(Invited) Energy-Efficient and Impurity-Tolerant Electrochemical Co-Generation of Acid and Base Solutions

Abstract

The sequential application of acid and base solutions can be used to perform chemical transformations that are of interest for carbon management and sustainable resource utilization, including CO2 capture, CO2 removal, production of slaked lime, and recovery of critical minerals. Electrochemical co-generation of acid and base from salt solutions enables their use in closed-loop processes that could be powered by renewable electricity, wherein acid and base are regenerated from the salt byproduct of their application. To be viable at scale, acid-base co-generating systems need to be energy-efficient and tolerant to impurities that are inevitably released into recycle loops. Conventional electrochemical methods for co-generating acid and base, such as bipolar membrane electrodialysis (BPMED), rely on the use of ion exchange membranes (IEMs) to inhibit H+ and OH– recombination, but IEMs cause large resistive losses that severely limit the energy efficiency and current density of these systems. In addition, cation exchange membranes (CEMs) do not tolerate polyvalent cations (e.g., Ca2+, Mg2+) in the presence of base, yet polyvalent cations are intrinsic to all high-impact applications. In this talk, I will describe our development of acid-base co-generating systems in which H+ and OH– recombination is inhibited by competitive transport of supporting electrolyte and H+ masking, which enables the use of a simple porous diaphragm separator instead of IEMs. We developed an ion transport model to predict the current efficiency for acid-base co-generation as a function of the electrolyte composition and operating parameters. Instead of a bipolar membrane, our systems use the H2 oxidation reaction (HOR) and H2 evolution reaction for H+ and OH– generation, respectively. We demonstrate IEM-free production of acid and base at useful concentrations in the presence of polyvalent cations with much higher energy efficiency and current density than state of the art BPMED systems. Individual cells can be stacked by combining HOR and HER electrodes into bipolar gas diffusion electrodes that recirculate H2 with near-unity efficiency. To demonstrate the utility of the acid and base solutions generated by this system, I will describe their use to extract Mg(OH)2 from ultramafic rocks for application in CO2 removal.