Limestone conversion to cement clinker precursor in a zero-gap electrolyzer

The carbon intensity of industrial cement production could be reduced if the high-temperature kilns used to decompose limestone (CaCO3(s)) were replaced. One possible solution is to use electrochemical reactors to convert CaCO3(s) into Ca(OH)2(s). The challenge is that the continuous-flow electrochemical reactors reported to date all require voltages that are too high (>4 V at 100 mA cm–2) to be put into practice. A key reason for these high voltages is that the reactors contain a chemical chamber, inserted between the anode and cathode chambers, that leads to a high Ohmic resistance. In this study, we present an electrolyzer that decomposes CaCO3(s) into reactive Ca2+ ions using only two chambers. This cell design, with an anode and cathode chamber separated by a membrane instead of a chemical chamber, follows a “zero-gap” design akin to hydrogen-producing electrolyzers and fuel cells. This cement electrolyzer is capable of operating at a full cell voltage (Ecell) of merely 0.38 V at 100 mA cm–2, and with 100% faradaic efficiency (FE). This strikingly low Ecell is 1.4 V lower than any other reported Ecell. We achieved this goal by not only eliminating the chemical chamber, but by also engaging the reversible redox activity of (hydro)anthraquinones to mediate oxidation and reduction within a narrow electrochemical window. This streamlined reactor is capable of operating at a record low voltages of 0.38 V at 100 mA cm–2, and 4.23 V at 1 A cm–2.