Cathode Catalyst Layer Structure Research Articles

Quantification of O2 Crossover and Concurrent Faradaic Efficiency Loss of H2 Production Via PEM Water Electrolysis

PEM water electrolyzers (PEMWEs) are considered to be a key technology for the decarbonization of major sectors by being able to store fluctuating renewable energy as chemical energy in the form of hydrogen gas. While the scarcity of the necessary iridium catalyst on the anode limits widespread application minding the high loadings still needed today,1 there are additional efficiency losses that are still to be fully understood.By employing a proton exchange membrane, PEMWEs are able to produce hydrogen at elevated pressure on the cathode while keeping the balance of plant costs low as the anode can still be operated with liquid water at close to ambient pressure. Hydrogen crossover to the anode compartment through the membrane is significantly increased at elevated cathode pressures, which is majorly considered a safety issue. As the anode catalyst does not serve as a good hydrogen oxidation reaction catalyst, explosive mixtures of hydrogen and oxygen can form at the anode (and its exhaust) depending on the membrane type and thickness, current density, temperature, hydrogen partial pressure at the cathode, cathode catalyst layer structure etc.2-4 Aside of safety concerns – which can be bypassed by the use of recombination interlayers in the membrane5 – gas crossover also reduces the Faradaic efficiency of the system as parts of the hydrogen produced cannot be used further because it does not reach the cathode exhaust.A so far not majorly considered additional Faradaic efficiency loss stems from oxygen crossover from the anode to the cathode. As the platinum based cathode catalyst is very active for the oxygen reduction reaction, basically all of the oxygen crossing over to the cathode compartment recombines with the hydrogen present, forming liquid water.6 The permeability of oxygen through common PEM materials is significantly lower than that of hydrogen.7 But due to the osmotic drag from the anode to the cathode as well as possibly high bubble pressures of oxygen at the anode catalyst layer / membrane interface,8 oxygen crossover deserves closer attention.In this study we show quantification of the oxygen crossover in operating PEMWE single cells by a full balance of the gases detected at both the anode and cathode exhaust using an online mass spectrometer. A direct correlation of Faradaic efficiency of the oxygen and hydrogen produced matches the expected chemical recombination on the cathode, which serves as a direct measure of the oxygen crossover to the cathode compartment during PEMWE operation. Depending on the exact operating conditions the Faradaic efficiency loss caused by oxygen crossover can easily be in the percentage range, stressing the importance of having a closer look at the oxygen crossover characteristics of PEMWE cells in order to improve the Faradaic efficiency of the system. Taie, Z.; Peng, X.; Kulkarni, D.; Zenyuk, I. V.; Weber, A. Z.; Hagen, C.; Danilovic, N., Pathway to Complete Energy Sector Decarbonization with Available Iridium Resources using Ultralow Loaded Water Electrolyzers. ACS Appl Mater Interfaces 2020, 12, 52701-52712.Bernt, M.; Schröter, J.; Möckl, M.; Gasteiger, H. A., Analysis of Gas Permeation Phenomena in a PEM Water Electrolyzer Operated at High Pressure and High Current Density. J. Electrochem. Soc. 2020, 167.Ito, H.; Miyazaki, N.; Ishida, M.; Nakano, A., Cross-permeation and consumption of hydrogen during proton exchange membrane electrolysis. Int. J. Hydrogen Energy 2016, 41, 20439-20446.Trinke, P.; Keeley, G. P.; Carmo, M.; Bensmann, B.; Hanke-Rauschenbach, R., Elucidating the Effect of Mass Transport Resistances on Hydrogen Crossover and Cell Performance in PEM Water Electrolyzers by Varying the Cathode Ionomer Content. J. Electrochem. Soc. 2019, 166, F465-F471.Zhang, Z.; Han, Z.; Testino, A.; Gubler, L., Platinum and Cerium-Zirconium Oxide Co-Doped Membrane for Mitigated H2 Crossover and Ionomer Degradation in PEWE. J. Electrochem. Soc. 2022, 169.Schalenbach, M.; Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., Pressurized PEM water electrolysis: Efficiency and gas crossover. Int. J. Hydrogen Energy 2013, 38, 14921-14933.Schalenbach, M.; Hoefner, T.; Paciok, P.; Carmo, M.; Lueke, W.; Stolten, D., Gas Permeation through Nafion. Part 1: Measurements. The Journal of Physical Chemistry C 2015, 119, 25145-25155.Weber, C. C.; Wrubel, J. A.; Gubler, L.; Bender, G.; De Angelis, S.; Buchi, F. N., How the Porous Transport Layer Interface Affects Catalyst Utilization and Performance in Polymer Electrolyte Water Electrolysis. ACS Appl Mater Interfaces 2023, 15, 34750-34763.

