Energy densities of various energy storage materials and technologies, illustrating the respective volumetric and gravimetric densities. 

Contexts in source publication

Context 1

... direct conversion of sunlight through a photocatalytic process that utilises solar energy to split water directly to its constituents, hydrogen and oxygen, without the attendant use of electricity. 17 This ideal production route, therefore, is to harness the power of the sun to split water from the oceans. The solar photodecom- position of water is probably the only major and long-term solution to a CO 2 -free route for the mass production of the huge volumes of hydrogen needed for the future hydrogen economy. Realisation of low-cost and efficient direct production of hydrogen from solar energy requires major scientific breakthroughs in developing innovative materials, emerging physical phenomena, novel synthetic techniques and new design concepts. The challenge is to develop a photocatalyst which is durable, both against the attack of the highly reactive oxygen species formed during the splitting of water and against damage from the light itself, is cheap to produce in large quantities, and makes use of abundant, visible-spectrum photons rather than ultraviolet light. A promising approach is to make use of nanostructured catalysts. The energy gap of many materials, in particular inorganic materials, may be altered by size effects in nanoparticles, possibly allowing us to tune photocatalytic materials to absorb wholly in the visible spectrum. Materials currently under investigation include nanostructures of titania, 18,19 cadmium (selenide/sulfide) supported in mesoporous silica, 20 and metal nanoparticles. An intriguing possibility is also to recruit biological catalytic molecules, that is, enzymes, and modify them by genetic engineering. A recent development in this field has been the modifi- cation of a bacterial hydrogenase (which produces hydrogen from formate) to increase by thirty-fold its rate of hydrogen production. 21,22 The energetic efficiency of photosynthesis in plants, which is relevant to the economics of biofuel production, depends upon an inefficient initial catalysis step by the enzyme Rubisco, 23 and efforts are under way to develop more efficient forms of the enzyme. 24 Nuclear energy can be used to produce hydrogen by the thermochemical electrolysis of water and this method has the potential to meet the future high demand for hydrogen. Next generation advanced nuclear reactors are also being developed that will enable high-temperature water electrolysis (with less electrical energy needed) or thermochemical cycles that will use heat and a chemical process to dissociate water. One additional benefit of high-temperature water electrolysis is the possibility to shift the main production between electricity and hydrogen during the day and night demand cycles thus using the extra energy output at night for storing energy in the form of hydrogen. Fusion power, if successfully developed, would also be a clean, abundant, and carbon-free resource for hydrogen production. The economic viability of any hydrogen production method will be strongly affected by regional factors (availability of renewable energy sources, delivery approaches, taxation etc. ). It is anticipated that in any fully developed hydrogen economy, hydrogen will be produced both centrally in large energy complexes and also locally – in refuelling stations, communities, and on-site at customers’ premises. 15,16 The development and implementation of such a diverse range of hydrogen production techniques 15,25 requires substantial technological advances, and with them the social acceptability of any such development. 26 In an extensive survey, Dutton has examined the principal technologies for producing hydrogen, for example from wind energy. 27 From a particular UK perspective, he finds that a fully developed hydrogen economy from renewable, carbon-free sources would require at least a doubling of current electrical energy demand! On-board hydrogen storage for fuel cell vehicles is another particularly challenging area, which is widely recognised as a critical enabling technology for the successful commercializa- tion and market acceptance of hydrogen-powered vehicles. 3–5,7,9 The accepted target is a reliable, safe and economical method to store between 4 to 5 kg of hydrogen (sufficient for a drive range of some 400–500 km) whilst minimising volume, weight, storage energy, cost and refuelling time and, just as important, in providing prompt hydrogen release on demand. At present, the critical technological barrier centres on the lack of a safe, low-weight, low-cost and high performance hydrogen storage method with a high energy density. 