User:Hitesh s18

Solar Hydrogen Production Hydrogen is an ideal, clean, carbon-free carrier of energy that producces only water vapor as a waste product and ahd potential application in automobiles, airplanes and also in home heating. As the reserver of fossil fuels are depleting at a fast pace and also lead to enviromental degradation hydrogen as fuel is receving attention. It is essential to accelerate the development of techniques for the production of hydrogen from an abundent source of energy, the sun so as to show down the thread of global warming. At present only about 5% of the commercial hydrogen production is primarily via water electrolysis and whereas 95% is mainly derived from fossil fuels. Effort have to be directed towards clean production of hydrogen through water electrolysis or other clean techniques. Ther are two principal methods for the solar production of hydrogen or solar electrolysis of water for hydrogen production.

Solar Production of Hydrogen   Active method   Photoelectrolysis   Photocatalysis     Passive method   <a href="/Green_Revolution/hydrogen/hydrogen/?Topic=electrolysishydrogen#Alkaline">Alkaline Electrolysis</a> </li> <li> <a href="/Green_Revolution/hydrogen/hydrogen/?Topic=electrolysishydrogen#Protonex">Proton Exchange Membrane cells(PEM)</a> </li> <li> Oxide Electrolysis </li> </ul> </li> </ul> The active methods consists of utilization of photogenerated charge carrier in the electrolysis of water and other products. In the passive method, the electrolysis is arried out in the dark at low temperature (80oC) using an <a href="/Green_Revolution/hydrogen/hydrogen/?Topic=electrolysishydrogen#Alkaline">alkaline electrolysis</a> such as NaOH or Proton Exchange Membrane cells <a href="/Green_Revolution/hydrogen/hydrogen/?Topic=electrolysishydrogen#Protonex">( PEM )</a> involving polymeric sulfonic acids, at intermediate ( 200oC - 500oC) temperatures for which suitable electrolysis are still being sought and at high (800oC) temperature using oxide electrolytes such as yttria- stabililized zirconia. Photovoltaic technology including the electrochemical photovoltaic cells could be useful in this method as a source of electricity Photovoltaic technology has the advantage that electricity is produced through solar energy which is a clean source of energy therefore hydrogen production through it is also clean.

<h4 id="Photoelectrolysis"><li>Photoelectrolysis</li>

Splitting of water to generate hydrogen and oxygen is an endothermic reaction, and is accompanied by a large increase in gibbs free energy dGo that can be provided by energetic photons and is expressed as H2O ->H2 + 1/2O2        dGo = 237.1 Kj/mol Hydrogen genration from watre is a two-electron process. however for oxygen generation each atom releases two electrons so that water splliting to generate a molecule of oxygen is four electron process. <image src="/Green_Revolution/hydrogen/hydrogen/solarhydrogen.jpg" style="height:200;margin:0 20 0 0;" onmouseover="imagezoomsize(this.src, 'image1sidediv')" onmouseout="imageantizoomsize('image1sidediv')"> In a typical photoelectrolysis cell, one electrode is a semiconductor, wheras the other is a metal. on illumination of the n-type semiconductor - electrolyte interface, the fermi level in the semiconductor rises towards the flat band potantial. the maximum possible rise in fermi level with the electrodes shorted together is given by the flat band potential "Ufb". For direct water photoelectrolysis to occur on illumination the flat band potential "Ufb" must be above the H+/H2 potential wheras the O2/H<sub2 O potential should be above the valance band of the n- type semiconductor. Usuelly the valance band of the n- type semiconductor would be below the O2 / H2O potential. However only few of semiconductors have "Ufb" above the H+/H2 potential. viz. n-SrTiO3, Nb2O3, n- ZrO2, n-Ta2O5, n-BaTiO3, n-KTaO3, n-InP, p-GaAs, n-CdS, n-SiC and n-TiO2(in alkaline electrolyte). An energy level diagram for an n-type semiconductor/metal photoelectrolysis cel in which the flat band potential "Ufb" lie above the H+/H2 potential whereas the O2/H2O potential lieabove the valanve band of the n-type semiconductor. Here the available photovoltage is in excess of the thermodynamic potential difference for water splitting ( 1.229 V at 25oC ). Its also overcomes the intrinsic energy level mismatches &#951;c and &#951;a. Where &#951;c is mismatch of the counterelectrode metal Fermi level Ef above the H+/H2potential required to sustain the current flow and &#951;a the mismatch due to the difference between O2/H2O potential and the valance band edge at the interface. Holes that are generated inthe n-type semiconductor within a diffusion lenght of the depletion region and diffuse into it, together with those generated in the depletion region itself are seperated and swept to the semiconductor surface by the built-infield. Because of the faverable band-edge positions and the available necessary overpotentials they can produce anodic oxidation of water molecule and generate oxgen. However the photo-generated electrons flow through the n-type semiconductoor bulk to the back contact and from there to a metal counter electrode where they are donated to reduce hydronium ion and generate hydrogen. The excess photovoltage is used to overcome the kinetic, diffusion and ohmic over-potential losses. Which are collectively termed polarization loss. Althougn SrTiO3, Nb2O3 and ZrO2 are suitable for direct water splitting their band gaps are too high in the ranf=ge 3.4 - 3.5 eV. Therefore these are to be illuminated by UV rediation which is hardly available in solar spectrum. CdS and SiC have favorable conduction and valance band position. However they have problems with photocorrosion.

