Cathodic Hydroxide Ions Induce Tetrose Formation during Glycolaldehyde Electroreduction to Alcohols: A Potential CO ₂ ‐to‐Carbohydrate Pathway

The electrochemical synthesis of organic compounds from CO₂ can potentially alleviate climate change by hampering the atmospheric accumulation of greenhouse gases. The production of carbohydrates from CO₂ reduction will have promising applications for the manufacturing of valuable, multi-carbon compounds that are traditionally produced from the petrochemical or agricultural industries. In this work, we analyzed the copper-catalyzed electrochemical reduction of glycolaldehyde, a commonly observed trace CO₂RR product that has been previously proposed as an intermediate for alcohol formation. We determine that glycolaldehyde is not the main intermediate on polycrystalline copper-based electrocatalysts that selectively produce ethanol. In an unbuffered electrolyte, the cathodic hydroxide ions produced induce the coupling of glycolaldehyde to tetroses in the solution phase, yielding a maximum glycolaldehyde-to-sugar conversion of 47.2% under ambient conditions. Using in situ infrared spectroscopy coupled with density functional theory (DFT) calculations, we show that glycolaldehyde reduction to alcohols proceeds via adsorption of its enol tautomer, η²(C,C)─CHOH═CHOH. Our findings not only shed light on the C₂ alcohol formation pathways during CO₂RR, but also imply that a CO₂ electrolyzer can potentially produce C₄ carbohydrates via CO₂ reduction to glycolaldehyde followed by C─C coupling in the solution phase, with only a high local pH needed to drive the tetrose formation step.