Since the beginning of the twenty-first century the limitations of the fossil age with regard to the continuing growth of energy demand, the peaking mining rate of oil, the growing impact of CO2 emissions on the environment and the dependency of the economy in the industrialized world on the availability of fossil fuels became very obvious. A major change in the energy economy from fossil energy carriers to renewable energy fluxes is necessary. The main challenge is to efficiently convert renewable energy into electricity and the storage of electricity or the production of a synthetic fuel. Hydrogen is produced from water by electricity through an electrolyser. The storage of hydrogen in its molecular or atomic form is a materials challenge. Some hydrides are known to exhibit a hydrogen density comparable to oil; however, these hydrides require a sophisticated storage system. The system energy density is significantly smaller than the energy density of fossil fuels. An interesting alternative to the direct storage of hydrogen are synthetic hydrocarbons produced from hydrogen and CO2 extracted from the atmosphere. They are CO2 neutral and stored like fossil fuels. Conventional combustion engines and turbines can be used in order to convert the stored energy into work and heat.
1. Energy and development
(a) Growing energy demand
Human beings developed on Earth based on plants, i.e. biomass, as the only energy carrier. The consumption of plants did not change the environment because the carbon dioxide that was liberated by the humans and animals was reabsorbed by the plants during the process of photosynthesis. In 1698, Thomas Savery built a simple steam engine able to pump water. Thomas Newcomen in 1712 developed an improved steam engine (Wilson 1981) able to deliver mechanical work. For the first time, a non-living machine worked for human beings. However, the steam engines of Savery and Newcomer were neither very powerful nor energy efficient. James Watt significantly improved the steam engine and, because the energy efficiency was increased by a factor of 4, the steam engine was widely introduced for the conversion of heat into mechanical work and enabled the start of industrialization (figure 1).
The energy for the steam engine was found in the form of mineral coal. The world energy consumption increased from 5×1012 kWh yr−1 in 1860 to 1.2×1014 kWh yr−1 today. Approximately 1.0×1014 kWh yr−1 (80%) is based on fossil fuels (coal, oil and gas). The population of human beings increased during the twentieth century by a factor of 6, but the energy consumption increased by a factor of 80 (Eidgenössischen Technischen Hochschule Zürich 2000). The worldwide average continuous power consumption today is 2 kW per capita.
(b) The efficiency paradox
The invention and development of the steam engine is an excellent historical example of the phenomenon that an increase in the efficiency of an engine leads to an increase in the overall energy consumption. In economics, the Jevons Paradox is an observation made by William Stanley Jevons that, as technological improvements increase the efficiency with which a resource is used, the total consumption of that resource tends to increase, rather than decrease. Increased energy efficiency tends to increase energy consumption by two means. First, increased energy efficiency makes the use of energy relatively cheaper, thus encouraging increased use (the rebound effect). Second, increased energy efficiency leads to increased economic growth, which increases energy use for the whole economy (Saunders 1992).
(c) Limited resources of fossil fuels
Most geologists view crude oil, like coal and natural gas, as the product of compression and heating of ancient vegetation over geological time scales (biogenic theory). According to this theory, it is formed from the decayed remains of prehistoric marine animals and plants. Three conditions must be present for oil reservoirs to form: a rich source rock, a migration conduit and a trap (seal) that forms the reservoir. The oil reserves on Earth are finite and will, therefore, not last for ever. The Hubbert peak theory (Hubbert 1956), also known as peak oil, is a theory concerning the long-term rate of production of conventional oil and other fossil fuels. It assumes that oil reserves are not replenishable (i.e. that abiogenic replenishment, if it exists at all, is negligible), and predicts that future world oil production must inevitably reach a peak and then decline as these reserves are exhausted. For various reasons (perhaps most importantly, the lack of transparency in accounting of global oil reserves), it is difficult to predict the oil peak in any given region. On the basis of available production data, proponents have previously (and incorrectly) predicted the peak for the world to be in years 1989, 1995, or 1995–2000. However, these predictions date from before the recession of the early 1980s, and the consequent reduction in global consumption, the effect of which was to delay the date of any peak by several years. A new prediction by Goldman Sachs (Engdahl 2008) gives 2007 for oil and some time later for natural gas. Just as the 1971 US peak in oil production was only clearly recognized after the fact, a peak in world production will be difficult to discern until production clearly drops off.
