Hydrogen Economy is not dead
– Some Recent Developments In Hydrogen Generation, Storage, Transport And Usage As Energy Carrier
- Dr N C Datta, Consultant, Modicon Pvt Ltd.
Hydrogen ecocomy has been touted for some time as a superior alternative to the hydrocarbon economy we are in today. This article covers various production processes, its transportation and storage aspects, particularly in terms of the latest advances in these areas.
“I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable.” - Jules Verne (The Mysterious Island, published in 1874, Chapter 33) Abstract
Hydrogen ecocomy has been touted for some time as a superior alternative to the hydrocarbon economy we are in today. Hydrogen is often seen as more attractive and cleaner than the conventional fuels because whether it is used in a fuel cell with air to produce electricity or burned to produce heat, the only by-product is water rather than carbon dioxide or other greenhouse gases and particulates. Much as hydrogen is a clean fuel and abundantly available in water, its production, storage and transportation poses many challenges.
This article covers various production processes, its transportation and storage aspects, particularly in terms of the latest advances in these areas.
Hydrogen economy is the vision of using hydrogen as the source of energy for several purposes. Currently, more than 70% of the crude oil is used in transportation. Proportionate amounts of CO , unburnt hydro
2 carbons, and NOx are released into the atmosphere, leading to global warming. Hence, for any meaningful abatement of global warming, it is necessary that a suitable substitute of oil is found, hydrogen can be the best alternative.
The concept was proposed almost 100 years ago in a paper presented by the famous scientist J.B.S. Haldane (1892-1964) before the Cambridge Society – The Heretics1. It was resurrected in 1970 when the first signs of an impending oil crisis loomed at the horizon. In a lecture at the Technology Centre of General Motors, the celebrated electrochemist, John O’M. Bockris (1923-2013), elaborated on the concept, and coined the term “Hydrogen economy”. Later in 1975,
1 he published a book, on the subject, entitled, “Energy: Solar Hydrogen Alternative”.
There are no two opinions that hydrogen is the cleanest fuel on earth. It burns in O /air, forming only
2 water, which, though a greenhouse gas in vapour form, is turned easily to liquid water. Energy output wise, 1 kg of hydrogen is almost equivalent to about 3.3 M3 of natural gas / 3.8-3.9 L of gasoline / 3.3-3.4 L of high speed diesel2. To illustrate more, a fuel cellpowered vehicle may travel upto 60 miles in USA conditions with 1 kg of hydrogen in the fuel tank, or the same quantity of hydrogen may provide electricity to an average USA household for 12 hours.
However, even today, the economy is driven almost completely by fossil fuels all around the world with little visibility of hydrogen as an alternative. To be specific, almost 85% of all energy requirements are still met from fossil fuels. Regarding the other energy resourc
4 es, nuclear energy is used only 2%, and renewables, 13%, with the following break up: biomass (wood, etc.) 10.2%, wind 0.2%, hydropower 2.3%, marine 0.0002%, geothermal 0.1%, and solar just 0.1% 4.
In 2016, about 65 million metric tons of hydrogen was produced worldwide, of which 10 million metric tons were produced in USA alone. Of this quan
5 tity, 48% was used in petroleum refining for a process known as hydrocracking, 43% was used in ammonia manufacture, about 4% in methanol production, and the balance 5% was used in metal fabrication, electronics manufacture and food processing. No hydrogen
5 was used as fuel. One reason for this is the economics. The prices of various energy carriers and resources, as of 2009, are shown in Fig.1. Hydrogen is one of the costliest in comparison with other energy carriers. The other reasons are technical, as will be discussed.
(1) No other energy carrier is as infinite as hydrogen, because it can be obtained from water, and carbohydrates (biomass), both of which are renewable resources.
(2) Hydrogen is non-toxic.
(3) Recharging of hydrogen-powered vehicles may be relatively easy – it may need just replacement of the exhausted hydrogen storage unit by a refill.
