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Cyclic voltammetry of three ferrocene derivatives – (ferrocenylmethyl)trimethylammonium (FcTMA+), ferrocenecarboxylic acid (FcCOOH), and ferrocenemethanol (FcCH2OH) – in aqueous solutions shows that the reduced form of the first two redox species weakly adsorbs onto freshly cleaved surfaces of highly oriented pyrolytic graphite (HOPG), with the fractional surface coverage being in excess of 10% of a monolayer at a bulk concentration level of 0.25 mM for both compounds. FcCH2OH was found to exhibit greater and stronger adsorption (up to a monolayer) for the same bulk concentration. The adsorption of FcTMA+ on freshly cleaved surfaces of high quality (low step edge density) and low quality (high step edge density) HOPG is the same within experimental error, suggesting that the amount of step edges has no influence on the adsorption process. The amount of adsorption of FcTMA+ is the same (within error) for low quality HOPG, irrespective of whether the surface is freshly cleaved or left in air for up to 12 hours, while – with aging – high quality HOPG adsorbs notably more FcTMA+. The formation of an airborne contaminating film is proposed to be responsible for the enhanced entrapment of FcTMA+ on aged high quality HOPG surfaces, while low quality surfaces appear less prone to the accumulation of such films. The impact of the adsorption of ferrocene derivatives on graphite for voltammetric studies is discussed. Adsorption is quantified by developing a theory and methodology to process cyclic voltammetry data from peak current measurements. The accuracy and applicability, as well as limits of the approach, are demonstrated for various adsorption isotherms. [ABSTRACT FROM AUTHOR]

The electron transfer (ET) kinetics of three redox couples in aqueous solution, IrCl62−/3−, Ru(NH3)63+/2+ and Fe(CN)64−/3−, on different grades of highly oriented pyrolytic graphite (HOPG) have been investigated in a droplet-cell setup. This simple configuration allows measurements to be made on a very short time scale after cleavage of HOPG, so as to minimise possible effects from (atmospheric) contamination, and with minimal, if any, change to the HOPG surface. However, the droplet-cell geometry differs from more conventional electrochemical setups and is more prone to ohmic drop effects. The magnitude of ohmic drop is elucidated by modelling the electric field in a typical droplet configuration. These simulations enable ohmic effects to be minimised practically by optimising the positions of the counter and reference electrodes in the droplet, and by using a concentration ratio of electrolyte to redox species that is higher than used conventionally. It is shown that the ET kinetics for all of the redox species studied herein is fast on all grades of HOPG and lower limits for ET rate constants are deduced. For IrCl62−/3− and Fe(CN)64−/3−, ET on HOPG is at least as fast as on Pt electrodes, and for Ru(NH3)63+/2+ ET kinetics on HOPG is comparable to Pt electrodes. Given the considerable difference in the density of electronic states (DOS) between graphite and metal electrodes, the results tend to suggest that the DOS of the electrode does not play an important role in the ET kinetics of these outer-sphere redox couples over the range of values encompassing HOPG and metals. This can be rationalised because the DOS of all of these different electrode materials is orders of magnitude larger than those of the redox species in solution, so that with strong electronic coupling between the redox couple and electrode (adiabatic electron transfer) the electronic structure of the electrode becomes a relatively unimportant factor in the ET kinetics. [ABSTRACT FROM AUTHOR]

The scanning droplet-based technique, scanning electrochemical cell microscopy (SECCM), combined with electron backscatter diffraction (EBSD), is demonstrated as a powerful approach for visualizing surface structure effects on the rate of the oxygen reduction reaction (ORR) at polycrystalline platinum electrodes. Elucidating the effect of electrode structure on the ORR is of major interest in connection to electrocatalysis for energy-related applications. The attributes of the approach herein stem from: (i) the ease with which the polycrystalline substrate electrode can be prepared; (ii) the wide range of surface character open to study; (iii) the possibility of mapping reactivity within a particular facet (or grain), in a pseudo-single-crystal approach, and acquiring a high volume of data as a consequence; (iv) the ready ability to measure the activity at grain boundaries; and (v) an experimental arrangement (SECCM) that mimics the three-phase boundary in low temperature fuel cells. The kinetics of the ORR was analyzed and a finite element method model was developed to explore the effect of the three-phase boundary, in particular to examine pH variations in the droplet and the differential transport rates of the reactants and products. We have found a significant variation of activity across the platinum substrate, inherently linked to the crystallographic orientation, but do not detect any enhanced activity at grain boundaries. Grains with (111) and (100) contributions exhibit considerably higher activity than those with (110) and (100) contributions. These results, which can be explained by reference to previous single-crystal measurements, enhance our understanding of ORR structure-activity relationships on complex high-index platinum surfaces, and further demonstrate the power of high resolution flux imaging techniques to visualize and understand complex electrocatalyst materials. [ABSTRACT FROM AUTHOR]