Enhancement of Electrocatalytic Oxygen Evolution Activity in Perovskite Oxides Via Fluorination Approach
Authors: Iwase Kazuyuki , Ohtaka Masaki , Honma Itaru 
Published: December 2024

The electrochemical oxygen evolution reaction (OER) is an important anode reaction for water electrolysis to generate green hydrogen using renewable energy sources. Numerous studies have been conducted on developing OER catalysts composed of transition metal oxides. Among them, perovskite oxides (PVs) are a group of metal oxides composed of alkali-earth metal ions and transition metal ions. It is reported that the OER activity of PVs can be improved by choosing the appropriate metal cations and tuning their composition. Recently, in addition to controlling the composition of metal cations, doping hetero-anions such as fluoride ions (F−) into the PV structure or substituting oxide ions (O2−) with F− in PVs have been employed to enhance their OER activity. Certain amounts of F− substitution (F-substitution) are expected to be a promising approach to tune the valence states of metal cations in PVs due to the difference in valence of F− and O2−. Since the valence state of the metal active center is an important factor determining the OER activity of PVs, F-substitution in PVs have been attempted in previous researches. However, since the synthesis of F-containing PVs was often conducted under high-temperature heat treatment conditions, the content of fluoride ions and enhancement of OER activity were limited. Based on these backgrounds, here we synthesized barium (Ba), iron (Fe), and cobalt (Co) containing PVs in the form of BaFe1–xCoxO3–d (x = 0, 0.1, 0.2) and attempted to enhance their OER activity through broad-range fluorine substitution via low-temperature fluorination.

BaFe1–xCoxO3–d (x = 0, 0.1, 0.2) was synthesized by a sol-gel method using metal nitrates as precursors referring to a previous report.[1] F-substitution was performed by heat-treating a mixture of the as-synthesized BaFe1–xCoxO3–d with polyvinylidene fluoride (PVDF) by modifying previous reports.[2,3] The OER activity was evaluated using rotating disk electrodes in O2-saturated 1M KOH.

This abstract reports the results for BaFe0.8Co0.2O3-d (x = 0.2, BFCO-20), BaFe0.
9Co0.
1O3-d (x = 0.1, BFCO-10) and their F-substituted samples. The F/Ba ratios of F-substituted samples in this abstract are 1 and 0.8 for BFCO-20 (F-BFCO-20) and BFCO-10 (F-BFCO-10), respectively. Powder X-ray diffraction (XRD) patterns confirmed that single-phase PVs were successfully synthesized. No formation of any impurity due to the F-substitution process was observed. The peaks of BFCO-20 and BFCO-10 were slightly shifted to the lower angle region by the F-substitution process. The Co K-edge and Fe K-edge X-ray absorption near edge structure (XANES) spectra shifted to the lower energy region by F-substitution as shown in Fig. 1a, indicating that the valence was reduced by F-substitution. The cyclic voltammograms for pristine BFCO-20, BFCO-10, and F-substituted samples revealed that F-substitution enhanced the OER activity (Fig. 1b). The F-substitution increased the OER current at 1.65 V vs. RHE about 3.9 times and 2.6 times for BFCO-20 and BFCO-10 (Fig. 1b), respectively.[4] In the presentation, we will also discuss details of the structural analysis and the electrochemical analysis of the synthesized samples.

[1] A. el Hadri et al., ACS Catal.
7 (2017) 8653-8663. [2] R. Heap et al., Solid State Commun.
141 (2007) 467–470. [3] O. Clemens et al., J Solid State Chem., 198 (2013) 262–269. [4] K. Iwase et al., Chem. Mater., 35, (2023) 2773–2781.

Figure 1