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 Fe55@C240, to shed light on the catalytic properties of SCEINs and studied C60 and C240 fullerenes for comparison. Both the thermodynamic free energy approach (ΔGH) and kinetic (Volmer-Heyrovský/Tafel reaction barrier Ea) calculations were realized on these systems. Our calculations proved that Fe55 has a key role in enhancing the hydrogen binding on C240. Volmer-Heyrovský is the preferred mechanism, Heyrovský being the limiting reaction with Ea > 1 eV. Non-zero coverage of the carbon surface enhances ΔGH without significantly affecting Ea. Because the ΔGH-to-Ea 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. Ea proved to be highly site- and (C-Fe) BO-dependent, leading to the highly heterogeneous catalytic ability of Fe55@C240. ΔGH/Ea best pairs can then be optimized by playing with the surface coverage.
01/01/2015 → 31/12/2018
Projekti: Academy of Finland: Other research funding