Poly(bisphenol) polymers are identified as a new class of passivating agents for carbon electrodes in ionic liquids. They are inert and can readily be deposited as thin, conformal films by electropolymerization. Unlike conventional poly(monophenol) polymers, a single voltammetric scan is sufficient to accomplish their deposition. This is seen, for example, in the cases of poly(bisphenol A) and poly(bisphenol P). In each case, the thickness of the electropolymerized films is determined by the quantum tunneling distance of the faradaic electrons. Thus, film growth terminates when the faradaic electrons can no longer transit the film at a measurable rate. At that point, all the faradaic reactions cease, while the capacitive charging processes continue unabated. Experimentally, film thicknesses are observed in the range 4–30 nm. A challenging test for the poly(bisphenol) polymers is to coat them onto arrays of microelectrodes (RAM electrodes). Normally, microelectrodes are difficult to coat by electropolymerization due to the intense flux of soluble intermediates away from their surfaces. In the present work, however, coating is facile due to the extreme insolubility of the intermediates. This same property makes the films strongly adherent. Such remarkable behavior suggest that poly(bisphenol) films may have an important role to play as passivating agents in supercapacitors. They may also find application in other areas of technology that require thin-film passivity, such as nanostructural engineering and device physics.

In modern transition state theory, the rate constant for an electron transfer reaction is expressed as the product of four factors: an exponential factor, a pre-exponential factor, an electronic transmission coefficient, and a nuclear transmission coefficient. The activation energy of the reaction manifests inside the exponential factor, and on the conventional view, catalysis occurs by decreasing this activation energy below its catalyst-free value. In the present work we report the discovery of an unusual counter-example in which catalysis occurs by increasing the electron transmission coefficient far above its catalyst-free value. The mechanism involves the formation of a superexchange bridge between an electron donor (a graphite cathode) and an electron acceptor (a pentasulfide ion). The bridge consists of a dz2 orbital inside a cobalt phthalocyanine molecule. The dramatic result is the acceleration of the reduction of pentasulfide ions by more than 5 orders of magnitude compared with the catalyst-free case.

Many experimental studies of electrochromism have been reported in the scientific literature, but the terminology is still not settled, particularly for the case in which an electrochromic material forms the load in an electrical circuit. In the present work, two new terms are introduced to help clarify this situation. These terms are “dielectric electrochromism” and “faradaic electrochromism”. The first term applies to the case where an applied electric current causes a change in the energy of the electronic states in the material, while the second term applies to the case where an applied electric current causes a change in the occupation number of the electronic states in the material. Precise definitions are given of both terms.

The terminology and definition of surface tension are discussed. In particular, the surface tension is defined as the partial derivative of the surface excess Gibbs energy with respect to an infinitesimal increment of surface area at constant temperature and pressure. The surface tension is also formulated as the sum of a stress-free component and a stress-containing component. The stress-containing component is defined as the surface stress. Finally, the case of charged surfaces is analyzed, and the Gokhshtein relations are derived from the Gibbs potential in the special case that the electrode/solution interface is ideally polarizable.

In many modern technologies (such as batteries and supercapacitors), there is a strong need for redox-stable ionic liquids. Experimentally, the stability of ionic liquids can be quantified by the voltage range over which electron tunneling does not occur, but so far, quantum theory has not been applied systematically to this problem. Here, we report the electrochemical reduction of a series of quaternary ammonium cations in the presence of bis(trifluoromethylsulfonyl)imide (TFSI) anions and use nonadiabatic electron transfer theory to explicate the results. We find that increasing the chain length of the alkyl groups confers improved chemical inertness at all accessible temperatures. Simultaneously, decreasing the symmetry of the quaternary ammonium cations lowers the melting points of the corresponding ionic liquids, in two cases yielding highly inert solvents at room temperature. These are called hexyltriethylammonium TFSI (HTE-TFSI) and butyltrimethylammonium TFSI (BTM-TFSI). Indeed, the latter are two of the most redox-stable solvents in the history of electrochemistry. To gain insight into their properties, very high precision electrical conductivity measurements have been carried out in the range +20 °C to +190 °C. In both cases, the data conform to the Vogel-Tammann-Fulcher (VTF) equation with “six nines” precision (R2 > 0.999999). The critical temperature for the onset of conductivity coincides with the glass transition temperature Tg. This is compelling evidence that ionic liquids are, in fact, softened glasses. Finally, by focusing on the previously unsuspected connection between the molecular degrees of freedom of ionic liquids and their bulk conductivities, we are able to propose a new theory of the glass transition. This should have utility far beyond ionic liquids, in areas as diverse as glassy metals and polymer science.

