Water hyacinth (Eichhornia crassipes, WH) is cultivated in a hydroponic system containing various concentrations of Ni2+ to demonstrate phytoremediation techniques as a facile, low‐cost, and sustainable method for synthesizing high‐performance biocarbon electrode materials. A high specific capacitance of 541 F g−1 in 2 m potassium hydroxide (KOH) is achieved for WH‐5 biocarbon with an energy density of 30.5 Wh kg−1. Materials are assembled into a coin cell supercapacitor capable of lasting 10 000 cycles with 100% capacitance retention. Surface area characterizations support these results with an SBET of 3429 m2 g−1, a VBET of 2.13 cm3 g−1, and an Sp avg of 2.5 nm, indicating enhanced pore formation and functional group cleaving. Raman spectroscopy, attenuated total reflectance Fourier transform IR (ATR‐FTIR), and X‐ray diffraction (XRD) give further insight into physical characteristics of the biocarbon that lead to improved electrochemical performance. This work describes an optimal concentration of preabsorbed Ni2+ catalyst (5 ppm in H2O) capable of achieving 98% of theoretical capacitance and value‐added environmental cleanup associated with synergistic remedial techniques.