Electrochemical Deposition (ECD) of ZnO as the Photoanode in Dual‐Chamber Photocatalytic Fuel Cell (PFC) for Methyl Red Degradation

Photocatalytic fuel cell (PFC) is known to treat the organic‐based wastewater efficiently. The coating method of zinc(II) oxide (ZnO) as the photocatalyst on the zinc (Zn) plate of photoanode plays important role in maintaining the efficiency and power generation from PFC. In the present study, a new coating method of ZnO on the zinc plate is developed by using electrochemical deposition (ECD) and the removal efficiency as well as power generation are compared with typical ultrasonicate method. A dual‐chamber PFC is constructed with ZnO/Zn photoanode and graphite plate as the cathode to treat methyl red dye synthetic wastewater. The surface morphology of both coating methods of ZnO/Zn photoanode is observed using scanning electron microscope (SEM) before and after the treatments. The dual chamber PFC with ECD coating shows a faster complete degradation rate with higher power generation than ultrasonicate method. Although, both coating methods can reach to 100% degradation efficiency of methyl red dye, the maximum power output for ultrasonicate method is 134.67 mW cm−2 while for ECD is 285.30 mW cm−2 under optimum conditions.


Electrochemical Deposition (ECD) of ZnO as the Photoanode in Dual-Chamber Photocatalytic Fuel Cell (PFC) for Methyl Red Degradation
Antonio Choon Kiat anak Niang, Jia Chee Liu, and Meng Guan Tay* DOI: 10.1002/admi.202202366 The capability of wastewater degradation fuel cell has gained attention as it is considered green and eco-friendly technology toward the environment by using photocatalyst such as zinc oxide (ZnO), and titanium dioxide (TiO 2 ). Photocatalyst plays vital role in organic pollutant degradation in wastewater due to the production of electron (e − ) and hole (h + ) pair from the photoexcitation (hv) where highly reactive oxidation species (ROS) such as hydroxyl radical (OH•), superoxide anion radical (O 2 • − ), hydroperoxyl radical (•HO 2 ), and alkoxyl radical (RO•) are generated (Equations (1)(2)(3)(4)). [3][4][5] By incorporating photocatalyst in fuel cell, the organic-based wastewater can be further utilized as fuel supply, and during the degradation process, the mass transfer of electron can be harvested as electrical energy.
In the previous researches, [2,[6][7][8][9][10] the photocatalytic system has been proven successfully treated organic-based wastewater using single chamber photocatalytic fuel cell (PFC). However, the method of preparation for ZnO/zinc (Zn) photoanode affects the overall degradation efficiency and power generation in several aspects such as surface area to volume (SA/V) ratio, and surface morphology. So far, the improvement of ZnO/Zn photoanode coating method has not been done to further enhance the degradation efficiency and power generation. Hence, the alteration of preparation of ZnO/Zn photoanode was carried out in our research to increase the efficiency of organic-based wastewater degradation and power generation. In the present paper, we demonstrate an alternative electrochemical deposition method (ECD) to coat ZnO on a zinc plate. The ECD method was compared to typical ultrasonicate method in terms of the degradation efficiency of synthetic azo dye wastewater (methyl red) as well as the electricity generation in a dual chamber PFC (Figure 1) under different pH and hypochlorite (ClO − ) concentrations.
Photocatalytic fuel cell (PFC) is known to treat the organic-based wastewater efficiently. The coating method of zinc(II) oxide (ZnO) as the photocatalyst on the zinc (Zn) plate of photoanode plays important role in maintaining the efficiency and power generation from PFC. In the present study, a new coating method of ZnO on the zinc plate is developed by using electrochemical deposition (ECD) and the removal efficiency as well as power generation are compared with typical ultrasonicate method. A dual-chamber PFC is constructed with ZnO/Zn photoanode and graphite plate as the cathode to treat methyl red dye synthetic wastewater. The surface morphology of both coating methods of ZnO/Zn photoanode is observed using scanning electron microscope (SEM) before and after the treatments. The dual chamber PFC with ECD coating shows a faster complete degradation rate with higher power generation than ultrasonicate method. Although, both coating methods can reach to 100% degradation efficiency of methyl red dye, the maximum power output for ultrasonicate method is 134.67 mW cm −2 while for ECD is 285.30 mW cm −2 under optimum conditions.

