氢是自然界里最轻的元素,其分子量为2.016。在一个大气压和20℃下的密度为83.764g/m~3,其液化温度大约为-253℃。由于这种特性,如按它的能量密度算,氢是难于以适当的形式来贮存的,而且有时还要消耗很多的能量。 相反,氢作为燃料却有很好的特性;低热值大约是 33.3kWh/kg或3kWh/标m~3,燃烧时只产生水。当与空气发生燃烧时则会产生氮氧化物,但可以适当控制燃烧温度来减少氮氧化物。如果氢用在燃料电池上来产生电能的话,不会有污染物排放。所以说,氢是一种有利于环境的理想燃料。 不幸的是,氢不以游离状态存在,而是以化合物的形态存在,其中最为常见的是水。如果氢可用对环境无害的能源（太阳能、风能、生物质、水力等）来生产,那么这种有利的特 Hydrogen is the lightest element in nature with a molecular weight of 2.016. The density is 83.764 g / m 3 at one atmosphere pressure and 20 ° C, and the liquefaction temperature is about -253 ° C. Because of this property, hydrogen, as calculated by its energy density, is difficult to store in a suitable form, and sometimes consumes a lot of energy. In contrast, hydrogen has good properties as a fuel; the low calorific value is about 33.3 kWh / kg or 3 kWh / m³, producing only water when burned. Nitrogen oxide is produced when it is burnt with air but nitrogen oxides can be reduced with proper control of the combustion temperature. If hydrogen is used to generate electricity on fuel cells, there is no emission of pollutants. So hydrogen is an ideal fuel for the environment. Unfortunately, hydrogen does not exist in free form, but in the form of compounds, the most common of which is water. If hydrogen can be produced from environmentally unfriendly sources of energy (solar, wind, biomass, hydraulics, etc.), then this advantageous