Platinum-Nickel (Pt3Ni) Alloy Nanoparticles Decorated Graphene As a Catalyst of Oxygen Electrode in Li-O2 Cells

Development of an appropriate bi-functional catalyst for reversible O₂ electrode in rechargeable Li-O₂ cells is a challenging task. Graphene is an emerging material for several applications owing to its unique properties. In the present study, Pt₃Ni alloy nanoparticles anchored onto reduced graphene oxide (RGO) are prepared and investigated as catalyst for Li-O₂ cells. For the preparation of Pt₃Ni-RGO, commercial graphite powder is oxidized and exfoliated to form graphene oxide (GO). The required quantities of chloroplatinic acid and nickel chloride are dissolved in an aqueous suspension of GO and then reduced by NaBH₄ so that the mass ratio of Pt₃Ni to RGO is unity.  Pt⁴⁺ and Ni²⁺ ions adsorbed on GO sheets undergo reduction simultaneously with the reduction of GO to form Pt₃Ni nanoparticles decorated RGO. The formation and morphology of Pt₃Ni-RGO are studied by using powder XRD and TEM. The microscopy image (Fig. 1) indicates the presence of alloy nanoparticles distributed over RGO sheets.

The catalytic activity of Pt₃Ni-RGO is investigated  by cyclic voltammetry and rotating disk electrode (RDE) studies in a non-aqueous electrolyte containing tetrabutyl ammonium perchlorate. Cyclic voltammograms recorded at several sweep rates consist of O₂ reduction current peaks as well as oxygen evolution current peaks. Both peak currents increase with square root of sweep rate following the Randles Sevcik equation. The RDE studies provide diffusion limited current at several speeds of RDE. The data follow Levich and Koutecky-Levich equations. The values of number of electrons and diffusion coefficient of O₂ calculated using these equations are 0.93 and 1.75 x 10^-5 cm² s^-1, respectively.

Several Li-O₂ cells employing Pt₃Ni-RGO as the O₂ electrode bi-functional catalyst are assembled and subjected to charge-discharge studies. The discharge capacity obtained at a current density of 0.3 mA cm^-2 is 9016 mAh g^-1 (8.36 mAh cm^-2). It is seen that there is a decrease in discharge capacity initially on repeated charge-discharge cycling. Nevertheless, the discharge capacity stabilizes in a few cycles. The capacity obtained at the end of 10 cycles is 3641 mAh g^-1 (3.27 mAh cm^-2). Results of these studies will be presented.