Transcript MS PowerPoint - Catalysis Eprints database

Hydrogen - Production
Prospects and Challenges
National Centre for Catalysis Research (NCCR)
Indian Institute of Technology, Madras
17th March 2008
NCCR
HYDROGEN FUTURE: FACTS
AND FALLACIES
M. Aulice Scibioh and B. Viswanathan, Bulletin of the Catalysis Society of India,
vol.3, pp.72-81(2004)
•
•
•
•
•
•
A transition to a ‘hydrogen economy’ is a sea change in our energy infrastructure and
is not to be taken lightly.
only 50% can reach the end user due to losses in electrolysis, hydrogen compression
and the fuel cell.
The rush into a hydrogen economy is neither supported by energy efficiency
arguments nor justified with respect to economy or ecology.
In fact, it appears that hydrogen will not play an important role in a sustainable
energy economy because the synthetic energy carrier cannot be more efficient than
the energy from which it is made.
Renewable electricity is better distributed by electrons than by hydrogen.
Consequently, the hasty introduction of hydrogen as an energy carrier cannot be a
stepping stone into a sustainable energy future. The opposite may be true.
Because of the wastefulness of a hydrogen economy, the promotion of hydrogen may
counteract all reasonable measures of energy conservation. Even worse, the forced
transition to a hydrogen economy may prevent the establishment of a sustainable
energy economy based on an intelligent use of precious renewable resources. .
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Choice of fuel and oxidant Chemical & electrochemical data on various fuels
FUEL
G0
kcal/mol
E0theoretical
(V)
E0max
(V)
Energy
density
(kWh/kg)
Hydrogen (H2)
-56.69
1.23
1.15
32.67
Methanol (CH3OH)
-166.80
1.21
0.98
6.13
Ammonia(NH3)
-80.80
1.17
0.62
5.52
Hydrazine(N2H4)
-143.90
1.56
1.28
5.22
Formaldehyde(HCHO)
-124.70
1.35
1.15
4.82
Carbon monoxide(CO)
-61.60
1.33
1.22
2.04
Formic acid(HCOOH)
-68.20
1.48
1.14
1.72
Methane(CH4)
-195.50
1.06
0.58
-
Propane(C3H8)
-503.20
1.08
0.65
-
Oxidant ---- gaseous oxygen/air (In general, the oxygen needed by
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a fuel cell is supplied in the form
of air)
Comparison of fuel properties
Properties
Hydrogen Methane
(H2)
(CH4)
Gasoline
(-CH2-)
Lower heating value(kWhKg-1)
33.33
13.9
12.4
Self ignition temperature (°C)
585
540
228-501
Flame temperature (°C )
2.045
1.875
2.200
Ignition limits in air ( Vol %)
4-75
5.3-15
1.6-7.6
Minimal Ignition energy (mWs)
0.02
0.29
0.24
Flame propagation in air (ms-1)
2.65
0.4
0.4
Diffusion coefficient in air (cm2s-1)
0.61
0.16
0.05
Toxicity
No
No
High
L. Schlapbach et al Nature, 414 (2001) 353.
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Transition to hydrogen economy
 Production
Storage
Metal Hydride
MOF
Choice limited
Distribution
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Petrol dispensing station
Transition to a “Hydrogen Economy” in Indian context
• Broad-based use of hydrogen as a
fuel
– Energy carrier analogous to
electricity
– Produced from variety of primary
energy sources
– Can serve all sectors of the
economy: transportation, power,
industry,
buildings and residential
– Replaces oil and natural gas as the
preferred end-use fuel – Makes
renewable and nuclear energy
“portable” (e.g. transportation needs)
• Advantages:
– Inexhaustible
– Clean
– Universally available to all countries
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Hydrogen Production Technologies
Water
Various ways of production of hydrogen
•
•
•
•
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Steam Reforming and partial
oxidation
Thermal
Thermo-chemical (Fe-X2;S-I2)
Electrolysis
Electrochemical
Photolysis
Photochemical
Photoelectrochemical(PEC PVC)
Biological
Bio-chemical, photobiological
Technology awaited
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CO2emissions
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Principle of PEC
STATUS OF THERMOCHEMICAL CYCLES
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POTENTIAL THERMOCHEMICAL CYCLES
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Material selection for PEC or Photocatalysis

Photocatalysis
catalyst
 Conventional
reaction
assisted by
photons
redox
reaction
Oxidizing agent should have
more positive potential
Photocatalysis simultaneous oxidation and
reduction
The redox couple capable of
promoting both the reactions
can act as photocatalyst
Metals, Semiconductors and
Insulators
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CB
H+/H2
CB
CB
VB
Metals
H2O/O2
E
VB
S
VB C
Insulators
Metals: No band gap,Only reduction or
oxidation, Depends on the band
position
Insulators: High band gap, High
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SELECTION OF MATERIALS
Among them TiO2 is widely used
Though ZnO, CdS, ZnS and WO3 have wide band gap
they undergo photocorrosion
OR Type – Oxidation and Reduction
R Type
– Reduction
O Type – Oxidation
X Type
– None
H+/H2
0.00V
1.23V
H2O/O2
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CdS, ZnS, ZnO undergo
photo corrosion
Activity decrease as the
time increases
S deposition on the catalyst surface
reduce the light absorption ability of
catalyst.
