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Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures

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Abstract

Green plants convert CO2 to sugar for energy storage via photosynthesis. We report a novel catalyst that uses CO2 and hydrogen to store energy in formic acid. Using a homogeneous iridium catalyst with a proton-responsive ligand, we show 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 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. The extraordinary and switchable catalytic activity is attributed to the multifunctional ligand, which acts as a proton-relay and strong π-donor, and is rationalized by theoretical and experimental studies.

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Figure 1: Reversible H2 storage is achieved by switching the pH to protonate or deprotonate the catalyst.
Figure 2: Crystal structure of 2[Cl2]2+ and reversible formation of [2(OH2)2]4+ and 2′(OH2)2.
Figure 3: Proposed mechanism and gas-phase free energy calculations for 2′(OH2)2 after it loses protons and water.
Figure 4: Variation of the rate of H2 evolution (left y-axis, blue) with pH compared to the ionization of the thbpym ligand (right y-axis, red).

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Acknowledgements

The work at Brookhaven National Laboratory is funded under contract DE-AC02-98CH10886 with the US Department of Energy and supported by its Division of Chemical Sciences, Geosciences, & Biosciences, Office of Basic Energy Sciences. J.F.H. acknowledges support as a BNL Goldhaber Distinguished Fellow. Y.H. acknowledges support from the Japanese Ministry of Economy, Trade, and Industry. R.P. and B.H. were supported by the CCHF 101 (grant no. DE-SC0001298).

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J.F.H. and Y.H. conceived the project, carried out the bulk of the experimental work and wrote the manuscript. W.W. made significant contributions to the synthesis and characterization. R.P. and B.H. provided the thbpym ligand. D.J.S. solved the crystal structure. J.T.M. carried out theoretical work and E.F. oversaw all work.

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Correspondence to Jonathan F. Hull, Yuichiro Himeda or Etsuko Fujita.

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The authors declare no competing financial interests.

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Hull, J., Himeda, Y., Wang, WH. et al. Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures. Nature Chem 4, 383–388 (2012). https://doi.org/10.1038/nchem.1295

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