(d) Environmental impact
The consumption of fossil fuels together with deforestation leads to the liberation of 7×1012 kg yr−1 of carbon in the form of CO2. The two major natural sinks of carbon dioxide are plants, which absorb an additional 2×1012 kg yr−1 carbon by the photosynthesis process, and oceans, in which the same amount of carbon is dissolved. Therefore, the net increase in carbon in the form of CO2 in the atmosphere owing to human activities is approximately 3×1012 kg yr−1. This corresponds to an annual increase of 0.4 per cent of the CO2 concentration in the atmosphere.
Svante Arrhenius (1896) determined that a doubling of CO2 in the atmosphere would lead to an average temperature increase of 5–6°C. According to basic science, observational sensitivity studies (Petit et al. 1999) and the climate models referenced by the IPCC (Working Group I 2007), temperatures may increase by 1.4–5.8°C between 1990 and 2100. This is expected to result in other climate changes, including rises in sea level and changes in the amount and pattern of precipitation. Such changes may increase the frequency and intensity of extreme weather events such as floods, droughts, heat waves and hurricanes; these changes themselves cause changes in agricultural yield, glacier retreat, reduced summer streamflows or contribute to biological extinctions.
(e) Economic dependency
The demand for fossil fuels has a strong impact on social, political and economic interactions between the various countries. For example, two-thirds of the crude oil reserves are located in the Middle East region, but most of the petrol is consumed in the USA, Europe and Japan. Fossil energy has a very strong influence on the development of a society and the standard of living. There is a dependency of the gross national product on the average amount of energy consumed per capita. This leads to the conclusion that today the economic gain of the industrialized world is, to a significant extent, correlated to the energy consumption from fossil fuels. Furthermore, economic growth directly depends on the availability of fossil fuels, and the recent steep increase in the oil price coincided with a worldwide economic crisis (figure 2).
In summary, the four major challenges for the future energy economy are:
— Energy demand: increase in the world energy demand owing to the industrialization of the largest countries (J.-M. Martin-Amouroux, private communication 2002; Schilling et al. 1977).
— Limited resources: the rate of discovered oil has already decreased since 1970 but the mining rate has increased. Peak oil is expected to take place between 2008 and 2012 (Campbell 2003).
— Climate change: the average temperature on Earth has increased owing to the increase in the CO2 concentration in the atmosphere (Mann et al. 1999).
— Economic dependency: energy per capita versus gross domestic product per capita (http://muller.lbl.gov/teaching/physics10/PffP.textbook/PffP-10-climate.files/image022.gif) grows linearly in the beginning of industrialization and then saturates.
Fossil fuels are naturally occurring energy carriers and are, therefore, in an economic context called primary energy sources. In the physical context, the only primary energy resources are solar light (heat, precipitation, wind), geothermal heat and the tides. These sources deliver energy fluxes (power flow) that vary according to the day/night cycle and the seasonal cycles and have an underlying spatial distribution on Earth. Therefore, natural energy fluxes have to be processed for residential, industrial and mobile energy needs by:
— conversion into a usable form of energy, i.e. heat and work (electricity),
— storage in a synthetic energy carrier, e.g. hydrogen, and
— transport to the consumer and for mobility.
This involves a technical process and, therefore, an investment. The economic system that is based on fossil energy has to be changed while fossil fuels are still available. Otherwise, the investments necessary for the change to renewable energy supply will no longer be possible. In order to change from fossil to renewable energy, the world economy has to be converted to a sustainable economy based on investments instead of mining resources. Renewable energy requires, in general, an investment of approximately the energy produced in 5 years. This means an investment of about five times the annual energy cost. While the investment in energy is defined by technology and is therefore constant, the financial investment directly depends on the cost of the fossil fuels. The investment corresponds today to approximately 25 t of oil in a highly industrialized country. Each person, e.g. in Europe, has to invest about 250 barrels of crude oil. This depends on the price of fossil fuels but is between 20 and 50 per cent of the annual income in Europe.
(a) Hydrogen cycle
The natural cycles in the atmosphere are able to transport only a few compounds, namely oxygen, nitrogen, water and CO2. Therefore, the combustion product of any kind of fuel has to be water, CO2 or nitrogen. Hydrogen is a fuel that produces only water as a combustion product.
