^{Text is an excerpt from Andrea Bedon PhD thesis (2018)}

Batteries are probably the most at hand choice for the storage of power, but they are not the only one. Another possibility is to store energy not in the form it was obtained (electrical), but to transform it in something easier to store and manage, as a fuel. The conversion into a fuel has various benefits: a fuel is easier to store, can be directly used in already existing technologies, has a very high energy density, in general is more flexible, which is an advantage that must not be underestimated. Indeed, when we talk in general about energy demand we sum up a lot of necessities, environments and uses that are extremely diversified if compared singularly, and every one of them has found its best solution in the current scenario. This is why there are so many energy sources and so many ways to use them. An airplane requires high amount of energy with the minimum weight possible, houses have plenty of small ready to use devices that must be fed with a clean energy source (nobody wonders why there are gasoline lawn mowers and electrical dishwashers, and not the contrary), heavy industries use a lot of energy so they need when possible a cheap source, electronics devices use small amount of energy packed in the smallest volume possible and must be shut on/off in the matter of seconds. So, for some contexts electricity is the best choice, for some others it is not, and this is where fuels found application. Often this happened for energy density reasons, and for these cases one can expect it would be preferable to still have fuels also in the future. At least, as long as batteries do not reach the same performances as energy; but they still have not, in spite of the huge and surprising development of the last years, even if compared to the centennial internal combustion engine technology.

Electrolysis allows to convert electricity into fuels, it is a process known since centuries and already commonly used for small scale hydrogen production. In the last years, the development of fuel cells, as some kind of side effect, is boosting also the development of electrolysis for fuel production. The device for fuel production from electricity and electricity from fuel is essentially the same, i.e. a fuel cell that operates with a potential applied is the starting point for building an electrolyser. Traditionally, water electrolysis can be carried out in two ways, by means of an alkaline electrolyser (conventional setup with liquid electrolyte) or a Polymer Electrolyte Membrane, which makes the device analogous to a PEM fuel cell. Efficiency for these two techniques are similar, around 60%. PEM electrolysers can be twice as expensive compared to alkaline one, in part because they require platinum electrodes, but they should reach higher efficiencies in shorter times given their less developed stage; maximum theoretical efficiency for PEM electrolysers is 94%, although this is only a purely speculative value.

A third way to convert electricity into fuel, that is currently under development but with very promising premises are the Solid Oxide Electrolysis Cells (SOECs), which stand to Solid Oxide Fuel Cells (SOFCs) the same way the PEM electrolysers stand to the PEM fuel cells. A SOEC works at high temperatures (=800°C) and thank to this the reaction is less impeded than it is for the low temperature devices. It does not require platinum, but the costs are still high because of a more complicated fabrication technique. On the other hand, there are two major valuable advantages. SOECs already tested were able to reach much higher efficiencies than their low temperature counterparts: their energy-conversion efficiency can even surpass 100% in some operation mode due to the possibility to use heat from other processes as a source of energy. Second, SOECs are able to co-electrolyse water and carbon dioxide, producing syngas (H₂ + CO), in other words they can be used for large scale conversion of atmospheric CO₂ into a useful chemical; this process today is already possible by means of dry reforming, but requiring methane as a reactant, while a SOEC only needs water (some argue also methane is a greenhouse gas, so it could be removed from the atmosphere hence the reforming would not use any fossil fuel, but the 1.8ppm methane concentration would make this process hardly economically sustainable). Moreover, syngas produced by SOECs can be converted to hydrocarbons via a Fischer-Tropsch process. Hypothetically, it would be already possible today to produce methane, which is a common used fuel, with completely sustainable processes by coupling a renewable energy source to a SOEC and a Fischer-Tropsch reactor; current estimation is the first fuel from solar production prototypes should be presented within a decade.

A common objection to the production of carbon containing fuels and to the continuation of their use is the subsequent emission of CO₂, that would further increase greenhouse effect. At this purpose it is important to stress that the real issue is the release of carbon that was trapped under the soil, but the use of carbon that already is in the atmosphere does not alter the carbon budget; similarly, the use of fuels generated from crops (the so-called biofuels) is cleaner than extracted fuels because no new carbon is introduced in the atmosphere.

First prototypes of cell stacks beyond the lab scale have already been proposed, although there are still significant problems of durability due to excessive anode degradation. Very interesting is the possibility to operate the same cell alternatively in the fuel cell mode and in the electrolyser mode, analogously to a secondary battery that can be discharger and recharged. Such a device is called reversible SOFC and is already studied and developed, because it would allow both an easy storage of energy excess from renewable and an efficient re-conversion to electrical energy when needed. A SOEC would greatly benefit from the switching between the two modes, because this mode of operation has been found to significantly reduce the long term degradation of the cell.

In the challenge for the energy storage system of the future, batteries have an unquestionable advantage in terms of development stage, compared to electrolysers, However, they still have drawbacks that will make difficult for them to gain an exclusive supremacy as the only energy storage devices. As the figure below shows, their energy density is low compared with fuel cells, which on the other hand fail when high powers are needed. To combine energy density and power density, probably the best solution is to build hybrid devices, that could benefit from both the advantages of the fuel cells and the batteries.