Investigation of Impact of Carbon Surface Oxidation on Wettability of PEMFC Cathode Electrode

The oxidation reaction on carbon in proton exchange membrane fuel cells (PEMFCs) electrodes has been reported as carbon corrosion recently under several fuel-cell operation procedures [1-4]. The influence of cell design factors for the membrane electrode assembly (MEA) and gas transport in the gas diffusion layer and the catalyst layer were often investigated at start up and partial hydrogen starvation. These results suggested that the coexistence of oxygen in the anode is critical for carbon corrosion on the positive electrode. Although the impact of carbon loss under ordinary fuel-cell operation is less than during startup / shutdown and partial hydrogen starvation situations, it is estimated to be large enough to affect long-term durability of PEMFC. CO₂generation on Pt/C catalyst in a PEMFC occurs under potential dynamic condition such as cyclic voltammetry. They each suggested that platinum accelerated the oxidation of carbon supports under potential dynamic condition.  

                In order to elucidate the influence of platinum on carbon oxidation in more detail, carbon oxidation on Pt/C under potential dynamic model has been investigated. Four reactions were assigned as carbon oxidation reactions.

C + H₂O → CO₂ + 4H+ + 4e^-      (1)

C-C + H₂O → C-COH + H⁺ + e^-  (2)

        C-COH + H₂O → C-COOH + 2H⁺ + 2e^-      (3)

C-COOH + Pt-OH → CO₂ + Pt + COH + H⁺ + e^-           (4)

Carboxyl group and phenol group species were induced from graphite structure carbon on carbon surface through the electrochemical reactions with water.

                High potential hold at 1.3V and potential cycling 0.1 V  to 0.9 V were exposed to Pt/C catalyst under 0.1M H₂SO₄aq. in order to oxidize carbon surface.  The carboxyl group and phenol group on carbon surface were measured before and after potential tests by Boehm method. Carbon groups were increased after 1.3V hold and potential cycling between 0.1 and 0.9V in Table 1. High potential hold caused high oxidation state carbon groups than lower potential cycling test. The increases in carbon groups on carbon surface is estimated to affect on the wettability on carbon surface and causes the suppress of PEFC performance.

Acknowledgement

 Authors appreciated Dr. T. Abe and K. Miyazaki of Kyoto University for the discussion on this work.

References

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