Homogenous Solution Catalysts at Semiconductor Electrodes

Another method of stabilizing the surface of semiconductor electrodes relates to an electrolyte modification, using solution mediators that efficiently accept the electron from the semiconductor to subsequently reduce C02. In aqueous solution, Taniguchi et al. used a p-GaP photoelectrode in the presence of 15-crown-5 ether at a potential of-0.95 V (versus SCE) [119]. In this case, current efficiencies of 44%, 15%, and 4% were observed for methanol, formic acid, and formaldehyde, respectively. 

Similarly, when Bockris and Wass used various organic mediators in a DMF solution with 5% water and p-CdTe as the photocathode [120], the bare p-CdTe produced a faradaic efficiency for CO of 92%. Subsequently, by using their best catalysts, namely 15-crown-5 ether and 18-crown-6 ether, these authors were able also to produce methanol with faradaic efficiencies of 14% and 13%, respectively, with the remaining current going to CO production. Although the potential at which these electrolyses were carried out was not reported, the onset potential for C02 reduction was shifted some 400 mV anodically in the presence of the organic mediators. 

Beley et al. and Petit et al. reported the reduction of C02 to CO using Ni(cyclam)2+ catalysts in aqueous solution at p-GaAs and p-GaP photoelectrodes [121-123]. Here, the faradaic yields for CO approached 100%, at potentials of —1.0V and -0.44V (versus SCE), respectively, although carbon deposits on the surfaces of the electrodes led eventually to a degradation of the system. 

More recently, the use of a pyridinium mediator in an aqueous p-GaP photo-electrochemical system illuminated with 365 nm and 465 nm light has been reported [125], In this case, a near-100% faradaic efficiency was obtained for methanol production at underpotentials of 300-500 mV from the thermodynamic C02/methanol couple. Moreover, quantum efficiencies of up to 44% were obtained. The most important point here, however, was that this was the first report of C02 reduction in a photoelectrochemical system that required no input of external electrical energy, with the reduction of C02 being effected solely by incident fight energy. 

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