Supercapacitive decoupled electrolyzer towards enhanced green hydrogen production

Green hydrogen production through renewable-powered water splitting is pivotal for achieving net-zero carbon emissions. However, its large-scale adoption remains challenged by high energy demands, system complexity, and the need for flexible integration with intermittent renewable energy sources. Decoupled water electrolysis, in which hydrogen and oxygen are produced in temporally separate steps, provides a promising route for membrane-free and inherently safer operation. Here, we present a membrane-free supercapacitive decoupled electrolyzer that couples a nitrogen-doped activated carbon (N-AC) capacitive mediator with an earth-abundant CoFeP bifunctional catalyst. The primary objective is to investigate capacitive-electrode engineering as a strategy to enhance charge storage and prolong decoupled operation. Structural and morphological characterization by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), and nitrogen adsorption-desorption analysis confirmed successful nitrogen incorporation, increased structural disorder, and an ion-accessible porous architecture in N-AC. Despite a reduction in specific surface area, nitrogen modification improved electrochemical performance, increasing the specific capacitance from 95.5 to 148.8 F g −1 at 10 mV s −1 . Under identical conditions at 10 mA cm −2 , the charging duration increased from 275 s for activated carbon (AC) to 436 s for N-AC, while the full electrode charge-discharge cycle duration increased from approximately 530 s (AC) to 850 s (N-AC). These results demonstrate that nitrogen functionalization enhances charge-storage capacity and extends the duration of uninterrupted decoupled operation. The assembled 5 × 5 cm 2 membrane-free supercapacitive electrolyzer operated over a current-density range of 10-100 mA cm −2 and exhibited a full HER/OER cycle duration of approximately 2000 s at 10 mA cm −2 . In situ H 2 and O 2 microsensor measurements confirmed temporally separated gas evolution during the alternating HER and OER stages. The cell-level DC energy consumption was calculated to be approximately 53-65 kWh kg −1 H 2 on a lower-heating-value basis. Rather than improving all device-level performance metrics, this work demonstrates that nitrogen-functionalized capacitive mediators can substantially increase charge-storage capacity and prolong decoupled hydrogen-production operation, providing a scalable strategy for membrane-free electrolysis systems.