Coordination Environment Regulation Of MOF-Derived Single-Atom Ni Electrocatalysts For CO₂

Single-atom catalysts(SACs)can realize the atomic dispersion of metal atoms and achieve their efficient utilization of metals at the atomic scale.Meanwhile,SACs have exhibited excellent activity and selectivity for many reactions.Therefore,they have attracted extensive research interest in recent years.Given that single metal atoms are stabilized by the supports based on coordination interaction,the regulation of the coordination environment of SACs is of vital importance for the optimization of catalytic performance.The metal coordination number,a key factor of the coordination environment,significantly affects the electronic and geometric structure of the metal center anchored on the catalyst and plays an important role in enhancing catalytic activity/selectivity.Although there have been great advances in the controllable construction of SACs,an effective and universal strategy to precisely regulate the metal coordination number has not been developed yet.Metal-organic frameworks(MOFs)is a class of crystalline porous materials featuring highly tailorable and flexible chemical composition and spatial structure.In recent years,MOFs have been widely applied to control synthesis of SACs,and a few reports have been published about the regulation of metal coordination environments.The currently adopted strategies towards SACs are mostly limited to one-step pyrolysis of the MOF precursors doped with the target metal atoms and the higher pyrolysis temperature leads to lower metal-nitrogen coordination numbers.However,this method suffers some great limitations that hinder its application to regulate the coordination environment of SACs in a rational way.Firstly,the introduction of doped metal will interfere with the formation process of MOFs,so the synthesis procedure of metal-doped MOFs must be modified according to different doped metal species.Moreover,one-step pyrolysis process requires comprehensive consideration of multiple variables such as pyrolysis temperature and metal loading to ensure the dispersion of single atoms,which severely limits the flexibility and universality of the strategy.In term of the above issue,there is an urgent need to explore a more universal method to achieve precise control over the coordination environment of single-atom catalysts.Herein,we developed a post-synthetic metal substitution(PSMS)strategy.We first obtain nitrogen-doped carbon supports with different nitrogen coordination environments through the pyrolysis of a Zn-MOF,ZIF-8.Taking the MOF-derived nitrogen-doped carbon(N-C)as the operating platform,the nickel precursors are introduced to afford single-atom nickel embedded on the pre-designed carbon supports.The PSMS strategy successfully avoids tackling the variable complexity caused by the correlation between N-C support design and single atom modification,thereby realizing the precise construction of single-atom nickel catalysts with different nitrogen coordination numbers.In the electrocatalytic reduction of carbon dioxide,the Ni-N3-C catalyst with a nitrogen coordination number of 3 exhibits the highest CO Faraday efficiency of 95.6%,which is much higher than that of Ni-N4-C with a nitrogen coordination number of 4.Theoretical calculations show that the lower Ni coordination number in Ni-N3-C can significantly increase the formation rate of the rate-determining intermediate COOH*,thus significantly accelerating the electrocatalytic reduction of carbon dioxide.In addition,Ni-N3-C exhibits extremely high CO Faraday efficiency and excellent stability in Zn-CO2 batteries,demonstrating its great potential in practical applications.