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Hydrogen - Production Prospects and Challenges

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1 Hydrogen - Production Prospects and Challenges
National Centre for Catalysis Research (NCCR) Indian Institute of Technology, Madras 17th March 2008 NCCR

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. . 17th March 2008 NCCR

3 Energy density (kWh/kg)
Choice of fuel and oxidant Chemical & electro-chemical data on various fuels FUEL G0 kcal/mol E0theoretical (V) E0max Energy density (kWh/kg) Hydrogen (H2) -56.69 1.23 1.15 32.67 Methanol (CH3OH) 1.21 0.98 6.13 Ammonia(NH3) -80.80 1.17 0.62 5.52 Hydrazine(N2H4) 1.56 1.28 5.22 Formaldehyde(HCHO) 1.35 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) 1.06 0.58 - Propane(C3H8) 1.08 0.65 Oxidant ---- gaseous oxygen/air (In general, the oxygen needed by a fuel cell is supplied in the form of air) 17th March 2008 NCCR

4 Comparison of fuel properties
Hydrogen (H2) Methane (CH4) Gasoline (-CH2-) Lower heating value(kWhKg-1) 33.33 13.9 12.4 Self ignition temperature (°C) 585 540 Flame temperature (°C ) 2.045 1.875 2.200 Ignition limits in air ( Vol %) 4-75 5.3-15 Minimal Ignition energy (mWs) 0.02 0.29 0.24 Flame propagation in air (ms-1) 2.65 0.4 Diffusion coefficient in air (cm2s-1) 0.61 0.16 0.05 Toxicity No High L. Schlapbach et al Nature, 414 (2001) 353. 17th March 2008 NCCR

5 Transition to hydrogen economy
Production Storage Metal Hydride MOF Choice limited Distribution 17th March 2008 NCCR Petrol dispensing station

6 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 17th March 2008 NCCR

7 Hydrogen Production Technologies
Water Various ways of production of hydrogen Steam Reforming and partial oxidation Thermal Thermo-chemical (Fe-X2;S-I2) Electrolysis Electrochemical Photolysis Photochemical Photoelectrochemical(PEC PVC) Biological Bio-chemical, photobiological CO2emissions Technology awaited 17th March 2008 NCCR Principle of PEC

17th March 2008 NCCR

17th March 2008 NCCR

10 Material selection for PEC or Photocatalysis
Conventional 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 reaction assisted by photons catalyst Metals VB CB H+/H2 H2O/O2 Insulators SC E Metals: No band gap,Only reduction or oxidation, Depends on the band position Insulators: High band gap, High energy requirement 17th March 2008 NCCR 10

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 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. H2O/O2 17th March 2008 NCCR

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 band gap semiconductor reduced Various recombination process on the photo-excited semiconductor surface and inside the bulk. 17th March 2008 NCCR

13 Percentage ionic character
Selection Criterion 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 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 72.7 59.0 42.2 39.6 Ionic solids as the ionicity of the M-O bond increases, the top of the valence band (mainly contributed by the p- orbitals 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. 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 (%) = 17th March 2008 NCCR

14 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 % 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. 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. 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 17th March 2008 NCCR

15 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 photo-corrosion 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 suitable since these semiconductors do not have the necessary band gap value of 1.23 V. - the search for utilizing lower end of the visible region is not possible for direct water splitting reaction. If one were to use visible region of the spectrum, then only one of the photo-redox reactions in water splitting may be preferentially promoted and probably this accounts for the frequent observation that non-stiochiometric amounts of oxygen and hydrogen were evolved in the photo-assisted splitting of water There are some other aspects of photo-catalysts on which some remarks may be appropriate. Though they have been derived from the solid state point of view like flat band potential , band bending, Fermi level pinning, these parameters also can be understood in terms of the bond character and the redox chemical aspects by which the water splitting reaction is dealt. 17th March 2008 NCCR

16 Some Other Opportunities
TYPICAL PHOTOCATALYTIC PROCESS ENGINEERING THE SEMICONDUCTOR ELECTRONIC STRUCTURES without deterioration of the stability should increase charge transfer processes at the interface should improvements in the efficiency Photodecomposition of water Photo-catalytic formation of fuel 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 ) 17th March 2008 NCCR

