Cathode Cathode Catalyst Layer Research Articles

Empirical Degradation Models of the Different Indexes of the Proton Exchange Membrane Fuel Cell Based on the Component Degradation

To describe the degradation of proton exchange membrane fuel cells (PEMFCs), empirical degradation models of different indexes of PEMFCs are established. Firstly, the simulation process and assumptions of PEMFC degradation are proposed. Secondly, the degradation simulation results including the performance and distribution indexes under the different degradation levels are conducted by AVL FIRE M. Finally, the empirical degradation models of performance and distribution indexes are established based on the above simulation results and experimental data. The results show that the relationship between the experimental and simulation results is established by the index of current density. The empirical degradation models of current density, average equilibrium potential on the cathode catalyst layer (CL), average membrane water content, average oxygen molar concentration on the cathode CL, and average hydrogen crossover flux are the linear function. The empirical degradation models of average exchange current density on the anode CL, average hydrogen molar concentration on the anode CL, and average oxygen crossover flux are the quadratic function. The empirical degradation model of average activation overpotential on the cathode CL is the quintic function.

Revolutionizing high-temperature polymer electrolyte membrane fuel cells: Unleashing superior performance with vertically aligned TiO2 nanorods supporting ordered catalyst layer featuring Pt nanowires

For large-scale commercialization of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), the issues of high Pt loading and unsatisfactory durability must be addressed. Disordered cathode catalyst layer (CL) with the random distribution of Pt/C electrocatalysts, binders, and pores is proved as one main reason for the motioned issues due to the limited Pt utilization efficiency, tortuous mass transport, and poor stability of Pt/C. Herein, we construct an ordered cathode CL consisting of in-situ grown Pt nanowires on the vertically aligned TiO2 nanorods array, coupled with carbon nanotubes as conductors to enhance the mass transfer characteristics and durability. In the single cell tests, the membrane electrode assembly (MEA) with ordered cathode CL achieves 3.53-fold Pt mass-specific peak power density compared to a conventional MEA. The Pt loading is only 20 % of the latter. Meanwhile, much better durability is confirmed. The superior performance and durability are attributed to the synergistic effects of ordered structure, stable TiO2 nanorods, one-dimensional features of Pt nanowires, and strong metal-support interaction between TiO2 and Pt. This work provides new insight into CL design and facile fabrication method of the ordered CL, which is of great significance for the large-scale application of HT-PEMFC.

(Invited) Modeling Two-Phase Transport in the Cathode Catalyst Layer of a PEM Fuel Cell with a Focus on the Ionomer Phase Structure and Its Effect on Oxygen Transport

A hydrophobic treatment was developed to reduce the liquid water saturation level and increase its removal rate from the cathode catalyst layer (CL) of a PEM fuel cell. If this was achieved, the transport rate of oxygen to the reactive sites would be expected to increase and the fuel cell performance would be improved, especially in the high current-density mass-transport controlled region. The performance of the fuel cells with the hydrophobically treated cathode CLs tested with humidified air at 70oC shows significant improvement (> 200% increase in peak power).[1,2] The polarization curves obtained show improvements over the entire curve from the kinetic-controlled region to the mass-transport affected and mass-transport limited regions instead of only the mass-transport limited region as originally expected. A mathematical model was developed to provide insight into the causes for the observed responses with a focus on the oxygen-water two-phase transport phenomena in the catalyst layer. It suggests the hydrophobic treatment affects the CL in two ways. First, the interface of the ionomer layer exposed to the gas pores becomes more hydrophobic and facilitates less coverage and faster water drainage from the CL resulting in better performance at high current densities. Second, it reduces the hydration level in the ionomer phase which results in higher oxygen concentration in the ionomer phase on top and in the catalyst agglomerates leading to higher performance over the whole polarization curve. This presentation will discuss the major features of the new model and the findings obtained in this study.[3] These efforts show that the modeling work is not just a simulation of the experimental results using established theories and equations. It requires researching, finding, and understanding the mechanisms and phenomena that govern the physical processes in the fuel cell and result in the measured fuel cell performance. The modeling work provides insights that could not be gained from fuel cell testing.References Regis P. Dowd Jr., Cynthia S. Day, and Trung Van Nguyen, “Engineering the Ionic Polymer Phase Surface Properties of a PEM Fuel Cell Catalyst Layer,” J. Electrochem. Soc., 164 (14), F138-F146 (2017). 10.1149/2.1081702jesRegis P. Dowd, Yuanchao Li, and Trung Van Nguyen, “Controlling the Ionic Polymer/Gas Interface Property of a PEM Fuel Cell Catalyst Layer During Membrane Electrode Assembly Fabrication,” J. Appl. Electrochem., 50 (10) 993-1006 (2020). 10.1007/s10800-020-01453-wYuanchao Li and Trung Van Nguyen, “A One-Dimensional Model of the Cathode Catalyst Layer of a PEM Fuel cell Hydrophobically Treated for Water Management,” J. Electrochem. Soc., 169 114505 (2022). 10.1149/1945-7111/ac9bdf

