Utilizing waste plastic to produce carbon catalysts is one way to recycle waste plastic. Carbon catalysts derived from low-density polyethylene (LDPE) (LDPE-C catalyst) and polyurethane (PUK-C catalyst) can help to improve the performance of vanadium redox flow batteries (VRFBs). Especially, for forming the PUK-C catalyst that has abundant surface nitrogen functional groups and large surface area, carbonization process is needed. Electrochemical analysis discloses that when this PUK-C catalyst is doped onto graphite felt (GF), the reactivity for redox reactions of vanadium ions is significantly enhanced. Specifically, peak current density and peak potential separation for the redox reactions are more improved than those observed with bare GF. Additionally, charge transfer resistance for the redox reactions is reduced when using PUK-C catalyst doped GF. When the performance of VRFBs utilizing PUK-C catalyst doped GF is measured, they exhibit better energy efficiency than VRFBs operated without the catalyst by 8.1%. Furthermore, maximum power density of VRFBs utilizing PUK-C catalyst doped GF can generate 14.9% higher power at 30 mA cm−2 than that of VRFBs utilizing bare felt. These findings demonstrate that the PUK-C catalyst is highly effective in enhancing the performance of VRFBs.