This study proposes a strategy for phosphorus recovery from algae biomass while simultaneously enhancing the electrochemical capacitance of resulting biochar in supercapacitor applications. Use of sulphuric acid leaching process on both wastewater microalgae and seaweed biochar demonstrated high phosphorus recovery rates, ranging from 93.4% to 95.4%, while at the same time retaining 52.7% to 58.6% of Fe content in the biochars. By modifying the sequential order of leaching (L) and physical activation (A) as LA and AL, the strategy allows for the optimisation of phosphorus recovery and the electrochemical properties of activated biochars. During the LA process, a more porous structure formed and more S-containing functional groups occurred compared to AL which explained the higher specific capacitance (486.3F g−1 at a current density of 1 A g−1) of wastewater microalgae biocarbon with the LA process. Consequently, the life cycle assessment of this strategy revealed a significant global warming reduction potential of 1.34–2.94 tonnes of CO2-eq/yr for each tonne of biochar produced. This indicates that sulphuric acid-assisted biochar production could be a sustainable strategy for waste management and carbon sequestration, while simultaneously generating economic profits from valorised carbon material for energy storage applications.