international journal of hydrogen energy 34 (2009) 7358–7367
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen energy in changing environmental scenario: Indian context M. Sterlin Leo Hudson, P.K. Dubey, D. Pukazhselvan, Sunil Kumar Pandey, Rajesh Kumar Singh, Himanshu Raghubanshi, Rohit. R. Shahi, O.N. Srivastava* Hydrogen Energy Center, Department of Physics, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India
article info
abstract
Article history:
This paper deals with how the Hydrogen Energy may play a crucial role in taking care of the
Received 30 July 2008
environmental scenario/climate change. The R&D efforts, at the Hydrogen Energy Center,
Received in revised form
Banaras Hindu University have been described and discussed to elucidate that hydrogen is
25 May 2009
the best option for taking care of the environmental/climate changes. All three important
Accepted 25 May 2009
ingredients for hydrogen economy, i.e., production, storage and application of hydrogen
Available online 25 June 2009
have been dealt with. As regards hydrogen production, solar routes consisting of photoelectrochemical electrolysis of water have been described and discussed. Nanostructured
Keywords:
TiO2 films used as photoanodes have been synthesized through hydrolysis of
Climate change
Ti[OCH(CH3)2]4. Modular designs of TiO2 photoelectrode-based PEC cells have been fabri-
Nanostructured TiO2
cated to get high hydrogen production rate (w10.35 lh1 m2). However, hydrogen storage
Hydrogen production rate
is a key issue in the success and realization of hydrogen technology and economy. Metal
Modular PEC solar cells
hydrides are the promising candidates due to their safety advantage with high volume
Intermetallic hydrides
efficient storage capacity for on-board applications. As regards storage, we have discussed
Complex hydrides
the storage of hydrogen in intermetallics as well as lightweight complex hydride systems.
Hydrogen fueled vehicles
For intermetallic systems, we have dealt with material tailoring of LaNi5 through Fe substitution. The La(Nil xFex)5 (x ¼ 0.16) has been found to yield a high storage capacity of w2.40 wt%. We have also discussed how CNT admixing helps to improve the hydrogen desorption rate of NaAlH4. CNT (8 mol%) admixed NaAlH4 is found to be optimum for faster desorption (w3.3 wt% H2 within 2 h). From an applications point of view, we have focused on the use of hydrogen (stored in intermetallic La–Ni–Fe system) as fuel for Internal Combustion (IC) engine-based vehicular transport, particularly two and three-wheelers. It is shown that hydrogen used as a fuel is the most effective alternative fuel for circumventing climate change. ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The emission of green house gases, particularly, CO2 from industries and automobile exhaust makes serious impact on the climate [1–4]. Unlike petroleum fuels, hydrogen is clean
(pollution free), renewable and environmentally friendly. The Hydrogen energy is the only renewable energy which can provide clean commercial energies, the electricity and the fuel for transport. Cold combustion of hydrogen in fuel cell leads to the creation of electrical power and hot combustion in
* Corresponding author. Tel.: þ91 542 2368468; fax: þ91 542 2368889. E-mail address:
[email protected] (O.N. Srivastava). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.05.107
international journal of hydrogen energy 34 (2009) 7358–7367
internal combustion (IC) engines of motor vehicles provides power in the same way as fossil fuel does. Both the cold and hot combustion processes lead to production of water. Hydrogen can be produced through a variety of processes by dissociation of water. Thus, produced from water, hydrogen burns back to water. It is indigenous and unlike petroleum which has to be imported, hydrogen can be produced within the country. The main advantage of hydrogen as a fuel is the absence of CO2 emissions [2]. Hydrogen is considered as a renewable and sustainable solution for reducing global fossil fuel consumption and combating global warming [3]. Hydrogen is of paramount relevance for countries like 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 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 from this burden. (b) India is currently importing about 122 MT of oil per year. Hence, we have to pay a huge amount in foreign exchange to oil exporting countries (mostly the Middle East countries). The increase by merely 1US$ of price in the international market leads to an additional burden on India worth Rupees 3000 crores. This is a very big burden on India’s economy. Hydrogen will relieve this burden. (c) Environmental/climate change has become possibly the most important reason 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 Report, which was released in October 2006 [1]. The Stern Report, among its other virtues, quantifies the effect of climate change economically. It appears that India is already losing about 1% of GNP due to climate change. As per IPCC Report (C/o Dr. R.K. Pachauri) published in 2007 [4], in India change in temperature is observed to be 0.68 C increase per century. Several catastrophic changes are already occurring in India. Thus, for example as the report points out because of climate change effects, 944 mm of rainfall occurred in Mumbai (India) on 26–27 July 2005, killing 1000 people and loss of property worth US$250 million. It thus appears that even if oil may still be present, it may not be used as a fuel in view of the climate change, crucial calamities which it will lead to. We will now discuss how the hydrogen economy can be realized. For this, three main steps, i.e., production, storage and application will have to be accomplished. In the following, we will proceed to describe and discuss these with special reference to R&D efforts at the Hydrogen Energy Center (HEC), Banaras Hindu University (BHU), Varanasi. A detail of such R&D has also been given in one of our earlier publication [5]. However, the results reported here are different.
