Preparation of Titanium Oxynitride-Based Platinum-Free Cathode Catalyst for Polymer Electrolyte Fuel Cells by Ammonia Nitridation of Titanium Complexes

The 4^th and 5^th groups transition metal oxides have been explored as alternatives to platinum cathode catalysts in polymer electrolyte fuel cells. The oxygen reduction reaction (ORR) activity of oxide metal catalysts remains insufficient, and further improvement is required. The low ORR activity is attributed to two major factors: the quality of ORR active sites on metal oxide surface is poor and the low electronic conductivity inherent to metal oxides. Previous studies have reported that appropriate doping of N or Fe into metal oxides can improve the quality of ORR active sites. Also, the formation of electron-conduction path, such as carbon between the active sites on the metal oxides and the current collector, has been reported to effectively enhance electron transfer. In this study, we prepared metal oxynitride catalysts by ammonia nitridation of metal complexes. This approach aims to improve the quality of active sites by doping N into the metal oxides through ammonia nitridation, while utilizing carbon residues derived from the complexes during heat treatment as electron-conduction path. We investigated the relationship between catalyst structure and ORR activity. Furthermore, we discuss guidelines for catalyst preparation to achieve higher ORR activity.
Titanium-polyacrylic acid complex was synthesized as a catalyst precursor. The precursor was subjected to ammonia nitridation using a tubular furnace or rotary kiln to prepare the titanium oxynitride catalysts. The catalyst structure was controlled by adjusting the heat treatment conditions, including temperature, duration, and oxygen concentration in the ammonia atmosphere. The temperature ranged from 600°C to 1000°C, the duration from 1 to 6 hours, and the oxygen concentration in the ammonia atmosphere from 0% to 0.5%. The catalyst structure was evaluated using TEM, SEM, XRD, XAS, TG, and powder resistivity measurements. The electrochemical properties were evaluated using a static three-electrode cell.
TEM images revealed that the prepared samples had a morphology where nanoparticles were embedded in carbon residues. Each sample exhibited XRD patterns corresponding to titanium oxynitride phases. The diffraction peak positions varied according to the heat treatment conditions. Lower temperatures or shorter durations or higher oxygen content in the treatment atmosphere conditions tended to shift the diffraction peaks towards the TiO side. By adjusting the heat treatment conditions, the compositions determined with Vegard’s law varied from TiO_0.29N_0.71 to TiO_0.91N_0.09. Regarding the carbon residues, lower temperature or shorter durations conditions resulted in more carbon residue formation around the titanium oxynitride particles. However, these carbon residues showed little electronic conductivity. Conversely, under more intense heat treatment conditions, the amount of carbon residue decreased, but its electronic conductivity improved.
ORR activity was evaluated using Linear Sweep Voltammetry in an acidic medium, based on the open circuit potential and current density. The open circuit potential tended to be higher when the nitrogen doping level in the titanium oxynitride was lower. The current density values were higher when the titanium oxynitride had a low level of nitrogen doping and sufficient formation of electroconductive carbon residues.
To achieve higher current densities, it is promising to dope a small amount of nitrogen into titanium oxide while forming a large amount of electroconductive carbon residues. On the other hand, various experiments results suggest that there is a trade-off between the nitrogen doping level in titanium oxide and the amount of electroconductive carbon residues. To achieve higher activity, it will be crucial to find a balance within this trade-off relationship and explore strategies to overcome these limitations.