Highly Efficient Electroreduction of CO₂ on Nickel Single‐Atom Catalysts: Atom Trapping and Nitrogen Anchoring

Abstract

process, desired products can be easily adjusted by changing the type of electrocatalysts and the applied potentials. [3] Unfortunately, there are still many challenges for CO 2 electroreduction due to the highly stable chemical bound of CO 2 molecule and high energy barrier of proton-coupled multielectron transfer process. [4] Thus, it is critical to develop efficient and stable catalyst materials for CO 2 electroreduction.At present, many catalysts have been reported, including metal, metal oxide, MXenes (transition-metal carbides, nitrides, and carbonitrides), carbon-based materials (e.g., graphene, carbon nanotube (CNT), and carbon quantum dots (CQD)). [5] The ideal heterogeneous catalyst of CO 2 electroreduction should contain highly dispersed active species and sufficient accessible surface to prevent the problem of the mass transfer. [6] Recently, single atom catalysts (SACs) have drawn much attention for CO 2 RR, with advantages of unique electronic structure, unsaturated coordination configuration of active centers, maximum atom-utilization efficiency, quantum size effect, strong interactions between metal and support as well as foreign atom effect. [7] SACs materials afford high selectivity and stability toward desired products, exhibiting great potential to bridge the gap between the homogeneous and heterogeneous catalysis.To date, researchers have successfully developed efficient M-N-C-based (M = Ni, Fe, Co, Mn, Zn, Sn) electrocatalysts with Construction of single atom catalysts (SACs) with high activity toward electroreduction of CO 2 still remains a great challenge. A very simple and truly cost-effective synthetic strategy is proposed to prepare SACs via a impregnation-pyrolysis method, through one-step pyrolysis of graphene oxide aerogel. Compared with other traditional methods, this process is fast and free of repeated acid etching, and thus it has great potential for facile operation and large-scale manufacturing. Both X-ray absorption fine structure and high-angle annular dark-field scanning transmission electron microscopy images confirm the presence of isolated nickel atoms, with a high Ni loading of ≈2.6 wt%. The obtained 3D porous Ni-and N-codoped graphene aerogel exhibits excellent activity toward electroreduction of CO 2 to CO, in particular exhibiting a remarkable CO Faradaic efficiency of 90.2%. Density functional theory calculations reveal that free energies for the formation of intermediate *COOH on coordinatively unsaturated NiN sites are significantly lower than that on NiN 4 site, suggesting the outstanding activities of CO 2 electroreduction originate from coordinatively unsaturated NiN sites in catalysts.

Understanding the Origin of Highly Selective CO2 Electroreduction to CO on Ni,N‐doped Carbon Catalysts

“…This high selectivity is in agreement with many recent literature reports of selective CO production in Ni,N-doped carbon materials (summary of past reported Ni-N-C catalysts in SI Table 2). [13][14][15][16][17][18][19][20][21] Thep artial current density towards CO increases with applied electrode potential and reaches 21 mA cm À2 at À1.1 Vvs. RHE before the onset of hydrogen evolution which results in aplateau in j CO and an increase of j tot to 40 mA cm À2 at À1.3 Vv s. RHE ( Figure 3B).…”

“…TheN i-N-C for CO 2 Rr eports have,h owever, varied in their assignment of Ni oxidation state,c oordination number, and bonding environment, and have not shown conclusive evidence of nitrogen-nickel bonding. [13][14][15][16][17][18][19][20][21] In addition, previous reports have focused far more on characterizing Ni in the single atom form with less attention to the role of Ni aggregates in controlling the catalytic performance of Ni-N-C materials.T hus,o pen challenges remain in confirming that nickel-nitrogen bonding occurs in these materials,a nd in conclusively attributing catalytic activity to these dispersed Ni species.…”

“…[11,12] Among these materials,Ni-N-C has been reported by many groups to have Faradaic efficiencies toward CO exceeding 90 %. [13][14][15][16][17][18][19][20][21] The proposed active site structure for Ni-N-C is atomically dispersed Ni-N x sites,ahypothesis that has been made based on evidence from electron microscopy, [13] hard X-ray absorption spectroscopy, [18] and density functional theory. [17] This active site determination closely matches the conclusions of the ORR literature for the Fe-N-C system where activity is attributed to FeN 4 C x sites that bear strong structure resemblance to typical molecular catalysts such as metal porphyrins and metal phthalocyanines.…”

Optimizing Electron Densities of Ni‐N‐C Complexes by Hybrid Coordination for Efficient Electrocatalytic CO2 Reduction

“…Recently,t he influencing factorso fM -N-C catalysts have been explored, such as the type of transition metal,v alence states, and coordination unsaturation effects. [36][37][38][39][40][41] Even though the concentration of Cspecies is always greater than 95 at %i nt he resultingM -N-C composites, most studies have focused only on the effect of as mall amount of Nw hile ignoring the contribution of Ct oa ctive sites. Indeed, it has been recently reported in the literature that MÀNa nd MÀCb onds coexist in the M-N-C catalysts, [16][17][18][19][42][43][44] however,t he roles of Na nd Ca toms in the active sites of M-N-C complexes are still unclear.T herefore, as ystematic investigation of the activity trends and catalytic mechanisms of N-and C-coordinated metal complexes through ac ombination of theoretical calculations and experimental validation is essential foracomprehensive and indepth understanding of M-N-C electrocatalysts.…”

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).…”