Deciphering the Structural and Chemical Transformations of Oxide Catalysts during Oxygen Evolution Reaction Using Quick X-ray Absorption Spectroscopy and Machine Learning

Bimetallic transition-metal oxides, such as spinel-like Co$_x$Fe$_{3–x}$O$_4$ materials, are known as attractive catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Nonetheless, unveiling the real active species and active states in these catalysts remains a challenge. The coexistence of metal ions in different chemical states and in different chemical environments, including disordered X-ray amorphous phases that all evolve under reaction conditions, hinders the application of common operando techniques. Here, we address this issue by relying on operando quick X-ray absorption fine structure spectroscopy, coupled with unsupervised and supervised machine learning methods. We use principal component analysis to understand the subtle changes in the X-ray absorption near-edge structure spectra and develop an artificial neural network to decipher the extended X-ray absorption fine structure spectra. This allows us to separately track the evolution of tetrahedrally and octahedrally coordinated species and to disentangle the chemical changes and several phase transitions taking place in Co$_x$Fe$_{3–x}$O$_4$ catalysts and on their active surface, related to the conversion of disordered oxides into spinel-like structures, transformation of spinels into active oxyhydroxides, and changes in the degree of spinel inversion in the course of the activation treatment and under OER conditions. By correlating the revealed structural changes with the distinct catalytic activity for a series of Co$_x$Fe$_{3–x}$O$_4$ samples, we elucidate the active species and OER mechanism

Influence of the cobalt content in cobalt iron oxides on the electrocatalytic OER activity

Sub 10 nm cobalt ferrite Co$_x$Fe$_{3−x}$O$_4$ (x ≤ 1.75) nanoparticles and cobalt-rich wüstite (Co$_{x/3}$Fe$_{(1−x)/3}$)O nanoparticles (x ≥ 2) were synthesized in a solvothermal approach and characterized by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) as well as energy dispersive X-ray spectroscopy (EDX), IR, Raman, and $^{57}$Fe-Mössbauer spectroscopy. Their electrocatalytic activity in the oxygen evolution reaction (OER) was evaluated and the active state formation was tracked by operando X-ray absorption spectroscopy (XAS). Our studies demonstrate that the cobalt-rich wüstite (Co$_{x/3}$Fe$_{(1−x)/3}$)O nanoparticles underwent a phase-transformation into the spinels Co$_x$Fe$_{3−x}$O$_4$ (x ≥ 2) under the applied OER conditions. The overpotential $η$10 at 10 mA cm$^{−2}$, serving as a benchmark for the OER activity of the cobalt ferrite nanoparticles in alkaline media, was lower than that of magnetite Fe$_3$O$_4$ even with low cobalt concentrations, reaching a minimum of 350 mV for Co$_{2.25}$Fe$_{0.75}$O$_4$ with a Tafel slope of 50 mV dec$^{−1}$. Finally, we identified that the catalytic activity is linked to the nanoparticle size as well as to the degree of Co redox activity and change in coordination during OER