Unveiling the impact of pore structure of cathode catalyst layer on proton exchange membrane cell performance

This work quantitatively explores the impact of pore structure of cathode catalyst layer (CCL) on the overall performance of low-Pt loading electrodes in Proton Exchange Membrane Fuel Cell (PEMFCs), providing guidelines for designing high-performance CCL. The model predictions align closely with experimental measurements. The results reveal that the best cell performance occurs when the ratio of secondary to primary pores is 2, which is attributed to the balance between liquid water saturation and the diffusivity of gaseous species within the catalyst layer. In addition, higher total porosity or higher Pt loading require more primary pores to achieve a remarkable improvement in performance. Furthermore, the influence of total porosity on the peak power density of fuel cell depends on pore size distribution. The optima total porosities are 0.5 and 0.7 when the ratios of secondary to primary pores are 2 and 1, respectively.

Experimental and numerical analysis of a catalyst layer in the membrane electrode assembly of polymer electrolyte membrane fuel cells

Fuel cells constitute an important strategy for carbon-neutral Carbon Dioxide recycling because they utilize the green hydrogen produced from renewable electrical energy. To achieve low-cost and high-performance polymer electrolyte fuel cells, reducing active metals by optimizing the cathode catalyst layer structure is crucial. In this study, we combined image analysis, numerical analysis, and electrochemical approaches to clarify the diffusion characteristics of the cathode catalyst layer in a polymer electrolyte membrane fuel cell. The structure of a catalyst-supported carbon substrate and stacked catalyst-supported carbon substrate was determined using transmission electron microscopy and focused ion-beam scanning electron microscopy. Porosity was analyzed by image analysis and torsion by numerical analysis using the random walk method. Diffusivity near the catalyst and pores was evaluated using a recently developed method by analyzing the active metal surface resistance based on the dependence of the catalyst layer resistance on carbon monoxide coverage. These analytical methods were applied to catalyst-supported carbon materials with different structures. Diffusion through the catalyst surface, carbon support, and carbon support aggregates was also analyzed; furthermore, we obtained following guidelines for improving the specifications. Larger pores and smaller flexures in the cathode catalyst layer may improve the performance of Polymer Electrolyte Membrane Fuel Cells.

Effect of the pore structure of cathode catalyst layer on the PEM fuel cell cold start process

A transient numerical model was developed to investigate the effect of the pore structure in the cathode catalyst layer on the cold start process of polymer electrolyte fuel cells. The results prove the necessity of considering the subcooled water at subzero temperatures when simulating the cold start of fuel cells based on the Gibbs-Thomson effect. At ambient temperatures lower than −13 °C, the ice blockage is found to occur inside the cathode catalyst layer (CCL). However, due to the generation and flow of the subcooled water, the blockage occurs at the CCL and cathode microporous layer (CMPL) interface at a temperature higher than −13 °C. The pore size of the CCL also plays an important role in the cold start of PEM fuel cells, and there exists a critical pore size to shift the blockage location from the interior of the CCL to the CCL/CMPL interface. The CCL with this critical pore size gives the fuel cell the best cold start capability. It is also worth noting that the actual pore size distribution of the CCL complicates the blocking mechanisms in the fuel cell and remarkably influences the cold start simulation, especially at an operating temperature higher than −12 °C.

