It was revealed that this large reactivity/selectivity difference between DC and rAP in (hetero)arene reduction could be explained by considering the competing proton reduction of the solvent (THF/EtOH mixture) (Figure ). Cyclic voltammetry indicates that the onset potential for proton reduction of this solvent system is around -2 V (vs Ag/Ag+). Accordingly, (hetero)arenes having more negative reduction potential than -2 V such as 32 will not be reduced in such solvents. Indeed, no reduction of 32 occurred when DC electrolysis was applied. Observation of gas evolution from the cathode strongly indicates that proton reduction was dominated under these conditions (Figure , right picture). In contrast, Birch reduction product 33 was observed in moderate yield when rAP was applied instead under the same conditions. Moreover, gas evolution was not noticeable (Figure , left picture). These observations strongly indicates that rAP-based arene reduction could take place even outside the electrochemical window of solvent. The implication of this phenomenon in electrochemical synthesis is profound. For example, consider the reduction of bromopyrrole 34 (Figure ). Standard rAP-based reduction gave 35 in 40% yield, where C-Br bond was cleaved due to the fast bond dissociation at the radical anion stage. Then, the same reduction was performed in the presence of Brønsted acid, expecting that rAP-heteroarene reduction could still operate by outcompeting proton reduction. The reduction product was indeed observed and, surprisingly, C-Br bond was retained (!) presumably due to fast protonation of the radical anion. Another instructive case came from the first large-scale rAP reaction (Figure ). The scale-up instruments are a simple combination of a signal generator and an amplifier with appropriate current/voltage output. The reactor was designed at AbbVie, which was adopted without further modification. Reaction conditions were considerably modified from the standard small-scale conditions to remove Me2S, further increase conductivity by using methanol as solvent instead of THF/EtOH mixture and replace fragile RVC (reticulated vitreous carbon) into mechanically robust graphite electrodes. Under these conditions, the product 15 was obtained on multi-gram scale. Interestingly, the starting material 14 was untouched when DC electrolysis was applied under the same conditions due to proton reduction of the solvent, emphasizing the crucial role of rAP to enable practical heteroarene reduction in inexpensive and practical conditions.