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Hydrogen energy in Indian context and R&D efforts at Banaras Hindu University a

a

a

P.R. Mishra , D. Pukazhselvan , M. Sterlin Leo Hudson , Sunil a

Kumar Pandey & O. N. Srivastava

a

a

Hydrogen Energy Centre, Department of Physics , Banaras Hindu University , Varanasi‐221005, India Published online: 17 Dec 2007.

To cite this article: P.R. Mishra , D. Pukazhselvan , M. Sterlin Leo Hudson , Sunil Kumar Pandey & O. N. Srivastava (2007) Hydrogen energy in Indian context and R&D efforts at Banaras Hindu University, International Journal of Environmental Studies, 64:6, 761-776, DOI: 10.1080/00207230701775581 To link to this article: http://dx.doi.org/10.1080/00207230701775581

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International Journal of Environmental Studies, Vol. 64, No. 6, December 2007, 761–776

Downloaded by [b-on: Biblioteca do conhecimento online UA] at 06:38 28 May 2015

Hydrogen energy in Indian context and R&D efforts at Banaras Hindu University P.R. MISHRA, D. PUKAZHSELVAN, M. STERLIN LEO HUDSON, SUNIL KUMAR PANDEY AND O. N. SRIVASTAVA* Hydrogen Energy Centre, Department of Physics, Banaras Hindu University, Varanasi-221005, India Taylor and Francis Ltd GENV_A_277557.sgm

(Received 30 October 2007) International 10.1080/00207230701775581 0020-7233 Original 0Taylor 00 Prof. 2007 [email protected] 00000 O& NSrivastava Article Francis (print)/1029-0400 Journal of Environmental (online) Studies

This paper describes Hydrogen energy in India and R&D efforts at Banaras Hindu University. All the three important ingredients i.e. production, storage and application of hydrogen have been dealt with. As regards hydrogen production, we have described and discussed the solar route consisting of photoelectrochemical electrolysis of water. Nanostructured TiO2 films have been synthesized through hydrolysis of Ti[OCH(CH3)2]4. This has been used as photoanode. Modular designs of TiO2 photoelectrode based PEC cells have been fabricated to get high rate of hydrogen production (~10.35 Lh1 -2 m ). Regarding storage which appears to be most crucial issue at present, we have discussed the intermetallic as well as complex hydride systems. For intermetallic we have dealt with materials tailoring of LaNi5 through Fe substituion. The La(Nil-xFex)5 (x=0.16) has been found to yield to high storage capacity of ~2.40wt%. We have also described and discussed the hydrogen storage in carbon nanofibres. Here storage capacity in excess of ~10wt% has been obtained. We have shown that CNT admixing in NaAlH4 helps to circumvent the low desorption rate of hydrogen in NaAlH4. For 8 mol % CNT admixing, we have found the desorption rate to increase from ~3.3 in more than 50 hrs to within 2 hrs. Relating to applications, we have focused on use of hydrogen (stored in intermetallic LaNi-Fe system) as fuel for IC engine based vehicular transport particularly 2 and 3-wheelers (and small car). The 2 and 3-wheeler have nearly the same performance as the petrol fueled vehicles. At present we have vehicle range of ~60–80 kms for 2-wheelers and ~60 kms for 3-wheelers (at top speed of ~50 kms/hr). Commercialization efforts on hydrogen fueled vehicular transport is being done by BHU:HEC with the help of Indian auto industries. Keywords: Nanostructured; TiO2; Hydrogen production rate; Modular PEC solar cells; Intermetallic hydrides; Complex hydrides; Hydrogen vehicles

1. Introduction Hydrogen energy is the only renewable energy which can provide both commercial energies, the electricity and the fuel for transport. Cold combustion of hydrogen in fuel cell leads to creation of electrical power and hot combustion in the internal combustion (IC) engines of motor vehicles and provides power in the same way as petroleum does. Both the cold and hot combustion processes lead to production of water. Hydrogen can be produced through a

*Corresponding author. O.N. Srivastava, PI & Co-ordinator, Hydrogen Energy Centre, Department of Physics, Banaras Hindu University, Varanasi-221005, INDIA. Email: [email protected] Phone & Fax: +91-542-2368468 & +91-542-2369889 International Journal of Environmental Studies ISSN 0020-7233 print: ISSN 1029-0400 online © 2007 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/00207230701775581

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variety of processes by dissociation of water. Thus, produced from water, hydrogen burns back to water. In contrast to petroleum fuels, hydrogen is clean (pollution free), renewable and environmental/climate friendly. It is indigenous and unlike petroleum which has to be imported, hydrogen can be produced within the country. Hydrogen is of paramount relevance for India. Some important reasons for this are outlined in the following:


a)

