Design and synthesis of copper single-atom electrocatalysts for electrocatalytic nitrate-to-ammonia conversion

This project investigates copper single-atom catalysts on nitrogen-doped multi-walled carbon nanotubes for the electrochemical nitrate reduction reaction (NO3RR) to ammonia. Industrial ammonia production via the Haber–Bosch process consumes 1–2% of global energy and generates significant CO2 emissions. Electrochemical reduction of nitrate, a widespread pollutant in agricultural runoff and industrial wastewater, offers a sustainable and decentralised alternative that couples ammonia synthesis with environmental remediation under ambient conditions. However, conventional Cu nanoparticle catalysts suffer from poor NH3 selectivity due to N–N coupling at adjacent active sites and competing hydrogen evolution reaction (HER). Single-atom catalysts, with spatially isolated metal centres, are hypothesised to suppress these parasitic pathways and improve Faradaic efficiency. This project aims to determine whether Cu SACs synthesised from Cu(NO3)2 using benzene-1,4-dicarboxylic acid (BDC) as a coordination spacer on MNCNTs can achieve improved Faradaic efficiency over Cu nanoparticle benchmarks. Three catalysts were compared: Catalyst B (Cu SAC/MNCNT, target), Catalyst A (Cu NP/MNCNT, without BDC), and Catalyst C (BDC/MNCNT, metal-free control). All shared identical supports and pyrolysis conditions (800 °C, 2 h, Ar). Catalysts were characterised by SEM-EDX and XRD. Electrochemical performance was evaluated by CV, chronoamperometry, EIS, and OCP in 0.1 M KNO3 / 1 M KOH. Ammonia product was quantified by the indophenol blue method. BDC-mediated synthesis achieved partial single-atom dispersion on Catalyst B, consistent with (but not definitive proof of) sub-nanometre Cu dispersion, as indicated by the absence of SEM-resolvable nanoparticles and attenuated XRD Cu0 peak intensity compared to Catalyst A. Catalyst B achieved the highest peak NH3 yield rate of 42,251 µg h-1 mg-1 at −0.80 V vs. RHE, surpassing Catalyst A (32,221 µg h-1 mg-1) and Catalyst C (30,477 µg h-1 mg-1). Catalyst B also showed superior selectivity: FE of 73.5% at −0.4 V vs. RHE, compared to 56.7% for Catalyst A, giving a rough 17 percentage point advantage from geometric HER suppression.