Screen‐Printed Calcium–Birnessite Electrodes for Water Oxidation at Neutral pH and an “Electrochemical Harriman Series”

A mild screen‐printing method was developed to coat conductive oxide surfaces (here: fluorine‐doped tin oxide) with micrometer‐thick layers of presynthesized calcium manganese oxide (Ca–birnessite) particles. After optimization steps concerning the printing process and layer thickness, electrodes were obtained that could be used as corrosion‐stable water‐oxidizing anodes at pH 7 to yield current densities of 1 mA cm −2 at an overpotential of less than 500 mV. Analyses of the electrode coatings of optimal thickness (≈10 μm) indicated that composition, oxide phase, and morphology of the synthetic Ca–birnessite particles were hardly affected by the screen‐printing procedure. However, a more detailed analysis by X‐ray absorption spectroscopy revealed small modifications of both the Mn redox state and the structure at the atomic level, which could affect functional properties such as proton conductivity. Furthermore, the versatile new screen‐printing method was used for a comparative study of various transition‐metal oxides concerning electrochemical water oxidation under “artificial leaf conditions” (neutral pH, fairly low overpotential and current density), for which a general activity ranking of RuO 2 >Co 3 O 4 ≈(Ca)MnO x ≈NiO was observed. Within the group of screened manganese oxides, Ca–birnessite performed better than “Mn‐only materials” such as Mn 2 O 3 and MnO 2 .