Suppression of H2O2 Formation at Pt-Skin/Pt Alloy Hydrogen Anode Catalysts for Mitigation of Membrane Degradation

Abstract

Chemical degradation of polymer electrolyte membranes (PEMs) by ·OH radical attack is one of the most serious issues during the operation of fuel cells. To scavenge ·OH radicals, Ce3+ ions are incorporated in PEMs: ·OH + Ce3+ + H+ → Ce4+ + H2O, followed by Ce4+ + ½ H2 → Ce3+ + H+.1,2 However, Ce ions migrate through the PEM from the anode to the cathode, resulting in a loss of cathode performance due to decreased ionomer conductivity.2 Because ·OH is generated by a reaction of H2O2 with dominantly metal ions (such as Fe2+) and secondarily with the acidic PEM,3 it is essential to decrease the H2O2 formation rate itself at the anode catalyst. It has been recognized that O2 at the cathode crosses over through the PEM to the Pt-based anode, resulting in H2O2 production via a two-electron oxygen reduction reaction (ORR) and/or a chemical reaction of O2 with adsorbed H atoms on Pt. Recently, we have found that H2O2 formation was suppressed at PtxAL–PtCo/C (PtxAL denotes stabilized Pt-skin of 1 to 2 atomic layers) compared with commercial c-Pt/C, when used as the cathode catalyst for the ORR, at least, from 0.8 to 0.5 V vs. RHE.4 It was also found that PtxAL–PtM (M=Fe, Co, Ni)/C exhibited superlative activity for the hydrogen oxidation reaction (HOR) in H2-saturated 0.1 M HClO4 at 70 and 90 ºC.5 In this presentation, we demonstrate an excellent suppression of H2O2 formation during the HOR at our PtxAL–PtM/C anode catalysts. The PtxAL–PtM/C catalysts were prepared in the same manner as that described previously.6,7 A commercial c-Pt/C catalyst with the same carbon support (780 m2 g−1) as that of PtxAL–PtM/C was used for comparison. Each catalyst was uniformly dispersed on a glassy carbon substrate as the working electrode in the channel flow double electrode (CFDE)4 cell with a constant Pt loading, 8 µgPt cm−2. A Nafion film was coated on the catalyst layer with an average thickness of 0.10 µm. Figure 1(B) shows hydrodynamic voltammograms for the HOR at a Nafion-coated c-Pt/C working electrode (WE) in H2- or 10% air/H2-saturated 0.1 M HClO4 at 80 °C. The HOR currents in H2-saturated solution at Nafion-coated c-Pt/C and PtxAL–PtM/C (not shown) commenced at 0.00 V vs. RHE and reached diffusion limits at ca. 0.06 V. As shown in Fig. 1(A), the j C due to the HOR was also detected at the Pt collecting electrode (CE) located downstream of the WE, but the j C(H2) was found to be minimized at the high CE potential of 1.4 V. With a flow of 10% air/H2-saturated solution, the HOR current at the WE decreased slightly due to an overlap of the ORR. At the Pt CE, H2O2 emitted from the WE was detected as an oxidation current. Then, the H2O2 formation current density, j(H2O2), was calculated as a function of potential: j(H2O2) = [j C(10% air/H2) – j C(H2)]/N where N is the collection efficiency experimentally obtained (N = 0.29). As shown in Fig. 2, j(H2O2) on both catalysts increased at less positive potentials, and reached the highest value at 0 V (open circuit potential), which is consistent with an accelerated degradation of PEMs at open circuit in a single cell.1 It is very interesting that the values of j(H2O2) on the PtxAL–PtCo/C catalyst were less than 1/2 of those on c-Pt/C at 0 ≤ E ≤ 0.06 V (practical potentials for the HOR). Hence, PtxAL–PtCo/C is a promising anode catalyst with low j(H2O2), thus being able to mitigate the degradation of PEMs, as well as having high HOR activity. This work was supported by funds for the ‘‘Superlative, Stable, and Scalable Performance Fuel Cells” (SPer-FC) project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References E. Endoh, ECS Trans., 16 (2), 1229 (2008).A. M. Baker, S. K. Babu, R. Mukundan, S. G. Advani, A. K. Prasad, D. Spernjak, and R. L. Borup, J. Electrochem. Soc., 164, F1272 (2017).M. Aoki, H. Uchida, and M. Watanabe, Electrochem. Commun., 8, 1509 (2006).H. Nishikawa, H. Yano, J. Inukai, D. A. Tryk, A. Iiyama, and H. Uchida, Langmuir, 34, 13558 (2018).G. Y. Shi, H. Yano, D. A. Tryk, A. Iiyama, and H. Uchida, ACS Catal., 7, 267 (2017).M. Watanabe, H. Yano, D. A. Tryk, and H. Uchida, J. Electrochem. Soc., 163, F455 (2016).M. Chiwata, H. Yano, S. Ogawa, M. Watanabe, A. Iiyama, and H. Uchida, Electrochemistry, 84, 133 (2016). Figure 1