Electrochemical surface oxidation of Vulcan XC-72, a carbon black commonly used in proton exchange membrane ͑PEM͒ fuel cells, was studied following potentiostatic treatments up to 120 h at potentials from 0.6 to 1.2 V at room temperature and 65°C. Surface oxidation was followed using cyclic voltammetry ͑CV͒, thermal gravimetric analysis coupled to on-line mass spectrometry ͑TGA-MS͒, X-ray photoelectron spectroscopy ͑XPS͒, and contact angle measurements. The analytical techniques all indicate significant surface oxidation occurred during the first 16 h of 1.2 V potential holds at room temperature and a slow increase in surface oxide formation thereafter. An identification of ether, carbonyl, and carboxyl surface oxide species was made by deconvolution of XPS spectra and assigning these functional groups to the observed TGA-MS CO 2 evolution peaks ͑150-750°C͒. An increase in CO evolution ͑Ͼ800°C͒ determined by TGA-MS was consistent with electrochemical CV data, which detected electroactive hydroquinone/quinone species; these electrochemically detected species were a minor fraction of the electrochemically generated surface oxides. Potential holds at 1.0 V at room temperature only resulted in slight oxidation of Vulcan XC-72. However, experiments at 65°C showed clear signs of surface oxidation after only 16 h at potentials у0.8 V, verifying that surface oxides can be generated under simulated PEM fuel cell conditions. Overall, these results suggest that changes in component hydrophobicity, driven by carbon surface oxidation, are an important factor in determining long-term PEM performance instability and decay.
The life of proton exchange membrane fuel cells (PEMFC) is currently limited by the mechanical endurance of polymer electrolyte membranes and membrane electrode assemblies (MEAs). In this paper, the authors report recent experimental and modeling work toward understanding the mechanisms of delayed mechanical failures of polymer electrolyte membranes and MEAs under relevant PEMFC operating conditions. Mechanical breach of membranes/MEAs in the form of pinholes and tears has been frequently observed after long-term or accelerated testing of PEMFC cells/stacks. Catastrophic failure of cell/stack due to rapid gas crossover shortly follows the mechanical breach. Ex situ mechanical characterizations were performed on MEAs after being subjected to the accelerated chemical aging and relative humidity (RH) cycling tests. The results showed significant reduction of MEA ductility manifested as drastically reduced strain-to-failure of the chemically aged and RH-cycled MEAs. Postmortem analysis revealed the formation and growth of mechanical defects such as cracks and crazing in the membranes and MEAs. A finite element model was used to estimate stress/strain states of an edge-constrained MEA under rapid RH variations. Damage metrics for accelerated testing and life prediction of PEMFCs are discussed.
Electrocatalyst decay protocols were used to accelerate cathode performance loss for Pt catalysts. Four electrodes with average platinum particle sizes of 1.9, 3.2, 7.1 and 12.7 nm were evaluated to elucidate the impact of particle size on initial performance and subsequent decay, when subjected to identical potential cycles. The decay rates of Pt electrochemical surface area (ECA) and mass activity (i m ) were significantly greater for 1.9 and 3.2 nm Pt-on-carbon catalysts (Pt-C) compared to 7.1 nm Pt-C, which was stable for 10,000 potential cycles. As expected, the performance decay rate of the electrodes with the smallest Pt particle size was the highest and that of the largest Pt particle size was lowest. However, the initial performance of the largest Pt particle size electrode was significantly lower. Thus, a Pt particle size was identified that balanced performance and durability. The relative impact of operational conditions, such as relative humidity, cell temperature and upper potential limit on 3.2 nm Pt electrodes was also evaluated. Highest decay rates were found when the cathode was subjected to a higher upper potential limit. The decay was attributed to a combination of Pt dissolution, particle growth and carbon support corrosion.
Abstract:The initial performance and decay trends of polymer electrolyte membrane fuel cells (PEMFC) cathodes with Pt3Co catalysts of three mean particle sizes (4.9 nm, 8.1 nm, and 14.8 nm) with identical Pt loadings are compared. Even though the cathode based on 4.9 nm catalyst exhibited the highest initial electrochemical surface area (ECA) and mass activity, the cathode based on 8.1 nm catalyst showed better initial performance at high currents. Owing to the low mass activity of the large particles, the initial performance of the 14.8 nm Pt3Co-based electrode was the lowest. The performance decay rate of the electrodes with the smallest Pt3Co particle size was the highest and that of the largest Pt3Co particle size was lowest. Interestingly, with increasing number of decay cycles (0.6 to 1.0 V, 50 mV/s), the relative improvement in performance of the cathode based on 8.1 nm Pt3Co over the 4.9 nm Pt3Co increased, owing to better stability of the 8.1 nm catalyst. The electron OPEN ACCESSCatalysts 2015, 5 927 microprobe analysis (EMPA) of the decayed membrane-electrode assembly (MEA) showed that the amount of Co in the membrane was lower for the larger particles, and the platinum loss into the membrane also decreased with increasing particle size. This suggests that the higher initial performance at high currents with 8.1 nm Pt3Co could be due to lower contamination of the ionomer in the electrode. Furthermore, lower loss of Co from the catalyst with increased particle size could be one of the factors contributing to the stability of ECA and mass activity of electrodes with larger cathode catalyst particles. To delineate the impact of particle size and alloy effects, these results are compared with prior work from our research group on size effects of pure platinum catalysts. The impact of PEMFC operating conditions, including upper potential, relative humidity, and temperature on the alloy catalyst decay trends, along with the EMPA analysis of the decayed MEAs, are reported.
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