Numerical analysis of Pt utilization in PEMFC catalyst layer using random cluster model

A random cluster model was proposed to simulate the catalyst layer of PEMFC. The cluster model consists of a random distribution of three kinds of particles, i.e., Pt/C catalyst, Nafion and poly-tetra-fluoro-ethylene (PTFE), which were generated on a computer by means of Monte Carlo method. Based on such a cluster model, the catalyst utilization was calculated through counting the number of Pt/C clusters and Nafion particles clusters. It was assumed that the effective Pt/C clusters are those that not only have electron channels via carbon particles to current collector but also have proton channels via Nafion polymer particles to the Nafion membrane. The factors influencing catalyst utilization was thoroughly discussed. For the case of high catalyst utilization, numerical results showed that there is a threshold for ratio of Pt/C catalyst loading to Nafion. Beyond this threshold, the catalyst utilization may drop dramatically. The results also showed that the Pt/C catalyst with higher Pt content could allow a larger range of the ratio of the Pt/C catalyst loading to Nafion. Generally, there is high catalyst utilization around the ratio of 1. Results also showed that the lower the Teflon loading in the catalyst layer, the higher the catalyst utilization will be. However, the Pt/C catalyst with higher Pt content can tolerate relatively high Teflon loading than that with a lower Pt content.     

Process based reconstruction and simulation of a three-dimensional fuel cell catalyst layer

Fundamental understanding of catalyst layer nanostructure of hydrogen polymer electrolyte membrane (PEM) fuel cells is critical for improvement in performance and durability. A process based 3D mathematical model has been developed to elucidate the effect of electrode composition, porosity and ionomer weight fraction in catalyst layers on electrochemical and nano-scale transport phenomena. Numerical reconstruction of catalyst layer random structure has been performed through a controlled random algorithm, mimicking the experimental fabrication process. Nano-scale species transport properties, e.g., Knudsen diffusion of oxygen in nano-pores and proton transport in thin-film electrolyte, have been included in the model, allowing for more rigorous study of the catalyst layer. It was found that there is a threshold in both porosity and ionomer weight fractions, below which species percolation through the random structure becomes difficult due to reduced connectivity and increased isolation. The degree of mixing or size of agglomerates has been studied and it was discovered that increasing or decreasing the agglomerate number from the optimum value reduces the electrochemically active area (ECA) and deteriorates species transport, suggesting an optimum level of stirring of the catalyst ink during catalyst layer preparation is critical.     

Monte Carlo simulation of the PEMFC catalyst layer

The performance of the polymer electrolyte membrane fuel cell (PEMFC) is greatly controlled by the structure of the catalyst layer. Low catalyst utilization is still a significant obstacle to the commercialization of the PEMFC. In order to get a fundamental understanding of the electrode structure and to find the limiting factor in the low catalyst utilization, it is necessary to develop the mechanical model on the effect of catalyst layer structure on the catalyst utilization and the performance of the PEMFC. In this work, the structure of the catalyst layer is studied based on the lattice model with the Monte Carlo simulation. The model can predict the effects of some catalyst layer components, such as Pt/C catalyst, electrolyte and gas pores, on the utilization of the catalyst and the cell performance. The simulation result shows that the aggregation of conduction grains can greatly affect the degree of catalyst utilization. The better the dispersion of the conduction grains, the larger the total effective area of the catalyst is. To achieve higher utilization, catalyst layer components must be distributed by means of engineered design, which can prevent aggregation.     

A two-phase PEMFC model for process control purposes

A reduced nonlinear dynamic two-phase model of a polymer electrolyte membrane fuel cell is presented. The model is derived from a spatially distributed dynamic model and captures the detailed model's multiplicity behaviour. The reduced model is of considerably lower order than the detailed one and requires less computation time. The reduced and the detailed model are compared in steady-state and dynamic simulations. Good agreement is observed. The model can be used to investigate model-based control strategies for PEM fuel cells, especially under operating conditions close to flooding.     

Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes

The performance of a proton exchange membrane fuel cell (PEMFC) with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated with the aim at characterizing the effects of the Nafion content, the catalyst loading in the electrode and also of the membrane thickness and gases pressures. At high current densities the best fuel cell performance was found for the electrode with 0.35 mg Nafion cm⁻² (15 wt.%), while at low current densities the cell performance is better for higher Nafion contents. It is also observed that a decrease of the usual Pt loading in the catalyst layer from 0.4 to ca. 0.1 mg Pt cm⁻² is possible, without introducing serious problems to the fuel cell performance. A decrease of the membrane thickness favors the fuel cell performance at all ranges of current densities. When pure oxygen is supplied to the cathode and for the thinner membranes there is a positive effect of the increase of the O₂ pressure, which raises the fuel cell current densities to very high values (>4.0A cm⁻², for Nafion 112—50 μm). This trend is not apparent for thicker membranes, for which there is a negligible effect of pressure at high current densities. For H₂/air PEMFCs, the positive effect of pressure is seen even for thick membranes.     

