With cost advantages, low-dimensional carbon nanomaterials beyond graphene and carbon nanotube have aroused great interest in diverse fields. To optimize their electrochemical properties for energy storage, it is highly desired to exploit an efficient strategy to engineer their dimensionality and relevant physicochemical parameters. Kevlar was found to be able to serve as a universal precursor to produce 1-2D carbon nanomaterials of nanofibrils (diameter 20-60 nm) and nanosheets (thickness 2-5 nm). When combining thermal pyrolysis with mussel-inspired dopamine coating, an unprecedented type of carbon nanomaterials-fibrous carbon nanosheet (fiber diameter 14-16 nm and nanosheet thickness 30-50 nm) was also produced with high surface area, N-doping and hierarchical porosity. They not only compromised the dimensionality of carbon nanofibrils and carbon nanosheets, but also showed high capacitance and rate capability, in contrast to low capacitance while high rate capability of carbon nanofibrils, as well as high capacitance while low rate capability of carbon nanosheets. With the capacitive properties comparable to graphene and carbon nanotube, they would offer an alternative supercapacitor electrode for low-cost energy storage. Their unique morphologies and super physical properties can also find functional applications in catalysis, batteries, fuel cells, environmental adsorption and electrochemical sensing.