Having established enhanced HCOO -production with Cu-SPy, we used two thiols with different structural features to help understand the mechanism. We first immobilised thiophenol (SPh), which is structurally similar to SPy but lacks the nitrogen functionality, and conducted CCE experiments under the same conditions as for Cu-SPy. We observed that Cu-SPh GDEs showed comparable HCOO -selectivity to unmodified Cu electrodes (Table ) this suggested that the nitrogen plays a key role in favouring formate production. We next studied the nitrogen contribution by looking into the possibility that reduced pyridine could act as a hydride donor. In highly alkaline conditions at cathodic potentials, dihydropyridine formation is only possible at the 2 or 6 positions of the pyridine.(39) Therefore we synthesised 2,6-dimethyl-4-mercaptopyridine (DMSPy), for which these sites are blocked, to see if the selectivity was altered. At -300 mA cm -2 we observed similar HCOO -yields to Cu-SPy, and consequently showed that a pyridine-based hydride mechanism was not possible, in line with previous works (Table ). These experiments showed that the nitrogen plays an important role but its exact function was unclear. We used DFT calculations to identify possible alterations that could account for the selectivity enhancement.