India has only 0.9% of world oil reserves (as against 5% for China; 15% for the USA and 59% for the Middle East). India will always be fuel starved if it depends on oil alone. Thus hydrogen, which can replace oil, will relieve us of this burden. b) We are currently importing about 130 MT of oil. Because of a continuing rise in the international price, we have to pay a huge amount in foreign exchange to exporting countries (mostly Middle East countries). The increase by merely one dollar of price in the international market leads to an additional burden on India worth Rupees 3000 crores. Thus when oil prices have changed from $32 to $65–70 from 2005 to 2007, we have to pay extra money worth about Rupees 1,140,000 crores. This is a very big burden on our economy. Hydrogen will relieve us of this burden. c) Urban air pollution is another reason why hydrogen is badly required for India. The air quality index for New Delhi has reached 85 to 90 (100 is most dangerous index). d) Global warming (climate change) has become possibly the most important reason for India to switch over to hydrogen. Global warming will affect the whole world, but India will be one of the countries which will suffer most. This is evident from the illuminating Stern Review Report (UNDP Expert Team, headed by Sir Nicholas Stern), which was released in October 2006 [1]. The Stern Report, among its other virtues, quantifies the effect of climate change economically. We will now explain how the hydrogen economy can be realized in India. For this, three main steps, that is, production, storage and application will have to be developed. The hydrogen production routes can be categorized under solar and non-solar routes. Here we will describe hydrogen production through the solar route employing the photoelectrochemical (PEC) process.

2. Photoelectrochemical electrolysis route of hydrogen production Hydrogen can be produced by employing solar energy through two prominent ways: a) photovoltaic (PV) driven electrolysis of water; b) photoelectrochemical (PEC) electrolysis of water. The former is a compound process using first PV electricity generation and then electrolysis. Even though it is a feasible process, efficiencies will be limited. The efficiencies of the single-crystalline silicon PV cells at maximum power point have been found to be ∼15.4% under testing conditions of 1000 W/m2 solar irradiation, 250°C ambient temperature and 156.25 cm2 cell area [2]. The latter method, that is, PEC electrolysis, is a single step process; the dissociation of H2O is done by electrons and holes produced in the semiconducting photoelectrode upon illumination with solar light. Fujishima and Honda [3] first reported in 1972 the experiment of water electrolysis using solar energy as the sole driving force for water decomposition. Breakthrough R&D efforts by Bockris et al. [4,5] and Gerisher [6,7] established the scientific foundations of photoelectrochemical hydrogen generation.

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According to Veziroglu [8,9], the method of photoelectrochemical water decomposition using solar energy is the most promising method for the generation of hydrogen. This view has been supported by a recent comprehensive review on hydrogen generation [10]. The urgent need to develop hydrogen technologies has resulted in much R&D activity on materials for solar hydrogen [11–15]. The PEC solar cells can be improved by use of novel materials to increase the conversion efficiency of solar energy into chemical energy [16–19]. R&D on solar hydrogen production through the photoelectrochemical (PEC) route employing a suitable semiconductor as the photoanode is of considerable interest.

3. R&D efforts at BHU on hydrogen production employing PEC electrolysis 3.1. Investigation and optimization of photoelectrode area employing nanostructured TiO2 based PEC solar cells for hydrogen production through photoelectrochemical process The developments concerning particularly nanostructured (e.g. TiO2) photoelectrodes have received much attention, because of the very high effective surface area and the incident photon-to-current conversion efficiency. One important aspect, which has not been widely investigated, concerns the determination of the effective photoelectrode area that will lead to high electrical and hydrogen production output. This would help in making modular PEC cell configurations for hydrogen production. It is necessary to estimate the optimum photoelectrode geometric area in order to investigate photoelectrochemical solar cells with modular configuration which might lead to a ‘Hydrogen Production Reactor’. We shall now look at these issues and determine the optimum photoelectrode area and then fabrication of the modular PEC solar cells for efficient electrical/hydrogen production. The nanostructured TiO2 (ns-TiO2) films have been prepared through sol-gel route employing Ti [OCH(CH3)2]4 and carrying out hydrolysis. The chemical process can be represented as: Hydrolysis

[

Ti OCH( CH 3 ) 2

]

4

→ TiO 2 (sol − gel)

80°C Details of the preparation and characterization of nanostructured TiO2 (ns-TiO2) and fabrication of PEC solar cells based on these electrodes have been described in detail in our published papers [20–23]. The following is a general description of the experiments and conclusions. i) The nanostructured TiO2 films were prepared through sol-gel technique followed by their deposition over Ti-conducting substrate. ii) Gross structural characterizations were carried out through XRD. Microstructural features including formation of nanocrystalline TiO2 film were monitored by employing transmission electron microscopy. iii) These studies revealed that the deposited TiO2 film corresponds to the anatase phase (tetragonal: a = 3.785 Å and c = 9.514 Å) with some peaks from the rutile phase (tetragonal: a = 4.593 Å and c = 2959 Å) and Ti substrate [22] and the microstructural feature revealed a fine-grained network of nanoparticles, suggesting the formation of nanocrystalline film

