(674d) Ammonium Formate As a Safe, Energy-Dense Electrochemical Fuel Ionic Liquid

Hydrogen has been investigated widely as an energy carrier for clean electrons, however its low energy density and large energy losses incurred in compression have limited its use. For these reasons, solid and liquid carriers of hydrogen fuel have been investigated, including liquid ammonia, formic acid/formate salts, and methanol, to name a few. While these proposed fuels have significant potential, many of them do not meet the energy efficiency requirements necessary to outperform compressed hydrogen and enable facile transport and storage of renewable energy. For example, ammonia is an appealing energy carrier because of its high hydrogen content: 110 kg H₂/m³ for liquid ammonia compared to 40 kg H₂/m³ for compressed hydrogen at 700 bar and 288 K. It is already produced, transported, and stored at scale, and there has been recent progress in reducing the carbon footprint of ammonia production technologies. Formic acid is also appealing due to its hydrogen content (53 kg H₂/m³), its ease of transport and storage since it is a liquid, and its potential to be synthesized efficiently from carbon dioxide, water, and renewable electricity. However, both ammonia and formic acid suffer from safety issues because they are caustic and corrosive, as well as difficulties with efficient release of energy at scale.

In this work, we propose ammonium formate, a combination of ammonia and formic acid, as an energy carrier. Not only is its production as cheap and simple as its constituent molecules, ammonium formate is a benign 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 forms an intrinsically conductive ionic liquid at 116℃; as an ionic liquid, ammonium formate has negligible vapor pressure and is significantly safer than either ammonia or formic acid alone. In this system, hydrogen is evolved at the cathode with near 100% Faradaic efficiency and formate is oxidized to carbon dioxide at the anode with near 100% Faradaic efficiency. In theory, ammonia could be oxidized to nitrogen in the same cell, however we observe that the operating temperature makes it so that ammonia gas evaporates from the cell and can be oxidized using a second, modular ammonia fuel cell. Because molten ammonium formate is conductive, the electrolyte and feed can approach 100% ammonium formate fuel, resulting in a large thermodynamic driving force. Overall, this system represents a new class of electrochemical fuel ionic liquids and results in modular release of H₂ with the potential for zero net carbon emissions.