Optimal MEA structure and operating conditions for fuel cell reactors with hydrogen peroxide and power cogeneration

The cogeneration of hydrogen peroxide (H2O2) and power in proton exchange membrane fuel cell (PEMFC) reactors via two-electron oxygen reduction reaction on the cathode is an economical, low-carbon, and green route for the on-site production of H2O2. However, in practice, the H2O2 that cannot be collected timely will accumulate and self-decompose in the catalyst layer (CL), reducing the H2O2 generation efficiency. Thus, accelerating the mass transport of H2O2 within the cathode CL is critical to efficient H2O2 generation in PEMFC. Herein, we investigated the effects of the membrane electrode assembly (MEA) fabrication process, cathode CL thickness, and cathode carrier water flow rate on H2O2 generation and cell performance in a PEMFC reactor. The results show that the catalyst-coated membrane-type MEA exhibits high power output due to its lower proton transport resistance. However, the formed CL with a dense structure significantly limits H2O2 collection efficiency. The catalyst-coated gas diffusion electrode (GDE)-type MEA formed macroporous structures in the cathode CL, facilitating carrier water entry and H2O2 drainage. In particular, carbon cloth GDE with thin CL could construct rich macroscopic liquid channels, thus maximizing the generation of H2O2, but will impede fuel cell performance. These results suggest that the construction of a well-connected interface between CL and proton exchange membrane (PEM) in MEA and the establishment of a macroscopic pore structure of the CL are the keys to improve the cell performance and H2O2 collection efficiency.


Introduction
Hydrogen peroxide (H 2 O 2 ) has been widely used as an eco-friendly oxidant in the chemical industry and for environmental treatments, such as organic/inorganic chemical synthesis, pulp and paper bleaching, medical disinfection, and wastewater treatment.Moreover, H 2 O 2 can be used as a sustainable energy carrier as an alternative to oil and hydrogen in fuel cells since it has the advantages of easy storage and safe operation, and its only by-product is water [1][2][3].The demand for H 2 O 2 is growing yearly.However, the traditional industrial anthraquinone process for H 2 O 2 production is not considered a green method.It involves the sequential hydrogenation and oxidation of alkyl-anthraquinone precursors dissolved in a mixture of organic solvents followed by liquid-liquid extraction to recover the H 2 O 2 [4][5][6][7].High energy consumption and organic waste generation make H 2 O 2 produced this way costly, making the process incompatible with sustainable development.Therefore, it is crucial to develop a green and economical method for H 2 O 2 generation [8][9][10][11].
Direct electrosynthesis of H 2 O 2 by coupling the electrocatalytic two-electron oxygen reduction reaction (2e-ORR) and the H 2 oxidation reaction is a promising method with the output of green-electricity [12][13][14][15].The cost of H 2 O 2 can be greatly reduced when its generation and the electricity output are synchronized.Recently, the concept of distributed generation and energy storage systems based on the highly efficient Scheme 1. Schematic diagram of the direct electrochemical synthesis of H2O2 with power output in a proton exchange membrane fuel cell (PEMFC).
H 2 O 2 electrochemical cycle has received much attention [16,17].In this system, H 2 O 2 can be synthesized by the 2e-ORR in the fuel cell reactor while generating power when there is an electricity shortage.When there is surplus electricity, H 2 O 2 can be electrolyzed into H 2 and O 2 to store energy.Based on this concept, the cogeneration of power and H 2 O 2 in a fuel cell reactor is highly favored.There are few studies on H 2 O 2 synthesis based on fuel cell reactors [18][19][20], and most studies on H 2 O 2 electrosynthesis focus on improving the selectivity and stability of catalysts [13,14,21,22].However, the study of practical reactor design and system engineering to improve the H 2 O 2 production efficiency and power generation is still in its infancy [11,14,15,22,23].Even for some studies involving reactor design, the usual strategy is to use bipolar porous membranes to accelerate the removal of H 2 O 2 .However, the use of bipolar membranes greatly increases the internal resistance of the reactor and leads to H 2 O 2 generation at the cost of high energy consumption [11,23].In comparison, developing a proton exchange membrane fuel cell (PEMFC) reactor for the cogeneration of electrical power and a value-added chemical (H 2 O 2 ) to reduce overall external energy consumption is extremely important.In addition, the stability of proton exchange membranes (PEM) in H 2 O 2 and acidic solutions has been demonstrated, which lays the foundation for the long-term synthesis of H 2 O 2 using the PEMFC reactor [17,24,25].
In an ideal PEMFC for H 2 O 2 generation, H 2 O 2 is generated by the 2e-ORR and promptly removed from the cathode while power is released.In an experimental PEMFC reactor (scheme 1), if the H 2 O 2 generated in the cathode catalyst layer (CL) cannot be removed in time, its undesired chemical decomposition and electrochemical reduction reaction will occur, resulting in low H 2 O 2 concentration and Faraday efficiency (FE).Therefore, accelerating the mass transport of H 2 O 2 in the CL while ensuring the output of electric power is the key to efficient power and H 2 O 2 cogeneration.
In this work, we used a platinum group metal-free (Co-N-C) catalyst with high 2e-ORR selectivity as the cathode catalyst [13,14,21,22], and systematically examined the effects of the membrane electrode assembly (MEA) fabrication method, cathode CL thicknesses, and cathode carrier water flow rate on H 2 O 2 generation efficiency and cell performance.The results show that different MEA fabrication processes have varying abilities to build up gas, proton, and product transport channels, which affect fuel cell reactor performance and H 2 O 2 collection efficiency.For the catalyst-coated membrane (CCM)-type MEA (MEA CCM-P ), a well-connected interface between the CL and the PEM decreases proton transport resistance and improves fuel cell performance.However, for the gas diffusion electrode (GDE)-type MEA (coating the catalyst on carbon paper or carbon cloth; MEA GDE-P or MEA GDE-C , respectively), more pore structures within the CL provide liquid transport channels that facilitate H 2 O 2 collection.In addition, we compared PEMFC performance and H 2 O 2 collection efficiency using CLs with different thickness.The results suggest that thinner CL can effectively reduce the retention time of H 2 O 2 in the CL and improve the H 2 O 2 collection efficiency but at the cost of the partial loss of cell performance.Furthermore, we found that increasing the carrier water flow rate will reduce H 2 O 2 retention time in the cathode and improve the H 2 O 2 collection efficiency.By analyzing the diffusion mechanisms of gases, protons, and products in different MEAs, we clarify the effects of cathode CL structure, CL thickness, and carrier water flow rate on cell performance and H 2 O 2 collection efficiency.Our results suggest that the construction of well-connected interface between CL and PEM in MEA and the establishment of a macroscopic pore structure of the MEA are the keys to improve the cell performance and H 2 O 2 collection efficiency.This can be used to guide the optimization of practical reactor designs for the production of low-cost H 2 O 2 .

