Electrochemical Performance of Pre-Modified Birch Biochar Monolith Supercapacitors by Ferric Chloride and Ferric Citrate

This study investigated the electrochemical properties of supercapacitors by pre-modifying thick birch biochar monoliths with FeCl₃ or C₆H₅FeO₇ solutions prior to wood pyrolysis. The pre-modification introduced iron species to the surface, promoting the specific surface area, charge-stored species, and surface functionalities, which enhanced the gravimetric capacitance. X-ray diffraction confirmed the successful loading of Fe₃O₄ and Fe. SEM implied the wider distribution of iron-rich particulates and porous carbon via self-pyrolysis on the biochar surface modified with 1.0 M C₆H₅FeO₇. Contact angle measurements demonstrated the enhanced wettability of the biochar surfaces following pre-modification, with the C₆H₅FeO₇-modified samples exhibiting superior wettability compared to the other groups. The gravimetric capacitance of the supercapacitor was dramatically promoted and reached 210 F/g and 219 F/g, respectively, when modified with 1.0M C₆H₅FeO₇ and 1.0 M FeCl₃ at a 5 mA/g current density. Compared to the birch biochar modified with 1.0 M FeCl₃, the 1.0 M C₆H₅FeO₇ had a higher current response peak and capacitive behavior in the CV analysis, demonstrated better ion diffusion capacity, and had lower charge-transfer resistance in the EIS results. But, a slight irreversible process on the electrode of the 1.0 M C₆H₅FeO₇ group led to a lower level of the supercapacitor capacitance retention. The results using ferric solution pre-impregnation show how iron species doping can improve capacitance behavior, providing a feasible scheme for the modification of thick biochar monolith.