Simulation of Reaction and Mass Transport in PEFC Cathode Catalyst Layer Fabricated by Inkjet Printing Method

The structure of cathode catalyst layer (CCL) is critically important for improved performance, durability, and stability of polymer electrolyte fuel cells (PEFCs). In this research, two types of CCL are designed to do comparison, one is flat structure, and the other is 3D structure. Inkjet printing method was used in this research to fabricate the structures of cathode catalyst layer, then the MEA was taken to do electrochemical evaluation. The results showed that electrochemical properties of CCL with 3D structure were better than the flat one through I-V curve and impedance. Overall, it is demonstrated that with a 3D structure, it is possible to achieve favorable morphological and microstructural characteristics.

Effect of Cathode Catalyst Layer Structure on the Performance of PEFC

One of the routes to promote PEFC commercialization is to reduce its manufacture cost. Platinum represents more than 50% of PEFC cost. Therefore, two targets can be considered; reduction of platinum loading (g/cm2) at cathode, and increase specific catalytic activity towards reaction (mA/cm2 pt). On the other hand, the effectiveness factor has an inherent relation with current density and mass transfer of reactants (oxygen and proton), which are existing simultaneously. For investigating cathode behavior, two dimensionless moduli, representing diffusion limitations due to oxygen and proton mass transfer resistances, have been proposed 1. To estimate mass transport resistances, effectiveness factor and intrinsic reaction rate constant, experiments were carried out by varying cathode catalyst layer (CCL) structure on a membrane electrode assembly (MEA). The MEA, fabricated by decal transfer method, consisted of a Nafion membrane (NR-212, 50 μm) with Pt/C weight ratio of 0.3, 0.4 and 0.5 in 10 μm CCL. The ionomer (I)/C ratio was 1. The CCL was prepared by two methods. The first, Pt particles were supported on carbon (Ketjen black) relevant to the required Pt/C ratio (Standard catalyst preparation). The second, lower Pt/C ratios were manipulated by adding carbon black (CB) to a prepared catalyst mixture contained 50% Pt/C where Pt loading was 0.3 mg/cm2 (CB dilution method). The IR corrected voltage (electromotive force) was calculated by measuring the polarization curves and membrane resistance using an electrochemical measurement system. The impedance spectra were measured in a range of frequency from 10 mHz to 100 kHz. The analysis of the results using our dimensionless moduli approach may support having deeper insight that could accelerate the development of PEFC. Reference: M. Kawase et al., AIChE J., 63, 249–256 (2017) http://doi.wiley.com/10.1002/aic.15545. Figure 1

Effect of Cathode Catalyst Layer Structure on the Performance of PEFC

Reducing the Platinum utilization in PEFC is one route for its commercialization, by reducing the cathode platinum loading, and/or increase in its specific reaction catalytic activity. However, the effectiveness factor has an inherent relation with current density and reactants mass transfer. Two dimensionless moduli, representing diffusion limitations have been used to investigate cathode behaviour. Experiments were carried out with varying cathode catalyst layer (CCL) structure. The membrane electrode assembly (MEA) consisted of a Nafion membrane (NR-212, 50 μm) with Pt weight fraction of 0.30, 0.40 and 0.50 in 10 μm CCL. The CCL was prepared by two methods; (1) Pt particles were supported on carbon relevant to the required Pt/C ratio, (2) lower Pt contents were manipulated by adding carbon black (CB) to a prepared catalyst containing 50 wt.% Pt. The results showed proton transport resistance is the key parameter affected by the platinum inhomogeneity within the CCL.

High-performance and durable cathode catalyst layer with hydrophobic C@PTFE particles for low-Pt loading membrane assembly electrode of PEMFC

