Low-Temperature Water Electrolysis Under a Sustained pH-Gradient for the Electrochemically-Induced Decarbonation of Limestone into Hydrated Lime
Authors: Rouxhet Rémy , Delmelle Renaud , Proost Joris 
Published: August 2024

Conventional low-temperature water electrolysis processes operate either at high or low pH to enhance conductivity, minimizing electrical resistance. Neutral water electrolysis in a buffer solution is considered non-competitive due to higher energy consumption. Nevertheless, interest in water electrolysis using a salt-based electrolyte has grown due to its ability to generate a pH gradient inside the cell. Electrolysis using a neutral salt-based electrolyte has higher operating costs due to its higher standard equilibrium potential (due to the pH gradient) and lower conductivity. Although this approach incurs higher operating costs, it presents a unique advantage: the production of additional valuable elements thanks to the pH gradient generated.
In 2019, Ellis & al. [1] highlighted the possibility to use this pH gradient to electrify the production of hydrated lime (calcium hydroxide). Lime has applications in different areas such as the removal of impurities during steel production and aqueous or gaseous effluent treatments. It can also be used as a precursor of cement. Cement production is accounting for 8% of global CO2 emissions. The acidity generated by water oxidation at the anode dissolves the calcium carbonate inserted. Carbon dioxide from the dissolution is recovered with the oxygen generated. The calcium cations migrate through the cationic membrane towards the cathodic compartment to react with the hydroxide ions generated by water reduction at the cathode. Calcium hydroxide is finally collected after precipitation. Figure 1 compares the conventional process with the electrochemical way and describes more precisely the steps involved in Figure 1. More recently, another way of exploiting this gradient has been proposed by Ni & al. [2] for the recycling of spent Li-ion batteries. With the growing interest in process electrification, other applications may emerge.
In this paper, this hybrid electrochemical system for the simultaneous production of hydrogen, oxygen and calcium hydroxide has been intensively studied. Water electrolysis has been performed in a two-compartment cell at different applied currents. Calcium concentration and pH measurement enabled the study of the sub-step efficiencies and equilibria in anodic and cathodic chambers. Perfect faradic efficiencies were obtained with perchlorate salt electrolytes and platinum-free electrodes. The establishment of the pH gradient has been proven and correlated with the transport number of protons through the cationic membrane. Finally, the kinetics of calcium ion migration and calcium hydroxide precipitation have been investigated as well. Continuous addition of calcium carbonate as opposed to a single excess addition was shown to be able to tune the working pH in the anodic compartment and thereby reducing the risk of precipitation on the membrane. Our experimental results lead to the formulation of practical recommendations for achieving optimal reaction stoichiometry in each stage. These findings pave the way for scaling up a promising new sustainable process for lime and cement production.
[1] L. D. Ellis, A. F. Badel and M. L. Chiang, "Toward electrochemical synthesis of cement," PNAS, vol. 117, no. 23, pp. 12584-12591, 2019.
[2] N. Jihong, Z. Jiayin, B. Jinhong and G. Xiaofei, "Recycling the cathode materials of spent Li-ion batteries in a H-Shaped neutral water electrolysis cell," Separation and Putification Technology , vol. 278, p. 119485, 2022.

Figure 1 - Conventional and electrochemical production route of Ca(OH)2 and a simplified scheme of the hybrid electrolyzer used in this work, with the reactions involved.

Figure 1