The experimental and theoretical insights towards the CO induced Pd-Graphene and their multifunctional energy conversion applications

Abstract

Here, CO gas environment has been used for reduction of graphene oxide (GO) and Pd precursor for preparation of varieties of Pd Nanostructures (PdNSs) with different shapes, size and surface morphologies on graphene support (RG-PdNSs). The extensive ab-initio Molecular Dynamics (MD) simulations and electronic structure calculations have been carried to get theoretical insight for the reduction process of GO by CO. The reduction of GO by CO as observed in experiment are confirmed by ab-initio MD snapshots at different time steps, energetic of the process and the Partial Density of States character of O2p orbital of GO before and after interaction with CO. The discrete states in the Partial Density of States of O2p orbital after CO attack indicates detachment of O from GO. The as-prepared RG-PdNSs are thoroughly characterized by different techniques. The simulation reveals the change in electronic properties from semi-metallic in pristine graphene to metallic due to the attachment of Pd in RG-PdNSs. The electrocatalytic activity of the as-synthesized nanostructures has been investigated toward the multi-functional energy conversion applications, the methanol oxidation reaction, formic acid oxidation reaction and oxygen reduction reaction. The RG-PdNSs exhibit excellent electrocatalytic performance compared to that of unsupported PdNSs and commercial Pd/C.

Introduction

The Nanostructures (NSs) of noble metals like palladium and platinum have been attractive in the fuel cell technology owing to their unique electrocatalytic properties [1,2]. Recently, Pd NSs are of great research interest because of their high surface poisoning resistant and low cost [2]. The catalytic and electrocatalytic activity of these NSs strongly depends upon the shape, size and surface morphology [3]. The shape-controlled synthesis is the most facile technique to obtain different shaped NSs by tuning the various parameters such as concentration and nature of metal precursor, surfactant/stabilizing agent, the reducing agent, seed and temperature of the reaction and so forth [4]. It has been reported that anisotropic NSs can be easily achieved by controlling the reduction kinetics which is based on the nature of reducing agent [5]. Among the various reducing agents, gaseous H₂, CO, etc. have attracted wide attention. Further, CO gas has been proven as the vital reducing and capping agent to produce anisotropic NSs. For example, Zheng and coworkers first time prepared thin Pd nanosheets using CO as a surface confining agent [6]. Various research groups successfully synthesized anisotropic Pt, Pd, Au NSs and their alloys using CO as reducing agent [7,8]. However, the decisive role of CO gas as a reducing agent, capping agent and the role of the solvent during the production of anisotropic NSs are still not clear and need more insight investigations.

To improve the catalytic performance, the nanostructured catalyst requires supporting material with high conductivity and large surface area [9]. It not only enhances the catalytic activity but also provides stability by preventing the extensive aggregation of nanoparticles. As supporting material, carbon-based nanostructures such as carbon nanotube [10], CNF, CB, and fullerene, etc. are attractive owing to their unique inertness and conductivity. Graphene has been attractive material from its discovery by A.K. Geim in 2004. It finds wide potential applications in various technological fields such as fuel cell [9], battery [11] and solar cell [12], etc. The outstanding properties render graphene as the most successful and unique catalytic support. Therefore, the extensive research interest has been devoted to the synthesis and application of graphene supported NSs [[13], [14], [15], [16], [17]]. The metal NSs with anisotropic shapes exhibits the interesting physio-chemical properties [[18], [19], [20], [21], [22], [23], [24]] Recently, graphene supported anisotropic NSs have been treated as the hybrid nanomaterials with excellent performance [[25], [26], [27], [28], [29]]. The synthesis of graphene-based nanomaterials has been reported by various research groups [30]. To the best of our knowledge, it is the first report portraying the in-situ wet-chemical synthesis of graphene supported Pd NSs with a variety of shapes using gaseous CO as reducing/capping agent. In the present work, we devolved a new and facile synthetic approach for graphene supported anisotropic Pd NSs with excellent electrocatalytic performance. Electrochemical measurements were carried out to study the electrocatalytic activity of the hybrid materials using methanol, formic acid, and oxygen as model molecules. The activities of graphene decorated different shaped Pd NSs have been compared along with the commercial Pd/C. Experimental observations were theoretically supported by ab-initio Molecular Dynamics (MD) and Density Functional Theory (DFT) simulations.