3 years ago

Covalent bonds immobilization of cofacial Mn porphyrin dimers on an ITO electrode for efficient water oxidation in aqueous solutions

Covalent bonds immobilization of cofacial Mn porphyrin dimers on an ITO electrode for efficient water oxidation in aqueous solutions
We previously reported several manganese porphyrin dimers as the first manganese-containing molecular catalysts for water oxidation, however, in non-aqueous CH3CN solution containing 5% H2O. Here, we successfully fabricated these dimers with mono- and hexaphosphonic acid groups, Mn2DP-PO3H2 and Mn2DP-(PO3H2)6, to covalently assemble them on the surface of ITO electrode (ITO=indium-doped tin oxide) and use the Mn2DP-PO3H2 and Mn2DP-(PO3H2)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 44.9s−1) at a low overpotential (η =0.47V) in a neutral buffer solution. In acidic buffered solutions (pH = 1.5), they showed higher TOFs (up to 47.4s−1) at a very low overpotential (η =0.26V). The robustness of the mono-phosphonic acid fabricated catalysts, Mn2DP-PO3H2|ITO, was tested at a high overpotential (η =0.80V). Although they showed an oxygen evolution with 178.3s−1 TOF, the oxygen evolution completely stopped after 11h 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, Mn2DP-(PO3H2)6|ITO, however, showed a continuous oxygen evolution without stopping even after 23h of electrolysis with 199.3s−1 TOF. Tafel plots in different pHs give insights on the mechanism of H2O oxidation.

Publisher URL: www.sciencedirect.com/science

DOI: S002195171730194X

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