SOME CONCEPTUAL DESIGNS
Coupling involves mixing small band gap
semiconductor with a higher band gap one
Smaller band gap semiconductor absorbs in
visible region and transfers excitons to the
other semiconductor
Recombination in the small
17thgap
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band
semiconductor
reduced
Various recombination process on the photoexcited semiconductor surface and inside the
bulk.
Selection Criterion
•
•
•
Ionic solids as the ionicity of the M-O
bond increases, the top of the valence
band (mainly contributed by the porbitals of oxide ions) becomes less and
less positive (since the binding energy of
the p orbitals will be decreased due to
negative charge on the oxide ions) and
the bottom of the conduction band will be
stabilized to higher binding energy
values due to the positive charge on the
metal ions which is not favourable for the
hydrogen reduction reaction.
Semiconductor
M-O
Percentage ionic character
TiO2
SrTiO3
Fe2O3
ZnO
WO3
CdS
CdSe
LaRhO3
LaRuO3
PbO
ZnTe
ZnAs
ZnSe
ZnS
GaP
CuSe
BaTiO3
MoS2
FeTiO3
KTaO3
MnTiO3
SnO2
Bi2O3
Ti-O
Ti-O-Sr
Fe-O
Zn-O
W-O
Cd-S
Cd-S
La-O-Rh
La-O-Ru
Pb-O
Zn-Te
Zn-As
Zn-Se
Zn-S
Ga-P
Cu-Se
Ba-O-Ti
Mo-S
Fe-O-Ti
K-O-Ti
Mn-O-Ti
Sn-O
Bi-O
59.5
68.5
47.3
55.5
57.5
17.6
16.5
53.0
53.5
26.5
5.0
6.8
18.4
19.5
3.5
10.0
70.8
4.3
53.5
72.7
59.0
42.2
39.6
More ionic the M-O bond of the
semiconductor is, the less suitable the
material is for the photo-catalytic
splitting of water. The bond polarity can
be estimated from the expression
Percentage ionic character (%) =
(1  e
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
(  A   B )2
4
) 100
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Some Governing Principles
•
The oxide semiconductors though - suitable
for the photo-catalytic water splitting reaction
in terms of the band gap value which is
greater than the water decomposition
potential of 1.23 V.
•
Most of these semiconductors have bond
character more than 50-60 % and hence
modulating them will only lead to increased
ionic character and hence the photo-catalytic
efficiency of the system may not be increased
as per the postulates developed
•
Therefore from the model developed in this
presentation the following postulates have
been evolved.
The photo-catalytic semiconductors are
often used with addition of metals or with
other hole trapping agents so that the life
time of the excitons created can be
increased. In this mode, the positions of
the energy bands of the semiconductor
and that of the metal overlap
appropriately and hence the alteration can
be either way and also in this sense only
the electrons are trapped at the metal sites
and only reduction reaction is enhanced
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•
Hence we need stoichiometrically both
oxidation and reduction for the water splitting
and this reaction will not be achieved by one
of the trapping agents namely that is used for
electrons or holes. Even if one were to use the
trapping agents for both holes and electrons,
the relative positions of the edge of the valence
band and bottom of the conducting band may
not be adjusted in such a way to promote both
the reactions simultaneously
Normally the semiconductors used in photo-catalytic
processes are substituted in the cationic positions so as to
alter the band gap value. Even though it may be suitable
for using the available solar radiation in the low energy
region, it is not possible to use semiconductors whose
band gap is less than 1.23 V and any thing higher than
this may be favourable if both the valence band is
depressed and the conduction band is destabilized with
respect to the unsubstituted system. Since this situation is
not obtainable in many of the available semiconductors
by substitution at the cationic positions, this method has
not also been successful.
