Synthesis, characterization, and design of precious metal electrocatalysts supported on transition metal carbides

Fuel cells and electrolyzers require electrocatalysts that are stable under acidic and alkaline conditions in aqueous solutions, and also are active at room temperature. The stability and activity restrictions limit electrocatalysts to precious platinum (Pt)-group metals supported on carbon powder. Pt and carbon are electronically dissimilar, and because of this, Pt is unstable on the carbon surface, and costly high loadings of Pt are needed to compensate for this. The use of supports with Pt-like electronic properties, like the early transition metal carbides (TMCs), would result in higher Pt stability and lower loading requirements. The potential applications of TMCs were studied as ideal supports for Pt-group metals. The use of theoretical models and experiments on low loadings of monolayer (ML) metals on synthesized TMC thin films were used to discover trends in stability and activity. This allows for a creation of a library to design low-loading precious metal electrocatalysts based on TMC supports.;TMC thin films were synthesized by carburizing metal or metal oxides from a carbon source like methane, which can lead to excess surface carbon. The effects of surface carbon formation during synthesis were explored on the hydrogen evolution reaction (HER) activities of tungsten carbide (WC) and ML Pt supported on WC.;TMC (titanium, vanadium, zirconium, niobium, and tantalum carbide) thin films were characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical stability of the TMC thin films was mapped under a wide range of pH and potential using chronopotentiometric-titration measurements. The electrochemical stability of the TMC thin films was correlated to density functional theory (DFT)-calculated oxygen binding energy (OBE) of the parent metals.;The second part of the thesis focuses on a specific TMC, titanium carbide (TiC), as not only is TiC very stable, but can be synthesized with high surface area. TiC also has a strong affinity to Pt, making TiC an ideal support for low loading of Pt as an active and stable electrocatalyst. DFT calculations predict that ML Pt on TiC will be stable and active for the HER. ML Pt on TiC was synthesized and found to have comparable activity to bulk Pt. The HER results were extended to Pt on TiC powders with similar results.;The effects of pretreatment synthesis methods used to impregnate Pt on TiC powders were explored on the catalytic activity for the HER and the ORR. Direct reduction of Pt on TiC powders lead to higher Pt dispersion and superior ORR activity than Pt on TiC powders that were calcined in air. Low loadings of Pt on TiC powders reduced in hydrogen have comparable ORR activity to much higher loadings of commercial Pt on carbon powders.