Continuous Carbonation Enables Sub-1-Volt Dual-pH Water Electrolysis for Co-production of Hydrogen and Solid Bicarbonate

Hydrogen production through water electrolysis is a promising pathway for achieving sustainable energy conversion; however, its widespread deployment remains constrained by the high energy demand associated with conventional electrolysis systems. Here, we report an indirect CO₂-coupled dual-pH electrolysis architecture that leverages carbon-mediated pH regulation to enable low-voltage hydrogen production. In this system, a concentrated alkaline anolyte is combined with a bicarbonate catholyte separated by a K⁺-conducting membrane to establish a stable pH gradient that substantially reduces the thermodynamic voltage required for water splitting. Unlike in conventional CO₂ utilization strategies based on direct electrochemical reduction, CO₂ is indirectly utilized through bicarbonate equilibria that continuously regulate the catholyte environment, suppress pH drift, and sustain efficient hydrogen evolution. Simultaneously, spatially separated KHCO₃ precipitation mitigates membrane scaling and promotes long-term operational stability. Consequently, the asymmetric pH configuration lowers the theoretical equilibrium cell voltage to 0.875 V, and the developed alkaline-ion carbon electrolysis cell delivers current densities exceeding 2.0 A cm^‒2 at 2.0 V and 60 °C with a H₂ Faradaic efficiency of 96%. Furthermore, the electrolyzer operates stably for over 100 h using simulated flue gas (15% CO₂/85% N₂) without performance degradation, and the solid product is phase-pure KHCO₃, demonstrating compatibility with dilute industrial CO₂ streams. This integration of carbon-mediated chemical buffering with electrochemical water splitting establishes a new strategy for reducing the energy demand of hydrogen production while enabling carbon utilization, thus providing a scalable pathway toward energy-efficient and carbon-integrated electrolysis technologies.