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Some Possibilities

In addition the dissolution potential of the
substituted systems may be more favourbale
than the water oxidation reaction and hence
this will be the preferred path way. These
substituted systems or even the bare
semiconductors which favour the dissolution
reaction will undergo only preferential photocorrosion and hence cannot be exploited for
photo-catalytic pathway. In this case ZnO is a
typical example.
•
•
Therefore it is deduced that the systems which
has ionic bond character of about 20-30%
with suitable positions of the valence and
conduction band edges may be appropriate
for the water splitting reaction.
This rationalization has given one a handle to
select the appropriate systems for examining
as photo-catalysts for water splitting reaction
Very low value of the ionic character also is not
There are some other aspects of photo-catalysts on which
suitable since these semiconductors do not have
some remarks may be appropriate.
the necessary band gap value of 1.23 V. - the
search for utilizing lower end of the visible region
Though they have been derived from the solid state point
is not possible for direct water splitting reaction.
of view like flat band potential , band bending, Fermi
If one were to use visible region of the spectrum,
level pinning, these parameters also can be understood in
then only one of the photo-redox reactions in
terms of the bond character and the redox chemical
water splitting may be preferentially promoted
aspects by which the water splitting reaction is dealt.
and probably this accounts for the frequent
observation that non-stiochiometric amounts of
oxygen and hydrogen were evolved in the photoassisted splitting of water
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Some Other Opportunities
ENGINEERING THE SEMICONDUCTOR
ELECTRONIC STRUCTURES
TYPICAL PHOTOCATALYTIC
PROCESS
 without deterioration of the
stability
 Photodecomposition of water
 Photo-catalytic formation of fuel
 should increase charge
transfer processes at the
interface
 Photo-catalysis in pollution abatement
 Photo-catalysis route for chemicals
(G.Maghesh, B.Viswanathan, R.P.Viswanath
& T.K Varadarajan, PEPEEF, Research
Signpost (2007)pp.321-357.)
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 should improvements in the
efficiency
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Modifications and opportunites
What modifications?
•
THE AVAILABLE OPPORTUNITIES
various conceptual principles have been
• Identifying and designing new
incorporated into typical TiO2 system so
semiconductor materials with
as to make this system responsive to longer
wavelength radiations. These efforts can
considerable conversion efficiency
be classified as follows:
and stability
• Dye sensitization
• Surface modification of the semiconductor
to improve the stability
• Constructing multilayer systems or
• Multi
layer
systems
(coupled
using sensitizing dyes - increase
semiconductors)
absorption of solar radiation
• Doping of wide band gap semiconductors
like TiO2 by nitrogen, carbon
and
Sulphur
• Formulating multi-junction
• New semiconductors with metal 3d valence
systems or coupled systems band instead of Oxide 2p contribution
• Sensitization by doping.
optimize and utilize the possible
• All these attempts some kind sensitization
regions of solar radiation
and hence the route of charge transfer has
been extended and hence the efficiency
could not be increased considerably.
• Developing nanosize systems Success appears to be eluding the
efficiently dissociate water
17thresearchers.
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•
The opportunities
• The opportunities that are obviously available as
such now include the following:
-Identifying and designing new semi-conductor
materials with considerable conversion efficiency
and stability
– Constructing multilayer systems or using sensitizing
dyes so as to increase absorption of solar radiation.
– Formulating multi-junction systems or coupled systems
so as to optimize and utilize the possible regions of solar
radiation.
– Developing catalytic systems which can efficiently
dissociate water.
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Opportunities evolved
• Deposition techniques have been considerably
perfected and hence can be exploited in various other
applications like in thin film technology especially for
various devices and sensory applications.
• The knowledge of the defect chemistry has been
considerably improved and developed.
