Towards the Use of Low-Concentration CO₂ Sources by Direct Selective Electrocatalytic Reduction

The direct CO₂ reduction reaction (CO₂RR) from simulated flue gas of various CO₂ concentrations could minimize extra energy for pre-concentration processes to highly concentrated CO₂ as a feedstock. We investigate the challenges for CO₂RR caused by low CO₂ concentrations and provide strategies concerning the impact of the chosen electrocatalyst material and the selection of the electrolyte to attain high CO selectivity. We continuously feed CO₂ mixed with N₂ (the typical dilutant in flue gas) in various ratios to gas diffusion electrodes in a model flow-through electrolyzer. Operating the CO₂RR at lower CO₂ concentrations results in an overpotential shift to more cathodic values. We show that higher active catalysts can maintain high CO selectivity down to 5 % CO₂ by using NiCu-based catalysts. NiCu reached its limit when the CO₂ concentration was lowered to 2 %, due to low CO₂ availability and competition of carbonate formation. Employing near-neutral electrolytes with buffering capacity, we maintained high Faradaic efficiency at low overpotentials and higher CO₂ utilization at low CO₂ concentration.