Although MFC systems have shown high potential as a renewable energy source and wastewater treatment technology, further progress is needed to improve current MFC performances. One of the current bottlenecks is to develop air-cathodes with high oxygen reduction reaction (ORR) performance, long-term stability and low material cost. In the present study, air-cathodes composed of carbon nanofibers (CNFs) doped with metals (Co, Ni or Fe) have been synthesized using electrospinning followed by a thermal treatment. Physical characterization shows homogeneously dispersed Co, Ni or Fe nanoparticles through the macroporous CNF matrix with high porosity and increased surface areas up to 573 m(2) g(-1). The electrochemical abiotic characterization of the free-standing CNFs indicates that the presence of metal nanoparticles together with the increased values of mesoporosity improves the catalytic activity. Best results were obtained with Fe-doped CNFs. Up-scaling studies with a 1.5L MFC and using the free-standing Co-doped CNF sample (Co 2.5A) as air-cathode showed a long-term stability (t > 6000 h) and an enhanced performance (power densities up to 14.4 W m(-3) and 92 mW m(-2)). (C) 2017 Elsevier Ltd. All rights reserved.