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.