9,12 Hydrogen contains more energy on a weight-for-weight basis than any other element or substance. Unfortunately, since it is the lightest chemical element, it also has a very low energy density per unit volume. Present hydrogen storage options for automotive applications have centred upon high-pressure (up to 700 bar) gas containers or cryogenically cooled (liquefied) fluid hydrogen (Fig. 3). One downside of these methods is a significant energy penalty – up to 20% of the energy content of hydrogen is required to compress the gas and up to 40% to liquefy it. Another crucial issue that confronts the use of high-pressure and cryogenic storage centres on public perception and acceptability is associated with the use of pressurised gas and liquid hydrogen containment. Hydrogen storage requires a major technological breakthrough and this is likely to occur in the most viable alternative to compressed and liquid hydrogen, namely the storage of hydrogen in solids or liquids. Interestingly, several classes of solid state hydrogen storage materials demonstrate higher energy density than those of liquid hydrogen (Fig. 3) and they are centred on metal hydrides formed from alloys ( e.g. LaNi 5 ) and chemical hydrides of the light chemical elements ( e.g. MgH 2 or LiBH 4 etc. ). 28 Solid state hydrogen storage promises a real and significant breakthrough, but much research and development is still needed to understand the physical and chemical processes governing hydrogen storage and release, and to improve hydrogen absorption/desorption characteristics. Again, this is a complex challenge, necessitating close interactions between chemists, material scientists, physicists and engineers; a most attractive feature of this challenge is the ‘‘natural’’ multidisciplinarity of the problem at hand. From such a multidisciplinary approach, one hopes to see the evolution of new concepts and ideas, for as the recent US National Academies report notes 5 ‘‘ . success in overcoming the major stumbling block of on-board storage is critical for the future of transportation use of fuel cells.’’ Once hydrogen has been produced and stored, it is most efficiently used for power generation in a fuel cell, an electrochemical device in which hydrogen is combined with oxygen – without combustion – to produce dc electricity. Because fuel cells are not subject to the limitations of the Carnot cycle, they convert fuel into electricity at more than double the efficiency of internal combustion engines. In transportation, hydrogen fuel cell engines operate at an efficiency of up to 65%, compared to 25% for present-day petrol driven car engines. When heat generated in fuel cells is also utilised in Combined Heat and Power (CHP) systems, an overall efficiency in excess of 85% can be achieved. 12 The chemical energy density of hydrogen is significantly higher than that found in electric battery materials 29–31 (Fig. 3) and hydrogen fuel cells could also deliver much longer operational lifetime than that of electric batteries. Unlike internal combustion engines or turbines, fuel cells demonstrate high efficiency across most of their output power range. This attractive scalability makes fuel cells ideal for a variety of applications from mobile phones to large-scale power generation. The hydrogen economy will therefore involve the widespread use of fuel cells for efficient, clean, quiet and local generation of electricity for both static and transport uses (Fig. 2). Several types of fuel cells suitable for different energy applications at varying scales have been developed but all share the basic design of two electrodes (anode and cathode) sepa- rated by a solid or liquid electrolyte. Hydrogen (or a hydrogen- containing fuel) and oxygen are fed into the anode and cathode of the fuel cell and the electrochemical reactions assisted by catalysts take place at the electrodes. The electrolyte enables the transport of ions between the electrodes while the excess electrons flow through an external circuit to provide electrical current. Fuel cells are classified according to the nature of their electrolyte, which also determines their operating temperature, the type of fuel and a range of applications. The electrolyte can be acid, base, salt or a solid ceramic or polymer that conducts ions. However, various major technological hurdles must still be overcome before fuel cells can compete effectively with conventional energy conversion technologies. The key scientific and technical challenges facing fuel cells are cost reduction and increased durability and performance of materials and components. This requires intensive research in the development of safe operation of improved or new materials and could provide the commercial viability of fuel cells in both stationary and mobile applications. Hydrogen fuel cell cars, currently the focus of intense development activity worldwide, are not expected to reach mass market until 2015, or possibly beyond. A recent status report 31 on hydrogen fuel cell vehicles from General Motors points out that cost, range and refuelling figures are presently inferior as compared to internal combustion engines. However, the authors also note that the current figures are much better than those of advanced battery electric vehicles and projections show that hydrogen fuel cells can ultimately become cost ...

