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 Pt_xAL–PtCo/C (Pt_xAL 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 Pt_xAL–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 Pt_xAL–PtM/C anode catalysts.

The Pt_xAL–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 Pt_xAL–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 µg_Pt 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 Pt_xAL–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 Pt_xAL–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, Pt_xAL–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.

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