(124a) Ammonium Formate As a Safe, Energy-Dense Electrochemical Fuel Ionic Liquid

Hydrogen has been investigated as an energy carrier for clean electrons; however, its low energy density and compression losses limit its use. Accordingly, solid and liquid hydrogen carriers have been proposed, including liquid ammonia, formic acid, methanol, etc. While these fuels have significant potential, many do not meet the energy efficiency requirements to outperform hydrogen. For example, liquid ammonia has a high hydrogen content (110 kg H₂/m³) compared to compressed hydrogen gas (40 kg H₂/m³), and it is already produced, transported, and stored at scale. Formic acid has a slightly higher hydrogen content (53 kg H₂/m³) than compressed hydrogen gas and is also produced at scale. In both cases, there has been recent progress on production with renewable electricity. However, both chemicals suffer from safety issues because they are caustic and corrosive, as well as difficulties with efficient energy release at scale.

In this work, we investigate ammonium formate, a combination of ammonia and formic acid, as an energy carrier. Its production is as cheap and simple as its constituent molecules, and it is a solid at ambient conditions, making it safe to transport and store. We demonstrate an electrochemical cell that takes advantage of the fact that ammonium formate can form an ionic liquid at temperatures slightly above 100℃. In this cell, hydrogen evolves at the cathode with ca. 100% Faradaic efficiency and formate oxidizes to carbon dioxide at the anode with ca. 100% Faradaic efficiency. In theory, ammonia could be oxidized at the anode; however, ammonia evaporates at this operating temperature and can be oxidized using a second, modular ammonia fuel cell. Because molten ammonium formate is conductive, the electrolyte is majority fuel, generating a large thermodynamic driving force. Overall, this system represents a new class of electrochemical fuel ionic liquids and results in modular release of hydrogen with potentially zero net carbon emissions.