Cathode Cathode Catalyst Layer Research Articles

Large-scale cold-start simulations for automotive fuel cells

In this study, the three-dimensional transient cold-start model is applied to a real-scale polymer electrolyte fuel cell (PEFC) geometry, and transient cold-start simulations are carried out from subzero to normal temperatures. In order to reduce the computational turnaround time involved for a large numerical mesh with millions of grid points, the cold-start code is parallelized for parallel computing. The simulation results clearly show the evolution of ice, water content, temperature, and current density contours at different cold-start stages, characterizing freezing, melting, hydration, and dehydration processes. In addition, the model predictions emphasize the beneficial influence of vapor-phase diffusion from the cathode catalyst layer (CL) to the gas-diffusion layer (GDL) during cold starts, which can contribute to reducing ice accumulation in the cathode CL. As the effect of vapor-phase diffusion is substantial, more ice is accumulated in the cathode GDL than in the cathode CL. The total amount of ice accumulated inside a cell is therefore not always proportional to the amount of ice in the cathode CL, depending on the vapor-phase diffusion strength.

Use of mechanistic carbon corrosion model to predict performance loss in Polymer Electrolyte Membrane fuel cells

A membrane electrode assembly is subjected to carbon corrosion accelerated stress tests (ASTs) involving the application of square-wave voltage cycles with different upper limits to measure the resulting loss in the mass of the carbon support at the cathode catalyst layer (CL) and the associated degradation in performance. Carbon loss is determined from the amount of CO2 evolved during degradation, while the performance loss is measured from the reduction in the cell voltage at 1.5 A cm−2 relative to the performance obtained in its beginning-of-life state. A kinetic model for carbon corrosion describing the rate of evolution of CO2 is shown to accurately predict the carbon loss measured during the various carbon corrosion ASTs in this study. A plot of the performance losses from the various carbon corrosion ASTs versus the corresponding carbon losses reveals that all the data demonstrate consistent degradation regardless of the AST conditions, and these result can be fitted with an exponential function termed the ‘fingerprint’ expression. By combining the carbon corrosion model and fingerprint expression, both the carbon loss and performance loss of a cathode CL during a carbon corrosion AST can be accurately predicted based only on knowledge of the waveform of the input voltage.

Ice Formation and Mechanical Stress in the PEFC during Cold Start

Self-startup of polymer electrolyte fuel cell (PEFC) without external heating is advantageous for the system. It is generally recognized that the produced ice accumulating in the cathode catalyst layer (CL) reduce the electrochemically active area (ECA) for oxygen reduction reaction (ORR) and the open pore volume for oxygen diffusion. Visualization results obtained from a transparent PEFC shows the importance of the catalyst layer design and gas purge to freeze-starts.1,2 A cyclic voltammetry (CV) technique was developed to investigate the effect of ice formation in the cathode CL on electrochemically active Pt area during and post subzero startup of a PEFC.3Mechanical stress built in the cell during cold start may limit its lifetime. To our knowledge, no work has been done about mechanical stress during cold start in PEFC.A cyclic voltammetry (CV) technique is developed to further investigate the effect of membrane thickness and startup current density on cumulative product water, rate of ice formation in electrode and rate of water absorption by ionomer and membrane in PEFC during cold start. Figure 1a shows typical CVs of the cathode prior to and post cold start. Obviously, the electrochemically active Pt surface area of the cathode CL post cold start is lower than that prior to. It is found that thick membrane offers much more capacity for water to be absorbed by the membrane at −10oC and −20oC. The product water almost does not increase with membrane thickness for starting from −30oC because water absorption by the membrane is negligible. The rate of ice formation in cathode catalyst layer increases with current density but decreases with the startup temperature. The rate of water absorption by the membrane increases with the startup temperature, current density and membrane thickness. A mathematical model is developed to investigate mechanical stress during cold start in PEFC. Ice produced in the catalyst layer forces the catalyst layer to expand. Figure 1b shows the effect of cold-start temperature on stress during cold start. Cold starting from low temperature brings high compress mechanical stress. Cold starting from low temperature also bring high strain in CL, but low strain in the membrane. The membrane is compressed during cold start from -30oC. That is because the amount of water absorbed by membrane is negligible but CL expands a lot. This work demonstrates the effect of thickness of gas diffusion layer (GDL), micro-porous layer (MPL) and CL on the mechanical stress built in the membrane. Membrane constraint is considered in the model.4The simulation result shows that mechanical stressed during purge, cool down and cost start operation do not vary serious due to elastic and porous MPL and GDL. The model is to help elucidate the fundamental physics of ice formation in the PEFC during cold start.Figure 1. (a) Cyclic voltammograms of the cathode prior to and post cold start at startup temperature of −10oC and −30oC. (CCM-25 mm. m Pt=0.4 mg/cm2. Startup conditions: i start=0.05 A/cm2, x H2=14.35, x air=12.06, RHH2=RHair=0%, p H2=p air=1 atm. CV conditions: T CV=T start, n=50 mV/s, F N2=F H2=40 mL/min, RHN2=RHH2=0%.) and (b) Effect of cold-start temperature on stress during cold start (lInitial= 6.0).

