Nickel–Nitrogen–Carbon Molecular Catalysts for High Rate CO₂ Electro-reduction to CO: On the Role of Carbon Substrate and Reaction Chemistry

Abstract

Metal–nitrogen–carbon (M–N–C) molecular catalysts with NiN4 active structure have been extensively studied as selective and active catalysts toward electrochemical reduction of CO2 to CO. The key challenge for a practical M–N–C catalyst is to increase the density of atomic metal active sites that achieves the partial current density of CO (j CO) relevant to the industrial level at lower overpotentials. Here, we revealed the effect of physical and chemical properties of carbon substrates and synthetic processes on the tuning of the density of atomic metal active sites as well as the role of reaction chemistry in enhancing the j CO and reducing the overpotential. The achievable loading of NiN4 active site in the Ni–N–C is determined by the combined content of pyridinic and pyrrolic N functionalities and Ni–N coordination efficiency derived from the pyrolytic step rather than the uptake capability of Ni2+ in the adsorption step in the case of carbon black with high specific surface area (>1000 m2/g). The N dopant content can be improved by modifying oxygen functional groups on the surface of carbon black, optimizing the pyrolytic temperature, and iterating the doping step. Through a combination of all optimum factors, the resultant Ni–N–C catalyst has a maximum loading of ∼4.4 wt % for atomic Ni. This Ni–N–C catalyst exhibited Faradaic efficiency (FE) of CO of 97% and j CO of −152 mA cm–2 at −0.93 V vs RHE in a flow cell using 0.5 M KHCO3 electrolyte while showing 93% FE of CO and j CO of −67 mA cm–2 at −0.61 V vs RHE at 1 M KOH. Adding KI to the base electrolyte significantly magnified the j CO to larger than −200 mA cm–2 at a potential of −0.51 V vs RHE while maintaining the almost unity FE of CO. The Ni–N–C is compatible with the membrane-electrode-assembly-based electrolyzer in which the j CO also achieved >200 mA cm–2 at a cell voltage of around 2.7 V.

Highly selective and productive reduction of carbon dioxide to multicarbon products via in situ CO management using segmented tandem electrodes

“…Preparation of Fe-N-C catalyst. The Fe-N-C catalyst was synthesized according to our previous report 47 . Briefly, 1 g of the PBX 51 powder (Cabot) was oxidized by the oxygen plasma and then dispersed in 100 ml of 5 mg ml −1 ferric chloride hexahydrate (Millipore Sigma) solution.…”

Recent Advances in Strategies for Improving the Performance of CO2 Reduction Reaction on Single Atom Catalysts

“…The most common structure of Ni SACs consisting Ni atoms is coordinated by four pyridinic‐N atoms from the N‐doped carbon surpport. [ 35,49 ] Under the same conditions including preparation and measurement, Ni SACs always exhibit a more excellent ECR performance than other SACs, such as Fe, Co, Ni, and Cu. [ 42,50 ]…”

A Machine Learning Model on Simple Features for CO2 Reduction Electrocatalysts

“…Bulk metals and alloys, , nonmetals, , and nanostructured materials , have been applied to this domain. Moreover, experiments and computations have demonstrated successful applications of carbon-based materials toward electroreduction of CO 2 . − A kind of modified carbon-based catalysts with isolated single-atom sites, generally called single-atom catalysts (SACs), shows the potential for the electroreduction process due to its distinctive structures and properties. Pan et al designed such catalysts for CO 2 electroreduction, with N-doped porous carbon spheres as the substrate to disperse Co atoms.…”