The hydrogen cycle is analogous to the natural carbon cycle, except that no carbon is involved. The advantage is that, in contrast to biomass, no living matter is necessary for the production of hydrogen, but the disadvantage is that hydrogen is a gas at ambient conditions.
Electricity from a renewable energy source is used for the electrolysis of water. Hydrogen as a gas occupies a large volume (11 m3 kg−1) under ambient conditions, for storage. The main challenge in hydrogen storage is to reduce the volume of gas in equilibrium with the environment. Finally, the hydrogen is reacted with oxygen to produce water in an internal combustion engine or a fuel cell and to release the stored energy (figure 3).
The production of hydrogen requires renewable energy, i.e. solar energy or geothermal energy. The solar constant (intensity at the top of the atmosphere) is 1369 W m−2. Approximately 50 per cent reaches the surface of the Earth, and 50 per cent of the time it is night. Assuming a conversion efficiency of 10 per cent (e.g. photovoltaic cells), a surface area of 500 000 km2 or 80 m2 per capita (240 m2 per capita for Europeans) is necessary to cover the current world energy demand.
(b) Hydrogen production
Hydrogen is a renewable fuel only if it is produced directly from solar light or indirectly via electricity from a renewable source, e.g. wind power or hydro power. The direct thermal dissociation of H2O requires temperatures greater than 4000 K (Steinfeld & Palumbo 2001). Furthermore, the thermal dissociation of water not only produces H2 but also atomic hydrogen, oxygen and hydroxyl ions. This production method is therefore the subject of research activities and various challenges, e.g. the separation of hydrogen from oxygen at high temperature, have to be solved before the thermal dissociation becomes an applicable method for hydrogen production. Electricity from a renewable energy source can be used for the electrolysis of water. Electrolysis at ambient temperature and ambient pressure requires a minimum voltage of 1.481 V and therefore a minimum energy of 39.7 kWh kg−1 (34.7 kWh kg−1 electricity) hydrogen. Large scale alkaline electrolyser systems today consume approximately 47 kWh kg−1 hydrogen and work at 90°C, i.e. the efficiency is approximately 82 per cent (E. Burkholter 2001, personal communication). High-temperature electrolysis is based on oxygen ion-conducting ceramics. The electrical energy needed to split water at 1000°C is considerably less than electrolysis at 100°C, i.e. the minimum energy required is 34.7 kWh and 25 kWh electricity, respectively.
(c) Hydrogen storage
Hydrogen storage basically implies the reduction of the volume of the hydrogen gas; 1 kg of hydrogen at a temperature of 25°C and a pressure of 1 bar requires a volume of 11 m3. There are six methods to store hydrogen with a high volumetric and gravimetric hydrogen density (table 1).
The most common storage systems are high-pressure gas cylinders with a maximum pressure of 20 MPa. New lightweight composite cylinders have been developed that support pressures up to 80 MPa; therefore, the hydrogen reaches a volumetric density of 36 kg m−3, which is approximately half as much as in its liquid form at the normal boiling point. The gravimetric hydrogen density decreases with increasing pressure owing to the increasing thickness of the walls of the pressure cylinder. The volumetric density of hydrogen increases with pressure and reaches a maximum, depending on the tensile strength of the material, above 100 MPa. However, the gravimetric density decreases with increasing pressure and the maximum gravimetric density is found for zero overpressure! Therefore, the increase in volumetric storage density is sacrificed by the reduction in the gravimetric density in pressurized gas systems. The relatively low hydrogen density together with the very high gas pressures, leakage by diffusion and the cyclic stability of the cylinders are important drawbacks of the technically simple and on the laboratory scale well-established high-pressure storage method.
