solar water spliting cell

Solar Water Splitting Cells _________________________: 

1 Solar Water Splitting Cells _________________________ Ejaz Hussain HEJ Research Institute of Chemistry.

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2 Introduction Solar Energy is decentralized and inexhaustible clean energy natural resource with minimal environmental impact. In a single instance the available solar power striking the earth surface is approximately 130 million 500 MW. Goal? to utilize this solar energy Energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. Collecting and storing solar energy in chemical bonds is a highly desirable approach to solving the energy challenge

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3 Efficiently splitting water into usable hydrogen could become a new industrial photosynthesis that would provide clean fuel whose only waste product upon utilization is water. Photosynthesis is a process of storing solar energy in chemical bonds. Solution of our problem in nature. Some algae and bacteria, using sunlight as their energy source, even give off hydrogen under certain conditions.

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4 Why hydrogen? Clean – no greenhouse gases Energy security – can be produced from abundant sources Economic growth Efficient – fuel cells ~75% efficiency Portable: Car tanks, micro fuel cells… Honda FCX Clarity NASA uses hydrogen fuel to launch the space shuttles.

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5 New Materials for Photocatalytic Water Splitting, Fredrik Skullman , MATRL 286G, 05/26/2010 Problem Need to build up infrastructure Safety concerns ( hydrogen storage, the high reactivity of hydrogen) Production today – 95% from natural gas which is not renewable and produces CO 2 as a byproduct Solution – Split water with renewable energy sources

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6 For this purpose we need to develop an economically viable water splitting cell, composed of stable semiconductors designed to split water directly at the semiconductor surface.

Photoelectrochemical H2 generation: 

7 Photoelectrochemical H 2 generation 1.       Absorption of light near the surface of the semiconductor creates electron-hole pairs. 2.       Holes (minority carriers) drift to the surface of the semiconductor (the photo anode) where they react with water to produce oxygen: 2h + + H 2 O -> ½ O 2 (g) + 2H + 3.       Electrons (majority carriers) are conducted to a metal electrode (typically Pt) where they combine with H + ions in the electrolyte solution to make H 2 : 2e - + 2H + -> H 2 (g) 4.       Transport of H + from the anode to the cathode through the electrolyte completes the electrochemical circuit. The overall reaction : 2h n + H 2 O -> H 2 (g) + ½ O 2 (g)

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8 New Materials for Photocatalytic Water Splitting, Fredrik Skullman , MATRL 286G, 05/26/2010 Process 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: Adsorbed species (water) is reduced and oxidized by the electrons and holes. Domen et al. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. 2007 H 2 O → 2H 2 +O 2 ∆V=1.23V, ∆ G=238kJ/mol Photoelectrolysis

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9 Water splitting cells require semiconductor materials that are able to support rapid charge transfer at a semiconductor/aqueous interface Types of Semiconductor Materials The silicon doped with extra electrons is called an “ N type” semiconductor . The Silicon doped with material missing electrons that produce locations called holes is called “ P type” semiconductor . Multifunction configurations that use p- and n-type semiconductors with differing band gaps, and surface bound electro catalysts, have been the predominant approach for the development of efficient photo electrolysis cells Semiconductor materials

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10 Current Flow in N-type Semiconductors Current Flow in P-type Semiconductors

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11 Conduction and valence bands of an insulator; semiconductor; and conductor. A valence band whose potential is not sufficiently positive, for water oxidation, or whose conduction band is not sufficiently negative, for proton reduction, can lead to slow or negligible water splitting.

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12 Oxygen evolution reaction (OER) hydrogen evolution reaction (HER) conduction band-edge energy (Ecb) band gap energy (Eg) valence band-edge energy (Evb) The energy required for photoelectrolysis at a semiconductor photoelectrode is frequently reported as 1.6-2.4 eV per electron-hole pair generated Photoelectrolysis

Semiconductor materials and their band gapes: 

13 Semiconductor materials and their band gapes Adapted from M. Gr ä tzel, Nature 414 , 388 (2001) Oxides:- Stable but efficiency is low (large gap) :- Efficiency is good but surfaces corrode Approaches:- Dye sensitization (lifetime issues) Surface catalysis Efficiency and lifetime

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14 New Materials for Photocatalytic Water Splitting, Fredrik Skullman , MATRL 286G, 05/26/2010 Photocatalyst material requirements Band gap : Band gap>1.23eV and sufficiently small to make efficient use of solar spectrum (~<3eV). Band levels suitable for water splitting. High Crystallinity : Defects can act as recombination sites. Long term stability : Charge transfer used for water splitting and not corrosion. Domen et al. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. 2007

Efficiencies of solar water splitting cells: 

15 Efficiencies of solar water splitting cells The theoretical efficiency = conversion of incident solar energy to chemical energy Ŋ = J g μ ex. Φ conv /S J g = absorbed photon flux, μ ex = excess chemical potential generated by light absorption, Φ conv = quantum yield for absorbed photons, S = total incident solar irradiance (mW cm-2).