Multi-objective optimization of the cathode catalyst layer micro-composition of polymer electrolyte membrane fuel cells using a multi-scale, two-phase fuel cell model and data-driven surrogates

Polymer electrolyte membrane fuel cells (PEMFCs) are considered a promising alternative to internal combustion engines in the automotive sector. Their commercialization is mainly hindered due to the cost and effectiveness of using platinum (Pt) in them. The cathode catalyst layer (CL) is considered a core component in PEMFCs, and its composition often considerably affects the cell performance (Vcell) also PEMFC fabrication and production Cstack costs. In this study, a data-driven multi-objective optimization analysis is conducted to effectively evaluate the effects of various cathode CL compositions on Vcell and Cstack. Four essential cathode CL parameters, i.e., platinum loading (LPt), weight ratio of ionomer to carbon (wtI/C), weight ratio of Pt to carbon (wtPt/c), and porosity of cathode CL (εcCL), are considered as the design variables. The simulation results of a three-dimensional, multi-scale, two-phase comprehensive PEMFC model are used to train and test two famous surrogates: multi-layer perceptron (MLP) and response surface analysis (RSA). Their accuracies are verified using root mean square error and adjusted R2. MLP which outperforms RSA in terms of prediction capability is then linked to a multi-objective non-dominated sorting genetic algorithm II. Compared to a typical PEMFC stack, the results of the optimal study show that the single-cell voltage, Vcell is improved by 28 mV for the same stack price and the stack cost evaluated through the U.S department of energy cost model is reduced by $5.86/kW for the same stack performance.

Enhanced Triple-Phase Interface in PEMFC by Proton Conductor Absorption on the Pt Catalyst

In a proton exchange membrane fuel cell (PEMFC), the membrane electrode assembly (MEA) is the core component and the region of the oxidation–reduction. In order to obtain a great performance, Pt with excellent catalyst efficiency is usually adopted in PEMFC as the catalyst. However, the high cost and poor durability remain the two major challenges in the application of PEMFC; thus, it is worth paying attention to enhance the utilization of the Pt catalyst and the stability of PEMFC. In this work, the Nafion array membrane with a larger specific surface and higher proton conductivity was applied to the cathode catalyst layer (CL) to prepare the ordered MEA. In order to improve the three-phase interface of the cathode CL, Nafion was adsorbed on the Pt particles as the proton conductor to expedite the proton transfer efficiency based on the principle that sulfonic acid is easily adsorbed on the Pt surface. In this case, the peak power density of PEMFC with Nafion absorption on the Pt surface is up to 843 mW cm–2 at the Pt loading of 61.4 μg cm–2, which is much higher than that of the fuel cell without a proton conductor on the Pt catalyst in the cathode CL (710 mW cm–2). Besides, the durability tests show that PEMFCs with Nafion absorption on the Pt catalyst surface can work continuously for 100 h without obvious voltage attenuation, which is more stable than that of the bare Pt for 70 h. In conclusion, Nafion as the proton conductor was adsorbed on the Pt catalyst surface of the cathode CL to enhance the triple-phase interface in PEMFC, which is expected to be a universal method to prepare PEMFCs with high stability and peak power density at a low preparation cost.

Water transport analysis during cathode dry operation of anion exchange membrane water electrolysis

The effect of water diffusion through an anion exchange membrane (AEM) on the concentration overpotential (ηconc) during cathode dry operation of AEM water electrolysis was experimentally examined using electrolytic cells with different membrane electrode assemblies (MEAs). The specially designed MEAs were used in the cells to obtain reliable and reproducible data to clarify the influence of membrane thickness (tmem) and porosity of cathode catalyst layer (CL). The relative humidity of generated hydrogen (ϕH2) during electrolysis was also measured based on dew point measurements of the hydrogen. The ηconc analysis for cells with single- and double-AEM MEAs revealed that water diffusion through the membrane was the main contributor to ηconc. The quantitative agreement between ϕH2 data and ηconc revealed that the difference in ηconc between the two types of MEAs is explained by the water concentration difference between anode and cathode via the Nernst equation. The effect of the porosity of the cathode CL on cell performance and on water transport was also examined experimentally. The results revealed that a high-porosity cathode CL tended to keep the cathode in a drier state during electrolysis compared with a low-porosity cathode CL. When ϕH2 is lower than a threshold value in the range from 0.5 to 0.6, the ion conductivity of AEM and ionomer would decrease, and the cell performance would deteriorate due to an increase in cell resistance (Rcell) and/or activation overpotential (ηact).

