Ionic Liquid-inspired Redox Shuttles: Properties of a Ferrocenylimidazolium Salt as an Efficient Mediator for Dye-sensitized Solar Cells

Abstract

A ferrocenylimidazolium salt was found to be an efficient iodine/iodide-free redox mediator for a dye-sensitized solar cell (DSSC), giving rise to a photoconversion efficiency of 4% under optimized electrolyte conditions. The excellent redox mediation process was ascribed not only to the electrochemically reversible nature of the molecule with a large heterogeneous electron-transfer rate constant in the order of 10−2 cm s−1 at electrodes, but also to an exceptionally high solubility up to 2.7 M to produce a large redox gradient in the mediation layer, thus allowing the increase of diffusion-limited current. The high solubility originating from the imidazolium group in the mediator molecule was reminiscent of ionic liquids, which are typically characterized by small absorption coefficients in a visible region. These properties provided a mass-transfer-based concept of molecular design for organic redox mediators in DSSCs.

Polymers for Reversible Hydrogen Storage Inspired by Electrode-active Materials in Organic Batteries

“…The molecular design of hydrogen carrier polymers is based on the property of high-density redox polymers for reversible charge storage, bearing redox-active organic pendant groups per repeating unit of the polymer main chain characterized by electrochemical reversibility and rapid electron-transfer rates. [28][29][30][31][32][33][34][35][36][37][38][39][40][41] Based on their facile and highly efficient electron self-exchange reactions in the polymers, the high-density redox polymers are employed to develop organic electrode-active materials in organic batteries. In search of hydrogen carrier polymers, we focused on the significant changes in the properties of the redox polymers upon the charging.…”

Expanding the Dimensionality of Polymers Populated with Organic Robust Radicals toward Flow Cell Application: Synthesis of TEMPO-Crowded Bottlebrush Polymers Using Anionic Polymerization and ROMP

“…We have been focusing on robust organic radical-containing polymers with superior charge-transport and -storage properties based on the reversible electrode reactions of the radical molecules such as 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and their fast electron exchange reactions via the densely populated radical sites − and have proposed them as the electroactive materials for electronic devices such as organic batteries, − dye-sensitized solar cells, − nonvolatile memories, , electrochromic displays, − and electrochemical diodes . A radical polymer forms an amorphous layer on a current collector and moderately swells in electrolyte solutions, allowing the electrolyte ions and the solvent molecules to swim smoothly throughout the polymer layer concurrently with the redox reaction of the radical molecules.…”

Facile charge transport and storage by a TEMPO-populated redox mediating polymer integrated with polyaniline as electrical conducting path

“…38 Added support for the diffusional process has been obtained by the radical polymer layer sandwiched with two electrodes, which gave rise to the steady-state redox gradient across the layer to produce limiting current according to the layer thickness. 39 In rechargeable devices such as batteries [40][41][42][43][44][45][46] and electrochromic cells, [47][48][49] such polymers must be partitioned into anode and/or cathode sides to avoid the redox shuttling between the two electrodes, [50][51][52][53][54] and, for this purpose, the polymers are usually crosslinked to suppress dissolution into electrolytes and yet to maintain the swelling properties. 37 In addition, conductive additives such as vapor-grown carbon nanofiber, 29 carbon nanotube (CNT), graphene 55 and carbon foam 56 have been employed to fabricate polymer composite electrodes to reduce the resistivity of the organic layer for excellent rate performance, 57,58 which have spawned various methods to prepare the polymer/carbon composite layers with the minimum requisite amounts of the carbon additives to maintain the overall redox capacity, such as the CNT wrapping 59 and surface-initiated polymerization of the radical monomers from carbon surface 60 and other electroconductive material surfaces.…”