Production of Liquid Solar Fuels and Their Use in Fuel Cells

This review focuses on the production of liquid fuels using solar energy combined with their use in direct liquid fuel cells. The production of formic acid, which is the two-electron reduced product of CO₂, as a solar liquid fuel as well as a hydrogen storage material is discussed together with its use in direct formate fuel cells. Other CO₂ reduction products such as methanol and formaldehyde as solar liquid fuels as well as hydrogen storage materials are reviewed with the performance of the corresponding fuel cells. The production of nitrogen fixation products such as ammonia and hydrazine is also reviewed together with their fuel cells. Finally, the production of hydrogen peroxide from not only pure water but also seawater and dioxygen in the air using solar energy is discussed and combined with the recent development of one-compartment hydrogen peroxide fuel cells. As the concerns increase worldwide on rapid depletion of fossil fuels, the resulting global warming, climate change, and environmental damage by consumption of fossil fuels, conversion of solar energy into easily storable chemical energy such as liquid fuels and their use in liquid fuel cells have attracted more and more attention as the key technologies to ensure a stable supply of energy as renewable alternatives to fossil fuels. This review focuses on the production of liquid fuels using solar energy, so-called solar liquid fuels, combined with their use in direct liquid fuel cells. First, the production of formic acid, which is the two-electron reduced product of CO₂, as a solar liquid fuel as well as a hydrogen storage material is discussed together with its use in direct formic acid and formate fuel cells. Then, other CO₂ reduction products such as methanol and formaldehyde as solar fuels as well as hydrogen storage materials and the performance of the corresponding fuel cells are reviewed. The production of nitrogen fixation products such as ammonia and hydrazine is also reviewed together with the ammonia and hydrazine fuel cells. Finally, the production of hydrogen peroxide from not only from water but also seawater, which is the most earth-abundant material, and dioxygen in the air using solar energy is discussed along with the recent development of one-compartment hydrogen peroxide fuel cells without membrane. All the liquid fuels discussed in this review have both merits and demerits in terms of energy density, storage, toxicity, and safety for their production and their use in liquid fuel cells. In each case, development of more efficient and selective catalysts for both solar-light-driven production of liquid fuels and their use in liquid fuel cells is required to establish an energy-sustainable society with no global warming and no depletion of fossil fuels. The production of liquid fuels such as hydrogen peroxide from not only from water but also seawater, which is the most earth-abundant material, and dioxygen in the air using solar energy is combined with the recent development of one-compartment liquid fuel cells for an energy-sustainable society with no global warming and no depletion of fossil fuels.