Influence of Carbon Nanotube Support on Electrochemical Nitrate Reduction Catalyzed by a Cobalt Complex

Access to clean water is a pressing environmental and public health concern. Excess nitrate (NO 3 - ) from the overuse of fertilizer finds its way into our surface water and groundwater, which leads to adverse health effects such as cancer and causes significant damage to ecosystems. 1-4 The electrochemical nitrate reduction reaction (NO 3 RR) has emerged as a promising water treatment approach to address this issue. The NO 3 RR involves the transformation of waste NO 3 - into value-added nitrogen species, such as ammonia (NH 3 ), via reduction. NH 3 serves as a valuable chemical in various applications, including its use as a fertilizer, chemical precursor, fuel, and a potential hydrogen (H 2 ) carrier for energy storage and transportation. 5-7 Leveraging renewable energy, the NO 3 RR provides a sustainable approach to distributed NH 3 production, which stands in stark contrast to the energy-intensive Haber-Bosch process. 8-10 As a result, there is growing interest in designing catalysts for the selective reduction of NO 3 - to NH 3 . However, this requires that we obtain a clear understanding of the role of each component in the catalyst structure. Molecular catalysts have unique advantages over other electrocatalytic materials due to their well-defined structures, which can be tailored through the choice of the metal center and ligands. 11,12 Moreover, molecular catalysts can be immobilized on substrates to create heterogeneous electrocatalysts capable of achieving a high current density. 13,14 In previous investigations, we showcased the efficacy of multi-walled carbon nanotubes (CNTs) as a catalyst support for electrochemical carbon dioxide reduction reactions. 13,15 In heterogenized molecular catalysts, CNTs are generally believed to provide high surface area and electron conduction. However, our recent investigation reveals that CNTs can serve as catalysts themselves for the NO 3 RR, 16 which expands our understanding of the diverse roles CNTs play in NO 3 - reduction electrocatalysis. A major challenge in the field of electrocatalysis is the development of catalyst design strategies to control the reaction selectivity. We report on an underexplored approach to overcome this challenge by tuning the CNT support for a cobalt complex. With pristine CNTs as the support, the cobalt complex/CNT hybrid catalyst is selective for NO 3 - reduction to NH 3 with a maximum Faradaic efficiency of 70%. In contrast, the cobalt complex supported on oxidized CNTs (OCNTs) generates mostly H 2 under the same conditions. On the basis of kinetic measurements which reveal that the rate-determining step of NO 3 – reduction is limited by the first electron transfer without involving a proton, we propose that the oxygen functional groups on the OCNT support help deliver protons and steer the supported cobalt complexes from catalyzing NO 3 - reduction to H 2 evolution. The performance of both of these hybrid catalysts is compared with the controls of bare CNTs and OCNTs. Additional control experiments include tests conducted in the absence of NO 3 - . 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