Effect of Semiconducting Properties of Oxide-Based Compounds on Oxygen Reduction Activity in Acidic Media

Abstract

Polymer electrolyte fuel cells (PEFCs) are attractive as energy conversion systems with high conversion efficiency. Although theoretical energy conversion efficiency is high, 83% at 295 K, the actual efficiency is very low because of a large overpotential of oxygen reduction reaction (ORR). In addition, the durability of the catalysts becomes important for commercialization of PEFCs. Pt supported carbon cathode is easy to degrade at high potential region and potential fluctuation. Therefore, we have focused on group 4 and 5 oxide-based compounds as oxides with active sites and electro-conductive supports. We successfully demonstrated that the surface of the oxides had high quality active sites [1] and superior durability against start/stop and load cycle tests [2].Recently, DFT calculation by Sugino’s group revealed that Pd or Rh doped TiO2 had possibility that these surfaces reached equilibrium potential for the ORR [3]. However, the DFT calculation was only based on the adsorption Gibbs energies of reaction intermediates. Considering the actual active sites, sufficient electron supply is necessary to proceed ORR fluently. Group 4 and 5 oxide-based compounds are essentially semi-conductors. Therefore, the semi-conducting physico-chemical properties such as carrier density and Flat-band potential affect the supply of electrons to the active sites. However, the effect of the semi-conducting properties of the oxide-based compounds on the oxygen reduction activity in acidic media has not been investigated yet. In this study, we attempted to reveal the effect of Flat-band potential and capacitance of space charge layer on the ORR activity. Thus, we focused on titanium oxides and prepared Nb, Ta, and Zr added titanium oxide to investigate the effect of foreign element doping.The oxide-coating film was prepared on a Ti plate (10 mm× 10 mm× 1 mm) substrate by the conventional dip-coating method using a 0.5 M butanolic solution of metal salt: titanium tetrabutoxide, zirconium tetrabutoxide, niobium pentaethoxide or tantalum pentaethoxide. The titanium substrate was etched with 10wt% oxalic acid at 80 °C for 1 h and then rinsed with deionized water before the dipping procedure. Calcination of the dip-coated salts was con-ducted in air at 350 °C. The dip-drying/calcination (alternating 10 min each) procedure was typically repeated 15 times.Electrochemical measurements were carried out using a three-electrode cell at 30 ± 1 °C in 0.1 M H2SO4. Cyclic voltammetry was performed with 5 mV sec-1 in the potential range from 0.2 to 1.2 V in an oxygen and nitrogen atmosphere to obtain oxygen reduction current. The oxygen reduction current density, i ORR, was normalized by the geometric area of the electrode. The ORR activity was evaluated by onset potential for the ORR, E onset, defined as the electrode potential at i ORR = 0.02 mA/cm2 and i ORR at 0.2 V, |i ORR@0.2V|.Electrochemical impedance measurements were performed with amplitude of 0.01 V and frequency of 1000 Hz to obtain capacitance of space charge layer, C scl, every 0.1 V below 1.1 V. In addition, the values of the (1/C scl)2 were plotted against the electrode potential, that is, Mott-Schottky plot, to obtain the flat-band potential, E fb.Figure 1 shows the relationship between E fb and E onset of Nb, Ta, or Zr doped TiO2 electrodes. The relation of E onset－E fb≒0.3 V was obtained and it was independent of the doped elements. The difference between the electrode potential and E fb corresponded to the barrier height of space charge layer. When the electrode potential is higher than the E onset, the electrons cannot pass through the barrier of the space charge layer, resulting that the ORR cannot proceed because the supply of electrons from bulk of semi-conductor to the active sites at the interface between oxide-electrolyte. When the electrode potential reaches at Eonset, the ORR starts to proceed due to the continuous supply of the electrons because the electrons can pass through the barrier. According to the relation of E onset－E fb≒0.3 V, the barrier height of the space charge layer, which the electrons can pass through, was estimated to be 0.3 V.AcknowledgementThe authors wish to thank the support of the New Energy and Industrial Technology Development Organization (NEDO).