Catalysis in Liquid Organic Hydrogen Storage: Recent Advances, Challenges, and Perspectives

With the renewed interest for hydrogen as an energy carrier, means to produce, but most importantly store, transport, and distribute, ???green??? hydrogen over long distances has become important. In this context, liquid organic molecules that can be hydrogenated and dehydrogenated under mild conditions of temperature and pressure continue to attract significant attention. These liquid organic molecules referred as ???liquid organic hydrides??? in the early 1980s include molecules such as cyclohexane, methylcyclohexane, decalin, N-heterocycles, methanol, ethanol, and formic acid. However, current liquid organic hydrides still suffer from limitations along with the emergence of more effective catalysts to meet the requirement of competing (de)hydrogenation reactions, as well as new chemistry to enable their (de)hydrogenation reactions under milder conditions and extended cycle lives. Herein, we critically review common state-ofthe-art catalyst designs, which remain one of the main barriers to the effective emergence of liquid organic hydrogen carriers enabling the widespread transport and distribution of hydrogen. Many of the most effective current catalysts are based on noble metals. Transitioning away from these rare critical elements to enable hydrogen uptake/release from organic compounds under economically viable chemical routes is a necessity.