reaction and covalent bonding to the substrate. This chemical treatment ensures improved durability and stability, as demonstrated in previous studies [37,38,39,40,41].

To assess the effect of surface structures on bubble generation, the macroscopic process of bubble generation during electrolysis was observed using a high-speed camera, and the size and distribution of bubbles were evaluated using optical instruments under identical experimental conditions (Figure 6). Figure 6a,b show images of bubbles generated by the MP and MMN electrodes, respectively. In the MP electrode, large bubbles visible to the naked eye predominantly formed. In contrast, the MMN electrode exhibited very few visible bubbles, with most bubbles appearing as a mist due to their significantly smaller size. The macroscopic process of bubble generation can be viewed in Supplementary Materials Videos S1 and S2.

The optical evaluation of the bubbles, as shown in Figure 6c, revealed that while the mesh electrodes in the pristine configuration produced a higher rate of fine bubbles (below 100 nm) compared to the plate electrodes, the overall bubble generation rate showed no significant difference between the two. However, applying micro-nano structures to the electrode surface resulted in a more than twofold increase in bubble generation compared to the pristine configuration, with a particularly high concentration of fine bubbles in the 100–150 nm range. Consequently, the MMN electrode design was identified as the most effective configuration for enhancing EF performance.

3.3. Evaluation of EF Performance for Micro-Nano Structured Mesh Electrode

EF is typically driven by the generation of gas bubbles at the electrode surface, which rise to the water surface. As they ascend, these bubbles collide with and agglomerate foreign matter in the liquid, leading to the flotation and separation of contaminants (Figure 7a). For a typical bubble with a diameter of 50 µm, the flotation velocity ($V_{bubble}$) is expressed using Equation (7) [56]:

$$V_{bubble}={\displaystyle \frac{2\left({\rho }_{liquid}-{\rho }_{gas}\right)g}{9\eta }}{r}^2$$

(7)

where $\eta$ is the viscosity constant of the liquid.

However, when bubbles are smaller than 50 µm in diameter, they begin to be influenced by Brownian motion. Brownian motion refers to the random movement of small particles suspended in a liquid, and the velocity ($V_R$) can be described using Equation (8) [57]:

$$V_R=\sqrt{{\displaystyle \frac{3K_{B}T}{M}}}$$

(8)

where $K_B$ is the Boltzmann constant, $T$ is the absolute temperature, and $M$ is the mass of the particles.

As the bubble size approaches 50 µm or less, the flotation velocity and Brownian motion speed become comparable, causing the bubbles to ascend more slowly. For nanobubbles with a diameter below 1 µm, Brownian motion becomes dominant, resulting in extremely slow ascent. Moreover, these nanobubbles are stabilized by a layer of charged ions surrounding their surface, preventing rapid dissolution and allowing them to remain in the solution for extended periods [58,59].

This stability and prolonged presence of fine bubbles in the liquid offer distinct advantages for EF. In addition to the traditional collision-based agglomeration, fine bubbles benefit from Brownian motion-induced collisions and electrostatic interactions between the charged bubble surfaces and charged foreign particles. These effects significantly enhance the overall EF performance (Figure 7).

To verify this effect, EF tests were conducted using both the MP and MMN electrodes with a uniform oil-in-water mixture. The separation times were compared, and it was observed that the MP electrode required over 5 min to achieve phase separation, while the MMN electrode achieved the same result in approximately 30 s (Figure 8).

To further assess the purification efficiency of the MMN electrode in EF, additional experiments were performed with various oil types, including n-dodecane, octane, petroleum ether, n-hexane, and toluene. The EF performance of the MMN electrode was compared to that of spontaneous flotation. As shown in Figure 2, after the flotation process, the residual oil concentrations in the solution were measured using Total Organic Carbon (TOC) analysis. The initial concentration of oil in the emulsion was approximately 80,000 mg/L. Figure 9 demonstrates that the EF performance using the micro-nano structured mesh electrode showed improvements ranging from a minimum of 52.5% for diesel to a maximum of 92.5% for hexane, compared to spontaneous flotation.

4. Conclusions

In this study, we aimed to develop practical electrodes with improved durability and large-scale production capabilities by applying micro-nano structures to the electrode surface. To evaluate the enhanced performance, the electrochemical behavior of these electrodes was assessed using the EF process. Aluminum was selected as the optimal material based on its superior bubble generation rate compared to titanium and copper. When micro-nano structures were applied to the aluminum electrodes, bubble generation significantly increased, while bubble size decreased, resulting in improved flotation performance. The MMN electrodes, in particular, produced over twice the number of bubbles as pristine electrodes, with a high concentration of fine bubbles in the 100–150 nm range. This increase in fine bubble production allowed the bubbles to remain in the solution longer, facilitating more interactions with oil droplets and contaminants. Compared to spontaneous flotation, the MMN electrodes demonstrated a much higher purification efficiency, reducing oil concentrations by up to 92.5% in the case of hexane.

This research not only optimized the EF process but also demonstrated that rapid bubble release from the micro-nano structured electrodes enhanced the overall reactivity of the electrodes. The ability to generate a higher volume of smaller bubbles is crucial for improving separation efficiency in EF, as it increases the likelihood of particle–bubble collisions, particularly with fine contaminants that are difficult to separate using traditional methods. Additionally, the extended retention time of nanobubbles in the solution further amplified the separation efficiency by promoting electrostatic interactions with charged contaminants.

The findings of this study provide valuable insights into the design and fabrication of high-performance electrodes for a wide range of applications. The enhanced EF performance demonstrated by the MMN electrodes underscores the potential for these electrodes to be utilized in industrial wastewater treatment and other systems requiring efficient separation processes. Overall, this work contributes to advancing electrode technology by showing how micro/nano structures can significantly improve both the electrochemical and flotation efficiencies of electrodes, paving the way for future research and practical implementations.