Lithium batteries can easily compensate for short-term supply problems with intermittent renewable energy. However, they are not ideal for long term storage as they slowly discharge. They are also not suitable for large amounts of energy ̵
However, the researchers are now reporting a possible solution to some of these problems: a fuel cell that can be operated efficiently in either direction, either with hydrogen or hydrogen Methane for power generation or power generation Use of power to produce these fuels. Their measurements indicate that after a full cycle, they subtract 75 percent of the current that they begin with.
There are many limitations
Batteries, as mentioned above, do not work for long-term storage as they usually lose their charge slowly. They are also expensive because more batteries are needed to add capacity. Flow batteries solve some of these problems by storing the charged and discharged forms of a chemical in different tanks. larger or additional tanks are cheap, making the extended capacity relatively easy and inexpensive. However, continuous-wave batteries are not as efficient as conventional batteries, and the chemicals used can be toxic or corrosive.
An alternative to longer-term storage is to convert excess electricity into fuel. However, these reactions often have their own efficiency issues, meaning that some of the energy is lost. And the cost can be quite high, as hardware is usually needed for both fuel and power generation, as well as very pure water sources and expensive catalysts.
One way to reduce costs is the so-called reversible fuel cell. Fuel cells simply separate different parts of a chemical reaction so that the electrons that are transferred during the reaction can be used as a power source. When operating in the forward direction, fuel cells receive hydrogen or methane as fuel and generate electricity by connecting it with oxygen from the air. When working in reverse order, electricity is used to promote the production of hydrogen from water or methane when water and CO are added 2 .
This allows for a fully reversible cycle in which the electricity is substantially 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 supplies or can be used to transport various means of transportation. A reversible fuel cell is clearly extremely flexible. Why do not we use it?
A number of types have been built, but they all have problems. Some molds require high temperatures to work. All produce a mixture of hydrogen and water that is less valuable than pure, dry hydrogen. And the efficiency of the round-trip response is often significantly lower than that of an actual battery. In many cases, the catalysts were rapidly degraded.
A Moderate Solution
A group of researchers from the Colorado School of Mines decided to take a closer look at the technology known as the reversible protonic ceramic electrochemical cell. must remember). This technology has proven to be very efficient when driving forward and requires only what fuel cells are at moderate temperatures (400 to 600 ° C). Thus, they are within the range in which the operating temperature could be achieved by using waste heat sources from industrial processes or by traditional power generation.
Unfortunately, they also lose more than 30 percent of the energy used as electricity backwards. The research team performed some computer modeling to find out where this energy is going, starting with a combination of Ba / Ce / Zr / Y / Yb and Ba / Co / Zr / Y electrodes. The modeling indicated that the loss of power during operation is derived from holes, areas with fewer electrons that can roam around the material. They found that they could reduce hole formation by changing the electrolyte; Once that was done, they started to test the performance.
These tests determined the optimal current density in the hardware – too low and the hydrogen does not flow through the fuel cell. too high, and the system uses up all of its water before more can diffuse. The optimum operating temperature was around 500 ° C. At this time, over 97% of the electricity supplied was involved in the reaction of the chemical reaction. With water alone, the system would produce hydrogen; With water and carbon dioxide, it would produce methane.
Using the hydrogen reaction, the overall efficiency of the system – how much power you put in compared to what you put in – was 75 percent. Not as good as batteries, but remember that this can be adjusted to any amount of hydrogen storage that you can provide and that you can keep it indefinitely.
The most important thing, however, is the stability of the system. Researchers performed the reversal reaction for 1,200 hours without significant degradation of a critical species. The imaging of these materials showed no obvious structural changes after all this use. Of course we want this to last for years instead of over 50 days, but the fact that it worked so well after 50 days of use suggests that there will be no unpleasant surprises.
All these sounds are very promising, but there are still many hurdles to overcome. While there is nothing like the platinum catalysts that are commonly used to break down water, ytterbium still costs around $ 14,000 per kilogram, which could make the hardware a bit more expensive. Someone would also have to show that the hardware is scalable, both in terms of manufacturing and reversible fuel cell function, when surrounded by standard industrial parts rather than hand-held laboratory equipment.
At present, we do not have an energy architecture to use or store large amounts of hydrogen or to channel the waste heat into such hardware. So much remains to be done.
Nature Energy 2019. DOI: 10.1038 / s41560-019-0333-2 (About DOIs).