Carbonate gradient reverse electrodialysis (CG–RED) enabled by bipolar membrane–induced pH–swing

Reverse electrodialysis (RED) is a promising technology for harvesting salinity–gradient energy; however, its performance is fundamentally constrained by the limited ionic concentration differences available in natural water sources. Here, we propose a carbonate gradient reverse electrodialysis (CG–RED) system in which the conventional NaCl salinity gradient is coupled with pH–dependent carbonate speciation. Carbonate speciation is actively regulated using a bipolar membrane (BPM)–induced pH–swing, generated via water dissociation under reverse bias. This pH modulation alters the relative distribution of carbonate species, thereby modifying the effective Nernstian driving force through redistribution of ionic contributions within the membrane system. Under representative operating conditions, the CG–RED system exhibits changes in open circuit voltage and power density compared to conventional NaCl–based RED, reflecting the combined effects of carbonate addition and pH–dependent speciation control. The contribution of the pH–swing itself results in a modest incremental change in power output under otherwise identical electrolyte conditions. Importantly, the present system does not achieve net-positive energy output, as the electrical energy required for the BPM–driven pH–swing exceeds the incremental gain in CG–RED performance under the current configuration. Therefore, this work is intended to provide mechanistic insight into the coupling between carbonate equilibrium, pH modulation, and ion transport in multi-ionic RED systems, rather than to demonstrate an energetically optimized power generation device. These findings establish pH–driven speciation control as an additional electrochemical degree of freedom for modulating ion transport behavior in salinity gradient systems.