Electrochemical CO2 reduction presents a sustainable route to the production of chemicals and fuels. Achieving a narrow product distribution with copper catalysts is challenging and conventional material modifications offer limited control over selectivity. Here, we show that the mild cathodic potentials required to reach high currents in an alkaline gas-fed flow cell permits retention of a surface-bound thiol (4-mercaptopyridine), enabling molecule-directed selective formate generation at high reaction rates. Combined experimental and computational results showed that formate production is favoured due to the inhibition of a CO producing pathway caused by destabilising interactions with the anchored molecule. The immobilisation of molecules to inhibit specific carbon-based products therefore offers a novel approach to rationally tune the selectivity of heterogeneous catalysts.