2. Photoelectrochemical electrolysis route of hydrogen production The hydrogen production routes can be categorized under solar and non-solar routes. Hydrogen can be produced by employing
7359
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 w15.4% under testing conditions of 1000 W/m2 solar irradiation, ambient temperature and 156.25 cm2 cell area [6]. The latter method, 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 [7] 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. [8,9] and Gerischer [10,11] established the scientific foundations of photoelectrochemical hydrogen generation. According to Veziroglu [12,13], 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 [14]. The urgent need to develop hydrogen technologies has resulted in much R&D activity on materials for solar hydrogen [15–19]. The PEC solar cells can be improved by the use of novel materials to increase the conversion efficiency of solar energy into chemical energy [20–25]. R&D on solar hydrogen production through the photoelectrochemical (PEC) route employing a suitable semiconductor as the photoanode is of considerable interest.
3. Hydrogen production at HEC-BHU, 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 their very high effective surface area and high incident photon-to-current conversion efficiency [21]. 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 a sol–gel route employing Ti[OCH(CH3)2]4 and carrying out hydrolysis. The chemical process can be represented as: Hydrolysis TiO2 ðsol-gelÞ Ti OCHðCH3 Þ2 4 / 80 C
Details of the preparation and characterization of nanostructured TiO2 and fabrications of PEC solar cells based on these electrodes have been described in detail in our earlier papers [25–28].
7360
international journal of hydrogen energy 34 (2009) 7358–7367
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 Ticonducting substrate [26]. (ii) The structural characterization revealed that the deposited TiO2 film corresponds to the anatase phase (tetrag˚ and c ¼ 9.514 A ˚ ) with some peaks from onal: a ¼ 3.785 A ˚ and c ¼ 2.959 A ˚) the rutile phase (tetragonal: a ¼ 4.593 A and Ti substrate and the microstructural feature revealed a fine-grained network of nanoparticles, suggesting the formation of nanocrystalline film with average grain size w2 nm. The selected area electron diffraction pattern confirmed the formation of nano-sized anatase crystallites [27]. (iii) 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 w0.50 cm2 [27]. (iv) 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. 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. Keeping these aspects in view, 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 [28].
3.2.1.
reference and counter electrodes shorted together. The electrodes are electrically connected in a parallel manner; not with respect to water flow, because the water is not flowing.
3.2.1.1. 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 measured. 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 mAcm2 at w0.40 V versus SCE and on doubling the photoelectrode area (i.e. 3.70 cm2) its value decreases to w1.08 mAcm2 [28]. The value of photocurrent density for the PEC cells with photoelectrode area 0.40 cm2 and 1.60 cm2 at 0.52 V versus SCE corresponds to 2.77 mAcm2 and 1.75 mAcm2, respectively. It was found that the decrease in the photocurrent density is only 3% for the modular cell with area (4 0.40 cm2) [28]. 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 centers 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 center. 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 light beam used for the illumination on moving outwards from the center of the photoelectrode. Thus, it would be beneficial to fabricate the modular PEC cell with smaller area electrodes connected in parallel. 3.2.1.2. Photoconversion efficiency. Fig. 1 shows the schematic diagram of parallel-connected photoelectrodes in the modular
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. Alkaline water has been used as 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 from w1.0 to 5.0 V) between the working and reference electrode, keeping
Fig. 1 – Schematic representation of parallel-connected photoelectrodes in modular PEC cell.
7361
international journal of hydrogen energy 34 (2009) 7358–7367
PEC cell. The maximum photoconversion efficiency was found to be 2.52% for the PEC cell with photoelectrode area 0.40 cm2. However, the maximum photoconversion efficiency, for a PEC cell with a relatively small photoelectrode area as well as for a modular cell made of smaller photoelectrodes, corresponds to a lower applied potential (viz., Eapp ¼ 0.45 V) in comparison to the PEC cells and modular cell made of relatively large photoelectrodes (where h max corresponds to Eapp ¼ 0.57 V) [28].
