Photoelectrochemical Splitting of water for hydrogen generation: Detailed Description

Photoelectrochemical Splitting of
Water for Solar Hydrogen Generation
RUNJHUN DUTTA

Why Hydrogen?
 The use of fossil fuels is responsible for climate change and their deposits are limited. There is a need
for a fuel generated from the raw materials which is abundantly available and which is environmentally
safe.
 Hydrogen can be stored in liquid, compressed gas and hydride forms.
 Hydrogen is the most abundant element on earth.
 Hydrogen can be burned in an internal combustion engine or in a fuel cell. By-products are water and
heat.
2

Hydrogen
 Energy Released:
140 kJ/g
 Heat of combustion
34.18 kCal/g
 Energy storage capacity/mole
119 kJ/g
3

Why PEC route for Hydrogen
generation?
 Based on solar energy, which is a perpetual source of
energy, and water, which is a renewable resource.
 Environmentally safe, with no undesirable by products.
 It may be used on both large and small scales.
Other routes for Hydrogen
generation:
 Wind
 Tidal
 Geothermal
 Hydrothermal
 Biological

PEC Principle
 The principle of photoelectrochemical water decomposition is based on the conversion of light
energy into electricity within a cell involving two electrodes, immersed in an aqueous electrolyte, of
which at least one is made of a semiconductor exposed to light and able to absorb the light. This
electricity is then used for water electrolysis.
There are three options for the arrangement of photo-electrodes in the assembly of PECs:
 photo-anode made of n-type semiconductor and cathode made of metal
 photo-anode made of n-type semiconductor and photo-cathode made of p-type semiconductor
 photo-cathode made of p-type semiconductor and anode made of metal
5

6
 The main component of the PEC cell is the semiconductor, which converts incident photons to electron–hole
pairs.
 These electrons and holes are spatially separated from each other due to the presence of an electric field inside
the semiconductor.
 The photogenerated electrons are swept toward the conducting back contact, and are transported to the metal
counter-electrode via an external wire.
 At the metal, the electrons reduce water to form hydrogen gas.
 The photogenerated holes are swept towards the semiconductor/electrolyte interface, where they oxidize water to
form oxygen gas.

Working of PEC cell
Electrons on
illumination travels
from VB to CB
resulting in the
formation of holes in
the VB.
These electrons are
transferred to the
counter electrode .
Holes react
with water
molecules
and
oxidizes it
to release
oxygen
and
These transported
electrons react
with protons and
reduces them to
hydrogen gas.

8
Figure
Structure of photo-electrochemical
cell (PEC) for water photo-electrolysis
[Fujishima and etal_1972]
In 1972, Honda and Fujishima first
investigated water splitting using a single
TiO2 crystal as a photoanode and Pt as a
cathode.

9
The materials required for the photo-electrodes of PECs should perform two fundamental functions:
• Optical function required to obtain maximal absorption of Solar energy
• Catalytic function required for water decomposition.
The properties of photo-electrodes should satisfy several specific requirements in terms of
semiconducting and electrochemical properties, including:
• Band Gap
• Nat Band Potential
• Schottky Barrier
• Electrical Resistance
• Helmholtz Potential
• Corrosion Resistance
• Microstructure

10
For an alkaline electrolyte, the reduction and oxidation reactions can be written as
For an acidic environment, the appropriate reactions can be obtained from above reactions
by subtracting or adding the dissociation reaction of water into protons and hydroxyl ions:

12
At standard temperature (298 K) and concentrations (1 mol/L, 1 bar), the electrochemical cell
voltage delta E of 1.229 V corresponds to a Gibbs free energy change of +237 kJ/mol H2.
This shows that the water-splitting reaction is thermodynamically uphill.
Gibbs Free Energy Change

14
Formation of the valence and conduction bands in
covalent semiconductors from bonding and
antibonding sp3 orbitals, respectively.
Calculated electronic band structure of silicon.
The gray area indicates occupied states in the
valence band of the material.

