Energy can be stored in different forms: as mechanical energy (for example, potential energy or rotation energy of a flywheel); in an electric or magnetic field (capacitors and coils, respectively); as chemical energy of reactants and fuels (batteries, petrol or hydrogen); or as nuclear fuel (uranium or deuterium). Chemical and electric energy can be transmitted easily because they both involve electronic Coulomb interaction. Chemical energy is based on the energy of unpaired outer electrons (valence electrons) eager to be stabilized by electrons from other atoms. The hydrogen atom is most attractive because its electron (for charge neutrality) is accompanied by only one proton. Hydrogen thus has the best ratio of valence electrons to protons (and neutrons) of all the periodic table, and the energy gain per electron is very high.Hydrogen is the most abundant element on Earth, but less than 1% is present as molecular hydrogen gas H 2 . The overwhelming majority is chemically bound as H 2 O in water and some is bound to liquid or gaseous hydrocarbons. The clean way to produce hydrogen from water is to use sunlight in combination with photovoltaic cells and water electrolysis (see review in this issue by Grätzel, pages 338-344). Other forms of primary energy and other water-splitting processes are also used: the hydrogen consumed today as a chemical raw material (about 5 ǂ10 10 kg per year worldwide) is to a large extent produced using fossil fuels and the reaction of hydrocarbon chains (-CH 2 -) with H 2 O at high temperatures, which produces H 2 and CO 2 . Direct thermal dissociation of H 2 O requires temperatures higher than 2,000 ᑻC (>900 ᑻC with a Pt/Ru catalyst).The chemical energy per mass of hydrogen ) is at least three times larger than that of other chemical fuels (for example, the equivalent value for liquid hydrocarbons is 47 MJ kg -1 ). Once produced, hydrogen is a clean synthetic fuel: when burnt with oxygen, the only exhaust gas is water vapour, but when burnt with air, lean mixtures have to be used to avoid the formation of nitrogen oxides. Whether hydrogen can be considered a clean form of energy on a global scale depends on the primary energy that is used to split water 1 . The availability of free energy is often unsafe. The mechanical energy of a 1,000-kg car running out of control at 40 km h -1 can kill pedestrians. The process of burning hydrogen can be done in an efficient and controlled way to liberate energy at a desirable rate, or in an uncontrolled way with the potential to cause damage. For historical reasons hydrogen has a bad reputation, which is not altogether justified: a more recent analysis 2 of the Hindenburg catastrophe shows that the air ship caught fire because of a highly flammable skin material and not because of the hydrogen gas it contained. The safety of hydrogen relies on its high volatility and non-toxicity.Today, many scientists and engineers, some companies, governmental and non-governmental agencies and even finance institutions are convinced that hydrogen's physical...

The investigation of the field emission (FE) properties of carbon nanotube (CNT) films by a scanning anode FE apparatus, reveals a strong dependence on the density and morphology of the CNT deposit. Large differences between the microscopic and macroscopic current and emission site densities are observed, and explained in terms of a variation of the field enhancement factor β. As a consequence, the emitted current density can be optimized by tuning the density of CNTs. Films with medium densities (on the order of 107 emitters/cm2, according to electrostatic calculations) show the highest emitted current densities.

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Energy can be stored in different forms: as mechanical energy (for example, potential energy or rotation energy of a flywheel); in an electric or magnetic field (capacitors and coils, respectively); as chemical energy of reactants and fuels (batteries, petrol or hydrogen); or as nuclear fuel (uranium or deuterium). Chemical and electric energy can be transmitted easily because they both involve electronic Coulomb interaction. Chemical energy is based on the energy of unpaired outer electrons (valence electrons) eager to be stabilized by electrons from other atoms. The hydrogen atom is most attractive because its electron (for charge neutrality) is accompanied by only one proton. Hydrogen thus has the best ratio of valence electrons to protons (and neutrons) of all the periodic table, and the energy gain per electron is very high.Hydrogen is the most abundant element on Earth, but less than 1% is present as molecular hydrogen gas H 2 . The overwhelming majority is chemically bound as H 2 O in water and some is bound to liquid or gaseous hydrocarbons. The clean way to produce hydrogen from water is to use sunlight in combination with photovoltaic cells and water electrolysis (see review in this issue by Grätzel, pages 338-344). Other forms of primary energy and other water-splitting processes are also used: the hydrogen consumed today as a chemical raw material (about 5 ǂ10 10 kg per year worldwide) is to a large extent produced using fossil fuels and the reaction of hydrocarbon chains (-CH 2 -) with H 2 O at high temperatures, which produces H 2 and CO 2 . Direct thermal dissociation of H 2 O requires temperatures higher than 2,000 ᑻC (>900 ᑻC with a Pt/Ru catalyst).The chemical energy per mass of hydrogen ) is at least three times larger than that of other chemical fuels (for example, the equivalent value for liquid hydrocarbons is 47 MJ kg -1 ). Once produced, hydrogen is a clean synthetic fuel: when burnt with oxygen, the only exhaust gas is water vapour, but when burnt with air, lean mixtures have to be used to avoid the formation of nitrogen oxides. Whether hydrogen can be considered a clean form of energy on a global scale depends on the primary energy that is used to split water 1 . The availability of free energy is often unsafe. The mechanical energy of a 1,000-kg car running out of control at 40 km h -1 can kill pedestrians. The process of burning hydrogen can be done in an efficient and controlled way to liberate energy at a desirable rate, or in an uncontrolled way with the potential to cause damage. For historical reasons hydrogen has a bad reputation, which is not altogether justified: a more recent analysis 2 of the Hindenburg catastrophe shows that the air ship caught fire because of a highly flammable skin material and not because of the hydrogen gas it contained. The safety of hydrogen relies on its high volatility and non-toxicity.Today, many scientists and engineers, some companies, governmental and non-governmental agencies and even finance institutions are convinced that hydrogen's physical...