Unveiling the Effect of Varying Chain Length and Wettability on the Charge-Storage Performance in Donor–Acceptor–Donor-Type Electrode-Based Pseudocapacitors

Pseudocapacitors (PSCs) based on small organic molecule materials have attracted significant research interest due to their structural diversity, tunable redox properties, and ability to support widening the operational potential window, offering higher energy density and rapid power density. Nevertheless, the progress is hindered by their unsatisfactory cycling stability. An approach to tailoring the cycling life of the organic electrode materials for supercapacitors (SCs) is extending π-conjugation of the molecular architecture. To improve the overall performance of the supercapacitor device, the electrode materials, in particular conjugated donor–acceptor–donor (D–A–D)-type molecules, can store both positive and negative charge in a pseudocapacitor device. The D–A–D-type conjugation with good packing could facilitate charge transport and enhance the electrochemical characteristics of the SC cell configurations. In this work, new D–A–D-type 2,7-bis(10-(2-ethylhexyl)-10H-phenothiazin-3-yl) benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (PTZ-NDI-EH) and 2,7-bis(10-(2-hexyldecyl)-10H-phenothiazin-3-yl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (PTZ-NDI-HD) bearing two different alkyl chains are designed and synthesized. The optimized PTZ-NDI-EH and PTZ-NDI-HD active organic electrode materials in combination with conducting graphite foil (GF) exhibit outstanding functionalities such as higher specific capacitance, higher energy density, and longer cycling stability. Such higher PSC performance could be originating from the molecular packing, higher electronic conductivity, and enhanced charge delocalization during the Faradaic reversible redox processes. The higher cycling stability could be attributed to efficient ion transportation. In addition, the alkyl chain length influences the electrochemical properties of PTZ-NDI-EH and PTZ-NDI-HD. The increase in contact angle of PTZ-NDI-HD (higher alkyl chain length) leads to the reduction of the specific capacitance compared to PTZ-NDI-EH (lower alkyl chain length). This provides the basis for the wettability of the electrode in the presence of aqueous electrolytes and their impact on the electrochemical properties. These results indicate that the D–A–D materials design and their applications contribute to the development of high-performance electrical energy storage (EES) devices.