Manganese-Oxide Solids as Water-Oxidation Electrocatalysts: The Effect of Intercalating Cations

A set of birnessite-type manganese oxides with both monovalent alkaline cations (Li+, Na+, K+, Rb+ or Cs+) and divalent alkaline- earth cations (Ca2+ or Mg2+) has been synthesized using electrolytic deposition methods. X-ray diffraction gives typical birnessite-type patterns, with diffraction peaks at 2θ = ∼12.01° and 24.7° assigned to (001) and (002) planes. Electron paramagnetic resonance spectra suggest differences in the stability of Mn(III) species in manganese-oxide layer, which is more stable in birnessite samples with divalent cations than samples with monovalent cations. The activity of samples for an anodic O2-evolution reaction is considerably affected by electrostatic potential and Lewis acidity of the monovalent cation, following the sequence of Li+ > Na+ > K+ > Rb+ > Cs+. While for samples with intercalating divalent alkaline-earth cations, there was slightly lower activity for O2 evolution, which may be correlated to different stability of Mn(III) species in the birnessite layers. In addition, we observed much higher O2-evolution activity at an overpotential of ca. 220 - 230 mV if the samples were heat-treated between 300 - 500 °C once they were electrolytically deposited as LiMnOx films. Clearly dry heating causes changes in both the crystalline structure and electronic structure of manganese-oxide layers, as indicated by associated changes in water-oxidation activity