Bottling Clean Energy in Chemical Bonds

Hydrogen as the starting point

Discussions involving chemical storage often revolve around hydrogen as the most promising molecule of all possibilities, noted Autrey. It can be produced by splitting water into hydrogen and oxygen gases before being used as a carbon-free energy source. In a fuel cell, hydrogen combines with oxygen to produce electricity and water.

However, storing pure hydrogen as a gas or liquid is logistically difficult, requiring either large, high-pressure tanks or very low temperatures. Researchers are developing a variety of alternative storage solutions to hold hydrogen in molecules or materials.

At PNNL, Autrey and the team are developing hydrogen carrier systems that harness chemical reactions to add and remove hydrogen from stable molecules on demand. An entire subfield of chemistry studies the catalysts that perform hydrogen addition and removal. PNNL researchers specialize in designing catalysts that facilitate storing hydrogen in molecules such as formic acid, methylcyclohexane, and butanediol, among others.

PNNL chemist Ba Tran led work testing the suitability of hydrogen-rich ethanol, combined with an established catalyst, to cycle with ethyl acetate for long term storage. Hydrogen remains bonded to the ethanol until needed, when it can be released for use and the ethanol converted into ethyl acetate. The catalyst can add two molecules of hydrogen to a single ethyl acetate molecule, producing two stable ethanol molecules that store the hydrogens.

Analysis beyond the lab

In addition to understanding the fundamental chemistry of adding and releasing hydrogen from other molecules, Tran and his colleagues incorporated data from experimental measurements and advanced molecular simulations into studies of larger-scale systems. “We want to see how the process of storing hydrogen in ethanol—and other forms of chemical energy storage—would behave in an application-scale system,” said theoretical chemist Samantha Johnson.

The team is exploring reactor design scales relevant for seasonal energy storage in a neighborhood. (Photo by Tom Ipri | Flickr.com)

In the ethanol study, for example, the team analyzed a reactor design at a scale relevant for seasonal energy storage in a neighborhood. The chemistry of the reactions worked well and the project taught the team valuable lessons about the engineering needed for a practical system, taking them in new directions to explore different hydrogen carriers.

Grounding research in reality

Whether studying the molecular details of how a hydrogenation catalyst works or engineering a neighborhood-scale storage system, the researchers are always asking questions that will help move research from the lab into the world. The team takes a cyclical approach to problem solving, where different portions of their research inform one another and create a more complete picture of how an energy storage system works. And bringing together researchers with diverse technical backgrounds allows the team to identify solvable problems or challenges for the broader energy storage field.

The collaborative atmosphere and additional instrumentation of the new Energy Sciences Center fits with the work the team performs. Their project is part of the wide range of energy-related research at PNNL that will be accelerated by the presence of the new building. The Energy Sciences Center brings together researchers with different specialties to encourage collaboration. Said Autrey: “We want to help move our society towards a future focused on renewable energy.”

Source: PNNL