1. Photo Electrochemical Water
Splitting for Hydrogen Production-
Basics
Presented by – ANAMIKA BANERJEE

2. 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

3. 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.

4. PHOTOELECTROCHEMICAL
CELLS

5. 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.

6. 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.

7. 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

8. SCHEMATIC REPRESENTATION OF 3 ELECTRODE
SYSTEM

9. SEMICONDUCTOR
PROPERTIES

10. 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

11.  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

12. •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.
Eg
Valence Band
Conduction Band
 BAND GAP (Eg)

13. BAND EDGES
A semiconductor capable of spontaneous water splitting must have
conduction band energy (EC) higher and valence band energy (EV) lower
than 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

14. Energy level Diagram
(pH 1)
Band positions of several semiconductor materials in contact
with aqueous electrolyte at pH 1

15. .
 It is imp. because when a reference electrode is used to make
measurements, it compares Ef of semiconductor with its own
unchanging 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.

16. +
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
EREDOX
EREDOXEF
EF
n – type p – type
BAND BENDING :-
EF > Eredox - e- will be transferred from
electrode into the solution and the there
is a positive charge associated with the
space charge region.
EF < Eredox – 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 the
bulk 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)

17.  Also called as depletion 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.
EF
EF
EC
EC
EV
EV
Space charge
(depletion)
Space charge
(depletion)
n – type
p – type
SPACE CHARGE REGION :-

18. 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+/H2 couple, which depends on the pH) can split the water molecule
without the imposition of a bias.
If Vfb 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 :-

19. 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

20. STRATEGIES FOR THE IMPROVEMENT OF
SEMICONDUCTOR

21. 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

22. CHARACTERIZATION METHODS

23. 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.

24. SEM
(Scanning Electron Microscope)
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

25. 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.
Photocurrent
Density
Potential
V < 0, Cathodic current in forward bias
region.
V > 0, Anodic current in reverse bias
region.

26. 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

27. MOTT- SCHOTTKY
MEASUREMENTS

28. EFFICIENCY
MEASUREMENTS

29. 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)

30. IPCE
IPCE describes the maximum possible efficiency with which
incoming radiations can produce hydrogen from water.