Photoelectrochemical systems

Rajeshwar K, Singh P, Dubow J (1978) Energy conversion in photoelectrochemical systems - A review. Electrochim Acta 23 1117-1144... 

Reichman J, Russak MA (1984) 1-V Behavior of the CdSe/sulfide-polysulfide and CdSe/ferro-feriicyanide photoelectrochemical systems. J Electrochem Soc 131 796-798... 

As already mentioned in the introduction, various fundamental and many empirical results have been published. Although photoelectrochemical cells are easily made, many problems concerning the stability of semiconductors and the function of catalysts still remain to be solved. There are few other approaches such as for instance sensitization (see e.g.) which are not treated here. In addition it should be mentioned that photoelectrochemical systems have been used for light induced synthesis of organic compounds (see e.g.) which could also not be considered in this article. 

Salvador [100] introduced a non-equilibrium thermodynamic approach taking entropy into account, which is not present in the conventional Gerischer model, formulating a dependence between the charge transfer mechanism at a semiconductor-electrolyte interface under illumination and the physical properties thermodynamically defining the irreversible photoelectrochemical system properties. The force of the resulting photoelectrochemical reactions are described in terms of photocurrent intensity, photoelectochemical activity, and interfacial charge transfer... 

In photochemical and photoelectrochemical systems the overall reaction, as shown above, is separated into two processes which are the oxidation of water to form 02 and its reduction to form H2. 

In the discussions by many authors of the energy conversion efficiency of semiconductor photoelectrochemical systems, it has been tacitly assumed that the maximum theoretical photovoltages produced is the difference between E (in units of eV) and E(0x/R). The best conversion efficiency should then be obtained with a redox couple whose standard redox potential is as low as possible, with a reasonable margin x, say 0.3 V, above E (Fig. 11). From this it follows that the maximum photovoltage obtainable is equal to the band gap, Eg, in an eV unit, minus a small margin x plus A. 

Finally it is pointed out that these conclusions for n-GaP can be extended to other various n-type semiconductors for general criteria of the performance of photoelectrochemical systems. 

Photoelectrochemical Systems Involving Solid-Liquid Interfacial Layers of Chlorophylls... 

Whether or not Chi is regarded intrinsically as an organic semiconductor, the solid Chi layer in contact with a metal does display a p-type photovoltaic effect, and its efficiency depends significantly on the morphology of the Chi layer as well as the nature of the metal. The effect corresponding to a p-type photoconductor can also be expected at the junction of a metal / Chi / liquid in a photoelectrochemical system. Such a presumption is in fact compatible with the photoelectrochemical behavior observed for most of Chl-coated metal electrodes, as will be shown later. 

Photoelectrochemical Systems Involving Chlorophyll-Coated Semiconductor and Metal Electrodes... 

From this explanation for the observed differences in output stability, we may infer that the decomposition potential of any CdX depends on its crystal structure. In view of the results presented here and in ref. 2, it is likely that this potential is a function of exposed crystal face as well. Further insight in those dependences, development of optimal surface treatments, and use of electrodes with optimal grain size, may all be of help in devising strategies to obtain even more stable CdX/S- photoelectrochemical systems. n... 

Quantum chemistry has so far had little impact on the field of photoelectrochemistry. This is largely due to the molecular complexity of the experimental systems, which has prevented reliable computational methods to be used on realistic model systems, although some theoretical approaches to various aspects of the performance of nanostructured metal oxide photoelectrochemical systems have appeared in the last 10 years, see e.g. [139, 140, 141]. Here we have focussed on quantum-chemical cluster and surface calculations of a number of relevant problems including adsorbates and intercalation. These calculations illustrate the emerging possibilities of using quantum chemical calculations to model complicated dye-sensitized photoelectrochemical systems. 

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. 

Heterogeneous semiconductor systems involve either suspensions or slurries of larger-sized semiconductor powders, or smaller colloids in solution. In principle, these semiconductor particles may act as tiny photoelectrolysis cells, similar to the photoelectrochemical systems discussed above. However, as many of the materials used for bulk electrodes are also described here in particulate form, both similar and new problems may arise, most notably irreproducibility in particle preparation, stability issues, and low C02 reduction rates. 

Photoswitchable redox-enzymes 1. Amperometric transduction of optical information - biocomputers 2. Amplification of weak optical signals -photonic amplifiers 3. Multisensor arrays — biosensor and bioelectronics 4. Photoelectrochemical systems Enzyme immobilized on electronic transducer... 

Aggregates of Cyanine Dyes as the Sensitizing Agents in the Photoelectrochemical Systems... 

MoIV (CN)8 and WIV (CN)8. This results in visible light photosensitization of Ti02, due to electron injection from the excited state of the complexes to the conduction band of the semiconductor. Photoelectrochemical systems with photoelectrodes of polycrystalline Ti02 derivatized with the above complexes give quantum yields of up to 37% upon illumination at the absorption peaks of the complexes, around 420 nm. The photoresponse is extended up to 700 nm. 

Willner I, Willner B. Electrical communication of redox proteins by means of electron relay-tethered polymers in photochemical, electrochemical and photoelectrochemical systems. React Polym 1994 22 267-79. 

A number of advances have been made recently in the understanding of the catalysts able to selectively activate CO2 [26-29]. Another key issue is the need to optimize the multielectron transfer necessary for the reduction of CO2. This issue has been examined recently by Barton Cole et al. [30], Most electrochemical and photoelectrochemical systems for the reduction of CO2 produce only the 2 e reduction products of CO and formate, while few reported the formation of methane and methanol and even fewer reported >C1 products, as briefly discussed before. The main limitation in achieving such products has been attributed to the inability of most catalysts to affect multielectron transfers along with the required multiproton transfers. 

During the last 15 years, many photoelectrochemical systems have been studied with respect to their applicability for solar energy conversion. Since most of these results have been summarized in various review articles [114,107,191-197], only some more recent developments will be considered here. 

There is still a problem in calculation of energy-conversion efficiency when electrochemical or chemical bias is also applied in photoelectrochemical or photocatal5rtic reaction of positive Gibbs energy. For example, as shown in Fig. 5c, energy-conversion efficiency for a photoelectrochemical system consisting of an n-type semiconductor and metal counter electrodes with bias voltage A6 is possibly expressed as follows ... 

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