3.2 Ni SA
The single atom catalysts (SACs) with the multifarious advantages
including high atomic utilization, unique electronic structure, high
catalytic activity and low cost, attract great
interest.[62,63]Constructing atomically dispersed
metal centers that form coordination interactions with neighboring N
atoms in carbon-rich supports is an efficient way to synthesize
single-atom catalysts. The N atoms can stabilize the M atoms, preventing
them from aggregating into NPs during synthesis and reaction. In
addition, the ultimate structure design of atomic level is regarded as
an ideal model to explore the mechanism of catalytic reaction and
understand the structure-performance
relationship.[64,65]Ni-N-C, with its unique structure and coordination environment, exhibits
fast kinetics and excellent ECR activity, and has been paid special
attention.
Sun et al. reported a novel and extensible synthesis strategy of
monoatomic catalyst (NC-CNTs (Ni)), that Ni NPs embedded in carbon
nanotubes are converted directly into heat-stabilized Ni single atom via
in-situ thermal diffusion (Figure
3 a).[65] NC-CNTs (Ni) exhibits high
FECO over 90% at low overpotential, TOF (close to 12000
h−1), and metal mass activity of about 10600 mA
mg−1 for ECR.
Density functional theory (DFT) results predict that
Ni@N3 (pyrrole) group may be the main active site for
ECR to CO. Although Ni@N3 (pyrrole) (1.09 eV)
demonstrates a higher *COOH generating barrier in comparison with
Ni@N3 (pyridine) (−0.20 eV) and Ni (111) (0.29 eV), the
CO desorption free energy at Ni@N3 (pyrrole) is only
−0.03 eV, much lower than Ni@N3 (pyridine) (−1.08 eV)
and Ni (111) (−1.19 eV), which is conducive to CO selection and release
(Figure 3b). In
addition,
the free energy of COOH* formation
(ΔGCOOH*) at
Ni@N3 is more negative than the H* free energy
(ΔGH*), indicating that CO2 reduction
reaction is more competitive than HER in kinetics. Xu et al. developed
electrocatalyst (NiSA/N-C) with the isolated Ni active center anchored
on ultra-thin porous N-doped carbon nanosheet via a polydopamine (PDA)
assisted g-C3N4 template
strategy.[66] PDA was easily coated on the porous
g-C3N4 template surface and stabilized
Ni2+ through coordination to ensure the formation of
Ni-N4 sites in NiSA/N-C. The isolated
Ni-N4 active site and the large specific surface area of
N-C endow NiSA/N-C with the excellent activity, which yielded a 96% CO
selectivity and significant TOF value of 8483 h−1 at a
high current density (26.4 mA cm−2 at −0.86 V).
Except for high intrinsic activity (TOF value at single site), more
active sites (surface active site density (SASD)) can also significantly
promote the catalytic activity.[67] In order to
recover the lack of the key parameter, Strasser’s group exploited a new
synthesis strategy of single-metal atomic site
(C-TpDt-Ni) to realize the
adjustment of SASD (Ni-Nx content) through
controlling the pyrolysis temperature of high-porosity triazole-based
organic framework (TpDt-COF)
impregnated with Ni ions (Figure 3c). The C-TpDt-Ni with Ni single atom
confirmed by high angle annular-dark field−scanning transmission
electron microscopy (HAADF-STEM) and Fourier-transformed (FT)-extended
X-ray absorption fine structure (EXAFS) spectra (Figure 3d,3e), still
retains the surface area, micropore structure and surface chemical
composition, similar to that of TpDt-COF. The basic kinetic and
structural parameters of active site were revealed taking C-TpDt-Ni as
an example. The experimental results indicated that the surface mass
activity of conversing CO2 to CO over C-TpDt-Ni with
single metal atoms is highly depended on the SASD (Figure 3f).
Furthermore, it has been verified that selecting the rich
sp2-N precursors can significantly increase the amount
of coordination Ni in N-doped carbon and thus increase the SASD value.
This study provides unique ideas and basis for the development of Ni-N-C
materials with high SASD. However, apart from theoretical simulation
studies, more characterization observations are required to further
define the behavior of the ECR at Ni SA
sites.[68,69] Based on this challenge, Jaramillo
and co-workers adopted multiple advanced characterization
techniques,[68] including scanning transmission
electron microscopy (STEM), single-atom electron energy loss
spectroscopy (EELS), and time-of-flight secondary ion mass spectrometry
(ToF-SIMS), to provide conclusive evidences for the existence and
catalytic behavior of isolated, nitrogen-coordinated Ni single sites in
Ni-N-C materials. This offers a general method for the microscopic
characterization and reaction process detection over monatomic
catalysts.
To date, a number of different synthesis strategies have been developed
(through pyrolysis and regulation involving a wide variety of catalyst
precursors, such as polymers, zeolite imidazolium salt scaffolders
(ZIF), covalent and metal-organic scaffolders) to generate Ni (SA)-N-C
catalysts with Ni single atoms.[70-72] Although
these achievements greatly promoted the development of ECR, the
synthesis routes are complex and the product yield is low.