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with average grain size ∼2 nm. A selected area electron diffraction pattern confirmed the formation of nano-size anatase crystallites [22]. iv) Based on the studies on the optimization of the effective photoelectrode area it has been found that the optimum TiO2 photoelectrode area for adequate photoconversion efficiency and hydrogen production rate corresponds to ∼0.50 cm2 [22]. v) From PV-biased PEC cell experiments it is possible to have a PV-assisted photoelectrochemical water electrolysis which may be beneficial over dark PV powered water electrolysis, since only a small biasing potential is needed for water electrolysis. Further, such a water electrolysis system represents an all-solar powered device. 3.2. Modular designing of nanostructured TiO2 based PEC solar cells for hydrogen production To obtain improved efficiencies and hydrogen production rates, it is necessary to develop suitable photoelectrodes. These should be such that loss of photogenerated carriers is at its lowest. There is a need to design modular cell systems, which can lead not only to improved response and efficiency but can also reduce the cost and complexity of the photoelectrochemical cells systems. For this, we have taken three different configurations: i) a single cell; ii) parallel connected cells; and iii) a cell with the electrode area equivalent to the area of combined cells. For the parallel combination, two and four cells with larger and smaller area were combined. The photoconversion efficiencies of the cells (single cell, parallel combined cells and a cell with the electrode area equivalent to the area of combined cells) have been determined and compared for both the configurations [23]. 3.2.1. Physical arrangement of the PEC solar cells. The PEC solar cells were fabricated using nanostructured TiO2 photoelectrodes. The nanostructured TiO2 photoelectrodes so prepared were fixed over separate perspex mounts having a central hole of predefined area using a chemically inert epoxy resin. Perspex mounts were used in the rectangular PEC cell. The physical arrangement of the PEC cell is shown in figure 1. The projection in figure 1 shows the parallel connected photoelectrodes in the modular PEC cell configuration. Here alkaline water has been used as the electrolyte and as the source of hydrogen. It is in direct contact with nanostructured TiO2 photoelectrodes film (not flowing). To achieve PV assisted photoelectrochemical water electrolysis, the potential was applied through an external PV panel (with output ∼1.0–5.0V) between the working and reference electrode, keeping reference and counter electrodes shorted together. ‘Parallel connected’ means the electrodes are electrically connected in a parallel manner; not with respect to water flow, because the water is not flowing (as it in direct contact with the electrolyte, i.e. water). for Fig.H12 &(a) O2Parallel collection, connected 5 quartzphotoelectrodes window, 6 Electrolyte in the modular (alkaline PEC water), cell and 7 Perspex (b) Schematic cell. Projection representation shows the of aparallel photoelectrolysis connected cell photoelectrodes showing conventional in the modular three-electrode PEC cell configuration. controlled potential measurement system along with various components. CE: counter electrode terminal, RE: Reference electrode terminal, WE: working electrode terminal, 1 nanostructured TiO2-working electrode, 2 PtCE, fused in Pyrex glass, 3 saturated calomel electrode, 4 Inverted burettes

i) Photo-electrochemical characterization The variation of the photocurrent density (Jp) as a function of measured potential (Emeas) versus saturated calomel electrode (SCE) for the PEC solar cells with the electrode area, viz. 0.40 cm2 and its different combinations have been shown in figure 2. The photocurrent density for module cells has also been recorded. It was found that the photocurrent density for a PEC cell with photoelectrode area 1.85 cm2 is 1.50 mAcm−2 at ∼0.40 V vs SCE and on doubling the photoelectrode area (i.e. 3.70 cm2) its value decreases to ∼1.08 mAcm−2 [23]. The value of photocurrent density for the PEC cells with photoelectrode area 0.40 cm2 and 1.60 cm2 at −0.52 V vs SCE corresponds to 2.77 mAcm−2 and 1.75 mAcm−2, respectively. It



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Figure 1. (a) Parallel connected photoelectrodes in the modular PEC cell and (b) Schematic representation of a photoelectrolysis cell showing conventional three-electrode controlled potential measurement system along with various components. CE: counter electrode terminal, RE: Reference electrode terminal, WE: working electrode terminal, 1 nanostructured TiO2-working electrode, 2 PtCE, fused in Pyrex glass, 3 saturated calomel electrode, 4 Inverted burettes for H2 & O2 collection, 5 quartz window, 6 Electrolyte (alkaline water), 7 Perspex cell. Projection shows the parallel connected photoelectrodes in the modular PEC cell configuration.

