(PEFCE&E 21 Best Student Poster - Second Place) Hydrogen Peroxide Generation and Hydrogen Oxidation Reaction Properties of Pt/Ir(111) and Ir/Pt(111) Bimetallic Surfaces

Abstract

Introduction Hydrogen peroxide (H2O2) can be generated through oxygen reduction reaction (ORR) particularly on the anode Pt/C surface due to oxygen crossover during operation of polymer electrolyte fuel cell (PEFC): such the H2O2 generation causes degradation of PEFC through reactive radical generations which severely attack the PE.1 Therefore, development of a novel anode catalyst is crucial that less-active for the H2O2 generation and, yet, active for hydrogen oxidation reaction (HOR). According to Durst et al., HOR activity of the Ir/C was about 1/3 of that of the Pt/C,2 and furthermore, we found to date that Ir(111) generates much less H2O2 than Pt(111) in low potential (< 0.3 V).3 Therefore, Pt-Ir bimetallic alloy is expected to suppress H2O2 generation with keeping practical HOR activity. Nevertheless, to our best knowledge, neither H2O2 generation nor HOR properties of Pt-Ir bimetallic surfaces has been reported. Therefore, in this study, we prepared well-defined Pt/Ir(111) and Ir/Pt(111) surfaces and investigated the H2O2 generation and HOR activity. Experimental The Pt/Ir(111) and Ir/Pt(111) bimetallic surfaces were fabricated as follows. First, Ir(111) or Pt(111) surfaces were cleaned by repeated cycles of Ar+ sputtering and annealing at 1153 K in ultra-high vacuum (UHV). Then, 0.1 monolayer (ML)-equivalent Pt or Ir were deposited on the respective Ir(111) and Pt(111) surfaces by arc-plasma deposition (APD) at room temperature. The surface morphology was verified by scanning tunneling microscopy (STM) in UHV. The electrocatalytic properties were evaluated by scanning electrochemical microscope (SECM) using Pt microelectrode (diameter: ca. 20 μm) as a tip electrode. The H2O2 generation property was investigated in O2-saturated 0.1 M HClO4.4 The substrate potential (E S; Pt/Ir(111) or Ir/Pt(111)) was swept by 2 mV/s, and the tip current (i T) due to H2O2 detection was recorded with the constant tip potential (E T) of 1.26 V and tip-substrate distance of 50 μm. The HOR activity was evaluated in an electrolyte containing 0.01 M HClO4 and 0.1 M NaClO4 solution.5 The tip electrode was set as close as possible to the substrate, and E S was swept by 5 mV/s while E T was fixed at −0.74 V. The standard rate constants (k 0) for HOR were estimated by fitting the E S-dependent i T values to the theoretical ones, following to the previous reports.6 All potentials are referenced to that of the RHE. Results and Discussion Fig. 1 shows the UHV-STM images for the Pt/Ir(111) (a) and Ir/Pt(111) (b) surfaces. The dispersed nano-sized, particle-like structures that ascribable to the AP-deposited Pt (a) or Ir (b) can be observed with the atomic steps of the single crystal substrates. The surface area differences estimated from the images were 1.05 and 1.11 for the Pt/Ir(111) and Ir/Pt(111), respectively.The potential-dependent H2O2 generation for the corresponding surfaces are shown in Fig.2 (a) with the simultaneously recorded linear sweep voltammograms (LSVs) (b). The results for clean Ir(111) and Pt(111) are also presented. As shown in Fig.2 (a), much lower i T-values for the Ir(111) than that for Pt(111) in the low potential region (< 0.3 V) indicates that the Ir(111) is much less-active to H2O2 generation than the Pt(111). The H2O2 generation of the Ir(111) in the low potential region remained almost unchanged by the Pt deposition. Also, the i T values for the Ir/Pt(111) was smaller than that for Pt(111), suggesting effective suppression of H2O2 generation of the Pt(111) surface by the deposited Ir.The estimated k 0 for HOR (i.e. HOR activity) are summarized in Fig. 2 (c). The HOR activity of Ir(111) is about 70 % of Pt(111). Though the HOR activity of Pt/Ir(111) is similar to that of Ir(111), the Pt/Ir(111) is ca. 1.5 times more HOR active than Pt(111).In conclusion, through the model catalyst studies (Pt/Ir(111) and Ir/Pt(111)), co-existence of Ir and Pt sites at the topmost surface is effective for developing novel anode catalysts having suppressed H2O2 generation property and practical HOR activity. Acknowledgement This study was supported by the new energy and industrial technology development organization (NEDO) of Japan. References R. Borup et al., Chem. Rev., 107, 3904–3951 (2007) https://pubs.acs.org/doi/10.1021/cr050182l.J. Durst, C. Simon, F. Hasché, and H. A. Gasteiger, J. Electrochem. Soc., 162, F190–F203 (2015).K. Hayashi, T. Tomimori, K. Kusunoki, N. Todoroki, and T. Wadayama, to be submitted.C. M. Sánchez-Sánchez and A. J. Bard, Anal. Chem., 81, 8094–8100 (2009).J. Zhou, Y. Zu, and A. J. Bard, J. Electroanal. Chem., 491, 22–29 (2000).R. Cornut and C. Lefrou, J. Electroanal. Chem., 621, 178–184 (2008). Figure 1