(Invited) In Situ XAFS Spectroscopy at Bio-Inspired Multicopper Catalysts for Oxygen Reduction Reaction
Authors: Yagi Ichizo , Matsubara Naohiro , Muto Marika , Yoneuchi Tsubasa , Uemura Yohei , Kato Masaru 
Published: February 2020

Platinum metal group (PMG) catalysts are used for oxygen reduction reaction (ORR) at cathodes in state-ofthe-art polymer electrolyte fuel cells (PEFCs). Since the PMG catalysts contain expensive rare metals, they should be replaced with rare-metal-free catalysts for widespread PEFCs. Recently, electrocatalysts synthesized from carbon and transition metal complexes have been studied [1,2]. It is known that ORR catalysts with multi-nuclear-active sites have high ORR activity [3]. It is also known that natural ORR catalysts such as laccases have a multinuclear copper complex as an active center and show high ORR activity [4], suggesting that laccase-inspired artificial ORR catalysts prepared from carbon and a multinuclear copper complex would show high ORR activity.

In the present work, we incorporated a trinuclear copper complex ([Cu3(trz)3(μ3-OH)]Cl2·6H2O) [5] into graphene nanosheets to synthesize laccase-inspired electrocatalysts for ORR, where grapheme nanosheets worked as an electrical conductive material and the trinuclear copper complex with a triangle core imitated the active center of laccases.

The trinuclear copper complex was prepared from copper chloride and 1,2,4-triazole according to the literature [5]. This copper complex and graphene oxide (GO) were mixed and then reduced in two methods. The first method is heat treatment at high temperature for a short time. The second method is electrochemical reduction on a glassy carbon (GC) electrode (-1.1 V vs. Ag|AgCl, 300 s). We carried out powdery X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) to characterize the catalysts. In situX-ray absorption fine structure (XAFS) spectroscopy was also carried out to estimate the molecular structure during ORR. For electrochemical measurements of the catalysts, the powdery catalyst was suspended in an ethanol-Nafion mixture to make ink containing the catalyst. The catalyst ink was put on a GC electrode and dried under the air at room temperature. Linear Sweep Voltammetry (LSV) was carried out under a saturated O2atmosphere using a rotary disk electrode to obtain current-potential curves. Britton-Robinson buffered solutions were used as electrolyte solutions.

The heat reduction improved ORR activity. The catalyst obtained after the heat treatment showed higher ORR activity than that obtained after the electrochemical reduction. This result indicated that the two reduction methods might give different structures of ORR active sites. XPS spectra of the catalyst before and after the heat treatment were recorded to obtain information on structural changes. A peak originating from C-O was observed before the heat treatment. It disappeared after the heat treatment. This result indicated that GO was reduced to rGO by the heat treatment. The Cu2p spectra showed that CuII was also reduced to CuI. A new peak was observed in N1s spectra after the heat treatment. This peak might be attributed to new configuration that triazole rings were incorporated into graphene nanoshheets. In situ XAFS spectra clarified that the multi-copper arrangement was partially kept even after heat reduction and contributed to the higher ORR activity than other copper catalysts.

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