4.3 Constructing vacancy defects
Defect engineering as the effective regulatory strategy is widely applied to optimize the catalytic performance through ameliorating the electronic structure and electrical conductivity. Among which, vacancy is considered to be an extremely subtle defect, which can flexibly and effectively regulate the performance of various catalytic reactions.[97,98] This strategy has also achieved remarkable results in the regulation of ECR activity over Ni-N-C electrocatalysts, because vacancy defects can improve the exposure of active sites, enhance the adsorption of reaction intermediates, optimize the free energy of the reaction process and promote electron migration.[99-101]
Bao’s team successfully synthesized porous carbon material with the coordinatively unsaturated Ni-N sites (C-Znx Niy ZIF-8) containing ~5.44 wt% monodisperse Ni specie, via pyrolyzing bimetallic Zn/Ni ZIF-8 (Figure 7 a).[99] The optimized C-Zn1Ni4 ZIF-8 can maintain 92.0 ~ 98.0% FECO in the wide potential range of −0.53 ~ −1.03 V, and achieve the CO current density of 71.5±2.9 mA cm−2 at −1.03 V. Based on the systematic characterization and experimental results, which confirms that the coordinatively unsaturated Ni-N was the active site, the faultless (Ni-N4, Ni-N3) and defective (Ni-N3V, Ni-N2V2) models of single site are constructed in Figure 7b. DFT calculations suggest that ΔGCOOH* is only 0.43 eV (Figure 7c), while ΔGH* is 0.57 eV, implying that ECR reaction will be more competitive than HER for NiN2V2 (V represents the vacancy). Therefore, the high-load Ni SA site with the unsaturated coordination of N achieves high current density and FECO, breaking the mutual restriction in ECR.
Lu and coworkers developed a Ni SA catalyst with vacancy-defect (Ni-N3-V SAC) by annealing the chelates of Ni (II) with cyanic acid (CA) and benzene melamine (DPT) and carbon cloth in N2 at high temperature (Figure 7d).[100] The ECR performance of Ni-N3-V SAC is greatly improved with the current density of 65 mA cm−2 and FECO of 90 % at −0.9 V. By quantum calculation of the three optimal Ni-N structures (Ni-N4, Ni-N3, Ni-N3-V (Figure 7e)), the results affirm that the most favorable free energy of COOH* (0.68 eV, as rate-determining step of ERC reduction) and mild CO desorption free energy (-0.11 eV) are observed on the Ni-N3-V site (Figure 7f), thus Ni-N3-V is endowed with the perfect catalytic performance. This work opens up a new idea for the controlled synthesis of monatomic structures with vacancy defect to enhancing the electrocatalytic activity by engineering design and optimizing the coordination environment of SA electronic structure.