Influence of the Dopant Atom and the Oxygen Vacancy Site for the Oxygen Reduction Reaction on the Titanium Oxide
Authors: Saida Takahiro , Suzuki Tomoya , Niwa Etsuko , Maruyama Takahiro 
Published: February 2020

Polymer electrolyte membrane fuel cells (PEMFCs) are expected as one of the next generation power sources because its exhaust gas is, theoretically, only water. On the other hand, the selling price of the PEMFC mounted devices expensive, and this price inhibits the wide-spread commercialization. It is one reason for this thing that the platinum-based catalyst, which is high cost and low durability, is used as the electrocatalyst and is required large quantity to obtain the cell performance. As one of the ultimate solutions for this problem, usage of the non-platinum oxide catalyst is proffered. Recently, the group 4 and 5 transition-metal oxides are attracted attention as the non-platinum oxide catalyst since these show the activity of oxygen reduction reaction (ORR) and the high durability. In these oxides, introducing of the oxygen vacancy site in oxide surface to get high ORR activity is the common design guide for the new catalyst development. Addition, it is well known that doping niobium to the titanium oxide with the oxygen vacancy site is also effective for obtaining the high ORR activity. In other words, introducing the oxygen vacancy site and doping the other metal atom is necessary to prepare for the high ORR catalyst based on group 4 and 5 transition-metal oxide in this time.
Although the active site for ORR on the oxide surface is not clear, it is speculated that the oxygen vacancy site would play the important role for ORR. However, the relationship between the dopant atom and the oxygen vacancy site has not been discussed enough in the viewpoint from ORR. In fact, the dopant atom in the previous works of literature in acidic environmental is almost the niobium, and other atoms are very few. The other hand, it is reported the calculated formation energy of oxygen vacancy site on anatase Ti
M
O2 (101) depends on the dopant atom. Therefore, this study describes the influence of the dopant atom and the oxygen vacancy site on the ORR activity of titanium oxide. In this study, niobium, manganese, and ruthenium were chosen as the dopant atom because their formation energy of oxygen vacancy site decreased step by step in literature.
All samples were prepared by the typical sol-gel method. The oxygen vacancy site was introduced by the chemical reduction method using the sodium borohydride. Electrochemical studies were conducted in Ar- or O2-saturated 0.1 M HClO4 (60ºC) with carbon plate counter-electrode and Ag/AgCl reference-electrode. The ORR activity was evaluated by the difference current of current in Ar-saturated condition from the current in O2-saturated condition. The on-set potential of ORR current was notated as
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on-set in bellow.
Crystal structures before chemical reduction treatment were a mixture of anatase and rutile in all dopant species. The crystallinity of samples was decreased by chemical reduction treatment, but the clear phase transition did not observe. In the case of niobium doping, the
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on-set was increased to ~0.6 V from ~0.4 V by the chemical reduction treatment. This phenome agreed with previous works, and it is thought that the enhancement effect was provided by the formation of the oxygen vacancy site. The ruthenium doped titanium oxide was also started ORR at ~0.4V. Thus, the difference of enhancement effect between niobium and ruthenium for ORR activity of titanium oxide was little before introducing the oxygen vacancy site. After introducing the oxygen vacancy site,
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on-set of ruthenium doped titanium oxide was ~0.75 V. In addition, its ORR current was the highest in this study. If the ruthenium dopant was removed from titanium oxide lattice and formed the ruthenium oxide during the chemical reduction treatment, its quantity of ruthenium oxide is very low. Because the doping ratio of ruthenium was 5 mol%. Moreover, the
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on-set of ruthenium oxide was ~0.6 V which was a lower value than that of reduced ruthenium doped titanium oxide. After the introducing oxygen vacancy site, the enhancement effect for ORR ability depended on the dopant species. Herein, the formation energy of oxygen vacancy site of niobium doped titanium oxide was higher than that of ruthenium doped titanium oxide. In ruthenium doped titanium oxide with oxygen vacancy site, it is speculated that the oxygen molecule is easier adsorption/desorption in comparison with niobium doped titanium oxide.
At this moment, we think that the ORR activity of titanium oxide depends by the dopant atom but needs not only dopant species but also the oxygen vacancy site.