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16 Theoretical Efficiencies for Water Splitting Cells Efficiencies of solar water splitting cells are estimated based on number of photo systems and the minimum number of absorbed photons per H2 molecule. S2 and D2 Classes Class S2 indicates a single band gap device having a single semiconductor material with a band gap of 1.6 eV (threshold wavelength ) 775 nm) has an ideal maximum conversion efficiency at 1 Sun of 30%. It requires two photons to produce one molecule of H2

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17 A dual band gap configuration is given the classification of ( D2 ) or ( D4 ), indicating a system that requires two or four photons to produce one molecule of H2, respectively. Dual band gap solar water splitting system has theoretical efficiency of 41% but chemical conversion efficiency is 27% due to losses of the fraction of unused energy per absorbed photon Energy conversion devices that utilize multiple semiconductors with different band gaps can achieve higher efficiencies. Dual band gap device (D2)

Theoretical efficiency of Intermediate band solar cells: 

18 0 E i E g E c VB IB CB qV Luque et. al. PRL, 78 , 5014 (1997) Theoretical efficiency of Intermediate band solar cells Intermediate Band Solar Cells can be very efficient Max. efficiency for a 3-band cell=63% Max. efficiency for a 4-band cell=72% In theory, better performance than any other ideal structure of similar complexity But NO multi-band materials realized to date

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19 The semiconductor band-edge positions are plotted versus their integrated maximum photocurrent under Air Mass 1.5 illumination. Large band gap n-type materials are not capable of producing high current densities. In contrast, the smaller band gap (higher photocurrent) p-type materials have more negative conduction/ valence bands that are well suited to effect reactions at the H+/H2 potential.

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20 Calculation of Solar-to-Chemical Conversion Efficiencies Efficiencies of devices can be calculated using equation. ŋ = Ј mp (1.23V - V app )/P in V app = the applied voltage measured between the oxygen-evolving. photoanode and the hydrogen-evolving photocathode. J mp = the externally measured current density. P in = the power density of the illumination. The solar conversion efficiency of individual candidate photoelectrode materials that might be used in a multiple band gap photoelectrolysis cell to drive either the HER or OER can be calculated from current-voltage data obtained using a potentiostat in an illuminated three-electrode cell.

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21 The efficiency (ŋ) of a photoelectrode can be calculated from its current-voltage data using equation. ŋ= J mp . V mp / P in J mp = the current density at the maximum power point ( P PA ) P PA = J mp . V mp V mp = the voltage at the maximum power point, and P in (in W cm-2) is the power of the incoming illumination. Overall water splitting efficiencies (STH) for photoelectrolysis cells can be estimated by overlapping the individually tested J - V data for each photocathode/anode

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22 Overlaid current density-potential behavior for a p-type photocathode and an n-type photo anode, with overall efficiency projected by the power generated P STH J op (1.23 V) by the cell for splitting water. The intersection of the two curves indicates the maximum operating current density ( J op) for the complete cell. A theoretical p/n-PEC photoanode/photocathode device The power generated for each component of the cell (red shaded area) and the power generated at the maximum operating current density (blue shaded area).

Photoelectrolysis Cell Configurations: 

23 Photoelectrolysis Cell Configurations A basic photoelectrochemical water splitting device can be constructed from a single p-type or n-type semiconductor i.e. is single band gap or dual band gap a single band gap device requires, at a minimum, a semiconductor with a 1.6 to 1.7 eV band gap in order to generate the V oc required to split water. To obtain efficient water splitting devices using currently available semiconductor materials, a D4 photoelectrolysis cell configuration is advantageous, due to the ability to explore various combinations of smaller band gap semiconductor materials that have complementary absorption and stability characteristics

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24 (a) a single band gap photoanode

Semiconductor Photoelectrochemistry: 

25 Semiconductor Photoelectrochemistry To design an efficient solar water splitting cell we need to understand physics of semiconductors The thermodynamic and kinetic parameters of semiconductor-liquid contacts The function of surface electrocatalysts

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26 Photocathodes for Hydrogen Evolution A single photoelectron is created on the photocathode and then detected using an Advanced gas multiplier. 2H+ + 2e- → H2 (low pH) 2H2O + 2e- →H2 + 2OH (high pH)

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27 Effects of Catalyst Particles on Photocathode Surfaces Addition of a catalyst to the surface (often in the form of a nanoparticulate metal film) can improve the kinetics of the reaction. the most efficient system based on a p-type semiconductor has been achieved using p-InP decorated with Pt catalyst islands, yielding a 13.3% conversion efficiency to hydrogen.

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28 Photoanodes for Water Splitting An oxygen evolving photoanode material must be an n-type semiconductor, such that the electric field generated by band bending drives holes toward the surface. The material needs to be stable under water oxidization conditions. Most of the photoanode materials that have been investigated are metal oxides or metal oxide anions (oxometalates),In pure, mixed, or doped forms.

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29 Mechanism and Theory of the Hydrogen Evolution Reaction The HER is one of the most well-studied electrochemical reactions. The reaction undergoes through two primary steps. HA + e- * → H•* + A- (1) HA + H•* + e-* → H2 + A- (2a) 2H•* → H2 (2b) The asterisk (*) represents a binding site at the electrode surface. A- refers to the conjugate base of the reduced (acidic) proton.

Summary: 

30 Summary Numerous combinations of semiconductor materials and electro catalytic combinations are available. Dual band gap configurations are more efficient than single band gap configurations. Multiple junction configurations require innovative contacts and new electrocatalysts. Photoelectrode stability continues to be a major challenge for the development of efficient photocathodes and photoanodes. Nature uses a continually renewed dual band gap photosystem to capture light and store the energy in simple sugar molecules. similar photoelectrosynthetic strategy can be used to decompose water using two semiconductors and store the energy in the simplest chemical bond, H2.

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31 THANKS