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The morphology and ionomer distribution in the polymer electrolyte membrane fuel cell (PEMFC) cathode electrode (used in Toyota Mirai) are quantified with nano-scale resolution X-ray computed tomography (nano-CT). Using the nano-CT data, different shapes, sizes and compositions of agglomerates are extracted. Statistical information frommultiple techniques, including transmission electron microscopy (TEM), ultra-small angle X-ray scattering (USAXS), and Brunauer-Emmett-Teller (BET) gas adsorption porosimetry are combined to reconstruct the high surface-area porous carbon (HSC) support, exterior (on the surface of carbon) and interior (inside carbon pores) catalysts, ionomer, and primary pores in the extracted agglomerates. Application of capillary condensation theory to the reconstructed agglomerate structure is shown to accurately represent the experimentally-observed relative humidity (RH) dependence of the electrochemically-active surface area (ECA). Direct numerical simulations (DNS) show that the high local O2 transport resistance (RO2) under dry conditions is mainly associated with the reduced ECA. We demonstrate that the agglomerate shape and size affect RO2 only if the primary pores are poorly accessible (i.e., capillary-condensed water filled primary pores result in lower RO2 for lower aspect ratio agglomerates). We also estimate RO2 associated with the location of catalyst (whether inside or on the surface of HSC). [ABSTRACT FROM AUTHOR]

A numerical framework is developed for simulating pore-scale liquid water transport in polymer electrolyte fuel cell (PEFC) diffusion media (DM) as measured by X-ray computed tomography. Accuracy of the model is evaluated by benchmarking it with micro-scale tomographic observations of liquid water in DM at different inlet pressures. Application of the model to nano-scale morphology of the micro porous layer (MPL) shows that cracks in MPL facilitate water management by keeping nano-pores dry. Simulations in macro-scale morphology of DM indicate that liquid water saturation is approximately two-times higher in DM without an MPL. This is shown to be a consequence of water accumulation in the middle-lower height of the DM in the absence of an MPL which is demonstrated to limit the water intrusion into DM. Simulation of the land-channel configuration shows water accumulation predominantly under the land of DM with an MPL whereas considerable amount of water is also observed under the channel without an MPL. Additionally, we performed direct reactant transport simulations showing that MPL facilitates significantly better O2 transport in wet DM (~ two-times higher formation factor in land-channel configuration) but it induces slightly less facile O2 transport in dry DM (12% lower formation factor). [ABSTRACT FROM AUTHOR]

We present a direct numerical simulation model for probing reactant transport at nano scale in agglomerates extracted from nano X-ray tomography of a cathode electrode. For use in macroscale models, agglomerate effectiveness factors for dry and flooded states of primary pores are correlated in terms of a rate constant for oxygen reduction reaction (ORR) and agglomerate size. We introduce a robust technique, named path of least resistance, for quantification of O2 transport resistance for catalyst surfaces distributed in agglomerates. Our analysis indicates that the O2 transport resistance in the primary pores is negligible in comparison to the resistance in the ionomer film covering the catalyst if the pores are dry, but in large agglomerates, it greatly exceeds the ionomer resistance if the pores are flooded with water. With fast reaction kinetics, flooding of the primary pores completely blocks reactant transport to the catalyst in the inner sections of the agglomerates, and the majority of the current is generated by the catalyst near the agglomerate surface. Conversely, catalyst in the inner sections remain accessible to O2 even at high ORR rates if the primary pores are dry, and the current production follows the electrochemically active surface area distribution in the agglomerates. [ABSTRACT FROM AUTHOR]

This two-part study characterizes the structural and transport properties of polymer electrolyte fuel cell (PEFC) cathode catalyst electrode. The agglomerates comprising the electrode are characterized with nano scale resolution X-ray computed tomography (nano-CT). A hybrid reconstruction technique is used to further refine the 3-D agglomerate microstructure and obtain 1-nm voxel resolution of the constituent catalyst, ionomer, carbon support and primary and secondary pore phases. Our analysis of nano-CT data shows that the electrode composition is heterogeneous and the agglomerates have structural heterogeneities in terms of size and composition. There is a strong correlation between the ionomer to carbon weight ratio (I/C) and agglomerate microstructure, suggesting that ionomer plays role in formation of larger agglomerates. With the hybrid reconstructions, we show that ionomer film thickness is spatially non-uniform inside the agglomerate and is strongly dependent on the I/C. Smaller agglomerate size and high I/C are shown to favor the ionomer impregnation. We demonstrate that the majority of the primary pores are smaller than 15 nm for I/C = 0.8 and higher I/C reduces the primary pore size. We also characterize the uniformity of the catalyst distribution and show that the majority of the catalyst is located away from the agglomerate surface. [ABSTRACT FROM AUTHOR]

