One-Pot Synthesis of Copper-Embedded Nitrogen-Doped Carbon Nanofiber Catalysts for Cascade Electroreduction of Carbon Dioxide to Ethylene
Authors: Kim Ji-Yong , Lee Jae-Chan , Joo Wonhyo , Hong Deokgi , Oh Sang-Ho , Kim Beomil , Lee Gun-do , Kim Miyoung , Oh Jihun , Joo Young-Chang 
Published: May 2020

Electrochemical reduction reaction of CO2 (CO2 RR) to valuable products offers potential for storing excessed renewable energy as well as chance to close the carbon loop, displacing the fossil fuel in chemical industries. Major efforts have been made to direct conversion of CO2 to ethylene which is widely used building blocks for chemical industries with large market size. Recently, tandem electrocatalysts composed of Cu and promoter metals which produces CO such as Au, Ag, and Zn have been investigated to enhance ethylene selectivity by increasing local concentration of CO*, overcoming limitation of pure Cu. However, these catalysts fabricated by sequential deposition of promoter metals on copper have limits in difficulty of controlling proximity between activated CO production sites and copper surfaces.

Carbon is known for good supporter for the electrocatalyst due to the good electrical conductivity and electrochemical stability. In addition, Doped nitrogen in the pyridinic or pyrrolic configurations acts as the active site of CO2 reduction to CO. However, many researches have been reported carbon as supporter or catalyst producing CO gas, not as CO supplying co-catalyst with Cu. This may be caused by difficulty of composing nitrogen uniformly in carbon and in the proximity of Cu. Here, we have developed a new organic-metal hybrid tandem catalyst composed of Cu particles embedded in N-doped carbon nanofiber (Cu/N-CNF) and N atoms located on the periphery of Cu particles via “one-pot selective oxidation”.

In order to fabricate Cu/N-CNF, electrospun copper acetate (CuAc: Cu(CO2CH3)2)/polyacrylonitrile (PAN: (C3H3N)n) nanofibers were calcinated at thermodynamically designed oxygen-controlled conditions based on Ellingham diagram. Cu-embedded undoped carbon nanofiber (Cu/CNF) catalysts were also synthesized by same process as Cu/N-CNF using polyvinyl alcohol (PVA : (C2H4O)n) instead of PAN. During calcination, temperature was maintained at 800 ºC for 5 h under 50 mTorr of pO2 at which the carbon is combusted and the copper is reduced, besides, the nitrogen hardly reacted with oxygen during calcination. It was confirmed that metallic Cu particles are successfully formed in all catalysts by SEM and TEM analysis. Also, Cu/N-CNFs contains pyridinic N predominantly than other N species by SEM and XPS analysis.

The activity and faradaic efficiency (FE) of each product were examined by flow cell reactor with 5 M KOH electrolyte. Cu/N-CNFs show higher current densities compared to Cu/CNF catalyst at measured potential range (-0.2 ~ -0.8 V vs. RHE). Furthermore, Cu/N-CNF shows maximum C2H4 selectivity (62%) at much lower overpotential of -0.57 V vs. RHE compared to undoped Cu/CNF (-0.75 V vs. RHE). It can be inferred that increased CO production by doped N promotes CO* dimerization, whereas, without doped N, lower CO* generation limits the maximum amount of CO* dimerization. Furthermore, decrease of the CO dimerization energy barrier by CO production of doped N around Cu particles also will be discussed by DFT calculations.