Enabling the metal fuel economy: green recycling of metal fuels

Energy carriers are required to store and transport clean, renewable energy. Currently, most scientific efforts are invested in batteries and hydrogen despite several drawbacks, such as low energy densities and, for hydrogen, a serious risk of accidental or intentional explosions. Metal fuels, which present an inherently low safety risk and have high energy densities, have been proposed as advanced storage and transport systems for large quantities of clean energy but have received far less attention. Metal powders or sprays can be burned with air to produce heat, or can be reacted with water to produce heat and hydrogen. The hydrogen produced can then fuel either an internal combustion engine or a fuel cell. Metal fuel utilization produces metallic oxides, which can be captured and cyclically reduced back into metal fuels ad infinitum. The recycling process to produce metal fuels must use renewable energy to ensure an environmentally friendly cycle. A set of 5 constraints guiding the choice for potential metal fuels, along with the available commercial and research-stage green recycling techniques, lead to a selection of 7 different metals that could be used as recyclable fuels in the near future.