The electrochemical interface of the cathode in high temperature PEM fuel cells

Abstract

One of the most challenging areas towards the optimization of fuel cell technology is the development of catalytic layers with active, extended and stable electrochemical interfaces giving the ability to operate under a variety of operational conditions. In this work, the effect of parameters such as the amount of H3PO4 and Pt loading was examined regarding the formation of the electrochemical interface of an operating cathodic electrode in a high temperature PEM Fuel Cell. Electrodes beased on commercial 30 wt% Pt/C and H3PO4 imbibed TPS polymer electrolyte membrane were employed. Based on the thickness and catalyst loading of the catalytic layer, there is an optimum amount of acid in relation to the partial pressure of steam (pH2O) produced at the cathode that leads to uniform wetting of the catalyst avoiding catalyst poisoning and/or blocking due to flooding of the electrode. Interestingly high pH2O causes a decrease in H2 evolution reaction rate and increase in H2 adsorption coverage, implying stronger Had bonding with the Pt surface. Moreover, this work describes the development of a reliable procedure for quantitative evaluation of the real electrochemical active surface area (ECSA) of high temperature Pt electrodes by the combined analysis of CO stripping experiments and the simultaneous CO2 recording by means of online mass spectrometry. Regardless of the H3PO4 or Pt loading in the cathodic electrode, increase of steam's partial pressure progressively causes the degradation the Pt/C|H3PO4 electrochemical interface. At pH2O > 10 kPa, the ECSA decreased in all cases, but more intensely for high amounts of Η3PΟ4 within the electrode structure. The charge of the peak in the cyclic voltammogram did not completely correspond to the CO electrooxidation reaction. This difference is attributed to side oxidation reactions taking place within the same potential window as with the COad electrooxidation and depends on conditioning time (CO adsorption time), temperature and humidity.