Co-utilization of wastewater sludge and heavy metals for single-atom electrocatalytic reduction of gaseous CO₂

Synergetic management of waste activated sludge, heavy metals (HMs) and CO₂ for their valorization and cyclic utilization is rarely reported. Herein, we employed sludge-derived extracellular polymeric substances (EPS) and HMs in wastewater to fabricate a gas diffusion electrode (GDE) for electrochemical CO₂ reduction. This approach atomically dispersed Ni at each nanofiber of the GDE. Abundant N element in the EPS proved to play a key role in the formation of N_x-Ni (mixture of N₃-Ni and N₄-Ni) sites for highly efficient CO₂ to CO conversion. The atomical Ni³⁺ shows high catalytic activity. Direct gaseous CO₂ reduction in a membrane electrode assembly generated a current density up to 50 mA·cm⁻² with CO:H₂ ratio of ∼100 and ∼75% FE_CO under 2.69 cell voltage. This strategy takes advantage of all waste streams generated on site and consolidates traditionally separated treatment processes to save costs, produces value-added products and generates carbon benefits during wastewater treatment.