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| Fig. 1: Schematic of a Hydrogen Fuel Cell. |
Individual transportation is a topic that is very important to most people. It is an important part of personal freedom and as such deeply engrained in the culture of most developed countries. Currently, the vast majority of cars are powered by internal combustion engines (ICEs) that run on either diesel fuel or gasoline. Both of these fuels are hydrocarbons and were produced by refining crude oil.
There are good reasons for why gasoline and diesel are used so widely. First, they have very high energy densities. In fact, the energy density per unit volume of gasoline and diesel is higher than that of any other currently available chemical fuel. [1] Second, they are liquids at standard temperatures and pressures which makes it easy to store, transport and distribute them. Third, they can currently be produced rather cheaply from crude oil.
Unfortunately, there are several fundamental problems with our current use of these fuels. The oil reserves of our planet are finite, and most scientists agree that the reserves will be depleted sometime within the next two centuries. Furthermore, the combustion of fossil fuels like gasoline and diesel produces CO2, one of the main contributors to the greenhouse effect and the resulting climate change. Evidently, whether it is for the sake of the environment or simply because the reserves are gone, we will have to switch from fossil fuels to alternative fuels within the next centuries, or even within the next decades.
Evidently, hydrogen fuel cell technology is a promising candidate for the future of powering individual transportation.
Hydrogen can be produced easily from water using electrolysis. Ideally, the electricity used for this process will in the future come from renewable energy sources. If we assume so, then the hydrogen fuel cycle is inherently clean and emission-free. Since hydrogen is a very light and volatile substance, efficient storage is currently one of the greatest challenges for a prospective hydrogen fuel cell technology in automobiles. In order to reach reasonably high densities for pure hydrogen, it must either be stored under high pressures or in liquid form. This means that the tank in question will either have to withstand several hundred atm. of pressure or be equipped with a sophisticated cryogenic system, which in any case poses a significant engineering challenge when considering automobiles. [2] There also exist more advanced technologies to store hydrogen; in metal hydride storage for example the hydrogen is kept within the metal lattice of some carrier material, thereby more densely packing the single hydrogen molecules. Current research makes use of the same basic idea, but uses carbon nanotubes instead of metals. Hydrogen will be distributed similarly to the way gasoline and diesel are distributed nowadays. Car owners will refuel at the hydrogen equivalent of gas stations by plugging a nozzle into their car. It remains questionable as to how much of the currently existing infrastructure can be reused and how much will have to be built anew.
The basic chemical reaction driving an ICE and a fuel cell is the same. In an ICE, the combustion of the fuel is used to drive a heat engine. By the laws of thermodynamics such heat engines have inevitable losses and can only reach efficiencies below certain limits. In practice, gasoline engines reach efficiencies of about 20-25%, while diesel engines are slightly higher - about 40-45%. [2] In fuel cells, the electron transfer of the chemical reaction is used directly to drive an external circuit in which useful electric work is performed. The process is kept reversible, which eliminates the basic thermodynamic limitations that internal combustion engines exhibit. The efficiencies of fuel cells are therefore substantially higher than those of ICEs. Fuel cell efficiencies can reach up to 60%. [2]
In a hydrogen fuel cell, the fuel is molecular hydrogen (H2), which is combined with oxygen O2 from surrounding air to produce water (H2O). The hydrogen is inserted on one side of a so-called PEM (proton exchange membrane) which permits the flow of protons through it but doesn't allow the flow of electrons. Using catalysts, the hydrogen as well as the oxygen molecules are broken up into single atoms at the electrodes. In order to recombine with oxygen atoms on the other side of the membrane to form water molecules, the proton separates from its electron and moves through the membrane while the electron is guided through an external circuit where it performs useful work.
Currently, there are only very few sustainable alternatives to fossil fuels that seem to have the potential to replace them with all their functionality and convenience. First, there are battery powered electric vehicles (EVs). EVs are already commercially available which suggests that they are a somewhat viable alternative to fossil fuels. On the other hand, there are some major disadvantages. For example, batteries have a very low energy density compared to liquid fuels. The battery packs in currently available EVs weigh several hundred kilograms but only provide a range of about 200 miles. The amount of hydrogen required to provide for such a range, even if considering the weight of the storage tank, would be less. [2,3] Additionally, the charging time is currently considerably longer than the refueling time of a gas or diesel tank. While the full recharging time of an EV is on the range of hours, a refueling stop for hydrogen would work very similarly to the way current gas or diesel tanks are refueled, which would make it a matter of minutes.
Apart from batteries and hydrogen fuel cells, biofuels are another alternative to gasoline and diesel. Chemically, biofuels are very similar to diesel fuel made from oil, which means that they have almost equally high volume energy densities; higher than those of batteries or hydrogen. On the other hand, the combustion reaction of biofuels with oxygen produces the unwanted CO2. Theoretically, this CO2 is in a closed cycle, since the amount of CO2 released in the combustion should equal the amount of CO2 captured during the growth of the biomass from which the biofuel was made of. Currently though, the production process involves many inefficiencies and therefore there are net CO2 emissions. Furthermore, the amount of agricultural land that would be required to produce enough biofuels for the entire US car fleet is a substantial fraction of the current US farmland and it is questionable how much land the economy can afford to devote to biofuel production.
Hydrogen fuel cells are a promising alternative to current automobile fuels. They essentially combine the energy density and the convenience of liquid fuels with the clean and efficient operation of electric vehicles. Although certain aspects of the technology such as efficient on-board storage still require some improvement, there are no reasons why hydrogen couldn’t become an equally convenient and attractive transportation fuel as diesel or gasoline are today.
© Julian Kates-Harbeck. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. A. King and O. R. Inderwildi, "Future of Mobility Roadmap: Ways to Reduce emissions While Keeping Mobile," Smith School of Enterprise and the Environment, Oxford University, 2010.
[2] R. B. Gupta, Hydrogen Fuel: Production, Transport and Storage (CRC Press, 2008).
[3] A. V. da Rosa, Fundamentals of Renewable Energy Processes, (Academic Press, 2005).