• Optical collectors, mirrors and all optical analysis
capability have increased which can be exploited in
many other future optical devices.
• The understanding of the electronic structure of
materials has been advanced and this has helped to
our background in materials chemistry.
• Many electrodes have been developed, which can be a
useful for all other kinds of electrochemical devices.
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Limited success – Why?
The main reasons for this limited success in all these directions are
due to:
• The electronic structure of the semiconductor controls the
reaction and engineering these electronic structures without
deterioration of the stability of the resulting system appears to
be a difficult proposition.
• The most obvious thermodynamic barriers to the reaction and
the thermodynamic balances that can be achieved in these
processes give little scope for remarkable improvements in the
efficiency of the systems as they have been conceived and
operated. Totally new formulations which can still satisfy the
existing thermodynamic barriers have to be devised.
• The charge transfer processes at the interface, even though a
well studied subject in electrochemistry has to be understood
more explicitly, in terms of interfacial energetics as well as
kinetics. Till such an explicit knowledge is available, designing
systems will have to be based on trial and error rather than
based on sound logical scientific reasoning.
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• Nanocrystalline (mainly oxides like TiO2, ZnO, SnO and
Nb2O5 or chalcogenides like CdSe) mesoscopic
semiconductor materials with high internal surface area
If a dye were to be adsorbed as a monolayer, enough can
be retained on a given area of the electrode so as to absorb
the entire incident light.
• Since the particle sizes involved are small, there is no
significant local electric field and hence the photoresponse is mainly contributed by the charge transfer with
the redox couple.
• Two factors essentially contribute to the photo-voltage
observed, namely, the contact between the nano
crystalline oxide and the back contact of these materials
as well as the Fermi level shift of the semiconductor as a
result of electron injection from the semiconductor.
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21
Another
aspect of the nano crystalline state is the alteration of the band gap
to larger values as compared to the bulk material which may facilitate both
the oxidation/reduction reactions that cannot normally proceed on bulk
semiconductors.
The response of a single crystal anatase can be compared with that of the
meso-porous TiO2 film sensitized by ruthenium complex (cis RuL2 (SCN)2,
where L is 2-2’bipyridyl-4-4’dicarboxlate).
The incident photon to current conversion efficiency (IPCE) is only 0.13%
at 530 nm ( the absorption maximum for the sensitizer) for the single crystal
electrode while in the nano crystalline state the value is 88% showing nearly
600-700 times higher value.
This increase is due to better light harvesting capacity of the dye sensitized
nano crystalline material but also due to mesoscpic film texture favouring
photo-generation and collection of charge carriers .
It is clear therefore that the nano crystalline state in combination with
suitable sensitization is one another alternative which is worth investigating.
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22
New Opportunities
1.
2.
3.
4.
5.
New semi-conducting materials with conversion efficiencies and stability
have been identified. These are not only simple oxides, sulphides but also
multi-component oxides based on perovskites and spinels.
Multilayer configurations have been proposed for absorption of
different wavelength regions. In these systems the control of the
thickness of each layer has been mainly focused on.
Sensitization by dyes and other anchored molecular species has been
suggested as an alternative to extend the wavelength region of
absorption.
The coupled systems, thus giving rise to multi-junctions is another
approach which is being pursued in recent times with some success
Activation of semiconductors by suitable catalysts for water
decomposition has always fascinated scientists and this has resulted in
various metal or metal oxide (catalysts) loaded semi conductors being
used as photo-anodes
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New opportunities (Contd)
• Recently a combinatorial electrochemical synthesis and
characterization route has been considered for developing
tungsten based mixed metal oxides and this has thrown open
yet another opportunity to quickly screen and evaluate the
performances of a variety of systems and to evolve suitable
composition-function relationships which can be used to
predict appropriate
compositions for the desired
manifestations of the functions.
• It has been shown that each of these concepts, though has its
own merits and innovations, has not yielded the desired
levels of efficiency. The main reason for this failure appears
to be that it is still not yet possible to modulate the electronic
structure of the semiconductor in the required directions as
well as control the electron transfer process in the desired
direction.
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24
Where are we?
LIMITED SUCCESS –
WHY?