Introduction
Along with rapid development of industrialization and urbanization, the amount of wastewater discharged into water bodies from various sources such as textile and manufacturing industries has been increased which brought irreversible impacts to the environments. [1,2] Currently, climate change, environmental pollution, and energy crisis are the most formidable foes which are driven by rapid economic growth around the world. Hence, it is a need to develop an efficient, but with energy conservation function, wastewater treatment system. [3] www.advmatinterfaces.de

Surface Morphology
The morphology of ZnO/Zn photoanodes which were prepared using ECD and ultrasonicate methods was observed using scanning electron microscope (SEM, JEOL, JSM-6390) at 10 kV accelerating voltage and magnification ranging from 1000× to 10 000×. Figure 2a shows the surface morphology of freshly prepared ZnO/Zn photoanode by ultrasonicate method at 1000× magnification. Several ZnO structure can be observed from the SEM image in the shape of rock, flakes, and hexagonal rod structures. [11] The hexagonal rod ZnO structures can be observed clearer under 10 000 times magnification ( Figure 2b) alongside with rock shape ZnO structures. Figure 2c shows the surface morphology of ECD method prepared ZnO/Zn photoanode at 1000× magnification. The surface of the ZnO/Zn photoanode was mostly flower shape [12] or petal shape morphology structures. A clearer image of the structure can be seen under 10 000× magnification in Figure 2d.
The formation of different ZnO structure on the Zn plate surface is due to the random collision between each ZnO particle in ultrasonicate process. Thus, it can be said that the ZnO structures are mechanically attached on the Zn plate surface. On the other hand, the ZnO particles are chemically grown on the Zn plate surface through ECD method. The zinc(II) acetate, Zn(CH 3 COO) 2 solution acts as the precursor for the growth of ZnO surface. Through ECD process, Zn(CH 3 COO) 2 reacts with water producing zinc hydroxide, Zn(OH) 2 , and acetic acid, (CH 3 COOH). Then, Zn(OH) 2 decomposed to ZnO and H 2 O. [13,14] The reaction can be summarized as Equations (5) and (6).
By comparing method of ZnO/Zn photoanode preparation, the ultrasonicate method comprised of three types of ZnO microstructure ( Figure 2b) while ECD method comprised of only one type of the structure (Figure 2d). The uneven coating of ZnO using ultrasonicate method has decreased the surface area to volume (SA/V) ratio hence directly affecting the wastewater degradation and power generation. In contrast to ultrasonicate method, the ECD method provided a similar surface roughness and even coating, which has enhanced the power generation and the degradation of organic pollutants in the wastewater. [15,16] Indeed, the geometrical size of photocatalyst plays an important role in improving photocatalytic activity; the smaller the size, the higher the photocatalytic activity is. [9] Moreover, stacking of different ZnO structure can be observed in Figure 2a, while minimum stacking of ZnO structure can be observed in Figure 2c. The stacking of ZnO structure significantly reduces the specific surface area thus decreasing effective adsorption on the photoanode surface.
Both ZnO/Zn photoanodes were used in dual-chamber PFC for investigating its degradation efficiency and power generation in 10 ppm methyl red solution under pH 4 and pH 5. The surface morphologies of both ZnO/Zn photoanode after the reaction processes are shown in Figure 3a-d.
At pH 4 and pH 5, the ECD ZnO/Zn photoanode (Figure 3a,b) has shown less dissociation of ZnO on the surface. Since the ZnO structure is still intact on the photoanode surface, it has the potential for reusing without decreasing in degradation efficiency and power generation.
On the other hand, at pH 4 and pH 5, the ultrasonicate ZnO/Zn photoanode (Figure 3c,d) has shown severe dissociation of ZnO on the surface. The flake-like ZnO structures have been exposed after used in dual chamber PFC. The exposure of ZnO structure has increased the SA/V ratio, hence increased its active site and reduced its potential for reusing while maintaining its degradation efficiency as well as power generation. According to Liu et al., [17] the lifetime of the IrO 2 -ZrO 2 anode in  www.advmatinterfaces.de their study was shorter when the formation of needle-like ZrO 2 has reached 50% as the SA/V ratio and active site increased. Thus, as the active site increased, the lifetime of ZnO/Zn photoanode reduced significantly.

