In Situ Surface-Enhanced Raman Spectroscopy Analysis for Estimation of the Role of Pyridinic Nitrogen on Nitrogen-Doped Carbon Catalyst in CO 2 Electroreduction

For carbon-neutral transition, technologies are required which enable the efficient conversion of CO 2 into valuable feedstocks for chemicals and fuels (e.g., CO, methane, and ethylene). Electrochemical reduction of CO 2 (CO 2 ER) is one of the most promising pathways due to its high product selectivity as well as high activity at mild temperature and pressure, powered by electricity from renewable sources. Among the CO 2 reduction products with high selectivity through CO 2 ER, CO is a useful feedstock to produce methane, methanol, and olefins through conventional catalytic reactions. It is reported that noble metal CO 2 ER catalysts such as Au and Ag exhibit high CO selectivity [1] . Toward social implementation of CO 2 ER technology on a commercial scale, earth-abundant elements are more favorable as a catalyst. In recent years, nitrogen (N)-doped carbon materials have attracted much attention for their high CO selectivity comparable to Au and Ag. Previous studies indicate the N-doped carbon catalysts with high content of pyridinic N show high CO generation activity [2] . However, the specific role of pyridinic N (how N contributes to the activity during CO 2 ER) is still unclear. To establish a design strategy for the efficient N-doped carbon catalyst, the catalytic mechanism involving the pyridinic N must be clarified. In this study, carbon nanotube (CNT) is selected as a model carbon material whose commercial-scale production methods are relatively advanced among carbon materials. With electrochemical and spectroscopic measurements, the local pH dependence of the CO 2 ER catalytic performance of N-doped CNT (NCNT) was investigated, and the role of the pyridinic N on the CO 2 ER activity was discussed. The NCNT was prepared by pyrolysis of 1,10-phenanthroline on a multi-walled carbon nanotube as previously reported [2] . CO generation activity was examined in two types of electrochemical systems; the liquid phase electrolysis using an H-cell with a three-electrode system including 4 cm 2 NCNT-loaded glassy carbon as a working electrode, and the gas phase using a polymer electrolyte fuel-cell type reactor including membrane-electrode assembly (MEA) with 5 cm 2 active area. In the liquid phase electrolysis with CO 2 gas bubbling, NCNT showed high faradaic efficiencies toward CO in 1.0 M KHCO 3 electrolyte (pH 7.37), while no CO was detected in 1.0 M KHSO 4 (pH 0.55) at any applied potentials. In the case of the gas phase reaction, NCNT loaded on a carbo