Superabsorbent polymer binder for achieving MnO2 supercapacitors of greatly enhanced capacitance density

One common dilemma encountered in designing a supercapacitor electrode is that the specific capacitance (Cs) of the active material decreases significantly as the active-material loading (mass area− 1) increases. As a result, the geometric capacitance density (GCD; Farad area− 1) of the electrode does not scale up linearly but gradually levels off with increasing loading. For MnO2 supercapacitors, this problem has been solved to a great extent by introducing a superabsorbent polymer (SAP) binder, namely polyacrylic acid (PAA), to form composite particles with MnO2. Other than acting as a binder to bound together MnO2 particles, the SAP is believed to facilitate distribution of electrolyte throughout the active layer owing to its electrolyte-absorbing and swelling behaviors. The Cs of MnO2 remains almost unchanged as the oxide loading varies over a wide range (1.5–6.5 mg cm− 2) of heavy active-material loading. In addition, putting PAA throughout the entire active layer helps to magnify the specific interaction between PAA and MnO2 that is known to enhance the capacitance of individual MnO2 particles. The success in combining both high Cs and high active-material loading results in GCD of ca. 1.8–1.4 F cm− 2 even under very high current densities (ca. 35–260 mA cm− 2 or 5–40 A g− 1-MnO2).