Activation of Ni Particles into Single Ni–N Atoms for Efficient Electrochemical Reduction of CO₂

Abstract

within acceptable limits are needed. Among the many possible solutions, electrochemical CO 2 reduction (ECR) offers a potentially sustainable approach not only for depressing CO 2 concentration but also converting it into fuels and commodity chemicals. [2] Unfortunately, the CO chemical bond in CO 2 (≈806 kJ mol −1 ) is thermodynamically very stable and its conversion is an uphill energy process with a high activation barrier. Moreover, during electrochemical reduction of CO 2 , the hydrogen evolution reaction (HER) inevitably occurs as a competing reaction, which is a major stumbling block for CO 2 reduction especially in aqueous electrolytes. [3] From these scenarios, robust catalysts that can selectively reduce CO 2 in lieu of protons at high turnover frequency (TOF) and faradaic efficiency (FE) for CO 2 reduction are desired.Since Hori's pioneering study on electroreduction of CO 2 in the 1980s, [4] Cu, [5] Au, [6] Ag, [7] Zn, [8] Sn, [9] and Bi [10] among others, have been widely investigated for electrocatalysis of CO 2 reduction, due to their promising capability to convert CO 2 into valuable chemicals and fuels while the HER is largely suppressed. Earth-abundant first-row transition metals such as Fe, Co, and Ni, however, are highly active for HER and also easily Electrochemical reduction of carbon dioxide (CO 2 ) to fuels and value-added industrial chemicals is a promising strategy for keeping a healthy balance between energy supply and net carbon emissions. Here, the facile transformation of residual Ni particle catalysts in carbon nanotubes into thermally stable single Ni atoms with a possible NiN 3 moiety is reported, surrounded with a porous N-doped carbon sheath through a one-step nanoconfined pyrolysis strategy. These structural changes are confirmed by X-ray absorption fine structure analysis and density functional theory (DFT) calculations. The dispersed Ni single atoms facilitate highly efficient electrocatalytic CO 2 reduction at low overpotentials to yield CO, providing a CO faradaic efficiency exceeding 90%, turnover frequency approaching 12 000 h −1 , and metal mass activity reaching about 10 600 mA mg −1 , outperforming current state-of-the-art single atom catalysts for CO 2 reduction to CO. DFT calculations suggest that the Ni@N 3 (pyrrolic) site favors *COOH formation with lower free energy than Ni@N 4 , in addition to exothermic CO desorption, hence enhancing electrocatalytic CO 2 conversion. This finding provides a simple, scalable, and promising route for the preparation of low-cost, abundant, and highly active single atom catalysts, benefiting future practical CO 2 electrolysis.

A metal–organic framework/polymer derived catalyst containing single-atom nickel species for electrocatalysis

“…2d and S18 †); this is likely due to the dense graphitic carbon layer that protects the NPs. 55,58,59 Upon closer inspection of the CNTs, a considerable amount of single atom nickel species are found distributed throughout the CNTs (Fig. 2e).…”

“…53,54 Moreover, residual nickel nanoparticles in the carbon matrix can potentially act as a source for the formation of more single atom nickel sites during catalyst preparation if extra polymer and the second thermal treatment was adopted. 55,56 In addition to residual NPs, high-angle annular dark-eld (HAADF) STEM images and EDXS elemental maps clearly conrm the presence of single Ni atom sites in all three carbons ( Fig. S12-S16 † and Fig.…”

Recent Advances in Atomic‐Level Engineering of Nanostructured Catalysts for Electrochemical CO2 Reduction

“…Very recently, single Ni atoms with a NiN 3 moiety were prepared via the transformation of Ni particles confined in N‐doped carbon nanotubes upon a one‐step pyrolysis strategy. [ 89 ] The single Ni catalysts displayed low onset potential of −0.4 V and a high EF over 90% with an overpotential of 690 mV. According to DFT calculations, a lower free energy for the formation of *COOH was required on the Ni@N 3 (pyrrolic) sites compared with Ni@N 4 sites and thus was reasonable for the excellent ECR activity.…”

Emerging Metal Single Atoms in Electrocatalysts and Batteries

“…As one of the non-noble metal SAs, Ni SAs are the most widely investigated SAs for catalyzing the CO 2 reduction, but they are typically combined with the substrates to generate a stable format. [155] Carbonaceous materials are the commonly used substrates, including graphene sheets, [156][157][158][159] carbon nanotubes, [160,161] and porous carbon frameworks. [162][163][164][165] For instance, the dispersion of Ni SAs into NG (Ni-NG) sheets, without the involvement of Ni nanoparticles, was reported to serve as the active sites for catalyzing CO 2 reduction to carbon monoxide (CO) ( Figure 6).…”

“…Ni SAs were also obtained from Ni particles, and previously, these particles served as the catalysts for the growth of carbon nanotubes. [160] Ni SAs were postulated to be existing in the format of the NiN 3 moiety, surrounded by a porous N-doped carbon sheath. The electrocatalyst can appropriately facilitate the highly efficient electroreduction of CO 2 at a very low potential to yield CO, presenting a CO Faradaic efficiency (FE) of above 90%, high turnover frequency, and metal mass activity.…”