Using earth abundant materials for long duration energy storage: electro-chemical and thermo-chemical cycling of bicarbonate/formate

Abstract

Using hydrogen to store energy in chemical bonds is a key component of the global strategy to achieving a sustainable future and ameliorating climate change. The challenges associated with handling molecular hydrogen can be solved by using liquid hydrogen carriers. In this perspective, we discuss the concept of bicarbonate–formate cycle, where aqueous solutions of formate ions (HCO₂⁻) are used as hydrogen and energy carriers. Such solutions are composed by earth abundant elements and, in contrast to common liquid organic carriers, are non-flammable, and readily convert to the oxide forms (HCO₃⁻) under reaction with water to release hydrogen, or electrons, at moderate temperatures. We discuss thermodynamic aspects of the bicarbonate–formate cycle and show how it offers the opportunity of combining electrochemical and thermochemical operations, as well as of coupling CO₂ capture with energy/hydrogen storage. We emphasize the potential role of electrochemistry in the generation of formate and in the release of energy in the form of electricity. At present, more information on both the fundamental and systems level, is needed to identify the feasible scenarios for using formate/bicarbonate salts for hydrogen/energy storage. It is likely, however, that several strategies, including hybrid electrochemical–thermochemical approaches, will suit different applications. It is also clear that more integration between the disciplines of electrochemistry and heterogeneous catalysis is needed to overcome the challenges for advancing the HCO₃⁻–HCO₂⁻ system as a feasible green alternative for storing and transporting energy.