A three-dimensional functional silver electrode on silicon with a trench-like optical structure realizes both broadband light harvesting and spontaneous oxidation reaction in 1 M sodium chloride solution through the silver chloride examined during hydrogen preparation. A standard electrode potential (Eo) of -0.23 V in the complete electrochemical reaction benefits hydrogen preparation without external bias. The proposed solar harvester under no power operation achieves 10 to 100 times the photoresponsivity and broader solar harvesting of previous studies. This work also provides the open-circuit voltage of 0.35V under one sun (100mW cm-2) illumination to accelerate the electrochemical reaction. The actual demonstration of hydrogen preparation, relying only on outdoor solar power, confirms the effectiveness of the proposed strategy.