Figure 3. (a) Schematic illustration of the formation of
NC-CNTs (Ni). (b) Free energy diagram of Ni@N3(pyrrolic)
and Ni@N3(pyridinic) for electrochemical
CO2 reduction to CO. Reproduced with
permission.[65] Copyright 2019, Wiley-VCH. (c)
Scheme for the synthesis of C-TpDt-Ni (carbon atoms are shown in grey,
nitrogen in blue, oxygen in red, Ni in green). (d)
High-resolution HAADF-STEM image of
C-TpDt-Ni-900. (e) The absolute values of Fourier-transformed (FT)-EXAFS
spectra of specimens. (f) The FECO of various COF-derived Ni-N-C and
reference catalysts. Reproduced with
permission.[67] Copyright 2022, Wiley-VCH.
3.3 Coexistence of
Ni NPs and Ni SA
Although single-atom catalysts contribute the greatest atomic
utilization and higher unit activity, metal loading concentrations are
usually very low, resulting in the insufficient apparent
activity.[73,74] In contrast, metal NPs with
easily adjustable electronic properties due to their variable crystal
structure and surface are endowed with appreciable apparent
electrocatalytic activity, but low atomic utilization reduces the
turnover frequency (TOF).[75] The coexistence of
SA-substrate interactions and
metal-substrate interactions confers
higher freedom degrees of hybrid systems in electron modulation,
creating more positive sites with diverging electron
properties.[76,77] Therefore, the combination of
metal NPs and SA in the system is conducive to develop advantages and
eliminate disadvantages for metal NPs and single atom. Based on this
result, relevant research has been carried out in sixpenny Ni-N-C
materials.
For instance, Deng and Ye have jointly reported a hybrid material
composed of metallic Ni cores and Ni-N-C layers (Ni-NC@Ni) by low
temperature vapor deposition strategy (Figure
4 a).[78] The lamellar open nanostructures have
abundant isolated Ni-N in the carbon layer (about 4.23 at%) (Figure
4b,4c), more open space to promote mass transfer and excellent
conductivity, showing a high FECO (87%) and a current
density of 14.8 mA cm−2. This provides an effective
way to improve the mass transport, electronic conductivity and exposure
of the catalytic active center. However, whether the dominant active
site is Ni single atoms or Ni NPs or both has not been conclusively
demonstrated.
Based on the above arguments, Liu’s team synthesized several
representative catalysts (Ni-NG),
including Ni single atoms, nitrogen-doped carbon-encapsulated Ni NPs,
and both. Surprisingly, the activity of Ni-NG catalyst, which was
treated with acid to remove the embedded Ni NPs, was not decreased for
the ECR performance (Figure 4d).[79] The reason is
that the N-doped carbon-encapsulated Ni NPs catalyst contained the
unexpressed Ni single atoms, which played a dominant role in ECR.
Theoretical calculations show that when three or more layers of N-doped
graphene carbon were coated by Ni NPs (3NGR@Ni), the charge of surface
carbon atoms can hardly be regulated (Figure 4e). Therefore, the free
energy of intermediates in CO2 reduction on carbon layer
before and after encapsulating Ni NPs did not change significantly. And
the higher ΔGCOOH* of
NGR@Ni and
3NGR@Ni showed the insufficient
CO2 conversion ability. Moreover, the thermodynamic
limiting potentials for ECR and HER
(UL(CO2) and
UL(H2)) revealed that N-doped
carbon-encapsulated Ni NPs (NGR@Ni) possessed high catalytic selectivity
for HER (Figure 4f), again indicating that Ni NPs are not the ECR active
site. Lin et al. successfully achieved the ratio tunability of
single-atom Ni and Ni NPs (1:0 ~ 1:5) on N-doped carbon
nanorods via controlling the HCl etching process, to adjust the ratio of
CO and H2 in syngas product.[80]The high H2 precipitation ratio of 90% and CO
selectivity of as low as 10% of Ni NPs further indicated that Ni NPs
favors HER. By contrast, the sample containing only single atom Ni-N
site displayed the highest CO selectivity, more than 95%.
Although the above research results provide sufficient evidence for
determining Ni single atoms as the active species in ECR, it does not
deny that the existence of Ni NPs can still have a positive effect on
the conductivity of material and the modulation of active site/carrier
from the edge/surface metal Ni. Recently, Hu and coworkers reported a
cooperative Ni single-atom-on-NPs (NiSA/NP) though the simple direct
solid-state pyrolysis of melamine.[81] The NiSA/NP
provided an industrial current density of 131 mA cm−2at −1.0 V in H-cell over CO (Figure 4g). Based on the characterization
and experimental results, Ni-N3@Ni model was constructed
and corresponding theoretical calculation was performed. The results
suggest that Ni NPs can afford the electrons for Ni−N−C on the surface
to achieve electronic modulation of Ni-N3. The analysis
of Bader charge further shows that the introduction of Ni (111)
(Δq=0.013 e−) causes the Ni single atom to gather more
electrons (Figure 4h), again indicating the microscopic modulation
effect of Ni NPs on the single atom. Afterwards, the calculated free
energy value (0.53eV) of COOH* at NiN3@Ni, is lower than
that of NiN3 (0.70 eV), suggesting that COOH* possesses
stronger binding strength with NiN3@Ni. The above
results demonstrate that the presence of Ni NPs optimizes the charge
distribution of Ni SA sites and enhances the ECR activity. In addition,
Hou et al. proposed a possible ECR mechanism on the atomically dispersed
Ni-Nx site in carbon
materials coated at the Ni NPs
(Figure 4i), and the Ni−N−C catalyst (Ni@NiN4CM) was
synthesized by the carbonization of nickel nitrate and
o-phenylenediamine (Figure 4j), which reveals the role of active
components in the protonation process from the perspective of
kinetics.[82] Also, the experiments and
theoretical calculations have confirmed that Ni NPs can promote the
ionization and capture of protons, accelerate the capture of protons
between the active center of Ni-N4 and the adsorption
intermediates, thus optimizing the ECR reaction kinetics.
Undoubtedly, single-atom Ni-Nx (x = 1, 2, 3, 4)
is the main active site for ECR reaction. However, the coexistence of Ni
NPs with SA offering the positive/negative impact on the activity needs
to be analyzed in combination with specific structures in ECR. The
current structural analysis is incomplete. The differences of
experimental and theoretical results also appear in different
preparation methods, such as the coordination mode of Ni NPs and Ni
single atom, the electron transport capacity of the material itself, the
apparent active site density and the theoretical model.