The urgent demand for global energy underscores the importance of earth-abundant electrocatalysts for massive-scale hydrogen production by electrocatalytic water splitting. The advancement of highly active and stable electrocatalysts with a detailed understanding of the oxygen evolution reaction (OER) has been a longstanding research focus.[1] Mixed 3d transition-metal oxides, such as Co-Mn spinel oxides, show promising electrocatalytic performance towards OER.[2] However, the potential effects of Mn in the Co spinel oxides on electrocatalytic activity remain unclear.[3,4] In this context, this research aims to investigate the role of Mn in the activation of Co-Mn spinel oxides during OER, and correlate the structure and composition changes with OER performance.

Herein, Co2MnO4 and CoMn2O4 were synthesized via a one-pot method. The OER activity of Co2MnO4 is significantly higher than that of CoMn2O4, as indicated by the overpotentials of Co2MnO4 (~334 mV) and CoMn2O4 (~460 mV) measured by linear sweep voltammetry at 10 mA cm-2 under the OER conditions. Additionally, Co2MnO4 exhibits good stability as its overpotential increases only by 14 mV after 1000 cyclic voltammetry (CV) cycles, indicating that the addition of a small amount of Mn can improve both activity and stability of cobalt spinel oxides. To understand the effects of Mn on the activity and stability improvement, X-ray absorption spectroscopy (XAS), X-ray spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and atom probe tomography (APT) were employed to investigate the changes of the oxidation state, structure and composition of Co2MnO4 and CoMn2O4 nanoparticles in the pristine state and after various CV cycles towards OER. Our XAS data reveals an apparent increase in oxidation state and an increase in the octahedrally coordinated Co after 500 CV cycles, implying that tetrahedrally-coordinated Co2+ might serve as the active sites. Furthermore, we observed an Mn-lean region by APT and an amorphous surface layer by HRTEM in CoMn2O4 after 500 CV cycles, which is consistent with the Inductively coupled plasma mass spectrometry (ICP-MS) results showing a preferential Mn leaching. In contrast, the surface concentration remains unchanged after 20 CV cycles for Co2MnO4, suggesting most likely the formation of stable active species. Overall, our work demonstrates that Co2MnO4 outperforms CoMn2O4 as the Mn leaching and surface amorphization are inhibited during OER.

References

[1] J.T. Mefford; A.R. Akbashev; M. Kang et al. Nature, 2021, 593, 67-73.

[2] A. Li; S. Kong; C. Guo et al. Nature Catalysis, 2022, 5, 109-118.

[3] W. Wang; L. Kuai; W. Cao et al. Angewandte Chemie, 2017, 129, 15173-15177.

[4] J. Villalobos; D.M. Morales; D. Antipin et al. ChemElectroChem, 2022, 9, e202200482.