Introduction

For highly efficient hydrogen generation by water electrolysis, enormous efforts have been devoted to the development of oxygen evolution reaction (OER) catalysts. Although cation doping is a general design strategy, its effectiveness is reaching the limitations and alternative strategies are demanded. Anion defect engineering is one of promising strategies for the development of oxide-based OER catalysts. For instance, OER activity of BaFe_1-xCo_xO_3-δ were improved by substituting part of O with F.^[1] To realize flexible control of anion composition, we developed a new anion-doping technique by using an electrochemical reactor and succeeded electrochemical F-doping into a perovskite oxide La_0.5Sr_0.5CoO_3-δ (LSC55).^[2] In this study, by evaluating the OER activity of electrochemically F-doped LSC55, we aimed to elucidate the influence of F defect on OER activity.

Experiment

LSC55 was synthesized by Pechini method. For the F-doping to LSC55, the electrochemical cell, LSC55-BaF₂|BaF₂|PbF₂-Pb, was fabricated. Here, BaF₂ and PbF₂-Pb were a fluoride-ion conductor and an F source respectively. 20 mol% of F was electrochemically supplied into LSC55 under the constant current 60 μA/g at 250°C. The crystal structure and surface morphology of the prepared samples were characterized by x-ray diffractometry (XRD) and annular bright field scanning transmission electron microscopy (ABF-STEM). To check the existence of F in the sample, x-ray photoelectron spectroscopy (XPS) F1s spectrum was measured.

For the OER test, the catalyst ink was prepared by mixing the catalyst powder, K⁺-exchanged Nafion and tetrahydrofuran. 6.4 μl of the catalyst ink was sonicated and dropped onto the glassy-carbon rotating disk electrode. Electrochemical measurement was performed with a Hg/HgO reference electrode and a Pt wire counter electrode in O₂-saturated 0.1 M KOH. To evaluate the OER activity, cyclic voltammetry (CV) was performed from 0.3 to 0.9 V at 10 mVs^-1 with a rotation rate of 1,600 rpm. The current density was normalized by the surface area of the disk electrode.

Results and discussion

By XRD measurements, it was revealed that the crystal structure of both the pristine and the F-doped LSC55 was a rhombohedral perovskite structure (space group: R3c ). An XPS F1s peak was clearly observed from the F-doped LSC55, which supports successful F-doping on the surface of the sample. The ABF-STEM observation on the pristine sample revealed both the surface and the bulk region maintain highly crystalline state. On the other hand, surface layer composed of amorphous and nano-size crystalline domains was observed in F-doped LSC55 particles.

OER activities were evaluated by CV. The initial OER activity of the F-doped LSC55 was higher than that of the pristine. The reaction current at 1.8 V of the F-doped LSC55 was about 12% higher than that of the pristine. Moreover, the F-doped LSC55 showed better stability upon CV cycles than the pristine. The degradation rate upon 100 CV cycles of the F-doped LSC55 was 83% while that of the pristine was 61%. These indicate that electrochemical F-doping can improve both catalytic activity and its stability.

In the presentation, the influence of the F-doping condition (F concentration, F-doping rate) on the OER activity of LSC55 is discussed.