Engineering the Surface of a Polymeric Photocatalyst for Stable Solar‐to‐Chemical Fuel Conversion from Seawater

The design of an efficient and highly selective organic polymeric semiconductor photocatalyst consisting of Earth‐abundant elements for solar fuel generation using seawater, and also deionized water, as a proton source is reported. The mesoporous g‐C 3 N 4 synthesized using a conventional precursor (urea) shows significant H 2 generation (ca. 33 000 μmol h −1  g −1 ) and drives the photoreduction of CO 2 to CH 4 , along with trace amount of methanol. However, when the chosen precursor cyanamide is used, drastic improvement in H 2 generation (ca. 41 600 μmol h −1  g −1 ) and CO 2 photoreduction is observed. The introduction of a surface nitrogen deficiency and modification of the surface with Cu 0 further enhances solar H 2 generation (ca. 50 000 μmol h −1  g −1 ) and CO 2 photoreduction (3.12 μmol h −1  g −1 ) activity, respectively, owing to improvement in light harvesting and charge separation, as revealed by a shorter average lifetime of 3.52 ns and higher Stern–Volmer quenching constant value of approximately 11.2  m −1 . In addition, improved selectivity in CO 2 photoreduction to only CH 4 is also observed. The designed photocatalytic system is stable, with the solar H 2 generation rate increasing even after 20 h under continuous illumination with a turnover number of 6500. When seawater used instead of deionized water, the overall solar fuel generation efficiencies of all photocatalysts marginally decreased owing to a decrease in the photogenerated charge‐carrier separation efficacy.