3.2.1.3. Hydrogen production measurements. The hydrogen production rate was determined for an applied bias Eapp corresponding to maximum in photoconversion efficiency for both types 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 lh1 m2 and 5.31 lh1 m2, 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 lh1 m2 and 10.35 lh1 m2, respectively. This is shown in Fig. 2. 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
Fig. 2 – Hydrogen production rate from single cell (area [ 0.40 cm2) and modular cell (area [ 4 3 0.40 cm2).
parallel-connected photoelectrodes of smaller area. The variations of different PEC parameters for the fabricated modular PEC solar cells are given in Table 1.
4.
Hydrogen storage
Storage is a key issue for the hydrogen economy. It cuts across the production, distribution, safety and applications aspects [29–31]. One gram of hydrogen gas occupies w11 L (2.9 gallons) of space at STP, so storage implies a need to reduce the enormous volume occupied by hydrogen. All the practical storage options have disadvantages, but still, the most promising appears to be hydrides. A viable means of hydrogen storage excludes inefficient and risky high-pressure cylinders, expensive cryogenic cylinders and all covalent hydrocarbon compounds. 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 to be a potential storage method and has attracted considerable interest [30,31].
4.1.
Intermetallic hydrides
Hydrides hold promise for safe mode of hydrogen storage. The BHU group has widely investigated hydrides of intermetallic compounds as typified by AB, AB5, AB2 and A2B for 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. The intermetallic hydrides can be able to meet the volumetric storage efficiency of w60 Kg/m3, required for vehicular applications. However, decades of extensive research on traditional intermetallic hydrides have led to achieve only a moderate increase in gravimetric storage efficiency, not sufficient for vehicular applications. One alternative option to improve the hydrogen storage capacity is by partial substitution of another element, having higher electron attractive power [32]. Thus, for and AB5 type intermetallic system (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: 8
7
6
Ni..3d 4s2 ; Co..3d 4s2 ; Fe..3d 4s2
Table 1 – Area of electrodes, photocurrent density and photoconversion efficiency of the modular cells. SL. No.
Type of the cell
1.
Single Cell
2.
Modular cell
Area of the electrodes (cm2)
Photocurrent density (mAcm2)
Photoconversion efficiency (%)
Hydrogen production rate (L/hm2)
0.40 1.60 1.85 3.70
2.77 1.75 1.50 1.08
2.52 1.63 1.17 0.85
6.72 – 4.15 –
4 0.40 ¼ 1.60 2 1.85 ¼ 3.70
2.69 1.38
2.45 1.07
10.35 5.31
7362
international journal of hydrogen energy 34 (2009) 7358–7367
Table 2 – Key materials investigated by BHU group in the last 10 years. AB AB5 AB2 A2B Composites
TiFe, TiFe0.9Mn0.1, TiFe0.8Ni0.2, LaNi5, MmNi5, MmNi4.15Al0.85, MmNi4.5Al0.5, MmNi4.6Fe0.4, La(Nil x yFexSiy)5, MmNi4.3Al0.3Mn0.4, La0.2Mm0.8Ni3.7Al0.48Co0.3Mn0.5Mo0.02, La0.2Mm0.75Ti0.05Ni3.7Al0.48Co0.3Mn0.5Mo0.02 ZrFe2, TiCr2, TiV0.6Fe0.15Mn1.25, FeTi1 xMmx, Zr(Fe1 xCrx)2 0 x 0.4, Zr1 2xMnxTixFe1.4Cr0.6 (x ¼ 0.0, 0.1, 0.2, 0.05) Mg2Ni, Mg2Fe, Mg2Cu La2Mg17, La2Mg17–x wt% LaNi5, Mg–x wt% FeTi, Mg–x wt% CFMNi5, LaNi5/La2Ni7, Mg–x wt% MmNi4.6Fe0.4 etc.