Power Generation
The coated ZnO/Zn photoanodes were subsequently used to degrade 10 ppm synthetic methyl red wastewater under room temperature at pH4 in the dual-chamber PFC. The color of methyl red dye was degraded until colorless solution was obtained and at the same time, the power generation from the PFC was measured and recorded. The entire process normally took about 3-5 h, depending on the concentrations of ClO − in cathode chamber. Figure 4a,b shows the highest power density (mW cm −2 ) of dual chamber PFC with the ECD and ultrasonicate method prepared ZnO/Zn photoanode in anode chamber, whereas for cathode chamber, different ClO − concentration namely 125, 250, and 375 mL L −1 NaClO solution were investigated under pH 4 and pH 5 conditions. Along the study, the dual-chamber PFC with the photoanode prepared from ECD method showed high power density at 285.31, 209.83, and 190.18 mW cm −2 at pH 4 whereas, the power density only reached 105.33, 98.85, and 133.92 mW cm −2 at pH 5 ( Figure 4a). While in contrary, the dual-chamber PFC with photoanode prepared from ultrasonicate method shows lower power density compared to ECD method prepared photoanode. The power density obtained from the ultrasonicate prepared ZnO/Zn photoanode is 102.083, 134.67, and 105.603 mW cm −2 at pH 4 while at pH 5 showed lower power generation 17.86, 29.60, and 13.94 mW cm −2 ( Figure 4b). Meanwhile, the polarization plots for ECD method are shown in Figure 5a,b, and ultrasonicate method are shown in Figure 5c,d.
Increasing the surface area of ZnO/Zn photoanode which was produced by ECD method has improved the power generation of the PFC by double compared to the ZnO/Zn photoanode produced using ultrasonicate method.
Indeed, the pH of the solution also greatly affects the power generation of the dual-chamber PFC as we can see from the results in Figure 4a,b. The higher power generation from dualchamber PFC is due to the increase in H + ion in the solution. Since the pH of the solution was adjusted using 10% HCl, the decrease in pH of the solution was caused by the presence of H + ion from HCl. Hydrogen can be said to be one of the most efficient energy carriers with the energy density of 140 MJ kg −1 compared to conventional solid fuel which only yields 50 MJ kg −1 . [18,19] Therefore, the increase in supply of H + ion in the solution enhanced the performance of dual chamber PFC by increasing the mass transfer of electrons. According to Ong et al., [8] the increase in H + ion in the solution resulted in more Zn 2+ ion formation. The Zn 2+ ion will then react with OH − and water to form [Zn(OH) 4 ] 2− ion (Equations (7) and (8) Table 1 compares the internal resistance values and highest power density in between ECD and ultrasonicate photoanodes at pH 4 and pH 5 with different ClO − concentrations in cathode chamber. It is obvious to observe that the internal resistance values in the ECD photoanodes were much lower than the one in ultrasonicate method. In addition, we also noticed that the  www.advmatinterfaces.de concentration of ClO − was in the reverse order with the highest power density, where the higher the concentration of ClO − , the lower the power density was. Hence the optimum ClO − concentration in our study was found at 250 mL L −1 .
As the H + ion in the dual chamber PFC increased, the internal resistance of the system decreased and hence increasing the power generation. [2] Repeatability test has been performed on the same piece of ZnO/Zn photoanode in order to know the power generation after several cycles of reuse. Figure 6 shows that the ZnO/Zn photoanode which was prepared using ECD method can be recycled for three times before the power generation decreased over 50% compared to the first use. Meanwhile, the ZnO/Zn photoanode which was prepared using ultrasonicate method can be recycled for two times only. The power density obtained at the 4th time from the one using ECD method was 11.161 mW cm −2 which was 96.1% loss compared to the first use. However, for ultrasonicate ZnO/Zn photoanode, the power density obtained at the 3rd time was only 1.141 mW cm −2 which is 99.2% loss compared to the first use.
The decrease of power density in both ZnO/Zn photoanodes was due to the reaction between ZnO and H + ion in the solution as shown in Equation (7) above. After recycling the ZnO/Zn photoanode, the degree of dissociation on the photoanode surface increased because of the increase in reaction rate of ZnO and H + ion forming Zn 2+ ion which greatly decreases the power generation efficiency. Since the ZnO coating on photoanode using ECD method is through chemical reaction, it is believed that the ECD coating method has better durability compared to ultrasonicate method. This is because the ZnO coating was only mechanically attached on the zinc plate. Thus, the ECD prepared ZnO/Zn photoanode has shown better performance than the one using ultrasonicate method.