was found that the decrease in the photocurrent density is only 3% for the modular cell with area (4 × 0.40 cm2) [23]. Upon illumination (energy of the incident photon being more than the band gap of TiO2), electrons and holes are produced in the conduction and valance band of the semiconductor. Here, in the case of parallel connected cells, overall photocurrents would be additives of individual cells (as they are electrically connected to each other through the Ti-substrate). It is expected that if the area of the photoelectrode is smaller, the defects and hence recombination centres for electrons and holes will be smaller leading to higher photocurrent; and vice versa for large area photoelectrodes. From these observations, it is evident that the photocurrent density decreases rapidly on increasing the photoelectrode area which is due to increase in the defect states originating mostly from grain boundaries/surface defects acting as the recombination centre. In contrast to this, the slight decrease in the photocurrent density in the case of modular cells may be related to the shadow effect at the edges of the finger mask, or to the decrease in photon density in the light beam used for the illumination on moving outwards from the centre of the photoelectrode. Thus, it would be beneficial to fabricate the modular PEC cell with smaller area electrodes connected in parallel. Fig. 2 I-V Characteristics under illumination for single (area=0.40 cm2), larger cell (area=1.60 cm2) and modular cell (area=4 × 0.40 cm2). (Intensity of illumination ∼ 85 mWcm−2).

ii) Photo-conversion efficiency Figure 3 shows the dependence of percent photoconversion efficiency on the applied potential Eapp for the individual photoelectrodes, along with the modules with the electrode having smaller area. The maximum photoconversion efficiency was found to be 2.52% (figure 3) for the PEC cell with photoelectrode area 0.40 cm2. However, the maximum photo conversion efficiency, for a PEC cell with a relatively small


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Figure 2. I-V Characteristics under illumination for single (area=0.40 cm2), larger cell (area=1.60 cm2) and modular cell (area = 4×0.40 cm2). (Intensity of illumination ∼ 85 mWcm−2).

photoelectrode area as well as for a modular cell made of smaller photoelectrodes, corresponds to a lower applied potential (viz., Eapp = 0.45 V) in comparison to the PEC cells and modular cell made of relatively large photoelectrodes (where h max corresponds to Eapp = 0.57 V) [23]. Fig. 3 Photo-coversion efficiency (η %) as a function of applied bias Eapp for single (area=0.40 cm2), larger cell (area=1.60 cm2) and modular cell(area=4×0.40 cm2) (Intensity of illumination ∼ 85 mWcm−2).

iii) Hydrogen production measurements The hydrogen production rate was determined for an applied bias Eapp corresponding to maximum in photoconversion efficiency (figure 2) for both type of cells. The rates of hydrogen production for a single cell with photoelectrode area 3.70 cm2 and for a module (2 × 1.85 cm2) have been found to be 4.15 lh–1m−2 and 5.31 lh–1m−2, respectively. Similarly, for the case of a PEC cell with photoanode area 1.6 cm2 and a modular cell (4×0.4 cm2), the measured values of hydrogen production rate correspond to 6.72 lh–1m−2 and 10.35 lh–1m−2, respectively. Thus, in the former case, the hydrogen production rate increases only by 27% on using a modular cell of the same effective area. On the other hand, for the module with individual electrodes of relatively smaller area (i.e. for 4×0.4 cm2 module) the hydrogen production rate increases by 54%. Therefore, the hydrogen production rate can be improved by employing modular PEC cells in the form of parallel-connected photoelectrodes of smaller area. The variations of different PEC parameters for the fabricated modular PEC solar cells are given in table 1.



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Figure 3. Photo-coversion efficiency (η %) as a function of applied bias Eapp for single (area=0.40 cm2), larger cell (area=1.60 cm2) and modular cell(area=4×0.40 cm2) (Intensity of illumination ∼ 85 mWcm−2).

4. Need for hydrogen storage Storage is a key issue for the hydrogen economy. It cuts across the production, distribution, safety and applications aspects [24]. Hydrogen can be stored in high compression cylinders, cryogenic tanks, solid hydrogen absorbing materials and liquid hydrides, etc. One gram of hydrogen gas occupies ∼11 litres (2.9 gallons) of space at atmospheric pressure, so storage implies a need to reduce the enormous volume occupied by hydrogen. All these storage options have disadvantages, but still, the most promising appears to be hydrides. A viable means of hydrogen storage excludes inefficient high pressure cylinders, expensive cryogenic

Table 1. S. No. 1.

2.

Comparative PEC parameters, e.g. photocurrent density and photo-conversion efficiency of the modular. Type of the cell Single cell

Modular cell

Area of the electrodes (cm2)

Photocurrent density (mAcm−2)

Photoconversion efficiency (%)

Hydrogen production rate (L/hm2)

