Here we demonstrate a remarkable enhancement of oxygen reduction reaction (ORR) activity on a novel Pt/TaO x /GC electrocatalyst where at first tantalum oxide (TaO x ) and next Pt were deposited electrochemically on a glassy carbon (GC) surface. An excellent electrocatalytic activity of the Pt/TaO x /GC electrocatalyst for ORR was found to be more than 12 times that of the unmodified Pt/GC one as evaluated from the kinetic currents at 0.80 V. SEM images showed no significant differences in the size and distribution of Pt nanoparticles between these two electrocatalysts, indicating that these are not factors causing the observed ORR activity. The spillover of oxygen-containing species resulting from the electronic interaction between Pt and TaO x , which is evidently demonstrated from the XPS analysis, is strongly suggested as the crucial factor for the ORR enhancement. Interestingly, the spillover effect also results in a remarkable increase in the electrochemically active “apparent” surface area of Pt on the Pt/TaO x /GC electrocatalyst. Moreover, the rotating ring-disk electrode voltammetric measurements obviously showed the increase in limiting current as well as the decrease in ring current on this novel electrocatalyst relative to the unmodified one, confirming a complete four-electron reduction pathway. On the basis of these findings a plausible mechanism has been proposed for the observed enhancement in ORR where the role of TaO x is to reduce the formation of OH on the Pt surface by spillover effect and to promote d orbital vacancy of Pt for oxygen adsorption by electron donation to Ta.

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Through the use of XPS and controlled Ar + etching, the surface composition and oxide species of tantalum oxides (TaO x ), which were electrodeposited on glassy carbon electrodes by cyclic voltammetric and constant-potential electrolyses, are quantified along the depth profile. Electrodeposition exhibits efficacy in depositing TaO x with a distribution of various TaO x : TaO, TaO 2 , and Ta 2 O 5 . The distribution gradient from the outer surface of TaO x is such that Ta 2 O 5 > TaO 2 >TaO. TaO is found to be the dominant species in the underlying layer of TaO x . Such a unique structure of the electrode surface is analogous to that of nanoparticles with a core-shell structure, with the core being suboxides and the surface being that of the saturated pentoxide, Ta 2 O 5 . The electrochemically induced nonhydrolytic condensation route is proposed to be capable of producing TaO x with a distribution gradient of Ta 2 O 5 , TaO 2 , and TaO in the depth direction.

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