Ice formation and distribution in the catalyst layer during freeze-start process - CRYO-SEM investigation

Ice distribution in the catalyst layer and gas distribution layer (GDL) of proton exchange membrane (PEM) fuel cells under isothermal constant voltage (ICV) operation at a subzero temperature was determined using a field emission scanning electron microscope with a cryogenic stage and sample preparation unit (CRYO-FESEM). The analysis method was designed to ensure that the entire experiment, from sample preparation to CRYO-FESEM characterization, are carried out under subzero (°C) conditions so that the water is always kept in a frozen state without thawing.Under a moderately wet shutdown, the porosity of the cathode catalyst layer decreased from an initial dry porosity of 65% to 15.9% for the frozen-only sample and to 8.2% for the sample which was operated at subzero temperatures (ICV). After the completion of the ICV, the catalyst surface was completely covered with ice and the gas was not able to reach the active sites and the reaction ceased. Two distinct regions with different porosities in the catalyst layer were observed at the half ICV state, which indicates that ice in the catalyst layer melted at the beginning of ICV operation.     

Performance enhancement of PEMFC through temperature control in catalyst layer fabrication

Pore size distribution and specific pore volume in the catalyst layer of polymer electrolyte membrane fuel cells were modified by controlling the temperature during the catalyst layer fabrication. Raising the temperature of the gas diffusion layer where the platinum catalyst is coated facilitated evaporation of the solvent in the catalyst ink and induced a large pore volume especially in the secondary pore. Fuel cell electrodes with large amounts of pores exhibit 30% improved single cell performance. The microstructure and electrochemical properties of electrodes were investigated by field emission scanning electron microscopy, mercury intrusion porosimetry, electrochemical impedance spectroscopy, and current-voltage polarization measurement. The results indicate that increased volume of the secondary pore reduces the mass transfer resistance and improves the performance.     

Characteristics of subzero startup and water/ice formation on the catalyst layer in a polymer electrolyte fuel cell

This work experimentally explores the fundamental characteristics of a polymer electrolyte fuel cell (PEFC) during subzero startup, which encompasses gas purge, cool down, startup from a subfreezing temperature, and finally warm up. In addition to the temperature, high-frequency resistance (HFR) and voltage measurements, direct observations of water or ice formation on the catalyst layer (CL) surface have been carried out for the key steps of cold start using carbon paper punched with microholes and a transparent cell fixture. It is found that purge time significantly influences water content of the membrane after purge and subsequently cold-start performance. Gas purge for less than 30 s appears to be insufficient, and that between 90 and 120 s is most useful. After gas purge, however, the cell HFR relaxation occurs for longer than 30 min due to water redistribution in the membrane-electrode assembly (MEA). Cold-start performance following gas purge and cool down strongly depends on the purge time and startup temperature. The cumulative product water measuring the isothermal cold-start performance increases dramatically with the startup temperature. The state of water on the CL surface has been studied during startup from ambient temperatures ranging from −20 to −1 °C. It is found that the freezing-point depression of water in the cathode CL is 1.0 ± 0.5 °C and its effect on PEFC cold start under automotive conditions is negligible.     

Heat flux from the catalyst layer of a fuel cell

We report exact solutions to the problem of heat transport in the catalyst layer (CL) of a fuel cell. The solutions are obtained for the low- and high-current regimes of CL operation. The approximate equation for the heat flux from the CL valid for the whole range of current densities is suggested. This equation is suitable for CFD calculations of heat transport in cells and stacks. Heat fluxes from the catalyst layers of PEMFC, HT-PEMFC and DMFC are discussed.     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part I. Experimental investigation

The cathode catalyst layer (CL) in direct methanol fuel cells (DMFCs) has been optimized through a balance of ionomer and porosity distributions, both playing important roles in affecting proton conduction and oxygen transport through a thick CL of DMFC. The effects of fabrication procedure, ionomer content, and Pt distribution on the microstructure and performance of a cathode CL under low air flowrate are investigated. Electrochemical methods, including electrochemical impedance, cyclic votammetry and polarization curves, are used in conjunction with surface morphology characterization to correlate electrochemical characteristics with CL microstructure. CLs in the form of catalyst-coated membrane (CCM) have higher cell open circuit voltages (OCVs) and higher limiting current density; while catalyzed-diffusion-media (CDM) CLs display better performance in the moderate current density region. The CL with a composite structure, consisting both CCM and CDM, shows better performance in both kinetic and mass-transport limitation region, due to a suitable ionomer distribution across the CL. This composite cathode is further evaluated in a full DMFC and the cathode performance loss due to methanol crossover is discussed.     

