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

Chemical transformations that are effected by sequential addition of acid and base can be performed in closed-loop processes powered by electricity if the acid and base are produced electrochemically from water. 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 scalability. Here we use an ion transport model to guide the design of an acid-base co-generating 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 co-generation 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 Mg(OH)2 * 2 MgSiO3, which are shown to be competent for removing CO2 from dilute streams to form Mg carbonates.