Membraneless Hydrogen Peroxide Fuel Cells as a Promising Clean Energy Source

Summary

This protocol introduces the design and evaluation of innovative three-dimensional electrodes for hydrogen peroxide fuel cells, utilizing Au-electroplated carbon fiber cloth and Ni-foam electrodes. The research findings highlight hydrogen peroxide's potential as a promising candidate for sustainable energy technologies.

Abstract

In an in-depth investigation of membraneless hydrogen peroxide-based fuel cells (H₂O₂ FCs), hydrogen peroxide (H₂O₂), a carbon-neutral compound, is demonstrated to undergo electrochemical decomposition to produce H₂O, O₂, and electrical energy. The unique redox properties of H₂O₂ position it as a viable candidate for sustainable energy applications. The proposed membraneless design addresses the limitations of conventional fuel cells, including fabrication complexities and design challenges. A novel three-dimensional electrode, synthesized via electroplating techniques, is introduced. Constructed from Au-electroplated carbon fiber cloth combined with Ni-foam, this electrode showcases enhanced electrochemical reaction kinetics, leading to an increased power density for H₂O₂ FCs. The performance of fuel cells is intricately linked to the pH levels of the electrolyte solution. Beyond FC applications, such electrodes hold potential in portable energy systems and as high surface area catalysts. This study emphasizes the significance of electrode engineering in optimizing the potential of H₂O₂ as an environmentally friendly energy source.

Introduction

A fuel cell is an electrochemical device that utilizes fuel and oxidant to convert chemicals into electrical energy. FCs have higher energy conversion efficiency than traditional combustion engines since they are not bound by the Carnot Cycle¹. By utilizing fuels such as hydrogen (H₂)², borohydride-hydrogen (NaBH₄)³, and ammonia (NH₃)⁴, FCs have become a promising energy source that is environmentally clean and can achieve high performance, offering significant potential to reduce human dependence on fossil fuels. However, FC technology faces specific challenges. One prevalent issue is the internal role of a proton exchange membrane (PEM) in the FC system, which acts as a safeguard against internal short circuits. The integration of an electrolytic membrane contributes to increased fabrication costs, internal circuit resistance, and architectural complexity⁵. Moreover, transforming single-compartment FCs into multi-stack arrays introduces additional complications due to the intricate process of integrating flow channels, electrodes, and plates to enhance power and current outputs⁵.

Over the past decades, concerted efforts have been made to address these membrane-related challenges and streamline the FC system. Notably, the emergence of membraneless FC configurations using laminar co-flows at low Reynold numbers has offered an innovative solution. In such setups, the interface between two flows functions as a "virtual" proton-conducting membrane⁶. Laminar flow-based FCs (LFFCs) have been widely studied, leveraging the benefits of microfluidics^7,8,9,10. However, LFFCs require stringent conditions, including high energy input for pumping laminar fuels/oxidants, mitigation of reactant crossover in fluidic streams, and optimization of hydrodynamic parameters.

Recently, H₂O₂ has gained interest as a potential fuel and oxidant due to its carbon-neutral nature, yielding water (H₂O) and oxygen (O₂) during electrooxidation and electroreduction processes at electrodes^11,12. H₂O₂ can be mass-produced using a two-electron reduction process or by a two-electron oxidation process from water¹². Subsequently, in contrast to other gaseous fuels, liquid H₂O₂ fuel can be integrated into existing gasoline infrastructure ⁵. Besides, the H₂O₂ disproportionation reaction makes it possible to serve H₂O₂ as both fuel and oxidant. Figure 1A shows a schematic structure of a facile H₂O₂ FC's architecture. In comparison to traditional FCs^2,3,4, the H₂O₂ FC utilizes the advantages of device "simplicity." Yamasaki et al. demonstrated membraneless H₂O₂ FCs, playing the role of both fuel and oxidant. The described mechanism of electrical energy generation has inspired research communities to continue this research direction⁶. Subsequently, electrooxidation and electroreduction mechanisms using H₂O₂ as a fuel and oxidant have been represented by the following reactions^13,14

