Opportunities and challenges in direct ocean capture of carbon dioxide

Abstract

Direct air capture removes CO ₂ directly from ambient air, where the concentration is ca. 400 ppm (ca. 0.04 vol%) and gas-phase mass transfer is intrinsically challenging. By contrast, seawater typically contains ca. 2.3 mmol L ⁻¹ dissolved inorganic carbon, providing a substantially higher aqueous carbon concentration and motivating direct ocean capture (DOC) as a complementary carbon dioxide removal route. This Perspective argues that electrochemical DOC cannot be assessed from cell-level performance alone. Three representative architectures, namely (1) bipolar-membrane electrodialysis, (2) membrane-less chloride-mediated cycling, and (3) electrochemically induced mineralisation, are used as case studies to examine how reported performance translates into system-level operation. Particular emphasis is placed on consistent energy and material boundaries, seawater throughput, durability, balance-of-plant requirements, and air-sea re-equilibration.