(Invited) Identification of Rate Limiting Mass Transport Phenomena in Cathode Catalyst Layer of Pemfcs

Proton exchange membrane fuel cell (PEMFC) is one of promising next-generation power sources for both passenger owned vehicles (POVs) and heavy duty vehicles (HDVs). For its wider distributions, challenging performance targets for POVs and HDVs were set, for example, by US DRIVE Fuel Cell Tech Team, Million Mile Fuel Cell Truck and the New Energy and Industrial Technology Development Organization in Japan [1]. Both targets indicate that the efficiency and power density need to be significantly enhanced to realize better fuel economy and compact implementations in mobility systems. In this study, multi-physics simulations of a single cell [2] were carried out to examine a set of realistic properties of components that can realize the target performances. After discussing the significance of reaction and mass transport resistances in the cathode catalyst layer (CL), we will show detailed breakdowns of relevant overpotentials in the state-of-the-art cathode CLs determined by a combined experimental and theoretical analysis. The theoretical analysis is based on a mathematical single cell model [3] which assumes that the porous CL is composed of homogeneous spherical carbon particles involving primary pores, where oxygen-accessible Pt nanoparticles are deposited. The proton and oxygen mass transports inside and outside the primary particles are analytically and numerically solved using effective mass transport resistances determined to reproduce experimentally measured limiting current densities and impedance spectra. The analysis indicated that the oxygen diffusion in the secondary pores outside the primary particles does not significantly contribute to the overpotential while the oxygen transport is significantly limited in the primary pores and at the ionomer-catalyst interface. Significant reduction of the catalytic activity is also caused at the ionomer-catalyst interface. The small contribution of the secondary pores was supported by a microscopic model of the porous CL, too. In this analysis, the microscopic secondary porous model was reconstructed from 3-dimensional scanning electron microscopy (3D-SEM) images, and the molecular and Knudsen oxygen diffusion in the reconstructed model was simulated. The combined analyses indicate that the ionomer-Pt interfacial structure is a key for the design of the cathode CL. Further detailed analysis using microelectrode experiments and molecular dynamics simulations [4-6] indicate that both the activity and the interfacial permeation can be significantly enhanced by using a highly oxygen permeable ionomer while the advanced interface still partially suffer from the interfacial resistance.

Understanding the Origin of Gas Transport Resistance in Fuel Cell Catalyst Layers

For the widespread use of proton exchange membrane fuel cells (PEMFCs), cost reduction of the stack is indispensable. For this purpose, it is effective to reduce the use of Pt and other cell components by reducing the cell area with a high-power density of the cell while reducing the use of Pt catalyst per geometric area. However, automobile manufacturers have reported that reducing the amount of Pt used per geometric area increases the resistance of gas transport in the catalyst layer, resulting in greater performance loss during high power generation.(1-3) This gas transport resistance is found to be inversely proportional to the electrochemical Pt surface area per geometric area (ECSA), and is considered to stem from near the Pt surface.(4) Previously, our group proposed a mechanism in which a high-density layer of ionomer formed at the ionomer / Pt interface is the cause of the increase in gas transport resistance at a low Pt content (low Pt area) based on the analysis of oxygen transport resistance using a microelectrode and the molecular dynamics calculation.(5-9) In this presentation, we report the results of investigating the origin of gas transport resistance in the cathode catalyst layer (CL) using CLs with a low Pt loading using Vulcan® as the catalyst support. In the case of a catalyst layer using a solid carbon support such as Vulcan®, the resistance can be divided into two categories: resistance independent of pressure and Pt surface area (Knudsen diffusion resistance in secondary pores formed among carbon particles) and resistance independent of pressure and dependent on Pt surface area (resistance near Pt surface). We modified a method of Katayama et al.,(10) which uses CO to control the ECSA of an identical catalyst layer and measures the limiting current of oxygen reduction reaction, and examined a method of separating the Knudsen diffusion resistance and the resistance near Pt using the limiting current of hydrogen oxidation reaction.(11) Furthermore, this technique was applied to catalyst layers with different ionomer-to-carbon weight ratios with a constant ionomer coverage to separate the contribution of gas transport resistance in the ionomer from that at the ionomer / Pt interface. This electrochemical analysis revealed that the gas transport resistance at the interface of ionomer and Pt surface is dominant in the range of practical ionomer-to-carbon weight ratio. The small contribution of the secondary pores was supported by a microscopic model of the porous CL. In this analysis, the microscopic secondary porous model was reconstructed from 3-dimensional scanning electron microscopy (3D-SEM) images, and the molecular and Knudsen oxygen diffusion in the reconstructed model was simulated. The combined analyses indicate that the ionomer-Pt interfacial structure is a key for the design of the cathode CL. Y. Ono, T. Mashio, S. Takaichi, A. Ohma, H. Kanesaka and K. Shinohara, ECS Trans., 28, 69 (2010). N. Nonoyama, S. Okazaki, A. Z. Weber, Y. Ikogi and T. Yoshida, Journal of The Electrochemical Society, 158, B416 (2011). T. A. Greszler, D. Caulk and P. Sinha, Journal of the Electrochemical Society, 159, F831 (2012). A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 7, 1127 (2016). R. Jinnouchi, K. Kudo, K. Kodama, N. Kitano, T. Suzuki, S. Minami, K. Shinozaki, N. Hasegawa and A. Shinohara, Nat Commun, 12, 4956 (2021). R. Jinnouchi, K. Kudo, N. Kitano and Y. Morimoto, Electrochimica Acta, 188, 767 (2016). K. Kudo, R. Jinnouchi and Y. Morimoto, Electrochimica Acta, 209, 682 (2016). T. Suzuki, K. Kudo and Y. Morimoto, Journal of Power Sources, 222, 379 (2013). T. Suzuki, H. Yamada, K. Tsusaka and Y. Morimoto, Journal of the Electrochemical Society, 165, F166 (2018). S. Katayama and S. Sugawara, Journal of Power Sources, 483, 229178 (2021). T. Schuler, A. Chowdhury, A. T. Freiberg, B. Sneed, F. B. Spingler, M. C. Tucker, K. L. More, C. J. Radke and A. Z. Weber, Journal of the Electrochemical Society, 166, F3020 (2019).