0.40 1.60 1.85 3.70 4×0.40=1.60 2×1.85=3.70

2.77 1.75 1.50 1.08 2.69 1.38

2.52 1.63 1.17 0.85 2.45 1.07

6.72 4.15 10.35 5.31


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cylinders and all covalent hydrocarbon compounds (liquid hydrides, releasing hydrogen if being heated above 800°C). The solution to these obstacles appears to be storage of hydrogen in the form of hydrides. A hydrogen storage alloy is capable of absorbing and releasing hydrogen without compromising its own structure. Hydrogen storage in metal hydrides is considered a potential storage method and has attracted considerable interest. Hydrogen storing materials can be classifieds as: 1) traditional intermetallic or interstitial hydrides (alloys of transition elements and/or rare earth elements); 2) large surface area materials (nano-tubes and fibres, zeolites, aerogels, etc.); 3) lightweight complex hydrides (alanates, borates, amides and imides); and 4) metal organic frameworks and glass microspheres, etc. [25]. 4.1. Intermetallic hydrides The solid state (hydrides) storage materials holds considerable promise. The BHU group has widely investigated hydrides of intermetallic compounds as typified by AB, AB5, AB2 and A2B in the last two decades. ‘A’ corresponds to the binary hydride forming transition element and ‘B’ is arbitrarily any transition element. Some key materials which have been investigated by BHU group in the last 10 years are given in table 2. One attractive quality of the intermetallic hydrides is that they nearly always meet the volumetric storage efficiency of ∼60 Kg/m3. Therefore for vehicular applications where volumetric storage efficiency is very important, the intermetallic hydrides are very attractive. However, decades of extensive research works on traditional intermetallic hydrides have led to only a moderate increase (highest storage capacity) in gravimetric storage efficiency. One option to improve hydrogen storage capacity is the substitution with an element which has higher electron attractive power [26]. Thus, example, with AB5 type intermetallic (e.g. LaNi5) one such feasible substitution would correspond to partial replacement of Ni by Fe or Co or both. This is evident by their electronic configurations as in the following: Ni ...........3d 8 4s 2 Co ...........3d 7 4s 2 Fe ...........3d 6 4s 2 Hydrogen absorption would increase if the substituting atom has the capacity to attract this electron (H atom). Thus, more H atoms can be incorporated (absorbed) and the storage capacity will increase. Since both Fe and Co have more vacancies in the 3d shell than the parent Ni atom, their substitution is likely to increase the hydrogen storage capacity. Another element which has higher electron attractive power than Ni (….3d64s2) is V (….3d34s2). Similar considerations may apply for the other metallic ingredient, that is, La in LaNi5. The La corresponds to 5d16s2 and Ce to 4f15d16s2. Thus partial substitution of Ce for La may enhance the Table 2. AB = AB5 =

Key materials investigated by BHU group in the last 10 years

TiFe, TiFe0.9 Mn0.1, TiFe0.8 Ni0.2,

LaNi5, MmNi5, MmNi4.15Al0.85, MmNi4.5Al0.5, MmNi4.6Fe0.4, La(Ni1-x-yFexSiy)5, MmNi4.3Al0.3Mn0.4, La0.2Mm0.8Ni3.7Al0.48Co0.3Mn0.5Mo0.02, La0.2Mm0.75Ti0.05Ni3.7Al0.48Co0.3Mn0.5Mo0.02 AB2 = ZrFe2, TiCr2, TiV0.6Fe0.15Mn1.25, FeTi1-xMmx, Zr(Fe1-xCrx)2 0 ≤ x ≤ 0.4, Zr1−2xMnxTixFe1.4Cr0.6(x = 0.0, 0.1, 0.2, 0.05) A2B = Mg2Ni, Mg2Fe, Mg2Cu Composites: La2Mg17, La2Mg17–x wt% LaNi5, Mg–x%FeTi, Mg–x% CFMNi5, LaNi5/La2Ni7, Mg-x wt% MmNi4.6Fe0.4 etc.



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storage capacity. (It should be pointed out that the concentrations of the substituting atoms have to be found out through a series of substitutions for determining the optimum hydrogen storage material which possesses the highest reversible hydrogen storage capacity). One example of our efforts in this direction is on AB5 type intermetallic LaNi5. We carried out material tailoring through substitution on the Ni site by the most feasible elements Co and Fe. Although substitutions by Co and Fe were investigated, optimum results were obtained only through Fe. The higher electron attractive power of Fe (3d6 as against 3d8 of Ni) is expected to lead to the possibility of putting higher number of hydrogen atoms in the unit cell, thus resulting in higher storage capacity. The size of the Fe atom (1.72 Å) is higher than that of Ni (1.67 Å) by about 2.99%. This may lead to the larger size of interstitial voids. Thus there may be higher number of interstitial voids occupied by hydrogen for the Fe substituted version as compared to Ni alone. It has been found that all the phases of La(Ni1-xFex)5 exhibit better hydrogen storage characteristics than the parent material LaNi5. Thus storage capacity of La(Ni1-xFex)5 for × = 0, 0.05, 0.10, 0.16, 0.25, 0.30 are ∼1.50, 1.93, 1.95, 2.40, 1.78 and 1.60 wt%, respectively. The material La (Ni0.84 Fe0.16)5 exhibits the highest storage capacity ∼2.40 wt%. Such improvements still do not take us to the required storage capacity of ∼3 wt% (WE-NET, Japan limit) or 6 wt% (US DOE limit). Material tailoring of intermetallic hydrides is being pursued so as to increase storage capacity further. Considerable attention has also been paid to study: a) MgH2 and Mg based composite materials; b) carbon nanosystems; and c) built in lightweight hydrides such as NaAlH4 (storage capacity (S.C) ∼5.5wt%), Mg(AlH4)2 (S.C. ∼69 wt%) etc. 4.2. Lightweight hydrogen storage materials and composites Recently, attention has turned to the hydrides of light metals such as Li, B, C, N, Na, Mg and Al, etc. [27]. One attractive candidate which is under focus is MgH2 [28]. The prime reason for interest in Mg based storage materials is that Mg is a light element. Further, it is comparatively less expensive. Most important, the storage capacity of the Mg binary hydride itself is ∼7.6 wt%. But, Mg is not a practical storage material because hydriding and dehydriding kinetics of Mg are very slow at ambient conditions. In order to enhance the hydrogen storage characteristics, new Mg bearing composites have been developed [29]. Some elucidative examples of the Mg bearing composites which have been extensively investigated are: ● ●