• Difficulties on controlling the semiconductor electronic structure without
deterioration of the stability
• Little scope on the thermodynamic
barriers and the thermodynamic
balances for remarkable improvements
in the efficiency
• Incomplete understanding in the
interfacial energetic as well as in the
kinetics
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THE OPPORTUNITIES
EVOLVED
Deposition techniques -thin film technology,
for various devices
and sensory applications.
Knowledge of the defect chemistry has been
considerably improved and developed.
Optical collectors, mirrors and all optical
analysis capability have increased
Understanding of the electronic structure of
materials
Many electrodes have been developed- useful
for all other kinds of electro-chemical devices.
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Amount of Hydrogen (micro moles / 0.1g )
AMOUNT OF HYDROGEN EVOLVED BY CdS
PHOTOCATALYST
700
CdS - Y
CdS - Z
CdS - 
CdS - with HY
CdS (bulk)
600
500
400
300
200
100
0
0
1
2
3
4
5
6
Time (h)
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26
TEM IMAGE OF CdS NANOPARTICLES
Particle Size
(nm)
Surface
area
(m2/g)
Rate of hydrogen
production
(  moles /h)
CdS - Y
8.8
36
102
CdS - Z
6
46
68
CdS - 
11
26
67
CdS - Bulk
23
14
45
Catalyst
CdS- 
CdS-Z
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CdS-Z
100 nm
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100 nm
27
SCANNING ELECTRON MICROGRAPHS
CdS-Z
CdS-Y
CdS-
CdS- bulk
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28
Effect of Metal loading on SC
PHOTOCATALYSIS ON Pt/TiO2 INTERFACE
Effect of metals on hydrogen evolution rate
Pt
Pd
Rh
Au
Cu
Ag
Ni
Fe
Ru
3%
Electrons are transferred to metal surface
Reduction of H+ ions takes place at the metal
surface The holes move into the other side of
semiconductor The oxidation takes place at the
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semiconductor
surface
Pt, Pd & Rh show higher
activity. High reduction
potential. Hydrogen over
voltage is less for Pt, Pd & Rh
Activity of the catalyst is directly
proportional to work function of the
metal and M-H bond strength.
(Amount of hydrogen (micro moles/ 0.1g))
PHOTOCATALYTIC
HYDROGEN EVOLUTION
OVER METAL LOADED
CdS NANOPARTICLES
3500
H beta
Pt / CdS
Pd / CdS
Rh / CdS
CdS (Bulk)
Ru / CdS
3000
2500
2000
1500
1000
500
0
0
1
2
3
4
5
6
4
5
6
Time (h)
H-ZSM-5
Pt / CdS
Pd / CdS
Rh / CdS
CdS (Bulk)
Ru / CdS
3500
3000
2500
Amount of Hydrogen (micro moles / 0.1g )
Amount of Hydrogen (micro moles / 0.1g)
4000
2000
1500
1000
500
0
0
1
2
3
4
5
6
HY
Pt / CdS
Pd / CdS
CdS (Bulk)
Rh / CdS
Ru / CdS
2500
2000
1500
1000
500
0
0
1
2
3
Time (h)
Time (h)
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3000
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30
HYDROGEN PRODUCTION ACTIVITY OF METAL
LOADED CdS PREPARED FROM H-ZSM-5
Metal
Redox
potential
(E0)
Metal- hydrogen
bond energy
(K cal mol-1)
Work
function
(eV)
Hydrogen
evolution rate*
(µmol h-1 0.1g-1)
Pt
Pd
Rh
Ru
1.188
0.951
0.758
0.455
62.8
64.5
65.1
66.6
5.65
5.12
4.98
4.71
600
144
114
54
*1 wt% metal loaded on CdS-Z sample. The reaction data is presented
after 6 h under reaction condition.
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M. Sathish,
B. Viswanathan, R. P.NCCR
Viswanath Int. J. Hydrogen Energy ()
31
EFFECT OF SUPPORT ON THE CdS PHOTOCATLYTIC
ACTIVITY
2, 5,10 and 20 wt % CdS on support - by dry impregnation method
80
CdS (ZSM-5)/MgO
Rate of hydrogen production
-1
-1
(µmol h 0.1g )
75
CdS (ZSM-5)/Al2O3
70
Alumina & Magnesia
supports enhance
photocatalytic activity
CdS (ZSM-5)
65
Bulk CdS/MgO
60
55
Bulk CdS/Al2O3
MgO support has higher
photocatalytic activity favourable band position
50
Bulk CdS
45
40
0
2
4
6
8
10 12 14 16 18 20
22
CdS (Wt %)
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32
Pb2+/ ZnS
 Absorption at 530nm (calcinations at 623-673K)
 Formation of extra energy levels between the band gap by Pb
6s orbital
 Low activity at 873K is due to PbS formation on the surface
(Zinc blende to wurtzite)
Eg
(a) 573 K, (b) 623 K, (c) 673 K, (d) 773 K, and (e) 873K
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Band structure of ZnS doped with Pb.