Catalyst layer design with inhomogeneous distribution of platinum and ionomer optimal for proton exchange membrane fuel cell cold-start

Performance of proton exchange membrane fuel cells under subfreezing environments is restrained by the properties of electrode materials, for instance, the platinum and ionomer loadings. Herein, we propose an optimization strategy for the cold-start performance based on comprehensive multiphysics simulation, employing a functional inhomogeneous cathode catalyst layer (CL) with appropriate platinum and ionomer loading gradients. Numerical analysis reveals the interaction modes between spatial evolution of ice deposition and local species transport in CLs during cold start. As a result, increased loadings towards the membrane side of the CL and near the cathode outlet improve the cold-start performance by promoting the ice uniformity and uplifting the critical ice fractions at the failure time. A practically relevant, multilayer graded cathode CL has been optimally designed, which proves to significantly expand the feasible operation domains of cold start while benefiting the nominal working cell performance compared with homogeneous CLs.

Design and optimization of gradient wettability pore structure of adaptive PEM fuel cell cathode catalyst layer

The design of cathode catalyst layer (CL) is essential to improve the mass transfer capacity of proton exchange membrane (PEM) fuel cell and increase power density. In this work, cathode CL is divided into three sub-layers, and each sub-layer is added nano-particles with different wettability. The gradient CL with hydrophilic SiO2 particles at inner layer and hydrophobic polytetrafluoroethylene (PTFE) particles at outer layer significantly enhances the performance of membrane exchange assembly (MEA). Its performance is 895 mW/cm2@0.6 V, which is 24.1 %higher than CL without any particles (721 mW/cm2@ 0.6 V). Under operating conditions of high current density, high cathode humidity and high air stoichiometry, the gradient CL has only a little voltage loss. Through Electrochemical Impedance Spectroscopies (EIS) impedance analysis under high current density (1.8A/cm2), mass transfer resistance of gradient CL is 25.4 Ω, and is much smaller than the mass transfer resistance of the homogeneous CL of 35.1 Ω, which reflects the significant enhancement in mass transfer capacity of gradient CL. The gradient catalyst layer is suitable for a wider range of current density, humidity, and stoichiometry, but excessive cathode gas stoichiometry causes a decrease in performance, which is caused by excessive drainage capacity. In addition, 18 different gradient CLs are designed and manufactured, and the gradient CL with catalyst coated membrane (CCM) structure has the best performance. In gradient CL, increasing the capillary pressure difference between sublayers is the key to performance improvement. It is confirmed that the property of MEA with appropriate wettability gradient design can be significantly improved.

