To widen the practical application of supercapacitors in acid electrolyte, carbon/molybdenum nitride (C/MoxN) nanofibers are synthesized via combination of electrospinning method and thermal treatment in mixed gas atmosphere of N2 and NH3. When the mass fraction of ammonium molybdate of 1.13 wt.% is added in the precursor mixture solution, as-prepared C/MoxN composite shows the nanofibers of about 200 nm diameter with MoxN nanoparticles uniformly embedded in the N-doped carbon substrate, and delivers a specific capacitance of 251 F g−1. Three types of supercapacitor devices of PANI||C/MoxN, AC||C/MoxN and C/MoxN||C/MoxN are configured in acid electrolyte. The asymmetric supercapacitor of PANI||C/MoxN shows a maximum energy density of 14.1 Wh kg−1 and power density of 312 W kg−1, which was larger than those of AC||C/MoxN device (12 Wh kg−1, and 325 W kg−1) and symmetric supercapacitors of C/MoxN||C/MoxN device (4.51 Wh kg−1, and 250 W kg−1). The C/MoxN||C/MoxN device exhibits good cycle life of specific capacitance retention of 78.6% at 2 A g−1 after 15, 000 cycles.