MEA fabrication
Cathode catalyst ink was prepared by mixing Co-N-C catalyst (20 mg, synthesized using a published method [16]) with DI-water (2.0 mL), IPA (2.7 mL), and D521 Nafion dispersion (424 µL, 5 wt%).The mixture was then sonicated in an ultrasonic bath for 2 h.Scheme 2 illustrates the two MEA fabrication methods used in this study: MEA CCM and MEA GDE .An anodic commercial GDE (0.2 mg Pt /cm 2 ; Fuel Cell Store; 20 wt% Pt/C on carbon paper; Sigracet 22 BB) was hot-pressed to one side of a Nafion NR211 membrane to form a half-MEA at 120 • C, 5.3 MPa for 5 min.In this work, we used a conventional Pt/C anode for the hydrogen oxidation reaction.High Pt loading of 0.2 mg Pt /cm 2 is used for our research purpose to ensure a minimum low anode potential.In the future development of more practical and economical reactors, the anode Pt-loading may be reduced to less than 0.05 mg Pt /cm 2 (the current state-of-the-art anode loading in PEM fuel cells) or replaced with low-cost PGM-free catalysts.For the MEA CCM , catalyst ink was sprayed onto the cathode side of the membrane at 80 • C to form the cathode CL.The MEAs were fabricated by adding a GDL (215 µm; Sigracet 22 BB) to the top of the cathode CL.For the MEA GDE-P , the cathode catalyst ink was sprayed onto a carbon paper microporous layer (MPL) (215 µm; Sigracet 22 BB) at 80 • C to form the cathode.For the MEA GDE-C , the cathode catalyst ink was sprayed onto a carbon cloth (330 µm; Ce-Tech W0S1009) at 80 • C to form the cathode.The cathode GDE and half-MEAs were then assembled into the full MEA.The full MEA was then placed in the fuel cell hardware, comprising a 4 cm 2 single-channel (width/depth = 1.27/0.8mm) serpentine flow-field graphite plate sealed with two fluoro-rubber gaskets (thickness = 200 and 100 µm).The MEAs were compressed to ca. 60% of their original thickness.