Liquid hydrogen is stored in cryogenic tanks at 21.2 K at ambient pressure. Owing to the low critical temperature of hydrogen (33 K), liquid hydrogen can only be stored in open systems, because there is no liquid phase above the critical temperature. The pressure in a closed storage system at room temperature could increase to about 104 bar before hydrogen solidifies. The volumetric density of liquid hydrogen at the boiling point (21 K) is 70.8 kg m−3. The challenges for liquid hydrogen storage are improving the energy efficiency of the liquefaction process (currently 10 kWh kg−1 H2, theoretically 4 kWh kg−1 H2) and the thermal insulation of the cryogenic storage vessel in order to reduce the boil-off of hydrogen (typically 0.4% d−1 for tanks with a storage volume of 50 m3, 0.2% for 100 m3). The large amount of energy necessary for liquefaction and the continuous boil-off of hydrogen limits the possible applications for liquid hydrogen storage systems. Applications in which the cost of hydrogen is not an important issue and the hydrogen is consumed within a rather short period of time, e.g. air and space, are possible.
The origins of the physisorption of gas molecules on the surface of a solid are resonant fluctuations of the charge distributions and are therefore called dispersive interactions or van der Waals interactions. In the physisorption process, a gas molecule interacts with several atoms at the surface of the solid. The potential energy of the molecule shows a minimum at a distance of approximately one molecular radius of the adsorbate and the energy minimum (London 1930a,b) is of the order of 0.01–0.1 eV (1–10 kJ mol−1). Owing to the weak interaction, significant physisorption is observed only at low temperatures (less than 100 K). Once a monolayer of adsorbate molecules is formed, the gaseous molecules interact with a surface of the liquid or solid adsorbate. Therefore, the binding energy of the second layer of adsorbate molecules is similar to the latent heat of sublimation or vaporization of the adsorbate. Consequently, the adsorption at a temperature equal to or greater than the boiling point of the adsorbate at a given pressure leads to the adsorption of one single monolayer (Brunauer et al. 1938). In the case of carbon as the substrate and hydrogen as the adsorbate, the maximum specific surface area of carbon is Sspec=1315 m2 g−1 (single side graphene sheet) and the maximum amount of adsorbed hydrogen (monolayer liquid hydrogen at the surface) is mads=3.0 mass%. From this theoretical approximation, we may conclude that the amount of adsorbed hydrogen is proportional to the specific surface area of the adsorbent with mads/Sspec=2.27 10−3 mass% m−2 g and can only be observed at low temperatures, i.e. close to the critical temperature of the gas, such as hydrogen. Despite the numerous publications during the last 10 years about hydogen adsorption in nanoporous materials, carbon nanotubes, metal organic frameworks, polymers and spillover from metal catalysts to carbon, no scientific evidence for a new phenomenon beyond physisorption has been found.
Thomas Graham (1867) described the absorption of hydrogen by the metal palladium. Many metals, intermetallic compounds and alloys in general react with hydrogen and form mainly solid metal–hydrogen compounds. Hydrides exist as ionic, polymeric covalent, volatile covalent and metallic hydrides according to the Allred–Rochow electronegativity (Huheey 1983). Hydrogen reacts at an elevated temperature with many transition metals and their alloys to form hydrides. The electropositive elements are the most reactive, i.e. scandium, yttrium, the lanthanides, the actinides and the members of the titanium and vanadium groups. The lattice structure is that of a typical metal with atoms of hydrogen on the interstitial sites; for this reason, they are also called interstitial hydrides. This type of structure has the limiting compositions MH, MH2 and MH3; the hydrogen atoms fit into octahedral or tetrahedral holes in the metal lattice, or a combination of the two types. Especially interesting are the metallic hydrides of intermetallic compounds, in the simplest case the ternary system ABxHn, because the variation in the elements allows the properties of the hydrides to be tailored. The A element is usually a rare-earth or an alkaline earth metal and tends to form a stable hydride. The B element is often a transition metal and forms only unstable hydrides. Some well-defined ratios of B to A in the intermetallic compound x=0.5, 1, 2, 5 have been found to form hydrides with a hydrogen to metal ratio of up to 2. The metal lattice expands proportional to the hydrogen concentration by approximately 2–3 Å3 per hydrogen atom (Fukai 1989). At greater hydrogen concentrations in the host metal (H/M>0.1), a strong H–H interaction owing to the lattice expansion becomes important and the hydride phase (β-phase) nucleates and grows. Metal hydrides have, owing to the phase transition upon hydrogen absorption, the very useful property of absorbing large amounts of hydrogen at a constant pressure, i.e. the pressure does not increase with the amount of hydrogen absorbed as long as the phase transition takes place. The characteristics of the hydrogen absorption and desorption can be tailored by partial substitution of the constituent elements in the host lattice. Some metal hydrides absorb and desorb hydrogen at ambient temperature and close to atmospheric pressure. A special feature of the metallic hydrides is the very high volumetric density of the hydrogen atoms present in the host lattice. The highest volumetric hydrogen density known today is 150 kg m−3 found in Mg2FeH6 and Al(BH4)3. Both hydrides belong to the complex hydrides and will be discussed below. Metal hydrides are very effective for storing large amounts of hydrogen in a safe and compact way. All the metallic hydrides that work around ambient temperature and atmospheric pressure consist of transition metals and, therefore, the gravimetric hydrogen density is limited to less than 2 mass%. Exploring the properties of the lightweight metal hydrides is still a challenge (figure 4).