<h4 id="Photocatalysis"><li>Photocatalysis</li> Many researchers have prepared nano-sized semiconductors with very large reactive surface area so as to enhance surface adsorption and photocatalytic reduction and oxidation reactions for the generation of H2 and O2 by water splitting. there is meither a separte anote for oxidation of water to hydrogen nor a seperations of suitable semiconductor matrials such as TiO2, SrTiO2, CdS and MoSe2 in a electrolyte are employed to carry out photo catalytic activity for water splitting. When a molecule in a solution absorbs light, an electron in the lightest occupied moleculer orbital creating a hole. The electron is provided to a molecule in the solution to reduce it, whereas the hole is provided to a molecule in solution to oxidise it. There are three type of semiconductor materials are used for the process. <image src="/Green_Revolution/hydrogen/hydrogen/solarhydrogen1.jpg" style="height:200;margin:0 20 20 0" onmouseover="imagezoomsize(this.src, 'image2sidediv')" onmouseout="imageantizoomsize('image2sidediv')">

<image src="/Green_Revolution/hydrogen/hydrogen/solarhydrogen2.jpg" style="height:200;margin:0 20 20 0" onmouseover="imagezoomsize(this.src, 'image2sidediv')" onmouseout="imageantizoomsize('image2sidediv')">

<image src="/Green_Revolution/hydrogen/hydrogen/solarhydrogen3.jpg" style="height:200;margin:0 20 20 0" onmouseover="imagezoomsize(this.src, 'image2sidediv')" onmouseout="imageantizoomsize('image2sidediv')">

1) Large semiconductor particles. 2) Q-Semiconductor. 3) Semiconductor with metal particle deposition. Optical properties of large ( &#8805;15nm) semiconductor particles in suspesion and colloidal solution are similar to those of extended (bulk) crystal. Quantization of energy levels occur in small particles when the electron and holes are limited by the potential walls of particale having size of order of the De Broglie wavelenght of charge carriers. Such particle are called quantized or Q- semiconductors. Electrons and holes generated by illumination in small semiconductor carry out redox process before they recombine. Therefore quantum yield can be high. But for small particles of redius ~ 10nm the very small potential drop at the center of the particle cannot aid in the seperation of electron-hole pair. A large band bending can be achieved when a metal particle of appropriate work function is deposited on semiconductor. Electrons would be driven to the metal particles whereas holes to the semiconductor surface. The Photocatalytic reactions consist of two key process. 1) Photogeneration of charge carriers. 2) Reduction and oxidation of molecules. The photogeneration occurs very fast, the reduction occur slowly ( 100 nsec ) and the oxidation occurs very slowly. ( milisecond )