Abstract

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.

Catalysis is at the core of previous energy transition. It has enabled the use of oil and natural gas as our primary energy sources in unprecedented ways and led to feedstocks enabling exceptionally high living standards in human history. In a decarbonized economy with hydrogen as the new energy vector, catalysis is already playing a key role in producing hydrogen. However, catalysts for the effective storage of hydrogen must be advanced. Many solid hydrogen storage materials such as magnesium‐based hydrides, alanates, and/or borohydrides display promising hydrogen densities far superior to the current state of compressed or liquid hydrogen. These solid materials have thermodynamic and kinetic barriers which severely hinder their practical hydrogen uptake and release. To date, most of these barriers for solid hydrides (especially boron or nitrogen compounds) are modified via catalysis; however, the catalytic species per se and their roles are obscure. Herein, a comprehensive overview of various catalysts for solid hydrogen storage materials, their catalytic roles, and the underpinning mechanisms is provided. The current state of knowledge is critically reviewed and gaps where further research intensification is needed to support rapid hydrogen generation and storage in solid materials for the emerging hydrogen economy are identified.

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