Direct ammonia and dihydroxyacetone production in an unbiased photoelectrochemical cell

Photoelectrochemical production of ammonia usually suffers from a low solar-to-ammonia efficiency and a high overpotential, which influences the bias-free operation of sustainable photoelectrochemistry. Herein, we realize solar-driven ammonia production from waste nitrate by constructing copper-osmium catalysts deposited on the Sb₂(S,Se)₃ semiconductor, enabling optimized photo-carrier transport pathways and a beneficial co-adsorption configuration of *NO₃-H₂O moieties. The photocathode reaches a photocurrent density of 5.6 mA cm⁻² at 0 V_RHE with a low onset potential of 0.86 V_RHE and a Faradaic efficiency of 96.98% at 0.6 V_RHE under AM 1.5 G illumination. We further employ glycerol oxidation reaction on ruthenium doped bismuth oxide catalyst decorated on titanium oxide photoanode, requiring an onset potential of 0.3 V_RHE to enable bias-free operation. The unbiased photoelectrochemical system shows Faradaic efficiencies of over 97% for ammonia products and above 77% for glycerol oxidation product under AM 1.5 G illumination. The large-sized photoelectrodes maintain a stability for 24 h without noticeable degradation. Our works indicate that unassisted and stable PEC ammonia production is feasible with in situ glycerol valorization using the photoanode and photocathode.