The Photoelectrochemical Reduction of CO

Light energy may be used to reduce the necessary electrical potential in photoelectrochemical reactions. The overpotential is decreased by 700 mV for the photoelectrochemical reduction of CO on p-CdTe, compared to that on indium - the best metal electrode for CO2 reduction. For these semiconductors which involve a high concentration of surface states, the double layer at the semiconductor-electrolyte interface plays an important role in the kinetics of photoelectrochemical reactions. In this paper, we report spectroscopic and impedance aspects of the electrode-electrolyte interface as affected by reactants and radicals involved in CO reduction. 

The surface state capacitance calculated by a similar procedure is shown in Fig. 19 as a function of bias potential. Fig. 20 shows the surface states density as a function of bias potential. Surface states density is an order of magnitude less than that at the CdTe interface under similar conditions. This is consistent with the fact that the photocurrent for GaP is les compared to CdTe for the photoelectrochemical reduction of CO (cf. the model suggested of mediator surface states). The surface state density increases from its minimum at 0.2V NHE, a potential which lies close to the pzc value determined for the system (0.17V NHE). Correspondingly, the form of the surface state density as a function of bias potential resembles an adsorption isotherm. These results support the concept that surface dates are induced by adsorbed ions at the interface. 

Ammonium ions are adsorbed at the semiconductor electrolyte interface and the reduced ammonium ion radical acts as mediator for the photoelectrochemical reduction of CO. ... 

The photoelectrochemical reduction of CO at semiconductor o produces in appreciable amour depends on the nature of the semiconductor surface. 

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