Solar-driven selective conversion of millimolar dissolved carbon to fuels with molecular flux generation

Abstract The capture and utilization of the dissolved inorganic carbon in seawater, e.g., bicarbonates, is a promising strategy for accessing fuels on demand and anywhere. We report unbiased photoelectrochemical (PEC) CO2 reduction (CO2R) devices, which can facilitate sustainable sunlight-to-syngas conversion. However, there have been very few reports on the use of dissolved inorganic carbon for direct light-driven CO2 conversion to produce solar fuels. In this work, we design and implement 3D-printed PEC devices that employ a boundary layer flow. The flow over photoanode-photocathode pairs facilitates the efficient transport of in-situ generated CO2(aq), which is produced upstream at BiVO4 photoanodes, to downstream CO2R Si photocathodes. In flowing seawater, the solar-to-fuels (STF) efficiency improved from 0.4–0.71%, a record for PEC CO2R devices compared with BiVO4-Si systems operating in static bicarbonate electrolytes with continuous CO2 purging. Even in 2.3-mM HCO3− seawater, CO selectivity significantly increased from 3–21% with flow. The boundary layer flow confines the in-situ generated CO2(aq) to the surface of BiVO4 and Si photocathodes. Thus, an optimized flow field can increase the CO2(aq) and proton transport flux and simultaneously reduce the CO2(aq) residence time for its efficient utilization at Si photocathodes. Our process also features a high carbon efficiency: ~ 1 mmol CO2 is additionally released per 4 mmol CO produced.