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 (.3d84s2) 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 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 system (LaNi5). We have carried out material tailoring through substitution on the Ni site by the most feasible elements Co and Fe substitution. Although substitutions by Co and Fe were investigated, optimum results were obtained only through Fe substitution. 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 ˚ ) is higher than that of Ni (1.67 A ˚ ) by about 2.99%. atom (1.72 A 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(Nil xFex)5 exhibit better hydrogen storage characteristics than the parent material LaNi5. Thus, storage capacity of La(Nil xFex)5 for x ¼ 0, 0.05, 0.10, 0.16, 0.25, 0.30 are w1.50, 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 of w2.40 wt% H2. Such improvements still do not take us to the required storage capacity of w3 wt% (WE-NET, Japan limit) or >6.5 wt% (US DOE limit). Material tailoring of intermetallic hydrides is being pursued so as to increase storage capacity further. In the case of intermetallic alloys, besides material tailoring to obtain high storage capacity, computer simulation of P–C Isotherms has also been done to check the thermodynamic viability of the material. Fig. 3 shows the experimental and simulated P–C Isotherm of La(Ni0.84 Fe0.16)5 using mathematical model described by Singh, R. K. et al. [33]. Considerable attention has also been paid to study: (a) MgH2 and Mg-based composite materials and (b) built-in lightweight complex hydrides such as NaAlH4, Mg(AlH4)2, LiAlH4, LiNH2/Li2NH etc.
4.2. Lightweight composite materials for hydrogen storage Recently, attention has turned on hydrides of light elements such as Li, B, C, N, Na, Mg and Al, etc. One promising material which is under focus is MgH2 [34]. The prime reason is that Mg is a light element and it is comparatively less expensive. Most importantly, the storage capacity of the Mg binary hydride is w7.6 wt% H2. However, Mg is not a practical storage material because hydriding and dehydriding kinetics of Mg is very slow at ambient conditions. In order to enhance the hydrogen storage characteristics, new Mg-based composites have been developed [35]. Some elucidative examples of the Mg-based composites which have been extensively investigated are: Mg–x wt% LaNi5 type storage materials Mg–x wt% Mg2Nil xCox (or Fex) type storage materials These Mg-based composite materials may be the ideal hydrogen storage systems for vehicular transport since these are lightweight materials and also the decomposition temperature ranges from w50 to w100 C, which can be easily available through the engine exhaust.
4.3. Fig. 3 – Experimental and simulated desorption P–C Isotherms of La(Ni0.84Fe0.16)5.
Carbon nanomaterials
Porous carbon structure is another interesting system for hydrogen adsorption [36]. Many improvements have been
international journal of hydrogen energy 34 (2009) 7358–7367
achieved in synthesizing microporous carbonaceous materials with very high hydrogen adsorption properties [37–42]. Carbon nanotubes (CNTs) and graphitic nanofibers (GNFs) have been considered as promising candidates for reversible hydrogen storage [37,38]. However, considerable confusion still exists regarding hydrogen storage after the early rather dramatic results [39] in regard to very high hydrogen storage capacities (up to w67 wt%) for GNF. Several investigators have attempted to find the reason for such high hydrogen storage capacities. Although it has not been possible to pin point the exact reason for the high hydrogen storage capacities exhibited by GNF, some general features governing the high storage capacities have become discernible. It has been suggested that mobility of the hydrogen may get suppressed and hydrogen molecules get agglomerated in a liquid like configuration. We have in our lab carried out investigations on hydrogenation behavior of carbon (graphitic) nanofibers [40] and are trying to establish the reproducible capacity. The formation of graphitic nanofibers is achieved through catalyst-assisted thermal cracking of hydrocarbons. 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) or benzene (C6H6) [41]. The yield obtained through these 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 [42]. It has been 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 catalysts where the resulting GNFs are randomly oriented. Seeking analogy from the formation of carbon nanotubes, where oriented CNTs grow when the catalyst particles are patterned, it may be taken that with catalyst in the form of a sheet with juxtapose grains, oriented GNFs will result. The GNFs grown by us have shown storage capacity of in excess of 10 wt% and up to 17 wt% H2 [42].
4.4.
Complex metal hydrides
Complex hydrides are actually built-in hydrides. Unlike intermetallic hydrides, the hydrogen in the complexes is tightly bonded with the parent material by strong covalent and/or ionic bonding. The number of hydrogen atoms per metal atom (H/M ratio) is two in many cases [43]. These complex hydrides show high gravimetric storage capacity at room temperature (e.g. LiBH4–18 wt% H2). But, the low hydrogen liberation kinetics even at very high temperature and irreversibility is the disadvantage for the practical use of these hydrides. Various studies on the alkali/alkaline earth metal aluminium hydrides and borohydrides have been carried out [44,45]. Among all complex-based aluminum hydrides, sodium alanate has received considerable attention due to its high hydrogen capacity (7.44 wt%) and favorable thermodynamics for reversible hydrogen storage [46]. The two step reactions together (3NaAlH4 / Na3AlH6 þ 2Al þ 3H2 and Na3AlH6 / 3NaH þ Al þ 3/2H2) liberate 5.55 wt% H2, which is very close to the US DOE limit of gravimetric hydrogen capacity. The decomposition temperature looks still higher
7363
Fig. 4 – (a). Desorption kinetics with respect to CNT concentration in NaAlH4 (for details please see ref [48]). (b). Recycling behavior of Mm doped NaAlH4 (for details please see ref [49]).
