Catalytic effects of [Ag(H2O)(H3PW11O39)]3− on a TiO2 anode for water oxidation

Abstract A [H3AgI(H2O)PW11O39]3−-TiO2/ITO electrode was fabricated by immobilizing a molecular polyoxometalate-based water oxidation catalyst, [H3AgI(H2O)PW11O39]3− (AgPW11), on a TiO2 electrode. The resulting electrode was characterized by X-ray powder diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Linear sweep voltammetry, chronoamperometry, and electrochemical impedance measurements were performed in aqueous Na2SO4 solution (0.1 mol L−1). We found that a higher applied voltage led to better catalytic performance by AgPW11. The AgPW11-TiO2/ITO electrode gave currents respectively 10 and 2.5 times as high as those of the TiO2/ITO and AgNO3-TiO2/ITO electrodes at an applied voltage of 1.5 V vs Ag/AgCl. This result was attributed to the lower charge transfer resistance at the electrode-electrolyte interface for the AgPW11-TiO2/ITO electrode. Under illumination, the photocurrent was not obviously enhanced although the total anode current increased. The AgPW11-TiO2/ITO electrode was relatively stable. Cyclic voltammetry of AgPW11 was performed in phosphate buffer solution (0.1 mol L−1). We found that oxidation of AgPW11 was a quasi-reversible process related to one-electron and one-proton transfer. We deduced that disproportionation of the oxidized [H2AgII(H2O)PW11O39]3− might have occurred and the resulting [H3AgIIIOPW11O39]3− oxidized water to O2.