Mitigation of the Start-up and Shut-down Degradation in Pemfcs By Means of a Selective H2 Oxidation Catalyst

Abstract

A major durability issue of fuel cells is the long-term stability of the cathode, especially during start-up and shut-down (SUSD) events, when a H2/air gas front passes through the anode.1 In the air-filled segment of the anode, the oxygen reduction reaction (ORR) takes place, while carbon oxidation as well as H2O oxidation take place at the adjacent segment of the cathode, leading to its degradation. Several mitigation strategies have been explored, including reduced residence times of the H2/air front and/or using corrosion resistant graphitized carbon or conductive oxide supports for the cathode catalyst.1, 2 However, retarding/preventing the ORR of the anode catalyst without compromising its activity towards the hydrogen oxidation reaction (HOR) would significantly reduce the degradation of the cathode during SUSD.Here we report on the mitigation of SUSD induced cathode degradation by the implementation of a HOR-selective catalyst, which exhibits poor ORR activity while retaining high HOR activity.3 The selectivity is a result of a strong metal-support interaction (SMSI) between Pt and TiO2, where the encapsulation of the Pt nanoparticles by a partially reduced TiOx film hinders the ORR of this carbon supported Pt/TiOx/C catalyst.3, 4 SUSD stability measurements were performed using 5 cm2 membrane electrode assemblies (MEAs) with a 0.4 mgPt cmMEA -2 loaded Pt/C cathode (TEC10V50E, Tanaka), comparing the H2/air performance and the SUSD degradation obtained with either a conventional Pt/C anode (0.05 mgPt cmMEA -2 loading; TEC10V20E, Tanaka) or with an anode prepared from an HOR-selective Pt/TiOx/C catalyst3 (referred to as mitigated MEA furtheron). SUSD cycling was carried out at 80°C, a relative humidity (RH) of 90%, and using a constant H2 or air flow rate at the anode of 550 nccm at 150 kPag inlet gauge pressure, resulting in a front residence time of 0.03 s. The time between start-up and shut-down events was 60 s.With regards to the H2/air performance at beginning of test (BOT), the mitigated MEAs showed a performance loss of ≈ 40 mV at 2 A cmMEA -2 that can be attributed to two main factors. First, an additional HOR overpotential of 30 mV due to the use of the selective Pt/TiOx/C catalyst that exhibits a ≈ 3 times smaller HOR activity; and second, an increase in the proton sheet resistance of the cathode catalyst layer due to TiO2 dissolution and Ti3+ migration to the cathode (shown by XPS of the cathode catalyst layer). During SUSD cycling, the electrochemically active surface area (ECSA) of the cathode quickly faded when using a Pt/C anode (fig. 1a), whereas 75% of the initial ECSA where maintained when using a Pt/TiOx/C anode (fig. 1b). Consequently, the mitigated MEA lost only 40 mV at 2 A cmMEA -2 after 3200 SUSD cycles vs BOT in contrast to a loss of 170 mV in H2/air performance for the reference MEAs with a conventional Pt/C anode.