PEC Presentation Sample

8/18/2019 PEC Presentation Sample

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Photo Electrochemical Water

Splitting for Hydrogen Production

-

Basics

Presented by ANAMIKA BANERJEE


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It is estimated that the global energy consumption will increase from

13.5 TW (in 2001) to 27- 41 TW (by 2050).

MAJOR SOURCES OF ENERGY


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HYDROGEN

1. Combustion

generates only

steam & water.

2.Heat of

combustion is

34.18 kcal/g.

3.High energy

storage capacity

i.e. 119 kJ/g.

4.Easily

assimilated

into the

biosphere.

5. It is non toxic.

6. Can be used in

the chemical

industry, for the

production of

chemicals &

conventional

petrochemicals.

7.Suitable fuel

for use in fuel

cells.

8.Transmission

of energy in the

form of

Hydrogen

is economical.


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PHOTOELECTROCHEMICAL

CELLS


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PEC technology is based on solar energy, which is a

perpetual source of energy and water, which is a

renewable source.

PEC technology is environmentally safe, with no

undesirable byproducts.

PEC technology may be used on both large and small

scales.

PEC technology is relatively uncomplicated.


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PHOTO- ELECTROCHEMISTRY OF WATER

DECOMPOSITION

The principle of photoelectrochemical water decomposition is based

on the conversion of light energy into electricity within a cell

involving two electrodes(or three), immersed in an aqueous

electrolyte, of which at least one is made of a semiconductor

exposed to light & able to absorb light. This electricity is then used

for water electrolysis.


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The performance of PECs is considered in terms of:

Excitation of electron

–

hole pairs in

photo

–

electrodes.

Charge separation in photo electrodes.

Electrode processes & related

charge transfer within PECs

Generation of PEC voltage

required for water decomposition


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SCHEMATIC REPRESENTATION OF 3 ELECTRODE

SYSTEM


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SEMICONDUCTOR

PROPERTIES


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ENERGY BAND IN SEMICONDUCTOR

Consist of a large number of closely spaced energy levels.

Bands are made up large number of atomic orbitals and the

difference in energy between adjacent orbitals within a given

energy band is so small so that band can be considered a

continuum of energy levels


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VALENCE BAND CONDUCTION BAND

Energy band: highest occupied energy level is called the

valence band and the lowest unoccupied energy level is

called the conduction band.

Conduction Band

Valence Band


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•The band gap, Eg

, is the smallest energy difference between

the top of the valence band and the bottom of the conduction

band

•The required band gap of the semiconductor for water splitting

should be 1.8 – 2.2 eV.

•Large band gap semiconducting oxides are stable in aqueous

electrolyte but absorb in UV region which is only about 4 of

the solar spectrum, whereas small band gap semiconductor &

optimum band gap semiconductor have the potential to absorb

visible part of solar spectra but corrode when dipped in

electrolyte.

E

g

Valence Band

Conduction Band

BAND GAP (Eg

)


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BAND EDGES

A semiconductor capable of spontaneous water splitting must have

conduction band energy (E

C

) higher and valence band energy (E

V

) lower

than that of reduction potential E

red

(H

2

/H

+

) & oxidation potential E

ox

(OH

-

/O

2

) of water respectively.

0.0

1.23

H

+

/H

2

O

2

/ H

2

O

E

C

E

V

h

+

e

-

E

g

≥ eV

V vs NHE

Ideal straddling condition of Conduction & valence band edges of a

semiconductor


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Energy level Diagram

(pH 1)

Band positions of several semiconductor materials in contact

with aqueous electrolyte at pH 1


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.

It is imp. because when a reference electrode is used to make

measurements, it compares E

f

of semiconductor with its own

unchanging Fermi level.

E

C

E

F

E

V

Intrinsic

FERMI ENERGY (E

F

)

In an extrinsic semiconductor:

E

C

E

F

E

V

n – type

E

C

E

F

E

V

p – type

In an intrinsic semiconductor:

It is the energy level where probability of occurrence of an electron is half.


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+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

E

REDOX

E

REDOX

E

F

E

F

n

–

type p

–

type

BAND BENDING :-

E

F

> E

redox

- e

-

will be transferred from

electrode into the solution and the there

is a positive charge associated with the

space charge region.

E

F

< E

redox

e

-

must transfer from the

solution to the electrode to attain

equilibrium and generates a negative

charge in the space charge region .

It is the difference between the potential at the surface and potential in thebulk of the semiconductor.

The electric field that is formed in space charge region results in bending of

bands.

Band bending acts as a barrier for the recombination of charge carriers.

Band bending becomes zero only at flat band potential(Vfb

)


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Also called asdepletion region.

An insulating region within a

conductive, doped semiconductor

material where the charge carriers are

diffused or forced away by an electric

field.