and it is kinetically very slow (>50 h for first step reaction at w150 C and 30 h for second step reaction at >200 C). Besides, the products of decomposition do not combine with hydrogen to form the initial alanate phase again. However, in 1997, Bogdanovic and Schwickardi [47] demonstrated that sodium alanate is a viable means of reversible hydrogen storage system by deploying transition metal catalysts (Ti, Zr, etc.). This has triggered the 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 ambient conditions. While transition elements (mainly Ti, Zr and Fe) have been proposed as promising catalysts by several workers, we have investigated the feasibility for the use of NaAlH4 by new alternative catalysts [48,49]. We have introduced carbon nanotubes (CNTs) as a catalyst for NaAlH4
7364
international journal of hydrogen energy 34 (2009) 7358–7367
Fig. 5 – Desorption kinetics of alanate mixture: xMg(AlH4)2 D yNaAlH4 (0 < x < 1, y ‡ 1) (for details please see ref [45]).
system. The main advantage of using CNT as a catalyst is its high outer surface area (w1315 m2/g). In addition to the nanosize, its high aspect ratio (diameter from w10 to w30 nm and length from w1 to w10 mm) can provide additional surface/ interface in the host material, where they are admixed. The CNTs are also known to possess significant catalytic activity via p and s bonds, particularly the latter associated with carbon in graphitic sheets. Multi-walled carbon nanotubes (MWNTs) were synthesized in our lab through thermal spray technique. A suitable amount of CNTs in 2,4,6,8 and 12 mol% has been taken with NaAlH4 and this mixture was mechanically admixed for 5 min at a milling speed of w5500 rpm (vial volume w40 cm3) under argon atmosphere. Out of various
Fig. 6 – Recycling behavior of ball-milled 2:1.1 molar mixtures of LiNH2 and MgH2 at 200 8C (for details please see ref [50]).
proportions, NaAlH4–x mol% CNT (x ¼ 2, 4, 6, 8 and 12), we have found that the material with x ¼ 8 mol% is the optimum material (as shown in Fig. 4(a)). It shows the highest desorption rate, leading to w3.3 wt% of H2 at w160 C within 2 h. The CNT admixed NaAlH4 has also been found to exhibit good rehydrogenation characteristics [48]. 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 CNTs). We have observed very interesting results when nanoform of Mischmetal (Mm: Ce-42 at%, La-31 at%, Nd-18 at%, Pr-9 at%) is used as catalyst [49] (refer to Fig. 4(b)). Apart from sodium alanate, other alanates such as lithium alanate and magnesium alanate are attractive candidates because of their high gravimetric hydrogen capacity. We have observed some interesting results on mixtures of NaAlH4 and Mg(AlH4)2. In the alanate the dehydrogenation mixture, 0.5Mg(AlH4)2 þ NaAlH4, temperature of NaAlH4 gets lowered by w50 C (from w190 C to 140 C) with 4 times faster desorption kinetics [45]. Fig. 5 shows the desorption kinetics of xMg(AlH4)2þyNaAlH4 (0 < x < 1, y 1). In the series of complex hydrides, we have also studied lithium amide/imide. One interesting result obtained by us relates to the formation of Mg(NH2)2 on ballmilling LiNH2 and MgH2 [50]. The hydrogen uptake capacity of prolonged ball-milled LiNH2 and MgH2 at 200 C has been observed to be 4.3 wt% in 2 h. Fig. 6 shows the recycling behavior of ball-milled LiNH2 and MgH2 mixture. The unfavorable thermodynamics of these hydrides for reversible hydrogen storage restricts their use. 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 fueled vehicular transport
As pointed out in the ‘Introduction’ section for India, climate/ environmental change and its deleterious effects require immediate change to hydrogen. India being an agricultural country, the economy is dominantly dependent on it. The climate change out of its various manifestations will influence agriculture in India in a significant way. It may be pointed out that about a decade back, the climate change was mostly gauged by temperature rise. Preliminary predictions of the influence of temperature rise in various areas including agriculture were made. However, based on IPCC reports and the Nicholas Stern Report (October 2006), specific consequences of climate change on agriculture have become available. As for example, it was getting about 12 monsoon depressions in a year. By 2000, it has dropped to about 4 per year. It has also become proven now that northern India will become warmer. Rainfall will decrease in Punjab, Rajasthan, Tamil Nadu and some other states. All this will lower our agricultural production. Rainfall will increase particularly along the western coast and west central India, affecting agriculture in Gujarat, Maharastra, and Karnataka. India has been marked in the Nicholas Stern report as one of the most sensitive countries in relation to effect of climate change on agriculture.