Mg-xwt% LaNi5 type storage materials – carbon nanomaterials; Mg-xwt% Mg2Ni1-xCox (or Fex) type storage materials – carbon nanomaterials.

These Mg bearing viable composite materials may be the ideal hydrogen storage systems for vehicular transport since these are lightweight materials and also their desorption temperature of ∼50 to ∼100°C can be easily available through engine exhaust. 4.3. Carbon nanomaterials Another interesting system for hydrogen absorption is porous carbon structure. There are significant breakthroughs in synthesizing microporous and ultra-microporous carbonaceous materials with very high hydrogen adsorption properties[30–35]. Carbon nanotubes (CNT) and graphitic nanofibres (GNF) have also been considered as interesting candidates for reversible hydrogen storage [30,31]. But, considerable confusion exists about hydrogen storage after the early rather dramatic results [32] in regard to very high hydrogen storage capacities (up to ∼67 wt%) for GNF. Several investigators have attempted to find the reason for


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such high hydrogen storage capacities. Although it has not been possible to identify the exact reason for the high hydrogen storage capacities exhibited by GNF, some general features have become discernible. It has been suggested that mobility of the hydrogen may get suppressed and hydrogen molecules may become agglomerated in a liquid-like configuration. We have, at BHU, carried out investigations on the hydrogenation behaviour of carbon (graphitic) nanofibres [33] and are trying to establish the reproducible capacity. The formation of graphitic nanofibres is achieved through catalyst assisted thermal cracking of hydrocarbon. The catalyst employed is often nickel, copper powder or a mixture of the two (e.g. 98 wt% Ni and 2 wt% Cu). The hydrocarbons employed are acetylene (C2H2), ethylene (C2H4) and benzene (C6H6) [34]. The yield obtained through the catalyst assisted cracking process is rather poor and also the resulting GNFs are randomly oriented. In order to improve upon this, we employed Fe, Co, Ni, Mo, Pd catalysts in the form of films and sheets [35]. It was found that Pd sheets give optimum results in regard to the yield and orientation of the as grown GNFs. This is in contrast to the earlier results on growth of GNFs employing Cu and Ni powders as catalyst where the resulting GNFS are randomly oriented. Seeking analogy from the formation of graphitic nanotubes (GNT) where oriented GNTs grow when the catalyst particles are patterned, it may be inferred that with a catalyst in the form of a sheet with juxtaposed grains, oriented GNFs will result. The GNFs grown by us have shown storage capacity in excess of 10 wt% and up to 17 wt%. 4.4. Complex hydrides Complex hydrides are known as built-on hydrides. The hydrogen in the complexes, unlike intermetallics hydrides, is tightly bonded with the parent material by strong covalent and/or ionic bonding at the time of synthesizing itself. The number of hydrogen atoms per metal atom (H/M ratio) is two in many cases [36]. These complex hydrides show high gravimetric storage capacity at room temperature (e.g. LiBH4-18 wt%). But, the low kinetics of the hydrogen releasing process even at very high temperature is a major problem for the practical use of these hydrides. Various studies on the alkali- alkaline earth metal aluminium hydrides and borohydrides have been carried out [37,38]. Among all complex based hydrides, sodium alanate has received considerable attention due to its high hydrogen capacity (7.44 wt %) and favourable thermodynamics for reversible hydrogen storage [39]. The two step reactions together (3NaAlH4 → Na3AlH6 + 2Al + 3H2 and Na3AlH6 → 3NaH + Al + 3/2H2) liberates 5.55 wt % which is very close to the US DOE limit of ∼6 wt % gravimetric hydrogen storage capacity. The decomposition reaction temperature looks still higher and the reaction is kinetically very slow (>50 hours for first step reaction at ∼150°C and 30 hours for second step reaction at >200°C). Besides, the products of decomposition do not combine with hydrogen to form the alanate phase again. In 1997, Bogdanovic [40] and coauthors demonstrated that sodium alanate is a viable means of reversible hydrogen storage system by deploying transition metal catalysts (Ti, Zr, etc.). This has triggered interest in NaA1H4 as a reversible hydrogen storage system and there has been a great deal of effort to find better catalysts like Ti for sodium alanate. Better alternative catalysts may exist that can improve the dehydrogenation kinetics and long-term reversibility in low ambient conditions. While transition elements (mainly Ti, Zr and Fe) have been proposed as promising catalysts by several workers, we have investigated the feasibility of use of NaAlH4 by new alternative catalysts [41,42]. We have introduced a new lightweight nano-solid catalyst with high surface area, carbon nanotubes (CNT). The main advantage in using CNT as a catalyst is the high outer surface area of CNT (1 gm of material has ∼1315 square m). In addition to the



