I’m sometimes asked why the gravimetric energy density of hydrogen is so high, and much higher than batteries. For example, the Tesla Model S (85 kWh) requires a battery pack weighing about 540 kg to achieve a 400 km range, but a similarly sized gasoline vehicle achieves the same range with 25 kg of petrol, and a fuel cell electric vehicle (FCEV) with 5 kg of hydrogen. To be fair, the differences are much less stark when the full drive chain mass is included, and high-pressure hydrogen is difficult to contain. But nonetheless, the weight-range trade-off is a key challenge of battery electric vehicles (BEVs).
The main reason is surprisingly simple and offers an important insight into the long-term value of hydrogen as an energy carrier – by convention, the energy density of hydrogen and hydrocarbons is expressed relative to the fuel and not to the ‘fuel plus oxidiser’. In the case of hydrogen energy, the hydrogen component comprises only 11% of the reactant mass – the oxidiser (in this case oxygen), comprises 89%. Similarly, the reactant mass of methane, diesel and gasoline comprises 20, 23, and 22% respectively.
The role of oxygen
Combustion engines and fuel cells release energy through redox chemistry. Redox reactions require a reductant (fuel) and an oxidizer. Common reductants include methane, kerosene, ammonia, and hydrogen. The most common, but not the only oxidizer, is oxygen. The case of rockets illustrates the ‘power’ of oxygen – rockets must carry both ‘fuel’ and ‘oxidizer’ and oxygen is the most common oxidizer.
But for atmospheric engines (and fuel cells), oxygen is conveniently available everywhere – after all, we inhabit a lower atmosphere composed of 21% oxygen. Since oxygen is not scarce, by convention it is not normally part of the calculation of energy density. In the example above, only 5 kg of hydrogen is required for 400 km range, but if we also include the mass of oxygen, the total reactant mass is 45 kg. Similarly, for petrol, the total is 114 kg.
The energy released by fossil fuel combustion is commonly assumed to derive predominantly from the enthalpy difference between hydrocarbons and carbon dioxide. However, close inspection of the enthalpy balance of hydrogen or hydrocarbon oxidation indicates that the majority of the energy released derives not from the enthalpy difference between methane and carbon dioxide, as might be expected, but from the difference between oxygen and water. The large difference between oxygen and water enthalpies is due to the relatively weak double bond of the oxygen molecule
Framed in this way, it is evident that much of the benefit of hydrogen and fossil fuels derives from the unlimited accessibility to oxygen – we metaphorically inhabit a virtual ‘tank’ of oxygen.
When we see bush fires, we identify the trees as the source of fuel but don’t think about the reactivity of oxygen. We inhabit a remarkably reactive lower atmosphere, but since air (including gaseous oxygen) is transparent, we don’t notice it.