Hydrogen Evolution Reaction on the Single-Shell Carbon-Encapsulated Iron Nanoparticle: A Density Functional Theory Insight

Platinum (Pt)-free catalysts for the hydrogen evolution reaction (HER) is currently a blooming research topic in view of the high cost and scarcity of Pt. Experiments on single-shell carbon-encapsulated iron nanoparticles (SCEINs) have proven comparable HER catalytic efficiency with the best Pt catalyst. However, an understanding of the structure-to efficiency is missing. We performed ab initio density functional theory calculations on a realistic model of SCEINs, namely Fe-SS@C-240, to shed light on the catalytic properties of SCEINs and studied C-60 and C-240 fullerenes for comparison. Both the thermodynamic free energy approach (Delta G(H)) and kinetic (Volmer-Heyrovsky/Tafel reaction barrier E-a) calculations were realized on these systems. Our calculations proved that Fe-SS has a key role in enhancing the hydrogen binding on C-240. Volmer-Heyrovsky is the preferred mechanism, Heyrovsky being the limiting reaction with E-a > 1 eV. Non-zero coverage of the carbon surface enhances Delta G(H) without significantly affecting E-a. Because the Delta G(H)-to-E-a relationship is nonlinear, we proposed a computationally efficient strategy based on the DDEC6 bond order (BO) method to preselect potential HER sites before any calculations. E-a proved to be highly site- and (C-Fe) BO-dependent, leading to the highly heterogeneous catalytic ability of Fe-SS@C-240. Delta G(H)/E-a best pairs can then be optimized by playing with the surface coverage.