Figure 11. (a) Scheme of the structures and products for the electrocatalytic CO reduction reaction on Cu-Ni SACs. (b) Faradaic efficiencies of CH4 on dual Cu SAC, Cu1.4Ni SAC, CuNi SAC, and Ni SAC at different potentials. (c) CH4 formation pathway and Gibbs free energy diagram for ECR (the path to CH4) on dual Cu-Cu SAC and Cu-Ni SAC at U = 0 V. Reproduced with permission.[119] Copyright 2021, American Chemical Society. (d) Schematic description of Ni/Cu-N-C fabrication. (e) HAADF-STEM of Ni/Cu-N-C; (f) FECO of the prepared catalysts. (g) Free energy profiles of the ECR. (h) Calculated partial density of states (PDOS) for Ni-N4/CuN4and NiN4 (Nr and Ni represent the N atoms located on the right and left sides of the Ni atom, respectively). Reproduced with permission.[120] Copyright 2021, American Chemical Society. (i) PDOS of d-band for Cox Ni1-x alloy. Free energy diagram of (j) ECR and (k) HER on CoxNi1-x (111) facet. Reproduced with permission.[121] Copyright 2019, Elsevier.
Ni-Cu. The monometallic Ni-N-C catalyst in the reported study can only achieve CO products. The copper element with the potential of multi-carbon products is introduced to regulate the product type of Ni-N-C catalyst.[119,120] Zhang and colleagues synthesized the porous and thin carbon frameworks containing two adjacent Cu-Ni sites (Figure 11 a).[119] The optimized Cu1.4Ni SAC showed high CH4 selectivity (FECH4 = 53%) and partial current density of CH4 (60 mA cm−2) at −1.66 V (Figure 11b). Based on the simulated model of Cu-Ni-N6, the computational results reveal the rate-determining step of CH4 formation is hydrogenation from CO* to CHO*. The ΔGCHO* of Cu-Ni SAC (1.26 eV) is lower than that of Cu-Cu SAC (1.40 eV) (Figure 11c). Using ZIF-8 as a template, He, et al. fabricated N-C catalyst with precise control of atomically dispersed Ni/Cu double sites (Figure 11d,e).[120] The sample containing N4Ni/CuN4 displays the excellent ECR performance, which shows 99.2% FECO at −0.79 V, higher than that of most similar catalysts reported previously (Figure 11f). The electron rearrangement and band gap narrowing caused by adjacent NiN4 and CuN4 sites enhance the conductivity and strengthen bonding interactions between COOH*, CO* and Ni centers, thereby reducing the overall reaction energy barrier (Figure 11g,h).
Ni-Co. Cobalt species can also regulate the activity of Ni-N-C catalysts. For example, Gao, et al. fabricated Cox Ni1-x nanoalloys with engineering electronics configuration anchored to N-doped carbon nanofibers (Cox Ni1-x /N-C NFs) through the electrospinning-pyrolysis method.[121] The optical Co0.75Ni0.25/N-C electrocatalyst displayed high ECR (FECO = 85.0%) activity. With increasing Co content in CoNi alloy, the d-band center shifted positively, resulting in lower binding energy and lower reaction free energy of key intermediates (COOH*) (Figure 11i,j). Furthermore, the more negative ΔGH* suggested that ECR process was promoted while HER was inhibited (Figure 11k). Based on the potential of syngas for thermal catalytic applications, ECR producing syngas (CO and H2) is considered a promising method for CO2 emission reduction. Chen et al. designed a nitrogen-doped carbon-supported Co/Ni bimetallic catalyst (CoNi-NC) by pyrolyzing the mixtures of glucose, dicyandiamide, and Co/Ni salts.[122] The regulation of Co/Ni ratio in CoNi-NC can adjust the CO/H2 ratio (0.23~2.26) at higher gas production rate, which is suitable for downstream thermochemical reactions. The reason is attributed to that single-atom Ni-N possesses nearly 100% CO selectivity, while Co-N is superior in HER activity. This finding provides a feasible electric/thermal hybrid catalytic process for CO2 reduction.
In addition, the introduction of other secondary metal (Zn, Mn, Sn, etc.) had also been proven to enhance ECR activity of Ni-N-C catalyst.[113,123,124]