From Beth Mundy
With renewable energies and energy storage technologies, variation is the order of the day.
The intensity of the natural resources that provide renewable energy varies from day to day and from season to season. Spring brings strong winds to comb the deserts and fill rivers with melted snow. Summer is synonymous with long hours of sunshine before the days shorten as autumn turns into winter.
We need a variety of ways to store renewable energy that suits our use, from batteries to fuel cells. Batteries are well suited for shorter storage periods, on the order of hours to days. Of the various methods of storing renewable energy, one stands out that offers a way of storing energy for months: storing energy in the chemical bonds of molecules like hydrogen.
Through decades of basic research, scientists at the Pacific Northwest National Laboratory (PNNL) have contributed detailed information on how catalysts help convert energy into molecular bonds, store that energy by forming bonds, and release it by breaking bonds.
Now, a team led by chemist and laboratory fellow Tom Autrey is working to transform chemical energy storage into practical facilities that could one day help energize neighborhoods, infrastructure and industry. To do this, the team examines entire systems, from catalysts to reactors to end products – and everything in between.
“Our work takes into account everything from electrons to dollars,” said chemist Mark Bowden, a longtime collaborator on the project. The interdisciplinary team combines knowledge of chemistry, engineering, technoeconomics and theoretical calculations in order to examine the practical suitability of chemical energy storage systems for large-scale storage.
The team will have a supportive home at PNNL’s Energy Sciences Center, due to open later this year. The building will house over 250 employees and an array of advanced scientific tools previously scattered around campus, fostering a collaborative environment to build on the team’s long history of progress. Research at the Center for Energy Sciences also includes work focused on the development of new catalysts for converting electricity into chemical bonds by the Center for Molecular Electrocatalysis.
Hydrogen as a starting point
Discussions about chemical storage often revolve around hydrogen as the most promising molecule of all possibilities, Autrey noted. It can be made by splitting water into hydrogen and oxygen gases before using it as a carbon-free energy source. In a fuel cell, hydrogen combines with oxygen to form electricity and water.
However, the storage of pure hydrogen as a gas or liquid is logistically difficult and requires either large high pressure tanks or very low temperatures. Researchers are developing a variety of alternative storage solutions to store hydrogen in molecules or materials.
At the PNNL, Autrey and the team are developing hydrogen carrier systems that use chemical reactions to add and remove hydrogen from stable molecules as needed. A whole branch of chemistry studies the catalysts that perform hydrogen addition and removal. PNNL researchers specialize in developing catalysts that facilitate the storage of hydrogen in molecules such as formic acid, methylcyclohexane and butanediol, among others.
PNNL chemist Ba Tran led the work to test the suitability of hydrogen-rich ethanol in combination with an established catalyst for cycling with ethyl acetate for long-term storage. Hydrogen remains bound to the ethanol until needed, then it can be released for use and the ethanol converted to ethyl acetate. The catalyst can add two hydrogen molecules to a single ethyl acetate molecule, creating two stable ethanol molecules that store the hydrogen.
Analysis beyond the laboratory
Tran and his colleagues not only understood the basic chemistry of adding and releasing hydrogen from other molecules, but also incorporated data from experimental measurements and advanced molecular simulations into studies of larger systems. “We want to see how the process of storing hydrogen in ethanol – and other forms of chemical energy storage – would perform in an application-scale system,” said theoretical chemist Samantha Johnson.
For example, in the ethanol study, the team analyzed a reactor design on a scale relevant to seasonal energy storage in a neighborhood. The chemistry of the reactions worked well, and the project taught the team valuable lessons about the engineering required for a practical system and took them in new directions to explore different hydrogen carriers.
Earth research in reality
Whether it’s investigating the molecular details of how a hydrogenation catalyst works or developing a storage system on a neighborhood scale – the researchers keep asking questions that help bring research out of the laboratory into the world. The team takes a cyclical approach to problem solving, where different pieces of their research educate each other and create a more complete picture of how an energy storage system works. And bringing together researchers from diverse technical backgrounds enables the team to identify solvable problems or challenges for the broader field of energy storage.
The collaborative atmosphere and the additional instrumentation of the new Energy Sciences Center fit in with the work of the team. Your project is part of the broad spectrum of energy-related research at the PNNL, which is being accelerated by the new building. The Energy Sciences Center brings together researchers from different disciplines to promote collaboration. Autrey said: “We want to help lead our society into a future that is focused on renewable energies.”
The researchers confirm the support from the Hydrogen and Fuel Cell Technologies Office of the Office of Energy Efficiency and Renewable Energy through the Hydrogen Advanced Research Consortium (HyMARC), which was formed as part of the US Department of Energy’s Energy Materials Network.
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