(4) H system can be integrated well into the power
2 grid and be very useful in grid stabilisation during demand fluctuations, as excess power generation could be utilised in electrolysis of water to make more hydrogen and oxygen, and any shortfall in power supply may be augmented from hydrogen-powered fuel cell stacks. This grid stabilisation through flexible input and output has become a necessity today in advanced countries because of sharply diminishing prices of alternate energy resources and change of user preferences.
(5) Like petroleum crude / oil, hydrogen may be transported over long distances through pipelines and vast quantities of hydrogen may be stored in large underground caverns.
(6) Some of the advantages of hydrogen are equally possible with other energy carriers such as methanol
and ethanol, which, too, may be obtained from renewable resources like biomass. However, hydrogen is unique and superior to other energy carriers because of one fundamental reason.
Table 1 shows the values of maximum available useful energy (DG) that could be obtained when some of these covalent chemical bonds such as H – H, C – H, C – C, and N – H, as present in different energy carriers, are broken by reaction with O . Table 1 shows also
(i) the number of electrons (n) involved in each of these reactions, if the reactions are carried out electrochemically, and (ii) the corresponding cell voltage (E), which is a measure of the available useful bond energy per electron. This available useful bond energy per electron is an important parameter, because fuel cells work only through flow of electrons.
On this basis, H (or H – H bond) contains the maxi
2 mum available useful bond energy per electron (1.23 V) in comparison with other covalent chemical bonds. Of course, the reactions 4, 7, and 8 of Table 1, viz, oxidation of NH by O to form N and H O, and reduc
3 2 2 2 tion of CO to form CH OH and C H OH, may have
2 3 2 5 similar useful bond energy per electron with E = 1.17, 1.213, and 1.145 V, respectively, but these reactions are not commercially viable because CO is present in
2 the atmosphere at a concentration of about 400 ppm only, and the oxidation of NH3 involves the handling of a highly hazardous substance. Also, the synthesis of NH3 requires a huge amount of energy and pure H .
2 (7) The splitting of water molecule into H and O
2 2 is the easiest; thanks to absence of any side reactions, faster kinetics and relatively lower activation barrier.
Critical Issues with hydrogen
(1) Being the lightest gas, it occupies a very large volume in gaseous state. Therefore, for transportation in vehicles as fuel tanks, it must be compressed to very high pressure and / or liquefied. For liquefaction, hydrogen must be cooled to below its critical temperature, 33 K. Therefore, adequate cryogenic cooling is necessary for storage and transportation of liquid hydrogen.
(2) Hydrogen is an extremely inflammable gas, may form explosive mixture with air, and explode if heated. In air, it has very wide flammability limits: 4 – 75% (v/v), and detonation limits: 13 – 70% (v/v).
(3) Hydrogen may displace oxygen rapidly and without notice, causing suffocation.
(4) It burns with pale blue flame, which is almost invisible in day light. While burning, it does not produce any infra-red radiation, but produces a lot of UV radiation – so any person standing nearby would not experience any heat, but would experience sun-burn like effect on the skin due to the exposure to UV radiation.
(5) So, any hydrogen leakage must be detected. The detection may be done by an electronic sensor or by an odorant. For efficient detection, an odorant should have similar molecular weight and diffusion characteristics as the bulk gas so that it spreads at the same rate. So far no odorant has been found which has similar speed (1.78 km/s) and diffusivity (0.61x10- m2/s) as hy
(6) Hydrogen exhibits a positive Joule-Thompson effect at temperatures above 193 K, which is its inversion temperature. It means that the temperature of the hydrogen gas increases upon depressurization, and this may lead to its ignition. Hydrogen has a very low ignition energy 0.0019 Joule.
(7) At elevated temperatures and pressures, hydrogen, being a tiny molecule, diffuses inside the metal
matrix of the storage container. As hydrogen spreads inside the metal, gradually the metal loses its ductility and becomes brittle. This is hydrogen embrittlement. This failure of metal is a serious concern in any situation involving storage or transfer of hydrogen gas under pressure.
