Capacitive-faradaic fuel cells (CFFCs) for ion separation: Macro-scale configurations and polarization mechanism

The polarization mechanism of capacitive-faradaic fuel cells is reported. The mixed potentials of faradaic (Pt and Ti/Pt-IrO2) and capacitive (activated carbon powder and fleece) electrodes were measured in aerated and hydrogenated NaCl solutions at various pH values. Next, two configurations featuring divided and undivided macro-scale CFFCs were investigated: (i) a system with two fixed electrodes made of Ti/Pt-IrO2 and activated carbon fleece, and (ii) a CFFC with one fixed Ti/Pt-IrO2 electrode and one flowing carbon electrode. The salt adsorption capacities obtained in both systems were low compared to conventional capacitive deionization, and therefore optimization of materials and cell architecture is needed. The behaviour of pH and Cl−, SO42− anions conforms to the proposed capacitive-faradaic mechanism, but that of NH4+ and Na+ cations contradicts it. This phenomenon is attributed to faradaic processes and the competitive adsorption–desorption of H+ and OH− ions, which require further detailed investigation.