<p>Inefficient charge separation and poor interfacial reaction selectivity constitute major barriers to semiconductor-driven photoelectrocatalytic synthesis of high-value-added chemicals. Herein, we find that the co-oxidation of polyols and NH₃ on four typical and unprotected photoanodes [i.e., BiVO₄, α-Fe₂O₃, TiO₂ and WO₃] generates even higher photocurrent densities than those commonly used hole scavengers. Detailed research on BiVO₄ photoanodes shows that the co-oxidation process induces the in situ formation of Bi/V-rich surfaces and enables the interfacial charge transfer efficiency approaching 100%. The achieved photocurrent density of 7.3 mA cm⁻² at 1.23 V_RHE approaches the theoretical limit of BiVO₄ on the unprotected photoanodes, which delivers formamide production of 171.5 μmol cm⁻² h⁻¹. By using an amplified flow photoelectrochemical cell, the photocurrent reaches 1.2A, producing formamide at the rate of 17.5 mmol h⁻¹ and achieving the gram-scale synthesis. The co-oxidation method illustrates an efficient strategy for designing photoelectrochemical systems at ampere-level photocurrents.</p>