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

Abstract A [H 3 Ag I (H 2 O)PW 11 O 39 ] 3− -TiO 2 /ITO electrode was fabricated by immobilizing a molecular polyoxometalate-based water oxidation catalyst, [H 3 Ag I (H 2 O)PW 11 O 39 ] 3− (AgPW 11 ), on a TiO 2 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 Na 2 SO 4 solution (0.1 mol L −1 ). We found that a higher applied voltage led to better catalytic performance by AgPW 11 . The AgPW 11 -TiO 2 /ITO electrode gave currents respectively 10 and 2.5 times as high as those of the TiO 2 /ITO and AgNO 3 -TiO 2 /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 AgPW 11 -TiO 2 /ITO electrode. Under illumination, the photocurrent was not obviously enhanced although the total anode current increased. The AgPW 11 -TiO 2 /ITO electrode was relatively stable. Cyclic voltammetry of AgPW 11 was performed in phosphate buffer solution (0.1 mol L −1 ). We found that oxidation of AgPW 11 was a quasi-reversible process related to one-electron and one-proton transfer. We deduced that disproportionation of the oxidized [H 2 Ag II (H 2 O)PW 11 O 39 ] 3− might have occurred and the resulting [H 3 Ag III OPW 11 O 39 ] 3− oxidized water to O 2 .