15
 The valence band is mainly composed of
O-2p orbitals, whereas the conduction
band is primarily Ti-3d in character.
Molecular orbital diagram of rutile TiO2 Electronic band structure and density-of-states of TiO2.
 The black parts of the DOS indicate completely filled
bands.
 The highest DOS occur at energies where the E–k curves
are flat (horizontal)

BAND-GAP
The bandgap of a material can be determined from a measurement of the absorption coefficient vs.
wavelength.
If the bottom of the conduction band and the top of the valence band are assumed to have a parabolic
shape, the absorption coefficient (units: per m) can be expressed as follows:
α = A (hv-Eg)m / hv
Here, A is a constant and m depends on the nature of the optical transition:
 m = ½ for a direct bandgap
 m= 2 for an indirect gap
Extrapolation of a plot of (α hv)1/2 vs. hv plot gives the indirect bandgap,
while a plot of (α hv)2 vs. hv yields the direct bandgap of the material.
Such a plot is called a “Tauc plot”.
Absorbance
t
 

17
 A semiconductor with a large band gap has sufficient potential difference to perform the catalytic reaction, but such a
material would absorb the light below UV region.
 On the other hand, a semiconductor with a smaller band gap might not have sufficient energy to perform the same
catalytic reaction, but it would absorb light in the visible region (400–700 nm), which constitutes the largest portion of
the total solar radiation.
 Additionally, the band gap energy should compensate for inevitable losses during charge transfer steps.
 The minimum bandgap is determined by the energy required to split water (1.23 eV) plus the thermodynamic losses
(0.3–0.4 eV) and the overpotentials that are required at various points in the system to ensure sufficiently fast reaction
kinetics (0.4–0.6 eV).
 In total, the photoactive material should have a band gap of 1.6–2.4 eV to perform a spontaneous PEC water splitting.
BANDGAP RANGE

18
Schematic representation demonstrating the chemical stability of materials used in PEC devices without any
bias voltage.

19
Type of Junctions
Both electrons and holes
migrate from semiconductor
A to semiconductor B, which
results in an inefficient charge
transfer.
An electron migrates from A
to B, but holes relocate from
B to A, resulting in efficient
overall charge transfer.
An electron migrates from A
to B, but holes relocate from
B to A, but has a more
pronounced driving force for
charge separation.

20
Band to Band transition
When semiconductor absorbs a photon of energy, hv > Eg, optical excitation results in a delocalized
electron in CB, leaving behind hole in VB.
1) Direct Band Gap: Momentum is conserved and top of VB and bottom of CB are located at
Electron Wave Vector = 0 (k).
2) Indirect Band Gap: Requires absorption or emission of phonon (lattice vibration).

21
 DOPING
 Involves addition of small amount of an element in a given semiconductor sample.
 Increases charge carrier density.
 Improves overall conductivity of the semiconductor.
 Extends absorption in visible region.
 Increases the lifetime of photo generated carriers.
 Improves electrical conduction.
 DYE SENSITIZATION
 A dye/sensitizer/catalyst is coated on the surface of semiconductor which serves the function of absorption of solar light.
 Improves solar energy absorption.
 Increases photo response of the electrode.
 ION IMPLANTATION
 Modify electronic structures of semiconductor to improve visible light response.
 Referred as ‘second generation photo catalyst’.
MODIFICATION TECHNIQUES

22
 SWIFT HEAVY ION IRRADIATION
 Ions with velocity 2.18 X 108 m/s hits target atoms.
 Shifts absorption range to visible region.
 Shifts flat band potential of photoanode to more negative value.
 Increases photocurrent density and hence water splitting efficiency.
 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
 Inbuilt electric field at the heterojunction
 Two different semiconductors with different properties are joined together to form a heterojunction.
 Results in favorable band gap required for water splitting.
 Allows broad range of spectrum for absorption.
 Reduced recombination rate due to the electric field generated at the interface of heterojunction.
 Enhancement in efficiency of electrode.

23
Charge Carriers and Doping
Under equilibrium conditions (i.e., no illumination and no net current flow), the
concentration of free electrons in the conduction band and free holes in the valence band is
given by the following expressions:

24
The conductivity of the material is given by
So increasing n or p will compensate for a small value of µe or µh

25
 If the dopant level is within ~2kT of EC or EV, it will be (almost) fully ionized at room
temperature – this is referred to as a Shallow Dopant.
 For deep donors and acceptors, the degree of ionization can be calculated with the
following equations:
 ED and EA are the donor and acceptor energies.
 gA and gD are the corresponding degeneracy factors that reflect the multiplicity of the energy
state.

