LAUSR

Abstract We previously reported several manganese porphyrin dimers as the first manganese-containing molecular catalysts for water oxidation, however, in non-aqueous CH 3 CN solution containing 5% H 2 O. Here, we successfully fabricated these dimers with mono- and hexaphosphonic acid groups, Mn 2 DP-PO 3 H 2 and Mn 2 DP-(PO 3 H 2 ) 6 , to covalently assemble them on the surface of ITO electrode (ITO = indium-doped tin oxide) and use the Mn 2 DP-PO 3 H 2 and Mn 2 DP-(PO 3 H 2 ) 6 |ITO assemblies as heterogeneous catalysts for electrochemical water oxidation in aqueous buffer solutions. The mono-phosphonic acid fabricated assemblies showed unprecedent high turnover frequencies (TOFs) (up to 44.9 s −1 ) at a low overpotential ( η  = 0.47 V) in a neutral buffer solution. In acidic buffered solutions (pH = 1.5), they showed higher TOFs (up to 47.4 s −1 ) at a very low overpotential ( η  = 0.26 V). The robustness of the mono-phosphonic acid fabricated catalysts, Mn 2 DP-PO 3 H 2 |ITO, was tested at a high overpotential ( η  = 0.80 V). Although they showed an oxygen evolution with 178.3 s −1 TOF, the oxygen evolution completely stopped after 11 h electrolysis. UV–vis spectra monitored during the electrolysis clearly indicated the gradual detachment of the catalysts from the ITO surface is likely the main reason of stopping the oxygen evolution. The hexa-phosphonic acid catalyst assembly, Mn 2 DP-(PO 3 H 2 ) 6 |ITO, however, showed a continuous oxygen evolution without stopping even after 23 h of electrolysis with 199.3 s −1 TOF. Tafel plots in different pHs give insights on the mechanism of H 2 O oxidation.