Abstract This paper mainly focuses on the optimization and design of cathode catalyst layer structure for low-Pt loading membrane assembly electrode. When Pt loading at cathode increased from 0.025 mg cm−2 to 0.4 mg cm−2, the apparent current density of electrode increases gradually, while the electrochemical active surface area (ECSA) and the utilization rate of catalyst show a downward trend. As Pt loading at cathode keeps at 0.1 mg cm−2, commercial Vulcan XC-72 carbon are doped into the conventional Pt/C catalyst layer to improve the utilization rate of the catalyst. The results show that the optimum doping carbon content leads to ∼2 times increase of the thickness of the cathode catalyst layer, and achieves a 32.93% increase of ECSA and better durability than that of conventional catalyst layer. Meanwhile, gas transport in the catalyst layer becomes more difficult with the increase of the amount of doping carbon. Based on this, a more stable Vulcan XC-72@PTFE particles are prepared and selected to construct internal gas transport channels under the same carbon content. The surface contact angle of the catalyst layer is obviously increased to 118.4°. The hydrophobicity of the cathode catalyst layer is enhanced, and the oxygen gain voltage is significantly reduced. The peak power density of PEMFC reaches 539 mW cm−2, which is 30.57% higher than that of one with a conventional catalyst layer structure. Therefore, this work provides a good way for the preparation of high-performance and durable cathode catalyst layer for low-Pt loading membrane assembly electrode.

Influence of Cathode Catalyst Layer Structure on PEFC Performance Using Two-Dimensional Carbon Support for Pt Catalyst

Introduction Polymer electrolyte fuel cell (PEFC) has been attracting much attention due to its high efficiency at low operating temperature. However, since platinum (Pt), which is used for the electrode catalyst in the cathode, is a metal of low abundance and high cost, reduction of the Pt usage is required. Generally, nano-size Pt particles supported on carbon black are used to increase the mass activity. However, nanoparticles have problems in durability. Moreover, since some Pt particles are located inside the pore of the support, diffusion of supplied oxygen and produced water are suppressed. Recently, nanostructured thin film (NSTF)1) and Pt thin film fabricated by atomic layer deposition (ALD)2) are reported as alternative catalysts. However, they are fabricated on non-conducting materials. Takenaka et al. fabricated metal oxide nanofilms using graphene oxide (GO) as a template to form two-dimensional (2-D) catalysts3). 2-D catalysts are expected to show high mass activity due to the high surface area, and high conductivity during power generation due to the reduced GO and/or metal oxide. In this study, membrane electrode assembly (MEA) was prepared using Pt/GO catalyst, and the relationship between the performance and the structure of the electrode catalyst layer using 2-D carbon support for Pt catalyst was investigated to obtain information towards low Pt loaded PEFC. Experimental Catalyst inks were prepared by mixing Pt /Graphene oxide (Pt/GO), carbon black (CB), n-Propyl alcohol, and distilled water, and then the mixture was sonicated. 5 wt.% Nafion® solution was added to the mixture which was sonicated again. Catalyst ink was mixed by rotation revolution mixer. Then it was coated onto polytetrafluoroethylene (PTFE) sheet using doctor blade method to fabricate catalyst layer (CL). CL and Pt/carbon paper were hot-pressed onto each side of the Nafion® membrane. After the PTFE sheet was peeled off, carbon paper was hot-pressed onto CL to fabricate the MEA. Terminal voltage vs. current density of the MEA was measured at 80 ˚C. Results and Discussion Terminal voltage vs. current density of MEA with various ionomer/carbon ratios (I/C) using Pt/GO are shown in Fig. 1. As it can be seen in the figure, the cell performance in the low current density region was higher when I/C was higher. This indicates that reactive sites have increased due to the coating of catalyst by ionomer. On the other hand, MEA prepared with low I/C showed higher cell performance in the high current density region. It is considered that ohmic resistance was reduced due to the thin coating of catalyst by ionomer. References 1) M. K. Debe, J. Electrochem. Soc, 160 (6), F522-F534 (2013) 2) M. Inaba, T. Suzuki, T. Hatanaka, Y. Morimoto, J. Electrochem. Soc, 162 (7), F634-F638 (2015) 3) S. Takenaka, S. Miyake, S. Uwai, H. Matsune, M. Kishida, J. Phys. Chem. C, 119 (22), 12445-12454 (2015) Figure 1

Self-humidifying membrane electrode assembly with dual cathode catalyst layer structure prepared by introducing polyvinyl alcohol into the inner layer

Short-term constant voltage discharge test of the self-humidifying MEAs with different cathode structure at 60 °C and 30 psi under 20% RH.

