Photoelectrochemical CO2 Reduction Devices Employing A Boundary Layer Flow for Direct Ocean Carbon Capture and Conversion

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.