Grafting of the carbon allotropes and polypyrrole via a Kevlar-type organic linker: the correlation of carbon structure/morphology with electrochemistry of the composite electrode

The important requirement that needs to be fulfilled for an effective capacitor is fast energy uptake and release [ 1 , 2 ]. Other important features are a long-lasting cycle life, stability and high specific capacitance. They all can be improved by proper chemical, structural and morphological modification of the electrode materials [ 3 ]. Currently, in field of energy storage/conversion electronics, one of the most important task is to replace/decrease the usage of toxic heavy metals by carbonaceous materials that operate with relatively mild electrolytes [ 4 ]. Carbon allotropes such as carbon nanotubes and graphene are the most promising, and thus became essential components that could reach characteristics similar or greater than metallic electrodes [ 5 ]. This is however very challenging since carbons have lower energy density, and consequently, smaller specific capacitance [ 6 ]. On the other hand, carbon-based supercapacitors can store energy and be charged very fast during the reversible adsorption/desorption process of ions in double-layer capacitors, or by the counter-ion doping in pseudo-capacitors [ 7 , 8 ]. Both pseudo-capacitors and electrical double-layer capacitors (EDLCs) have possibilities to improve their specific capacitance by expanding the specific surface and increasing the number of redox active centers (for pseudo-capacitors) [ 9 , 10 ]. Many studies have shown that a combination of both types results in a synergistic improvement of the total capacitance, especially when pseudo-capacitors from the group of conjugated polymers (e.g., polypyrrole or polyaniline) are used. These carbon-polymer hybrid materials or composites outperform their individual components [ 11 – 13 ]. The synergistic improvement is generated by two effects. First, is the increase in conductivity for the pseudo-capacitors resulting in faster ion diffusion that restricts the capacitance [ 14 ]. This process is particularly effective when a strong covalent bond (e.g., amide-type linker) between the polymer and carbon is created. The covalently grafted polymer benefits from improved mechanical stability provided by the carbon support. This combination also demonstrates more dynamic (flexible) structure that can accommodate volumetric changes taking place upon the ion uptake/release in the polymer pseudo-capacitor. The high surface area carbon facilitates uniform distribution of the polymer particles, which is critical during the extensive charge–discharge (shrinking–stretching). Such volumetric changes for a thick and uneven polymer electrode accelerate its degradation due to the local inhomogeneity of the charge distribution (surface with less conductive fractions). All these phenomena shorten the electrode cycle life [ 15 ]. Another important factor is the electronic interaction at the carbon-linker-polymer junction. In the reverse donor–acceptor system, the covalently bound conductive polymer acts as an electron acceptor and the carbon allotrope as the electron donor, resulting in a more electrochemically stable system [ 16 ].

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