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nano-size, its high aspect ratio (diameter ∼10 to ∼30 nm and length ∼1 to ∼10 µm) can provide additional surface/interface in the host material where they are admixed. The CNTs are also known to possess significant catalytic activity via π and σ bonds, particularly the latter associated with carbon in graphitic sheets. Carbon nanotube (MWCNT) was synthesized in our lab through thermal spray technique. Suitable amounts of CNT in 2,4,6,8 and 12 mol % was taken with NaAlH4 and this mixture was mechanically admixed for the period of 5 minutes at milling speed of ∼5500 rpm (vial volume ∼40 cm3) under argon atmosphere. Out of the various materials corresponding to NaAlH4–x mol% CNT (x = 2, 4, 6, 8 and 12), we have found that the material with × = 8 mol% is the optimum material. It shows the highest desorption rate, leading to ∼3.3 wt % of H2 at ∼160°C within 2 h. The CNT admixed NaAlH4 has also been found to exhibit good rehydrogenation characteristics. CNT catalyst is found to be better than other carbon based catalysts such as graphite and activated carbon. We have continued our studies for finding new catalysts (other than CNT). We have observed very interesting results when Mischmetal (Mm: Ce-42 at %, La-31 at %, Nd-18 at %, Pr-9 at %) is used as new catalyst [40]. Apart from sodium alanate, other alanates such as lithium alanate and magnesium alanate are attractive candidates because of their high gravimetric hydrogen capacity. The unfavourable thermodynamics of these hydrides for reversible hydrogen storage restricts their use. However, we have observed new interesting results on mixed or composite type alanates [38]. These systems need extensive R&D works to achieve a matured hydrogen storage technology. We are continuing our R&D efforts in this direction. 5. Applications 5.1. Hydrogen fuelled vehicular transport After production and storage, the next step towards the hydrogen economy is the application. The main focus for the application aspect at the BHU is vehicular transport. This is because out of the two commercial energies required – electricity and oil (for motive power) – for India, the latter poses a bigger challenge. Whereas coal powered super thermal, nuclear and hydroelectric generation are expected to take care of electrical requirements, the same is not true for oil. India requires at present about ∼130 MT of oil of which we have to import 110 MT. With urban air pollution, climate change (CO2 emission), available land area (inadequacy of use of biofuels and food versus fuel crisis through use of biofuels like biodiesel), the economic costs of oil dependence (including price rise risks), hydrogen as a fuel is a clear winner. Thus, use of hydrogen as road transport fuel is of utmost importance. The Hydrogen Energy Centre (HEC), BHU, in regard to the application question, has focused on R&D and demonstration of hydrogen fuelled road transport, particularly twowheelers, three-wheelers (and small cars). It is difficult to make an internal combustion engine run on hydrogen fuel, because of significantly different properties of hydrogen as compared to petroleum, particularly the density (density 0.0892 g/l, 7% density of air) and the self-ignition energy (0.02 mJ as compared to petroleum for which it is 0.29 mJ), among other things. One key to increasing the power of a hydrogen fuelled IC engine is to increase the compression ratio. The self-ignition temperature of H2 is ∼630°C as compared to petrol where it is 230°C. Thus higher compression ratios leading to higher thermal efficiency can be achieved with hydrogen as a fuel. This aspect is being studied at BHU. We have found

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that compression ratio for our hydrogen fuelled two- and three-wheelers can be increased from ∼8 to ∼11.


