Photoelectrochemical nitrate reduction reaction (p-NORR) has been one of the most promising ways for ambient ammonia synthesis. However, current trials haven't realized efficient ammonia production due to low ammonia yield. Herein, we present that silicon (Si) photocathode with CoNi as co-catalyst can achieve low onset potential, high ammonia yield, and close-to-unity selectivity in p-NORR. The optimal photocathode (CoNi/Si) exhibits a record-high ammonia yield of 2054 µg h cm with an excellent Faraday efficiency (FE) of 98.6% at only −0.1 V vs. reversible hydrogen electrode (RHE), which outperforms reported photocathodes in literature. Our characterizations and calculations demonstrate that the metallic Co dominantly exists as the interior of CoNi/Si that accelerates electron transfer, while Ni is introduced into CoO on the surface that optimizes the adsorption behavior and improves the activity and selectivity. We believe that the findings provide insightful guidance for enhanced performance towards large-scale ammonia synthesis by photoelectrochemical nitrate reduction.