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Electrocatalysis on PtZn surface - fundamental studies and the applications in PEM fuel cells Sode, Aya

Abstract

The PtZn alloy was characterized and studied as a possible candidate for proton exchange membrane fuel cells (PEMFCs). Zn was electrochemically deposited on a bulk Pt substrate and the alloy was spontaneously formed at ambient conditions, as confirmed by Auger electron spectroscopy (AES). PtZn decreased the oxygen reduction reaction (ORR) overpotential by 30 mV with respect to that measured for Pt. The alloy was also prepared in a similar manner on a membrane electrode assembly (MEA) and studied in a single stack fuel cell. The PtZn MEA also showed an improved performance compared to the Pt MEA. Carbon monoxide oxidation catalysis was also investigated. Zn modification of a Pt substrate makes the resulting surface rough, and the roughness was compensated through the measurement of capacitance. Although, in this case, Zn modification did not enhance the catalytic activity compared to Pt, cycling the potential in a CO saturated acidic solution made the PtZn surface smoother and more Pt-rich. The ORR catalytic activity was also compared to Pt with equivalent roughness. PtZn showed enhancement in the kinetics of the reaction, indicating Zn modification had impact on improving the ORR catalytic activity beyond the roughness effect. The PtZn surface becomes Zn-rich after cycling the potential in an oxygen saturated acidic solution. The stability of the alloy under fuel cell operating conditions was also studied. PtZn nanoparticles were formed on glassy carbon substrates and the surface and bulk alloy compositions were determined before and after the alloy was polarized in an oxygen saturated acidic solution. Approximately 18% of Zn originally present in the alloy was lost after 40 hours of polarization at 0.8 V vs. RHE. It was also confirmed PtZn particles adhere well to carbon and were free from sintering effects. The cumulative results presented support that the PtZn alloy is thus a promising candidate for the cathode of PEMFCs.

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