Utilizing sustainable electrocatalysis to upcycle glycerol (GLY; the by-product of bio-diesel) to lactic acid (LA; the key monomers for biodegradable polymer) is an efficient way to reduce the cost of biodiesel, which is also consistent with the waste-to-wealth principle. However, current research still suffers from issues of low LA selectivity (<80%), especially at high current density.

Herein, we designed a cooperative catalyst by modifying BiO_x nanolayers on gold nanoparticles (BiO_x/Au), achieving high LA selectivity (82%) at high current density (226 mA cm^-2 at 1.05 V vs. RHE), outperforming most of the reported works in the literature. We reveal that BiO_x could promote the adsorption of the middle hydroxyl of GLY, which facilitates the formation of DHA and thus enables high LA selectivity. Moreover, the electron-deficient Au in BiO_x/Au is responsible for the enhanced current density, which is more beneficial to generate reactive oxygen species (OH*). To further reveal the application potential of this electrocatalytic strategy, we realized that the conversion of crude GLY extracted from cooking oil to LA coupled with H₂ production using a membrane-free flow electrolyzer, demonstrating a sustainable fashion to convert bio-diesel waste into high value-added product and H2 fuel.