In addition to organic hydrogen storage materials such as methanol and methane, researchers have been interested in formic acid (HCO₂H) and its salts, known as formates, for the generation of hydrogen. These chemical compounds are nontoxic and stable with a hydrogen content of 4.4 wt % (FA) and 2.35 wt % (for NaHCO₂/H₂O). As formates are noncorrosive and nonirritating, they are also easy to handle, Boddien et al. note in their paper.

A fundamental problem with the use of these storage materials is the separation of the carbon dioxide formed when the hydrogen is released. The team from Rostock has now successfully used a special ruthenium catalyst that catalyzes both the release and uptake of hydrogen to establish a reversible, CO₂-free hydrogen storage cycle.

Ideally, the hydrogen carrier, CO₂, should be re-used and a closed carbon cycle should be achieved. Therefore, trapping carbon dioxide in the dehydrogenation process is fundamental. Unfortunately, this has not yet been achieved. Here, we describe for the first time the design of a reversible hydrogen storage cycle based on the redox system bicarbonate/formate.

Our concept has the following advantages: Compared to carbon dioxide, solid bicarbonates are easy to handle and highly soluble in aqueous media (96 g L^-1 NaHCO₃ at 20 °C in H₂O). The resulting aqueous bicarbonate solution can be catalytically converted to a formate solution under much milder conditions than those required for reactions of methanol or methane. The nontoxic aqueous solution of formate is easily stored and transported. Finally, hydrogen can be released on demand in the presence of a suitable catalyst. Again, this hydrogen discharge can be performed at or below room temperature.

Most importantly, after full conversion of the formate, the bicarbonate solution may be recharged with hydrogen to close the cycle. To the best of our knowledge, no catalyst system has yet been described that can facilitate both reaction pathways under basic conditions and also trap the CO₂ formed in the dehydrogenation.

—Boddien et al.

Bicarbonates are a component of many natural stones and are also commonly used as baking powder or sherbet (sodium bicarbonate, NaHCO₃).

In their study, they achieved hydrogenation of NaHCO₃ to sodium formate in 96% yield at 70 °C in water/THF without additional CO₂. Dehydrogenation of sodium formate was achieved with high conversion (> 90 %) under ambient temperature (30 °C). In contrast to their earlier studies on the dehydrogenation of formic acid/amine adducts, this new process is amine-free.

Our new concept has a number of advantages. In comparison to CO₂, solid bicarbonate is easy to handle and is very soluble in water. The resulting bicarbonate solution can be catalytically converted to a formate solution under much milder conditions than those required for the reactions to form methane or methanol.

—Matthias Beller

In addition, the harmless solid could easily be stored and transported. Retrieval of the hydrogen occurs at room temperature or even lower.

Most important is that a closed carbon cycle is now possible because the resulting bicarbonate can simply be loaded up with hydrogen again.

—Matthias Beller

Resources

• Albert Boddien, Felix Gärtner, Christopher Federsel, Peter Sponholz, Dörthe Mellmann, Ralf Jackstell, Henrik Junge, and Matthias Beller (2011) CO₂-“Neutral” Hydrogen Storage Based on Bicarbonates and Formates. Angewandte Chemie International Edition doi: 10.1002/anie.201101995