| Summary: | This study reveals a simple approach to recycle wasted coffee grounds into highly valuable carbon material with superior electrochemical performance. Activated carbon prepared from wasted coffee grounds has been formed via hydrothermal acidic hydrolysis followed by a KOH chemical activation at 800<inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow></semantics></math></inline-formula>. To understand the electrochemical properties of the sample, a set of characterization tools has been utilized: N2 and CO2 adsorption–desorption isotherms, thermal gravimetric analysis, Fourier transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy. The specific surface area obtained from a Brunner–Emmett–Teller (BET) analysis reached <inline-formula><math display="inline"><semantics><mrow><mn>2906</mn><mo>(</mo><mn>19</mn><mo>)</mo></mrow></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mi mathvariant="normal">m</mi></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><msup><mrow></mrow><mn>2</mn></msup></semantics></math></inline-formula>/<inline-formula><math display="inline"><semantics><mi mathvariant="normal">g</mi></semantics></math></inline-formula>. Prepared sample (designated as ACG-800KOH) was tested as electrode material in an electric double layer capacitor (EDLC) device with ionic liquid PYR13-TFSI as an electrolyte. The EDLC test was conducted at temperatures ranging from 20 to 120 <inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow></semantics></math></inline-formula>. The specific material capacitance reached 178<inline-formula><math display="inline"><semantics><mi mathvariant="normal">F</mi></semantics></math></inline-formula>/<inline-formula><math display="inline"><semantics><mi mathvariant="normal">g</mi></semantics></math></inline-formula> measured at 20<inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow></semantics></math></inline-formula> and 50/<inline-formula><math display="inline"><semantics><mi mathvariant="normal">g</mi></semantics></math></inline-formula> and was in the range 182<inline-formula><math display="inline"><semantics><mi mathvariant="normal">F</mi></semantics></math></inline-formula>/<inline-formula><math display="inline"><semantics><mi mathvariant="normal">g</mi></semantics></math></inline-formula>–285<inline-formula><math display="inline"><semantics><mi mathvariant="normal">F</mi></semantics></math></inline-formula>/<inline-formula><math display="inline"><semantics><mi mathvariant="normal">g</mi></semantics></math></inline-formula> at the 20<inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow></semantics></math></inline-formula>–120<inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow></semantics></math></inline-formula> temperature range.
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