Performance prediction of PEM fuel cell cathode catalyst layer using agglomerate model

Abstract Computation of Proton Exchange Membrane (PEM) fuel cell's cathode Catalyst Layer (CL) is performed using agglomerate models in this paper, and the results are compared with homogenous one. Following our earlier homogenous model for cathode CL (see Khajeh-Hosseini et al., 2010), the focus of the present study is on agglomerate model. In this study, the derivation of agglomerate model is performed in such a way that in the simplified case when agglomerate sizes shrink to zero, the homogeneous model condition is retrieved. Validations versus two sets of experimental data are performed. For example, in one of the validation cases, Case (II), it is observed that in I tot = 3000 [A m −2 ] the homogeneous model overestimates the performance by 80%. But the agglomerate model agrees well with the validating test cases. A set of parametric studies are performed using the agglomerate model, in which the influences of some CL structural- and cell operating-parameters are studied. A sensitivity study on the cell performance is performed to rank the influence of the parameters, with rank 1 for the most influential parameter. It is observed the agglomerate sizes possess rank 1. These results give useful guidelines for manufactures of PEMFC catalyst layers.

A study of the agglomerate catalyst layer for the cathode side of a proton exchange membrane fuel cell: Modeling and optimization

A comprehensive mathematical model for the cathode catalyst layer (CL) of a proton exchange membrane fuel cell (PEMFC) is developed to investigate its performance, which is determined by activation overpotential. Numerous cathode CL parameters such as the saturation and eight structural parameters, namely, ionomer film thickness covering each agglomerate, agglomerate radius, platinum loading, carbon loading, ionomer volume fraction, gas diffusion layer penetration content and its porosity, and CL thickness are taken into account. The main effects of each parameter and their interactions are investigated using the analysis of means (ANOM) and interaction plots, respectively. Analysis of variance (ANOVA) is used to identify the influence of the CL structural parameters on the activation overpotential. Finally, an evolutionary optimization algorithm is employed to maximize the cathode CL's performance.

Cold start characteristics of proton exchange membrane fuel cells

In this study, the effects of the start-up temperature, load condition and flow arrangement on the cold start characteristics and performance of a proton exchange membrane fuel cell (PEMFC) are investigated through in-situ experiments with the simultaneous measurements of the current and temperature distributions. Rather than the commonly recognized cold start failure mode due to the ice blockage in cathode catalyst layer (CL), another failure mode due to the ice blockage in flow channel and gas diffusion layer (GDL) leading to significantly high pressure drop through cathode flow field is observed at a start-up temperature just below the lowest successful start-up temperature. Three ice formation mechanisms are proposed, corresponding to the ice formations in cathode CL, GDL and flow channel. The general distributions of current densities and temperatures during the constant current cold start processes are similar to the constant voltage cold start processes, except that the temperatures at the end of the constant current cold start processes are more evenly distributed over the active reaction area because of the increased heat generation rates. The cold start characteristics are mainly dominated by the cathode flow, and changing the flow arrangement has unimportant impact on the cold start performance.

Prediction and analysis of the cathode catalyst layer performance of proton exchange membrane fuel cells using artificial neural network and statistical methods

A mathematical model was developed to investigate the cathode catalyst layer (CL) performance of a proton exchange membrane fuel cell (PEMFC). A numerous parameters influencing the cathode CL performance are implemented into the CL agglomerate model, namely, saturation and eight structural parameters, i.e., ionomer film thickness covering the agglomerate, agglomerate radius, platinum and carbon loading, membrane content, gas diffusion layer penetration content and CL thickness. For the first time, an artificial neural network (ANN) approach along with statistical methods were employed for modeling, prediction, and analysis of the CL performance, which is denoted by activation overpotential. The ANN was constructed to build the relationship between the named parameters and activation overpotential. Statistical analysis, namely, analysis of means (ANOM) and analysis of variance (ANOVA) were done on the data obtained by the trained neural network and resulted in the sensitivity factors of structural parameters and their mutual combinations as well as the best performance.