Degradation Durations
The degradation durations in different ClO − concentrations were recorded along the process. As shown in Figure 7, the  The degradation duration of the methyl red dye for each experiment vary potentially due to the free radical reaction induced by hydroxyl radicals and other non-radical reactive species such as ClO − . As shown in Figure 7, the shortest degradation rate was found at the concentration of ClO − of 250 mL L −1 under pH 4 (138 min) with the ECD coated ZnO. Meanwhile at the concentration of ClO − of 375 mL L −1 , the degradation duration of methyl red has become longer (170 min) compared to the one at 250 mL L −1 .
The role of ClO − in the cathode chamber is as the oxidizing agent to attract electrons flow from anode chamber toward cathode chamber. This also increases the power generation and degradation rate of the methyl red. But based on the results in Table 1 and Figure 7, increasing ClO − concentration to 375 mL L −1 did not result in a higher power generation as well as a faster degradation rate. This could be due to the saturation of electrons at the cathode chamber which promote the formation of OH − ion (Equation (9)) and subsequently increase the pH of the solution.

ClO H O 2e
Cl Figure 8 shows the degradation rate of 10 ppm methyl red dye solution at pH4 using the ZnO/Zn photoanode prepared by ECD and ultrasonicate method, respectively. The degradation duration of the methyl red dye solution was around 40 min. However, the degradation rate of ECD prepared ZnO/Zn photoanode was faster (k = 0.0696 min −1 ) than ultrasonicate prepared ZnO/Zn photoanode (k = 0.0621 min −1 ). From Figure 8, the kinetic order of methyl red dye in dual-chamber PFC was in the first kinetic order. where, [A] = concentration of methyl red dye solution k = rate reaction t = time (min) Although both ECD and ultrasonicate prepared ZnO/Zn photoanode degradation rates have slight differences, the methyl red dye solution has degraded within 40 min.
This result was parallel with a study reported by Li and co-workers [15,20] where the group investigated the kinetic study of the degradation of rhodamine B (RhB) using gC 3 N 4 /Ag/GO (CNAG) nanocomposite catalyst, where gC 3 N 4 is a metal-free graphite carbon nitride polymeric organic semiconductor and GO is the oxide of graphene. The kinetic model that the group obtained was a pseudo-first order kinetic model. [15]

Conclusion
ECD method has been successfully used to coat ZnO on the Zn plate and also been used as the ZnO/Zn photoanode in a dualchamber PFC.
Based on the SEM images, the ZnO coated using ECD method has larger surface area than the one using typical ultrasonicate method. Hence, this resulted ZnO/Zn photoanode prepared by ECD method has significantly higher power generation than the one using ultrasonicate method at different sodium hypochlorite (NaClO) solutions. The highest power density generated from the dual chamber PFC was 285.30 mW cm −2 at pH 4 with ClO − solution concentration of 125 mL L −1 , whereas the power density generated from the ZnO/Zn photoanode prepared by ultrasonicate method was 134.67 mW cm −2 under the same pH. Nevertheless, the optimal concentration of ClO − was found at 250 mL L −1 due to the shortest degradation duration (138 min) of methyl red was achieved at this concentration. Meanwhile, the ZnO/Zn photoanode prepared by ECD method can be reused for three times before the power density dropped below 50%, whereas for the ZnO/Zn photoanode prepared by ultrasonicate method, it only can be reused for two times.
As a conclusion, the ZnO/Zn photoanode prepared by ECD method has demonstrated a higher power generation as well as higher degradation efficiency compared to the one using ultrasonicate method. Meanwhile, kinetic model of degradation in both photoanodes was known as first kinetic model.

