Redox electrolytes

An interesting idea has been to prepare the photosensitive electrode on site having the liquid play the dual role of a medium for anodic film growth on a metal electrode and a potential-determining redox electrolyte in the electrochemical solar cell. Such integration of the preparation process with PEC realization was demonstrated initially by Miller and Heller [86], who showed that photosensitive sulfide layers could be grown on bismuth and cadmium electrodes in solutions of sodium polysulfide and then used in situ as photoanodes driving the... 

Mishra et al. [198] discussed in an exemplary way the dark and photocorrosion behavior of their SnS-electrodeposited polycrystalline films on the basis of Pourbaix diagrams, by performing photoelectrochemical studies in aqueous electrolytes with various redox couples. Polarization curves for the SnS samples in a Fe(CN) redox electrolyte revealed partial rectification for cathodic current flow in the dark, establishing the SnS as p-type. The incomplete rectification was... 

Fig. 3.8 Electron energy distribution at the contact between a semiconductor and a redox electrolyte for two different redox systems at equilibrium, (a) n-type semiconductor, and (b) p-type semiconductor.

For an interface described by a constant Helmholtz potential electron exchange between the semiconductor and redox electrolyte solution. The result is that dV = d(psc, and for a non-equilibrium system one can obtain the current-voltage relation ... 

The dye-sensitised solar cell (DSSC) is constructed as a sandwich of two conducting glass electrodes filled with a redox electrolyte. One of the electrodes is coated, using a colloidal preparation of monodispersed TiOj particles, to a depth of a few microns. The layer is heat treated to rednce resistivity and then soaked in a solution of the dye until a monomolecnlar dispersion of the dye on the TiO is obtained. The dye-coated electrode (photoanode) is then placed next to a connter electrode covered with a conducting oxide layer that has been platinised , in order to catalyse the reduction of the mediator. The gap between the two electrodes is filled with an electrolyte containing the mediator, an iodide/triodide conple in acetonitrile. The structure is shown schematically in Fignre 4.29. 

One problem for the coated system is that the film is peeled off after prolonged irradiation. In order to have a more adhesive film, the surface of n-Si was modified with N-(3-trimethoxysilyIpropyl)pyrrole (22). Pyrrole was then electrodeposited on this modified electrode as shown in Eq. (24) 85). The durability of the coated poly(pyrrole) was improved by such a treatment of n-Si surface. The n-Si electrode coated only with poly(pyrrole) gave a declined photocurrent from 6.5 to 1.8 mA cm-2 in less than 18 h, while the poly(pyrrole) coated n-Si treated at first with 22 as Eq. (24) gave a stable photocurrent of 7.6 mA cm-2 for 25 h. When an n-Si electrode was coated with Pt layer before the deposition of poly(pyrrole), the stability of the semiconductor was improved remarkably (ca. 19 days)85b). A power conversion efficiency of 5.5% was obtained with iodide/iodine redox electrolytes. 

Fig. 23. Time dependence of steady-state I-V behaviour (scan rate 100 mV/s) for (a) polymer-coated n-GaAs electrode (area —0.1 cm2) and (b) bare n-GaAs photoanode in contact with the I /I (0.5/0.5 M) redox electrolyte (pH = 5). The illuminated electrode (light intensity 53mW/cm2) was maintained at approximately short-circuit condition (0.3 V vs. SCE) for the duration shown, after which the potential scans were initiated. The initial level of the I-V curve for the bare electrode was dose to that seen at 0 min for the coated sample. The electrolyte was stirred in all cases...

It is known that the photoelectrochemical cell (PEC), which is composed of a photoelectrode, a redox electrolyte, and a counter electrode, shows a solar light-to-current conversion efficiency of more than 10%. However, photoelectrodes such as n- and p-Si, n-and p-GaAs, n- and p-InP, and n-CdS frequently cause photocorrosion in the electrolyte solution under irradiation. This results in a poor cell stability therefore, many efforts have been made worldwide to develop a more stable PEC. 

It has been considered that the high stability of the dye in a DSSC system could be obtained by the presence of I - ions as the electron donor to dye cauons. Degradation of the NCS ligand to the CN ligand by a intramolecular electron-transfer reaction, which reduces consequently the Ru(III) state to the Ru(II) state, occurs within 0.1-1 sec [153], whereas the rate for the reduction of Ru(in) to Ru(II) by the direct electron transfer from I ions into the dye cations is on the order of nanoseconds [30]. This indicates that one molecule of N3 dye can contribute to the photon-to-current conversion process with a turnover number of at least 107—10s without any degradation [153]. Taking this into consideration, N3 dye is considered to be sufficiently stable in the redox electrolyte under irradiation. 

Figure 4.2(d) shows that an energy barrier forms at the semiconductor/redox electrolyte interface, similar to the Schottky barrier at a metal/semiconductor interface. The most important quantity is the barrier height (q ) or the flat band potential U, which essentially determines the surface band positions of the semiconductor with respect to the energy levels of solution species. The q B is given for an n-type semiconductor by... 

Such a dilemma can be overcome by using semiconductor electrodes coated with sparsely scattered, extremely small (nanometer-sized) metal dots,42 45) such as shown schematically in Fig. 4.9 with n-Si used as a semiconductor. The naked Si surface is covered with naturally grown thin Si02 layer and passivated. The photocurrent flows through the metal dots. The photocurrent can be stable in aqueous redox electrolyte because the Si surface is covered and protected by coating with metal dots and Si02. 

Dye sensitization of a nanometer-sized Ti02 powder film soaked in an organic medium containing iodine/iodide redox electrolytes successfully generated open circuit photovoltage (Foe) 0.68 V, Jsc 11.2 mAcm-2, Fill factor (FF) 0.68, and... 

The optimization of seven factors will contribute to the improvement of DSC As far as Ti02 is concerned, characterization of T]a and T] is crucial. With regard to dye molecules, factors affecting r]e, r)lh and phi should be clarified and optimized. To optimize or solidity the redox electrolyte solution, materials shuuld be developed without lowering rjhl, i/ht and rjhc. 

In a report on the electron transfer between dye/hole transport electrolyte, the electron transfer rate from redox electrolytic solution (0.3 M KI and 0.03 M I2 ethylene carbonate/propylene carbonate (1 1) solution) to oxidized state of Dye 2 was determined to be 110 nsec based on the lifetime of the Dye 2 cation.41) It is much faster (by an order of 103 fold) than the back electron-transfer from Ti02 to oxidized Dye 2. 

Fig. 4.2 Schematic illustrations of (a) the charge distribution, (b) the charge-density distribution, (c) the potential distribution, and (d) the band bending at the semiconductor/redox electrolyte interface, assuming that no surface charge nor surface dipole is present.

---