Effects of different vacancies in nickel hydroxides on the electrooxidation towards 5-hydroxymethylfurfural

The electrocatalytic oxidation of 5-hydroxymethylfurfural (HMFOR) emerges as an alternative pathway for the green production of valuable oxygenated chemicals owing to the catalyst innovation. The performance of nickel hydroxides, which are well-known for HMFOR, is still far from satisfactory as a result of the weak adsorption towards HMF, limited intrinsic activity and poor conductivity. Vacancy engineering provides an effective way to improve the catalytic ability of nickel hydroxides, nevertheless the role of different vacancies played in the HMFOR process is still unclear. Herein, a carbon-paper supported nickel hydroxide (Ni(OH)2/CP) with anionic vacancies (av-Ni(OH)2/CP) or cationic vacancies (cv-Ni(OH)2/CP) is employed for HMFOR study, with a special focus on the structure–function relationship between vacancy type and HMFOR performance. The successful introduction of different vacancies was manifested by electron paramagnetic resonance spectroscopy. X-ray photoelectron spectroscopy revealed the opposite roles of the anionic and cationic vacancies in modulating the electronic structure of Ni(OH)2. The time-dependent open-circuit potential measurements verified the different adsorption behavior of HMFOR on the surface of the catalysts. Benefiting from the well-tuned electronic structure and the enhanced adsorption ability, the av-Ni(OH)2/CP exhibits superior catalytic performance than the pristine Ni(OH)2/CP and the cv-Ni(OH)2/CP. More than 98.97% HMF can be converted using av-Ni(OH)2/CP as catalyst, with a remarkable yield of FDCA (98.51%) and a ultrahigh faradaic efficiency (98.46%) at 1.45 V. This work not only reports an excellent HMFOR electrocatalyst, but also elucidates the crucial effect of different vacancies on the electrochemical oxidation performance of Ni(OH)2 materials.