(Invited) Photoinduced Hydrogen Evolution with Ni-Fe Hydrogenase-Viologen-Porphyrin Immobilized ITO Electrode
Authors: Asakura Noriyuki 
Published: December 2020

Hydrogenase-viologen-porphyrin triad was immobilized on ITO electrode and carried out photoinduced hydrogen evolution. This system can reductively generate hydrogen by electron donation from the photoexcited porphyrin to hydrogenase via viologen.
Hydrogen production by water splitting (electrochemical, photochemical, or thermal) offers an obvious way to capture solar energy. Catalysts are a crucial issue for Hydrogen production, and there is intense interest in finding alternatives to noble metals that are capable of converting water into hydrogen close to the thermodynamic potential.
As a biological alternative, enzymes known as hydrogenases catalyze the reduction of protons into hydrogen at active sites composed of iron- or nickel/iron complexes: importantly, the electrochemical reaction is reversible with a small overpotential. Determination of the structures of hydrogenases from several organisms paired with detailed spectroscopic and electrochemical investigations have considerably improved our understanding of these enzymes in recent years.
Hydrogenases also show high H2 production yields in photocatalytic schemes within sacrificial electron donors in pH neutral aqueous solution. In these systems with hydrogenase, a photoexcited photosensitizer provides electrons to the active site where proton reduction occurs via an internal electron transfer iron sulfur clusters. Examples are the immobilization of a hydrogenase on Ru-sensitised TiO2, on Cd-based, quantum dotsas well as homogeneous systems using the hydrogenases with a covalently linked photosystem I or in combination with an organic dye,and multi-component systems with a dye and a soluble redox mediator.
In this study, we report on ITO electrode system with hydrogenase, where Ni-Fe hydrogenase immobilized on ITO surface using a linker contains both a viologen and a porphyrin. [NiFe]-Hydrogenase from Desulfovibrio vulgaris (Miyazaki) is approximately 90 kDa in size, with large and small subunits of 60 kDa and 30 kDa, respectively. The Ni-Fe active site is located to the surface of the enzyme. Three Fe-S clusters from the putative electron transfer pathway between the active site. The proximal [4Fe4S] cluster lies closest to the active site while the distal [4Fe4S] cluster is found near the surface of the enzyme. A [3Fe4S] cluster is located in between the two [4Fe4S] clusters and is known as the medial cluster. The distal [4Fe4S] cluster works as the first electron acceptor of the [NiFe]-hydrogenase in the case of hydrogen evolution.
Figure 1 shows the ITO electrode system prepared in this study. Hydrogenase was immobilized via the 4,4’-(3-aminopropyl)-bipyridinium (DAPV) by a glutamate-rich patch of hydrogenase where the distal Fe-S cluster is located.
The photoexcited state of Tetrakis(4-carboxyphenyl)porphyrin (TCPP) donates electrons to [NiFe]-hydrogenase via the 4,4’-(3-aminopropyl)-bipyridinium (DAPV) bridging between the two. DAPV was linked to acidic amino acid near the distal [4Fe4S] cluster, electron entrance, of [NiFe]-Hydrogenase in order to efficiently donate electrons to the [NiFe]-Hydrogenase.
Photoinduced hydrogen evolution were carried out under applied -0.3V vs.SHE. During light irradiation, cathodic continuous current was observed due to the hydrogen evolution. The photoexcited state of TCPP donate electrons to hydrogenase via the viologen, and ITO electrode play an electron donor to HOMO of the TCPP. Therefore, photoinduced hydrogen evolution occur in TCPP-DAPV-hydrogenase triads on ITO electrode.

Figure 1