Microfluidic fuel cell (MFC) with active area of 0.11 cm2 is fabricated. Formic acid is used as fuel and KMnO4 as oxidant for MFC operation. CNx nanofibers as cathode catalyst exhibits maximum power density of 3.43 mW cm−2. CNx nanofibers power density comparable to Au (2.72 mW cm−2), Pt (3.09 mW cm−2). MFC power density at flow rate of 300 μL min−1 higher against 400 and 500 μL min−1.