Fuel cell test and H 2 O 2 collection
Fuel cell performance was measured using an FC workstation (Scribner 850e).The cell temperature was 25 • C. H 2 and O 2 were humidified at 25 • C and flowed through the anode and cathode at 200 sccm without back pressure.Steady-state polarization curves were recorded by polarizing the cell using an open circuit voltage (OCV) up to 0.2 V in 0.02 V steps with a 20 s hold time at each step.
Cyclic voltammograms (CVs) for the Co-N-C cathode were determined using a bipotentiostat (CHI 760E) after measuring the polarization curves.The anode was used as a counter/quasi-reference electrode.Fully humidified H 2 and N 2 flowed at 200 sccm at atmospheric pressure through the anode and cathode.The CVs were recorded from 0 to 1.0 V vs. RHE at a scan rate of 50 mV s −1 .A constant capacitance of 30 µF cm −2 was used to evaluate the electrochemical surface area (ECSA), which was used to estimate the loading of the Co-N-C catalysts [26].
During the H 2 O 2 collection experiment, DI-water was pumped into the oxygen electrode at various flow rates (0, 10, 20, 30, and 40 mL min −1 ) to drainage the generated H 2 O 2 (figures 1(b) and (c)).The DI-water and oxygen were fed into the cathode of the fuel cell reactor simultaneously through a Y-shaped connector.The amount of H 2 O 2 collected was determined via a standard potassium permanganate titration process, as shown in the following reaction [27]: where 0.5 M H 2 SO 4 was utilized as the H + source.The number of moles of collected H 2 O 2 (n H2O2 ) and the rate of H 2 O 2 production (r H2O2 ) during the test were calculated as follows: where C KMnO4 (1 M) and V KMnO4 are the molarity and volume of the KMnO 4 solution, respectively, used for the titration process.A is the area of the MEA (A = 4 cm 2 ), and t is the collection time.
The Faraday efficiency (FE) of the H 2 O 2 production was calculated using the following equation:  where j is the discharge current density when collecting H 2 O 2 .
The E iR-free was obtained via the following equation: where E cell is the measured cell voltage, j is the current density, and HFR is the high frequency resistance (HFR).

