Graphene-Supported Palladium Nanostructures as Highly Active Catalysts for Formic Acid Oxidation Reaction

Palladium (Pd) has recently emerged as a stable and active yet cheaper alternative electrocatalyst to costly platinum for the formic acid oxidation reaction (FAOR). Crystal engineering, wherein particle morphologies, defects, and facets are selectively altered, can enhance the electrocatalytic activities of Pd nanostructures. Herein, we have demonstrated that a combination of crystal engineering and supporting the nanostructures on a conductive catalyst support (few-layered graphene (FLG)) leads to highly enhanced catalytic activities. FLG was prepared by using liquid-phase exfoliation in aqueous solution of a surfactant. Swollen liquid crystals promoted the nanostructuring of Pd as well as nanocomposite formation as they acted as "soft" templates. Spherical nanoparticles (Pd0D), nanowires (Pd1D), and nanosheets (Pd2D) of Pd were formed and preferentially deposited on graphene sheets on the exposure of mesophases containing graphene along with Pd2(dba)3 to hydrazine vapor, H2, and CO, respectively. The Pd1D/FLG nanocomposite exhibited an exceptional electrocatalytic activity for FAOR. It had many folds higher electrocatalytically active surface area (ECSA), current density, and stability than the other nanocomposites as well as other Pd-based catalysts reported in the literature. Increased presence of more active Pd(100) facets was identified as the major reason for the enhanced catalytic activity of Pd1D. Supporting the Pd nanostructures on graphene led to enhanced electrocatalytic activities owing to the preserved surface sites of Pd nanoparticles, enhancement in electronic conductivities, and mass transfer and charge transfer from graphene to Pd.