Figure 9. (a) Schematic illustration for the preparation of Ni-Px Ny . (b) The atomic structures of Ni SSCs with the calculated Bader charge on central Ni atom. (c) The free energy diagram for CO2electroreduction to CO. Reproduced with permission.[108] Copyright 2022, Springer Nature. (d) Schematic illustration of the synthesis of the Ni-NPC catalyst. (e) Free energy diagram of CO2 reduction to CO on the NC, NPC, Ni–N4, and Ni-N3P active sites. (f) Potential differences in limiting steps of NC, NPC, Ni–N4, and Ni-N3P. Reproduced with permission.[109] Copyright 2022, Royal Society of Chemistry. (g) Schematic for synthesis of Ni-SNC electrocatalyst. Reproduced with permission.[110] Copyright 2022, Elsevier.
4.6 Bimetallic activity sites
Although the Faraday efficiency obtained on most Ni-N-C catalysts is satisfactory (generally FECO > 90%), their practical application is still limited by high overpotential and product uniformity. The introduction of other active species with lower energy barrier to form adsorbent COOH intermediates (*COOH) induces strong electron interactions at adjacent sites in the bimetal, which can more rationally optimize the free energy barrier of the intermediates and maximize the potential of Ni-Nx for ECR.[111-113]
Ni-Fe. The Fe-N center can significantly reduce the overpotential of ECR due to the rapid transfer of the proton-coupled electron. The synergistic effect of Fe and Ni active centers significantly affects the different reaction steps on two isolated active centers, thereby improving the performance and selectivity of ECR.[114,115]
Zhao’s team successfully synthesized isolated diatomic Ni-Fe sites (Ni/Fe-N-C) anchored on carbon nitride by heat-treating ZIF-8 template (Figure 10 a).[116] After the introduction of Fe species, the overpotential of Ni/Fe-N-C is significantly decreased, and the FECO exceeds 90% in the potential range from −0.5 to −0.9 V (The maximum FECO value is 98% at −0.7 V) (Figure 10b). After 30 hours of electrolysis, Ni/Fe-N-C can still maintain high CO selectivity (99%), which proves the remarkable durability of Ni/Fe-N-C. The ECR mechanism on bimetal-N sites is established by theoretical simulation. Specifically, the adjacent Fe-Ni sites passivated by CO* exhibit lower COOH* and CO* free energies than bare Ni/Fe-N-C. The synergistic action of Ni-Fe center increases the ΔGCOOH* to 0.47 eV and decreases the ΔGCO* to 0.27 eV, greatly promoting CO formation (Figure 10c). Arbiol’s group also reported the Ni/Fe-N-C with quasi-double-star Ni/Fe sites via thermal treatment of Ni, Fe co-doped zinc-based MOFs synthesized by simple one-pot method (Figure 10d).[117] The optimized Ni7/Fe3-N-C catalyst exhibits unique CO selectivity of 98% at the overpotential of 390 mV, superior to other state-of-the-art M-N-C catalysts. The bimetallic Ni7/Fe3-N-C with adjacent Ni and Fe centers is more favorable to the COOH* formation than Ni-N-C catalyst (Figure 10e), due to the presence of Fe near the Ni center affecting the electron density and configuration environment of two active centers. Recently, Wu, et al. confirmed that the 2N-bridged (Fe-Ni)N6 was the most active N-coordinated dual-metal configuration through in-depth structural characterization and theoretical calculation (Figure 10f).[118] The synergistic interaction from FeN4 and NiN4 groups shared with two N atoms endows 2N-bridged (Fe-Ni)N6 with better free energy of COOH* adsorption and CO* desorption than these of single metal sites, while the HER is restrained, ensuring better intrinsic activity and selectivity (Figure 10g,10h).