Investigation of Active Factors for Oxygen Reduction Reaction by Titanium Oxide Model Electrode

Polymer electrolyte fuel cells (PEFCs) are one of the low-environmental-impact power sources. However, the broad commercialization of PEFCs has been slow. One reason for this is that the electrocatalyst of PEFCs is the high manufacturing cost and low durability. Titanium oxide-based catalysts have attracted attention as the candidate for low-cost and highly durable electrocatalysts. Titanium oxide-based catalysts need to address the drawback of poor catalytic activity.<br/>Currently, introducing the oxygen vacancies and doping the other metal atoms are known to enhance the oxygen reduction-reaction (ORR) activity on titanium oxide. Generally, the catalytic reaction progresses at the low coordination site on the surface. Introducing oxygen vacancies leads to an increase in low-coordination sites, which means increasing the activity sites. As a result, the ORR activity of titanium oxide is improved by introducing oxygen vacancies. In addition, the oxygen vacancy changes the electron structure of titanium oxide and provides electron conductivity. From the results of the DFT calculation, it was reported that the low-coordination metal site, which was formed by oxygen vacancy, and the neighborhood structural oxygen played an important role in ORR. On the other hand, the structural analysis results using the pair distribution functions (PDFs) method using X-ray showed that the active site was related to the crystal distortion site and no confirmed oxygen vacancies. One of the problems in titanium oxide-based catalysts is that the active site and reaction mechanism are still unclear. For this reason, the design of new catalysts must rely on rules of thumb. Clarifying the active control factor for ORR on titanium oxide is one of the effective approaches to the high activity close to platinum-based catalysts.<br/>In this study, a titanium oxide model electrode formed by a mono-layer of the titanium oxide nanosheets was used to separate the effects from the low coordination site and the crystal distortion. A titanium oxide nanosheet is composed of octahedrons centered on Ti atoms. Thus, the variation of Ti valence is linearly related to the number of low-coordination sites.<br/>The crystal distortion was evaluated by in-plane XRD and grazing-incidence XAFS measurements in synchrotron radiation facilities. In particular, the local coordination structure of the titanium-oxide model electrode was investigated from pre-peak before the adsorption edge and EXAFS region in the obtained XAFS spectrum. The Ti valence was estimated from the energy of half value at the edge jump intensity in the XAFS spectrum. The number of oxygen vacancies on a titanium-oxide model electrode was roughly controlled by the calcination temperature with hydrogen flow conditions. Electrochemical measurements were performed in a three-electrode cell with 0.1 M HClO₄ electrolyte.<br/>The structure of the titanium-oxide model electrode was changed from the lepidocrocytes structure to the anatase structure with increasing calcination temperature. The local coordinate structure was also transformed with increasing calcination temperature. At that time, the Ti valence was gradually elevated from 3.47 to 3.92. The ORR activity of the model electrode was enhanced by the calcination with hydrogen flowing until the beginning of the phase transition to the anatase structure. The distortion of the crystal structure was most significant after the phase transition. However, the ORR activity was low compared to before the phase transition. Introducing the oxygen vacancies to the titanium-oxide model electrode clearly enhanced the ORR activity and gave its structure a very slight crystal distortion. At this time, it is forecasted that the crystal distortion is more effective in ORR activity on titanium oxide rather than the number of oxygen vacancies. However, the permissible crystal distortion rate may be narrow, and the phase transition probably leads to the degradation of the ORR activity.