A novel process for fabricating membrane-electrode assemblies with low platinum loading for use in proton exchange membrane fuel cells.

摘要

A novel method based on pulse current electrodeposition (PCE) employing four different waveforms was developed and utilized for fabricating membrane-electrode assemblies (MEAs) with low platinum loading for use in low-temperature proton exchange membrane fuel cells. It was found that both peak deposition current density and duty cycle control the nucleation rate and the growth of platinum crystallites. Based on the combination of parameters used in this study, the optimum conditions for PCE were found to be a peak deposition current density of 400 mA cm-2, a duty cycle of 4%, and a pulse generated and delivered in the microsecond range utilizing a ramp-down waveform. MEAs prepared by PCE using the ramp-down waveform show performance comparable with commercial MEAs that employ ten times the loading of platinum catalyst. The thickness of the pulse electrodeposited catalyst layer is about 5-7 mum, which is ten times thinner than that of commercial state-of-the-art electrodes.;MEAs prepared by PCE outperformed commercial MEAs when subjected to a series of steady-state and transient lifetime tests. In steady-state lifetime tests, the average cell voltage over a 3000-h period at a constant current density of 619 mA cm-2 for the in-house and the state-of-the-art MEAs were 564 mV and 505 mV, respectively. In addition, the influence of substrate and carbon powder type, hydrophobic polymer content in the gas diffusion layer, microporous layer loading, and the through-plane gas permeability of different gas diffusion layers on fuel cell performance were investigated and optimized.;Finally, two mathematical models based on the microhardness model developed by Molina et al. [J. Molina, B. A. Hoyos, Electrochim. Acta, 54 (2009) 1784-1790] and Milchev [A. Milchev, "Electrocrystallization: Fundamentals of Nucleation And Growth" 2002, Kluwer Academic Publishers, 189-215] were refined and further developed, one based on pure diffusion control and another based on joint diffusion, ohmic and charge transfer control developed by Milchev [A. Milchev, J. Electroanal. Chem., 312 (1991) 267-275 & A. Milchev, Electrochim. Acta, 37 (12) (1992) 2229-2232]. Experimental results validated the above models and a strong correlation between the microhardness and the particle size of the deposited layer was established.