4, 4 '-Bipyridine as a molecular catalyst for electrochemical hydrogen production

Hydrogen evolution reaction (HER) on Cu and Ag electrodes in neutral and alkaline media (but not in acidic media) is significantly accelerated by the addition of 4,4′-bipyridine (BiPy). The mechanism of the HER acceleration is discussed at a molecular level by using surface-enhanced infrared absorption spectroscopy (SEIRAS) coupled with electrochemical techniques and density functional theory (DFT) calculations. Simultaneous SEIRAS and cyclic voltammetry measurements reveal that the onset potential of the accelerated HER coincides with the 1e−+1H+ reduction potential of adsorbed BiPy to yield monohydro BiPy radical (BiPy-H). Tafel plot analysis and DFT calculations suggest that BiPy-H is further reduced to N,C(3)-dihydro BiPy via a subsequent 1e−+1H+ step and finally H2 is evolved by the combination of two H atoms on N and C(3) atoms (BiPy-H2→BiPy+H2). Since adsorbed BiPy is readily reduced again to BiPy-H in the potential range of HER, the reaction process is catalytically cycled. The enhanced HER is not observed on Au electrodes due to the desoption of BiPy from the surface. Also the enhanced HER is not observed on Pt electrodes despite strong adsorption on Pt because the redox potential of adsorbed BiPy, the first step of the HER, is more negative than the onset of HER on bare Pt surface.