Electronic Structure Investigation of BSCF Catalysts during Electrochemical Water Electrolysis by Using Time Zero Analysis and Advanced Operando X-Ray Absorption Spectroscopy
Authors: Liao Kuowei , Cao Weijie , Thakur Neha , Kumar Mukesh , Watanabe Toshiki , Matsunaga Toshiyuki , Orikasa Yuki , Kuroda Yoshiyuki , Mitsushima Shigenori , Uchimoto Yoshiharu 
Published: August 2024

The rapid increase in global energy demands and environmental awareness regarding the excessive use of fossil fuel has spurred significant academic interest in the development of alternative energy conversion and storage technologies that prioritize both efficiency and environmental sustainability. Among all kinds of renewable energy, hydrogen energy holds the potential to create a carbon free clean society, and a possible solution to the environmental issues.1 Electrochemical water splitting is considered as a greener approach for pure H2, which involved cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). The anodic OER is kinetically sluggish and considered as a rate determining step of the water splitting reaction. Therefore, it is necessary to develop OER catalysts with high activities.
Perovskite oxides knows as a promising oxygen electrode catalyst due to their low cost, flexibility, and tailorable properties. However, most perovskite oxides possess poor electrical conductivity at room temperatures, and this limits their oxygen catalytic activity. In our work Ba0.5Sr0.5CoxFe1-xO3-δ (BSCF) catalysts with different cobalt and iron amounts where X=0, 0.2, 0.8, 1 were synthesized and investigated by time zero analysis and flow rate infinitization analysis which are electrochemical testing method that uses pressure to remove generated oxygen gas bubbles and provides accurate activity of the catalyst.2We find the volcano plot of the Fe amount and the OER activities, showing samples with X=0.8 have the best OER activities as Figure 1. Electronic structure of B-Site transition metal elements, oxygen vacancies, crystal structures and electrical conductivity are the key factors for determining the activities of Perovskite catalysts in OER.3Also it is quite important to determine the real active sites during the OER process by using advance operando techniques.
Therefore, in order to investigate the in-depth mechanism of OER activities of Perovskite catalysts, the newly designed operando soft X-ray Absorption Spectroscopy (XAS) cells was utilized to analyze the subtle change as shown in Figure 2 on the oxygen K-edge (531 eV - 532 eV). Ba0.5Sr0.5Co0.8Fe0.2O3-δ has the most obvious changes in integral surface area at the O K-edge pre-edge region, which leads to the best activity among other BSCF samples. According to the Energy Dispersive X-ray Spectroscopy and Lattice Oxygen Evolution Mechanism, ions on the A sites (Ba2+,Sr2+) tend to dissolve and ions on the B sites (Co, Fe) are forming hydroxides (BOOH).
Keywords: oxygen evolution reaction, BSCF catalysts, Time zero analysis, operando X-ray absorption spectroscopy

Acknowledgements

This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

Reference:

(1) Rose, P. K.; Neumann, F. Hydrogen refueling station networks for heavy-duty vehicles in future power systems. Transportation Research Part D: Transport and Environment
2020, 83, 102358.
(2) Nagasawa, K.; Matsuura, I.; Kuroda, Y.; Mitsushima, S. A Novel Evaluation Method of Powder Electrocatalyst for Gas Evolution Reaction. Electrochemistry
2022, 90 (1), 017012-017012.
(3) Zhu, Y.; Zhou, W.; Shao, Z. Perovskite/carbon composites: applications in oxygen electrocatalysis. Small
2017, 13 (12), 1603793.

Figure 1