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.