4.5 Heteroatom Regulation
The unsaturated Ni-Nx coordination can improve the ECR ability of the catalyst by optimizing the charge distribution density of the Ni-Nx site and the adsorption energy of the intermediate CO* and COOH*. By doping the atoms with larger radius and lower electronegativity, such as S and P, the stable Ni-N4 structure is changed and obtained the unbalanced coordination structure and effectively regulate the local electron density of the Ni-Nx site at the atomic level, thus significantly improving the activity and selectivity of monatomic catalysts.[105-107]
Recently, Chen et al. designed and constructed the Ni SA center catalyst (SSC) with dual coordination of P and N atoms (Figure 9 a).[108] By controlling the added amount of triphenylphosphine, the coordination mode of N, P to Ni SA can be regulated, such as Ni-N4, Ni-P1N3 and Ni-P2N2. In the ECR reaction, the jCO of Ni-Px Ny is significantly higher than that of unmodified Ni-N4 catalyst. In addition, the Ni-P1N3possesses highest FECO (85.0 ~ 98.0%) in the potential range from −0.65 to −0.95 V. The monodisperse Ni-P1N3 site is as the source of high activity. The calculated Bader charge indicates that more electrons are retained with the gradual introduction of P atoms in the Ni-N4 (Figure 9b), benefiting to enhance the interaction of adsorbates. The adsorption/desorption of COOH* and CO* intermediates on the Ni-P1N3 site can be optimized by doped P atoms, resulting in higher catalytic activity and selectivity of Ni-P1N3 than those without P introduction (Figure 9c).
Cheng and coworkers synthesized N, P co-coordinated Ni SA embedded in 3D carbon materials (Ni-NPC) (Figure 9d).[109] Ni-NPC exports the ECR performance of 92% FECO at −0.8 V, much superior to that of NC, NPC and Ni-NC catalysts. The coordination environment and charge distribution of Ni SA in Ni-NPC have been clearly defined by XANES and EXAFS. The doped P atoms can transport electrons to the Ni atoms, resulting in the free energy of rate-limiting step (CO2→ COOH*) reduce to 0.97 eV from 2.06 eV of Ni-N4, which benefits to overcome the limiting-step bottleneck towards ECR (Figure 9e). In addition, compared with Ni-N4 (0.71 eV), NPC (0.82 eV), and NC (0.92eV), Ni-N3P (0.23 eV) displayed the lowest limit potential difference (UL(CO2) - UL(H2)) of ECR and HER (Figure 9f), indicating the best competitive ability of CO2 reaction. Subsequently, this team reported a novel, stable S, N co-coordinated Ni SA electrocatalyst (Ni-SNC) by calcining SO42− doped Zn/Ni ZIF (Figure 9g).[110] The 95% FECO at −0.8 V proves the high activity of Ni-SNC for ECR. The intrinsic activity of Ni-SNC comes from the doped S modulating the local spatial charge distribution at the unsaturated Ni-N3-S site, optimizing the energy barrier of ECR and enhancing the CO selectivity and current density. These findings provide the effective basis and method for designing and constructing new metal energy conversion center with asymmetric bicoordination for improving the ECR performance.