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.