Bifunctional Nitrogen and Cobalt Codoped Hollow Carbon for Electrochemical Syngas Production

Abstract

Electrochemical conversion of CO2 and H2O into syngas is an attractive route to utilize green electricity. A competitive system economy demands development of cost‐effective electrocatalyst with dual active sites for CO2 reduction reaction (CO2RR) and hydrogen evolution reaction (HER). Here, a single atom electrocatalyst derived from a metal–organic framework is proposed, in which Co single atoms and N functional groups function as atomic CO2RR and HER active sites, respectively. The synthesis method is based on pyrolysis of ZnO@ZIF (zeolitic imidazolate framework). The excess in situ Zn evaporation effectively prevents Co single atoms (≈3.4 wt%) from aggregation and maintains appropriate Co/N ratio. The as‐prepared electrocatalyst is featured with high graphitic degree of carbon support for rapid electron transport and sponge‐like thin carbon shells with hierarchical pore system for facilitating active site exposure and mass transport. Therefore, the electrocatalyst exhibits a nearly 100% Faradic efficiency and a high formation rate of ≈425 mmol g−1 h−1 at 1.0 V with the gaseous product ratio (CO/H2) approximating ideal 1/2. With the assistance of an extensive material characterization and density functional theory (DFT) calculations, it is identified that Co single atoms are uniformly coordinated in the form of Co–C2N2 moieties, and act as the major catalytic sites for CO2 reduction.

Supported Single Atoms as New Class of Catalysts for Electrochemical Reduction of Carbon Dioxide

“…[61a] This result was further confirmed by Song et al, who synthesized a Co SACs (3.4 wt%) constructed from Co–C 2 N 2 moieties . The Co SACs reached a nearly 100% FE at potential less than −0.7 V. Interestingly, the CO selectivity of Co SAC dramatically decreased to 9.8% after the addition of potassium thiocyanate, confirming that the active site for CO 2 RR is Co–N 2 …”

“…I) CO FEs of Co–N 2 , Co–N 3 , and Co NPS at different applied potentials. Reproduced with permission . Copyright 2017, Wiley‐VCH.…”

“…These have been demonstrated by the dramatic decrease of CO 2 RR activity after blocking of the metal active single‐atom sites using SCN − . [53b,62] Strasser and co‐workers investigated the trends of M–N x species (M is Mn, Fe, Co, Ni, Cu) in carbon for CO 2 RR . The initial LSV analysis revealed that the Mn‐, Fe‐, Ni‐, and Cu‐doped carbons show lower η for the CO 2 RR as compared to that for HER.…”

“…The Co–N–C catalysts showed H 2 FE of 80% at all applied potential, revealing that it is a poor catalyst for CO 2 RR, consistent with the reported results that Co–N 4 is poor for CO 2 RR. [61b,62] Fe–N–C and especially Ni–N–C catalysts are promising for selective CO production, with a CO production FE of 65% at −0.55 V for Fe–N–C and a CO production FE of 85% on Ni–N–C . Three distinct reaction dynamics regions that determine the CO 2 RR activity have been identified through the investigation of the relationship between the predicted DFT theoretical energy diagrams and the experimental TOF for CO formation ( Figure ).…”

A Disquisition on the Active Sites of Heterogeneous Catalysts for Electrochemical Reduction of CO2 to Value‐Added Chemicals and Fuel

“…In a control experiment where Co–N 2 , Co–N 3, and Co–N 4 catalysts were tested for CO 2 RR, it was observed that Co–N 2 demonstrated the highest FE CO of 95% at −0.68 V compared to 63% at −0.53 V for Co–N 3 and 7% with Co–N 4 catalysts . These results were also verified by a Co single atom on hollow carbon structure (Co‐HNC) catalyst that also consisted of Co–N 2 active sites and demonstrated an unprecedented FE CO of ≈100% . In recent times, Co–N 5 sites anchored on polymer derived N‐doped porous carbon spheres were also reported to be active for CO 2 RR…”