  This study aims at demonstrating that wastes from anaerobic biodigester
plants can be effectively valorized as functional materials to be implemented
in technologies that enable efficient energy management and water treatment,
therefore simultaneously addressing the Water-Energy-Waste Nexus challenges.
Lignin, the main solid residue of the biodigester plant, has been valorized
into activated biochar with a mild activation agent, like KHCO3, to produce
electrode of supercapacitors and microbial fuel cells. In addition, the same
sludge that is the liquid effluent of the biodigester plant has been exploited
as inoculum and electrolyte for the MFC. The lignin derived carbons obtained at
lignin/KHCO3 mass ratios of 1:0.5 (LAC-0.5) and 1:2 (LAC-2) comprised of
mesopores and micropores displaying BETs of 1558 m2/g and 1879 m2/g,
respectively. LAC-2 carbon exhibited a superior specific capacitance of 114 F/g
in 2.5 M KNO3 with respect to LAC-0.5. A supercapacitor with LAC-2 electrodes
was built displaying specific energy specific power up to 10 Wh/kg and 6.9
kW/kg, respectively. Durability tests showed that the device was able to
maintain a capacitance retention of 84.5% after 15,000 charge-discharge cycles.
The lignin-derived carbons were also studied as electrocatalysts for ORR in a
neutral medium. The LAC-2 showed higher ORR electrocatalytic activity than
LAC-0.5. The interconnected porous network and the high surface area made the
lignin-derived porous carbons suitable electrode materials for dual
applications. The feasibility of the use of LAC 2 carbon incorporated in an air
breathing cathode for MFC applications is also reported.