How palladium is useful
In view of the problems of storing and transportation of compressed and liquefied hydrogen, researches have been done to develop solid absorbents, which would absorb a large volume of hydrogen and desorb it reversibly on user demand. Several metals and alloys have been developed for this purpose. These metals and alloys absorb H to form hydrides and these hy
2 drides decompose at higher temperatures, liberating the absorbed hydrogen and regenerating the original metals and alloys. Table 2 shows the temperature and pressure required for the formation of some of these metal hydrides, their composition, and quantity of H
2 these may carry.
It is essential that the molecular H should dissoci
2 ate into atoms before it is incorporated into the metal / alloy lattice to form the hydrides. It is commonly established that palladium has an extraordinary ability to dissociate molecular H rapidly, and this property of
2 palladium is at the root of its use as a very efficient catalyst for hydrogenation reactions in organic synthesis.
As shown in Table 2, most metals and alloys, other than palladium, require some pressure and / or temperatures to overcome an activation barrier. But palladium absorbs hydrogen under ambient conditions upto 900 times of its own volume, forming palladium hydride of composition: PdHx, where x varies from 0.015 to 0.607. Still, palladium is not acceptable as a hydrogen storage material because it is too expensive, and the total quantity of hydrogen that can be stored in Pd is not very high – it is just 0.56% by weight. But Pd has the potential to play a major role in all areas of hydrogen economy such as hydrogen purification, storage, detection, and fuel cells.
(a) Hydrogen Storage
The US Department of Energy has concluded that for a good hydrogen storage device: (i) it must be able to absorb at least 5.5 wt% hydrogen for the time being and should be able to absorb upto 9 wt% later after further development, (ii) it should be light-weight, inexpensive and readily available, (iii) the sorption - desorption kinetics should be fast and reversible, and (iv) it should have long-term stability after repeated recycling. From Table 2, it is apparent that MgH meets 7
2 all these criteria, but it is highly susceptible to be attacked by both acids and alkalis. Also, the rate of H
2 sorption by Mg is very sluggish, and the hydrogen gets desorbed only at temperatures higher than 300oC.
All these problems may be solved, if Mg is alloyed first with Ti, forming a thin film of an alloy of composition MgyTi1- y, where y = 0.80 optimally, and then if Pd is deposited electrochemically on this alloy upto a thickness of 3-4 nm. This capping of MgyTi1- y flm by Pd makes it not only acid – alkali resistant, but also its hydrogen sorption – desorption kinetics become reasonably fast. It has been observed that the hydrogen stor
8 age capacity of this film of Pd-capped MgyTi1- y alloy approaches 1750 mAh/g, when used in fuel cell, and this is equivalent to 6.4 wt% of hydrogen storage. Pd
8 capped Mg-Sc alloys of similar composition also have shown identical properties.
(b) Hydrogen detection7
Pd may be used to make some very efficient sensors to detect hydrogen. In one type of sensors, its electrical resistivity increases sharply as hydrogen gets absorbed in Pd. In another type of sensors, Pd is coated with an optically active material, which sends an optical signal proportional to the concentration of hydrogen absorbed. In both types of sensors, Pd must be in nano-form.
(c) Hydrogen purification
Among all transition metals and metal oxides, platinum has been found to be the most effective catalyst in all types of fuel cells, but it is extremely susceptible to poisoning by CO, H S, and other poisons.
When H is obtained by reforming hydro
2 carbons such as steam methane reforming reaction (SMR) or from carbohydrates by oxidation, some quantities of CO and CO
2 are invariably formed in course of the reac-
tions. Even after stringent purification, some residual CO remain in the product hydrogen, and the Pt catalyst in the fuel cell is irreversibly poisoned, if H feed
2 contains more than 10 ppm of CO. This is one major road block for use of fuel cells in automobiles, because most of the hydrogen is made by SMR as on now. It has been observed that the palladium or palladium alloy based membranes may be useful to make 99.9999% pure hydrogen. But there is some problem here, too. H adsorption in Pd is accompanied by phase
2 change and lattice expansion. At lower concentration of H-absorption, it forms an α-phase, and as the absorption increases, the lattice gradually expands and forms a β-phase. Finally, beyond a certain critical limit of the lattice expansion, the membrane cracks and breaks into pieces. This also is called as hydrogen embrittlement.
