Modeling of Bipolar Semiconductor Photoelectrode Arrays for Electrolytic Processes

The effect of light flux, redox couple concentration, and the number of panels on the electrical and chemical efficiency of a bipolar semiconductor photoelectrode array has been examined using a model based on the power characteristic curves of a single photoelectrochemical cell and the current-voltage characteristics of any desired electrolytic process. Data from a single bipolar CdSe//CoS photoelectrode were used to simulate water splitting array performance under a variety of conditions. Experimental data from a 6 photopanel array were consistent with the simulated results. The optimum number of panels was shown to be independent of light intensity over a range of 0.1-1 AM2 and of the concentration of polysulfide in the interior of the arrays from 0.1 to 1M. The efficiency of the CdSe/CoS array for electrical generation (after correcting for light absorption due to the electrolyte) is about 6%. Recent papers from this laboratory described bipolar semiconductor photoelectrode (BSP) arrays and their application to light-driven water splitting and electrical power generation (1, 2). The term bipolar electrode refers to a configuration consisting of a nor p-type semiconductor surface in ohmic contact with a clark catalytic electrode surface that serves as the counterelectrode to the facing semiconductor of the next bipolar electrode. These electrodes can be used in a wireless series configuration array to provide sufficiently high voltages to drive electrolytic reactions of interest. The principles of such an array are shown in Fig. 1A. These arrays obviate two problems: (i) the location of the semiconductor bandedges relative to the water decomposition potentials and (ii) the requirement of semiconductor stability during photohole generation and oxygen production in aqueous solution. Studies of water photoelectrolysis ("water splitting") date from the work of Honda and Fujishima working with TiO2 and Pt electrodes (3). The inadequate photopotential generated at the TiO~/solution interface required the application of an external bias and hence the expenditure of energy in addition to the incident radiant energy. An alternative strategy involves series arrays of PEC cells. The basic principles of PEC devices have been discussed in detail *Electrochemical Society Active Member. (4-7). In bipolar semiconductor photoelectrodes, vectorial charge transfer occurs: photogenerated minority carriers move to the semiconductor surface and majority carriers move to the electrocatalytic surface. This arrangement optimizes light collection and obviates the need for connecting wires. With five n-TiOj/Pt (where // represents an ohmic contact) (1) BSP's in series, hydrogen was evolved at the Pt surface of panel 1 with oxygen evolution on the nTiO2 surface of panel 5 (1). The electrodes on the inside panels behaved as photovoltaic cells and carried out a regenerative reaction (in this case photoproduction and reduction of oxygen) to provide a bias for the end electrodes. This initial version of the BSP array suffered from the requirement of contact of the end photoanodic semiconductor surface (electrode 1 in Fig. 1) with the solution where O2 is evolved, thereby limiting the BSP material to large bandgap semiconductors stable under conditions where photogenerated holes oxidize water. However, a (dark) electrocatalytic electrode can be used on the anodic (O2-evolution) end, instead of the n-type semiconductor, removing this need for semiconductor stability (Fig. 1B). The anodic side of the dark bipolar electrode 1 should be composed of an electrocatalyst for oxygen evolution on the substrate of choice, while the cathodic side is an electrocata~yst appropriate for the redox couple (O/R) used in the P JP array. In these arrays, the il luminated array of Downloaded 02 Feb 2009 to 146.6.143.190. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp