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We investigate the effect of the surface electronic structure and compositionof LiMn$_2$O$_4$ nanoparticles on the electrocatalytic oxygen evolutionreaction (OER). Scanning transmission electron microscopy (STEM) electronenergy loss spectroscopy (EELS) studies combined with density functional theory(DFT) based simulations of the EEL spectra reveal in pristine nanoparticles a 4nm thick surface layer with reduced average Mn oxidation state and increased Mnconcentration. This is attributed to Mn$^{2+}$ partially replacing Li$^+$ atthe tetrahedral sites of the spinel lattice accompanied by Mn 3d-state fillingof octahedrally coordinated Mn. During electrocatalytic OER cycling, thisnear-surface tetrahedral Mn is leached out, thereby increasing the oxidationstate of octahedrally coordinated Mn. Using rotating ring-disc electrode (RRDE)based detection of O and Mn during the OER, we show that the oxygen evolutionremains constant while the Mn$^{2+}$ is removed, revealing that near-surfacetetrahedrally coordinated Mn has no effect on the OER activity ofLiMn$_2$O$_4$. This is surprising since the e$_g$ occupancy of Mn in octahedralsites, which is widely used as a descriptor of OER activity, changessignificantly in the surface layer during cycling. The fact that e$_g$ emptyingfails to correlate with OER activity here indicates that octahedral cationvalence is not a fundamental measure of activity, either because the activesurface state is not affected by tetrahedral Mn or because other details of theband structure or metal-oxygen bonding character, more strongly regulate therate-limiting steps for OER.

A criticial view on e$_g$ occupancy as a descriptor for oxygen evolution catalytic activity in LiMn$_2$O$_4$ nanoparticles