Fig. 1: Comparison of photovoltaic-electrochemical (PV + EC) systems and photoelectrochemical (PEC) systems for separations.

a PV + EC systems consist of separate but connected PV and EC systems, where the PV system absorbs sunlight to produce the electric current that then drives the electrochemical separation in the EC system. b Internally, the EC system consists of a working electrode (WE) and counter electrode (CE), of which at least one electrode is functionalized with a redox interface. Current from the PV system charges the interface, which then selectively binds ions in the solution flowing between the electrodes. c On the other hand, solar-to-current conversion and the electrochemical process occur in a single unified unit operation that absorbs sunlight and also performs the separation. d Internally, a semiconductor layer between the conducting electrode surface and the redox interface absorbs light to produce charge carriers that separate within the electrode to charge the interface. The redox functionalization in (b, d) takes the form of a polymer coating², though other implementations are also possible²⁵. Batteries may be included in both PV+EC and PEC systems to store excess energy and modulate cell potentials.