In the acidic media:

Anode: H₂O₂ → O₂ + 2H⁺ + 2e^-; E_a1 = 0.68 V vs. SHE
Cathode: H₂O₂ + 2H⁺ + 2e^- → 2H₂O; E_a2 = 1.77 V vs. SHE
Total: 2 H₂O₂ → 2H₂O + O₂

In the basic media:

H₂O₂ + OH^- → HO₂^- + H₂O
Anode: HO₂^- + OH^- → O₂ + H₂O + 2e^-; E_b1 = 0.15 V vs. SHE
Cathode: HO₂^- + H₂O + 2e^- → 3OH^-; E_b2 = 0.87 V vs. SHE
Total: 2 H₂O₂ → 2H₂O + O₂

Figure 1B illustrates the working principle of H₂O₂ FCs. H₂O₂ donates electrons at the anode and accepts electrons at the cathode. Electron transfer between the anode and cathode occurs through an external circuit, resulting in the generation of electricity. The theoretical open circuit potential (OCP) of H₂O₂ FC is 1.09 V in acidic media and 0.62 V in basic media¹³. However, numerous experimental results have shown lower values, reaching up to 0.75 V in acidic media and 0.35 V in basic media, compared to the theoretical OCP. This observation can be attributed to the presence of a mixed potential¹³. Additionally, the power and current output of H₂O₂ FCs cannot compete with the mentioned FCs^2,3,4due to the limited catalytic selectivity of the electrodes. Nevertheless, it is noteworthy that current H₂O₂ FC technology can outperform H₂, NaBH₄, and NH₃ FCs in terms of overall cost, as shown in Table 1. Thus, the enhanced catalytic selectivity of electrodes for H₂O₂ electrooxidation and electroreduction remains a significant challenge for these devices.

In this study, we introduce a three-dimensional porous structure electrode to improve the interaction between the electrode and H₂O₂ fuel, aiming to increase the reaction rate and enhance power and current output. We also investigate the impact of solution pH and H₂O₂ concentration on the FC's performance. The electrode pair used in this study comprises a gold-electroplated carbon fiber cloth and nickel foam. Structural characterization is conducted using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), with Open Circuit Potential (OCP), polarization, and power output curves serving as the primary parameters for FC testing.

Protocol

1. Pre-processing of materials

NOTE: Ni-foam (commercially available, see Table of Materials) with 25 mm x 25 mm x 1.5 mm is used for the H₂O₂ FC's anode.

1. Immerse the Ni-foam sample into alcohol and deionized (DI) water, sonicate for three times, 5 min in solvent and water. Subsequently, place the Ni-foam on a clean glass substrate.
2. Utilize the carbon fiber cloth (see Table of Materials) as the cathode substrate. Cut the carbon cloth into 25 mm x 25 mm square pieces using scissors.
3. Immerse the carbon cloth sample into acetone, 75% alcohol, DI water, and sonicate three times for 5 min, respectively. Then, flush the carbon cloth with DI water to remove residues of alcohol. Place the carbon cloth on a glass substrate.
   ​NOTE: Based on the discussed research results^15,16, Au as the cathode and Ni as the anode have been chosen as catalysts for H₂O₂ FCs. Metals like Pt, Pd, Ni, Au, and Ag have specific catalytical selectivity towards H₂O₂ oxidation or reduction reaction, making them suitable electrode materials. The Au@carbon fiber electrode offers a combination of electrocatalytic activity, stability, and enhanced conductivity, making it a suitable choice for membraneless hydrogen peroxide fuel cells.