It has been found that if the absorption of H occurs
2 at 570 K and above, there is no lattice expansion and no hydrogen embrittlement. But absorption of H at 570
K and above would reduce the quantity of absorbed hydrogen further – also, this would involve an expenditure of energy. However, this temperature of 570 K may be reduced to lower temperatures, say, to 393 K, by alloying Pd with Ag (23 wt% Ag) or with Cd (15
9 at% Cd) 10. Such alloying not only prevents hydrogen embrittlement, but also reduces the cost of hydrogen storage by using less expensive metals.
(d) Pd as catalyst in fuel cells
Platinum is the established electrode material in all types of fuel cells. A proton exchange membrane or polymer electrolyte membrane fuel cell (PEMFC) is shown in Fig. 2.
But very high cost of platinum, its limited supply, and susceptibility to poisoning are its major limitations. Also, the cathodic oxygen reduction reaction
(see Fig.2) is not very fast on Pt, although Pt is the fastest catalyst for this reaction among most metals. Pd alloys and combinations of Pd with other platinum group metals such as Ru, Ir, Pt, etc. have been widely investigated in fuel cells using methanol, ethanol, or formic acid as fuel. Pd-Pt bimetallic catalysts have
12 been found to be better than Pt in many reactions, and Pd is shown to be a far superior catalyst than Pt in formic acid oxidation. For the oxygen reduction reaction, Pd-alloys have also demonstrated improved performance when compared to Pt. The change from Pt to
Pd-based catalysts in fuel cells is being considered seriously, but the price of palladium has increased drastically in recent times due to increased usage and other geopolitical reasons. It is not clear if such a change will bring down ultimately the fuel cell cost.
Hydrogen is the most abundant element in the universe – 75% of all matter in the universe is made of hydrogen, but the earth’s atmosphere contains just 1 ppm of H . Therefore, it has to be obtained always from its
2 combined forms such as water, hydrocarbons, and carbohydrates, which are available in plenty. The various commercial processes, which are presently used to make H , are: reforming of natural gas or steam meth
2 ane reforming (SMR), gasification of coal or biomass in air /O , pyrolysis of coal or biomass in absence of O ,
2 2 and electrolysis of water.
Table 3 shows the efficiencies of energy conversion in various technologies of H production, as cal
2 culated in the Hydrogen Tools Portal of the Pacific Northwest National Laboratory with support from the US Department of Energy. As shown in Table 3, in all
14 such processes, almost 30-60% of energy is wasted.
14 Therefore, any process to use hydrogen as energy carrier would be economically viable only if the energy to isolate hydrogen from its compounds is available cheaply. And what could be cheaper source than the energy from the Sun, which is available freely and abundantly around the Globe?
There are three processes by which CO, CO -free
H may be made using solar radiations. These are: (1)
2 water splitting by direct concentrated solar radiation,
assisted by photocatalysts, (2) solar thermochemical hydrogen (STCH) cycles, and (3) electrolysis of water, assisted by electrocatalysts, using electricity generated by photovolatics. These processes are discussed briefly in the following.
