Grand-canonical DFT applied to the electro-oxidation of 5-hydroxymethylfurfural on (substituted) NiOOH(0001)

The oxidation of organic molecules such as 5-hydroxymethylfurfural (HMF) is a promising alternative to water oxidation in electrolysers generating green hydrogen. While the thermodynamics of HMF oxidation is considerably more favorable than water oxidation (difference of ∼ 1 V), the onset potentials on Earth-abundant catalysts such as NiOOH are only 0.1 to 0.2 V lower than the ones for water oxidation. One of the reasons is the generation of NiOOH itself, which already requires a minimal potential of 1.4 to 1.5 V vs RHE. Second, the oxidation reaction itself is not very fast, as mainly ”chemical” (rather than electrochemical) reaction steps dominate. To understand these well-established experimental observations, we here present a detailed grand-canonical density functional theory-based atomistic mechanism and determine key activation energies as well as their electrochemical potential dependence. We show that the C–H activation of the aldehyde functional groups is a limiting step (activation energy of ∼ 1 eV), while the oxidation of the alcohol functional group to the aldehyde can be achieved in a single step with a low activation energy ( ∼ 0.4 eV). A second limitation is the favorable desorption of carboxylate intermediates, which amounts to a reduced selectivity when the reaction is not driven to completion. Substituting part of the Ni atoms by Cr or Mn lowers the oxidation potential to reach the NiOOH-like phase. The mechanistic investigation of Cr and Mn substituted NiOOH surfaces confirms that the oxidative power of the corresponding NiIII centers is still sufficient to drive HMF oxidation, suggesting that such substituted catalysts should enable lower onset potentials compared to pure Ni catalysts. Thereby, our study contributes to the rationalization of mechanistic insights under realistic conditions and to the design of more efficient nickel oxyhydroxide catalysts. • C-H activation of aldehydes is limiting HMF electro-oxidation. • Activation energies for limiting steps barely depend on the electrochemical potential. • Substituting Ni by Mn or Cr leaves mechanism unchanged, but lowers onset potential.