5.2. Examples of hydrogen fuelled vehicular transport developed at BHU: two-, three- and four-wheelers The hydride powder is filled in the heat exchanger system which is coupled to IC engine exhaust gas (which is mostly steam in the case of hydrogen). The hydride of choice has been MmNi4.6Fe0.4. Some trials have also been done using Ti admixed NaAlH4. The total quantity of hydride employed is ∼18 kg. The exhaust gas coming out at temperatures of ∼60°C in the case of two-wheelers is circulated in the hydride heat exchanger bed. Since thermal conductivity of the hydride is very poor (0.5 to 1.0 W/mK), a hydride heat exchanger tank (HHET) has been designed. The HHET is located below the driver’s seat. To heat the hydride effectively by the exhaust heat of the engine, 18 kg of hydride (storage capacity ∼1.8 wt%) is distributed in the HHET by 18 aluminium tubes of 1 inch diameter and 12 inch length. The exhaust heat from the 100CC two-wheeler engine is able to raise the temperature of hydride to ∼60°C. This results in continuous emission of hydrogen which is sent to the engine. An important point is the site and timing of hydrogen injection in the engine. We have found that for knock-free operation hydrogen injection near the engine inlet valve is best. Hydrogen is introduced during the suction stroke at 25°C before the top dead centre. The hydrogen entry on and off time is controlled through a cam located in the engine head. Most of our work on development of the hydrogen fuelled IC engine is on two-wheelers. These have achieved ∼60 to ∼80 km range in a single charge. With higher storage capacity hydride, this range can be increased. We have recently developed Mm (La rich, i.e. La > 35%) -Ni-Fe hydride with a storage capacity of ∼ 2.4 wt%. We are now in the process of using this hydride. The range is expected to become ∼80 to ∼100 km. The work is being extended to three-wheelers and small cars. Previously, we mounted the hydride tank at the side of the vehicle; this is the position of the silencer for conventional two-wheelers. For the hydrogen/hydride fuelled two-wheeler, the hydride heat exchanger tank is so designed that it works as silencer. In India, two- and three-wheelers are the poor man’s vehicles for intra-city road transport. These form about ∼70% of the total vehicle population in India. These are also the most polluting vehicles; for example, the three-wheeler when used as a motor rickshaw. Whereas two-wheelers are used for personalized transport, three-wheelers are used for passenger transport. The HEC at Banaras Hindu University seeks to persuade industry to produce hydrogen fuelled two- and three-wheelers. Thus, we have converted a petrol-driven threewheeler manufactured and provided by International Cars and Motors Limited (ICML), Jallandhar (Punjab) to run on hydrogen stored in Mm-Ni-Fe hydride. A 40 kg hydride tank which we have developed was interfaced with the 1.75 HP internal combustion engine exhaust of the three-wheeler. The hydrogen was injected through a timed manifold injection. The average distance travelled by a three-wheeler vehicle per day is ∼30 km. The average of the three-wheeler is about 60 km at a top speed of ∼50 kmph. The ICML engineers have been trained to convert these wheelers to run on hydrogen. The ICML is in the process of manufacturing 10 three-wheelers by the end of 2007. These will run between the Central Secretariat and Lodhi Road, New Delhi. This naturally makes hydrogen motorized transport obvious to all and sundry at the heart of the nation’s capital. This will be followed by production of 100 hydrogen three-wheelers in 2008. Similar efforts are being made for two-wheelers with the help of the Society for Indian Auto Manufacturers (SIAM), which has access to various


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two-wheeler manufacturers in India. Both these efforts to introduce hydrogen fuelled small vehicles in India are being made in collaboration with the Ministry of New and Renewable Energy (MNRE), Government of India.

6. Conclusions This paper has surveyed the production, storage and application of hydrogen energy, especially the R&D and hydrogen fuelled device demonstration activities at the Hydrogen Energy Centre, Banaras Hindu University. We have developed hydrogen fuelled two-wheelers. This work is being extended to three-wheelers and small cars. India’s use of hydrogen as a fuel needs more investment, more R&D and more general acceptance as an idea whose time has come. Hydrogen/Hydride Prof. Test run T. N. of Veziroglu, Hydrogen Fueled fueled President, 2-wheeler car in International presence developed ofAssociation Dr. at BHU. R. Chidambaram, Notice of Hydrogen the schematic Principal Energydiagram Scientific having awhich ride Advisor, on shows the Govt. hydrogen howofhydrogen India fueled andistwo Prof. liberated wheeler Panjab from developed Singh, hydride the at Vice-Chancellor heat Hydrogen exchanger Energy tank of BHU Centre, and fed held BHU toon the22.7.2005. (2005). IC engine. The scheme through which hydrogen is converted to steam (water) on combustion in the engine has been indicated.

Acknowledgements The authors would like to thank Prof. A.R. Verma, Prof. C.N.R. Rao, Prof. R. Chidambaram, Prof. S.K. Joshi, Prof. S. P. Thyagrajan and Prof. Panjab Singh for their encouragement and support. Financial support from the Ministry of New and Renewable Energy Sources and the University Grants Commission is thankfully acknowledged. References [1] Stern Review, 2006, The Economics of Climate Change. Available online at: www.sternreview.org.uk, accessed 30 October 2006. [2] Jie, Ji, Pei Gang, Chow Tin-tai, Liu Keliang, He Hanfeng, Lu Jianping and Han Chongwei, 2007, Experimental study of photovoltaic solar assisted heat pump system. Solar Energy, doi:10.1016/j.solener.2007.04.006.



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