Air-breathing bio-cathodes based on electro-active biochar from pyrolysis of Giant Cane stalks

An innovative low-tech solution to fabricate electro-active biochar (e-biochar) electrodes for bio-electrochemical systems (BES) is proposed. Ligno-cellulosic stalks of Giant Cane (Arundo Donax L.) were subjected to pyrolysis treatment at 900 °C for 1 h. The material kept its original hollow cylindrical shape, rigid morphology and porous texture, as confirmed by 3DX-ray micro-computed tomography. These characteristics are suitable for its use at the air-water interface in BES, as air-breathing bio-cathodes. BET (Brunauer-Emmett-Teller) specific surface area was equal to 114 ± 4 m2 g−1, with more than 95% of pores in the microporosity range (pore diameter < 1 nm). Surface electrocatalytic activity was sufficient to sustain oxygen reduction reaction at pH 7, in terms of both onset potential (−0.02 V vs Ag/AgCl) and reduction limiting current density (1 A m−2). Electrical resistivity measurements confirmed sufficient conductivity (8.9 × 10−3 ± 1 × 10−4 Ω m) of the material and Raman spectroscopy allowed to estimate a graphitization degree in relation to the ID/IG, equal to 2.26. In parallel, the e-biochar were tested as air-exposed bio-cathodes in BES, coupled to carbon cloth bio-anodes. After inoculation with wastewater from swine-farming, current densities were generated in the range of 100–150 mA m−2, along more than 2 months of operation, under sodium acetate feeding. Confocal laser scanning imaging revealed consistent biofilm formation on the water-side surface of the cathodes, while a nearly-complete absence of it at the air-side. These e-biochar electrodes might open innovative perspectives to scale-up BES for different applications. Here, consistent salts depositions on the material after 70 days of exposure to the wastewater, suggest that e-biochar biocathodes might serve to recycle nutrients to agricultural soils, through minerals-enriched biochar.