Hydrogen storage as liquid solar fuels

Because hydrogen (H2) is an explosive gas and the volumetric energy density is quite low, it is highly desired to develop liquid or solid solar fuels as safe hydrogen storage alternatives. This review article focuses on catalytic interconversion between hydrogen and liquid or solid solar fuels using metal complexes as redox catalysts. The first candidate of a liquid solar fuel is formic acid (HCOOH), which is liquid at room temperature and has a fairly high volumetric density of 1.22 g cm−3. HCOOH is produced by hydrogenation of CO2 by H2 with various metal complex catalysts under mild conditions. HCOOH thus produced is used as a fuel in direct HCOOH fuel cells. Hydrogen peroxide that is produced from the catalytic reaction of H2 with O2 is used as a fuel in formate-peroxide fuel cells as well as direct H2O2 fuel cells. Cyclic alkanes and N-heterocyclic species have also been used as liquid organic hydrogen carriers (LOHCs) which can liberate H2 on demand using dehydrogenation catalysts and photocatalysts under photoirradiation, because dehydrogenation of organic molecules to release H2 is an uphill process. LOHCs are also used as fuels in direct liquid fuel cells (DLFCs). On the other hand, plastoquinol is used as an effective hydrogen carrier in natural photosynthesis. In the artificial system, hydroquinones (plastoquinol analogues) are much safer and easier to handle for storage as well as transportation, because hydroquinones unlike H2 are nonflammable and nontoxic and can be safely stored in liquids or as solids. Hydroquinones are produced by photocatalytic reduction of p-benzoquinones with water using a molecular functional model of photosystem II in photosynthesis. Hydrogen is produced by photocatalytic oxidation of hydroquinones using a molecular functional model of photosystem I in photosynthesis. In addition, direct hydroquinone/quinone fuel cells have been developed using anthrahydroquinone-2,7-disulfonic acid (AQDS) as an LOHC.