Effect of proton sources on the electrocatalytic hydrogen evolution reaction mediated by a copper complex of...

The widespread utilization of hydrogen fuel is one of the important strategies to fix the issues of energy shortage. It is thereby vital to develop robust, low-cost efficient catalysts for the hydrogen evolution reaction (HER). In this work, a new bistriazolylpyridine dimethyl-2,2'-(pyridine-2,6-diyl bis(1H-1,2,3-triazole-4,1-diyl))diacetate (dbes) and its copper complex [Cu(dbes)₂](ClO₄)₂ (Cudbes) were prepared. The electrocatalytic activity of Cudbes toward the HER was studied in CH₃CN solution using acetic acid (AcOH), trifluoroacetic acid (TFA), and p-toluene sulfonic acid (TsOH) as proton sources. In the presence of AcOH, TFA, and TsOH, the overpotentials for the HER mediated by Cudbes were 550 mV, 525 mV, and 520 mV, respectively. The faradaic efficiency was above 98%. The rate constant for the HER (k_obs) was 1377 s^-1 with AcOH, much higher than those with TFA (535 s^-1) and TsOH (519 s^-1). The acids profoundly affected the reaction pathways of the HER mediated by Cudbes. The catalytic HER mediated by Cudbes with AcOH followed the EECC or EEECC pathway. By contrast, in the presence of TFA or TsOH, the catalytic HER followed the CEEC or CECE pathway. The catalytic reactions invoked the proton-coupled electron transfer step and the proton relay on the triazolyl moiety of dbes. This work demonstrates that bistriazolylpyridines are good organic scaffolds to build transition metal complexes that are promising catalyst candidates for the HER and gives some new insights into the catalytic HER mediated by copper complexes using different proton sources.