Photoelectrochemical Generation of Hydrogen

	Photoelectrochemical Generation of Hydrogen by Vikrant Urade School of Chemical Engineering, Purdue University, West Lafayette, IN-47907


1. Limited fossil fuels, growing demand 2. Hydrogen: Fuel of the Future 3. Methods of hydrogen production from solar energy 4. Introduction to photoelectrochemical hydrogen production 5. Properties of semiconducting electrode material 6. Lowering of the band gap of titania 7. Various configurations of the PEC cell 8. Optimization of cell performance 9. Nanomaterials Aspects of PEC Generation of Hydrogen Links to Other Resources on the Web Back to CHE697M Hillhouse Research Group Website	4. Introduction to Photoelectrochemical Hydrogen Production: In its simplest form, a photoelectrochemical (PEC) hydrogen production cell consists of a semiconductor electrode and a metal counter electrode immersed in an aqueous electrolyte. When light is incident on the semiconductor electrode, it absorbs part of the light and generated electricity. This electricity is then used for the electrolysis of water. Fujishima and Honda first demonstrated the lysis of water using solar energy in a PEC cell about 30 years ago [8]. A schematic of their cell is shown in the figure below: Schematic showing the structure of a PEC cell (Fujishima and Honda, [8]) As seen from the diagram, the cell consists of a semiconductor photo anode which is irradiated with the electromagnetic radiation. The counter electrode is a metal. Following processes take place in the cell when light is incident on the semiconductor electrode: 1. Photo generation of charge carriers (electron and hole pairs) 2. Charge separation and migration of the holes to the interface between the semiconductor and the electrolyte and of electrons to the counter electrode through the external circuit. Now, holes are simply vacancies created in the valence band due to promotion of electrons from the valence band to the conduction band. However, in the study of electronic behavior of materials, "holes" are considered to be independent entities, with their own mass. 3. Electrode processes: oxidation of water to H⁺ and H₂O by the holes at the photo anode and reduction of H⁺ ions to H₂ by electrons at the cathode. The representation of the same process in band energy terms is shown in the following diagram: A schematic illustrating the operating principles of a photoelectrochemical cell producing hydrogen. The cell depicted in the figure is a single photoelectrode type cell, with the anode being the active photoelectrode The lower yellow band is the valence band of the n-type semiconductor, while the upper yellow band is the conduction band. The energy difference between the top of valence band and the bottom of conduction band is termed as the band gap of semiconductor, E_g. Photons having energy greater than E_g are absorbed by the semiconductor and free electrons are generated in the conduction band and free holes in the valence band. 2hν = 2e^- + 2h⁺ The electrons and holes are separated due to the potential generated at the interface of the semiconductor-electrolyte due to band bending. The holes move to the interface and react with water producing oxygen: 2h⁺ + H₂O = 1/2 O_2(gas) + 2H⁺_(aq) The electrons travel in the external circuit and arrive at the interface between the counter electrode and electrolyte. There, they reduce the H⁺ ions to H₂: 2e^- + 2H⁺_(aq) = H_2(gas) The complete reaction is absorption of photon and splitting of water into hydrogen and oxygen. Some other configurations of the PEC cell are also possible: 1. The semiconducting material may be a p-type material. In this case, it will act as photo cathode, and reduction of H⁺ ions to H₂ will take place at this electrode. The counter electrode may me a metal in this case. 2. Both electrodes, the cathode and anode, are photo active semiconducting materials. In this case, the n-type electrode will act as anode and oxidation of water to oxygen and H⁺ will take place at this electrode. The p-type electrode will act as cathode, where H⁺ ions will be reduced to H₂. Having discussed the fundamentals of how hydrogen can be generated from water using a photoelectrochemical cell, now is time to discuss what are the properties that we are looking for in the most important part of the PEC cell, viz., the photoelectrode and how to maximize the efficiency of PEC cells by altering these properties? >> Website created and maintained by: Vikrant Urade, School of Chemical Engineering, Purdue University.