Introduction To Photoelectrochemical Hydrogen Production

. 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 lyses of water using solar energy in a PEC cell about 30 years ago. A schematic of their cell is shown in the figure below:

Schematic showing the structure of a PEC cell (Fujishima and Honda) 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 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 ntype 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, Eg. Photons having energy greater than Eg 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 O2(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) = H2(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 ntype 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. PHOTOELECTROCHEMICAL PRODUCTION

HYDROGEN

Developing high-efficiency, potentially low-cost, photoelectrochemical (PEC) systems to produce hydrogen directly from water using sunlight as the energy source. The main thrust of the work has been the development of integrated multijunction photoelectrodes, comprising semiconductor, catalytic, and protective thin-films deposited on low-cost substrates (such as stainless steel), for solar hydrogen production (Rocheleau et al. 1998). In the illustration of a generic hydrogen photoelectrode shown in Figure 1, sunlight shining on photoactive regions of the electrode produces electric current to drive the hydrogen and oxygen evolution reactions (HER, OER) at opposite surfaces.

Photoelectrochemical hydrogen production

goals, a PEC system must be low-cost, operate at solar-to-chemical conversion efficiencies greater than 10%, and have long operating lifetimes. research, our approach has been to develop photoelectrodes incorporating multijunction thin-film photoconvertors (for high voltage) and thin-film catalyst and protective layers (for stability).Specifically, our work has provided strong evidence that direct solar-tohydrogen conversion efficiency of up to 10% can be expected using photoelectrodes fabricated from lowcost, multijunction amorphous silicon (a-Si) (Rocheleau and Miller 1997), while conversion efficiencies approaching 15% are possible using

advanced photoelectrode designs based on multijunction copper-indium-gallium-diselenide (CIGS) cells stacked in a side-by-side Configuration (Miller and Rocheleau 2000). Both the a-Si and CIGS photoelectrode systems have the potential for low cost based on the very thin semiconductor layers involved of photo electrolysis of water directly into oxygen at a TiO2 electrode and hydrogen at a Pt electrode by the illumination of light greater than the band gap of TiO2 [3.1 eV] In the photo electrochemical cell, the photoelectro process involves the generation of charge carriers in the semi conducting electrodes and the transfer of these charge carriers across the electrode/electrolyte interface. For the direct photo electrochemical decomposition of water to occur, several key criteria have to be met with. These can be stated at the first level as follows: 1. The band edges of the electrode must overlap with the acceptor and donor states of water decomposition reaction, thus necessitating that the electrodes should at least have a band gap of 1.23 V, the reversible thermodynamic decomposition potential of water. This situation necessarily means that appropriate

semiconductors alone are acceptable as electrode materials for water decomposition. The situation is shown pictorially in Fig.1. 2. The charge transfer from the surface of the semiconductor must be fast enough to prevent photo corrosion and shift of the band edges resulting in loss of photon energy. Employing these two essential criteria and considering the physics of semiconductor solution interface, various materials have been examined as anodes for direct photo electrochemical decomposition of water. The efficiency of this process is still a point of contention, even though double digit efficiencies up to 18.3% has been reported using complex electrodes in an electrolytic cell. However, realizing this level of efficiency routinely and cheaply and sustaining it appear to be still eluding. In the last three decades, a large number of semiconductor materials have been investigated for photo electrochemical applications. The semiconductor electrode system in efficient solar energy converters should have optimized band gap so as to be able to make maximum utilization of solar radiation, and should also have sufficient chemical stability against photo or other corrosion processes.

Unfortunately most of the materials that satisfy the first criterion, namely semiconductor materials with band gap around 1.4 eV are susceptible for photo corrosion, while stable materials with a wider band gap absorb light only in the UV region. In order to match these two opposing factors, various conceptual principles have been incorporated into typical TiO2 system so as to make this system responsive to longer wavelength radiations. These efforts can be classified as follows: • Dye sensitization • Surface modification of the semiconductor to improve the stability • Multi layer systems (coupled semiconductors) • Doping of wide band gap semiconductors like TiO2 by nitrogen, carbon and Sulphur • New semiconductors with metal 3d valence band instead of Oxide 2p contribution • Sensitization by doping. All these attempts can be understood in terms of some kind sensitization and hence the route of charge transfer has been extended and hence the efficiency could not be increased considerably. In spite of these options being elucidated, success appears to be eluding the researchers.

PRASHANT MISHRA B.TECH (AUTOMOTIVE DESIGN ENGG.) UNIVERSITY OF PETROLEUN AND ENERGY STUDIES