international journal of hydrogen energy 34 (2009) 7358–7367
In view of the above, it is clear that India should be the first country to start using hydrogen as a fuel. This is the best way to combat climate change. For realizing this, we must have working modules of devices, particularly vehicles running on hydrogen as fuel. In the following, we proceed to describe and discuss these developments on hydrogen-fueled vehicles carried out at the Hydrogen Energy Center (HEC), BHU. 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), the latter possesses 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 w160 MT of oil of which we have to import 122 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. BHU Hydrogen Energy Center, in regard to the application question, has focused on R&D and demonstration of hydrogen fueled road transport, particularly two-wheelers, 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 selfignition 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 fueled Internal Combustion (IC) engine is to increase the compression ratio. The self-ignition temperature of H2 is w630 C as compared to petrol, 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 that compression ratio for our hydrogen fueled two and three-wheelers can be increased from w8 to w11.
7365
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 center. 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 fueled IC engine is on two-wheelers. These have achieved from w60 to w80 km driving 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 w2.4 wt%. We are now in the process of using this hydride. The range is expected to become from w80 to w100 km. The work is being extended to threewheelers and small cars (refer to Fig. 7). 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 fueled two-wheeler, the hydride heat exchanger tank is so designed that it works as a silencer. In India, 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 fueled two and three-wheelers. Thus, we have converted a petrol-driven three-wheeler manufactured and provided by International Cars and Motors Limited (ICML), Jallandhar (Punjab) to run on hydrogen stored in Mm–Ni–Fe hydride (refer to Fig. 8). 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 traveled by a three-wheeler vehicle per day is
5.2. Examples of hydrogen fueled 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 (storage capacity w1.8 wt% H2). Some trials have also been done using Ti admixed NaAlH4. The total quantity of hydride employed is w20 kg. The exhaust gas coming out at temperatures of w60 C in the case of two-wheelers is circulated in the hydride heat exchanger bed. Since thermal conductivity of the hydride is very poor (0.5–1.0 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, 20 kg of hydride is distributed in the HHET by 20 aluminum tubes of 1-in diameter and 12-in length. The exhaust heat from the 100 cc two-wheeler engine is able to raise the temperature of hydride to w60 C. This results in continuous emission of hydrogen which is sent to the engine. An important point is the site and
Fig. 7 – Hydrogen/hydride heat exchanger tank coupled with Internal Combustion engine of a four-wheeler.
7366
international journal of hydrogen energy 34 (2009) 7358–7367
The use of environmentally friendly and renewable fuel, hydrogen, needs more investment, more R&D and more general acceptance as an idea whose time has come.
Acknowledgements The authors would like to thank Prof. T.N. Veziroglu, Prof. A.R. Verma, Prof. C.N.R. Rao, Prof. R. Chidambaram, Prof. S.K. Joshi, Prof. S. P. Thyagrajan and Prof. D.P. Singh (VC:BHU) for their encouragement and support. Financial support from the Ministry of New and Renewable Energy and the University Grants Commission are thankfully acknowledged.
references
Fig. 8 – Demonstration of 3-wheeler developed by Physics Department, Banaras Hindu University for International Cars and Motors Ltd., under BHU-ICML-UGC-MNES programme in AUTO-EXPO 2006 held at New Delhi.
w30 km. In a single charge, the average distance travelled by the hydrogen fueled three-wheeler is about 60 km at a top speed of w50 kmph. The ICML engineers have been trained to convert these wheelers to run on hydrogen. In India, the ICML is in the process of manufacturing 10 three-wheelers. 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. Similar efforts are being made for two-wheelers with the help of the Society for Indian Auto Manufacturers (SIAM), which has access to various two-wheeler manufacturers in India. Both these efforts to introduce hydrogen fueled 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 reviews the R&D activities of Hydrogen Energy Center, BHU in production, storage and application of Hydrogen Energy. We have developed hydrogen fueled twowheelers. This work is being extended to three-wheelers and small cars. The hydrogen fueled vehicles developed at Hydrogen Energy Center, BHU have nearly the same performance as that of the petrol fueled vehicles but with no impact on the climate change. At present we have developed vehicles running in the range of w60–80 km for two-wheelers and w60 km for three-wheelers (at top speed of w50 km/hr) for single charging. Commercialization efforts on hydrogen fueled vehicular transport are being done by Hydrogen Energy Center, BHU with the help of Indian auto industries.