Influence of Cathode Catalyst Layer with SiO2-Coated Pt/Ketjen Black Catalysts on Performance for Polymer Electrolyte Fuel Cells

In this study, we investigated the effect of silica (SiO2) layer included in a cathode catalyst layer (CL) on the performance for polymer electrolyte fuel cells (PEFCs). Porous carbons such as Ketjen black (KB) have been widely used as a support for Pt catalysts in PEFCs. Such KB-supported Pt catalyst (Pt/KB) was used as a cathode CL with low ionomer content (a condition of low proton conductivity). The Pt/KB was then coated with SiO2. In addition, the Pt/KB and SiO2-coated Pt/KB (SiO2-Pt/KB) were measured and analyzed under relative humidity (RH) conditions (100% and 20%). The catalyst ink of SiO2-Pt/KB showed higher stability and dispersion compared to Pt/KB, due to the hydrophilic surface characteristics of SiO2, which act as a binder-like ionomer. The performance of the SiO2-Pt/KB at 100% RH, was significantly lower than that of Pt/KB, whereas the performance of the Pt/KB at 20% RH, was significantly improved by SiO2 coating. This is due to an increase in the proton conductivity, which can be attributed to the hydrophilic properties of SiO2. Based on these results, the effect of SiO2 coating on performance, depending on carbon supports of SiO2-coated Pt/Carbon catalysts, could be evaluated.

(Invited) Highly Durable and Active Electrocatalysts Using SnO2 Supports for Polymer Electrolyte Fuel Cells

Toward the widespread use of polymer electrolyte fuel cells (PEFCs) in electric vehicles, highly durable and active catalysts are required. Pt catalysts loaded on high surface area carbon blacks are promising candidate catalysts with high oxygen reduction reaction (ORR) activity [1]. In the future, the improvement of the catalyst durability is also desired for PEFCs for heavy use applications. Alternative candidate catalysts with high activity and high durability, during both startup/shutdown and load cycling, are Pt catalysts supported on non-carbon ceramic supports [2,3]. Our group confirmed that Pt catalysts supported on SnO2 (Pt/M-SnO2, M = Nb, Ta) without carbon additive were superior in ORR activity and durability (startup/shutdown, load cycling) to those of commercial Pt catalysts, e.g., supported on either graphitized carbon black (Pt/GCB, TEC10EA30E, TKK) or carbon black (Pt/CB, TEC10E30E, TKK), by evaluation of membrane-electrode assemblies [4-8]. The non-carbon support of SnO2 nanoparticles has a unique carbon-like microstructure consisting of a fused-aggregate network structure, which enhances the electronically conducting pathways via the aggregated microstructure and gas diffusion pathways via the open pores in the microstructure. In addition, the interface control between Pt and SnO2 support also affects the decrease of both the overpotential for the ORR and the resistance of the cathode catalyst layer (CL). We have also proposed ways to design the three-phase boundary within the CL to diminish the gas diffusion resistance (Fig. 2). The cell performance using the Pt/SnO2 cathode CL at operating temperatures from 80oC to 120oC is quite promising for the development of high power density with simultaneous high durability. Acknowledgments This work was partially supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cell” (SPer-FC) project and the “Advanced Research Program for Energy and Environmental Technologies” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and JSPS KAKENHI Grant Number (B24350093, B17H03410) from the Ministry of Education, Culture, Sports, Science and Technology. References N. Ramaswamy, W. Gu, J.M. Ziegelbauer, S. Kumaraguru, J. Electrochem. Soc., 167, 064515 (2020).A. Masao, S. Noda, F. Takasaki, K. Ito, K. Sasaki, Electrochem. Solid-State Lett., 12, B119 (2009).E. Fabbri, A. Rabis, R. Kötz, T.J. Schmidt, Phys. Chem. Chem. Phys., 16, 13672 (2014).K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, M. Watanabe, Electrochim. Acta, 56, 2881 (2011).Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, M. Watanabe, RSC Adv., 6, 321800 (2014).Y. Chino, K. Taniguchi, Y. Senoo, K. Kakinuma, M. Watanabe, M. Uchida, J. Electrochem. Soc., 162, F736 (2015).K. Kakinuma, R. Kobayashi, A. Iiyama, M. Uchida, J. Electrochem. Soc., 165, J3083 (2018).K. Kakinuma, K. Suda, R. Kobayashi, T. Tano, C. Arata, I. Amemiya, S. Watanabe, M. Matsumoto, H. Imai,Iiyama, M. Uchida, ACS Appl. Mater. Interfaces 11, 34957 (2019). Figure 1

Structural characteristics, diffusion mechanism and mechanical behaviour of cathode catalyst layer

