Figure 11. (a) Scheme of the structures and products for the
electrocatalytic CO reduction reaction on Cu-Ni SACs. (b) Faradaic
efficiencies of CH4 on dual Cu SAC,
Cu1.4Ni SAC, CuNi SAC, and Ni SAC at different
potentials. (c) CH4 formation pathway and Gibbs free
energy diagram for ECR (the path to CH4) on dual Cu-Cu
SAC and Cu-Ni SAC at U = 0 V. Reproduced with
permission.[119] Copyright 2021, American Chemical
Society. (d) Schematic description of Ni/Cu-N-C fabrication. (e)
HAADF-STEM of Ni/Cu-N-C; (f) FECO of the prepared
catalysts. (g) Free energy profiles of the ECR. (h) Calculated partial
density of states (PDOS) for Ni-N4/CuN4and NiN4 (Nr and Ni represent the N atoms located on the
right and left sides of the Ni atom, respectively). Reproduced with
permission.[120] Copyright 2021, American Chemical
Society. (i) PDOS of d-band for
Cox Ni1-x alloy. Free
energy diagram of (j) ECR and (k) HER on
CoxNi1-x (111) facet. Reproduced with
permission.[121] Copyright 2019, Elsevier.
Ni-Cu. The monometallic Ni-N-C catalyst in the reported
study can only achieve CO products. The copper element with the
potential of multi-carbon products is introduced to regulate the product
type of Ni-N-C catalyst.[119,120] Zhang and
colleagues synthesized the porous and thin carbon frameworks containing
two adjacent Cu-Ni sites
(Figure 11 a).[119] The optimized
Cu1.4Ni SAC showed high CH4 selectivity
(FECH4 = 53%) and partial current density of
CH4 (60 mA cm−2) at
−1.66 V (Figure 11b). Based on the
simulated model of Cu-Ni-N6, the computational results
reveal the rate-determining step of CH4 formation is
hydrogenation from CO* to CHO*. The ΔGCHO* of Cu-Ni SAC
(1.26 eV) is lower than that of Cu-Cu SAC (1.40 eV) (Figure 11c). Using
ZIF-8 as a template, He, et al. fabricated N-C catalyst with precise
control of atomically dispersed Ni/Cu double sites (Figure
11d,e).[120] The sample containing
N4Ni/CuN4 displays the excellent ECR
performance, which shows 99.2% FECO at −0.79 V, higher
than that of most similar catalysts reported previously (Figure 11f).
The electron rearrangement and band gap narrowing caused by adjacent
NiN4 and CuN4 sites enhance the
conductivity and strengthen bonding interactions between COOH*, CO* and
Ni centers, thereby reducing the overall reaction energy barrier (Figure
11g,h).
Ni-Co. Cobalt species can also regulate the activity of
Ni-N-C catalysts. For example, Gao, et al. fabricated
Cox Ni1-x nanoalloys with
engineering electronics configuration anchored to N-doped carbon
nanofibers (Cox Ni1-x /N-C
NFs) through the
electrospinning-pyrolysis
method.[121] The optical
Co0.75Ni0.25/N-C electrocatalyst
displayed high ECR (FECO = 85.0%) activity. With
increasing Co content in CoNi alloy, the d-band center shifted
positively, resulting in lower binding energy and lower reaction free
energy of key intermediates (COOH*) (Figure 11i,j). Furthermore, the
more negative ΔGH* suggested that ECR process was
promoted while HER was inhibited (Figure 11k). Based on the potential of
syngas for thermal catalytic applications, ECR producing syngas (CO and
H2) is considered a promising method for
CO2 emission reduction. Chen et al. designed a
nitrogen-doped carbon-supported Co/Ni bimetallic catalyst (CoNi-NC) by
pyrolyzing the mixtures of glucose, dicyandiamide, and Co/Ni
salts.[122] The regulation of Co/Ni ratio in
CoNi-NC can adjust the CO/H2 ratio
(0.23~2.26) at higher gas production rate, which is
suitable for downstream thermochemical reactions. The reason is
attributed to that single-atom Ni-N possesses nearly 100% CO
selectivity, while Co-N is superior in HER activity. This finding
provides a feasible electric/thermal hybrid catalytic process for
CO2 reduction.
In addition, the introduction of other secondary metal (Zn, Mn, Sn,
etc.) had also been proven to enhance ECR activity of Ni-N-C
catalyst.[113,123,124]