One-Pot Synthesis of Carbon-Supported Pd Nanoparticles with Ni and Carbon Matrix Protection for Durable Alkaline Hydrogen Oxidation Reaction

Protecting nanoparticles with a carbon matrix can enhance the durability of the catalysts in alkaline fuel cells (AFC) and is well-documented. While others have tried complex syntheses to produce small nanoparticle catalysts, in this work, in order to scale-up batches of 15 g or more, carbon-cap (or carbon-coating) protected Vulcan XC72-supported Pd-Ni (1-2) nanoparticles (Pd-Niacc/C) were successfully synthesized via a one-step dry-synthesis process. This catalyst was compared with a commercial Pd-Ni/Vulcan XC72 material (PdNi/C, Premetek) in terms of hydrogen oxidation reaction (HOR) activity and stability. To produce less ordered carbon caps (versus graphite/graphene), a low-temperature heat-treatment (below 500 degrees C) was used, resulting in Pd-Niacc/C of unique electrochemical properties: easily electrochemically activated, this catalyst outperforms PdNi/C for alkaline HOR and proves more durable under highly oxidizing accelerated stress test (AST) conditions. Identical location transmission electron microscopy (ILTEM), X-ray photoelectron spectroscopy (XPS) and rotating disk electrode (RDE) measurements demonstrate how the Ni-rich surface plays an important role in HOR activity for both PdNi/C and Pd-Niacc/C and that the protective carbon-coating of the latter ensures better durability of performance and better resistance to materials degradation. The class of electrocatalyst with protected metal active sites (e. g. the present carbon-capped and carbon-supported nanoparticles) is not new in the literature and is being applied in many domains of electrochemistry, due to the peculiar properties of such materials, like high tolerance towards passivation, hydridation or degradation (e. g. metal dissolution, nanoparticles agglomeration) that is provided by the protection (herein the carbon cap). However, the synthesis strategies used to elaborate such materials are not always compatible with upscaling and practical deployment of the electrocatalysts. This work highlights a methodology to prepare such catalyst that is easily scalable. Alkaline hydrogen oxidation reaction materials are prepared using non-Pt nanoparticles as active metal site (M-site, here M being Pd) that are combined with metal-oxide sites (MOx-site, here MOx being Ni-oxides) protected by some layers of amorphous carbon (carbon cap). Once properly electrochemically activated, these nanoparticles bare contiguous Pd sites and NiOx sites that are optimal for the alkaline HOR (for which metal/metal-oxide surfaces are suited), and a carbon-cap that protect them from dissolution/agglomeration (degradations) as well as passivation/hydridation (deactivation). This strategy enables faster and more durable HOR electrocatalysis, which cannot be obtained when the initial PdNi/C nanoparticles are not protected by a well-covering carbon cap; in that latter case, the Ni initially alloyed with Pd in bimetallic nanoparticles tends to leach out and precipitate as Ni-oxides, possibly away from the remaining Ni-depleted PdNi nanoparticle, which is not an optimal configuration for the alkaline HOR. image