Figure 9. (a) Schematic illustration for the preparation of
Ni-Px Ny . (b) The atomic
structures of Ni SSCs with the calculated Bader charge on central Ni
atom. (c) The free energy diagram for CO2electroreduction to CO. Reproduced with
permission.[108] Copyright 2022, Springer Nature.
(d) Schematic illustration of the synthesis of the Ni-NPC catalyst. (e)
Free energy diagram of CO2 reduction to CO on the NC,
NPC, Ni–N4, and Ni-N3P active sites.
(f) Potential differences in limiting steps of NC, NPC,
Ni–N4, and Ni-N3P. Reproduced with
permission.[109] Copyright 2022, Royal Society of
Chemistry. (g) Schematic for synthesis of Ni-SNC electrocatalyst.
Reproduced with permission.[110] Copyright 2022,
Elsevier.
4.6 Bimetallic activity
sites
Although the Faraday efficiency obtained on most Ni-N-C catalysts is
satisfactory (generally FECO > 90%), their
practical application is still limited by high overpotential and product
uniformity. The introduction of other active species with lower energy
barrier to form adsorbent COOH intermediates (*COOH) induces strong
electron interactions at adjacent sites in the bimetal, which can more
rationally optimize the free energy barrier of the intermediates and
maximize the potential of Ni-Nx for
ECR.[111-113]
Ni-Fe. The Fe-N center can significantly reduce the
overpotential of ECR due to the rapid transfer of the proton-coupled
electron. The synergistic effect of Fe and Ni active centers
significantly affects the different reaction steps on two isolated
active centers, thereby improving the performance and selectivity of
ECR.[114,115]
Zhao’s team successfully synthesized isolated diatomic
Ni-Fe sites (Ni/Fe-N-C) anchored on
carbon nitride by heat-treating ZIF-8 template
(Figure
10 a).[116] After the introduction of Fe species,
the overpotential of Ni/Fe-N-C is significantly decreased, and the
FECO exceeds 90% in the potential range from −0.5 to
−0.9 V (The maximum FECO value is 98% at −0.7 V)
(Figure 10b). After 30 hours of electrolysis, Ni/Fe-N-C can still
maintain high CO selectivity (99%), which proves the remarkable
durability of Ni/Fe-N-C. The ECR mechanism on bimetal-N sites is
established by theoretical simulation. Specifically, the adjacent Fe-Ni
sites passivated by CO* exhibit lower COOH* and CO* free energies than
bare Ni/Fe-N-C. The synergistic
action of Ni-Fe center increases the ΔGCOOH* to 0.47 eV
and decreases the ΔGCO* to 0.27 eV, greatly promoting CO
formation (Figure 10c). Arbiol’s group also reported the
Ni/Fe-N-C with quasi-double-star
Ni/Fe sites via thermal treatment of Ni, Fe co-doped zinc-based MOFs
synthesized by simple one-pot method (Figure
10d).[117] The optimized
Ni7/Fe3-N-C
catalyst exhibits unique CO selectivity of 98% at the overpotential of
390 mV, superior to other state-of-the-art M-N-C catalysts. The
bimetallic Ni7/Fe3-N-C with adjacent Ni
and Fe centers is more favorable to the COOH* formation than Ni-N-C
catalyst (Figure 10e), due to the presence of Fe near the Ni center
affecting the electron density and configuration environment of two
active centers. Recently, Wu, et al. confirmed that the 2N-bridged
(Fe-Ni)N6 was the most active N-coordinated dual-metal
configuration through in-depth structural characterization and
theoretical calculation (Figure 10f).[118] The
synergistic interaction from FeN4 and
NiN4 groups shared with two N atoms endows 2N-bridged
(Fe-Ni)N6 with better free energy of COOH* adsorption
and CO* desorption than these of single metal sites, while the HER is
restrained, ensuring better intrinsic activity and selectivity (Figure
10g,10h).