26
Under the assumption that n= ND
+ , the following useful expressions relate the free
electron concentration directly to the position of the donor level in the bandgap

27
FIGURE
 Energy levels of shallow and deep donors
(SD,DD) and acceptors (SA, DA) in a
semiconductor.
 Deep donor or acceptor states can also
occur below or above midgap,
respectively.
 Midgap states (RC) are often very
efficient recombination centers and can
be either donor- or acceptor-like in
nature.
 Deep dopants can act as optically active
centers or as catalytically active surface
sites.

28
Space Charges and Band Bending
Origin of the Space Charge Layer
Figure Simplified illustration of the formation of a space charge region (SCR) at a metal oxide
semiconductor surface when exposed to (humid) air

29
The total amount of charge is related to the depletion layer thickness via
Expression for the Space Charge Width

30
Deep Depletion, Inversion, and Accumulation Layers
 Inversion layer:
 If the number of adsorbed negative (positive) surface charges increases beyond a certain number for an n-type (p-type)
semiconductor, the Fermi level crosses the middle of the bandgap and the surface region becomes p-type (n-type).
 Deep depletion layers :
 If the dominant charge carriers in the inversion layer are annihilated faster than they are (thermally) generated, no free
charge carriers are present and the surface remains insulating.
 A deep depletion layer is then formed which are fairly common in photoelectrode materials with a relatively large
bandgap (>2 eV) because generation of minority carriers is difficult in these materials.
 They can also be formed in the presence of surface-adsorbed species that consume the minority carriers through fast
oxidation or reduction reactions.
 Accumulation layer:
 An accumulation layer can form when an excess of positive (negative) charges is adsorbed at the surface of an n-type (p-
type) semiconductor. To compensate these surface charges, free majority carriers will accumulate near the surface,
forming the accumulation layer.

32
 When a semiconductor is immersed in an aqueous solution, H+ and OH ions in the solution will
continuously adsorb and desorb from the surface.
 A dynamic equilibrium will be established, which can be described by the following protonation and
deprotonation reactions:
Semiconductor/Electrolyte Interface
 The equilibrium of these reactions depends on the pH of the solution and the Brønsted
acidity of the surface.
 Depending on these conditions, the net total charge adsorbed at the surface will be positive,
zero, or negative.
 The pH at which the net adsorbed charge is zero is called the point of zero charge (PZC)
of the semiconductor.

34
 The potential drop across the Helmholtz layer
~0.1–0.5 V
 Width of the Helmholtz layer
~2–5 A °
 Dielectric constant of water
~6
 Helmholtz capacitance
10–20 mF/cm 2

35
This important result shows that the Helmholtz potential
changes with -59 mV (2.3kT/e) per pH unit at 25 OC.

36
Applying A Bias Potential
Effect of applying a bias voltage (VA) to an n-type semiconductor electrode.
Any change in applied potential falls across the space charge layer, whereas VH remains constant.

Flat Band Potential
 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.
37

3838
Flat Band
Potential
Positive
Negative
More electrons are attracted towards
space charge region
Space charge region decreases
Increase in the recombination of
charge carriers
Space charge region becomes
broadened
Decrease in recombination of charge
carriers

“
The performance of PECs is
considered in terms of:
• Excitation of electron–hole pair in
photo-electrodes.
• Charge separation in photo-electrodes.
• Electrode processes and related charge
transfer within PECs.
• Generation of the PEC voltage required
for water decomposition.
39

40
Incident photon to current conversion efficiency (IPCE)-
It is the measure of the percentage of incident photon that have been converted into electrons in PEC cell
as a function of wavelength of illuminating light.
IPCE(λ) = 1240 X JSC (A/cm2)
λ (nm) X Pin (W/cm2)
X 100%
JSC is photocurrent density
Pin is the incident photon intensity
Solar to hydrogen efficiency (STH)-
It is defined as the ratio of chemical energy stored in hydrogen gas to the solar energy input.
STH =
𝐉 𝐒𝐂
𝐦𝐀
𝐜𝐦𝟐
𝐗 𝟏.𝟐𝟑 𝐕 𝐗 𝐧𝐅
𝐏𝐢𝐧 (𝐦𝐖/𝐜𝐦𝟐)
JSC is the photocurrent density obtained without any external bias
Pin is the intensity of incident light and nF is faradic efficiency