A parametric study of cathode catalyst layer structural parameters on the performance of a PEM fuel cell

This paper is a computational study of the cathode catalyst layer (CL) of a proton exchange membrane fuel cell (PEMFC) and how changes in its structural parameters affect performance. The underlying mathematical model assumes homogeneous and steady-state conditions, and consists of equations that include the effects of oxygen diffusion, electrochemical reaction rates, and transport of protons and electrons through the Nafion ionomer (PEM) and solid phases. Simulations are concerned with the problem of minimizing activation overpotential for a given current density. The CL consists of four phases: ionomer, solid substrate, catalyst particles and void spaces. The void spaces are assumed to be fully flooded by liquid water so that oxygen within the CL can diffuse to reaction sites via two routes: within the flooded void spaces and dissolved within the ionomer phase. The net diffusive flux of oxygen through the cathode CL is obtained by incorporating these two diffusive fluxes via a parallel resistance type model. The effect of six structural parameters on the CL performance is considered: platinum and carbon mass loadings, ionomer volume fraction, the extent to which the gas diffusion layer (GDL) extends into the CL, the GDL porosity and CL thickness. Numerical simulations demonstrate that the cathode CL performance is most strongly affected by the ionomer volume fraction, CL thickness and carbon mass loading. These results give useful guidelines for manufactures of PEMFC catalyst layers.

Ionomer Degradation in Polymer Electrolyte Membrane Fuel Cells

The structural degradation of the polymer electrolyte in both the bulk membrane and the cathode catalyst layer (CL) was investigated. An accelerated stress test (AST) was developed to degrade the ionomer in two membrane electrode assembly (MEA) designs, with cathode catalyst structures comprised of 23 and 33 wt % Nafion, respectively. During the AST the air cathode potential was held at (where RHE is reference hydrogen electrode) at and 100% relative humidity for up to 440 h. The MEA with 33 wt % Nafion had a greater platinum content in the membrane and a higher fluoride washout rate, suggesting the higher ionomer content in the cathode CL facilitated the mass transfer of contaminants (such as dissolved platinum) into the membrane. It is proposed that was produced at the anode, diffused into the membrane, and decomposed at the platinum and iron sites bound in the membrane structure. The decomposition products attacked the ionomer both in the bulk phase and CL, causing (i) membrane thinning, which exacerbated the crossover, (ii) lower membrane conductivity, and (iii) CL structure degradation manifested by enhanced reaction penetration depth into the CL and decreased effective oxygen diffusivity due to the changes in CL water content. These effects acting in synergy had profound negative repercussions on the fuel cell polarization for the MEA with 33 wt % Nafion in the cathode CL.

Effect of hydrophilic SiO 2 additive in cathode catalyst layers on proton exchange membrane fuel cells

Hydrophilic nanosized SiO 2 and sulfonated SiO 2 particles were added to the cathode catalyst layer (CL) to improve the water wettability and the performance of a proton exchange membrane fuel cell (PEMFC) at low humidity. It was found that both nanosized SiO 2 and sulfonated SiO 2 additive improved the hydrophilicity of the cathode CL by the contact angle measurement. Contrary to nanosized SiO 2, sulfonated SiO 2 improved the conductivity of the cathode CL. Increased wettability of the cathode CL from SiO 2 maintained fuel cell at hydration conditions. This phenomenon had a profound influence on electrode performance at low humidity. Since the sulfonic groups in sulfonated SiO 2 improved the proton conductivity of the cathode CL, the cell with sulfonated SiO 2 showed better performance.

