Sp-D Orbital Hybridization Driven Metal-Graphene Catalysts for Electrochemical CO2 Reduction to Formic Acid

Abstract

The efficient transformation of readily-available CO2 into renewable energy sources is a crucial for the decarbonization. Formic acid (FA), the simplest combination of hydrogen and CO2, is one of the most promising hydrogen storage compounds for its high hydrogen density, non-toxicity, and great stability under ambient temperature and pressure. However, thermochemical conversion of CO2 to FA on the noble metal and metal alloy catalysts has been a challenge for a decade since the reaction is thermodynamically unfavorable and kinetically difficult due to the nature of relatively weak adsorption of CO2 on the metal surfaces in comparison to HCOO, and thus requires very high pressure/temperature. Electrochemical conversion of CO2 on the conventional carbon black supported noble metal and metal alloy catalysts, in contrast, operates at ambient conditions, but suffer low stability through dissolution and agglomeration, and requires high overpotentials to overcome a competitive hydrogen evolution reaction (HER). In this context, we investigate a highly active and stable electrocatalyst obtained by depositing transition metal monolayer on graphene (M/Gr) using DFT method, with several experimental whenever tractable. It is found that the hybridization of sp and d orbitals between graphene and metal significantly reduces the local density of states of d-band of metals and increases s-and p-orbital of graphene near the fermi level at the same time, which leads to a strong covalent bonding between metal monolayer and graphene. The covalent bonding manifests in an epitaxial-like metal growth on Gr, which has been experimentally observed for Pt/Gr, Au/Gr and Pd/Gr systems. The calculation of the orbital-resolved density of states for d band of M specifically reveals that sharp dz 2, dyz, and dxz orbital peaks below the fermi level play a decisive role in the formation of covalent bonding between M and Gr. In addition, it is found that the charge polarization on graphene in Pt/Gr and Pd/Gr not only enables a deposition of another metallic monolayer on the graphene, thus forming Pt/Gr/M or Pd/Gr/M sandwich structures, but also induces a transition from physisorption to chemisorption for M = Ag, Au, and Cu cases. Finally, by evaluating the overpotentials required for CO2 reduction to FA, it is discovered that, among M/Gr, Pt/Gr/M, and Pd/Gr/M electrocatalysts, Ni/Gr, Pd/Gr, Pt/Gr/Ag, and Pt/Gr have excellent activity and selectivity toward electrosynthesis of FA.