The cathode catalyst layer (CL) is an important part of proton exchange membrane fuel cells (PEMFCs), and its performance directly determines the durability and lifetime of PEMFCs, therefore it is necessary and significant to understand its basic properties. In this study, the structural characteristics, diffusion mechanism, and mechanical behaviour of the CL are studied by molecular dynamics. The interaction between Pt and graphene, Pt and oxygen, and sulfonic groups and hydronium ions increases when the water content rises, however, oxygen has little effect on the interaction between particles. Increasing oxygen content can improve the uniformity of self-distribution. The diffusion of oxygen occurs at the interface of different phases and in the water region, the former takes precedence. The change of diffusion mode from interphase form to solution form will cause the change of diffusion coefficient. Compared with water, oxygen has a little effect on the mechanical properties of CL. During stretching deformation, holes are formed initially in the hydrophilic region, causing the structure to be broken with large deformation, and Pt/graphene particles are always encapsulated by ionomers. In addition, the corresponding properties of anode CL, including hydrogen and water effects, has been discussed and compared with those of cathode CL. This work provides theoretical understanding and prediction of the microstructure and physical properties of CL, the results are helpful for designing and optimizing its structure.

Cathode Catalyst Layer Design with Graded Porous Structure for Proton Exchange Membrane Fuel Cells

Cathode catalyst layer (CL) design is important for improved performance, durability and stability of proton exchange membrane fuel cells (PEMFCs). An effective cathode CL must serve multiple functions simultaneously: electron and H+ conduction, O2 or H2 supply, and effective water management. The reaction in the CL requires three-phase boundaries (or interfaces) among ionomer (for proton transfer), platinum (for catalysis), and carbon (for electron transfer), as well as voids (reactant diffusion); an optimized cathode CL structure should have micropores and mesopores to balance water egress and O2 diffusion. Our previous research showed that increasing microporosity in the cathode CL increases the power production of the PEMFCs [1]. Modeling indicates that high CL microporosity enhances water evaporation, which reduces mass transport resistances to enable higher power production [2]. Meanwhile, low microporosity encourages liquid water retention. It is known that catalyst support type has a profound effect on electrode porosity, influencing both micro- and mesoporosity [3, 4]. By constructing electrodes with layers of different carbon supported catalyst types, we demonstrate that we can create porosity gradients in the CLs that vary the electrode water management properties and the power of the PEMFC. Two different types of commercial platinum/carbon electrocatalysts are used in dual cathode CLs – 40 wt. % Pt on Vulcan carbon (Pt/VC) having low micropores and low porosity and 40 wt. % Pt on Ketjen black (Pt/KB) having large micropores and high porosity. CLs are created with both homogeneous porosity in the Z direction (Figure 1a), and compared to ones with non-uniform porosity across the Z-direction (Figure 1b). The microstructure of these cathode CLs is characterized using N2-sorption porosimetry. The CLs are assembled into PEMFCs and probed with polarization curves (at 100, 50 and 25% RH), cyclic voltammetry, O2 gain, electrochemical impedance spectroscopy, and limiting current measurements to quantify the sources of mass transport resistance. The results indicate that the regions with low microporosity encourage liquid water retention, while high microporosity encourages water evaporation. These countering actions are balanced for high CL hydration and gas diffusion, and better PEMFC performance.

Development of Graphene-Based PEFC Catalyst Layer for Reduction of Oxygen Transport Resistance

Polymer electrode fuel cells (PEFCs) have various positive aspects for improving the worldwide environmental problems. But PEFCs have some issues that restrict commercialization of PEFCs. The biggest issue is its high cost because of using platinum, expensive metal, as catalyst. So, efficient utilization of expensive platinum is necessary to reduce the cost. The resent study reported that graphene has not only high electrical conductivity but also enough proton conductivity (1). It was also reported by the authors that the oxygen transport resistance in the cathode catalyst layer (CL) is dominant at interface of ionomer (2). In this study, we introduced graphene nano platelets (GNPs) as support material for platinum instead of conventional carbon. This idea has possibility that we can reduce the amount of ionomer and the oxygen transport resistance in the CL drastically, resulting in more effective utilization of platinum. In this study, cathode CLs with various ionomer-carbon ratio (I/Cs) were fabricated using self-made Pt-supported GNPs by decal transfer method. The structure and the performance of fabricated CL were evaluated by I-V measurement, SEM observation, and N2 physisorption analysis. These were also compared with fabricated CLs using commercial Pt-supported Ketjen Blacks. At the anode side, the CL with the Pt loading 0.20mg/cm2 and I/C 0.8 was used. Operation temperature was 80℃ and relative humidity (RH) of supply gases was 80%. First, the effects of I/C on the cell performance were investigated. Measured I-V curves for the graphene-based CL with I/C 0.4 was better than that with I/C 0.8. This tendency is different from that for the Ketjen Black based CL, suggesting the possibility that ionomer can be reduced. However, the performance of the graphene-based CLs was lower than the Ketjen Black based CL. This was estimated to be caused by the poor pore structure in the graphene-based CLs. The N2 physisorption analysis showed that the graphene-based CL has few secondary pores around 100nm and the porosity with I/C 0.4 is about half of that of I/C 0.8 Ketjen Black based CL. On the other hand, the CL porosities were also estimated from the used component volumes and the thickness measured by SEM images, and the porosities for the two different CL were similar. These indicate that there are a lot of pores that oxygen cannot permeate enough in the I/C 0.4 graphene-based CL. Then, we tried to evaluate the I/C 0.0 graphene-based CL. Although it is difficult to fabricate CL with I/C 0.0, more effective pores for oxygen paths are expected with enough proton conductivity by graphene itself. Figure 1 shows the results with the I/C 0.0 graphene-based CL and the I/C 0.8 Ketjen Black CL. It can be confirmed that the graphene-based CL with Pt loading only 0.044mg/cm2 achieves high performance similar to Ketjen Black based CL with Pt loading 0.21mg/cm2. This indicates that graphene could play a significant role in large reduction of platinum in PEFC CLs. References (1) S. Hu, et.al, Nature, 516, 227 (2014). (2) T. Hayashi, et.al, ECS Trans., 75(14), 373 (2016). Figure 1

