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Japan-US team develops novel catalyst for reversible hydrogen storage under mild conditions using CO2, formate and formic acid

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This diagram shows the new catalyst in its protonated and deprotonated states as it reversibly converts hydrogen and CO2 gas to and from liquid formate or formic acid at ambient temperature and pressure. The gases can thereby be stored and transported as a liquid, and used later in carbon-neutral energy applications, simply by adjusting the pH. Click to enlarge.

Scientists at the US Department of Energy’s (DOE) Brookhaven National Laboratory and collaborators have developed a novel catalyst that uses CO2 and hydrogen to store energy in formic acid. The work—described in a paper published in Nature Chemistry—could lead to efficient ways to safely store and transport hydrogen for use as an alternative fuel.

Using a homogeneous iridium catalyst with a proton-responsive ligand, they showed the first reversible and recyclable hydrogen storage system that operates under mild conditions using CO2, formate and formic acid. This system is energy-efficient and green, the researchers say, because it operates near ambient conditions, uses water as a solvent, produces high-pressure CO-free hydrogen, and uses pH to control hydrogen production or consumption.

Hull
H2 is stored by the hydrogenation of CO2 in the presence of hydroxide (basic aqueous conditions) and then regenerated by the addition of acid. Hull et al. Click to enlarge.

The team attributed the extraordinary and switchable catalytic activity to the multifunctional ligand, which acts as a proton-relay and strong π-donor, and is rationalized by theoretical and experimental studies.

The new work builds on earlier efforts to combine hydrogen with carbon dioxide to produce a liquid formic acid solution that can be transported using the same kind of infrastructure used to transport gasoline and oil.

This is not the first catalyst capable of carrying out this reaction, but it is the first to work at room temperature, in an aqueous (water) solution, under atmospheric pressure—and that is capable of running the reaction in forward or reverse directions depending on the acidity of the solution.

—Brookhaven chemist Etsuko Fujita, who oversaw Brookhaven’s contributions to this research

When the release of hydrogen is desired for use in fuel cells or other applications, one can simply flip the ‘pH switch’ on the catalyst to run the reaction in reverse.

— Brookhaven chemist James Muckerman, a co-author

Muckerman noted that the liquid formic acid might also be used directly in a formic-acid fuel cell.

Collaborator Yuichiro Himeda of the National Institute of Advanced Industrial Science and Technology (AIST) of Japan had been making substantial progress toward the goal of developing this type of catalyst for a number of years. He used iridium metal complexes containing aromatic diimine ligands (groups of atoms bound to the metal) with pendent, peripheral hydroxyl (OH) groups that can serve as acidic sites that release protons to become pendent bases.

Himeda recently entered into collaboration—via the US-Japan Collaboration on Clean Energy Technology program—with Fujita, Muckerman, and Jonathan Hull (a Goldhaber Fellow working on Fujita’s team). The Brookhaven group carried out coordinated experimental and theoretical studies to understand the sequence of chemical steps by which these catalysts converted H2 and CO2 into formic acid. Their goal was to design new catalysts with improved performance.

We were inspired by the way hydrogen bonds and bases relay protons in the active sites of some enzymes. Good catalysts efficiently move protons and electrons around, taking them from some molecules and placing them onto others to produce the desired product. Nature has many ways of doing this. Under the right conditions, the hydroxyl groups on the diimine ligand of the catalyst help hydrogen react with carbon dioxide, which is difficult to do. We thought we could improve the reactivity by placing the pendent bases near the metal centers, rather than in peripheral positions.

—Jonathan Hull

Once the Brookhaven team understood how Himeda’s catalysts worked, Hull realized that a novel ligand that had been synthesized by collaborators Brian Hashiguchi and Roy Periana of The Scripps Research Institute for an entirely different purpose would possibly be ideal for accomplishing this goal. The Brookhaven group designed a new iridium metal catalyst incorporating this new ligand.

Collaborator David Szalda of Baruch College (City University of New York) determined the atomic level crystal structure of the new catalyst to “see” how the arrangement of its atoms might explain its function.

Tests of the new catalyst revealed superior catalytic performance for storing and releasing H2 under very mild reaction conditions. For the reaction combining CO2 with H2, the scientists observed high turnovers at room temperature and ambient pressure; for the catalytic decomposition of formic acid to release hydrogen, the catalytic rate was faster than any previous report.

We were able to convert a 1:1 mixture of H2 and CO2 to formate (the deprotonated form of formic acid) at room temperature, successfully regenerate H2, and then repeat the cycle. It’s a design principle we are very fortunate to have found.

—Jonathan Hull

The regenerated high-pressure gas mixture (hydrogen and carbon dioxide) is quite pure; importantly, no carbon monoxide (CO)—an impurity that can ‘poison’ fuel cells and thus reduce their lifetime—was detected. Therefore, this method of storing and regenerating hydrogen might have a use in hydrogen fuel cells.

Further efforts to optimize the hydrogen storage process are ongoing using several catalysts with the same design principle.

This research was funded by the DOE Office of Science, a Goldhaber Distinguished Fellowship, and by the Japanese Ministry of Economy, Trade, and Industry.

Resources

  • Jonathan F. Hull, Yuichiro Himeda, Wan-Hui Wang, Brian Hashiguchi, Roy Periana, David J. Szalda, James T. Muckerman & Etsuko Fujita (2012) Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures. Nature Chemistry doi: 10.1038/nchem.1295

Comments

Davemart

By weight, formic acid only contains 4.3% hydrogen.
That is less than the ~5% by weight in a carbon fibre tank containing hydrogen, so this sounds more like a transport medium than something you have in your car, although to be sure weight is not the only consideration.
Storage would be cheaper than in a carbon fibre tank, and at 50kwh/kg hydrogen you are still getting a couple of kilowatt hours/kilogram liquid.
At 60% efficiency through a fuel cell that is still 1.4kwh/kg, many times better than batteries.

Its all of no use whilst it uses iridium though:
'Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes.' (Wiki)

A D

Put all these marvelous hydrogen gadjets on the market before chrismas of this year. There will be a big economic recession if by 21 december 2012 there no hydrogen on the market. Please follow the science of nostradamus and start selling hydrogen technology right now, it's written now so no more fuss..

kelly

"Its all of no use whilst it uses iridium though:
'Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes.' (Wiki)"

It's always something..

Darius

Smart positioning: 'hydrogen storage'. I would name it 'new type of synthetic fuel'.

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