Abstract: The catalytic, electrocatalytic, or photocatalyticconversion of CO2 into useful chemicals in high yield forindustrial applications has so far proven difficult. Herein, wepresent our work on the electrochemical reduction of CO2 inseawater using a boron-doped diamond (BDD) electrodeunder ambient conditions to produce formaldehyde. Thismethod overcomes the usual limitation of the low yield ofhigher-order products, and also reduces the generation of H2.In comparison with other electrode materials, BDD electrodeshave a wide potential window and high electrochemicalstability, and, moreover, exhibit very high Faradaic efficiency(74%) for the production of formaldehyde, using eithermethanol, aqueous NaCl, or seawater as the electrolyte. Thehigh Faradaic efficiency is attributed to the sp3-bonded carbonof the BDD. Our results have wide ranging implications for theefficient and cost-effective conversion of CO2.