The electro-reduction of carbon dioxide in a continuous reactor

This paper reports an investigation into the electro-reduction of CO2 in a laboratory bench-scale continuous reactor with co-current flow of reactant gas and catholyte liquid through a flow-by 3D cathode of 30# mesh tinned-copper. Factorial and parametric experiments were carried out in this apparatus with the variables: current ( 1 - 8 A), gas phase CO2 concentration ( 16 - 100 vol%) and operating time ( 10 - 180 min), using a cathode feed of [CO2 + N-2] gas and 0.45 M KHCO3(aq) with an anolyte feed of 1 M KOH(aq), in operation near ambient conditions (ca. 115 kPa(abs), 300 K). The primary and secondary reactions here were respectively the reduction of CO2 to formate (HCOO-) and of water to hydrogen, while up to ca. 5% of the current went to production of CO, CH4 and C2H4. The current efficiency for formate depended on the current density and CO2 pressure, coupled with the hydrogen over-potential plus mass transfer capacity of the cathode, and decreased with operating time, as tin was lost from the cathode surface. For superficial current densities ranging from 0.22 to 1.78 kA m(-2), the measured values of the performance indicators are: current efficiency for HCOO- = 86 - 13%, reactor voltage = 3 - 6 Volt, specific energy for HCOO- = 300 - 1300 kWh kmol(-1), space-time yield of HCOO- = 2 x 10(-4) - 6 x 10(-4) kmol m(-3) s(-1), conversion of CO2 = 20 - 80% and yield of organic products from CO2 = 6 - 17%.