Aluminium and boron build complex hydrides of the type M+x[AlH4−]x and M+x[BH4−]x. One electron is almost completely transferred from the cation to each [AlH4−] and [BH4−] anion, while the hydrogen is covalently bound to the aluminium or boron. The alkali, alkali earth and many of the transition metals build together with boron and aluminium a large variety of lightweight metal–hydrogen complexes. They exhibit a gravimetric hydrogen density up to an order of magnitude greater than that of metal hydrides. The number of hydrogen atoms per metal atom is in many cases 2. The hydrogen in the complex hydrides is often located in the corners of a tetrahedron with boron or aluminium in the centre. The negative charge of the anion, [BH4]− and [AlH4]−, is compensated by a cation, e.g. Li or Na. The hydride complexes of borane, the tetrahydroborates (boranate) M[BH4], and of alane, the tetrahydroaluminate (alanate) M[AlH4], are interesting storage materials; however, they are known to be stable and decompose only at elevated temperatures and often above the melting point of the complex. One of the compounds with the highest gravimetric hydrogen density at room temperature known today is Li[BH4] (18 mass%), compared with Be[BH4]2 (28.9 mass%), because beryllium is not a feasible hydride (figure 5).
The stability of the tetrahydroboranates and the tetrahydroalanates is related to the Pauling electronegativity of the cation element. The empirical equation for the enthalpy of formation (Nakamori et al. 2006) of the tetrahydroboranates is ΔH (kJ mol−1 BH4) = 247.4 En−421.2. The borohydride Al[BH4]3 is one of the few liquid hydrides at room temperature and releases hydrogen spontaneously below room temperature. The application of the complex hydrides is a challenge, because complex hydrides decompose into several phases upon hydrogen desorption according to the equation: M+x[BH4−]x→MHx+x/2 B2H6→MHx+xB+3x/2H2. According to Pauling (1929), the stability of the elemental hydride MHx is given by the empirical equation: ΔH=ΔHMM+194 [En(M) − En(H)] (kJ mol−1). The stability of the alkali and alkali earth alanates and boranates varies over a wide range. Most of the physical properties of the complex hydrides have not yet been investigated; some of the transition metal boranates may even be liquids at room temperature.
3. Energy density
The photosynthesis of plants is inefficient because it is based on a two-photon process. The process converts CO2 that is absorbed from the atmosphere by the leaves and water collected by the roots into carbohydrates (-HCOH-) and oxygen and is therefore binding the same amount of CO2 as is released by the combustion of the carbohydrates (CO2 neutral). The maximum annual energy converted in the most efficient plants (C4-plants) is 10 kWh m−2. Furthermore, the cultivation and harvesting is energy intense, especially in heavily industrialized regions of the world, which leads to an additional reduction in the efficiency. The reason for the low efficiency of the energy conversion in plants is the two-photon process in the photosynthesis and the large surface area required for the absorption of CO2. The conversion of solar energy to electricity by photovoltaics is one order of magnitude more efficient and delivers 100 kWh m−2 electricity per year in central Europe and twice that value in the Sahara (North Africa). The storage of electricity in today’s batteries is limited to 0.2 kWh kg−1 and 200 kWh m−3. An average European car today consumes 8.6 l petroleum 100 km−1, which corresponds to about 20 kWh of mechanical energy for 100 km. Therefore, the battery required for a 500 km driving range would have a weight of 500 kg, a volume of 2.5 m3 and would cost 100 000 (1000 kWh−1; K. Hirose, private communication). Alternatively, renewable energy is stored in a synthetic energy carrier, e.g. hydrogen, hydrocarbons or ammonia, and the energy of the synthetic fuel is converted in an internal combustion engine, a turbine or a fuel cell.