2. Electroplating of Au on a carbon cloth

1. Prepare reagents for electroplating as given by the following: chloroauric acid (HAuCl₄), potassium chloride (KCl), hydrochloric acid (HCl), and DI water (see Table of Materials).
2. Prepare 80 mL solutions (based on the volume of the beaker) in a clean beaker with 0.005 M HAuCl₄, 0.1 M KCl, and 0.01 M HCl. Seal the opening and stir the solution for 15 min.
3. Prepare the electroplating material carbon cloth, and the plating solution. The electroplating process is run by Electrochemical Station (ES) (see Table of Materials).
   NOTE: Three electrode method is selected here for plating: carbon cloth as the Working Electrode (WE), graphite rod as the Counter Electrode (CE), and Ag/AgCl (saturated 1 M KCl solution) as the Reference Electrode (RE).
4. Ensure each electrode is clamping the correct object. Immerse electrodes into the plating solution.
5. Start the ES. Set the program to the Chronoamperometry Method, as shown in Figure 1C. Ensure a single depositing circle is as follows: working potential 0.1 V for 0.1 s and resting potential 0.2 V for 0.2 s. As the result, the AuCl₄^- ion diffuses uniformly around the WE.
   1. Set Electroplating Circles at 800, 1600, 2400, and 3200 circles. Run the program.
      NOTE: Typically, the Chronoamperometry method program in ES cannot achieve 1600 cycles. Alternatively, Multi-Potential Steps program of ES can also be used for the electroplating method, the same selections as the Chronoamperometry method (see manufacturer's instructions).
6. After electroplating, close the ES, pack the reagents, and collect Au electroplated carbon fiber cloth (Au@CF).
7. Immerse the Au@CF into the DI water three times to remove the solution residues. Place it on a glass substrate for drying in the air.
8. Cut the un-plated part of the Au@CF caused by the clamps to prevent part of CF from contacting solutions.
9. Measure the size of Au@CF (a: length, b: width) with a ruler for calculating current/power densities.

3. Performance characterization of an FC

1. Prepare solutions with two concentrations, one solution for pH gradient (1 mol H₂O₂, pH = 1, 3, 5, 7, 9, 11, 13), while the other one for H₂O₂ (C_HP) gradient (pH = 1, C_HP = 0.25 mol, 0.5 mol, 1 mol, 2 mol).
2. Characterize the FC performance by ES with two electrodes for OCP and three electrodes for the polarization and power output curves (steps 3.3-3.6).
3. Re-wash Ni-foam and Au@CF again with DI water two times. Place them aside for standby.
4. Obtain OCP data during the testing of an FC: OCP is an essential parameter in the FC performance.
   1. Use Ni-foam as both RE and CE, and Au@CF as WE. Add the solution to the test beaker. Connect electrodes to the ES. Turn on the ES.
   2. Set the program to Open Circuit Potential - Time Method; Run Time: 400 s, Sample Interval: 0.1 s, High E Limit: 1 V, Low E Limit: -1 V. Run the program.
      NOTE: It often takes time for the FC output to stabilize. Run measurements until stable FC results are obtained.
   3. Measure the data. Close the program. Wash the beaker and electrodes. Add other solutions for specific tests.
5. Test output performance of FC based on OCP data. Here, only original Linear Sweep Voltammetry (LSV) curve data is required. Further output data can be calculated from the LSV curve.
   1. Re-wash Ni-foam and Au@CF with DI water (repeat two times). Use Ni-foam as RE and CE, Au@CF as WE. Add the solution to the test beaker.
   2. Set the program to LSV, OCP as Initial E, 0 V as Final E, scan rate as 0.01 V/s, corresponding to the conditions of open circuit (OCP) and short circuit (0 V). Run the program.
   3. Collect the data, close the program, wash the beaker and electrodes, and add other required solutions for specific tests.
6. Wash the electrodes after experiments and store them on a glass.
   ​NOTE: Experiment data can be stored in EXCEL format.

4. Structural characterization of electrodes

NOTE: XRD is a facile and reliable method to analyze samples. XRD is taken to detect elements of the electrodes, such as electroplated Au on the carbon cloth. XRD tests are done before and after FC characterization to analyze potential corrosion and degradation of electrodes. For example, Au particles' can detach from CF, and nickel corrosion may occur in acidic solutions⁵.

