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.