Figure 7. (a) Schematic illustration of synthesis of C-Znx Niy ZIF-8. (b) Optimized atomic structures of different Ni-N structures with Ni atoms coordinated with NiN4, NiN3, NiN3V, NiN2V2. (c) Free energy diagrams with implicit (solid lines) and explicit (dashed lines) solvation effect corrections for ECR pathway on Ni sites of different Ni-N structures at 0 V. Optimized atomic structures for COOH* intermediates adsorbed on Ni sites are shown on the top. Inset: COOH* intermediates adsorbed on the NiN2V2site with one water molecule layer to simulate the solvent environment. The white, grey, red, blue, purple balls represent H, C, O, N, Ni atoms, respectively. Reproduced with permission.[99]Copyright 2018, Royal Society of Chemistry. (d) Illustration for the synthesis of Ni-N3-V. (e) The optimized structures of Ni-N3-V. (f) Calculated free-energy diagram for the conversion of CO2 to CO. Reproduced with permission.[100] Copyright 2020, Wiley-VCH. (g) Schematic illustration of microwave induced plasma assisted synthesis for SACs. (h) Raman spectra of two catalysts and HAABF-STEM image (inset) of SA-NiNG NV. (i) Electron paramagnetic resonance (EPR) spectra. The insets show the expanded peaks (left) where g values are around 2, and the spin numbers (right). (j) FECO of SA-NiNG-NV and SA-NiNG. Reproduced with permission.[101] Copyright 2021, Wiley-VCH.
In addition, Pan and Zhao reported an easy plasma-assisted and N-vacancy (NV)-induced coordination reconstruction strategy for constructing Ni-pyridine N2 groups with abundant defect (SA-NiNG-NV) (Figure 7g).[101]X-ray absorption spectroscopy (XAS) confirms that the local reconstruction of active site can be implemented due to easily removing the doped N atoms by high-energy plasma. Under continuous plasma impact, the as-preformed pentagonal pyrrole N defects around Ni SA can be converted into the pyridine N-dominated Ni-N coordination structure (Figure 7h,i), benefiting to promote the kinetics from CO2 conversion to CO. The CO selectivity and production efficiency of reconstructed SA-NiNG-NV are significantly improved with a 96% FECO at −0.59 V and jCO (CO current density) of 33 mA cm−2 at −0.89 V (Figure 7j). Ex-situ XAS and DFT calculations indicate that the highly defective pyridine N reduced the constraint of N on the central Ni atom, making Ni atoms easier to shift outward and provide enough space for adsorbing and activating CO2, thus reducing the energy barrier of CO2 reduction.
4.4 Surface Embellish
The capture and concentration of CO2 can enhance the reduction efficiency of CO2. The introduction of specific functional groups or functional molecules on the catalyst surface can promote CO2 dissolution and enhance CO2 transfer. These functional species can adjust the affinity and electronic structure of active species to optimize the reaction activity of electrocatalysts.[102-104]
For instance, Chen et al. fabricated a hydroxyl modified cellular carbon catalyst (H-Ni/NC) with Ni SA (Figure 8 a).[102]H-Ni/NC maintains a CO selectivity of ≥ 88.0% in the 0.5 M KHCO3 at −0.5 ~ −0.9 V. Various characterization and experimental results indicate that the synergistic effect between atomically dispersed Ni active sites and surface hydroxyl groups promotes the ECR activity of H-Ni/NC (Figure 8b). DFT results further confirm that the nearby hydroxyl groups of Ni SA can induce and regulate the electron distribution of Ni atoms with higher positive charges, which can reduce the energy barrier of rate-limiting step from 1.25 eV (Ni/NC) to 1.05 eV (H-Ni/NC) (Figure 8c).
Liu’s group modified the Ni SA-nitrogen-carbon catalyst with amino (Ni-N4/C-NH2) via the generalized amination strategy, making the jCO of Ni-N4/C-NH2 being generally incremental (Figure 8d).[103] The FEco of Ni-N4/C-NH2 can be maintained above 85% in a wide working potential from −0.5 to −1.0 V. The CO2adsorption capacity of Ni-N4/C-NH2 is significantly better than that of the initial Ni-N4/C by the CO2 adsorption isotherm (Figure 8e), accelerating the ECR rate. The DFT results display that the d-band center of −2.30 eV for Ni 3d DOS in Ni-N4/C-NH2 is more positive than that of −3.35 eV in Ni-N4/C (Figure 8f), indicating that the electronic structure of ammoniated catalyst is regulated. The positive shift of d-band center suggests the enhanced adsorption energy of CO2* and COOH* intermediates, which makes that the ΔGCOOH* being decreased by 0.57 eV and ΔGH* being increased by 0.38 eV, promoting the ECR and inhibiting the HER (Figure 8g).