The Electrochemistry Group Medal Lecture. Electron self-exchange dynamics between redox sites in polymers

Abstract

Using microelectrochemical techniques, our laboratory has explored selfexchange-based electron transport in a variety of mixed-valent polymeric media. The transport rate is measured as the electron diffusion coefficient, D,, or the self-exchange rate constant k,, . The basic variables for electron transport in mixed-valent polymer materials include: ( a ) the physical mobility of the counterions of the polymer that migrate due to electroneutrality requirements, ( b ) the physical diffusion coefficient, Dphy\, of the monomeric or polymeric oxidized and reduced molecular sites or ions relative to the rate of electron hopping or tunnelling between donor/acceptor pairs, ( c ) the observational timescale relative to these mobilities which provides the distinction between transient and steady-state experiments, and ( d ) the chemical environment of the polymer, whether dry and solvent-free or contacted by solvent vapour or liquid. Experimental strategies and results are presented for the measurement of rates of ion diffusion, D , , in N,-dry and solvent-wetted mixed valent polymers. I n a dry, mixed-valent osmium complex polymer, the electron-transport rate measured under steady-state conditions, where no ion transport occurs concurrently, is much faster than the diffusion rate of the ion as estimated in a transient electrolysis experiment. In a solvent-wetted osmium complex polymer, the electron-transport rate measured under transient conditions is much slower than that of the ion which was measured under steady-state conditions. These circumstances allow isolation of individual processes and are interpreted as giving electrontransport rates not strongly influenced by macroscopic ion-transport rates. Cyclic voltammetry of [Co(bpy),]*+ and of Li+TCNQ-in dry poly(ethy1ene oxide) polymer electrolyte solvents exhibits differing measured diffusion coefficients, Dapp, for the oxidation us. the reduction of each compound, reflecting the coupling of physical diffusion and electron self-exchange transport. Microdisc electrode voltammetry of solutions of a synthesized ferrocene mono-tagged poly(ethy1ene oxide) in a polymer solvent of comparable molecular weight gives Dapp values smaller than those for ferrocene monomer dissolved in the same polymer solvent. The Dapp in the former case measures the self-diffusion rate of a linear chain polymer within a linear chain polymer solvent. Measurability of this rate has implications for assumptions about diffusive mobility of redox molecules attached to polymer chains.

Solid-State Voltammetry — Analytical Prospects

“…The potential materials for solid-state electrochemical measurements are expected to contain three-dimensionally distributed, highly concentrated redox centers between which fast electron selfexchange (hopping) is feasible [13][14][15][16]. These redox centers are fixed and, although they may have short range mobility about an equilibrium position, they classically are macroscopically immobile.…”

“…In the latter case, the charge-compensating counterions originate from an infinite, external source, the liquid supporting electrolyte. For solid-state measurements in the absence of bulk liquid phase, the systems to be investigated are expected to contain not only the redox centers, which are highly concentrated and practically immobile [13][14][15], but also a sufficiently large 'ionic budget' [14] of mobile, structural counterions.…”

“…Potential applications of solid-state electrochemical measurements include such diverse areas as charge storage in batteries and redox supercapacitors [18][19][20], electrochromism [21,22], electrosynthesis [23], molecular electronics [11,24,25], characterization of mixed-valence ionically conducting materials [14][15][16], and the development of novel amperometric sensors for gases [26][27][28]. In the latter case, the measurement concept is based on electrocatalysis at the gas-solid interface.…”

Solid-state electrochemical behavior of Keggin-type borotungstic acid single crystal

“…Among the important requirements for a material to reveal electroactivity in solid, rigid, or semirigid state, in the absence of external liquid electrolyte, essential is the existence of mixed-valence redox centers and mobile charge-compensating ions [16][17][18][19][20]. Moreover, to minimize such parameters as ohmic resistance and migration effect, the concentration of charged species, like mobile ions and electrons, should be fairly large in the system.…”

Development of copper-stabilized conducting-polymer/polyoxometalate hybrid materials for effective electrochemical charging

“…Because charge displacement in supercapacitors could be largely interfacial and no diffusional limitations there exist, they can become high power devices. To fabricate high-energy density rechargeable battery-type materials, one has to refer to fundamental concepts of fast redox transitions within thin solid films of redoxconducting materials [6][7][8][9]. Applicable systems include protonically/electronically conducting mixed valence inorganic materials (e.g., polyoxometalates of molybdenum or tungsten [10][11][12][13]) and certain organic conjugated conducting polymers (e.g., pristine or derivatized poly (3,4-ethylenedioxythiophene), PEDOT [14,15]).…”