In situ grown carbon nanotubes on carbon paper as integrated gas diffusion and catalyst layer for proton exchange membrane fuel cells

In situ grown carbon nanotubes (CNTs) on carbon paper as an integrated gas diffusion layer (GDL) and catalyst layer (CL) were developed for proton exchange membrane fuel cell (PEMFC) applications. The effect of their structure and morphology on cell performance was investigated under real PEMFC conditions. The in situ grown CNT layers on carbon paper showed a tunable structure under different growth processes. Scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) demonstrated that the CNT layers are able to provide extremely high surface area and porosity to serve as both the GDL and the CL simultaneously. This in situ grown CNT support layer can provide enhanced Pt utilization compared with the carbon black and free-standing CNT support layers. An optimum maximum power density of 670 mW cm⁻² was obtained from the CNT layer grown under 20 cm³ min⁻¹ C₂H₄ flow with 0.04 mg cm⁻² Pt sputter-deposited at the cathode. Furthermore, electrochemical impedance spectroscopy (EIS) results confirmed that the in situ grown CNT layer can provide both enhanced charge transfer and mass transport properties for the Pt/CNT-based electrode as an integrated GDL and CL, in comparison with previously reported Pt/CNT-based electrodes with a VXC72R-based GDL and a Pt/CNT-based CL. Therefore, this in situ grown CNT layer shows a great potential for the improvement of electrode structure and configuration for PEMFC applications.     

A thin-film/agglomerate model of a proton-exchange-membrane fuel cell cathode catalyst layer with consideration of solid-polymer-electrolyte distribution

A thin-film/agglomerate model for the cathode part of a proton-exchange-membrane fuel cell is developed. Parameter estimation is employed to determine the exchange current density in the catalyst layer, proton conductivity of the recast ionomer, and oxygen diffusivity in the solid polymer electrolyte. The effects of catalyst and polymer electrolyte loadings in the catalyst layer on the cell performance are demonstrated using this model. The influence of polymer electrolyte distribution in the catalyst layer is correlated with the oxygen diffusion and proton migration rates within the electrolyte. It is found that proton migration in the polymer electrolyte is the dominant factor for cell current density under normal operating conditions. A better cell performance is achieved by a concentrated polymer electrolyte near the catalyst layer/membrane interface.     

Multi-variable optimization of PEMFC cathodes using an agglomerate model

A comprehensive numerical framework for cathode electrode design is presented and applied to predict the catalyst layer and the gas diffusion layer parameters that lead to an optimal electrode performance at different operating conditions. The design and optimization framework couples an agglomerate cathode catalyst layer model to a numerical gradient-based optimization algorithm. The set of optimal parameters is obtained by solving a multi-variable optimization problem. The parameters are the catalyst layer platinum loading, platinum to carbon ratio, amount of electrolyte in the agglomerate and the gas diffusion layer porosity. The results show that the optimal catalyst layer composition and gas diffusion layer porosity depend on operating conditions. At low current densities, performance is mainly improved by increasing platinum loading to values above 1 mg cm⁻², moderate values of electrolyte volume fraction, 0.5, and low porosity, 0.1. At higher current densities, performance is improved by reducing the platinum loading to values below 0.35 mg cm⁻² and increasing both electrolyte volume fraction, 0.55, and porosity 0.32. The underlying improvements due to the optimized compositions are analyzed in terms of the spatial distribution of the various overpotentials, and the effect of the agglomerate structure parameters (radius and electrolyte film) are investigated. The paper closes with a discussion of the optimized composition obtained in this study in the context of available experimental data. The analysis suggests that reducing the solid phase volume fraction inside the catalyst layer might lead to improved electrode performance.     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part II: Computational modeling and design

