Figure 5 Scheme of the fabrication of NiSA-Nx -C catalysts by the host-guest cooperative protection strategy for ECR. (b) FECO of NiSA-Nx -C catalysts at different applied potentials, and (c) the corresponding TOFs of CO production over NiSA-Nx -C. (d) Free-energy diagram of CO2 reduction to CO over NiSA-Nx -C catalysts. Reproduced with permission.[86] Copyright 2020, Wiley-VCH. (e) Illustration of synthesis process of Ni@Nx Cy -1000 catalyst. (f) Fourier-transformed (FT) k3-weighted EXAFS profiles of Ni@C-800, Ni@Nx Cy -1100, Ni@Nx Cy -1000, Ni@Nx Cy -900, and Ni@Nx Cy -800 catalysts at the Ni K-edge. (g) Free energy diagram for CO2 reduction to CO and (h) potential differences in the limiting steps of UL(CO2) and HER UL(H2) on Ni sites in the Ni−N4, Ni−N3C1, Ni−N2C2, Ni−N1C3 active sites. Reproduced with permission.[87] Copyright 2021, American Chemical Society.
4.2 Designingmorphology
Due to the different hybrid modes, the carbon is stacked in a variety of ways at the nano and micron scales, resulting in the abundant appearance morphology, such as 1D carbon nanotubes/fibers, 2D graphene/graphene oxide (GO), 3D hollow/array nanocarbon structures, etc. The morphology of N-doped carbon matrix can affect the contact area of the reaction liquid, exposure rate of the active site, and mass transfer efficiency of reactants at the gas-catalyst-electrolyte phase interface.[88-90] Reasonable design of carbon matrix structure, such as high specific surface area and abundant voids, can make more metal sites exposed, simultaneously enhance the mass transfer process and catalytic efficiency.
Three-dimensional (3D) hierarchical carbon nanostructure, integrating the advantages of 2D graphene and 1D carbon nanotubes can offer the large active surface area and good stability, thus acting as an ideal electrocatalytic carrier.[91,92] Li and his colleagues reported the 3D hierarchical nitrogen-(N)-doped composite carbon nanosheet/nanotube catalyst with the co-existing of Ni SA and graphite-carbon-coated Ni NPs via one-step chemical vapor deposition (CVD) method (Figure 6 a,6b).[91] The hierarchical structure of CNT and CNS in Ni-N-CNS/CNT displayed higher specific surface area and lower charge transfer resistance, significantly enhancing the ECR activity. Specifically, under H-cell condition, the FECO of optimized Ni-N-CNS/CNT catalyst is stable at 91%, and the CO partial current density is 28.9 mA cm−2 at −0.74 V. Meanwhile, Ni-N-CNS/CNT exhibits more than 85% CO selectivity at −2.0 V and a commercially viable current density of 600 mA cm−2 in the flow cell.
The porous carbon can inhibit the formation of metal NPs and facilitate the rapid transport of reactants and products.[92-94]Hollow carbon spheres have been widely studied as electrocatalyst substrates. For instance, Lou’s team synthesized hollow porous N-doped carbon nanostructures (Ni-NC(HPU)) at low temperature, using urchin-like Ni particles synthesized by hydrothermal method as the template (Figure 6c,6d).[94] The urchin-like Ni particles have three functions, including serving as solid template formed by hollow structure, catalyzing the ordered growth of NC coating on the surface of Ni particles, and the Ni source of single Ni sites entering the NC layer. Unique surface hollow spines, good electrical conductivity and large surface area facilitate single Ni site being exposed and electron/mass transfer. Compared with hollow nanospheres and dodecahedral monatomic Ni-N-C catalysts, Ni-NC(HPU) exhibited higher EDR activity of 91% FECO, CO partial current density (24.7 mA cm−2 at −0.8 V) and prominent durability (27 h), further confirming the advantages of this structure.