synthesisAppropriate amounts of polyvinylpyrrolidone (PVP) were dis-solved into N,N-dimethylformamide (DMF) at room temperatureunder stirring until the solution became clear. Then, the stoi-chiometric amounts of Cu(NO3)2·3H2O and Co(NO3)3·6H2O witha molar ratio of 1:2 were added into the above solution to get ahomogeneous precursor solution of 10 wt.% PVP and 15 wt.% metalsalts for electrospinning. The precursor solution was loaded into aplastic syringe equipped with a flat stainless steel needle of 0.9 mmin diameter. A high voltage supply was connected to the spin-neret (stainless steel needle) and a nickel mesh collector (whichwas placed 15 cm away from the orifice). The electric voltage wasset at 20 kV. CuCo–CFs were prepared by carbonizing the electro-spun Cu(NO3)2/Co(NO3)3/PVP fibers at 900◦C for 2 h in N2. Forcomparison, Co–CFs, FeCo–CFs, NiCo–CFs, and MnCo–CFs were alsoprepared by using the same procedures.