(217e) Density-Functional-Theory Studies of Face-Centered-Cubic Tungsten Carbide and Pt Core-Shell Nanoparticles Catalysts for the Hydrogen Evolution Reaction

Platinum is among the best electrocatalysts for the hydrogen evolution reaction (HER)—a potentially fossil-free route for hydrogen production—but the scarcity and high cost of this noble metal catalyst makes it economically unviable at commercial scales. Reduction in Pt loading and overall catalyst cost, without sacrificing catalytic activity, can be achieved by the synthesis of highly stable core-shell nanoparticles of transition-metal carbides and Pt. A large body of prior theoretical work has focused on the hexagonal phase of WC (α-WC) as a support material for Pt monolayers (ML), while the face-centered-cubic phase of WC (β-WC) has attracted lesser attention. Motivated by the recent preparation of stable β-Ti_xW_1-xC@Pt core-shell nanoparticles, we employ density-functional theory (DFT) calculations to study and contrast the suitability of the α and β phases of WC as support (core) materials for Pt ML (shells). We examine the thermodynamic stability of Pt MLs on α- and β-WC surfaces, carefully accounting for the delicate balance between epitaxial mismatch strains and chemical bonding between cores and shells. Furthermore, we study the effects of alloying β-WC with Ti to modulate the stability and electronic structure of the core-shell structures. Based on our thermodynamic studies, we find that β-WC is a better support material for Pt MLs than α-WC and that alloying with Ti improves the stability of Pt/β-WC interfaces without adversely affecting the activity of the Pt shell. We compare the electronic structures of Pt MLs supported on α-WC and β-Ti_xW_1-xC surfaces and compare their activities for HER using the hydrogen binding energy (HBE) as a descriptor of catalytic activity. Of the various cases studied, we find that 2ML Pt supported on β-WC (111) and β-Ti_12.5W_87.5C (111) show higher HER activities than Pt (111) making these core-shell structures very promising candidates for HER electrocatalysts; considering the improved thermodynamic stability of β-Ti_12.5W_87.5C over β-WC, the former is predicted to be a particularly suitable core material for Pt shells. Overall, our results provide detailed insight into experimental observations of the high catalytic activity of β-Ti_xW_1-xC@Pt core-shell nanoparticles.