(a) Water splitting by photocatalysts
Direct splitting of water by solar radiation, assisted by photocatalysts, has been a dream for decades. A large number of metal oxides, sulphides, nitrides, nano-composites, doped materials and organo-metallic complexes have been tried with varying degrees of success. So far TiO , and catalysts based primarily
2 on TiO have been found to be most successful. But
2 no process has been found to be viable for commercialisation as yet because of (i) wide band gap (~ 3.2 eV), (ii) large overpotential for hydrogen evolution, and (iii) rapid recombination of electron-hole pairs in TiO based catalysts. Recently a nano-hybrid of Au on
TiO has been found to make as high as 647,000 mol
2 of H per hour per gram of the catalyst, but it is still in
2 laboratory level only. In fact, as on now, the other two methods, viz. thermochemical cycle and photovoltaics based electrolysis appear to be more promising than the photocatalytic splitting of water. (b) Water splitting by thermochemical Cycle
In a thermochemical cycle, one highly endothermic decomposition reaction is carried out at a very high temperature using solar radiation, which is intensely concentrated by a ring of parabolic mirrors. O
2 is evolved during this decomposition reaction. This is called the reduction step. In the next step, one of the decomposition product is reacted with water at a relatively lower temperature or electrolysed in aqueous medium generating H and
2 the original reactant. Since H is eliminated from water
2 in this step, this is called an oxidation step. The process is shown schematically in Fig. 3. When electrolysis is done in the oxidation step, it is called a hybrid cycle.
Innumerable thermochemical processes are possible on the basis of thermodynamic data, but only a few are considered to be commercially viable. Some of the promising thermochemical cycles are shown in Table 4. Two such processes are discussed below for illustration and these are: (a) zinc oxide cycle, which is a direct thermochemical cycle, meaning all steps are chemical,
and (b) hybrid sulphur cycle.
Zinc Oxide Cycle: As shown in Table 4, in the reduction step zinc oxide is dissociated into Zn powder and O at a very high temperature of 1800-2000oC by
2 intensely concentrated solar radiation. In the next step, zinc oxide is regenerated and H is formed by hydro
2 lysis of zinc powder with H O at 450oC. This cycle has
2 attracted considerable attention because zinc oxide is a non-hazardous, easily available, and a relatively benign material. But the major technical problems are the recombination of Zn powder with O inside the reactor
2 to form ZnO back again, and the rapid deterioration of the reactor materials at such high temperatures. The problem of back reaction to ZnO could be solved by rapid quenching of Zn powder in argon, but this led to the loss of some sensible heat. A 10-kW demonstration plant was established18, but the actual efficiency of the process in the pilot plant was found to be much less than the theoretical efficiency and the problem of reactor damage could not be solved, too. Therefore,
19 there is some skepticism on the commercial viability of this process, and according to some recent studies, the non-stoichiometric perovskites, which lose O at lower
2 temperatures, may probably be a more promising option.
19, 20 The Hybrid Sulphur cycle or the HyS process: It consists of two steps: (a) a high temperature (at ~ 850oC) decomposition of H SO
2 4 to SO and O , followed
2 2 by (b) a low temperature (at ~ 100oC) electrolysis step of oxidizing SO to
H SO at the anode and
2 4 generating pure H at the
2 cathode. The reactions are shown in Table 4. On the basis of the standard potential of the overall reaction (0.158 V), only 12.8% of the electrical energy is required for the electrolysis step of this cycle in comparison with the electrical energy required for the electrolysis of water (1.23 V).
The electrochemical oxidation of sulphur dioxide was discovered by Westinghouse in 1970s and has since been intensively investigated on many electrode systems using platinum, gold, graphite, palladium, palladium oxide, platinum oxide, and platinum-gold alloys in various configurations. It has been found that a high concentration of sulphuric acid is required in the electrolysis cell to maximize the overall energy efficiency of the cycle. But the Nafion
21 membrane in the electrolyser cell, which requires to be hydrated for proton transfer across the cell, is also responsible for water migration into the anode compartment. This, consequently, leads to dilution of sulphuric acid, and decrease in cell efficiency. However, two developments in recent years have given a push for a serious consideration of this cycle: (1) the development of a bayonet-type reactor using silicon carbide as material of construction to carry out efficiently the thermal decomposition of the sulphuric acid under solar radiation, (Fig.4), and (2) use of sulphuric acid
22 doped polybenzimidazole-based membranes in place of Nafion in the electrolysis part.