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I. Tsuji, et al J. Photochem. Photobiol. A. Chem 622 (2003) 1
33
PREPARATION OF MESOPOROUS CdS NANOPARTICLE
BY ULTRASONIC MEDIATED PRECIPITATION
250 ml of 1 mM
Cd(NO3)2
Rate of addition
20 ml / h
Ultrasonic waves
 = 20 kHz
The resulting precipitate was
washed with distilled water
until the filtrate was free from
S2- ions
250 ml of 5 mM
Na2S solution
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34
N2 ADSORPTION - DESORPTION ISOTHERM
 The specific surface area and pore volume are 94 m2/g and
0.157 cm3/g respectively
 The adsorption - desorption isotherm – Type IV (mesoporous nature)
140
 The maximum pore
volume is contributed by
45 Å size pores
100
Relative volume (%)
3
 Mesopores are in the
range of 30 to 80 Å size
Volume (cm /g)
120
8
6
4
2
80
0
0
60
20
40
60
80
100
Pore range (A)
40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
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35
X- RAY DIFFRACTION PATTERN
 XRD pattern of as-prepared CdS -U shows the presence of cubic
phase
 The observed “d” values are 1.75, 2.04 and 3.32 Å corresponding
to the (3 1 1) (2 2 0) and (1 1 1) planes respectively - cubic
(111)
The particle size is calculated
using Debye Scherrer Equation
Intensity (a.u.)
 The peak broadening shows the
formation of nanoparticles
(220)
(311)
 The average particle size of asprepared CdS is 3.5 nm
20
30
40
50
60
70
2 theta
17th
2008and
M.March
Sathish
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R. P. Viswanath Mater.
Res. Bull (Communicated)
36
ELECTRON MICROGRAPHS
 The growth of fine spongy particles of CdS-U is observed on the
surface of the CdS-U
 The CdS-bulk surface is found with large outgrowth of CdS particles
 The fine mesoporous CdS particles are in the nanosize range
 The dispersed and agglomerated forms are clearly observed for
the as-prepared CdS-U
TEM
SEM
CdS-U
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CdS - Bulk
CdS-U
100 nm
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37
PHOTOCATALYTIC HYDROGEN PRODUCTION
Amount of hydrogen/M 0.1g
-1
10000
Na2S and Na2SO3 mixture
used as sacrificial agent
Amount of hydrogen (µM/0.1 g)
Metal
CdS-U
CdS-Z
CdS
bulk
Pt / CdS-U
Pd / CdS-U
Rh / CdS-U
CdS-U
8000
6000
4000
2000
0
0
-
73
68
45
Rh
320
114
102
Pd
726
144
109
Pt
1415
600
275
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1
2
3
4
5
6
Time (h)
1 wt % Metal loaded CdS – U
is 2-3 times more active than
the CdS-Z
NCCR
38
Where are we?
LIMITED SUCCESS –
WHY?
• Difficulties on controlling the semiconductor electronic structure without
deterioration of the stability
• Little scope on the thermodynamic
barriers and the thermodynamic
balances for remarkable improvements
in the efficiency
• Incomplete understanding in the
interfacial energetic as well as in the
kinetics
17th March 2008
THE OPPORTUNITIES
EVOLVED
Deposition techniques -thin film technology,
for various devices
and sensory applications.
Knowledge of the defect chemistry has been
considerably improved and developed.
Optical collectors, mirrors and all optical
analysis capability have increased
Understanding of the electronic structure of
materials
Many electrodes have been developed- useful
for all other kinds of electro-chemical devices.
NCCR
Thank you all for
your kind attention
17th March 2008
NCCR
17th March 2008
NCCR
17th March 2008
NCCR