Publication date: 31st March 2013
Sunlight is a clean, renewable and abundant energy source on the earth. Its conversion to hydrogen has been considered an ideal solution to counter the depletion and environmental problems of fossil fuels. Photoelectrochemical water splitting is an idealtechnology for the purpose, since H2 could be produced directly from abundant and renewable water and solar light from the process.
Oxidation: H2O + 2 h+ → 2 H+ + 1/2 O2
Reduction: 2 H+ + 2 e- → H2
Overall: H2O → H2 + 1/2 O2
The key to the technology is photoelectrodes of high efficiency, high stability, and low cost. In addition of the discovery of new materials, the structure and morphology of the known materials could be designed to extract the maximum capability of the materials and then to enhance the performance of the photoelectrodes.
As shown in Fig. 1, the solar energy to hydrogen conversion process is made of a series of steps; light absorption, charge separation, charge transport, charge collection and water splitting reactions. To improve the efficiency of the overall process, each of these steps has to be optimized Thus, the solar to hydrogen efficiency (hSTH) could be expressed as:
hSTH= hA x hCS x hCT x hCC
where hA= light absorption efficiency, hCS= Charge separation efficiency, hCT= charge transport efficiency and hCC= charge collection efficiency. In this presentation, the concepts of materials design and their examples are proposedto optimize these individual stepsfor fabrication of efficient photoelectrodes of photoelectrochemical (PEC) cellsfor visible light water splitting.We discuss the material designs including;i) p-n heterojunction photoanodes for effective electron-hole separation, ii) electron highway to facilitate interparticle electron transfer, iii) cation or aniondoping to improve conductivity of the semiconductor, iv) one-dimensional nanomaterials for vectoral electron transfer, and v) gas evolving co-catalysts.
We focused on metal oxide semiconductors for photoanode materials considering durability and cost. High efficiency has been demonstrated for all these examples by minimizing electron-holerecombination, which is the main source of efficiency loss. Modern material processing techniqueshave been explored to materialize these concepts [1-4].
[1]. W.J. Jo, J.-W. Jang, K. Kong, H.J. Kang, J.Y. Kim, H. Jun, K.P.S. Parmar and J.S. Lee ¡°Phosphate oxoanion doping into monoclinic BiVO4 for enhanced photoelectrochemical water oxidation activity¡± Angew. Chemie Int. Ed. 51, 3147 (2012). [2]. H. Jun, J. Kim, B. Im, S.J. Hong, E. Kim and J.S. Lee," Photoelectrochemical water splitting over ordered honeycomb hematite electrodes stabilized by alumina shielding" Energy Env. Sci. 5, 6375 (2012) [3]. H.G. Kim, P.H. Borse, J.S. Jang, C.W. Ahn, E.D. Jeong, J.S. Lee, "Engineered Nanorod Perovskite Film Photocatalysts to Harvest Visible Light" Adv. Mater. 23, 2088-2092, (2011) [4]. S.J. Hong, S. Lee, J.S. Jang, and J.S. Lee. "Composite Electrodes of BiVO4 and WO3 for Enhanced Photoactivity of Water Oxidation", Energy Env. Sci. 4, 1781-1787 (2011)
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