[1] Stern Review. The economics of climate change, Available from: www.sternreview.org.uk. [2] Marban G, Valdes-Solis T. Towards the hydrogen economy? Int J Hydrogen Energy 2007;32:1625–37. [3] Midilli A, Dincer I. Hydrogen as a renewable and sustainable solution in reducing global fossil fuel consumption. Int J Hydrogen Energy 2008;33:4209–22. [4] IPCC Fourth Assessment Report. Impacts, adaptation and vulnerability [chapter 10]. Available from www.ipcc.ch/ ipccreports/ar4-wg2.htm. [5] Mishra PR, Pukazhselvan D, Hudson MSL, Pandey SK, Srivastava ON. Hydrogen energy in Indian context and R&D efforts at Banaras Hindu University. Int J Environmental Studies 2007;64:761–76. [6] Ji J, Gang P, Tin-tai C, Keliang L, Hanfeng H, Jianping L, et al. Experimental study of photovoltaic solar assisted heat pump system. Solar Energy 2008;82:43–52. [7] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37–8. [8] Bockris JOM, Uosaki K, Kita H. Interfacial electron transfer as a significant step in photoelectrochemical reactions on some semiconductors. J Appl Phys 1981;52:808–10. [9] Bockris JOM, Khan SUM. Surface electrochemistry: a molecular level approach. New York: Plenum Press; 1993. [10] Gerischer H. Topics in applied physics. In: Seraphin BO, editor. Solar energy conversion. Amsterdam: Elsevier; 1979. p. 115–72. [11] Gerischer H. Electrochemical techniques for the study of photo-sensitization. Photochem Photobiol 1972;16:243–60. [12] Veziroglou TN. Dawn of the hydrogen age. Int J Hydrogen Energy 1998;23:1077–8. [13] Veziroglu TN. Quarter century of hydrogen movement 1974–2000. Int J Hydrogen Energy 2000;25:1143–50. [14] Bak T, Nowotny J, Rekas M, Sorrell CC. Photoelectrochemical hydrogen generation from water using solar energy: materialsrelated aspects. Int J Hydrogen Energy 2002;27:991–1022. [15] Nozik AJ. Photoelectrolysis of water using semiconducting TiO2 crystals. Nature 1975;257:383–6. [16] Gratzel M. Photoelectrochemical solar cells. Nature 2001;414: 338–44. [17] Zou Z, Sayama K, Arakawa H. Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature 2001;414:625–7. [18] Chandra S. Photo-electrochemical solar cells. London: Gordon and Breach; 1985. [19] Khaselev O, Turner JA. A monolithic photovoltaicphotoelectrochemical device for hydrogen production via water splitting. Science 1998;280:425–7.
international journal of hydrogen energy 34 (2009) 7358–7367
[20] Khan SUM, Al-Shahry M, Ingler WB. Efficient photochemical water splitting by a chemically modified n-TiO2. Science 2002;297:2243–4. [21] Tributsch H. Photovoltaic hydrogen generation. Int J Hydrogen Energy 2008;33:5911–30. [22] Zhaoyue L, Kai P, Min L, Qinglin Z, Tun L, Yang L, et al. Influence of the binder on the electron transport in the dyesensitized TiO2 electrode. Thin Solid Films 2005;484:346–51. [23] Mor GK, Varghese OK, Paulose M, Ong KG, Grimes CA. Fabrication of hydrogen sensors with transparent titanium oxide nanotube-array thin films as sensing elements. Thin Solid Films 2006;496:42–8. [24] Gong D, Grimes CA, Varghese OK, Hu WC, Singh RS, Chen Z, et al. Titanium oxide nanotube arrays prepared by anodic oxidation. J Mater Res 2001;16:3331–4. [25] Karn RK, Srivastava ON. On the synthesis of nanostructured TiO2 anatase phase and the development of the photoelectrochemical solar cell. Int J Hydrogen Energy 1999; 24:27–35. [26] Mishra PR, Singh AK, Srivastava ON. Synthesis of nanostructured TiO2 by sol–gel technique for PEC applications and its advantages. Indian J Phys 2004;78A(1):81–4. [27] Mishra PR, Shukla PK, Singh AK, Srivastava ON. Investigation and optimization of nanostructured TiO2 photoelectrode in regard