Effects of MEA fabrication method on cell performance and H 2 O 2 production
We studied the effects of the MEA fabrication method on cell performance and H 2 O 2 production rate using a cathode with the same loading of Co-N-C catalyst (0.7 mg cm −2 ). Figure 2 shows the top-view and cross-sectional SEM images of the different MEAs.For MEA CCM-P , the cathode CL was continuous and evenly covered the Nafion membrane (figures 2(a) and (b)).The CL was densely packed, with a uniform thickness of ca. 25 µm (figure 2(c)).As the catalyst was sprayed directly onto the PEM, a well-connected interface was formed between the CL and the PEM.This well-connected interface was intended to reduce internal resistance and facilitate the mass transport process, thus improving the performance of the fuel cell [28].For MEA GDE-P , the catalyst was sprayed directly onto the carbon paper MPL.The rougher surface of the MPL compared to the PEM led to the construction of a layer with more catalyst aggregates (figure 2(d)).However, this CL surface did not form a good contact interface with the PEM, which may have led to increased internal resistance and mass transport resistance, thus reducing the performance of the MEA.The cathode CL of the MEA GDE-P contained large macropores compared to the MEA CCM-P (figures 2(e) and (f)) [29,30].The more macroporous structure may have promoted the drainage of H 2 O 2 and improved the H 2 O 2 collection efficiency.For the carbon cloth MEA GDE-C as the GDL, the catalyst could not form a continuous thin layer on the carbon cloth surface; the texture of the carbon cloth remained visible (figure 2(g)).Due to the voids between the carbon fiber bundles in the carbon cloth, the catalyst particles were introduced into the pores between the carbon fiber bundles during the spraying process (figures 2(h) and (i)).Catalyst particles deeply penetrated into carbon cloth cannot obtain protons due to incomplete contact with the PEM, which is extremely detrimental to MEA performance enhancement.However, the macroscopic pores formed by the catalyst on the carbon cloth surface established an effective transport channel for liquid.The carbon cloth as the GDL in the MEA GDE-C was better for H 2 O 2 collection due to the low tortuosity of the pore structure and its rough textural surface that facilitated droplet detachment [31,32].Figures 3(a) shows the polarization curves of MEA CCM-P and MEA GDE-P in H 2 -O 2 fuel cells.With the same cathode catalyst loading, the two MEAs exhibited extremely close OCVs of ca.0.75 V.However, as the current density (j) increased, the performance of the MEA CCM-P was significantly better than that of the MEA GDE-P .The maximum discharge current density of the MEA CCM-P was ca.258 mA cm −2 (at 0.25 V), while the MEA GDE-P was up to 190 mA cm −2 (at 0.25 V).Correspondingly, the maximum power density of the MEA CCM-P was ca.67 mW cm −2 , larger than the 50 mW cm −2 of the MEA GDE-P .Further analysis of the polarization curves (figure 3(a)) suggested that the MEA GDE-P performance loss at high current densities was due to ohmic loss and mass transfer limitations.The HFR of MEA GDE-P was 0.31 ohm cm 2 , which was higher than that of MEA CCM-P (0.25 ohm cm 2 ).We then evaluated the H 2 O 2 generation capacity of both MEAs (figure 3(b)).For MEA CCM-P and MEA GDE-P , the H 2 O 2 generation rate (r H2O2 ) was ca.40 µmol cm −2 h −1 and ca.116 µmol cm −2 h −1 , respectively, at an E cell of 0.6 V.The corresponding FEs for the H 2 O 2 synthesis were ca.5% and ca.32.5%, respectively.r H2O2 was higher for the MEA GDE-P than the MEA CCM-P .Figures 3(c)-(e) show the Tafel plots of polarization curves with measured cell voltage (E cell ) and internal resistance corrected voltage (E iR-free ).The voltage difference between the E cell and the E iR-free is the ohmic loss (η ohmic ).The linear region of the logj-E iR-free curve can be fitted with the Tafel equation.The Tafel slopes of the three MEAs varied slightly from 63.1 to 64.5 mV dec −1 , indicating the similar kinetic performance of the cathodes.Assuming a constant Tafel slope is maintained within the full potential range, the relationship between kinetic current (j k ) and electrode potential (E) can be predicted and shown as the straight lines of logj k -E in figures 3(c)-(e).The voltage difference between the logj k -E curve and the logj-E iR-free curve represents the mass transport loss (η mass transport ).At j of 100 mA cm −2 (logj = 2), the η ohmic of MEA CCM-P , MEA GDE-P , and MEA GDE-C were ca.26, 32, and 32 mV, and their corresponding η mass transport were 80, 158, and 176 mV, respectively.The significantly higher η mass transport in MEA GDE is attributed to the macroscopic pore structure within the CL, which may channel the carrier water entry and H 2 O 2 drainage while limiting the O 2 gas transport.For MEA CCM-P , a close contact between the CL and the PEM reduced the η ohmic .The continuous dense CL structure (figures 2(a)-(c)) facilitated the mass transport process, leading to the MEA CCM-P exhibiting higher fuel cell performance [28,33].However, the well-connected interfaces and continuous dense CL structure were detrimental to the timely drainage of generated H 2 O 2 , resulting in a low H 2 O 2 collection efficiency for MEA CCM-P .For MEA GDE-P , the CL with a rough surface and large macropores were unfavorable for the mass transport process.The large macropore structure facilitated the drainage of H 2 O 2 and reduced the H 2 O 2 retention time within the CL.
We then compared the effects of different types of GDLs (carbon paper and carbon cloth) on the H 2 O 2 generation rate and fuel cell performance using GDE-type MEA. Figure 3(a) shows the polarization curves of the two GDE-type MEAs.With the same anode catalyst loading, both MEAs exhibited comparable performances, with an OCV of ca.0.7 V and a maximum output current density and maximum output power density of ca.190 mA cm −2 (at 0.25 V) and ca.50 mW cm −2 , respectively.However, due to the well-connected interface between the CL and PEM in MEA GDE-P compared to MEA GDE-C , the HFR of MEA GDE-P (0.31 ohm cm 2 ) was slightly lower than that of MEA GDE-C (0.34 ohm cm 2 ).MEA GDE-C showed a higher r H2O2 of ca.157 µmol cm 2 h −1 at 0.6 V compared to the carbon paper MEA, corresponding to an FE of ca.50% (figure 3(b)).These results suggest that using different GDLs does not impact fuel cell performance but significantly alters the H 2 O 2 collection efficiency.We believe that the substantially more macroporous structure provided by carbon cloth establishes effective liquid transport channels that are more favorable for DI-water entry and H 2 O 2 drainage than carbon paper (figures 2(g)-(i)) [34,35].

