Sulfur-Doped Graphene-Supported Nickel-Core Palladium-Shell Nanoparticles as Efficient Oxygen Reduction and Methanol Oxidation Electrocatalyst

The design of novel platinum-free highly efficient electrocatalysts for the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) has been regarded as the core challenge toward the development of commercially available fuel cell devices. In this contribution, a facile strategy was applied to prepare surfactant-free nickel-core palladium-shell nanoparticles, abbreviated as Pd@Ni-NPs, uniformly distributed on sulfur-doped graphene (SG). The Pd@Ni-NPs/SG hybrid material was realized by employing a modified polyol method for the in situ preparation of Ni-NPs/SG, followed by deposition of a Pd shell through the galvanic replacement method and complementary characterization by Raman and IR spectroscopy, STEM/EELS, SEM, and EDS as well as XRD and TGA. The Pd-to-Ni molar ratio was optimized in view of the hybrid's electrocatalytic performance. Interestingly, the Pd@Ni-NPs/SG hybrid was proved to be a highly efficient and stable electrocatalyst toward ORR and MOR. It exhibited comparable initial ORR performance with the benchmark Pd/C catalyst and importantly greater stability, as after 2,000 potential cycles it possesses 36% and 67% higher diffusion-limited and kinetic current density, respectively, having a minimal loss of its initial activity. Further investigations on the reaction kinetics showed a four-electron direct reduction of oxygen to water for the Pd@Ni-NPs/SG as well as Tafel slopes of -48 mV dec(-1)/-116 mV dec(-1), values much closer to those proposed for the polycrystalline platinum. In addition, Pd@Ni-NPs/SG revealed enhanced MOR specific activity by 82% over Pd/C (2.26 mA cm(-2) vs 1.24 mA cm(-2)) and by far better antipoisoning abilities. Overall, Pd@Ni-NPs/SG hybrid is a highly efficient and low-cost electrocatalyst, revealing huge potential for application in high-performance energy conversion devices.