Monitoring hydration in lime-metakaolin composites using electrochemical impedance spectroscopy and nuclear magnetic resonance spectroscopy

This paper describes a study of the hydraulic reactions between metakaolin (MK) and air lime using electrochemical impedance spectroscopy (EIS) and nuclear magnetic resonance spectroscopy (NMR). Tests were carried out at 20, 25 and 30 degrees C on lime-MK pastes with 10:1 w/w ratio. Tests over 28 days allowed identification of relevant changes in the EIS signals and characterization of pastes using thermal analysis (TGA/DSC), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and uni-axial compressive tests. Tests over shorter periods of time (up to 42 h) allowed more detailed studies of the hydraulic phases formed at the very beginning of the reactions. Results of thermal analyses demonstrate formation of hydraulic compounds such as CSH, C(4)AH(13) and C(3)ASH(6) and show their evolution over time. MIP analysis demonstrates changes in pore size distribution related to the formation and trasformation of hydraulic phases. Variations of impedance response with time are shown to be associated with reaction kinetics. Changes in the NMR signal within the first 42 h of reaction are shown to be associated with the dissolution of calcium hydroxide in the pore solution. Overall, this paper demonstrates the importance of NMR in the study of hydraulic reactions in lime based materials and the ability of EIS to detect the formation of hydraulic compounds and the end of the calcium hydroxide dissolution process

Transient study of the oxygen reduction reaction on reduced Pt and Pt alloys microelectrodes: evidence for the reduction of pre-adsorbed oxygen species linked to dissolved oxygen

Using chronoamperometry at preconditioned oxide-free Pt microdisc electrodes in aqueous media, we investigated the oxygen reduction reaction (ORR) on the millisecond timescale and obtained results consistent with the reduction of oxygen species which adsorb on the electrode before the ORR is electrochemically driven. Furthermore these adsorbed species are clearly linked to oxygen in solution. At long times, the amperometric response is solely controlled by the diffusion of dissolved oxygen towards the microelectrode. However, at short times, typically below 50 ms, the reduction of pre-adsorbed oxygen produces a large extra current whose magnitude depends on the oxygen concentration in solution, deliberate electrode poisoning and the rest time before the potential step. Using sampled current voltammetry we show that this extra current affects the entire potential range of the ORR. Using microdisc electrodes made with Pt alloys we find that the amperometric response is sufficiently sensitive to distinguish oxygen coverage differences between Pt, Pt0.9Rh0.1 and Pt0.9Ir0.1 microdiscs. These unexpected and, to our knowledge, never previously reported results provide new insight into the oxygen reduction reaction on Pt. The existence over a wide potential range of irreversibly adsorbed oxygen species arising from dissolved oxygen and different from Pt oxide is particularly relevant to the development of oxygen reduction catalysts for low temperature fuel cells

The unexpected activity of Pd nanoparticles prepared using a non-ionic surfactant template

Pd deposits on vitreous carbon substrates were prepared by electrodeposition from liquid crystal phases (both micellar and hexagonal phases) consisting of self-assembled non-ionic surfactant molecules. The morphology of the deposits varied significantly with the concentration of the surfactant but all are made up of aggregated nanoparticles circa 9 nm in diameter. The deposits from the micellar phase of the surfactant offer the largest electroactive area and specific activity for the hydrogen evolution, oxygen evolution and reduction reactions and formic acid and ethanol oxidations. Unexpectedly the deposits lead to an increase in catalytic activity far in excess of that expected from an enhancement in electroactive are

Second-order kinetics for EC' reactions at a spherical microelectrode

A second-order (nonlinear) model is derived for steady-state kinetics of an EC' (catalytic electrochemical) reaction at a spherical microelectrode in the case where the electron transfer process is followed by a homogeneous chemical reaction regenerating the electroactive species. An asymptotic analysis of the model is performed, and the asymptotic results are compared with those from a numerical solution of the full nonlinear model. It is shown that in the fast reaction limit, where the current at the electrode takes its maximum possible value, the concentrations of the reactants are controlled by diffusion both close to and far from the electrode, with significant chemical activity occurring only in a narrow zone standing off the electrode. Also, it is shown that an equation obtained from a different asymptotic limit may be used to predict the limiting current at the microelectrode in all circumstances. The reasons for the surprising measure of agreement at the surface of the electrode are discussed, the predictions from the model of the limiting current are compared (favourably) with experimental results, and the model is compared with the standard pseudo-first-order model, which, although also based on a linearization of the governing equations, has a restricted range of validity

Improvement to the equation for the steady-state limiting currents at a microelectrode - Ec' processes (1st and 2nd-order reactions)

The aim of this communication is to present a more general expression to cover the situation when each species is allowed an individual diffusion coefficient and also to show the enhanced agreement with experimental data

Multiscale modelling and analysis of lithium-ion battery charge and discharge

A microscopic model of a lithium battery is developed, which accounts for lithium diffusion within particles, transfer of lithium from particles to the electrolyte and transport within the electrolyte assuming a dilute electrolyte and Butler-Volmer reaction kinetics. Exploiting the small size of the particles relative to the electrode dimensions, a homogenised model (in agreement with existing theories) is systematically derived and studied. Details of how the various averaged quantities relate to the underlying geometry and assumptions are given. The novel feature of the homogenisation process is that it allows the coefficients in the electrode-scale model to be derived in terms of the microscopic features of the electrode (e. g. particle size and shape) and can thus be used as a systematic way of investigating the effects of changes in particle design. Asymptotic methods are utilised to further simplify the model so that one-dimensional behaviour can be described with relatively simpler expressions. It is found that for low discharge currents, the battery acts almost uniformly while above a critical current, regions of the battery become depleted of lithium ions and have greatly reduced reaction rates leading to spatially nonuniform use of the electrode. The asymptotic approximations are valid for electrode materials where the OCV is a strong function of intercalated lithium concentration, such as Li xC 6, but not for materials with a flat discharge curve, such as LiFePO 4. © 2011 Springer Science+Business Media B.V

Applications of the boundary element method in electrochemistry: scanning electrochemical microscopy

Boundary element method (BEM) simulations are presented for a range of scanning electrochemical microscopy applications. Calculations are performed to quantify the effects of the surrounding shield for a range of tip geometries and produce three-dimensional images of electrodes embedded in substrate surfaces. Approach curves are presented for a range of experimentally exploited probes, including the sphere?cap electrode. In addition, the BEM is used to generate a line scan across the interface between a conducting and nonconducting substrate for different tip geometries. The comparative resolution at a fixed tip?substrate separation for a microdisk and microhemisphere probe is noted. Finally, three-dimensional images of raised and recessed hemispherical electrodes embedded in nonconducting flat substrates are generated and the results compared to the image of a microdisk electrode. <br/