The cathode catalyst layer in direct methanol fuel cells (DMFCs) features a large thickness and mass transport loss due to higher Pt loading, and therefore must be carefully designed to increase the performance. In this work, the effects of Nafion loading, porosity distribution, and macro-pores on electrochemical characteristics of a DMFC cathode CL have been studied with a macro-homogeneous model, to theoretically interpret the related experimental results. Transport properties in the cathode catalyst layers are correlated to both the composition and microstructure. The optimized ionomer weight fraction (22%) is found to be much smaller than that in H₂ polymer electrolyte fuel cells, as a result of an optimum balance of proton transport and oxygen diffusion. Different porosity distributions in the cathode CLs are investigated and a stepwise distribution is found to give the best performance and oxygen concentration profile. Influence of pore defects in the CLs is discussed and the location of macro-pores is found to play a dual role in affecting both oxygen transport and proton conduction, hence the performance. The reaction zone is extended toward the membrane side and the proton conduction is facilitated when the macro-pores are near the gas diffusion layer.     

The regimes of catalyst layer operation in a fuel cell

A generalized Perry-Newman-Cairns model for performance of a generic catalyst layer (CL) with the Butler-Volmer conversion function is considered. The CL polarization curve, the rate of electrochemical conversion S(x) and the thickness of the conversion domain l_∗ are derived for the cases of ideal transport of ions or feed molecules. In both cases, the CL may work in the low- or high-current regime. In the low-current regime with poor ionic transport, l_∗ is given by the Newman’s current-independent reaction penetration depth. In the high-current regime, l_∗ is inversely proportional to the cell current, regardless of the origin of transport loss. The position and width of the transition region between the low- and high-current branches of the polarization curve are calculated. Based on these results, the features of catalyst layer performance in PEMFC, HT-PEMFC, DMFC and SOFC are discussed.     

Numerical optimization of proton exchange membrane fuel cell cathodes

A fuel cell gradient-based optimization framework based on adaptive mesh refinement and analytical sensitivities is presented. The proposed approach allows for efficient and reliable multivariable optimization of fuel cell designs. A two-dimensional single-phase cathode electrode model that accounts for voltage losses across the electrolyte and solid phases and water and oxygen concentrations is implemented using an adaptive finite element formulation. Using this model, a multivariable optimization problem is formulated in order to maximize the current density at a given electrode voltage with respect to electrode composition parameters, and the optimization problem is solved using a gradient-based optimization algorithm. In order to solve the optimization problem effectively using gradient-based optimization algorithms, the analytical sensitivity equations of the model with respect to the design variables are obtained. This approach reduces the necessary computational time to obtain the gradients and improves significantly their accuracy when compared to gradients obtained using numerical sensitivities. Optimization results show a substantial increase in the fuel cell performance achieved by increasing platinum loading and reaching a Nafion mass fraction around 20-30 wt.% in the catalyst layer.     

Doping-level control of atomic layer deposited YSZ surface layer on platinum catalyst

While local surface composition of fuel cell electrode is critical to performance, precise control of the surface composition at the thickness scale of <10 nm has been challenging. Here we report on the fabrication of ultra-thin (5 nm-thick) yttria-stabilized zirconia (YSZ) surface layers whose compositions are precisely controlled by using atomic layer deposition (ALD), and their applications to cathodic overlayers of low-temperature solid oxide fuel cells. Exchange current density was improved by 80% by optimizing the doping level (16 mol%) of YSZ overlayer. Activation resistance of the cell with 16 mol% doped ALD YSZ overlayer decreased by 27%, and the cell performance improved by 70% compared to the cell without the overlayer. Such improvement was attributed to the ability of the doping-controlled ALD YSZ overlayer to locally improve O^2? incorporation, which is the rate-determining step of oxygen reduction reaction, at the cathode surface.     

Model‐based design and optimization of the microscale mass transfer structure in the anode catalyst layer for direct methanol fuel cell

Microscale mass transfer structure in the anode catalyst layer (CL) can significantly alter the performance of a direct methanol fuel cell (DMFC) because it changes both the oxidation rate and crossover flux of methanol. The microscale mass transfer structure can be modified by changing the loading of the pore former (PF). An empirical model was developed for the microstructural design and optimization of anode CL by incorporating the PF into the anode CL. The optimal loading of PF is 100 g/m² according to the calculated results. Experimental results confirmed the accuracy of the calculations, and the passive DMFC performs 37% better by incorporating the optimal loading of PF into the anode CL as compared to the conventional anode CL. The validity of the proposed empirical model can also be proven by comparing the calculated polarization results with the previously reported experimental data. © 2012 American Institute of Chemical Engineers AIChE J, 59: 780–786, 2013