PEFCのカソード触媒層構造が氷点下起動特性に及ぼす影響

PEFCのカソード触媒層構造が氷点下起動特性に及ぼす影響

Analysis of Cathode Catalyst Layer Structure and Oxygen Transport Resistance Depending on Fabrication Condition in PEFC

For efficient use of Pt catalyst in polymer electrolyte membrane fuel cells (PEFCs), the effects of the cathode catalyst layer (CL) structure on the cell performance were evaluated experimentally. In particular we focused on two parameters used in analysis by agglomerate models, the agglomerate size and the oxygen dissolution resistance to the ionomer surrounding the carbon agglomerate. Various ratios of ionomer to carbon were selected as the CL fabrication condition with same amount of Pt-supported carbon. To estimate the agglomerate size, pore size distribution (PSD) was measured by nitrogen physisorption method, and an estimation method of agglomerate size distribution from the measured PSD was applied. It was shown that the model analysis using the estimated parameters with relatively-small agglomerate size can well simulate the measured IV characteristics, and that the balance of the oxygen diffusion and dissolution resistances in the ionomer dominates the cell performance under the condition here.

Analysis of Cathode Catalyst Layer Structure and Oxygen Transport Resistance Depending on Fabrication Condition in PEFC

In polymer electrolyte fuel cell (PEFC), the cathode catalyst layer (CL) needs a large amount of Pt because of the slow oxygen reduction reaction. Since electron, proton, and oxygen are necessary for the cathode reaction, achieving the optimum structure of electrode CL and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. In this study, the effects of the cathode CL structure on the IV characteristics were evaluated experimentally using catalyst coated membranes (CCMs) fabricated under several conditions. Pore size distribution of the CL was measured by nitrogen physisorption method and the estimation method of agglomerate size distribution from the measured PSD was developed. From the estimated CL structural parameters, the relationship between the CL structure and the cell performance was elucidated using the analysis with an agglomerate model. In particular, we focused on the agglomerate size and the polymer film thickness, and the effects of the oxygen diffusion and dissolution resistances in the polymer electrolyte (ionomer) on the cell performance are discussed. Figure 1 shows the I-V characteristics for different ratios of ionomer to carbon in the CL (I/C ratios). To investigate the effects of ionomer thickness surrounding the carbon agglomerate, the weights of the carbon with Pt were set to be similar; decrease in the I/C ratio corresponds to decrease in the amount of ionomer in the CL. The results with the I/C ratio from 2.0 to 0.75 shows the expected tendency of decrease in the concentration overpotential and improvement of the cell performance with the lower I/C ratio. This is considered because the oxygen diffusion resistance in the ionomer decreases with smaller amount and thinner ionomer film. However, an increase in concentration overpotential and a deterioration of cell performance are observed with the I/C ratio from 0.75 to 0.5. To evaluate the factors dominating the concentration overpotential using the model analysis developed by the authors (1), the estimation of the CL structure was conducted. Figure 2 shows the pore size distributions measured by nitrogen physisorption method for different I/C ratios, 0.5 and 0.75. The mode pore diameters of the CL are similar, but the porosities are different: 0.67 and 0.58 for the I/C ratios of 0.5 and 0.75. This is estimated to be because of decrease in the amount of the ionomer filling the pores among the carbon frame of the CL. Here, the thickness of the ionomer is expected to become thinner with the lower I/C ratio, 0.5. From the measured pore size distribution in Fig. 2, the structural parameters of the CL were identified using an estimation model of the agglomerate diameter, developed by the authors (2). The estimated parameters are summarized in Table 1. It is confirmed that the ionomer thickness with the I/C ratio of 0.5 is thinner as expected, and the carbon agglomerate diameters are around 30nm. Figure 3 shows the I-V characteristics calculated by the model analysis using the estimated parameters in Table 1. The result can simulate the tendency of the measured IV characteristics, the concentration overpotential and the limiting current density. The calculation reveals the reasons why the performance with the I/C ratio of 0.5 is lower than that of 0.75 in spite of the thinner ionomer thickness. In the case with the I/C ratio of 0.5, the surface area of the ionomer surrounding the carbon agglomerate is smaller, and the oxygen flux into the ionomer at the surface is larger at the same current density, compared with that of 0.75. This increases the effect of oxygen dissolution resistance, resulting in deterioration of the cell performance. The result means that the oxygen dissolution resistance into the ionomer is also a dominant factor to the concentration overpotential as well as the oxygen diffusion resistance in the ionomer. It can be concluded that the proper CL structure with smaller oxygen transport resistance is determined by the balance between the oxygen diffusion and dissolution resistances in the ionomer.Reference(1) Y. Tabe, M. Nishino, H. Takamatsu, and T. Chikahisa, J. Electrochem. Soc., 158, B1246 (2011).(2) S. Akabori, K. Suzuki, Y. Tabe, and T. Chikahisa, ECS Trans., 64(3), 305 (2014). Figure 1

