Photoelectrochemical CO₂ Reduction with a Rhenium Organometallic Redox Mediator at Semiconductor/Aqueous Liquid Junction Interfaces

Abstract

Electrochemical and photoelectrochemical CO 2 reductions were carried out with Re(bh-bipy)(CO) 3 (OH 2 ) cocatalysts in aqueous electrolytes.C ompetition between hydrogen evolution and CO 2 reduction was observed under (photo)electrochemical conditions for both glassy carbon and CuInS 2 electrodes.T he partial current density for CO generation is limited even though the additional potential is applied. However,e lectrochemical hydrogen evolution was suppressed under photoelectrochemical conditions,and the selectivity and partial current density for CO were considerably increased when compared to the electrochemical reduction in an identical electrode/electrolyte system. This finding may provideinsights into using semiconductor/liquid junctions for solar fuel devices to overcome the limitations of electrolysis systems with an external bias.Artificial photosynthesis is the process wherein the CO 2 reduction reaction (CRR) takes place with hydrogen (water) and energy (sunlight) sources.T his process might be one of the key technologies for the future of human society because it is useful for producing valuable chemicals from waste contributing to global warming. [1] There are several ways to obtain solar fuel or chemicals by light irradiation, for example,photoelectrochemical cells (PECs) which are based on semiconductor/liquid junctions.I nt he PEC system, solar energy is directly converted into chemical energy at the semiconductor-electrolyte interface junction. Meanwhile,i n photovoltaic (PV) cell electrolysis systems,s olar energy is converted into electrical energy by photovoltaic cells,a nd then the electrical energy is used for electrochemical (EC) reactions. [2] Forb oth systems,i nput photons are finally converted into chemical energy forms.T herefore,t hese two different systems are not well distinguished.However,t he basic principles of PEC and PV-EC are clearly different. In particular,the electrode/electrolyte interface of aP EC is much different from that of EC reactions because of the properties of semiconductors.Unlike conducting electrodes,electron transfer between semiconductors and redox couples is limited, even though the Fermi level is shifted by applied potential due to rectifying properties of band pinning behavior. [3,4] When cocatalysts are combined with photoelectrodes,t herefore,t he energy structure between cocatalysts and semiconductor electrodes is more important than the one in the cocatalyst/non-semiconductor electrode system. In this research, we are reporting on different selectivities and reaction rates for PEC and EC systems having an identical cocatalyst, [(Re(bh-bipy)(CO) 3 (OH 2 )] + (bh-bipy = 4,4'-bis (hydroxymethyl)-2,2'-bipyridine), which is known to be very selective for CO production, using aC O 2 saturated aqueous electrolyte. [5] Firstly,t he CO 2 reaction was carried out with ag lassy carbon (GC) electrode in aC O 2 saturated electrolyte containing 1mm of the Re complexe (Figure 1a). From the linear sweep voltammetry (LSV) results,areduction peak of the Re complex is f...

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