Hydrogen fuel is a zero-emission fuel when burned with oxygen (if one considers water not as an emission) or used in a contained cell (allowing to react galvanically) also capable of 'reversing' the reaction if needed. It often uses electrochemical cells, or combustion in internal engines, to power vehicles and electric devices. It is also used in the propulsion of spacecraft and might potentially be mass-produced and commercialized for passenger vehicles and aircraft.
Contents
Hydrogen lies in the first group and first period in the periodic table, i.e. it is the first element on the periodic table, making it the lightest element. Since hydrogen gas is so light, it rises in the atmosphere and is therefore rarely found in its pure form, H2. In a flame of pure hydrogen gas, burning in air, the hydrogen (H2) reacts with oxygen (O2) to form water (H2O) and releases energy.
2H2(g) + O2(g) → 2H2O(g)(If carried out in atmospheric air instead of pure oxygen (as is usually the case), hydrogen combustion may yield small amounts of nitrogen oxides, along with the water vapor.)
The energy released enables hydrogen to act as a fuel. In an electrochemical cell, that energy can be used with relatively high efficiency. If it simply is used for heat, the usual thermodynamics limits on the thermal efficiency apply.
Since there is very little free hydrogen gas, hydrogen is in practice only an energy carrier, like electricity, not an energy resource. Hydrogen gas must be produced, and that production always requires more energy than can be retrieved from the gas as a fuel later on. This is a limitation of the physical law of the conservation of energy. Most Hydrogen production induces environmental impacts.
Production
Because pure hydrogen does not occur naturally on Earth in large quantities, it takes a substantial amount of energy in its industrial production. There are different ways to produce it, such as electrolysis and steam-methane reforming process. In electrolysis, electricity is run through water to separate the hydrogen and oxygen atoms. This method can use wind, solar, geothermal, hydro, fossil fuels, biomass, nuclear, and many other energy sources. Obtaining hydrogen from this process is being studied as a viable way to produce it domestically at a low cost. Steam-methane reforming, the current leading technology for producing hydrogen in large quantities, extracts the hydrogen from methane. However, this reaction causes a side production of carbon dioxide and carbon monoxide, which are greenhouse gases and contribute to global warming.
General Chemistry textbooks and laboratory manuals have students prepare hydrogen by reacting many metals (iron, zinc, and other metals but not copper, silver, platinum, or gold) with hydrochloric, sulfuric, phosphoric acid and other acids.
Energy
Hydrogen is locked up in enormous quantities in water, hydrocarbons, and other organic matter. One of the challenges of using hydrogen as a fuel comes from being able to efficiently extract hydrogen from these compounds. Currently, steam reforming, or combining high-temperature steam with natural gas, accounts for the majority of the hydrogen produced. Hydrogen can also be produced from water through electrolysis, but this method is much more energy intensive. Once extracted, hydrogen is an energy carrier (i.e. a store for energy first generated by other means). The energy can be delivered to fuel cells and generate electricity and heat, or burned to run a combustion engine. In each case hydrogen is combined with oxygen to form water. The heat in a hydrogen flame is a radiant emission from the newly formed water molecules. The water molecules are in an excited state on initial formation and then transition to a ground state; the transition unleashing thermal radiation. When burning in air, the temperature is roughly 2000 °C. Historically, carbon has been the most practical carrier of energy, as more energy is packed in fossil fuels than pure liquid hydrogen of the same volume. The carbon atoms have classic storage capabilities and releases even more energy when burned with hydrogen. However, burning carbon base fuel and releasing its exhaust contributes to global warming due to the greenhouse effect of carbon gases. Pure hydrogen is the smallest element and some of it will inevitably escape from any known container or pipe in micro amounts, yet simple ventilation could prevent such leakage from ever reaching the volatile 4% hydrogen-air mixture. So long as the product is in a gaseous or liquid state, pipes are a classic and very efficient form of transportation. Pure hydrogen, though, causes metal to become brittle, suggesting metal pipes may not be ideal for hydrogen transport.
Uses
Hydrogen fuel can provide motive power for liquid-propellant rockets, cars, boats and airplanes, portable fuel cell applications or stationary fuel cell applications, which can power an electric motor. The problems of using hydrogen fuel in cars arise from the fact that hydrogen is difficult to store in either a high pressure tank or a cryogenic tank.