Effects of cathode CL thickness on cell performance and H 2 O 2 production
We studied the effects of CL thickness on H 2 -O 2 fuel cell performance and H 2 O 2 production.The CL thickness was controlled by cathode catalyst loading, and the thin and thick CLs shown here corresponded to a catalyst loading of 0.7 and 1.3 mg cm −2 , respectively.Cyclic voltammetry curves were used to determine the cathode catalyst loading for different MEAs; the double-layer capacitance and the ECSA increased as the catalyst loading increased from ca. 0.7-1.3mg cm −2 (figure 4(a)).The CL thicknesses increased with catalyst loading, as shown in the cross-sectional SEM graphs (figure 4(b)).Comparing fuel cell performance at low loading and high loading, the maximum discharge current density of the fuel cell was ca.211 and 234 mA cm −2 , respectively.The maximum output power densities were ca.48 and ca.61 mW cm −2 , respectively (figure 4(c)).This difference in cell performance was attributed to the increasing number of active sites with increasing catalyst loading.
Figure 4(d) compares the H 2 O 2 production rate of MEA GDE-C with thin and thick CLs; the r H2O2 was ca.157 and 100 µmol cm −2 h −1 at 0.6 V, respectively.The corresponding FE were ca.60% and ca.21%, respectively.We attributed this r H2O2 variation to the effect of CL thickness on the carrier water and H 2 O 2 diffusion resistance.Figure 4(e) demonstrates the mechanism of the CL thickness effect on H 2 O 2 collection.The thicker CL is denser than the thin CL, as shown in the SEM graphs (figure 4(b)), which makes it difficult for the carrier water from the outside to enter the layer to carry the H 2 O 2 , and the H 2 O 2 is diffused from the inside to the outside the layer.The dense CL structure led to the localized H 2 O 2 not being brought out in time, resulting in thermochemical decomposition or further electroreduction to H 2 O, which eventually led to a decrease in r H2O2 .Therefore, constructing a sparse CL structure to reduce retention time and decrease H 2 O 2 consumption in the layer is essential.

Effects of cathode carrier water flow rate on cell performance and H 2 O 2 production
We investigated the effect of the cathode carrier water flow rate on cell performance and H 2 O 2 production.We used a carbon paper GDE-type (MEA GDE-C ) with a cathode catalyst loading of 0.7 mg cm −2 .Figure 5(a) shows that carrier water affected the polarization performance of the fuel cell.The cell performance decreased significantly as the flow rate of the carrier water increased, with the maximum output power decreasing from ca.48 mW cm −2 to ca. 36 mW cm −2 .However, as the carrier water flow rate increased from 10 to 40 mL min −1 , the fuel cell polarization performance remained almost independent.The maximum discharge current density and maximum output power density of the fuel cell remained at ca. 138 mA cm −2 and ca.36 mW cm −2 , respectively.We then compared the OCV and the output current density at 0.6 V for the fuel cell at different carrier water flow rates (figure 5(b)).When the carrier water flow rate increased from 0 to 10 mL min −1 , the OCV was maintained at 0.75 V, but the discharge current density at 0.6 V decreased from ca. 25 mA cm −2 to ca. 17 mA cm −2 .The OCV and discharge current density remained stable as the carrier water flow rate increased.The gradually increasing carrier water flow rate increased the mass transport resistance at the reaction interface and thus decreased cell performance, but this effect was limited.
According to equation (3) and figures 5(c), a higher current corresponds to more H 2 O 2 production.As the discharge voltage decreased, the rate of H 2 O 2 production increased from 157 to 297 µmol cm −2 h −1 , and the corresponding FE decreased from 50% to 12% (the carrier water flow rate was 20 mL min −1 ).We believe that the accumulation and consumption of high concentrations of H 2 O 2 within the CL were more severe under low-voltage operating conditions, and the timely drainage of H 2 O 2 was crucial.Figure 5(d) shows the effect of the carrier water flow rate on H 2 O 2 collection efficiency and r H2O2 under low-voltage operating conditions.Increasing the carrier water flow rate facilitated the collection of H 2 O 2 , with an increase in FE from 10% to 15% and a corresponding increase in r H2O2 from 253 to 369 µmol cm −2 h −1 .

