Global energy scenarios and principle of photoelectrochemistry

Global energy scenarios and

1. Global Energy Scenarios and Principle of Photoelectrochemistry SWARNENDU BADURI by

2. Consumption of Energy Increased by 85% Between 1970 and 1999 By 2020, Consumption will Triple

3. Global Energy Crises Global Energy demand increases continuously Depletion of fossil Fuel Oil price increases continuously, It puts pressure on country economic World oil and gas reserves are estimate at just 45 years and 65 years respectively Coal is likely to last a little over 200 years We need to find alternate energy sources

4. Energy sources Primary or natural Secondary or synthetic Conventional Conventional Non-conventional or alternatives Non-conventional or alternatives Fossil fuels (Coal, petroleum and natural gas), Water power (Hydro and pumped storage), Nuclear Fission Electricity, Gasoline, Kerosene Solar, Wind, Water power (tidal and wave), Geothermal, Ocean thermal, Bio-fuels (biogas and biomass ) Chemical (Hydrogen, methanol etc) Electrochemical (Fuel cells and batteries), Nuclear fusion

5. What is Photocatalysis ? A photocatalyst is a material that functions as a catalyst (alters the rate of a chemical reaction) when exposed to light. Conventional redox reaction Photocatalyts + LIGHT Self cleaning effect Water cleaning Air cleaning Anti-bacterial effect Water cleavage Water splitting

6. Solar Energy-driven Artificial Photosynthesis Semiconductor particles as photo harvester Needs co-catalysts (e.g., Pt, MoS 2 , and Co 3 O 4 ) Return reaction of H 2 O need to be prevented e - or h + scavengers can be used (electrolytes) to generate only O 2 or H 2 . Osterloh and Parkinson, MRS Bulletin , 2011, 36, 17 A. Kudo and Y. Miseki . Chem. Soc. Rev . 2009, 38, 253-278

7. Photoelectrolysis of Water Step 1 : Photon with energy above 1.23eV ( λ <~1000 nm) is absorbed. Step 2 : Photoexcited electrons and holes separate and migrate to surface. Step 3 : H 2 O is reduced by the electrons to H 2 and oxidized by the holes to O 2 Photocatalyst material requirements Band gap : Band gap>1.23eV and sufficiently small to make efficient use of solar spectrum (UV-VIS region). Band levels suitable for water splitting: B and edges of the conduction band (CB) and valence band (VB) of a semiconductor photocatalyst must be located at a potential more negative than that for H 2 evolution (0 V vs SHE) and a potential more positive than that for water oxidation (+1.23 V vs SHE), respectively. High Crystallinity : Defects can act as recombination sites. Long term stability : Charge transfer used for water splitting and not corrosion. Process : H 2 O→2H 2 +O 2 ∆V=1.23V, ∆ G=237kJ/mol

8. Principle of water splitting using semiconductor Photocatalysts Suitable band gap (Eg) of the Semiconductors: Eg > 1.23eV . Energy of incident light : h n > Eg Generation of electrons at conduction band (CB) and holes at valence band (VB).

9. H 2 or O 2 evolution reaction in the presence of sacrificial reagents —Half reactions of water splitting

10. Band positions of semiconductors with respect to the thermodynamic potentials of water splitting.

11. THANK YOU