Photo Electrochemical Water

Splitting for Hydrogen Production-

Basics

Presented by – ANAMIKA BANERJEE

It is estimated that the global energy consumption will increase from13.5 TW (in 2001) to 27- 41 TW (by 2050).

MAJOR SOURCES OF ENERGY

HYDROGEN

1. Combustion generates only steam & water.

2.Heat ofcombustion is34.18 kcal/g.

3.High energystorage capacityi.e. 119 kJ/g.

4.Easilyassimilatedinto thebiosphere.

5. It is non toxic.

6. Can be used in

the chemicalindustry, for theproduction ofchemicals &conventionalpetrochemicals.

7.Suitable fuelfor use in fuelcells.

8.Transmission of energy in the form of Hydrogen is economical.

PHOTOELECTROCHEMICAL CELLS

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.

PHOTO- ELECTROCHEMISTRY OF WATER DECOMPOSITION

The principle of photoelectrochemical water decomposition is basedon the conversion of light energy into electricity within a cellinvolving two electrodes(or three), immersed in an aqueouselectrolyte, of which at least one is made of a semiconductorexposed to light & able to absorb light. This electricity is then used

for water electrolysis.

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

SCHEMATIC REPRESENTATION OF 3 ELECTRODE SYSTEM

SEMICONDUCTOR PROPERTIES

ENERGY BAND IN SEMICONDUCTOR

Consist of a large number of closely spaced energy levels.

Bands are made up large number of atomic orbitals and thedifference in energy between adjacent orbitals within a givenenergy band is so small so that band can be considered acontinuum of energy levels

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

•The band gap, Eg, is the smallest energy difference betweenthe top of the valence band and the bottom of the conductionband

•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 aqueouselectrolyte but absorb in UV region which is only about 4% ofthe solar spectrum, whereas small band gap semiconductor &optimum band gap semiconductor have the potential to absorbvisible part of solar spectra but corrode when dipped inelectrolyte.

Eg

Valence Band

Conduction Band

BAND GAP (Eg)

BAND EDGES

A semiconductor capable of spontaneous water splitting must have

conduction band energy (EC) higher and valence band energy (EV) lowerthan that of reduction potential Ered (H2/H+) & oxidation potential Eox(OH-

/O2) of water respectively.

0.0

1.23

H+ /H2

O2 / H2O

EC

EV

h+

e-

Eg ≥ 2eV

V vs NHE

Ideal straddling condition of Conduction & valence band edges of a semiconductor

Energy level Diagram

(pH 1)

Band positions of several semiconductor materials in contactwith aqueous electrolyte at pH 1

.

It is imp. because when a reference electrode is used to makemeasurements, it compares Ef of semiconductor with its ownunchanging Fermi level.

EC

EF

EV

Intrinsic

FERMI ENERGY (EF)

In an extrinsic semiconductor:

EC

EF

EV

n – type

EC

EF

EV

p – type

In an intrinsic semiconductor:

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

++

+

+

+

+

+

+

-

-

-

-

-

-

-

-

EREDOX

EREDOXEF

EF

n – type p – type

BAND BENDING :-

EF > Eredox - e- will be transferred fromelectrode into the solution and the thereis a positive charge associated with thespace charge region.

EF < Eredox – e- must transfer from thesolution to the electrode to attainequilibrium and generates a negativecharge 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 ofbands.Band bending acts as a barrier for the recombination of charge carriers.Band bending becomes zero only at flat band potential(Vfb)

Also called as depletion region.

An insulating region within aconductive, doped semiconductormaterial where the charge carriers arediffused or forced away by an electricfield.

It is called so because it is formedfrom a conducting region by removalof all charge carriers leaving none tocarry a current.

EF

EF

EC

EC

EV

EV

Space charge(depletion)

Space charge(depletion)

n – type

p – type

SPACE CHARGE REGION :-

The flat band potential corresponds to the externally applied potentialfor 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 withnegative flat-band potentials (relative to the redox potential of theH+/H2 couple, which depends on the pH) can split the water moleculewithout the imposition of a bias.

If Vfb is positive, then more electrons are attracted towards spacecharge region, hence this region decreases and it leads to increase inthe recombination of charge carriers.

If Vfb is negative, then space charge region becomes broadened as aresult there is decrease in recombination of charge carriers.

FLAT BAND POTENTIAL :-

BENCHMARKS EMERGE TO MAKE PEC TECHNOLOGY VIABLE

PHOTOANODE CHARACTERISITICS

MATERIAL REQUIREMENTS

Band gap energy around 2 eV

Strong optical absorption

High electron mobility

Long life time of charge carriers

Must straddle with redox potential of water

Stability in strong electrolytes

Good catalytic properties

Cost effective

Conversion Efficiency- 10%

Current Density (JPC) – 10-15 mA/cm2

Material Durability ˃2000 h

Economically Feasible

STRATEGIES FOR THE IMPROVEMENT OF SEMICONDUCTOR

DOPING:•Extends absorption invisible region.

• Increases the lifetimeof photo generatedcarriers.

•Improves electricalconduction

DYE SENSITIZATION

•Use ofsensitizer/catalyst/dyes

• Improvement inabsorption of solarenergy

•Enhanced photo -response with dyes

ION IMPLANTATION

• Modify electronicstructures ofsemiconductor toimprove visible lightresponse.

•Referred as ‘secondgeneration photocatalyst’.

SWIFT HEAVY ION IRRADIATION

•For modification insurface properties ofmaterial throughelectronic excitationsresulting in alterationin the photo response ofthe material in PEC cell.

BILAYERED SYSTEMS

• Broad absorption

• Good charge transportation

• Reduced recombination rate

• Inbuilt electric field at the

heterojunction

CHARACTERIZATION METHODS

X- Ray Diffractometer

• For measurement of Phase & Particle size

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

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 hetero- junction thin film.

Potentiostat – For PEC study.

SEM (Scanning Electron Microscope)

Scans the surface of the sample by releasing electrons and makingthe electrons bounce or scatter upon impact. The machine collects thescattered 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

PHOTOCURRENT – VOLTAGE CHARACTERISTICS

• They are the useful tool in determining the operating characteristicsof a device by showing its possible combinations of current &voltage. As a graphical aid visually understand better what ishappening.

• These curves show the relationship between the current flowingthrough an electrical or electronic device & the applied voltage acrossits terminals.

Ph

otoc

urr

ent

Den

sity

Potential

V < 0, Cathodic current in forward bias

region.

V > 0, Anodic current in reverse bias

region.

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 (Voc): 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 (Isc) : 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

MEASUREMENTS

EFFICIENCY MEASUREMENTS

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 = Jph (mA/cm2) X [1.23 – Vb (V)]/P (mW/cm2)

APCE = Jph (mA/cm2)X[1.23 – Vb (V)]/P mono(mW/cm2) X λ(nm)(1-10-A)

STH = [|jsc(mA/cm2)| X(1.23V) X ηF]/P (mw/cm2)

IPCE

IPCE describes the maximum possible efficiency with whichincoming radiations can produce hydrogen from water.