Nanoscale Surface Engineering of Perovskite Oxide Electrocatalyst by Electrochemical Fluorine Doping

Perovskite oxides have emerged as promising noble metal-free catalysts for oxygen evolution reaction (OER) in alkaline water electrolysis. Anion doping and surface modification have attracted attention as developing strategies for highly active OER catalyst materials. Here, we report an original electrochemical F-doping technique that simultaneously controls anion composition and nanoscale surface structure to enhance the OER activity. 20 mol % F-doped La 0.5 Sr 0.5 CoO 3-δ (LSC55) samples with different surface states were prepared by changing the current density, which corresponds to the F-doping rate. Low-rate F-doping (60 μA g –1 ) created a thin amorphous surface layer (∼3 nm), while high-rate doping (3000 μA g –1 ) created a thick F-enriched nanocrystalline layer (∼20 nm). Both of the F-doped samples exhibited significantly enhanced OER activity compared to the pristine LSC55 sample. The high-rate sample showed the highest activity among the samples tested in this work. Remarkably, the OER activity of all of the samples was improved with increasing cyclic voltammetry cycles. Scanning transmission electron microscopy analyses revealed progressive amorphization by surface reconstruction in all of the samples during the OER. Soft X-ray absorption spectroscopy revealed that F remained in the reconstructed amorphous surface layer of the F-doped samples even after the OER measurements. These results indicated that the highly active F-containing surface layer was reconstructed on the high-rate F-doped LSC55. This study demonstrates the utility of nanoscale surface engineering by electrochemical anion doping, providing pathways for advanced electrocatalyst development.