Functionalization of Polypyrrole Nanopipes with Redox-Active Polyoxometalates for High Energy Density Supercapacitors

Hybrid materials are very attractive for the fabrication of high-performance supercapacitors. Here, we have explored organic-inorganic hybrid materials based on open-end porous 1 D polypyrrole nanopipes (PPy-NPipes) and heteropolyoxometalates (phosphotungstate ([PW₁₂ O₄₀ ]^3- , PW₁₂ ) or phosphomolybdate ([PMo₁₂ O₄₀ ]^3- , PMo₁₂ )) that display excellent areal capacitances. Two different hybrid materials (PMo₁₂ @PPy and PW₁₂ @PPy) were effectively synthesized and used for symmetric supercapacitors. The anchoring of the inorganic nanoclusters onto the conducting polymer nanopipes led to electrodes that stood up to our best expectations exhibiting outstanding areal capacitances that are almost 1.5 to 2 fold higher than that of pristine PPy-NPipes. In addition, symmetric cells based on PMo₁₂ @PPy and PW₁₂ @PPy hybrid electrodes were fabricated and showed significant improvement in cell performance with very high volumetric capacitances in the range of 6.3-6.8 F cm^-3 (considering the volume of whole device). Indeed, they provide extended potential windows in acidic electrolytes (up to 1.5 V) which led to ultrahigh energy densities of 1.5 and 2.2 mWh cm^-3 for PMo₁₂ @PPy and PW₁₂ @PPy cells, respectively. Thus, these unique organic-inorganic hybrid symmetric cells displayed extraordinary electrochemical performances far exceeding those of more complex asymmetric systems.