Intrinsic Catalytic Activity of Carbon Nanotubes for Electrochemical Nitrate Reduction
Nia J. Harmon, Conor L. Rooney, Zixu Tao, Bo Shang, Neera Raychaudhuri, Chungseok Choi, Huaping Li, Hailiang Wang
Abstract
We study a variety of carbon nanotubes (CNTs) as cost-effective catalysts for the electrochemical nitrate (NO3–) reduction reaction, which holds promise for converting environmental NO3– pollutants into useful ammonia (NH3) products. We discover that pristine multi-walled carbon nanotubes (MWCNTs) exhibit notable electrocatalytic activity for NO3– reduction to NH3 in a near-neutral electrolyte. Single-walled carbon nanotubes have even higher activity and selectivity than MWCNTs for this reaction. Results from control experiments confirm that this activity originates from the native carbon surface rather than from metal impurities. Contrary to the prevalent notion that heteroatom doping into carbon materials increases their electrocatalytic performance, we find that introducing oxygen and nitrogen functional groups into MWCNTs reduces their electrocatalytic activity for NO3– reduction. After analyzing the potential-dependent electrocatalytic performance of the CNT catalysts, we further report an interesting positive correlation between total current density and Faradaic efficiency for NH3 formation irrespective of the type of CNT. This correlation suggests deeper NO3– reduction at higher reaction rates regardless of the catalyst identity. The findings presented in this work suggest a unique type of active site for electrocatalytic NO3– reduction, therefore broadening the role of carbon materials in electrocatalysis.