Temperature Effect on Hydrogen Underpotential Deposition for Pt/C

Abstract

The charge associated with hydrogen underpotential deposition (HUPD) is routinely used to estimate the apparent electrochemically active Pt surface area (ECSA) of Pt-based electrocatalysts. However, the ECSA estimation is not flawless. The HUPD overlaps with the H2 evolution region, leading to poor deconvolution of the Hupd charge, which becomes more serious as the electrolyte temperature is increased, which in turn leads to an apparent decrease in the HUPD charge decrease with increasing electrolyte temperature.1,2 In this study, we have re-investigate the change in Hupd at various temperatures and attribute the decay of HUPD charge with rise in the electrolyte temperature to the disorder arrangement of surface platinum atoms with exposure to high temperature electrolyte. The experimental procedure is shown in Scheme 1. Figure 1 shows the HUPD charge of Pt/C (TEC10E50E) as a function of potential cycle number. The HUPD charge of Pt/C increased by break-in potential cycles between 0.05 and 1.3 V vs. RHE until 30 cycles, owing to surface cleaning. The HUPD charge gradually decreased after 30 cycles due to dissolution of Pt and/or particle growth. By increasing the electrolyte temperature from 25°C to 60°C, the HUPD charge rapidly decreased from 625±16 mC at 109 cycles to 509±5 mC at 112 cycles. The decrease in Hupd charge cannot be attributed to the above degradation mechanism, since the decay rate of HUPD does not decrease with the same number of break-in cycles. After the temperature was turned back from 60°C to 25°C, the HUPD charge did not change (509±22 mC at 115 cycles). Additional potential cycling lead to a gradual increase in the HUPD charge, and the HUPD charge recovered to 589±16 mC at 145 cycles, which is similar to the the initial HUPD charge at 25°C). It has been reported that the hydrogen adsorption energy on platinum decreases when the d-band center is upshifted.3 Since the d-band center upshifts with increasing roughness factor (decreasing the average coordination number) of surface platinum,2 we interpret our findings to the decay of HUPD charge with increasing electrolyte temperature to the disorder arrangement of surface platinum. This leads to the low adsorption energy of hydrogen due to upshift of d-band center with decreasing the average coordination number of surface platinum. Finally, we note that additional potential cycles for room temperature ECSA measurements is necessary to avoid underestimation of HUPD (apparent ECSA) for Pt-based electrocatalysts after high temperature measurements such as accelerated durability test. This research was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References A. Zolfaghari, M. Chayer, and G. Jerkiewicz, J. Electrochem. Soc, 144, 3034 (1997).N. M. Marković, T. J. Schmidt, B. N. Grgur, H. A. Gasteiger, R. J. Behm, and P. N. Ross, J. Phys. Chem. B, 103, 8568 (1999).I. A. Pašti, N. M. Gavrilov, and S. V. Mentus, Adv. Phys. Chem., 2011, 1 (2011). Figure 1