We report the discovery and analysis of curved Tafel slopes from the electrochemical reduction of hexamminecobalt(III) under steady-state conditions. In order to confirm the existence of the curvature, random assemblies of carbon microelectrodes (RAM™ electrodes) were employed to obtain experimental data over more than three orders of magnitude, without significant double layer charging currents and without ohmic distortion. Since the rate-determining step in the reduction reaction is electron transfer, and no ligand substitution reactions occur on the timescale of experiments, the curvature of the Tafel plot is attributed to the dependence of the symmetry factor on electrode potential.

Recently, the science of comparative genomics has begun to revolutionize our understanding of the biological world. In the light of these developments, a new world view is emerging, more coherent than before, and bringing with it exciting opportunities for electrochemical research. In this essay, the author briefly traces the general outlines of the new landscape. An attempt is also made to set modern developments within a historical context. Strong emphasis is placed on the role of the electron in biology, and the name “electronomics” is suggested for this general field of research.

This article provides an overview of the theory of electron transfer. Emphasis is placed on the history of key ideas and on the definition of difficult terms. Among the topics considered are the quantum formulation of electron transfer, the role of thermal fluctuations, the structures of transition states, and the physical models of rate constants. The special case of electron transfer from a metal electrode to a molecule in solution is described in detail.

Tafel slopes for multistep electrochemical reactions are derived from first principles. The derivation takes place in two stages. First, Dirac’s perturbation theory is used to solve the Schrödinger equation. Second, current–voltage curves are obtained by integrating the single-state results over the full density of states in electrolyte solutions. Thermal equilibrium is assumed throughout. Somewhat surprisingly, it is found that the symmetry factor that appears in the Butler–Volmer equation is different from the symmetry factor that appears in electron transfer theory, and a conversion formula is given. Finally, the Tafel slopes are compiled in a convenient look-up table.

In a previous paper (S Fletcher, J Solid State Electrochem 11:965, 2007) a non-Marcus theory of electron transfer was developed, with results applicable to the normal region of thermodynamic driving forces. In the present paper, the theory is extended to highly exergonic reactions (the inverted region) and to highly endergonic reactions (the superverted region). The results are presented mathematically and in the form of Gibbs energy profiles plotted against a charge fluctuation reaction coordinate. The new theory utilizes the concept of donor and acceptor “supermolecules,” which consist of conventional donor and acceptor species plus their associated ionic atmospheres. The key findings are as follows. (1) In the inverted region, donor supermolecules are positively charged both before and after the electron transfer event. (2) In the normal region, donor supermolecules change polarity from negative to positive during the electron transfer event. (3) In the superverted region, the donor supermolecule is negatively charged both before and after the electron transfer event. This overall pattern of events makes it possible for polar solvents to catalyse electron transfer in the inverted and superverted regions. Because this new effect is predicted only by the present theory and not by the Marcus theory, it provides a clear means of distinguishing between them.

Based on recent developments in the theory of electron transfer, we prove that a non-polar environment is needed to maintain the high efficiency and chemical integrity of the photosynthetic reaction centre. We also determine the Gibbs energy diagram for the primary act of charge separation in photosynthesis, and propose an equivalent circuit that captures the principal features of the entire acceptor side of the electron transport chain in photosystem II.

