Metal-free activated biochar as an oxygen reduction reaction catalyst in single chamber microbial fuel cells

Metal-free catalysts are promising candidates for Bio-electrochemical Systems (BESs) due to their high surface area, chemical stability, good electrical conductivity, and enhanced mass-transport capabilities. In this study, biochars derived from olive mill waste (OMW) with and without supercritical CO2 pretreatment, and salted pistachio nut shells, were produced via pyrolysis and/or chemical and physical activation. The catalytic activity towards oxygen reduction reactions (ORR) of the biochars was investigated by cyclic and linear sweep voltammetry in neutral media. The electrochemical characterization of the samples revealed that olive mill waste biochar showed the highest catalytic activity toward ORR, in terms of reaction rate (Epr V vs. RHE = 0.537 +/- 0.00), density of active sites, and number of electrons exchanged (ne- E-@ -0.6 V vs. RHE = 3.9 +/- 0.2). These biochars were used as catalysts in air cathode microbial fuel cells. The power density obtained by MFCs equipped with an OMW cathode achieved a maximum power density of 271 +/- 34 mWm(-2) (R-ext = 250 Omega). This value was approximately 15 times higher than the power density obtained by a commercial carbon black used as control.