Understanding the interplay between cathode catalyst layer porosity and thickness on transport limitations en route to high-performance PEMFCs

We examine the interplay between cathode catalyst layer (CL) porosity/thickness on mass transport limitations in single cell fuel cells comprised of Pt/C-based CLs fabricated via ultrasonic spray deposition onto polymer electrolyte membranes. We determine that the pore size distribution remains unchanged as CL thickness increases from 3.8 to 11.8 μm, but porosity decreases with increasing CL thickness. The decrease in porosity results in an increase in mass transport resistance for thicker CLs, but does not result in an increase in charge transfer resistance for the oxygen reduction reaction. We found that a fuel cell comprising a 7.5 μm-thick cathode CL delivers the highest performance (1 A cm−2 at 0.60 V at 80 °C in H2|Air at a relative humidity of 50% under ambient pressure). We attribute this high performance to the CL striking an optimal balance between solid and void networks, with the solid networks facilitating transport of H+/e− to the Pt electrocatalyst, and the void network ensuring adequate transport of O2 to, and H2O away from, the Pt electrocatalyst.

Quantitative Analysis Effect of the Cathode Catalyst Layer with Various Ionomer Ratio on PEMFC by Protonic Resistance

Cathode catalyst layer (CL) is responsible for main efficiency losses in polymer electrolyte membrane fuel cells because of both insufficient oxygen reduction kinetics and proton conduction losses. Quantitative analysis was used to study the impact of the ratio of ionomer/carbon on membrane electrode assemblies. The protonic impedance was estimated by electrochemical impedance spectroscopy (EIS) in the absence of oxygen using a physical impedance model. The performance of the prepared electrode was comprehensively evaluated by polarization curves, cyclic voltammetry, and EIS in different gas supply conditions (H2/air, H2/N2). Examining the data of EIS analysis and oxygen reduction reaction testing together, we inferred that the optimized ionomer/carbon ratio is the key role for effective reducing the ionic resistance and boosting mass transfer. The results verify the critical influence of ionomer ratio on the performances of cathode CL and could be used to fabricate an optimized cathode electrode.

Carbon Corrosion on the Wettability Change of Cathode Catalyst Layers of PEM Fuel Cells

It is well-known that water management plays a critical role in operation of PEM fuel cells especially under the dynamic operating conditions and relatively low temperatures, in which both liquid and gaseous water is present. In this study, the wettability of the cathode catalyst layer (CL) on MEAs is characterized by ex-situ sessile-drop method. The method was applied on ten cathode CLs with varying carbon loss (0 to 13 wt% of the cathode) under both potential cycling and holds. The apparent contact angles of the cathode CL decreased from 143.5° to 130° after severe corrosions. Atomic Force Microscopy and X-ray Photoelectron Spectroscopy were used to elucidate structural and compositional changes that account for the decrease in apparent contact angles.