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... to dissociate water. One additional benefit of high-temperature water electrolysis is the possibility to shift the main production between electricity and hydrogen during the day and night demand cycles thus using the extra energy output at night for storing energy in the form of hydrogen. Fusion power, if successfully developed, would also be a clean, abundant, and carbon-free resource for hydrogen production. The economic viability of any hydrogen production method will be strongly affected by regional factors (availability of renewable energy sources, delivery approaches, taxation etc. ). It is anticipated that in any fully developed hydrogen economy, hydrogen will be produced both centrally in large energy complexes and also locally – in refuelling stations, communities, and on-site at customers’ premises. 15,16 The development and implementation of such a diverse range of hydrogen production techniques 15,25 requires substantial technological advances, and with them the social acceptability of any such development. 26 In an extensive survey, Dutton has examined the principal technologies for producing hydrogen, for example from wind energy. 27 From a particular UK perspective, he finds that a fully developed hydrogen economy from renewable, carbon-free sources would require at least a doubling of current electrical energy demand! On-board hydrogen storage for fuel cell vehicles is another particularly challenging area, which is widely recognised as a critical enabling technology for the successful commercializa- tion and market acceptance of hydrogen-powered vehicles. 3–5,7,9 The accepted target is a reliable, safe and economical method to store between 4 to 5 kg of hydrogen (sufficient for a drive range of some 400–500 km) whilst minimising volume, weight, storage energy, cost and refuelling time and, just as important, in providing prompt hydrogen release on demand. At present, the critical technological barrier centres on the lack of a safe, low-weight, low-cost and high performance hydrogen storage method with a high energy density. 9,12 Hydrogen contains more energy on a weight-for-weight basis than any other element or substance. Unfortunately, since it is the lightest chemical element, it also has a very low energy density per unit volume. Present hydrogen storage options for automotive applications have centred upon high-pressure (up to 700 bar) gas containers or cryogenically cooled (liquefied) fluid hydrogen (Fig. 3). One downside of these methods is a significant energy penalty – up to 20% of the energy content of hydrogen is required to compress the gas and up to 40% to liquefy it. Another crucial issue that confronts the use of high-pressure and cryogenic storage centres on public perception and acceptability is associated with the use of pressurised gas and liquid hydrogen containment. Hydrogen storage requires a major technological breakthrough and this is likely to occur in the most viable alternative to compressed and liquid hydrogen, namely the storage of hydrogen in solids or liquids. Interestingly, several classes of solid state hydrogen storage materials demonstrate higher energy density than those of liquid hydrogen (Fig. 3) and they are centred on metal hydrides formed from alloys ( e.g. LaNi 5 ) and chemical hydrides of the light chemical elements ( e.g. MgH 2 or LiBH 4 etc. ). 28 Solid state hydrogen storage promises a real and significant breakthrough, but much research and development is still needed to understand the physical and chemical processes governing hydrogen storage and release, and to improve hydrogen absorption/desorption characteristics. Again, this is a complex challenge, necessitating close interactions between chemists, material scientists, physicists and engineers; a most attractive feature of this challenge is the ‘‘natural’’ multidisciplinarity of the problem at hand. From such a multidisciplinary approach, one hopes to see the evolution of new concepts and ideas, for as the recent US National Academies report notes 5 ‘‘ . success in overcoming the major stumbling block of on-board storage is critical for the future of transportation use of fuel cells.’’ Once hydrogen has been produced and stored, it is most efficiently used for power generation in a fuel cell, an electrochemical device in which hydrogen is combined with oxygen – without combustion – to produce dc electricity. Because fuel cells are not subject to the limitations of the Carnot cycle, they convert fuel into electricity at more than double the efficiency of internal combustion engines. In transportation, hydrogen fuel cell engines operate at an efficiency of up to 65%, compared to 25% for present-day petrol driven car engines. When heat generated in fuel cells is also utilised in Combined Heat and Power (CHP) systems, an overall efficiency in excess of 85% can be achieved. 