Emergence of Oxygen Reduction Activity in Niobium-Doped Titanium Oxides As Non-Platinum Cathode for PEFCs

Development of non-platinum electrocatalysts for oxygen reduction reaction (ORR) is required for wide commercialization of polymer electrolyte fuel cells. We focused on titanium oxide-based electrocatalysts, because of their high chemical stability and much less expensive than platinum. We found that Ti-Nb complex oxide mixed with magneli phase titanium oxide (Ti4O7) had high ORR onset potential and superior durability [1]. However, the ORR current was small because of their large particle size. In this study, we have tried to prepare titanium oxide-based nano particles using multi-walled carbon nano-tubes (CNTs) as support via hydrolysis method. We examined the effect of heat-treatment condition such as temperature and time on the ORR activity. We prepared Nb doped titanium oxide [mass ratio of TiO2:Nb2O5= 8:2] supported CNTs with total oxide mass ratio of 20wt%. These catalysts were heat treated at 600-900 oC for 0-60 min under 4%H2/Ar atmosphere. The catalyst without heat-treatment had poor ORR activity. The heat-treatment under reductive atmosphere enhanced the ORR activity of the catalysts. Figure 1 shows the dependence of the ORR current at 0.6 and 0.7 V of the catalysts prepared at several temperatures for 10 min under 4%H2/Ar in 0.1M H2SO4 at 30 oC. The catalyst prepared at 800 oC showed highest ORR activity. The SEM observation indicated that the sizes of the oxide-based particles were around 20 nm for all catalysts with and without heat treatment. Therefore, the surface area of the oxide-base particles was almost the same. The dependence of the ORR activity was not responsible for the change of the surface area. The XRD patterns revealed that the catalyst without heat treatment was composed of anatase phase of TiO2, and the peaks due to the rutile TiO2 apparently appeared at around 700-800oC. In particular, the peaks due to Rutile TiO2 shifted at lower angular, suggesting that the niobium doped into the TiO2 phase. Thus, the Nb doping into rutile TiO2 could affect the ORR activity. Figure 2 shows the Ti 2p XPS spectra of these catalysts. As shown in Fig.2, the peak at 457.8 eV corresponding to Ti3+ (Ti2O3) as well as the peak at 459.2 eV corresponding to Ti4+ (TiO2) were observed. The peak due to Ti3+ increased from 600 to 800 oC, and decreased at 900oC. This behavior was well corresponded to the ORR activity. Therefore, we concluded that Ti3+ could act as active sites for the ORR. According to the XRD patterns, the Nb doping into rutile TiO2 proceeded with increasing the temperature from 700 to 800oC. The niobium doping generated the Ti3+ via electro-neutral principle. The formation of the Ti3+ in TiO2 rutile phase was considered to be essential for emergence of the ORR. [1] A. Ishihara et al., J. Electrochem. Soc, 163(7), F603 (2016). Figure 1