<p>Integration of efficient value-added electrochemical oxidation with hydrogen evolution reaction presents a promising sustainable route for both hydrogen energy and the electrochemical refinery industry. However, the serious competition of oxygen evolution reaction (OER) with targeted oxidation reactions at high current densities forms a substantial hurdle for industrial application. Here we report a straightforward approach to inhibit OER side reaction by introducing a trace amount of Cu²⁺ into the electrolyte for efficient glycerol oxidation reaction (GOR). Such a strategy enables improved Faradaic efficiency of glycerol to the target product formic acid from 62.2% (without Cu²⁺ addition) to 99.3% at a high current density of 800 mA cm⁻². The underlying mechanism is that a reversible redox process of Cu²⁺/Cu⁺ fully suppresses the formation of OER-active-phase hydroxy peroxide on the surface of GOR-active Co₃O₄ catalyst. The current strategy also applies to other important electrochemical oxidation reactions, paving the way for developing efficient non-OER electrochemical oxidation reactions for various chemical conversion processes.</p>