The energy systems of the future will probably require storage capabilities for medium and long term energy storage. One of the options is the conversion of (excess) electricity into a chemical energy carrier called an electrofuel. We interviewed Ning Yan, assistant Professor at Van ‘t Hoff Institute for Molecular Sciences (HIMS).
When we want to convert excess electricity to hydrogen for storage, what are the main technologies that can be used for this conversion?
The most straightforward approach is the electrolysis of water. By passing an electric current through the water, we can split it into oxygen and hydrogen.
What would be the advantages of hydrogen for energy storage?
Hydrogen has one of the highest energy density values per unit mass, higher than conventional fossil fuels. Besides, converting hydrogen back into electricity does not generate greenhouse gases or pollutants.
And what are the disadvantages?
Because the hydrogen molecular is so small, light and highly flammable, the efficient and safe storage of hydrogen is challenging. Besides, converting excess electricity to hydrogen via electrolysis is not very efficient either.
At some point the hydrogen will have to be converted back to electricity. Which technologies are available for this process?
Fuel cells are perhaps the best technology. They are electrochemical devices that enable direct conversion of hydrogen into electricity with high efficiency.
What would be the efficiency of the whole cycle from electricity to hydrogen to electricity?
This depends on the type of the applied electrolyzer and fuel cells. Typically, the practical efficiencies for both electricity-hydrogen and hydrogen-electricity conversions are around 50%, making the whole-cycle efficiency equal to 25%.
Do you expect that there is much room for improving this efficiency?
Considering that the theoretical efficiency for both processes is much higher, I would say that there is yet much room for improvement.
Has the principle of storing hydrogen for energy been applied on an industrial scale?
I would say Yes and No. A number of automobile manufacturers have launched their fuel cells powered cars. These vehicles are fuelled by hydrogen which is stored in the pressurized tank. Nonetheless, storing excess power from the grid in the form of hydrogen on an industrial scale has not been implemented yet.
How does hydrogen energy storage compare to battery storage in terms of energy density, efficiency and cost?
Compressed hydrogen has much higher weight energy density than typical Li-ion batteries (can be up to 100 times). The cost for storing of hydrogen is also significantly cheaper than most batteries counterparts. Therefore, it is usually believed that the battery is unsuitable for large scale energy storage though it is highly efficient.
If we want to create more complex electro fuels like transport fuels, which ones show the most promise?
In addition to hydrogen, hydrocarbons, such as methane, methanol and formic acid also show promise as transport fuels.
What are the technologies that can be used to create these fuels?
Conventionally, all these fuels are synthesized from syngas, a mixture of carbon monoxide and hydrogen. Electrochemical and photo electrochemical reductions of CO2 are also viable approaches for such synthesis.
Which carbon sources could be used for these processes? Is it for instance possible to use CO2 from point sources as a feedstock?
Natural gas, biogas and syngas are all possible feedstocks. Recently, CO2 has attracted much attention as a sustainable carbon source. In the laboratory, researchers have successfully converted CO2 into fuels with help tailored catalysts. However, the high energy cost associated with the CO2 capture and conversion prevents its practical use as the feedstock at the moment.
Can you mention one or more research projects at HIMS in the area of electro fuels?
In the Sustainable Chemistry Research Priority Area program at HIMS (www.suschem.uva.nl), we have a number of projects focusing on the sustainable synthesis of such fuels. For example, my team are now researching carbon and perovskite oxide materials, which are much more cost-effective and abundant than noble metal catalysts, for efficient water splitting. The research team led by Prof. Joost Reek are studying molecular catalyst for the generation of “solar fuels” using the sun light.
Are electro-fuels an economically viable option at this moment for energy storage and transportation fuels or do we need major technological breakthroughs to accomplish this?
Though such electro-fuels are not a good option for energy storage at the moment, I see a bright future for its real-life application if we can increase the efficiency of the conversion using the affordable catalysts.