Analysis of Cathode Catalyst Layer Structure and Cell Performance in PEFC

For efficient use of Pt catalyst in polymer electrolyte membrane fuel cells (PEFCs), the effect of the cathode catalyst layer (CL) structure on the I-V characteristics was evaluated by a model analysis, and superior fabrication method was investigated experimentally. The experimental results showed that reducing the hot-pressing pressure and increasing the mixing intensity of the catalyst ink in the fabrication process of the catalyst coated membrane (CCM) improve the cell performance, and the measured pore size distribution (PSD) in the CL indicated the effect of the increase in agglomerate surface area on the performance improvement. In addition, we developed a method for estimating the mean diameter and the dispersion of the agglomerates from the PSD. The effect of the estimated CL structure on the cell performance was also examined by the model analysis for the CCMs fabricated by the different conditions.

Analysis of Cathode Catalyst Layer Structure and Cell Performance in PEFC

In polymer electrolyte fuel cell (PEFC), the catalyst layer (CL) of the cathode needs a large amount of Pt because of the slow oxygen reduction reaction. Since electron, proton, and oxygen are necessary for the cathode reaction, achieving the optimum structure of electrode CL and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. In this study, the effects of the cathode CL structure on the IV characteristics were evaluated by a model analysis, and superior fabrication methods were investigated experimentally for efficient use of Pt catalyst.Figure 1 shows a schematic diagram of the cathode CL model, here the identical carbon agglomerates with the electrolyte film are dispersed uniformly in the CL (1). In the z-axis of the thicknesses direction, the origin and z = δCL are the interfaces between polymer membrane and CL, and between CL and gas diffusion layer (GDL), and number of the carbon agglomerates per unit volume in the CL, nag , is calculated from the parameters for the CL structure. The proton current density in the CL, iH+ , can be expressed by Eq. 1 from the balance of generated current and oxygen consumption.Here, F is Faraday’s constant, Sag is the surface area of a carbon agglomerate is the effective proton conductivity. The oxygen consumption, JO2 ag , corresponds to the oxygen flux in the radial direction of carbon agglomerate at the surface. From the model analysis with various CL structures, it was shown that the increase in total surface area of agglomerates is effective for better IV characteristics. Figure 2 shows the effects of the catalyst preparation conditions on the IV characteristics. A catalyst coated membranes (CCMs) were made by decal transfer method, and two types of the mixing methods of the catalyst ink in the fabrication process, with ball mill and with stirrer, were examined (the hot-press conditions: 2.0 MPa, 120 °C, 30 min). Comparing the IV curves with the CCMs by the same time, 0.5 h, the limiting current density for the ball mill is slightly higher than that for the stirrer. Further, the limiting current density for the ball mill is increased with increasing the mixing intensity of the catalyst ink. To evaluate the cause of the improvement of the IV characteristics, the pore size distributions were compared between with the ball mill and with the stirrer mixing, as shown in Fig. 3. The distribution of the secondary pore diameters (20-100nm) is affected by the mixing method of the catalyst ink. For the ball mill with strong mixing intensity, the mode pore diameter is slightly smaller and the total volume of the secondary pores is larger than those for the stirrer, indicating increase in the total surface area of the secondary pores. This means that the total surface area of the agglomerate may also increases, contributing to the performance improvement. Thus, it was confirmed by the measured polarization curves and pore size distributions in the catalyst layer that the increase in total surface area of the agglomerates is effective for improving the cell performance. However, the quantitative relationships between the pore size distribution of the secondary pores and the diameter distribution of the agglomerate particles are not clear. We are investigating these relationships and the detailed effect of the measured CL structure on the measured IV characteristics by model analyses. Elucidation of appropriate CL structure and proposal of the structure control method for efficient use of Pt catalyst in the fabrication process are our future works.Reference(1) Y. Tabe, M. Nishino, H. Takamatsu, and T. Chikahisa, J. Electrochem. Soc., 158, B1246 (2011).