12 The chemical energy density of hydrogen is significantly higher than that found in electric battery materials 29–31 (Fig. 3) and hydrogen fuel cells could also deliver much longer operational lifetime than that of electric batteries. Unlike internal combustion engines or turbines, fuel cells demonstrate high efficiency across most of their output power range. This attractive scalability makes fuel cells ideal for a variety of applications from mobile phones to large-scale power generation. The hydrogen economy will therefore involve the widespread use of fuel cells for efficient, clean, quiet and local generation of electricity for both static and transport uses (Fig. 2). Several types of fuel cells suitable for different energy applications at varying scales have been developed but all share the basic design of two electrodes (anode and cathode) sepa- rated by a solid or liquid electrolyte. Hydrogen (or a hydrogen- containing fuel) and oxygen are fed into the anode and cathode of the fuel cell and the electrochemical reactions assisted by catalysts take place at the electrodes. The electrolyte enables the transport of ions between the electrodes while the excess electrons flow through an external circuit to provide electrical current. Fuel cells are classified according to the nature of their electrolyte, which also determines their operating temperature, the type of fuel and a range of applications. The electrolyte can be acid, base, salt or a solid ceramic or polymer that conducts ions. However, various major technological hurdles must still be overcome before fuel cells can compete effectively with conventional energy conversion technologies. The key scientific and technical challenges facing fuel cells are cost reduction and increased durability and performance of materials and components. This requires intensive research in the development of safe operation of improved or new materials and could provide the commercial viability of fuel cells in both stationary and mobile applications. Hydrogen fuel cell cars, currently the focus of intense development activity worldwide, are not expected to reach mass market until 2015, or possibly beyond. A recent status report 31 on hydrogen fuel cell vehicles from General Motors points out that cost, range and refuelling figures are presently inferior as compared to internal combustion engines. However, the authors also note that the current figures are much better than those of advanced battery electric vehicles and projections show that hydrogen fuel cells can ultimately become cost competitive with internal combustion engines. Any possible transition to a widespread hydrogen economy will probably require many decades. However, one notes that Iceland plans to create the world’s first hydrogen economy by 2030. The timescale and evolution of such a transition is the focus of many ‘‘roadmaps’’ emanating from Japan, Canada and the EU (amongst many others). 2–4,32,33 For example, the European Commission has very recently endorsed the concept of a Hydrogen and Fuel Cell Technology Platform with the expenditure of V 2.8 billion over a period of ten years, and the European Parliament has voted overwhelmingly in favour of such a major advance in energy policy. 34 The introduction of hydrogen as an energy carrier has also been identified 33 as a possible strategy for moving the UK towards its voluntary adopted targets for carbon dioxide reduction of 60% of current levels by 2050. Table 1 summarises the forecasts of several roadmaps for deployment status and targets for hydrogen technologies and fuel cell applications today and up to 2050. One can therefore summarise the key scientific and technological challenges for any transition to a hydrogen economy; these are: (a) The cost of hydrogen production must be lowered to a level comparable with the energy cost of petrol, a target which becomes easier to achieve as the cost of hydrocarbon fuels continuously increases. Such a ‘‘cost equivalence’’ is necessary in order that the penetration of hydrogen into the fuel sector becomes economically possible. This initial step may not neces- sarily involve truly sustainable methods of hydrogen production; transitional methods such as hydrogen production from hydrocarbons may still be useful in enabling (or demonstrating) the transition from an economy dominated by carbon-fuel- consuming combustion systems to hydrogen fuel cell systems. (b) A sustainable and CO 2 -free route to the mass production of hydrogen at a competitive cost is vital in the long term. As we have seen, it is likely that solar methods, either using electrolysis by photoelectricity or using photocatalytic methods, will be the dominant technology; however, wind, wave, geothermal and nuclear electricity generation will all contribute. (c) A safe and efficient national infrastructure for hydrogen delivery and distribution must be developed. Industrialised nations already have systems in place which achieve safe and efficient distribution of gaseous, liquid and solid fossil fuels; however, the fundamentally different ...

