Water splitting

Water splitting means a chemical transformation that separates water into its elements which are molecular oxygen (O2) and molecular hydrogen (H2). The transformation can be done by various processes such as: electrolysis, photochemical, photocatalytic, thermal decomposition. Electrolysis is a method of separating elements chemically bounded into compounds by passage of an electric current through them: in the case of electrolysis of water the latter is decomposed (splitted) into gases O2 and H2. When energy is obtained from light, photochemical and photocatalytic devices can be used and the system that splits water into hydrogen and oxygen is also named artificial photosynthesis (Devens et al. 2009). Watersplittingover a semiconductor photocatalyst using solar energy is a promising process for the large-scale production of clean, recyclable H2 at temperatures close to the ambient one. Numerous attempts have been made to develop photocatalysts that function under visible-light irradiation to efficiently utilize solar energy (Kazuhiko 2011; Valdes et al. 2012). The photocatalytic splitting of water under visible light irradiation is agreatly desired reaction system for hydrogen production. However, powdered photocatalysts always produce a gas mixture of hydrogen and oxygen in such a reaction and, thus, a separation process for the gas mixture is required before the hydrogen can be effectively utilized. Construction of a photocatalytic system enabling the separate evolution of hydrogen and oxygen from water (e.g. by means a membrane) under visible light irradiation and absence of sacrificial agents using a Z-scheme (Abe 2010) is, therefore, of vital interest. In this Z-scheme methodology, hydrogen and oxygen are generated photocatalytically in different cells that are illuminated and separated by a membrane. An electrolyte is used to ensure the electroneutrality in each cell and to allow charge transfer from one compartment to the other. One suitable electrolyte is the Fe2+/Fe3+ redox pair. A Nafion membrane is a suitable barrier between the two compartments of the reactor because of its chemical and physical properties (Seger et al. 2007). The uptake characteristic of different cations (Fe3+, Cu2+ and Ni2+) by Nafion 117 which is commonly used as separator for different chemical processes were also investigated (Ramirez et al. 2010). Thermal decomposition (or thermolysis) is another approach for water splitting: it is a chemical transformation that breaks a substance into at least two chemical substances when heated. At elevated temperatures (e.g. 2200 °C ) water molecules can decompose but, using catalysts, lower temperatures can be used to obtain hydrogen and oxygen. Solar energy can be used for thermal decomposition of water via an integrated thermo-chemical reactor/receiver system (Agrafiotis et al. 2005). A multi-channel honeycomb ceramic supports coated with active redox reagent powders, in a configuration similar to that encountered in an automobile catalytic converter, has been used. Iron-oxide-based redox materials, capable to operate under a complete redox cycle, could take oxygen from water producing pure hydrogen at reasonably low temperatures (800 °C) and could be regenerated at temperatures below 1300 °C. Ceramic honeycombs capable of achieving temperatures in that range when heated by concentrated solar radiation were assembled in a dedicated solar receiver/reactor. The operating conditions of the solar reactor were optimized to achieve adjustable, uniform temperatures up to 1300 °C throughout the honeycomb, making thus feasible the operation of the complete cycle by a single solar energy converter (Agrafiotis et al. 2005). References Abe R (2010) Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation. Journal of Photochemistry and Photobiology C: Photochemistry Review 11:179-209 Agrafiotis C, Roeb M, Konstandopoulos AG, Konst Nalbandian L, Zaspalis VT, Sattler C, Stobbe P, Steele AM (2005) Solar water splitting for hydrogen production with monolithic reactors. Solar Energy 79: 409-421 Devens G, Moore TA, Moore AL (2009) Solar Fuels via Artificial Photosynthesis. Accounts of Chemical Research 42:1890-1898 Kazuhiko M (2011) Photocatalytic water splitting using semiconductor particles: History and recent developments. J Photochem & Photobiol C-Photochem Reviews 12: 237-268 Ramìrez J, Godìnez LA, Mèndez M, Meas Y, Rodriguez FJ (2010) Heterogeneous photo-electro-Fenton process using different iron supporting materials. Journal Appl. Electrochem. 40:1729-1736 Seger B, Vinodgopal K, Kamat PV (2007) Proton activity in Nafion films. Probing exchangeable protons with methylene blue. Langmuir 23:5471-5476 Valdes A, Brillet J, Gratzel M, Gudmundsdottir H, Hansen HA, Jonsson H, Klupfel P, Le Formal F, Man IC, Martins RS, Norskov JK, Rossmeisl J, Sivula K, Vojvodic A, Zach M (2012) Solar hydrogen production with semiconductor metal oxides: new directions in experiment and theory. Phys Chem Chem Phys 14-1: 49-70