Characteristics of PEMFC mass transport for H 2 O 2 production
To demonstrate the effect of the MEA structure and CL on cell performance and H 2 O 2 production, figure 6 shows a schematic diagram of the MEA and GDE, depicting the through-plane transport of reactants and products in the CL during the 2e-ORR [36][37][38].The O 2 reactant was fed as a gas to the GDL and gradually diffused into the CL to be reduced to H 2 O 2 .The carrier water then entered the GDL to drainage the generated H 2 O 2 .It should be noted that in GDEs, the O 2 and generated H 2 O 2 move in opposite directions.This opposing flow implies that the O 2 diffusion is compromised, and the fuel cell suffers severe flooding with high liquid flow rates, eventually leading to cell performance degradation.Compared with MEA GDE-P , MEA CCM-P provides good mass transport channels with a well-connected interface between the CL and PEM and a dense CL structure.This can reduce the gradient of proton transport resistance (R H+, interface ) variation at the interface and improve the performance of the fuel cell.However, a dense CL inhibits the diffusion of H 2 O 2 , leading to increased retention time of H 2 O 2 within the CL and decreased H 2 O 2 concentration outside Comparing thin and thick CLs with MEA GDE-C , increased CL thickness leads to a longer diffusion distance for O 2 , protons, and H 2 O 2 , which results in enhanced gradient changes in the corresponding substances and reduced fuel cell performance.However, a thick CL provides more active sites, which improves the discharge capacity of the fuel cell.We believe that a thick CL implies a denser CL structure, which inhibits the entry of the carrier water and the diffusion of H 2 O 2 and increases the H 2 O 2 retention time within the layer (figures 6(c) and (d)).This may lead to an increase in the gradient of the H 2 O 2 concentration change from inside to outside the CL.Establishing the well-contact between the CL and PEM and building a CL structure with suitable pores are critical to simultaneously improving fuel cell performance and H 2 O 2 collection efficiency.

Conclusion and outlook
We successfully achieved cogeneration of H 2 O 2 and power using a PEMFC and a platinum group metal-free cathode catalyst (Co-N-C).By systematically analyzing the effects of the MEA fabrication method, cathode CL thickness, and cathode water flow rate on the H 2 O 2 generation efficiency and fuel cell performance, we clarified the transport mechanisms of gas, protons, and products by the MEA structure.The MEA CCM-P structure provides dense proton and mass transport channels that maximize cell output power.However, the dense CL structure of the MEA CCM-P leads to long retention times for H 2 O 2 in the CL and low H 2 O 2 yield.In contrast, the MEA GDE-C structure achieved a higher H 2 O 2 yield by building large macroscopic pore structures at the expense of partial cell performance loss.In addition, CL thickness can significantly affect cell performance and H 2 O 2 yield.A thinner CL can reduce the chemical consumption of H 2 O 2 within the layer and significantly increase the H 2 O 2 yield.Our study demonstrates how the structure of the MEA and CL are crucial parameters that influence mass transport and product concentration.The results of this study provide guidance for the design of PEMFC reactors based on the low-cost electrochemical synthesis of H 2 O 2 .
Development of more active catalysts, more efficient MEA structures, and flow field designs can further improve the cell performance, although the single-cell reactor for H 2 O 2 synthesis shows a lower OCV than the conventional fuel cell due to the thermodynamic limits of 2e-ORR.The capability of fuel cell reactors to convert chemical energy into electric power while producing valuable high-purity H 2 O 2 makes the system energetically efficient and attractive to industrial applications.

Scheme 2 .
Scheme 2. Two membrane electrode assembly (MEA) fabrication methods used in this study.

Figure 1 .
Figure 1.(a) Experimental setup of the PEMFC reactor used to produce H2O2.The photographs show the color change of the KMnO4 solution (b) before and (c) after titration.

Figure 6 .
Figure 6.Schematic of the concentration changes gradient of the reactants (O2) and products (H2O2), the gradient of proton transport resistance (R H+, interface ), and their respective transport directions during H2O2 production through different MEA structures: (a) CCM-type MEA, (b) GDE-type MEA with carbon paper, (c) GDE-type MEA with carbon cloth and thin CL, and (d) GDE-type MEA with carbon cloth and thick CL.