PEMFC catalyst layers: the role of micropores and mesopores on water sorption and fuel cell activity.

The effects of carbon microstructure and ionomer loading on water vapor sorption and retention in catalyst layers (CLs) of PEM fuel cells are investigated using dynamic vapor sorption. Catalyst layers based on Ketjen Black and Vulcan XC-72 carbon blacks, which possess distinctly different surface areas, pore volumes, and microporosities, are studied. It is found that pores <20 nm diameter facilitate water uptake by capillary condensation in the intermediate range of relative humidities. A broad pore size distribution (PSD) is found to enhance water retention in Ketjen Black-based CLs whereas the narrower mesoporous PSD of Vulcan CLs is shown to have an enhanced water repelling action. Water vapor sorption and retention properties of CLs are correlated to electrochemical properties and fuel cell performance. Water sorption enhances electrochemical properties such as the electrochemically active surface area (ESA), double layer capacitance and proton conductivity, particularly when the ionomer content is very low. The hydrophilic properties of a CL on the anode and the cathode are adjusted by choosing the PSD of carbon and the ionomer content. It is shown that a reduction of ionomer content on either cathode or anode of an MEA does not necessarily have a significant detrimental effect on the MEA performance compared to the standard 30 wt % ionomer MEA. Under operation in air and high relative humidity, a cathode with a narrow pore size distribution and low ionomer content is shown to be beneficial due to its low water retention properties. In dry operating conditions, adequate ionomer content on the cathode is crucial, whereas it can be reduced on the anode without a significant impact on fuel cell performance.

On the micro-, meso-, and macroporous structures of polymer electrolyte membrane fuel cell catalyst layers.

In this work, N(2) adsorption was employed to investigate the effects of carbon support, platinum, and ionomer loading on the microstructure of polymer electrolyte membrane fuel cell catalyst layers (CLs). Brunauer-Emmett-Teller and t-plot analyses of adsorption isotherms and pore-size distributions were used to study the microstructure of carbon supports, platinum/carbon catalyst powders, and three-component platinum/carbon/ionomer CLs. Two types of carbon supports were chosen for the investigation: Ketjen Black and Vulcan XC-72. CLs with a range of Nafion ionomer loadings were studied in order to evaluate the effect of an ionomer on the CL microstructure. Regions of adsorption were differentiated into micropores associated with the carbon primary particles (<2 nm), mesopores ascribed to the void space inside agglomerates (2-20 nm), and meso- to macroporous space inside aggregates of agglomerates (>50 nm). Ketjen Black was found to possess a significant fraction of micropores, 25% of the total pore volume, in contrast to Vulcan XC-72, for which the corresponding fraction of micropores was 15% of the total pore volume. The microstructure of the carbon support was found to be a significant factor in the formation of the microstructure in the three-component CLs, serving as a rigid porous framework for distribution of platinum and the ionomer. It was found that platinum particle deposition on Ketjen Black occurs in, or at the mouth of, the support's micropores, thus affecting its effective microporosity, whereas platinum deposition on Vulcan XC-72 did not significantly affect the support's microstructure. The codeposition of ionomer in the CL strongly influenced its porosity, covering pores < 20 nm, which are ascribed to the pores within the primary carbon particles (pore sizes < 2 nm) and to the pores within agglomerates of the particles (pore sizes of 2-20 nm).

The effect of relative humidity on binary gas diffusion.

The dependence of diffusion coefficient of O2-N2 mixture in the presence of water vapor was experimentally determined as a function of relative humidity (RH) with different temperatures using an in-house made Loschmidt diffusion cell. The experimental results showed that O2-N2 diffusion coefficient increased more than 17% when RH increased from 0% to 80% at 79 degrees C. In the experiments, the RH in both top and bottom chambers of the diffusion cell were the same, and the pressure inside the diffusion cell was kept as ambient pressure (1 atm.). Maxwell-Stefan theory was employed to analyze the mass transport in the diffusion cell, and found that there was no effective water vapor diffusion taking place, indicating that the gas diffusion in this ternary (O2-N2-water vapor) system could be considered binary gas (O2-N2) diffusion. The Fuller, Schettler, and Giddings (FSG) empirical equation of the kinetic theory of gases was generalized to accommodate the dependence of the binary diffusion coefficient on the RH. The prediction of the generalized equation was found to be consistent with experimental results with the difference of less than 1.5%, showing that the generalized equation could be applied to calculate the diffusion coefficients of the binary gaseous mixture with different temperature and RH values. The effect of water vapor on the increase of O2-N2 diffusion coefficient was discussed using molecule theory.