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McGill team develops simple system for reversible H2 storage using organic cyclic hydrocarbons; alternative route to solar fuels

A team at McGill University in Canada has developed a reversible hydrogen storage/release system based on the metal-catalyzed hydrogenation and photo-induced dehydrogenation of organic cyclic hydrocarbons at room temperature. The system, they suggest in a paper published in the Journal of the American Chemical Society, provides an alternative route to artificial photosynthesis for directly harvesting and storing solar energy in the form of chemical fuel.

The system easily switches between hydrogen addition (>97% conversion) and release (>99% conversion) with superior capacity of 7.1 H2 wt% using a rationally optimized platinum catalyst with high electron density, simply regulated by dark/light conditions. In a paper published in the Journal of the American Chemical Society, the researchers reported that the photodriven dehydrogenation of cyclic alkanes gave an excellent apparent quantum efficiency of 6.0% under visible light illumination (420–600 nm) without any other energy input.

Since the first discovery of water photolysis on TiO2 electrodes by Honda and Fujishima in the early 1970s, utilizing solar energy to either produce renewable fuels or trigger valuable transformations has become one of the most sustainable and promising strategies to solve the growing global concerns on energy supply and environmental issues, which are among the biggest challenges facing our society today.

… clean H2 produced from the water-splitting reaction is a very attractive energy storage carrier due to its exceptional mass energy density (142 MJ kg−1), which is at least three times higher than that of other chemical fuels. Unfortunately, the problem of hydrogen storage and transportation is a major obstacle to employing hydrogen as a universal fuel since hydrogen has a very low volumetric density at ambient conditions. This has prompted great efforts to replace the current method of using compressed gas hydrogen (30−70 MPa) or liquid hydrogen (−253 °C) in tanks.

The major challenge is how to design a high capacity system with superior reversibility that can realize both hydrogen release and regeneration processes under practical conditions. … To achieve economical and high-density hydrogen storage, it is preferable to construct the system based on abundant and lightweight elements. In this regard, a promising strategy is to add hydrogen to the unsaturated aromatic hydrocarbons that contain only carbon and hydrogen atoms. For instance, benzene, the simplest aromatic, can bear six hydrogen atoms to form cyclohexane through a metal-catalyzed hydrogenation reaction.

—Li et al.

The researchers said that the cleavage of six covalent C−H bonds from one cyclohexane molecule to form benzene and hydrogen (Eq 1) stores comparable amount of energy to that of splitting one water molecule (Eq 2), supporting the potential for a new approach to the solar energy harvesting.

C6H12(g) → C6H6(g) + 3H2(g),  ΔH(298K) = 206 kJ/mol (1)

2H2O(g) → 2H2(g) + O2(g),  ΔH(298K) = 484 kJ/mol (2)

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The reaction starts with the activation of cyclohexane C−H bonds on the exposed Pt facets. Upon visible light irradiation, cyclohexane loses two hydrogen atoms to form cyclohexene. 1,3-Cyclohexadiene produced from the further dehydrogenation of cyclohexene can quickly go through the dehydrogenation process or react with cyclohexene to form benzene.

After the dehydrogenation reaction, the produced hydrogen can very slowly add back into benzene to form cyclohexane again under darkness at room temperature in the presence of Pt@ TiO2−N. The hydrogenation process can be accelerated dramatically under slightly higher temperature. Credit: ACS, Li et al.

In summary, our detailed studies have shown that the abundant and low-cost petrochemicals can be made possible for large scale reversible hydrogen storage/release. The apparent quantum efficiency for solar-to-hydrogen conversion can reach 6% under white light illumination, which can be further improved by optimizing the photon absorption. Compared with on-board hydrogen storage, this strategy is more promising for stationary applications due to the requirement of light. The efforts to further reduce the amount of Pt and develop other inexpensive metal catalysts are in progress.

—Li et al.

Resources

  • Lu Li, Xiaoyue Mu, Wenbo Liu, Zetian Mi, and Chao-Jun Li (2015) “Simple and Efficient System for Combined Solar Energy Harvesting and Reversible Hydrogen Storage” Journal of the American Chemical Society doi: 10.1021/jacs.5b03505

Comments

DavidJ

Don't see how it is storing solar energy, and nor is it a convenient hydrogen storage mechanism given that it requires light to release the hydrogen. It does provide a method for solar harvesting, but at 6% is currently worse than solar cells.

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