In Situ Surface-Enhanced Raman Spectroscopy Analysis for Estimation of the Role of Pyridinic Nitrogen on Nitrogen-Doped Carbon Catalyst in CO2 Electroreduction

Abstract

For carbon-neutral transition, technologies are required which enable the efficient conversion of CO2 into valuable feedstocks for chemicals and fuels (e.g., CO, methane, and ethylene). Electrochemical reduction of CO2 (CO2ER) 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 CO2 reduction products with high selectivity through CO2ER, CO is a useful feedstock to produce methane, methanol, and olefins through conventional catalytic reactions. It is reported that noble metal CO2ER catalysts such as Au and Ag exhibit high CO selectivity[1]. Toward social implementation of CO2ER 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 CO2ER) 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 CO2ER catalytic performance of N-doped CNT (NCNT) was investigated, and the role of the pyridinic N on the CO2ER 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 cm2 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 cm2 active area. In the liquid phase electrolysis with CO2 gas bubbling, NCNT showed high faradaic efficiencies toward CO in 1.0 M KHCO3 electrolyte (pH 7.37), while no CO was detected in 1.0 M KHSO4 (pH 0.55) at any applied potentials. In the case of the gas phase reaction, NCNT loaded on a carbon paper (a gas diffusion electrode) with a Nafion® ionomer (Nafion® DE2020CS, The Chemours Company, Delaware, U.S.) showed poor CO selectivity at any cell voltage in both cases using a proton and an anion exchange membrane, while the ionomer substitution to Sustainion® XA-9 (Dioxide Materials, Inc., Florida, U.S.), an anion exchange type, increased the faradaic efficiency toward CO remarkably. From those results, it was confirmed that the local pH near the catalyst surface affects the CO generation activity of NCNT.The local pH dependence on the CO generation activity could be attributed to the CO2 concentration in the electrolyte as well as the protonation of the pyridinic N of the catalyst as previously reported in the system of O2 electroreduction[3]. To observe changes in the local pH and the chemical state of pyridinic N directly, spectroscopic analyses were conducted. We applied our previously reported in situ surface-enhanced Raman spectroscopy measurement system[4] to this reaction to detect the local pH near NCNT surface in the liquid phase as well as the adsorbate species. Surface X-ray photoelectron spectroscopy was also performed to study the changes in the chemical state of N. Those spectroscopic analyses suggested that the factor which affects the CO2ER activity is not only the difference in dissolved CO2 concentration induced from the less acidic condition but also the change in the pyridinic N state.