It is called so because it is formed

from a conducting region by removal

of all charge carriers leaving none to

carry a current.

E

F

E

F

E

C

E

C

E

V

E

V

Space charge

(depletion)

Space charge

(depletion)

n – type

p – type

SPACE CHARGE REGION :-


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The flat band potential corresponds to the externally applied potential

for which there is no band bending at the semiconductor surface.

This potential is equal to the curvature of the bands in the absence of

any potential applied to the interface.

Photo-cells equipped with a photo-anode made of materials with

negative flat-band potentials (relative to the redox potential of the

H

+

/H

2

couple, which depends on the pH) can split the water molecule

without the imposition of a bias.

If V

fb

is positive, then more electrons are attracted towards space

charge region, hence this region decreases and it leads to increase in

the recombination of charge carriers.

If Vfb

is negative, then space charge region becomes broadened as a

result there is decrease in recombination of charge carriers.

FLAT BAND POTENTIAL :-


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STRATEGIES FOR THE IMPROVEMENT OF

SEMICONDUCTOR


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DOPING:

•Extends absorption in

visible region.

• Increases the lifetime

of photo generated

carriers.

•Improves electrical

conduction

DYE SENSITIZATION

•Use of

sensitizer/catalyst/dyes

• Improvement in

absorption of solar

energy

•Enhanced photo -

response with dyes

ION IMPLANTATION

• Modify electronic

structures of

semiconductor to

improve visible light

response.

•Referred as ‘second

generation photo

catalyst’

.

SWIFT HEAVY ION

IRRADIATION

•For modification in

surface properties of

material through

electronic excitations

resulting in alteration

in the photo response of

the material in PEC cell.

BILAYERED SYSTEMS

• Broad absorption

• Good charge transportation

• Reduced recombination rate

• Inbuilt electric field at the

heterojunction


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CHARACTERIZATION METHODS


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X- Ray Diffractometer

• For measurement of Phase & Particle size

• Scherrer’s Equation (for crystallite size): 0.9λ /β cosθ

UV Vis Spectrophotometer

• For the measurement of band gap.

• For direct- indirect, allowed or forbidden transition.

Atomic Force Microscope (AFM )

• To determine surface morphology of heterojunction

thin film.

Potentiostat – For PEC study.


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SEM

(Scanning Electron M icroscope)

Scans the surface of the sample by releasing electrons and making

the electrons bounce or scatter upon impact. The machine collects the

scattered electrons and produces an image.

Information on the

sample’s surface and its

composition

Shows the sample bit by

bit as area where the

sample is placed can be

rotated in different

angles.

3D image Resolution-0.4nm


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PHOTOCURRENT VOLTAGE CHARACTERISTICS

• They are the useful tool in determining the operating characteristics

of a device by showing its possible combinations of current &

voltage. As a graphical aid visually understand better what is

happening.

• These curves show the relationship between the current flowing

through an electrical or electronic device & the applied voltage across

its terminals.

P

o

u

n

e

n

s

t

y

Potential

V < 0, Cathodic current in forward bias

region.

V > 0, Anodic current in reverse bias

region.


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PARAMETERS OBTAINED FROM THE I-V PLOT :

1. Photocurrent Density: Difference of light current- dark current/area of

semiconductor.

More photocurrent density, more is hydrogen production.

2. Open Circuit Voltage (V

oc

): The voltage between the terminals when no

current is drawn (infinite load resistance)

In Electron recombination kinetics, high Voc

- low recombination rate

and high photocurrent density.

3. Short circuit current (I

sc

) : The current when the terminals are connected

to each other (zero load resistance)

Isc

increases with light intensity, as higher intensity means more

photons, which in turn means more electrons

MOTT

SCHOTTKY


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MOTT - SCHOTTKY

MEASUREMENTS


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EFFICIENCY

MEASUREMENTS


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They are split into two main categories:

(i) Benchmark efficiency

(a) solar-to-hydrogen conversion efficiency (STH)

(ii) Diagnostic efficiencies (to understand material performance)

(a) applied bias photon-to-current efficiency (ABPE)

(b) external quantum efficiency (EQE) = incident photon-to-

current efficiency (IPCE)

(c) internal quantum efficiency (IQE) = absorbed photon-to-

current efficiency (APCE)

ABPE = J

ph

(mA/cm

2

) X [1.23 – V

b

(V)]/P (mW/cm

2)

APCE = J

ph

(mA/cm

2

)X[1.23 – V

b

(V)]/P mono(mW/cm

2

) X λ (nm)(1-10-A)

STH = [|j

sc

(mA/cm

2

)| X(1.23V) X ηF]/P (mw/cm2)


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IPCE

IPCE describes the maximum possible efficiency with which

incoming radiations can produce hydrogen from water.


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