There has been a tendency for the automotive sector to view batteries and hydrogen as competing technologies when they are in fact highly complementary, says Dr Gareth Hinds, Science Area Leader in Electrochemistry at the National Physical Laboratory (NPL).
With the UK government’s aspiration to end the sale of new diesel and petrol vehicles by 2035, it’s hard at the moment to escape the ongoing debate: Battery electric vehicles (BEVs) or fuel cell electric vehicles (FCEVs)?
While the choice of technology to power their vehicles ultimately comes down to individual manufacturers, the fact that this change is being driven primarily by environmental imperatives rather than purely market forces makes this a very interesting debate. My view is that the response should be informed by the science.
Electrochemical energy storage involves the interconversion of electrical energy and chemical energy. Electricity generated from renewable sources, such as solar and wind, can be stored for later consumption by using the electrons to transform more stable materials into less stable materials containing higher amounts of energy. Examples include the electrolysis of water to form hydrogen and oxygen, or the insertion of lithium ions into the graphite electrode in a lithium ion battery (known as intercalation). Such a chemical store can then be raided when required by reversing the process and allowing the material to revert to its more stable form, with the release of low carbon electricity. In a FCEV, this is achieved by combining the hydrogen with air in a fuel cell.
Thermodynamics dictates that each time we convert energy between any of its forms we lose a fraction of this energy through irreversible heating of the system and its surroundings. Fortunately, the process of intercalation in a lithium ion battery is incredibly efficient (> 99%). This translates into a round-trip efficiency of around 70% for BEVs. Hydrogen fuel cells, on the other hand, have much lower efficiencies of around 60%, mainly due to losses associated with the oxygen reaction. When this is combined with the losses in producing the hydrogen from electrolysis in the first place, the round-trip efficiency of a FCEV drops to around 30%.
So why do we need hydrogen at all, given its much lower efficiency?
Firstly, there is an inherent weight limitation for BEVs because all of the energy in a battery is stored within its electrodes. If we want to power a bigger vehicle or drive a longer distance, we need to add more electrodes to store the additional charge. Battery electrodes are relatively heavy and the extra battery weight rapidly becomes impractical. In contrast, in a FCEV the fuel cell itself is inert and all of the energy is stored in the hydrogen tank. Since hydrogen is so light, the incremental mass associated with increasing the size or range of the vehicle is much lower than for a BEV.
Hybridisation of battery and fuel cell technologies is…likely to be explored more widely
So, for heavy duty applications such as vans, trucks, trains, ships and aircraft, it makes far more sense to power them with hydrogen. Vehicles that regularly travel long distances or that need to refuel quickly are also more suited to hydrogen. But for light duty passenger vehicles regularly travelling short distances, the superior efficiency of BEVs makes them ideal for this purpose.
Fast charging of BEVs may sound attractive but it carries several challenges, including the power demand on the electricity grid and the tendency to curtail the lifetime of the battery. Hybridisation of battery and fuel cell technologies is a potential solution that is likely to be explored more widely in the coming years.
Another important factor is that hydrogen is more suited to longer term storage of energy since it is a gas and can be easily stored in tanks, containers and underground caverns. Batteries tend to lose their charge over time due to side reactions within the cell and their lifetime suffers when they are not charged and discharged regularly.
Hydrogen can also be burned in boilers and cookers, which makes it an attractive candidate to replace methane in the gas grid. This would effectively create a country-scale storage facility that could be continuously topped up by electrolysis using renewable electricity. Efficiency losses would then be less of an issue.
Finally, hydrogen technologies are almost 100% recyclable and are much easier to handle at end-of-life than batteries, for which significant sustainability concerns exist. Another key role for hydrogen may be to limit the scale on which it is necessary to deploy battery technology in order to avoid major waste disposal issues in the longer term.
Hydrogen has a number of advantages that allow it to fill gaps in our decarbonisation strategy that batteries cannot address effectively on their own. Most of these benefits are associated with the fact that the energy in a fuel cell is not stored in the device itself but in the hydrogen gas.
It should also be noted that the requirement for decarbonisation of energy goes well beyond the transport sector, which comprises only a third of global CO2 emissions. There are many complex interdependencies between the different sectors, all of which need to be taken into account.
As the UK’s National Metrology Institute, the National Physical Laboratory (NPL) is working closely with academia and industry to develop more reliable standard test methods, novel diagnostic techniques and advanced modelling tools to support improvements in cost, performance, lifetime and safety of batteries and hydrogen cells, plus electrolysers. This important underpinning work will help to build a greener and more sustainable infrastructure to power industry and society in the future.
Dr Gareth Hinds is Science Area Leader in Electrochemistry at the National Physical Laboratory (NPL)
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