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

The direct utilization of dissolved inorganic carbon in seawater for CO₂ conversion promises chemical production on-demand and with zero carbon footprint. Photoelectrochemical (PEC) CO₂ reduction (CO₂R) devices promise the sustainable conversion of dissolved carbon in seawater to carbon products using sunlight as the only energy input. However, the diffusion-dominant transport mechanism and the near-zero concentration of CO₂(aq) (CO₂ dissolved in aqueous solution) in static seawater has made it extremely challenging to achieve high solar-to-fuel (STF) efficiency and high carbon-product selectivity. Here, where CO₂(aq) as a reactant generated in situ by acidification of HCO₃^- flows continuously from BiVO₄ photoanodes to Si photocathodes, enabling a single-step conversion of dissolved carbon into products. Our PEC device significantly increases the CO selectivity from 3% to 21%, which approaches the 30% theoretical limit according to multi-physics modeling. Meanwhile, the Si/BiVO₄ PEC CO₂R device achieved a STF efficiency of 0.71%. Such flow engineering achieves flow-dependent selectivity, rate, and stability in simulated seawater, thus promising practical solar fuel production at scale.