(c) Water splitting by photovoltaic electricity
The splitting of water into H and O by applying
2 2 electricity is not new, but the generation of electricity at commercial level solely by using sunshine, and applying it to split water is a technology under development for years. The success of hydrogen economy depends largely on how efficiently the solar radiation is
converted to electricity, and then how efficiently this electricity is used to generate H in the electrolyser cell,
2 or, in brief, on the solar-to-hydrogen (STH) efficiency. Thus, the development has two aspects: one, the development of cheaper solar cell; two, development of a better electrocatalyst that will reduce the overpotential of the O evolution reaction.
The cost of H produced by electrolysis is still sig
2 nificantly higher than that produced by steam methane reforming reaction (SMR). According to the US Department of Energy, for commercial viability, H
2 threshold cost should be USD 2.00–4.00 per gallon of gasoline equivalent, whereas the most up-to-date reported H production cost via electrolysis is USD 3.26–
6.62 per gallon of gasoline equivalent.
Among many developments, mention may be made of a recently developed photovoltaic-electrolysis system of a very high STH efficiency. It consists of two polymer electrolyte membrane electrolysers in series with one triple-junction solar cell which produces a large-enough voltage to drive both electrolysers with no additional energy input. The triple junction is made of InGaP (1.9 eV) / GaAs (1.4 eV) /GaInNAsSb (1.0 eV). The electrode assembly consists of carbon paper/ platinum black/Nafion/Nafion membrane /Nafion/ iridium black/titanium mesh. The system achieved a 48-h average STH efficiency of 30%., and according to the authors, this is the highest ever efficiency achieved so far.
(d) Hydrogen by enzymatic method25,
Among many new developments, mention may be made of a purely biological process because of its spectacular production of hydrogen from biomass, though it does not use any solar radiation. This process is known as cell-free synthetic enzymatic pathway biotransformation or shortly, as SyPaB. It uses a combination of 13 enzymes to convert carbohydrate (C H O ) to CO and H2 by complex pathways. But the
6 10 5 2 most striking features are that the reactions take place at 30oC and atmospheric pressure, and the hydrogen yield is very high – about 12 molecules of H per glu
2 cose equivalent in place of the usual 4. Also, it may be able to produce hydrogen from municipal sewage and industrial waste water containing very high degree of organics. It is estimated that after full scale up, this technology may be able to bring down the cost of H to
2 about USD 2 per kg, but as on now, the method is in the laboratory level.
Hydrogen economy comprises three aspects: hydrogen generation, storage & transport, and extraction of energy from hydrogen by fuel cells. This article has discussed very briefly each aspect and some of the recent developments. For more information, interested readers may visit the websites of the US Department of Energy, Office of Energy Efficiency and Renewable Energy. These websites27 provide in detail the latest developments in hydrogen economy and fuel cells. It is inevitable that hydrogen would be the main driver of the world economy in future. In January 2017, at the end of the Davos Summit, a global initiative has been taken by several leading energy, transport and industry companies of the world, and Hydrogen Council has been formed with a mission “to position hydrogen among the key solutions of the energy transition” 28. Mankind took a huge number of millennia to transit from wood and animals to coal, and a few hundred years from coal to oil. It may take now just a few decades to transit from oil to hydrogen.
The author expresses his deep gratitude to Mr Amit Modi, Director, Modicon Pvt Ltd, Mumbai, for providing research opportunities so that this article could be written. References
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Data source: Ref. 6
Source: Modified from Ref.3 (F in Column 5: Faraday constant = 96,485 coulombs)
Fig 2. Schematic diagram of a proton exchange membrane fuel cell
Source: Modified in SI units from Ref. 14
Fig 3. Schematic presentation of a solar thermochemical cycle
Fig 4. A Schematic Diagram of the Bayonet-type Reactor22