Carbon Surface Oxidation by Short-Term Ozone Treatment for Modeling Long-Term Degradation of Fuel Cell Cathodes

In regard to degradation phenomena in a polymer electrolyte fuel cell (PEFC), there is little information on voltage loss due to the excess accumulation of liquid water blocking pores for the gas supply in the cathode (flooding) observed in a long-term operation over . In this study, modeling of this degradation phenomenon was attempted using ozone treatment of the cathode catalyst layer (CL). The hydrophilicity of the layer was increased by the treatment, which was attributed to oxidation of the carbon surface of the catalyst (Pt-loaded carbon black, ) exposed to the gas phase in the CL. The flooding was observed at the PEFC with the cathode CL formed using and treated with ozone for , and the extent of the flooding was enlarged by the ozone treatment of . The flooding was caused by the ozone treatment when the Pt content was 20 or . These results indicated that modeling of the long-term degradation was achieved by the short-term ozone treatment, and that the excessive hydrophilicity of the carbon surface of was one of the highly potent candidates for causes of the degradation.

Durability of Membrane Electrode Assemblies under Polymer Electrolyte Fuel Cell Cold-Start Cycling

Electrochemical and microstructural measurements of membrane electrode assemblies (MEAs) cycled under cold-start conditions are reported. An experimental protocol using a single-cell fixture was developed for MEA durability tests under cold-start cycling. Electrochemical diagnostics using high-frequency resistance and pure found that MEA no. 1 cycled under from does not show any degradation after 100 cycles, MEA no. 2 cycled under from exhibits mild degradation after 150 cycles, and MEA no. 3 cycled under from suffers severe degradation after 110 cycles. Transmission electron microscopy and X-ray diffraction using cross-sectional samples of the aged MEAs further revealed three primary degradation mechanisms: (i) interfacial delamination between the cathode catalyst layer (CL) and membrane, (ii) cathode CL pore collapse and densification upon melting of a fully ice-filled CL, and (iii) Pt particle coarsening and Pt dissolution in perfluorosulfonic acid ionomer. The interfacial delamination and CL densification appear to be closely related to each other, and the key parameter to affect both is the ice volume fraction in the cathode CL after each cold-start step. Eliminating or minimizing these two degradation processes could improve the MEA cold-start durability by 280%. Mitigation strategies, such as improved gas purge prior to cold start, better MEA design, low startup current density, and low cell thermal mass, are proposed.

Cyclic Voltammetry Study of Ice Formation in the PEFC Catalyst Layer during Cold Start

A cyclic voltammetry technique has been developed to investigate the effect of ice formation in the cathode catalyst layer (CL) on electrochemically active Pt area during and post-subzero startup of a polymer electrolyte fuel cell (PEFC). It was found that the Pt area decreases after each cold start and the Pt area loss increases with the product water generated during cold start. We hypothesize that the Pt area loss is caused by ice precipitated between Pt particles and ionomers during cold start. After startup from a subzero temperature and warmup to with all ice in the CL melted, the cell remains subject to Pt area loss, but this loss at is substantially reduced. We also find that subsequent cell operation at and for is very effective for recovering the active Pt area and cell performance. Both permanent loss in the active Pt area and cell performance degradation due to structural alteration of the cathode CL by the presence of ice increase gradually with the cold-start cycle number and become more severe for cold start from than from and . In addition, startup from a subzero temperature appears to have no influence on the electrochemically active Pt area of the anode CL. It is suggested that the ice amount present in the CL holds a key to determine the temporary Pt area loss due to ice formation as well as permanent performance degradation resulting from cold-start cycling.

Optimization of polymer electrolyte fuel cell cathode catalyst layers via direct numerical simulation modeling

The cathode catalyst layer (CL), due to sluggish oxygen reduction reaction and several transport losses therein, plays an important role in the overall performance of polymer electrolyte fuel cells (PEFCs). The relative volume fractions of the constituent phases, i.e. the electronic, electrolyte and void phases, of the cathode CL need to be selected appropriately in order to achieve an optimal balance between oxygen diffusion and proton conduction. In this work, the influence of electrolyte and void phase fractions of the cathode CL on the cell performance is investigated based on a pore-level description of species and charge transport through a random CL microstructure via the direct numerical simulation (DNS) model. Additionally, the effects of inlet relative humidity and net water transport from the anode on the cathode performance have been studied which indicate the interdependence between the CL composition and the cell operating conditions. The results indicate that the low humidity operation benefits the performance by enhancing the oxygen transport especially under high current densities. Finally, the DNS model predicts the volume fractions of 0.4 and 0.26 for the void and electrolyte phases, respectively, as the optimal composition of the catalyst layer for the best performance.

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.