Pd/3D-Graphene Electrocatalysts for Ethanol Oxidation

Abstract

Versatile carbonaceous materials such as graphene have attracted increasing interest due to their excellent electrical conductivity, thermal stability and high specific surface areas.[1] These exceptional resourceful properties make graphene an ideal candidate for electrochemical applications. On the other hand, palladium (Pd) nanoparticles are also emerging as an alternative to Pt/C electrocatalysts owing to their high activity and stability. However, there is limited research focused on integrated these two resourceful materials for electrocatalytic applications such as fuel cells. In view of that, our present work adopts the sacrificial support method (SSM) [2-3] developed at UNM for the synthesis of highly porous 3D-Graphene nanosheets. The 3D-Graphene nanosheets were then utilized as a support structure for the deposition of palladium (Pd) nanoparticles and studied for the electrochemical oxidation of ethanol in alkaline media. Following SSM synthesis, the surface areas and morphology of 3D-Graphene were optimized by the controlled thermal and chemical reduction of graphene oxide to 3D-Graphene and characterized using Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). The optimized 3D-Graphene was then deposited with palladium nanoparticles using the Soft Alcohol Reduction Method (SARM) with varying ratios of ethyl alcohol and deionized water (EtOH : DI) to determine its correlation with electrochemical performance. Figure 1 is a SEM micrograph depicting the high porosity of chemically reduced 3D-Graphene with a surface area of 530 m2/g (as determined using the Brunauer–Emmett–Teller method). Figure 2 is a TEM micrograph showing the layer of thin nanosheets present in our 3D-Graphene. Figure 3 is a SEM micrograph of well dispersed Pd nanoparticles on 3D-Graphene. Figure 4 is a cyclic voltammogram of 1M ethanol electro-oxidation in 1M KOH scanned at 20mV/s. The electrochemical activity of SARM synthesized Pd/3D-Graphene was investigated using a rotating disk electrode (RDE). The Pd/3D-Graphene catalyst inks for RDE experiments were prepared by sonicating the catalyst powder with an optimized amount of Nafion™ in IPA/DI water solution. Our results (Figure 4) show a clear trend and significant increase in performance with a peak current density recorded for Pd/3D-Graphene that were chemically reduced and synthesized with 1:2 = EtOH:DI water ratio. Further analysis of Pd particle shape and size was confirmed and characterized using HRTEM and XRD. In summary, highly active palladium nanoparticles supported on 3D graphene nano sheets were prepared for ethanol oxidation in alkaline media and optimized for the highest surface area using chemical reduction and characterized using TEM, SEM and BET. Pd nanoparticles were deposited on 3D-GNS using a Soft Alcohol Reduction Method where varying the ratio of EtOH:DI-water during synthesis had a significant effect on electrochemical activity as determined from RDE cyclic voltammograms. These results from our study will extend our current capabilities by integrating experimental, electrochemical and surface analysis techniques for the rational design of hybridized metal/graphene electrocatalysts for fuel cells.