An electrochemical flow cell system with catalyst-ionomer ink deposited on glassy carbon is used to investigate the aqueous stability of commercial PtCo alloys under cyclic potentials. An on-line inductively coupled plasma-mass spectrometer, capable of real-time measurements, is used to resolve the anodic and cathodic dissolution of Pt and Co during square-wave and triangle-wave potential cycles. We observe Co dissolution at all potentials, distinct peaks in anodic and cathodic Pt dissolution rates above 0.9 V, and potential-dependent Pt and Co dissolution rates. The amount of Pt that dissolves cathodically is smaller than the amount that dissolves anodically if the upper potential limit (UPL) is lower than 0.9 V. At the highest UPL investigated, 1.0 V, the cathodic dissolution greatly exceeds the anodic dissolution. A non-ideal solid solution model indicates that the anodic dissolution can be associated with the electrochemical oxidation of Pt and PtOH to Pt2+, and the cathodic dissolution to electrochemical reduction of a higher Pt oxide, PtOx (x > 1), to Pt2+. Pt also dissolves oxidatively during the cathodic scans but in smaller amounts than due to the reductive dissolution of PtOx. The relative amounts Pt dissolving oxidatively as Pt and PtOH depend on the potential cycle and UPL. [ABSTRACT FROM AUTHOR]

In this study, we present microstructural characterization for polymer electrolyte fuel cell (PEFC) cathodes with low platinum group metal (PGM) loadings together with polarization data analysis. Three-dimensional pore morphology and ionomer distribution are resolved using nano-scale X-ray computed tomography (nano-CT). Electrode structural properties are reported along with the effective ion and reactant transport properties. The characterization results are incorporated with two-dimensional multi-physics model that accounts for energy, charge, and mass transport along with the effect of liquid water flooding. Defining total mass transport resistance for the whole polarization curve, contributions of transport mechanisms are identified. Analysis of the experimental polarization curves at different operating pressures and temperatures indicates that the mass transport resistance in the cathode is dominated by the transport processes in the electrode. It is shown that flooding in the electrode is a major contributor to transport losses especially at elevated operating pressures while the pressure-independent resistance at the catalyst surface due to transport through the ionomer film plays a significant role, especially at low temperatures and low catalyst loading. In addition, by performing a parametric study for varying catalyst loadings, the importance of electrode roughness (i.e, electrochemically-active surface area/geometric electrode area) in determining the mass transport losses is highlighted. [ABSTRACT FROM AUTHOR]

This work demonstrates that functionalizing annealed-Pt/Ketjen black EC300j (a-Pt/KB) and dealloyed-PtNi/Ketjen black EC300j (d-PtNi/KB) catalysts using p-phenyl sulfonic acid can effectively enhance performance in the membrane electrode assemblies (MEAs) of proton exchange membrane fuel cells (PEMFCs). The functionalization increased the size of both Pt and PtNi catalyst particles and resulted in the further leaching of Ni from the PtNi catalyst while promoting the formation of nanoporous PtNi nanoparticles. The size of the SO3H-Pt/KB and SO3H-PtNi/KB carbon-based aggregates decreased dramatically, leading to the formation of catalyst layers with narrower pore size distributions. MEA tests highlighted the benefits of the surface functionalization, in which the cells with SO3H-Pt/KB and SO3H-PtNi/KB cathode catalysts showed superior high current density performance under reduced RH conditions, in comparison with cells containing annealed Pt/KB (a-Pt/KB) and de-alloyed PtNi/KB (d-PtNi/KB) catalysts. The performance improvement was particularly evident when using reactant gases with low relative humidity, indicating that the hydrophilic functional groups on the carbon improved the water retention in the cathode catalyst layer. These results show a new avenue for enhancing catalyst performance for the next generation of catalytic materials for PEMFCs. [ABSTRACT FROM AUTHOR]

A model for coalescence/sintering of Pt nanoparticles is developed to analyze particle growth and electrochemical surface area (ECSA) loss measured in aqueous tests and in catalyst-coated membrane and gas diffusion electrode-containing single fuel cells. The model combines a non-ideal solid solution theory for Pt dissolution with the dynamics of particle size evolution considering particle growth by Ostwald ripening and coalescence/sintering. Results from the model indicate that the observed growth in particle size and loss in ECSA in accelerated tests are primarily due to coalescence/sintering resulting from Pt dissolution and redeposition between particles. An enthalpy of dissolution of 49.3 kJ.mol-1 and an effective heat of fusion of 28.2 kJ.mol-1 for particle coalescence/sintering have been empirically determined from the measured temperature dependence of particle growth. The model suggests that higher Pt solubility in the presence of oxygen is responsible for enhanced particle growth and ECSA loss in accelerated tests in H2/air as compared to H2/N2. The model also indicates that the dissolution rate constant must be reduced by two orders of magnitude to explain the measured decrease in ECSA loss of the Pt cathode catalyst after 10,000 square potential cycles at 80°C when the relative humidity is decreased from 100% to 30%. [ABSTRACT FROM AUTHOR]

The stability of dispersed high surface area carbon-supported platinum nano-particle electmcatalysts (Pt/C) was investigated as a function of particle size (mean diameters of 1.9, 3.2, 7.1, and 12.7 nm) and oxide coverage under potentiostatic and potentiodynamic conditions in aqueous perchloric acid electrolyte. A non-ideal solid solution theory was formulated to explain the observed dependence of the equilibrium dissolved Pt concentration on potential, Pt particle size, and oxide coverage, as inferred from cyclic voltammetry measurements. The activities of Pt and PtOx in Pt-PtOx solid solutions were correlated with the oxide coverage and Pt particle size. The theoretical framework was also used to determine the rate constants for Pt dissolution and PtOx formation and reduction. The results from the kinetic model were found to be consistent with the measured Pt dissolution for triangle potential cycles with different upper and lower potential limits and scan rates. [ABSTRACT FROM AUTHOR]