This study investigates the use of a hierarchical porous carbon electrode derived from oil palm leaves in a “water-in-salt” supercapacitor. The impact of anion identity on the electrical performance of the carbon electrode was also explored. The results show that the prepared carbon had a hierarchical porous structure with a high surface area of up to 1840 m² g⁻¹. When a 20 m LiTFSI electrolyte was used, the carbon electrode had a specific capacitance of 176 F g⁻¹ with a wider potential window of about 2.6 V, whereas the use of a cheaper 20 m LiCl electrolyte showed a higher specific capacitance of 331 F g⁻¹ due to the smaller size of the Cl⁻ anion, which enabled inner capacitance. Therefore, the anion identity has an effect on the electrochemical performance of porous carbon, and this research contributes to the understanding of using “water-in-salt” electrolytes in carbon-based supercapacitors. The study's findings provide insights into developing low-cost, high-performance supercapacitors that can operate in a wider voltage range.