SYSTEMS STRATEGIES TO MITIGATE CARBON CORROSION IN FUEL CELLS by xfo14057

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									ECS 210th Meeting, Abstract 0597.pdf




          SYSTEMS STRATEGIES TO MITIGATE                                 decay, without necessarily making any changes to the
          CARBON CORROSION IN FUEL CELLS                                 electrode structures. This is the approach that UTC
                                                                         Power has primarily implemented, to date. These
                                                                         strategies include: 1) minimizing the time that the adverse
          Mike L. Perry, Timothy W. Patterson, and Carl Reiser           conditions exist, and/or 2) controlling the potentials
                           UTC Power, LLC                                during start-up and/or shutdowns using external loads,
                       South Windsor, CT 06074                           and/or 3) minimizing the number of adverse cycles that
                                                                         occur in a given application. Some examples of these
          In 1999, UTC Power discovered a decay mechanism that           decay-mitigation strategies will be presented, along with
          is particularly aggravated by repeated start-stop cycles       some results. These solutions are relatively simple to
          [1]. The decay occurs when the anode is partially filled       implement and are not overly expensive. Furthermore,
          with air, which is a condition that is difficult to avoid      they can enable one to meet the performance and
          during prolonged start-stop cycles. Under these                durability requirements of almost any application,
          conditions, the electrochemical potential of the electrolyte   including those with a relatively large number of idle time
          can fall to values of -0.5V RHE, resulting in much higher      and start-stop cycles (e.g., automotive), using existing
          than expected overpotentials for carbon corrosion on the       materials.
          cathode. A detailed explanation of the “reverse-current
          decay mechanism,” as well as some experimental effects,
          has been presented elsewhere [1]. A mathematical model         References
          will also soon be published [2]. Additionally, it has been
          shown that this decay mechanism can also occur during          1.   C. Reiser, L. Bregoli, T. Patterson, J. Yi, J. Yang, M.
          steady-state operation if a portion of the anode is denied          Perry, and T. Jarvi, “A reverse-current decay
          fuel [3].                                                           mechanism for fuel cells,” Electrochemical and
                                                                              Solid-State Letters, Vol. 8, No. 6, pp. A273-A276
          Another carbon-corrosion concern is extended periods of             (2005).
          time spent at elevated potentials, e.g., idle conditions       2.   J. Meyers and R. Darling, "Model of Carbon
          and/or air on both anode and cathode immediately after              Corrosion in PEM Fuel Cells," Journal of the
          shutdown when the temperature of the stack is still                 Electrochemical Society, Vol. 153, No. 8 (2006).
          relatively high [4]. These concerns have led to the call for   3.   T. Patterson and R. Darling, “Damage to the cathode
          more corrosion-resistant catalyst supports and extended             catalyst of a PEM fuel cell caused by localized fuel
          testing at elevated potentials (e.g., ≥ 1.2 V RHE), at              starvation,” Electrochemical and Solid-State Letters,
          operating temperatures, to ensure adequate stability for            Vol. 9, No. 4, pp. A183-A185 (2006).
          the intended application (e.g., automotive).                   4.   M. Mathias, R. Makharia, H. Gateiger, J. Conley, T.
                                                                              Fuller, C. Gittleman, S. Kocha, D. Miller, C.
          One possible mitigation strategy is to develop alternative          Mittelsteadt, T. Xie, S. Yan, and P. Yu, “Two fuel
          catalyst supports that are more corrosion resistant in order        cell cars in every garage?” Interface, Vol. 14, No. 3,
          to eliminate, or at least reduce, performance decay that            pp.32-34 (2005).
          can result from these adverse conditions. Certainly, one       5.   U.S. DOE, “Research and development of fuel cell
          can improve the stability of catalyst layers by employing           technology for the hydrogen economy,” Funding
          carbon supports with lower surface areas and/or using               Opportunity Number: DE-PS36-06GO96017, Topic
          graphitized carbons, but this durability improvement                3, pp. 13-16, 1/24/2005.
          comes at the expense of performance and/or cost. More          6.   K. Kinoshita, Carbon Electrochemical and
          importantly, at the electrochemical potentials that can             Physicochemical Properties, J. Wiley & Sons, New
          result from the reverse-current mechanism, the durability           York, NY (1988).
          improvement is expected to be relatively minor. Most           7.   L. Roen, C. Paik, and T. Jarvi, “Electrocatalytic
          likely, the mitigation of this degradation will require the         corrosion of carbon support in PEMFC cathodes,”
          development of non-carbon-based catalyst supports, since            Electrochemical and Solid-State Letters, Vol. 7, No.
          even the most highly graphitized carbons are readily                1, pp. A19-A22 (2004).
          electrochemically oxidized at these potentials [6]. This is
          especially true of carbon-supported catalysts with high Pt
          mass fractions [7]. Therefore, technical breakthroughs
          are required to effectively address this decay solely via
          electrode material changes.

          An alternative strategy is to employ system changes and
          stack-design practices that can effectively mitigate this

								
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