Researchers at the College of Chemistry and Chemical Engineering of Hunan University, in collaboration with colleagues from the Research Center for X-ray Science and the Department of Physics at Tamkang University, have introduced an intelligent strategy that involves stocking oxygen vacancies with removable heteroatoms for the development of high-efficiency electrooxidation catalysts that convert polyols into formic acids.
With the development of nanocatalysts, researchers have recognized the decisive role of lattice defects (e.g., oxygen vacancies) in electrocatalysis based on defective nanocatalysts. The electrochemical conversion of polyols to formic acid over NiOxHy is an attractive strategy for the electrochemical upgrading of biomass-derived polyols. The team has proved that the electrochemical conversion of polyols to formic acid on NiOxHy depends on an oxygen vacancy-induced mechanism. However, high-energy oxygen vacancies are easily rendered ineffective, leading to the gradual decline in the activity of oxygen vacancy-rich catalysts, especially for those catalytic reactions involving oxygen vacancy-induced mechanisms, e.g., electrochemical conversion of polyols to formic acid. As for the development of oxygen vacancy-catalysts, the chief difficulty is ensuring the sustaining action of oxygen vacancies during catalytic reactions.
Although oxygen vacancies are easy to create on β-Ni(OH)2, unstable oxygen vacancies (especially for Schottky-type defects) are easily filled with reactive oxygen species (e.g., oxygen) upon exposure to the atmosphere, thus leading to the VSO-β-Ni(OH)2 deactivation. Due to the lack of oxygen vacancy-induced C–C bond cleavage, polyols can be electrochemically oxidized to form polymers as an insoluble passivation film, thus leading to the passivation of electrodes during polyol electrooxidation reaction over β-Ni(OH)2/VSO-β-Ni(OH)2. Therefore, ensuring the sustaining action of oxygen vacancy-induced catalytic mechanism is the prerequisite for achieving high-efficiency and stable electrochemical conversion performance of polyols to formic acid catalysts.
Using low temperature plasma technology, researchers filled oxygen vacancies with removable sulfur (S) heteroatoms to synthesize an oxygen vacancy-filling with sulfur β-Ni(OH)2 (S-VO-β-Ni(OH)2) catalyst, whose oxygen vacancies are protected by filling S heteroatoms. More importantly, during the electrochemical conversion of polyols to formic acid over S-VO-β-Ni(OH)2, the pre-electrooxidation-induced loss of sulfur and self-reconstruction cause the in-situ generation of stable Frenkel-type oxygen vacancies for guaranteeing the sustained action of oxygen vacancy-induced C–C bond cleavage process. Hence, S-VO-β-Ni(OH)2 can effectively and steadily catalyze the electrochemical conversion of polyols to formic acid involving the C–C bond cleavage, thus exhibiting excellent electrochemical conversion performance of polyols to formic acid.
This work represents a significant milestone in polyol electrooxidation and other electrooxidation reactions involving oxygen vacancy-induced catalytic mechanisms, offering a promising outlook for future advancements.