1. Wash the electrodes with DI water (two times) and dry them in the air at room temperature.
2. Scrape off metals on the electrodes with tweezers. Collect the metal powder and place it in a container.
3. Perform XRD tests¹⁷on the metal powder samples.
4. Take SEM to characterize the morphology of the electrodes and investigate infiltration and electroplating between the gold and carbon fiber cloth. In addition, characterize the corrosion of nickel by SEM.

5. Data processing and power output calculation

1. All data can be analyzed in EXCEL. Use Excel or Origin to analyze data and plot experimental graphs.
2. Use the OCP data to characterize the selectivity of electrodes, e.g., using a table or a line figure. Use average potential for table legends. Typically, a line figure is used to demonstrate the stability of the FC.
3. Use the LSV data to characterize the output performance of FC. There are two columns of data in the EXCEL file. Typically, one data set shows potential (U), and the other is recorded current (I). Calculate the power output using the following equation: P = U × I
   NOTE: A high current (I) value shows a satisfactory performance of the FC. For example, a large electrode surface area results in higher currents. A normalized parameter referred to FCs' performance is current density (I_D), which is equal to the current divided by the surface area (A) of the electrodes: I_D = I/A
4. Subsequently, calculate the power density (PD) as: P_D = U × I_D
   NOTE:It is essential to take the absolute value, as preliminary data values may be negative due to the direction of the current, which is not desirable during measurements.
5. Comparing parameters using U, I_D, and P_D within a single figure is straightforward. Assign I_D to the x-axis, U to the left y-axis, and PD to the right y-axis.

Results

Electroplating results
Figure 2 shows the electroplating results. Figure 2A indicates the X-ray diffraction result. Figure 2B,C are the micrographs. Figure 2D,E are SEM results. The effective deposition of gold (Au) on the carbon fiber cloth (CF) was first confirmed using the physical change of color in the carbon fiber cloth from black to go...

Discussion

Several parameters significantly influence the performance of a membraneless hydrogen peroxide fuel cell beyond solution pH and H₂O₂ concentration. The choice of electrode material dictates electrocatalytic activity and stability, while the electrode's surface area can enhance reaction sites. Operating temperature affects reaction kinetics, and the flow rate of reactants can determine the mixing efficiency of fuel and oxidant. The concentration of any catalyst used is pivotal for reaction rates,...

Materials

Name	Company	Catalog Number	Comments
Acetone	Merck & Co. Inc. (MRK)	67-64-1	solution for pre-process of materials
Alcohol	Merck & Co. Inc. (MRK)	64-17-5	solution for pre-process of materials
Carbon fiber cloth	Soochow Willtek photoelectric materials co.,Ltd.	W0S1011	substrate material for electroplating method
Electrochemistry station	Shanghai Chenhua Instrument Co., Ltd.	CHI600E	device for electroplating method and fuel cell performance characterization
Gold chloride trihydrate	Shanghai Aladdin Biochemical Technology Co.,Ltd.	G141105-1g	main solute for electroplating method
Hydrochloric acid	Sinopharm Chemical ReagentCo., Ltd	10011018	adjustment of solution pH
Hydrogen peroxide	Sinopharm Chemical ReagentCo., Ltd	10011208	fuel of cell
Nickel foam	Willtek photoelectric materials co.ltd(Soochow,China)	KSH-2011	anode material for hydrogen peroxide fuel cell
Potassium chloride	Shanghai Aladdin Biochemical Technology Co.,Ltd.	10016308	additives for electroplating method
Scanning electron microscope	Carl Zeiss AG	EVO 10	structural characterization for sample
Sodium hydroxide	Sinopharm Chemical ReagentCo., Ltd	10019718	adjustment of solution pH
X-Ray differaction machine	Bruker Corporation	D8 Advance	structural characterization for sample