to hydrogen production through photoelectrochemical process. Int J Hydrogen Energy 2003; 28:1089–94. [28] Mishra PR, Shukla PK, Srivastava ON. Study of modular PEC solar cells for photoelectrochemical splitting of water employing nanostructured TiO2 photoelectrodes. Int J Hydrogen Energy 2007;32:1680–5. [29] Berry GD, McAceves SM. The case for hydrogen in a carbon constrained world. J Energy Resour Technol 2005;127:89–94. [30] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007;32:1121–40. [31] Schalapbach L, Zuttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414:353–8. [32] Pandey SK, Srivastava A, Srivastava ON. Improvement in hydrogen storage capacity in LaNi5 through substitution of Ni by Fe. Int J Hydrogen Energy 2007;32:2461–5. [33] Singh RK, Lototsky MV, Srivastava ON. Thermodynamical, structural, hydrogen storage properties and simulation studies of P–C isotherms of (La, Mm) Ni5yFey. Int J Hydrogen Energy 2007;32:2971–6. [34] Janot R, Darok X, Rougier A, Aymard L, Nazri GA, Tarascon JM. Hydrogen sorption properties for surface treated MgH2 and Mg2Ni alloys. J Alloy Comp 2005;404–406:293–6. [35] Sai Raman SS, Davidson DJ, Srivastava ON. On the synthesis, characterization and hydrogenation behaviour of Mg-based
[36] [37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
7367
composite materials Mg-x wt% CFMmNi5 prepared through mechanical alloying. J Alloy Comp 1999;292:202–11. Zubizarreta L, Arenillas A, Pis JJ. Carbon materials for H2 storage. Int J Hydrogen Energy 2009;34:4575–81. Froudakis GE. Hydrogen interaction with carbon nanotubes: a review of ab initio studies. J Phys: Condens Matter 2002;14: R453–65. Simonyan VV, Johnson JK. Hydrogen storage in carbon nanotubes and graphitic nanofibers. J Alloy Comp 2002; 330–332:659–65. Chambers A, Park C, Terry R, Baker K, Rodriguez NM. Hydrogen storage in graphite nanofibers. J Phys Chem B 1998; 102:4253–6. Gupta BK, Srivastava ON. Synthesis and hydrogenation behaviour of graphitic nanofibres. Int J Hydrogen Energy 2000;25:825–30. Kim MS, Rodriguez NM, Baker RTK. The interaction of hydrocarbons with copper-nickel and nickel in the formation of carbon filaments. J Catal 1991;131:60–73. Gupta BK, Tiwari RS, Srivastava ON. Studies on synthesis and hydrogenation behaviour of graphitic nanofibres prepared through palladium catalyst assisted thermal cracking of acetylene. J Alloy Comp 2004;381:301–8. Arroyo ME, Ceder G. First principles investigations of complex hydrides AMH4 and A3MH6 (A ¼ Li, Na, K, M ¼ B, Al, Ga) as hydrogen storage systems. J Alloy Comp 2004;364:6–12. Amendola SC, Sharp-Goldman SL, Janjua MS, Spencer NC, Kelly MT, Petillo PJ, et al. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25:969–75. Hudson MSL, Pukazhselvan D, Sheeja GI, Srivastava ON. Studies on synthesis and dehydrogenation behavior of magnesium alanate and magnesium-sodium alanate mixture. Int J Hydrogen Energy 2007;32:4933–8. Zaluski L, Zaluska A, Stro¨m-Olsen JO. Sodium alanates for reversible hydrogen storage. J Alloy Comp 2000;298: 125–34. Bogdanovic B, Schwickardi M. Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. J Alloy Comp 1997;253–254:1–9. Pukazhselvan D, Gupta BK, Srivastava A, Srivastava ON. Investigations on hydrogen storage behavior of CNT doped NaAlH4. J Alloy Comp 2005;403:312–7. Pukazhselvan D, Hudson MSL, Gupta BK, Shaz MA, Srivastava ON. Investigations on the desorption kinetics of Mm-doped NaAlH4. J Alloy Comp 2007;439:243–8. Shahi RR, Yadav TP, Shaz MA, Srivastava ON. Effects of mechanical milling on desorption kinetics and phase transformation of LiNH2/MgH2 mixture. Int J Hydrogen Energy 2008;33:6188–94.
