Abstract
The increasing need for energy storage systems in the transition to renewable energy is accelerating the search for sustainable raw materials, with hyperaccumulator plants offering unique, untapped sources by capturing and processing electroactive metals like nickel (Ni) into their carbon matrix. In this study, we investigated broccoli's ability to absorb and process Ni, a metal that significantly contributes to enhanced energy storage capacity. Unlike traditional phytoremediation, we focused on producing carbon-based materials with electrochemical performance rather than environmental recovery. To demonstrate our ecoconscious concept, broccoli plants were exposed to varying Ni concentrations (0, 0.4, 0.8, 1.6, and 3.2 g/L) over 10 weeks, producing shoot dry biomass with Ni contents from <20 to 492 mg Ni/kg. Following pyrolysis at 650 °C, the resulting biochar was used as electroactive material for supercapacitor applications. Surprisingly, the best performance was observed in plants with 152 mg Ni/kg, with the bionanocomposite exhibiting a specific capacitance of 38 F/g at the scan rate of 100 mV/s and excellent capacitance retention of 94 % over 5000 cycles at 10 A/g. The supercapacitor prototype presented a maximum energy density of 0.762 Wh/kg at 184 W/kg power density. This unexpected finding suggests altered plant metabolism as a potential explanation, offering new insights and raising scientific questions about engineering hyperaccumulator plants for sustainable energy storage applications.