Membrane-free electrochemical production of acid and base solutions capable of processing ultramafic rocks

Abstract

Electrochemical production of acid and base from water enables their use as regenerable reagents in closed-loop processes, with attractive applications including CO2 capture or mineralization and low-temperature production of Ca(OH)2. Conventional systems utilize ion exchange membranes (IEMs) to inhibit H+/OH– recombination, which leads to high resistive losses that compromise energy efficiency and poor tolerance for polyvalent metal ions that complicates applications involving mineral resources. Here we use ion transport modeling to guide the design of a system that uses a simple porous separator instead of IEMs. Using H2 redox reactions for H+/OH– production, we demonstrate acid-base production at useful concentrations in the presence of polyvalent impurities with lower energy demand and higher current density than reported IEM-based systems. Cells can be stacked by combining H2 electrodes into a bipolar gas diffusion electrode, which recirculates H2 with near-unity efficiency. We show that the cell outputs extract alkalinity from olivine and serpentine as Mg(OH)2 and Mg3Si2O6(OH)2, which remove CO2 from ambient air to form 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.