(666d) Upscaled and Tunable Synthesis of M-N-C Type Single Atom Catalysts (SACs) Towards Selective Electrochemical CO2 Reduction to CO

The advent of fine-tuning single atom catalysts (SACs) enabled a plethora of research work on different carbon-based and metal oxide-based substrates spanning different electrochemical applications. So called anchor engineering of the SAC has been found to be detrimental towards the stability of the active sites at commercially relevant current densities and harsh electrochemical micro-environments. To that end, M-N-C class SACs have witnessed increasing interest due to the apparent SAC stability on the carbon support when coupled with nitride character, as well as the amalgam of different possible coordination structures leading to tunable reaction profiles. To the best of our knowledge, M-N-C type SACs are produced in milligram and gram scale quantities at lab-scale. Herein, we provide an overview of two patented methods that employ commercial carbon nanotubes (CNTs) as both the carbon support and SAC metal source from intrinsic impurities. These methods also utilize pharmaceutical wastes (sulfamethoxazole; CNT-SMX) or melamine (CNT-Mel) to act as sustainable and adaptable nitrogen precursors, making the M-N-C synthesis more scalable and allowing tens of grams per batch without affecting performance. Targeting CO as a model electrochemical CO₂ valorization product, both CNT-SMX and CNT-Mel were investigated within a flow cell wherein > 95% faradaic efficiency towards CO were measured at commercially relevant 400 mA cm^-2 of current density for both materials. Further, selectivity and electrochemical stability tests were performed between different batch-sizes to infer any variations to the catalytic performance. An array of characterization strategies including morphological (SEM, HRTEM, STEM, EDX), crystallinity (XRD, Raman), and surface chemical (XPS, XAS), as well as first-principle density functional theory (DFT) analyses were undertaken to investigate and confirm the identify and structural features of the attained catalyst for CO production. The presented work aims to highlight the importance and demonstrate the feasibility of upscaled M-N-C type SACs towards commercial applications.