(Invited) Confined Molecular Electrocatalyst for Electrochemical and Photoelectrochemical Hydrogen Evolution Reaction
Authors: Uosaki Kohei , Masuda Takuya , Kurniawan Cepi , Sun Yu , Noguchi Hidenori 
Published: February 2020

Hydrogen is the most important clean fuels in the future and production of hydrogen from water by solar energy is required. Photoelectrochemical (PEC) and photocatalytic decomposition of water has been studied for long time. Most serious problem of PEC production of hydrogen from water is that while semiconductor electrodes with small energy gap can absorb large fraction of solar energy but unstable, those with large energy gap are stable but can absorb only small fraction of solar energy. This can be solved by using semiconductor of small gap and separating the reaction site from the surface. We have constructed viologen molecular layer on hydrogen terminated Si(111) surface and then Pt complex was introduced within the molecular layer by ion-exchange reaction [1-3]. Significant decrease of overpotential for dark hydrogen evolution reaction (HER) at n-Si electrode was observed by this modification. Further improvement was achieved by constructing multi-viologen-Pt -layers. Viologen layers and Pt species act as electro relay and HER catalyst, respectively. Phoelectrochemical HER at p-Si(111) electrode is also accelerated by the modification with viologen-Pt layers. In situ XAFS study during HER showed that Pt complexes not Pt particles acted as HER catalysts, i.e., confined molecular catalysts, as schematically shown in the figure [3]. The decrease of white line intensity suggests the formation of a hydride complex as an intermediate of HER. The formation of Pt particles may have been inhibited since the complexes are separated by molecular layers.
Confined molecular catalyst by viologen-Pt layers for HER was also constructed on indium tin oxide (ITO). While two redox peaks corresponding to viologen dication/ radical cation (V2+/V+•) and viologen radical cation/neutral form (V+•/V0) were observed in cyclic voltammogram (CV) of viologen monolayer modified ITO electrode without Pt complex, no redox peaks but large current due to HER was observed at viologen monolayer modified ITO electrode with Pt complex. Spectroelectrochemical measurements at constant potentials and during potential scan showed stable spectra corresponding to radical cation (V+•) and neutral form (V0) of viologen at potentials between V2+/V+• and V+•/V0 redox peaks and more negative than V+•/V0 redox peak, respectively, at an ITO electrode modified by viologen monolayer without a Pt complex but no absorption peaks corresponding to V+• or V0 at an ITO electrode modified by viologen monolayer with a Pt complex. Time-resolved spectroelectrochemical measurements, however, showed that V+• is formed upon the potentials step to the potentials more negative than V2+/ V+• redox potential and disappeared within ca. 1 ms at the ITO electrode modified by viologen monolayer with Pt complex, confirming that electron is transferred from ITO electrode to proton via viologen moiety and Pt complex [4].
By incorporating various metal complexes, not only HER but also CO2 reduction can be accelerated [5, 6]

References

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[4] C. Kurniawan, H. Noguchi, T. Masuda, K. Uosaki, Electrochem. Commun., 62 (2016) 56–59.
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[6] T. Masuda, Y. Sun, H. Fukumitsu, H. Uehara, S. Takakusagi, W.-J. Chun, T. Kondo, K. Asakura, K. Uosaki, J. Phys. Chem. C, 120 (2016) 16200-16210.

Figure 1