Fig. 5: Evidence of lattice oxygen as reaction site under OER conditions.

In situ XPS spectra of Mn 2p (a) and O 1 s (b) of NaMn₃O₇ under open circuit, 1.25 and 1.55 V (versus RHE). c The process of oxygen vacancy refilling under 1.23 V and the associated energy barrier. d pH dependence of the OER activities of NaMn₃O₇. The inset is the proton reaction order estimated by ρ^RHE = (∂log(i)/∂pH), with ρ^RHE and i being the proton reaction order and current density at a fixed potential of 1.55 V vs. RHE. e Schematic OER pathway of acid–base nucleophilic attack involving the rate-limiting proton transfer decoupled from electron transfer step. The inset illustrates the formation of negatively charged oxide surface when equilibrated with the electrolyte. f Raman spectra of NaMn₃O₇ electrodes. These electrodes were respectively operated at 1.55 V versus RHE in 1 M KOH (black line) and TMAOH (red line) solution, then were thoroughly washed by rinsing with high-purity water and acetone for ex situ Raman spectroscopy measurement.