Hydrogen Storage in Formic Acid: A Comparison of Process Options

Formic acid (53 g H2/L) is a promising liquid storage and delivery option for hydrogen for fuel cell power applications. In this work we compare and evaluate several process options using formic acid for energy storage. Each process requires different steps, which contribute to the overall energy demand. The first step, i.e. production of formic acid, is thermodynamically unfavorable. However, the energy demand can be reduced if a formate salt is produced via a bicarbonate route instead of forming the free acid from hydrogen and carbon dioxide. This bicarbonate/formate approach also turns out to be comparatively more efficient in terms of hydrogen release than the formic acid route even though less energy efficient, catalytic decomposition of formic acid has the advantage of reaching higher volumetric power densities during hydrogen release. Efficiencies of all process options involve aqueous media and are dependent on concentration. Heating water leads to additional energy demand for hydrogen release and thus lowers the overall efficiency. Separation and purification of hydrogen contribute a minor impact to the overall energy demand. However, its effect on efficiency is not negligible. Other process options like thermal decomposition of formic acid or direct formic acid fuel cells thus far do not appear competitive.