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PEM fuel cell catalyst degradation mechanism and mathematical modeling
Wu Bi
Paperback. ProQuest, UMI Dissertation Publishing 2011-09-11.
ISBN 9781244095410
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Förlagets beskrivning
The durability of carbon-supported platinum oxygen reduction electrocatalysts is one of the limiting factors for their commercial applications in PEM fuel cell cathodes. In this work, we applied both experimental and numerical tools to study Pt/C catalyst degradation mechanisms. An accelerated catalyst degradation protocol through cycling the cathode potential in a square-wave profile was applied to study cell performances, Pt/C catalyst ORR activity, and active surface area losses. Post-mortem analyses of cathode Pt particle size were conducted by X-ray diffraction. Changes of platinum distributions in CCMs were studied by SEM/EDS analyses with surface coated Au as the reference element. The mechanisms of platinum deposition in membrane were investigated. It was confirmed by the SEM/EDS Pt distribution analyses that the deposited Pt atoms originated from the cathode. It was hypothesized that dissolved Pt ions from the cathode diffused into the membrane and were reduced by the permeated hydrogen from the anode. These deposited Pt atoms catalyzed the combustion of permeated oxygen and hydrogen. Pt band was predicted and experimentally confirmed at the location where the permeated hydrogen and oxygen completely reacted with each other. An active research thrust for PEM fuel cells is the development of membranes for high temperature (above 80°C) and low humidity operations. However a large tradeoff the benefits running fuel cell at relatively high temperatures was observed due to the accelerated cathode degradation processes. And at low humidity conditions, the cathode degradation rate decreased due to the slow transport of soluble platinum ions in possible narrowed/limited water (or ionic) channel networks in polymer electrolytes. From the Pt dissolution experiments in 0.5 M HClO4 solution, large positive effects of holding potentials on dissolution rates and soluble Pt concentrations were observed. Without an external holding potential, Pt dissolution rate was orders of magnitude higher under the air condition than in a non-reacting nitrogen environment. However, the difference was less than 100% between the nitrogen and air environments at a holding potential of 0.8 V (vs. RHE). Hence we believed that at an open-circuit condition, platinum was oxidized by oxygen molecule and further dissolved in acidic electrolyte. While at closed-circuit conditions, both chemical and electrochemical oxidation and dissolution might be involved. Platinum electrochemical oxidation kinetics was studied and simulated by cyclic voltammetry. In a simplified cathode degradation model, overall Pt particle growth by Pt mass exchange between small and large particles was clearly demonstrated through a favored Pt dissolution from small particles and Pt ion deposition onto large particles due to the particle size effect. The model also predicted the cycling upper potential and cycle frequency as the major positive effects on catalyst degradation, in an agreement with other literature results. We recommended further study of catalyst degradation especially on dissolution processes, and more durable electrode materials and an effective management of cell potentials will be needed to prolong cathode lifetime
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PEM fuel cell catalyst degradation mechanism and mathematical modeling
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