Towards the use of low-concentration CO2 sources by direct selective electrocatalytic reduction

Abstract

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 feed-stock. 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 Ag- and 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.

Keywords: CO2 reduction; electrocatalysis; flow-through electrolyzer; low CO2 concentration; simulated flue gas.