41
Applied bias photon-to-current efficiency (ABPE)-
It is also a measure of hydrogen evolution, but in biased condition.
ABPE =
𝐉 𝐩𝐡
(mA/cm2) 𝐗 (𝟏.𝟐𝟑−𝐕𝐚𝐩𝐩)(𝐕)
Pin(mW/cm2)
Jph is photocurrent density under an applied bias
Pin is intensity of light source.
Absorbed photon to current conversion efficiency (APCE)-
It is the measure of the percentage of absorbed photons that have been converted into electrons by
PEC cells as a function of wavelength of illuminating light.
APCE =
𝐉 𝐩𝐡
(mA/cm2) 𝐗 𝟏𝟐𝟒𝟎
Pin(mW/cm2) 𝑿 𝝀 𝒏𝒎 𝑿 (𝟏−𝟏𝟎− 𝑨
)

“
42
Key Factors Affecting the
Photoelectrochemical Cell
Activity
 Size and shape of the nanoclusters
 Electrolyte Temperature
 pH Effect
 Effects of Crystallinity
 Effects of Particle Size and Defects
 Band Gap
 Light Intensity

43
pH Effect
 It is pH of the electrolyte that decides whether the net transfer of charges is negative, zero, or positive.
 The erratic migration of ions in a high pH environment may lead to weakening the surface of electrodes and
possibly ends up in corroding the electrode.
 Efforts must be taken to design catalysts that can work in the entire pH range to increase the usability of the
PEC device from lab to commercial scale.

44
Electrolyte Temperature
 An increase in temperature assists in fast transport of charges and a surge in electrochemical
reactions, but continually reduces the open circuit voltage, negating the performance of the device
by accelerating more rapid recombination of charge carrier pairs, which ultimately results in a
decline in the device performance.
 The temperature also affects the contact resistance between the metal–semiconductor interfaces.
 Variations in temperature induce a metal migration through the semiconductor–metal interface,
which results in a decrease in shunt resistance of the photo-absorber and thus significantly affect the
catalytic performance of the device.

45
Effects of Crystallinity
 The efficiency of PEC is directly related to the crystallinity of material.
 Due to better charge-transfer property, direct contact of the electrolyte with a large internal surface
area of the tube.
 As the extent of crystallinity inside a material increases, the amount of defect successively decreases
in a similar trend, and the available sites for the e-/h+ recombination are dramatically suppressed.
 These features together assist in better hydrogen generation and increase the efficiency of the device.

46
 Annealing is a process to introduce crystallinity in materials as a simple method to boost efficiency.
 The principle that governs the defect annealing process during ultrasonication is the hot spot
mechanism.
 The particles are locally heated up, and they get a chance to reorganize themselves followed by a
quenching process, leaving locally annealed nanoparticles different from those during annealing at
high temperature.
 This repeated reaction of NCs with a hot spot in the ultrasonication bath creates defect relaxations
and interface defects in bulk of the material.
 Such defect relaxations are short lived, temporally discontinuous, and spatially localized to tune the
electronic structures of material to boost the efficiency.
Annealing

47
Dimensionality of Materials
0D Nanomaterial
 Quantum Dots (Qds) of Cds, Co3O4, Cdse, Graphene, Graphitic Carbon Nitride.
1D Nanomaterials
 Nanorods, Nanowires and Nanotubes
2D Nanomaterials
 Thin films
3D Nanomaterials
 Dendritic α-Fe2O3

48
PARAMETERS OBTAINED FROM THE I-V PLOT
Photocurrent Density:
 Difference of light current- dark current/area of semiconductor.
 More photocurrent density, more is hydrogen production.
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.
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.