17 Modifications and opportunites
What modifications? THE AVAILABLE OPPORTUNITIES Identifying and designing new semiconductor materials with considerable conversion efficiency and stability Constructing multilayer systems or using sensitizing dyes - increase absorption of solar radiation Formulating multi-junction systems or coupled systems - optimize and utilize the possible regions of solar radiation Developing nanosize systems - efficiently dissociate water various conceptual principles have been incorporated into typical TiO2 system so as to make this system responsive to longer wavelength radiations. These efforts can be classified as follows: Dye sensitization Surface modification of the semiconductor to improve the stability Multi layer systems (coupled semiconductors) Doping of wide band gap semiconductors like TiO2 by nitrogen, carbon and Sulphur New semiconductors with metal 3d valence band instead of Oxide 2p contribution Sensitization by doping. All these attempts some kind sensitization and hence the route of charge transfer has been extended and hence the efficiency could not be increased considerably. Success appears to be eluding the researchers. 17th March 2008 NCCR

18 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. 17th March 2008 NCCR 18

19 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. 17th March 2008 NCCR 19

20 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. 17th March 2008 NCCR 20

21 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 photo-response 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. 17th March 2008 NCCR 21

22 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 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. 17th March 2008 NCCR 22

23 New Opportunities 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 17th March 2008 NCCR 23

24 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. 17th March 2008 NCCR 24

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. Difficulties on controlling the semi-conductor 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 NCCR

17th March 2008 NCCR 26

Catalyst 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 CdS-Z 100 nm CdS-Z CdS-  100 nm 17th March 2008 NCCR 27

CdS-Z CdS-Y CdS- CdS- bulk 17th March 2008 NCCR 28

29 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 semiconductor surface Pt, Pd & Rh show higher activity. High reduction potential. Hydrogen over voltage is less for Pt, Pd & Rh 17th March 2008 NCCR

Activity of the catalyst is directly proportional to work function of the metal and M-H bond strength. 17th March 2008 NCCR 30

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. 17th March 2008 M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy () NCCR 31

2, 5,10 and 20 wt % CdS on support - by dry impregnation method Alumina & Magnesia supports enhance photocatalytic activity MgO support has higher photocatalytic activity - favourable band position 17th March 2008 NCCR 32

33 I. Tsuji, et al J. Photochem. Photobiol. A. Chem 622 (2003) 1
Pb2+/ ZnS Absorption at 530nm (calcinations at K) Formation of extra energy levels between the band gap by Pb s 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 Band structure of ZnS doped with Pb. 17th March 2008 NCCR I. Tsuji, et al J. Photochem. Photobiol. A. Chem 622 (2003) 1 33

BY ULTRASONIC MEDIATED PRECIPITATION 250 ml of 1 mM Cd(NO3)2 Rate of addition ml / h Ultrasonic waves  = 20 kHz 250 ml of 5 mM Na2S solution The resulting precipitate was washed with distilled water until the filtrate was free from S2- ions 17th March 2008 NCCR 34

The specific surface area and pore volume are 94 m2/g and cm3/g respectively The adsorption - desorption isotherm – Type IV (mesoporous nature) Mesopores are in the range of 30 to 80 Å size The maximum pore volume is contributed by Å size pores 17th March 2008 NCCR 35

36 M. Sathish and R. P. Viswanath Mater. Res. Bull (Communicated)
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 The peak broadening shows the formation of nanoparticles The particle size is calculated using Debye Scherrer Equation The average particle size of as- prepared CdS is 3.5 nm 17th March 2008 M. Sathish and R. P. Viswanath Mater. Res. Bull (Communicated) NCCR 36

37 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 100 nm CdS-U CdS - Bulk 17th March 2008 NCCR 37

38 1 wt % Metal loaded CdS – U is 2-3 times more active than the CdS-Z
PHOTOCATALYTIC HYDROGEN PRODUCTION Na2S and Na2SO3 mixture used as sacrificial agent Amount of hydrogen (µM/0.1 g) Metal CdS-U CdS-Z CdS bulk - Rh Pd Pt 73 320 726 1415 68 114 144 600 45 102 109 275 1 wt % Metal loaded CdS – U is 2-3 times more active than the CdS-Z 17th March 2008 NCCR 38

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. Difficulties on controlling the semi-conductor 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 NCCR

40 Thank you all for your kind attention
17th March 2008 NCCR

41 17th March 2008 NCCR

42 17th March 2008 NCCR

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