(a) Synthetic fuels
According to the empirical models available today the maximum energy density in hydrides is limited to 7 kWh kg−1 and 5800 kWh m−3, which is half of the energy density of hydrocarbons like octane (C8H18). Therefore, the combination of hydrogen from an ideal hydrogen storage material with a fuel cell is able to deliver the same amount of work as the combustion of petroleum in an internal combustion engine. The best of the recent hydrogen storage systems allows between 0.3 and 1.3 kWh kg−1 and 660 and 1320 kWh m−3 to be stored. This is approximately 20 per cent of the theoretical maximum energy density of the ideal hydrogen storage material. The main reason for the limited energy density reached in technical systems is that the best storage materials are known to exhibit 50 per cent of the storage density of the ideal material and the storage system contains a maximum of 50 per cent of the storage material.
Renewable electricity is used to dissociate water into hydrogen and oxygen in an electrolyser, which is an efficient (approx. 82%) process at large scale. The hydrogen can be transported in pipelines or absorbed in hydrides. Furthermore, atomic hydrogen at the surface of a metal hydride may react with CO2 to form methane and water. The methane may subsequently be reacted to longer hydrocarbon chains in order to produce a liquid fuel. This represents an efficient route to synthesize hydrocarbons from hydrogen and CO2. If the CO2 is extracted from the atmosphere by an adsorber, then the hydrocarbons from such a process represent a synthetic fuel and are CO2 neutral (figure 6). Furthermore, the synthetic hydrocarbons contain 75 per cent of the energy in the hydrogen.
Synthetic fuels are stored and used just like the fossil fuels and allow an efficient sustainable energy economy based on renewable energy. Besides the advantage that the well-established technology for the storage, transport and distribution of fuels can be directly used for synthetic hydrocarbons, the process also offers a real CO2 sink. Independent of where the CO2 would be produced and released, it can be extracted anywhere from the atmosphere. This represents a big advantage over the not sustainable and local sequestration of carbon in power plants.
(b) Energy carriers
The origin of primary energy worldwide is more than 75 per cent fossil fuels (hydrocarbons and coal), complemented by 6 per cent nuclear power (fission), 7 per cent hydropower and 12 per cent biomass. The importance of the hydrocarbons is due to the fact that these energy carriers were abundant a century ago, easy and safe to store and they exhibit an especially high energy density. Only hydrides and ammonia exhibit an energy density close to that of fossil fuels, as shown in figure 7; all other energy carriers are at least an order of magnitude less dense in energy except for nuclear reactions, i.e. fission and fusion. However, nuclear reactions are stationary, i.e. very system demanding for the application and continuous delivery of power. In addition, fusion is still not carried out in a controlled efficient way in a reactor. The energy density in batteries is very limited (less than 0.2 kWh kg−1 in a Li ion battery today). Furthermore, batteries exhibit time constants in the range of several minutes to hours. The potential for the improvement of current battery technology is estimated to be approximately 1 kWh kg−1. Such an ultimate battery could be derived from the Zn–air battery by replacing the Zn with lithium in a set-up with a non-aqueous electrolyte. New types of batteries, e.g. metal–air, where the energy is stored in metal clusters suspended in a non-aqueous electrolyte combined with an oxygen (air) electrode, have the potential for a significantly increased energy density and the cluster suspension would allow the battery to be refuelled.
In order to combat limited resources, primary energy has to be converted into heat and electricity. Electricity is partially used directly and also used to produce CO2-neutral synthetic fuels, which do not increase climate change. The science and technology for the conversion of renewable energy as well as the synthesis of fuels from electricity, water and CO2 is of great economic value because developing countries in Asia are becoming large market areas. Therefore, the new technology would allow the challenge of the increasing energy demand to be solved and would liberate heavily industrialized countries in the West from their economic dependence on fossil fuels.
One contribution of 13 to a Discussion Meeting Issue ‘Energy materials to combat climate change’.
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