49
Mott-Schottky Plots
The Mott–Schottky plots for obtaining the
flat band potential of a semiconductor

Mott-Schottky Measurements
50
 The conductivity type (n or p type) for a semiconductor can be determined by the sign of the slope of this
straight tangent line. Generally, if the slope of the straight tangent line is positive, the semiconductor is n
type; however, if the slope is negative, the semiconductor is p type.
 The charge-carrier density (ND) can be calculated by taking the derivative of Equation.
 The actual values of the dopant density and the depletion layer width can be determined by impedance
measurements

51
TiO2 nanotube 100; k = 320–400 nm 1MKOH 13 mA/cm2 at 0 V vs Ag/AgCl PCE = 6.8%
TiO2 nanotube 98; UV illumination 1MKOH 26 mA/cm2 at 0 V vs Ag/AgCl PCE = 16.25%
TiO2 nanowire 100; AM 1.5 1MKOH 0.8 mA/cm2 at 0 V vs Ag/AgCl PCE = 0.75%
IPCE = 90% @350 nm
TiO2/TiSi2 nanonet 80; Xenon lamp 0.05 M 0.6 mA/cm2 at 0 V vs Ag/AgCl PCE = 16.7% @330 nm
KOH
TiO2/Si nanowire 100; Xenon lamp 1MKOH 0.25 mA/cm2 at 0 V vs SCE N/A
WO3 Nanowire
100; AM 1.5; k > 400
nm 0.1 M 1.43 mA/cm2 @1.23 V (two-
IPCE = 60% @400 nm
and
Na2SO4 electrode method) 0.5 V
Mesoporous WO3 films 100; AM 1.5 3 M H2SO4 2.9 mA/cm2 @1.23 V vs RHE IPCE = 80% @400 nm
WO3 Nanowire 100; AM 1.5 0.5 M, 1.2 mA/cm2 1.23 V vs SCE
IPCE = 75% @400 nm
and
H2SO4
2.0 mA/cm2
1.2 V vs SCE
WO3/Mn-oxo catalyst 150 W Hg Lamp HCl (pH 4) at 1.23 V vs RHE
IPCE = 17% @400 nm
and
1.23 V vs RHE
Cauliflower-type Si-doped
Fe2O3 100; AM 1.5 1 M NaOH 2.3 mA/cm2 at 1.23 V vs RHE
IPCE = 30% @400 nm
and
1.23 V vs RHE
2.7 mA/cm2 at 1.23 V vs RHE
IPCE = 36% @400 nm
and
1.23 V vs RHE
Material Light intensity Electrolyte Photocurrent unit Efficiency

52
Cauliflower-Fe2O3 100; AM 1.5 1 M NaOH 3.0 mA/cm2 at 1.23 V vs RHE
IPCE = 36% @400 nm
and
1.23 V vs RHE
Ti–Fe–O nanotubes 100; AM 1.5 1 M NaOH 3.0 mA/cm2
at 0.6 V vs
Ag/AgCl
APCE = 7% @450 nm
and
0.7 V (two-electrode)
Fe2O3 nanotubes N/A 1 M NaOH
IPCE = 3% @400 nm
and
0.4 V (two-electrode)
Fe2O3 nanotubes 100; AM 1.5 1MKOH 1.41 mA/cm2 at 0.5 V vs Ag/AgCl PCE = 0.84%
Mesoporous Sn-doped Fe2O3 100; AM 1.5 1 M NaOH 1.0 mA/cm2 at 1.55 V vs RHE N/A
nanoparticles
2.3 mA/cm2 at 1.7 V vs RHEMesoporous Ti-doped Fe2O3 100; AM 1.5 1 M NaOH
IPCE = 36% @400 nm
and
nanoparticles mA/cm2 at 1.23 V vs RHE 1.63 V vs RHE
Fe2O3 film 100; AM 1.5 1 M NaOH 0.6 N/A2
at 1.23 V vs RHESi-doped Fe2O3 100; AM 1.5 1 M NaOH
1.6
mA/cm N/A
Material Light intensity Electrolyte Photocurrent unit Efficiency

Photoelectrochemical Splitting of water for hydrogen generation: Detailed Description

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Ähnlich wie Photoelectrochemical Splitting of water for hydrogen generation: Detailed Description

Ähnlich wie Photoelectrochemical Splitting of water for hydrogen generation: Detailed Description (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

Photoelectrochemical Splitting of water for hydrogen generation: Detailed Description

Hinweis der Redaktion