Photocatalytically Generated Pt/C-TiO2 Composite As ORR Catalyst
Authors: Vedarajan Raman , Badam Rajashekar , Matsumi Noriyoshi 
Published: February 2020

Metal decorated carbon materials have grabbed the attention of the world due to its tremendous application in various paths of life. Among the various metals decorated over carbon, transition metals like Pt is more talked about when it comes to catalytic reactions in energy devices. Very recently we reported exfoliated acetylene black/Pt nano-particles showing extraordinarily high ORR activity [1]. However, the general methods used for decoration of metal nanoparticles on to carbon substrates are basically carried out by employing sacrificial reducing agents (SRA) like alcohols or other reducing agents at elevated temperatures, which are against to the green approach. Lookout for alternative green procedures has recently ended up in looking into exploiting photo-reduction processes. However, research dealing with the photo-reduction of metal salts to metals in aqueous medium without a SRA is not fully studied [2]. Alongside ‘green’ aspect, large scale commercial applications are precluded by the high manufacturing cost of the ORR catalyst. Hence, researcher have been focusing on reducing the cost of the catalyst by decreasing the amount of Pt used with the introduction of transition metal oxides and doping of nitrogen in carbon. The present work deals with the photo-reduction of chloroplatinic acid on to graphite and graphene using TiO2as photo-catalyst irradiated by solar simulator. Graphite (carbon substrate) weighing 90mg was ultrasonicated in water for 2 hrs to obtain a uniform dispersion. To this uniform dispersion, 10 mg of TiO2and 800 μL of 0.045 M chloroplatinic acid solution was added and ultrasonicated for 15 min. The solution was irradiated using solar simulator for 5 hrs. The reaction completion time was confirmed by studying the open-circuit potential of the solution across Pt wire working electrode and Ag/AgCl reference electrode. The material was then subjected to morphological characterization by TEM and elemental analysis by XPS. Further, the material was studied for its ORR behavior using cyclic voltammetry. TEM micrographs showed uniform distribution of Pt-nps all over the graphite along with TiO2. Platinum nano particles were in the range of 5 to 10 nm in size Fig1 (A and B). Elemental characterization by XPS (Fig 1C) revealed the presence of Pt, O, Ti and C along with trace amounts of chlorine given in Fig. 1D. The reaction completion time was studied using change in the open circuit potential (OCP), which was found to be nearly 4 hrs (Fig. 1E). Oxygen reduction characteristics of the electrocatalyst was studied using cyclic voltammetry. The voltammograms (Fig 1F) confirmed that the prepared material to be a promising catalytic material. The electrochemical active surface area (ECSA) was calculated as 62.24 m2/gm. The ECSA was observed to be significantly high compared to commercial Pt/C catalyst. In conclusion, a highly reproducible, environmentally benign and ultrafast method was developed for the preparation of active ORR catalyst (for use in Fuel cell and Lithium-air battery). The method uses solar light as the only source of energy in the preparation of this catalytically efficient composite utilizing abundant and environmentally benign TiO2 as photo-catalyst. Using this green methodology, preparation of novel composites containing TiO2 particles, graphite substrates and platinum nanoparticles was successfully carried out. As prepared material showed excellent ORR activity comparable to the best of commercial ORR catalysts. It can also be confirmed that this method is feasible for all the conducting carbon substrates. The above results clearly demonstrates that the novel, green and a very viable method of Pt decoration method is as efficient as rather more efficient as that of well-established methods using SRA.

References 1) R. Badam, R. Vedarajan, N. Matsumi, Chem. Commun., 2015, 51, 9841. 2) F. Mahlamvana, R.J. Kriek, Appl. Cat. B, 2014, 148-149, 387.

Figure 1