Metal-encapsulated carbon nanotube arrays for enhancing electrocatalytic nitrate reduction in wastewater: importance of lying-down to standing-up structure transition

Abstract

Metal-encapsulated and nitrogen-doped carbon-based (M@NC) catalysts have emerged as promising options for electrochemical nitrate reduction reaction (ENRR) because of their excellent activity and stability. However, in the form of powder or the self-supporting metal nanoparticle-embedded carbon layer morphology, they show unsatisfactory denitrification performance when ENRR is performed at a high current density (e.g., >10 mA cm⁻²). In this work, we comprehensively determined the underlying reasons for this phenomenon and accordingly synthesized a free-standing M@NCNT (carbon nanotube) array on carbon cloth (CC) to resolve the issue. Experimental results show that the utilization efficiency of the active sites of M@NC plays a crucial role in guaranteeing superior ENRR activity under kinetically promoted conditions. Compared with the lying-down morphology, the unique standing-up structure enables more active centers that are accessible to the reactants, thus improving their utilization and overall ENRR performance. The as-prepared Co@NCNT/CC cathode delivered outstanding denitrification activity (with a removal proportion of nearly 100% over 3 h electrolysis at 10 mA cm⁻²) and good stability (continuously running for ENRR over 30 consecutive cycles without performance deterioration). When operated under the constant-current mode, it was also effective in removing nitrate from groundwater and real coking wastewater effluent. The findings provide important insights into the rational design of advanced M@NC catalysts toward ENRR.