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... and new design concepts. The challenge is to develop a photocatalyst which is durable, both against the attack of the highly reactive oxygen species formed during the splitting of water and against damage from the light itself, is cheap to produce in large quantities, and makes use of abundant, visible-spectrum photons rather than ultraviolet light. A promising approach is to make use of nanostructured catalysts. The energy gap of many materials, in particular inorganic materials, may be altered by size effects in nanoparticles, possibly allowing us to tune photocatalytic materials to absorb wholly in the visible spectrum. Materials currently under investigation include nanostructures of titania, 18,19 cadmium (selenide/sulfide) supported in mesoporous silica, 20 and metal nanoparticles. An intriguing possibility is also to recruit biological catalytic molecules, that is, enzymes, and modify them by genetic engineering. A recent development in this field has been the modifi- cation of a bacterial hydrogenase (which produces hydrogen from formate) to increase by thirty-fold its rate of hydrogen production. 21,22 The energetic efficiency of photosynthesis in plants, which is relevant to the economics of biofuel production, depends upon an inefficient initial catalysis step by the enzyme Rubisco, 23 and efforts are under way to develop more efficient forms of the enzyme. 24 Nuclear energy can be used to produce hydrogen by the thermochemical electrolysis of water and this method has the potential to meet the future high demand for hydrogen. Next generation advanced nuclear reactors are also being developed that will enable high-temperature water electrolysis (with less electrical energy needed) or thermochemical cycles that will use heat and a chemical process to dissociate water. One additional benefit of high-temperature water electrolysis is the possibility to shift the main production between electricity and hydrogen during the day and night demand cycles thus using the extra energy output at night for storing energy in the form of hydrogen. Fusion power, if successfully developed, would also be a clean, abundant, and carbon-free resource for hydrogen production. The economic viability of any hydrogen production method will be strongly affected by regional factors (availability of renewable energy sources, delivery approaches, taxation etc. ). It is anticipated that in any fully developed hydrogen economy, hydrogen will be produced both centrally in large energy complexes and also locally – in refuelling stations, communities, and on-site at customers’ premises. 15,16 The development and implementation of such a diverse range of hydrogen production techniques 15,25 requires substantial technological advances, and with them the social acceptability of any such development. 26 In an extensive survey, Dutton has examined the principal technologies for producing hydrogen, for example from wind energy. 27 From a particular UK perspective, he finds that a fully developed hydrogen economy from renewable, carbon-free sources would require at least a doubling of current electrical energy demand! On-board hydrogen storage for fuel cell vehicles is another particularly challenging area, which is widely recognised as a critical enabling technology for the successful commercializa- tion and market acceptance of hydrogen-powered vehicles. 3–5,7,9 The accepted target is a reliable, safe and economical method to store between 4 to 5 kg of hydrogen (sufficient for a drive range of some 400–500 km) whilst minimising volume, weight, storage energy, cost and refuelling time and, just as important, in providing prompt hydrogen release on demand. At present, the critical technological barrier centres on the lack of a safe, low-weight, low-cost and high performance hydrogen storage method with a high energy density. 9,12 Hydrogen contains more energy on a weight-for-weight basis than any other element or substance. Unfortunately, since it is the lightest chemical element, it also has a very low energy density per unit volume. Present hydrogen storage options for automotive applications have centred upon high-pressure (up to 700 bar) gas containers or cryogenically cooled (liquefied) fluid hydrogen (Fig. 3). One downside of these methods is a significant energy penalty – up to 20% of the energy content of hydrogen is required to compress the gas and up to 40% to liquefy it. Another crucial issue that confronts the use of high-pressure and cryogenic storage centres on public perception and acceptability is associated with the use of pressurised gas and liquid hydrogen containment. Hydrogen storage requires a major technological breakthrough and this is likely to occur in the most viable alternative to compressed and liquid hydrogen, namely the storage of hydrogen in solids or liquids. Interestingly, several classes of solid state hydrogen storage materials demonstrate higher energy density than those of liquid hydrogen (Fig. 3) and they are centred on metal hydrides formed from alloys ( e.g. LaNi 5 ) and chemical hydrides of the light chemical elements ( e.g. MgH 2 or LiBH 4 etc. ). 