Experimental Section
Materials: The chemicals used in the project were methyl red, ZnO, zinc acetate dihydrate, and 37% hydrochloric acid purchased from R&M Chemicals, potassium chloride (KCl) purchased from Univar, anhydrous sodium carbonate (Na 2 CO 3 ) was purchased from Uni-Chem, and zinc plate (Zn) was purchased from Bendosen. All chemicals including methyl red dye were used without further purification.
Preparation of Photoanode by Ultrasonicate Method: The ZnO/Zn photoanode was made by referring to the preparation method mentioned in Moksin et al. [4] with some modifications. A zinc plate (99.999% purity) with the thickness of 0.38 mm was cut to 6.0 cm × 2.0 cm dimension. The Zn plate was mechanically polished with P400 grit SiC sandpaper to achieve even surface coarseness and to remove oil stains from the surface. The purpose of polishing the Zn surface was to increase the coating effectiveness. The Zn plate was the rinsed at least three times with distilled water to remove zinc dust on the surface. Then, the Zn plate was immersed and sonicated (Elmasonic Easy 30H, Elma Schmidbauer GmbH) in ZnO suspension for 1 h. The ZnO suspension solution was prepared using 1.0 g ZnO powder suspended in 100 mL distilled water. After 1 h sonicating, the Zn plate was dried in oven for 24 h at 120 °C.
Preparation of Photoanode by Electrochemical Deposition Method: The ZnO/Zn photoanode was prepared using ECD method. The Zn plate (99.999% purity) with thickness of 0.38 mm was cut to 10.0 cm × 2.0 cm dimension. The Zn plate was then polished with P400 grit SiC paper to achieve even surface roughness. The Zn plate was then rinsed with distilled water for removing the Zn dust produced during polishing step. The Zn plate was connected to cathode of a direct current (DC) supply unit (Longwei 30 V/10A DC supply, LW-K3010D) while graphite plate with 5.0 cm × 4.0 cm with thickness of 0.2 cm was connected to anode. Both electrodes were dipped into 150 mL of 0.05 m of zinc acetate, Zn(CH 3 COO) 2 , solution with 10% w/v KCl salt added as electrolyte to increase the conductivity of the solution. The ECD process was carried out for 5 min at 5 V with a constant stirring. Then, the plate was rinsed with distilled water to remove excess precipitate from the Zn plate surface. The ZnO coated plate was dried in over for 24 h at 120 °C. The characteristics of ZnO/Zn photoanodes from both methods were observed using SEM (JEOL-JSM-6390) at accelerating voltage of 10 kV.
Preparation of Methyl Red Dye Solution: Methyl red dye was selected because it was characterized as an azo dye due to the presence of NN bond in its chemical structure. A 100 mL stock solution of 1000 ppm methyl red dye solution was prepared by dissolving 0.10 g of methyl red powder in 100 mL of distilled water. A 10% of HCl solution was added to the solution to ensure all the methyl red powder was dissolved, then the solution was top up to 100 mL calibration mark with distilled water. To prepare 10 ppm methyl red solution, 10 mL of stock solution was taken out and added with 990 mL of distilled water.
Preparation of Sodium Hypochlorite Solution: The NaClO solution was prepared by diluting commercially available bleach in distilled water. The amount of bleach used was 100, 200, and 300 mL then topped up to 800 mL with distilled water. The final concentrations of the NaClO solution were 125, 250, and 375 mL L −1 , respectively.
Preparation of 1.0 m KCl Solution: KCl (37.3 g, 0.5 mol) was dissolved in 500 mL distilled water to produce 1.0 m KCl solution. The KCl solution was then used as salt bridge in the dual chamber PFC setup.
pH Adjustment of Methyl Red Solution: The pH of the 10 ppm methyl red solution was adjusted by using 10% HCl solution and 10% w/v Na 2 CO 3 solution until desired pH was obtained. The pH of the solution was determined and monitored using pH meter (Eutech pH 700, Thermo Scientific).
Fabrication of Dual-Chamber PFC: A dual chamber PFC system with ZnO/Zn photoanode and graphite plate as cathode was fabricated. A salt bridge contained 1.0 m KCl solution was used in the dual chambers PFC. The anode chamber was filled with 800 mL of 10 ppm methyl red solution while the cathode chamber was filled with 800 mL of 125 mL L −1 NaClO solution. The graphite plate which acts as cathode was aerated with an air pump (Brand: Aquaspeed, Model: AP-446) at room temperature. An external resistor was connected to the dual chamber PFC and a multimeter (AVOSKY, XL830L) was connected parallel to the external resistor (Brand: DoMore, Model: ZX21d) for measuring the voltage across the resistance. A LED light bulb (Wishful 40 W, 220-240 V, 6500K) was used as light source for the dual chamber PFC. The voltage from dual chamber PFC was collected immediately after both electrodes were inserted into the solution and at an interval of 1 h until the methyl red solution is completely degraded. The voltage of different resistance ranging from 1 to 50 000 Ω were collected for obtaining the polarization plot and determining the higher power density from dual chamber PFC.
The power density generated was calculated based on Equation (11). max s c oc where, P max = Power density, J sc = current density, and V oc = voltage output The current of the dual chamber PFC was calculated based on Equation (12).
where, V = voltage (V), I = current (A), and R = resistance (Ω) The current density (J sc ) was calculated by dividing the current calculated from Equation (12) with the surface area of ZnO/Zn photoanode (14 cm 2 ).