Experimental Analysis of Oxygen Transport Resistance for Different Types of Ionomer in PEFC Catalyst

In polymer electrolyte fuel cells (PEFCs), 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 in PEFCs, achieving the optimum structure of cathode CL and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. Some researches reported that when Aquivion is used as the ionomer of the CL, the output voltage increases as compared with the CL using Nafion in the intermediate current density region (1). However, dominant factors improving the cell performance have not been elucidated quantitatively. In this study, the oxygen transport resistance at the Pt surface, which is thought to have a large contribution to the oxygen transport resistance inside the CL (2), was investigated using CLs fabricated with different ionomers. In this study, catalyst coated membranes was fabricated with various cathode structure: 6 types of cathode side CLs using three kinds of Pt-supported carbons (TEC10E30E, TEC10E50E, TEC10E70TPM) having different Pt carrying densities and two types of ionomer, Nafion and Aquivion. All of these CLs were fabricated with I/C = 0.8. First, to distinguish the oxygen transport resistance in the CL from the other resistance such channels, I-V measurement experiments were conducted with different total pressures (0.10MPa, 0.15MPa, 0.20MPa), and the limiting current density method (2) was applied. In the I-V measurement, the flow rate of cathode gas (oxygen and nitrogen mixture) was 2000 sccm with different oxygen concentration (1%, 2%, 3%) and the flow rate of anode hydrogen was 100 sccm with RH 80% (80°C). Next, the oxygen transport resistance at the Pt surface was evaluated. (The details will be described later). In this evaluation, the electrochemical surface area (ECSA [m2/gPt]) in the CL is needed. The ECSA was estimated by CV measurement. In the CV measurement, cathode was filled with N2 gas with RH 100% (80°C). The flow rate of anode hydrogen was 100 sccm with RH100% (80°C). Figure 1 shows I-V characteristics of the CL with Pt loadings of approximately 0.20 mg/cm2. The output current density for the Aquivion based CL is higher than that of the Nafion based CL at a voltages around 0.5 V, whereas the Nafion based CL has superior performance in the operation at 0.1 V where oxygen supply is limited by various transport resistances. In this paper, we focus on the oxygen transport resistance at low voltage, in particular the resistance at the Pt surface, because the Pt surface resistance is considered to be dominant part of oxygen transport resistance under the low Pt loading CL. Figure 2 shows the ECSA estimated from the CV measurement. The values of ECSA are similar with Nafion and Aquivion based CLs. As the Pt carrying density increases, the ECSA decreases. This is because the Pt particles aggregate on the carbon and the surface area becomes smaller. Using the simplified evaluation formula of oxygen transport resistance (2), the total oxygen transport resistance in the CL can be expressed as RCL = RPt + Rdiss + Rpore . The RPt means the resistance at the Pt surface, Rdiss means the resistance of oxygen dissolves into ionomer, and Rpore means the oxygen diffusion resistance in the CL pores. The RPt is formulated as RPt = 1 / (APt × γ × kPt ) (2). Here, APt is the reactive surface area of catalyst Pt per unit area of the CL, γ is ratio of oxygen concentration in the ionomer and CL pore, and kPt [m/s] is the oxygen transfer rate constant at the Pt surface. Figure 3 shows the result evaluating RPt with both ionomer based CLs. The ordinate represents RCL and the abscissa represents 1/APt . Using this result, kPt can be calculated as kPt = 1.5×10 -2 s/m with the Nafion based CL and kPt = 9.1×10 -3 s/m with the Aquivion based CL. As the oxygen transport resistance at the Pt surface is inversely proportional to the kPt , the resistance of Aquivion based CL is estimated to be larger. This indicates a possibility that Aquivion based CL is inferior due to its larger Pt surface resistance. The larger Pt surface resistance may be due to the characteristics of Aquivion: the shorter side chain and the hydrophilic property induce strongly condensed structure of Aquivion at the surface of Pt, and that results in inhibiting the transport of oxygen. Reference (1)C. Park, et al., J. Power Sources, 315,179 (2016). (2)Tabe, et al., ECS Trans., 80(8), 205 (2017). Figure 1

Numerical analysis of ice-induced stresses in the membrane electrode assembly of a PEM fuel cell under sub-freezing operating conditions

In this work, we report on numerical investigation on the behavior of a polymer electrolyte membrane fuel cell (PEMFC) during a non-optimized start-up procedure from (−20 °C) with a two-dimensional model that couple transport, electrochemical reaction, ice formation and accumulation and mechanical stress induced by ice. The results indicate that the regions with maximum values of ice saturation and von Mises stress are all located in the cathode catalyst layer (CL) and the micro pore layer (MPL) under the land; assuming a 5% ice expansion rate, predicts ice saturation of 0.95 in the cathode CL 240 s, and von Mises stress of 2.8 MPa in the membrane which is below the critical value for damage. However, in the CL the von Mises stress exceeds 15 MPa. Tensile stresses in particular are found to attain sufficiently high values to cause after only 150 s to potentially cause delamination and/or dislocation of particles resulting in higher bulk and interfacial resistance. The simulations indicate that ice-induced degradation may be prevented with measures that reduce the cold start time to under 75 s.