28 Solid state hydrogen storage promises a real and significant breakthrough, but much research and development is still needed to understand the physical and chemical processes governing hydrogen storage and release, and to improve hydrogen absorption/desorption characteristics. Again, this is a complex challenge, necessitating close interactions between chemists, material scientists, physicists and engineers; a most attractive feature of this challenge is the ‘‘natural’’ multidisciplinarity of the problem at hand. From such a multidisciplinary approach, one hopes to see the evolution of new concepts and ideas, for as the recent US National Academies report notes 5 ‘‘ . success in overcoming the major stumbling block of on-board storage is critical for the future of transportation use of fuel cells.’’ Once hydrogen has been produced and stored, it is most efficiently used for power generation in a fuel cell, an electrochemical device in which hydrogen is combined with oxygen – without combustion – to produce dc electricity. Because fuel cells are not subject to the limitations of the Carnot cycle, they convert fuel into electricity at more than double the efficiency of internal combustion engines. In transportation, hydrogen fuel cell engines operate at an efficiency of up to 65%, compared to 25% for present-day petrol driven car engines. When heat generated in fuel cells is also utilised in Combined Heat and Power (CHP) systems, an overall efficiency in excess of 85% can be achieved. 12 The chemical energy density of hydrogen is significantly higher than that found in electric battery materials 29–31 (Fig. 3) and hydrogen fuel cells could also deliver much longer operational lifetime than that of electric batteries. Unlike internal combustion engines or turbines, fuel cells demonstrate high efficiency across most of their output power range. This attractive scalability makes fuel cells ideal for a variety of applications from mobile phones to large-scale power generation. The hydrogen economy will therefore involve the widespread use of fuel cells for efficient, clean, quiet and local generation of electricity for both static and transport uses (Fig. 2). Several types of fuel cells suitable for different energy applications at varying scales have been developed but all share the basic design of two electrodes (anode and cathode) sepa- rated by a solid or liquid electrolyte. Hydrogen (or a hydrogen- containing fuel) and oxygen are fed into the anode and cathode of the fuel cell and the electrochemical reactions assisted by catalysts take place at the electrodes. The electrolyte enables the transport of ions between the electrodes while the excess electrons flow through an external circuit to provide electrical current. Fuel cells are classified according to the nature of their electrolyte, which also determines their operating temperature, the type of fuel and a range of applications. The electrolyte can be acid, base, salt or a solid ceramic or polymer that conducts ions. However, various major technological hurdles must still be overcome before fuel cells can compete effectively with conventional energy conversion technologies. The key scientific and technical challenges facing fuel cells are cost reduction and increased durability and performance of materials and components. This requires intensive research in the development of safe operation of improved or new materials and could provide the commercial viability of fuel cells in both stationary and mobile applications. Hydrogen fuel cell cars, currently the focus of intense development activity worldwide, are not expected to reach mass market until 2015, or possibly beyond. A recent status report 31 on hydrogen fuel cell vehicles from General Motors points out that cost, range and refuelling figures are presently inferior as compared to internal combustion engines. However, the authors also note that the current figures are much better than those of advanced battery electric vehicles and projections show that hydrogen fuel cells can ultimately become cost competitive with internal combustion engines. Any possible transition to a widespread hydrogen economy will probably require many decades. However, one notes that Iceland plans to create the world’s first hydrogen economy by 2030. The timescale and evolution of such a transition is the focus of many ‘‘roadmaps’’ emanating from Japan, Canada and the EU (amongst many others). 2–4,32,33 For example, the European Commission has very recently endorsed the concept of a Hydrogen and Fuel Cell Technology Platform with the expenditure of V 2.8 billion over a period of ten years, and the European Parliament has voted overwhelmingly in favour of such a major advance in energy policy. 34 The introduction of hydrogen as an energy carrier has ...