<![CDATA[Marine sediment microbial fuel cells (MS-MFCs) provide a sustainable approach for harvesting energy from benthic environments, but their performance is limited by slow electron-transfer kinetics, unstable power output, and charge-storage capability. This study investigates the coupled kinetic and capacitive enhancement of MS-MFCs through co-modification with biomass-derived hydrochar (HC) and acetate as complementary electron-transfer and metabolic modulators. Four sediment compositions containing 0, 5, 10, and 15% (v/v) HC were operated for 30 days under a 1 kΩ external load, with acetate introduced on Day 21 to stimulate microbial metabolism. Electrochemical behavior was evaluated using cyclic voltammetry, electron-transfer kinetic analysis, current-density monitoring, power-density measurements, and physicochemical characterization of the anolyte. The apparent electron-transfer rate constant (ks) increased from 1.77 s⁻¹ in the unamended control to 3.19 s⁻¹ and 3.49 s⁻¹ in the 10% and 15% HC systems, respectively. Maximum power densities reached 21.8–23.1 mW m⁻², approximately three orders of magnitude higher than the control. Hydrochar also improved redox stability, ionic conductivity, and apparent capacitive behavior by providing a porous, conductive, and pseudocapacitive scaffold that supported microbial attachment and facilitated microbe–electrode coupling. Meanwhile, acetate served as a readily metabolizable carbon source that accelerated microbial activity and enhanced electron delivery to the anode. The strongest performance was observed at 10–15% HC, although the 15% system showed mass-transfer limitations during operation. These findings demonstrate a synergistic relationship between kinetic enhancement, substrate utilization, and capacitive charge buffering, offering a mechanistic basis for designing robust, self-sustaining MS-MFCs for in situ coastal energy recovery and environmental monitoring]]>
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