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.