Estimation of the Active Surface Area on the Pt Catalyst during ORR Proceeds with the in-Situ XAFS Method
Authors: Kamino Rin , Kazuma Hisazaki , Negishi Mayu , Maruyama Takahiro , Saida Takahiro 
Published: December 2024

Electrocatalysts for polymer electrolyte fuel cells must be not only highly active and durable but also have a high Pt utilization rate to suppress their cost. Pt utilization is evaluated using the electrochemically active surface area (ECSA). However, ECSA is calculated from the electric charge required for the hydrogen adsorption as the Pt surface area, and it does not accurately reflect the three-phase interface where the oxygen reduction reaction (ORR) occurs. Additionally, it is impossible to estimate ECSA from the electrochemical measurements data progressing the ORR because the reduction current is dominant. Therefore, we focused on in-situ X-ray absorption fine structure (XAFS) measurements using hard X-rays as a new estimation method for the real active surface area of the electrocatalyst during ORR. Concretely, the height variation of the white line of the XANES spectrum, which corresponds to the valence changes from Pt0 to Ptn+, was converted to the active surface area of Pt. Pt valence on the surface fluctuates by oxygen molecule adsorption, reaction progression of ORR, and the detachment of generation water molecules. Below 1.0 V, this variation of Pt valence mainly arises on Pt atoms at the topmost surface. If the conversion value between the Pt valence variation and ECSA is determined, the real active area of the Pt catalyst under OCV can be estimated. The aim of this study was to determine the conversion value between the Pt valence variation and ECSA, and its value was verified using single-cell measurements.

TEC10E50E (Tanaka Kikinzoku Kogyo K.K., Japan) was used as an electrocatalyst. A half-cell was composed of a carbon counter electrode, a reversible hydrogen electrode (RHE) as a reference electrode, and a 0.1 M HClO4 electrolyte. To vary the Pt utilization rate, three different Pt loading on a working electrode, 17.3 mg-Pt/cm2 (the standard), 8.65 mg-Pt/cm2 (half of the standard), and 173 mg-Pt/cm2 (10 times the standard), were employed. The Pt utilization rate was calculated by dividing an ECSA obtained from electrochemical measurement by theoretical Pt surface area, which was assumed from the average Pt particle size. Calculated Pt utilization rates were used to determine the conversion value.

A sing-cell employed TEC10E50E as the cathode catalyst, Nafion NR212 as a polymer electrolyte membrane, TECPd (ONLY) E50E as the anode catalyst, gas diffusion layer, and carbon-based separator with serpentine channel. The catalyst loading in MEA was 0.5 mg-metal cm-2.

In-situ XAFS measurements were conducted at BL5S1 and BL11S2 of the Aichi Synchrotron Radiation Center and BL01B1 of SPring-8 in Japan. In half-cell, in-situ XAFS measurements were conducted in fluorescence mode. On the other hand, single-cell tests were evaluated in transmission mode. The white line area of Pt was calculated by fitting the measured XANES spectra in the range of 11.52 keV to 11.60 keV with the arctangent function and Lorentzian function.

From the results of ECSAs, Pt utilization rates, and white line areas indicating the Pt valence, the coefficient “α” was determined at 19.7, which was the conversion value between the Pt valence variation and ECSA. When this coefficient “α” was applied to the XANES spectra of OCV in single-cell tests, the real active surface area reflecting the three-phase interface was estimated, and an obtained value approximately agreed with ECSA in the inert gas flow conditions. Thus, the real active surface area of the Pt catalyst at progressing ORR would be estimated using the coefficient “α” discovering this study and in-situ XAFS measurements.