Lithium batteries can easily eliminate short-term hiccups in intermittent renewable energy supply. But they are not ideal for long-term storage, as they will discharge slowly. They are also not great for large amounts of energy: to store more, you have to keep buying more battery. Because of these problems, research has been conducted on a number of technologies that adapt better, such as flow batteries and renewable fuel production. But these pose their challenges, both chemical and economic.
But researchers are now pointing to a possible solution to some of these problems: a fuel cell that can work efficiently in both directions, using hydrogen or methane to produce electricity or using electricity ; electricity to produce these fuels. Their measurements suggest that, after completing a cycle, they get 75% of the electricity they put in to get started.
The batteries, as we have said above, do not work for long-term storage, as they generally lose their charge slowly. They are also expensive, because adding capacity means adding more batteries. Flow batteries solve some of these problems by storing the loaded and unloaded forms of a chemical in different tanks; Larger or larger tanks are inexpensive, making the expanded capacity relatively simple and inexpensive. But flow batteries are not as efficient as conventional batteries and the chemicals they use can be toxic or corrosive.
An alternative for long-term storage is to convert excess electricity into fuel. But these reactions often have efficiency problems, which means that part of the energy is lost in the process. And the costs can be quite large, since in general you need hardware for both fuel production and electricity generation, as well as pure water sources and expensive catalysts.
An option to cut costs is what is called a reversible fuel cell. Fuel cells simply separate the different parts of a chemical reaction so that the electrons that are transferred during the reaction can be used as a source of electricity. Operating forward, the fuel cells will take hydrogen or methane as fuel and produce electricity by combining it with the oxygen present in the air. Operating in reverse, they will use electricity to drive hydrogen production from water, or methane if given water and CO 2 .
This allows for a completely reversible cycle by which electricity is essentially stored in the form of hydrogen or methane, without the need for separate hardware for storage and use. In essence, it acts like a big battery. Alternatively, hydrogen and methane are valuable chemical titers or can be used to fuel various forms of transport. Clearly, a reversible fuel cell is extremely flexible. So why aren't we using them?
Different types have been created, but they all have problems. Some forms require high temperatures to function. All produce a mixture of hydrogen and water less precious than pure and dry hydrogen. And the efficiency of the round-trip reaction is often much lower than a real battery could provide. In many cases, the necessary catalysts degrade rapidly.
A moderate solution
A group of researchers from the Colorado Mining School decided to take a closer look at a technology called reversible proton ceramic electrochemical cells (no, you won). I must remember). This technology has been very efficient when going forward and only requires what, for fuel cells, are moderate temperatures (400-600 ° C). This places them in the field where the operating temperature could be reached by using residual heat sources from industrial processes or traditional energy generation.
Unfortunately, they lose over 30% of the energy input as electricity when they are running backwards. Then, the research team ran some computer models to understand where that energy was going, starting with a combination of Ba / Ce / Zr / Y / Yb and Ba / Co / Zr / Y electrodes. Modeling suggested that the current lost during the operation is carried away by holes, areas with a number of electrons lower than normal that can migrate around the material. They found that they could reduce the formation of the hole by changing the electrolyte; once this was done, they began to test its performance.
These tests included finding the optimal current density in hardware, too low, and hydrogen did not flow through the fuel cell; too high, and the system consumes all its water before it can spread more. The optimal operating temperature was around 500 ° C, at which point over 97% of the electricity supplied was involved in conducting the chemical reaction. Given the water alone, the system would produce hydrogen; given water and carbon dioxide, it would produce methane.
Using the hydrogen reaction, the overall efficiency of the system, the amount of electricity that is obtained compared to what was injected, has reached 75%. Not as good as the batteries, but remember that this can scale up to a quantity of hydrogen storage that can be supplied and can hold it indefinitely.
But the fundamental thing could be the stability of the system. The researchers performed the reverse reaction for over 1
All these sounds extremely promising, but there is still a large number of obstacles to be clarified. Although there is nothing like the platinum catalysts often used to split water, the ytterbium continues to operate at around $ 14,000 per kilogram, which could make the increase quite expensive dell & # 39; hardware. Someone should also prove that the hardware could scale, in terms of both production and operation as a reversible fuel cell when surrounded by standard industrial parts, rather than hand-tuned laboratory equipment.
Finally, we currently do not have an energy architecture in place to use or store large volumes of hydrogen or to channel waste heat into hardware like this. So there is still a lot of work to do.
Nature Energy 2019. DOI: 10.1038 / s41560-019-0333-2 (Information on DOI).