Energy-Efficient Seawater Carbon Removal using Reversible Phenazine-Mediated pH Swings: Insights from Experiments, Modeling, and Machine Learning

<title>Abstract</title> <p>Removing dissolved inorganic carbon (DIC) from seawater can draw down atmospheric CO₂, but electrochemical approaches have been constrained by high energy input, reliance on bipolar membranes or noble-metal catalysts, short demonstrations, and acidic effluents that re-release CO₂. Here we report an electrochemical ocean carbon removal process using a single redox-active phenazine–carbon nanotube electrode that drives pH swings using reversible proton-coupled electron-transfer (PCET). The electrode fully converts DIC to extractable CO₂ and restores seawater alkalinity within one cycle, and coupling it to automated batchwise delivery sustains &gt;50h of operation while returning a benign alkaline effluent. We achieve 79–96% removal from synthetic and real seawater at 114.8 ± 5.8 and 136.6 ± 17.7 kJ mol⁻¹ CO₂ — to our knowledge the lowest energy and longest demonstrations yet reported. A reaction–transport model, a SHAP interpreted machine-learning surrogate, and techno-economic analysis show that kinetic and transport losses, not thermodynamics, set performance, and yield transferable, material-agnostic design rules for scalable, low-cost deployment.</p>