Data corresponding to the isotherms for the equilibrium water content of a PolyBenzImidazole (PBI)-H3PO4 High Temperature Membrane Electrode Assembly (HT-MEA) are presented over a range of temperatures from 30°C to 180°C. The data show values for the ratio of water to acid which range from 4.0 at 70°C to 0.2 at −160°C at 30kPa of water vapor pressure. In addition to the equilibrium data, rates of adsorption are shown to have time constants on the order of 500s and 2500s at 70°C and 50°C, respectively. At temperatures above 100°C the time constants for adsorption are on the order of 100s. These data allow for the calculation of “acid dilution” in this HT-MEA during low-temperature excursions and start up/shut down.

The objective of this study is to aid the development of an understanding of the mechanisms of H(3)PO(4) loss and transport in PolyBenzImidazole (PBI)-type High Temperature Membranes (HTM) fuel cells. The isotherms for adsorption and desorption of water in PBI-type HTM are critical data and reported for the first time here for a commercially available membrane. Depending on their values, this adsorption/desorption could play an important role in the long-term dilution of acid and thereby affect (in the long-term) the transport of this acid and proton flux in the membrane. Inclusion of these data is necessary in the development of a mechanistic model that allows for design of new experiments and prediction of fuel cell performance.

This paper shows how the performance of a PEMFC with a Pt/Ru anode changes when nitrogen and carbon dioxide are present in the fuel. The data with N-2/H-2 mixtures show deviations of 10-30 mV from Nernst behavior at currents above 0.3 A/cm(2). Similar deviations were observed with CO2/H-2 mixtures and differences between the anode polarization with the two types of mixtures was within the experimental error of the load cell (i.e., 5 mV). However, closer inspection of the anode with Cyclic Voltammetry (CV) after exposure to these mixtures indicates that the CO2/H-2 mixtures produced some CO from the reverse water gas shift reaction. The CO stripping area showed dependence on the inlet composition of the anode gas and the coverage of anode was found to be in the order of 10,7 Mol/cm(2). Equilibrium calculations at different temperatures and pressures show that the equilibrium CO concentration can reach between 10-100 ppm.

Reformed gas containing CO2, N2, and H2 may be used in proton exchange membrane fuel cells (PEMFCs), and recent evidence has shown that CO2 can react in situ with H2 (i.e., a reverse water gas shift (RWGS) reaction) and produce adsorbed CO that can poison the electrode catalyst. Here, a study is presented to extend the previous observations by considering how pressure, gas composition, and temperature affect this reaction in a PEMFC for both Pt and Pt/Ru alloy catalysts. The coverage of CO produced on the electrodes was determined by stripping cyclic voltammetry (CV). The data show how the CO stripping potential depends on temperature, and how the analysis allows the determination of an activation energy. The data are shown to be consistent with a kinetic catalytic model and not with an equilibrium model.

Data are presented to quantify the effect of reformate components on the performance of proton exchange membrane fuel cells (PEMFCs) with Pt and Pt/Ru alloy anodes. The performance deviates by 10-70 mV from Nernst behavior at current densities above 0.3 A/cm(2) when the cell is operated with N-2 /H-2 mixtures. The same deviations were observed with CO2/H-2 mixtures for a PEMFC with a Pt/Ru anode. However, for a PEMFC with a Pt anode, CO2-diluted H-2 gives significantly larger polarizations than N-2-diluted H-2. Also, cyclic voltammetry indicates that, after the anode is exposed to the CO2/H-2 mixture, some CO is produced through the reverse water gas shift (RWGS) reaction. These data are consistent with equilibrium calculations showing that the CO concentration can reach between 10 to 170 ppm. Consistent with CO production from CO2, the CO stripping areas (which are a measure of CO coverage! also showed dependence on the inlet composition of the anode gas as well as the presence of O-2 on the cathode side. The CO coverage resulting from the RWGS reaction approaches 5 x 10(-7) mol/cm(2) for a 0.4 mg/cm(2) Pt anode. (C) 2004 The Electrochemical Society.