Targeted improvement of the low efficiency of water oxidation during the oxygen evolution reaction (OER) is severely hindered by insufficient knowledge of the electrocatalytic mechanism on heterogeneous surfaces. We chose LiMn2O4 as a model system for mechanistic investigations as it shares the cubane structure with the active site of photosystem II and the valence of Mn3.5+ with the dark‐stable S1 state in the mechanism of natural photosynthesis. The investigated LiMn2O4 nanoparticles are electrochemically stable in NaOH electrolytes and show respectable activity in any of the main metrics. At low overpotential, the key mechanistic parameters of Tafel slope, Nernst slope, and reaction order have constant values on the RHE scale of 62(1) mV dec−1, 1(1) mV pH−1, −0.04(2), respectively. These values are interpreted in the context of the well‐studied mechanism of natural photosynthesis. The uncovered difference in the reaction sequence is important for the design of efficient bio‐inspired electrocatalysts.
An improved understanding of the correlation between the electronic properties of Mn-O bonds, activity and stability of electro-catalysts for the oxygen evolution reaction (OER) is of great importance for an improved catalyst design. Here, an in-depth study of the relation between lattice structure, electronic properties and catalyst performance of the perovskite Ca1−xPrxMnO3 and the first-order RP-system Ca2−xPrxMnO4 at doping levels of x = 0, 0.25 and 0.5 is presented. Lattice structure is determined by X-ray powder diffraction and Rietveld refinement. X-ray absorption spectroscopy of Mn-L and O-K edges gives access to Mn valence and covalency of the Mn-O bond. Oxygen evolution activity and stability is measured by rotating ring disc electrode studies. We demonstrate that the highest activity and stability coincidences for systems with a Mn-valence state of +3.7, though also requiring that the covalency of the Mn-O bond has a relative minimum. This observation points to an oxygen evolution mechanism with high redox activity of Mn. Covalency should be large enough for facile electron transfer from adsorbed oxygen species to the MnO6 network; however, it should not be hampered by oxidation of the lattice oxygen, which might cause a crossover to material degradation. Since valence and covalency changes are not entirely independent, the introduction of the energy position of the eg↑ pre-edge peak in the O-K spectra as a new descriptor for oxygen evolution is suggested, leading to a volcano-like representation of the OER activity.
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