While semiconductors (like silicon, perovskites, or metal oxides) are exceptional at absorbing light and generating excited electrons, they are often terrible catalysts for complex, multi-electron reactions like the electrochemical reduction of CO2. If you attempt to reduce CO2 directly on the surface of a photoelectrode, it generally results in poor product selectivity, or the semiconductor physically corrodes away in the liquid electrolyte (photocorrosion). A PEC-EC system solves this by dividing the labor. It physically and electrically couples a solar-harvesting photoelectrode (the PEC side) with a highly optimized, specialized "dark" electrocatalyst (the EC side).

The PEC Photoanode (The Solar Engine): Positioned behind a quartz or specialized glass window, a semiconductor (such as BiVO4, Fe2O}3, or TiO2) absorbs incoming photons. The photon energy excites electrons, driving the Oxygen Evolution Reaction (OER) on the surface of the photoanode. It splits flowing liquid water into O2 gas, protons (H+), and electrons (e-). The flowing anolyte continuously sweeps the O2 bubbles away so they do not scatter the incoming sunlight.

The "Dark" EC Cathode (The Chemical Factory): The electrons generated by the sun are funneled through an external circuit (or a monolithic internal conductive layer) to the opposite side of the flow cell. This side utilizes the exact Gas Diffusion Electrode (GDE) architecture required for continuous gas conversion. Humidified CO2 gas is forced through the GDE.A highly engineered, purely electrochemical catalyst (like nanostructured copper, silver, or single-atom catalysts) sits on the liquid-gas boundary. This catalyst receives the solar-generated electrons and uses them to reduce the CO2 into specific, targeted products like ethylene, formate, or carbon monoxide, which are then swept out by the catholyte flow loop.

References:

1. Chanon Pornrungroj, et al., Bifunctional Perovskite-BiVO4 Tandem Devices for Uninterrupted Solar and Electrocatalytic Water Splitting Cycles, Adv. Funct. Mater., 2021, 31, 2008182. 

2. L. Zhang, et al. Decoupled Artificial Photosynthesis, Angew. Chem. Int. Ed., 2023, 62, e202219076.