We propose a new model for the elementary act of electron transfer between two species in solution. The central idea is that the solution in the immediate vicinity of each species may be represented by an equivalent circuit consisting of a Debye circuit shunted by a resistor. Based on this insight, we derive a new formula for the one-dimensional potential energy profile of a coupled donor–acceptor pair at finite (but large) separation d, along a charge-fluctuation reaction co-ordinate, at fixed radii of the transition states. The corresponding reorganisation energy of the reaction is also derived, and it is found to differ from that in the Marcus theory. In particular, the new model predicts that the reorganisation energy is independent of the static dielectric constant of the solution, whereas the old model predicts a strong dependence. The difference is traced to the fact that the Marcus theory omits consideration of the work required to form the charge fluctuations and focuses instead on the work required to localise the charge fluctuations. In general, the equivalent circuit approach permits many of the difficult-to-derive equations of non-equilibrium polarisation theory to be written down by inspection.

It is widely thought that the intercalation of ionic species in solids is unaffected by non-ionic species in solution. But in the present work we report that, in some rare cases, low concentrations of non-ionic species in solution can actually catalyse the intercalation of ionic species in the solid state. Cyclic voltammetry, quartz crystal gravimetry, X-ray diffraction, and molecular modelling are used to confirm the existence of this remarkable effect. The origin of the effect is traced to the co-incorporation of the non-ionic species in the solid state, which causes the crystal structure to expand. The rapid incorporation of the ionic species can then occur. This discovery suggests that other solid state reactions might be catalysed in a similar way. The solid state species which are shown to exhibit the catalytic effect include three rhenium carbonyl complexes, namely trans-Re(Br)(CO)(dppe)2, trans-Re(Br)(CO)(dppe)(dppm) (dppe=Ph2P(CH2)2PPh2; dppm=Ph2PCH2PPh2), and trans-Re(Br)(CO)(dpbz)(dppm) (dpbz=o-(Ph2P)2(C6H4); a chromium carbonyl complex, namely trans-Cr(CO)2(dppe)2; the high-stability electron shuttle tetraphenylviologen dichloride; and the propeller-shaped redox reagent decaphenylferrocene. The solvents which can act as catalysts include acetonitrile, acetone, methanol and ethanol.

The concept of massograms is described and their utility is demonstrated by the analysis of a wide range of interfacial reactions. Massograms are recorded on a quartz crystal microbalance, and show the rates of change of mass versus electrode potential. They are, therefore, the mass flux analogs of voltammograms, with which they can be productively compared. A Venn diagram is used to identify five different kinds of interfacial reaction which are distinguishable when massograms and voltammograms are recorded at the same time. What distinguishes the reactions is how charge transfer is coupled to mass transfer at the electrode surface. Examples of all five kinds of reaction are identified and discussed. The first section of the Venn diagram corresponds to faradaic processes that are associated with mass changes. Typical examples are electrodeposition, electrodissolution, and intercalation reactions. The second section of the Venn diagram corresponds to faradaic processes that are not associated with mass changes. Typical examples of these are gas evolution reactions. The third section of the Venn diagram corresponds to non-faradaic processes that are not associated with mass changes: capacitive charging is representative. The fourth section of the Venn diagram corresponds to non-faradaic processes that are associated with mass changes. An example of this is the specific adsorption of perchlorate ions on gold. The fifth and final section of the Venn diagram corresponds to mass changes that are associated with neither faradaic nor non-faradaic processes. An example is the Ostwald-like ripening of TCNQ microcrystals on gold.

We report the discovery and analysis of curved Tafel slopes from the electrochemical reduction of hexamminecobalt(III) under steady-state conditions. In order to confirm the existence of the curvature, random assemblies of carbon microelectrodes (RAM™ electrodes) were employed to obtain experimental data over more than three orders of magnitude, without significant double layer charging currents and without ohmic distortion. Since the rate-determining step in the reduction reaction is electron transfer, and no ligand substitution reactions occur on the timescale of experiments, the curvature of the Tafel plot is attributed to the dependence of the symmetry factor on electrode potential.