Parametric investigation of a high-yield decal technique to fabricate membrane electrode assemblies for direct methanol fuel cells

This study comprehensively investigates various technical aspects of a roll-press-based decal process that is used to fabricate membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs). Decal transfer yield, flexibility of processing conditions and electrochemical performance of MEAs are taken into account for monitoring the productiveness of the current method. A complete transfer of both electrodes is achieved even under a pressure as low as 1.0 MPa, which is 8–35 times lower than that of conventional decal processes. This method permits use of a H+-form Nafion membrane in a wide press temperature domain ranging from 140 to 180 °C without the occurrence of degradation problems that are generally encountered in the conventional decal processes. The effective hot-pressing time is successfully shortened to only 2–5 s, which is far less than those of the conventional decal processes (3–10 min). The structure of cathode catalyst layer is optimized by regulating the ionomer amount. The decal MEA prepared under optimal conditions delivers a peak power density of 115 mW cm−2 at 60 °C, which is substantially high in a DMFC operation. Superior throughput and flexibility of processing conditions over a wide range make the current method appropriate for use in the mass-production of MEAs.

406 固体高分子形燃料電池における触媒層構造制御と性能解析(熱学I)

406 固体高分子形燃料電池における触媒層構造制御と性能解析(熱学I)

Effects of Cathode Catalyst Layer Structure and Properties Dominating Polymer Electrolyte Fuel Cell Performance

Transport of electrons, protons, and oxygen are necessary for the cathode reactions in polymer electrolyte membrane fuel cells, and achieving the optimum structure of the electrode catalyst layer and the efficient transport of reactants is an effective avenue to reduce the use of platinum catalyst. This study applied three-phase boundary and cathode catalyst layer models to understand details of optimally efficient structures for the transport of reaction components. The factors dominating the effects of the catalyst layer structure and the properties identified in this manner are investigated using the models. Additionally, equations of evaluation are developed to evaluate the effects of the structure and the properties on the cell performance, and the effectiveness of the developed equations is confirmed by a comparison of the results calculated by the equations with the model simulations. From these results, the structure of the porosity, the catalyst layer, and the polymer electrolyte thicknesses, that are optimum for the gas transport and proton conduction, are determined. It is found that the solubility of oxygen in the polymer is one of the dominant factors in the processes of the cathode catalyst layer, and that increasing the solubility is highly effective to reduce the need for platinum.

Structural Effect of Cathode Catalyst Layer on the Performance of PEFC

The structure of cathode catalyst layer (CCL) has strong relationship with the performance of polymer electrolyte fuel cells (PEFCs). We investigated the relationship between the catalyst layer structure and the cell performance experimentally. Multi-layered CCL is used to investigate the effect of the layer design on the cell performance. Membrane side of CCL works, as a reaction area, more actively than the gas diffusion layer (GDL) side at low relative humidity (RH) due to the lower proton conductivity. On the other hand, when the cathode gas has less oxygen partial pressure at high RH, GDL side is more active than membrane side owing to low diffusivity of oxygen. We suggest that the volumetric catalyst concentration of the CCL membrane side should be higher at low RH, however at high RH with lower oxygen partial pressure in cathode gas, the GDL side should have higher concentration. Simple theoretical model is employed to see the behavior of the reaction distribution in the catalyst layer.

Publisher's Note: Effects of Cathode Catalyst Layer Structure and Properties Dominating Polymer Electrolyte Fuel Cell Performance [J. Electrochem. Soc., 158, B1246 (2011)

Publisher's Note: Effects of Cathode Catalyst Layer Structure and Properties Dominating Polymer Electrolyte Fuel Cell Performance [J. Electrochem. Soc., 158, B1246 (2011)