Adding Value to Biomass via Energy Applications: High-Performance Biocarbon from Sewage Sludge for Supercapacitors and Fuel Cells

A sewage sludge-derived biocarbon (SSB) was produced using biomass from a water treatment plant in Saltillo, Mexico. Its specific surface area was 412.0 m 2 g −1 , with an average pore diameter of 4.4 nm. Evaluation in an SSB||SSB symmetrical supercapacitor showed a high performance in several electrolytes (6 M KOH, 0.5 M Na 2 SO 4 , and 1 M NaClO 4 ). The supercapacitor in KOH exhibited the highest specific capacitance (C sp ) of 144.80 F g −1 at 1 mV s −1 . Nevertheless, the widest operating voltage window of 2.5 V was attained in NaClO 4 . This enhanced behavior led to achieving energy density (E D ) and power density (P D ) values of 22.52 Wh kg −1 and 508.0 W kg −1 , respectively, in the organic electrolyte. Moreover, rotating ring-disc electrode tests in 0.1 M KOH showed that SSB catalyzes the oxygen reduction reaction, with a low percentage of perhydroxyl ion produced ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>%</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> ) of 7.41% and a high electron transfer number (n) of 3.85 before an accelerated degradation test of 3000 cycles, sustaining a high stability after the cycling. This study provided evidence that high-performance active catalytic materials for symmetric supercapacitors and cathodes for anion exchange membrane fuel cells can be produced from Earth-abundant sewage sludge biomass.