3.1 Ni NPs
The facile synthesis procedure and cost effectiveness have made metal
NPs attractive for a long time.[53-56] However,
the exposed Ni NPs have been determined to be more conducive to HER
occurrence, especially at high potentials. Wrapping
Ni NPs with the protective layer to
isolate the Ni NPs from the electrolyte can greatly slow down the
occurrence of HER, while maintaining the high conductivity of the
internal active Ni NP, which is conducive to enhancing the selectivity
and conductivity of ECR. Therefore, Ni NPs loaded/encapsulated by
N-doped carbon (Ni NPs-NC) show the promising activity towards ECR.
For example, Sun et al. studied the influence of co-existing metal
particles and surface species (nitrogen ions) on the carrier, as well as
the interaction of Ni and N-doped C on the ECR
performance.[57] N-doped carbon-coated Ni NPs
catalyst (Ni-NC_ATPA@C) was prepared via direct pyrolysis process of
nickel-diamine-dicarboxylic metal-organic skeletons (MOFs).
Ni-NC_ATPA@C presents a 94% FE of CO
(FECO) at −0.59 V,
comparable to that of the single Ni site
(Figure 2 a). Based on the
models of Ni NPs and graphene-encapsulated Ni NPs in Figure 2b, the
catalytic mechanism of Ni-NC_ATPA@C in ECR reaction was studied by
theoretical calculation. Metallic Ni can significantly reduce the free
energy of *COOH formation (0.58~0.66 eV), but is not
favorable for CO desorption.[58,59] Compared with
pure graphene and Ni NPs, Ni NPs encapsulated by N-doped carbon layer,
especially the doped forms of pyrrole N and graphite N, exhibit optimal
*COOH stability and lower CO desorption free energy. Both Ni NPs and
N-doped carbon layer of Ni-NC_ATPA@C play the crucial roles on
enhancing ECR performance. Chen et al. developed the N-doped carbon
nanotubes in-situ encapsulating Ni nanoparticles material (Ni@NCNT) by
high-temperature treatment of Ni-MOF and melamine mixture
(Figure
2c).[60] Core-shell Ni@NCNT exhibits strong and
durable activity in the ECR reaction, specifically described with the
FECO (> 90%) in wide potential range from
−0.65 to −1.0 V, and lasting 18 h electric reduction without significant
decay. Compared with pure Ni (111) and N-doped carbon nanorods (NCNR)
without Ni NPs, Ni@NCNT can not only optimize the *COOH free energy
(Figure 2d), but also inhibit HER process, resulting in high ECR
activity and CO selectivity.
In order to further disclose the active sites of CO2adsorption and activation in Ni NPs encapsulated by N-doped carbon
catalysts (Ni@NC), Zhou and cooperators fabricated N-doped carbon-coated
nickel catalyst supported on reduced graphene oxide (Ni@N-C/rGO) (Figure
2e).[61] The
evaluation results suggest that the optimal Ni@N-C/rGO (4, 4´-bipy) can
react for 10 h with 88% FECO at −0.97 V. The relevant
results from well-designed toxic and single-variation experiments
suggest that both N-species and Ni NPs are essential for enhancing
catalytic activity (Figure 2f). By virtue of the theoretical
calculation, the activity site of Ni@NC catalyst is further defined as
pyrrole N in carbon coating, which can adsorb and activate
CO2 molecules. Meanwhile, the electron-rich Ni NPs
nuclear spontaneously transfers electrons to the π orbital of the N-C
skeleton (Figure 2g), which reduces the free energy of *CO desorption,
thus achieving the optimization of catalytic performance.