Membrane-Free Electrochemical Production of Acid and Base Solutions Capable of Processing Ultramafic Rocks

The electrochemical production of acid and base from water enables their use as regenerable reagents in closed-loop processes that drive chemical transformations by sequential acid and base addition. Such processes have attractive applications for industrial decarbonization including CO2 capture or mineralization and low-temperature production of Ca(OH)2. Conventional methods of electrochemical acid-base production utilize ion exchange membranes (IEMs) to inhibit proton (H+) and hydroxide (OH–) recombination, but these components lead to high resistive losses, low current densities, and poor tolerance for polyvalent metal ions, which compromise energy efficiency and complicate applications involving mineral resources. Here we use an ion transport model to guide the design of an electrochemical acid-base production system that inhibits recombination via competitive transport of the supporting electrolyte and masking H+ as HSO4–, which enables the use of a simple porous separator instead of IEMs. Using the H2 oxidation reaction (HOR) and H2 evolution reaction (HER) for H+ and OH– production, we demonstrate steady-state cogeneration of acid and base solutions at useful concentrations in the presence of polyvalent impurities with lower energy demand and higher current density than state-of-the-art reported IEM-based systems. Cells can be stacked by combining HER and HOR electrodes into a bipolar gas diffusion electrode, which recirculates H2 with near-unity efficiency. The acid and base outputs of the cell are capable of extracting alkalinity from olivine and serpentine in the form of Mg(OH)2 and Mg3Si2O6(OH)2, which can be used to remove CO2 from dilute streams, including ambient air, to form thermodynamically stable Mg carbonates. These studies establish the principles for membrane-free electrochemical acid-base production, enabling closed-loop resource recovery and material processing powered by renewable electricity.