Atomically precise nickel-sulfur clusters for efficient electrochemical 2,5-hydroxymethylfurfural oxidation

Nickel sulfide exhibits excellent catalytic activity in the electrochemical 2,5-hydroxymethylfurfural oxidation reaction (HMFOR). However, due to the polydispersity of nanoparticles, it is difficult to establish a clear structure-activity relationship at the atomic level. In this work, we have successfully synthesized atomically precise Ni₆(PET)₁₂ and Ni₄(PET)₈ clusters (PET: 2-phenylethanethiol) for HMFOR. Ni²⁺ and S^2- with atomic ratio of 1:2 were mainly existed in Ni₆(PET)₁₂ and Ni₄(PET)₈ to form Ni-S bond. The electrochemical test results have suggested both Ni₆(PET)₁₂ and Ni₄(PET)₈ displayed outstanding electrocatalytic ability for HMFOR. The Ni₆(PET)₁₂ exhibited better electrocatalytic ability than Ni₄(PET)₈ with higher current density, lower overpotential and faster reaction kinetics. The superior electrochemical ability of Ni₆(PET)₁₂ may be due to the enhanced adsorption towards HMF molecule with strong interaction towards hydroxyl group and furan ring. Moreover, it  found that the Ni²⁺ species in Ni₆(PET)₁₂ could rapidly oxidized into Ni³⁺ species, which could spontaneously capture electron and proton from HMF for oxidation. The theoretical calculation demonstrated that the Ni₆(PET)₁₂ process lower free energy barrier than Ni₄(PET)₈ to display excellent electrocatalytic performance. This work is of great significance for designing efficient electrocatalysts for HMFOR.