Under fixed limits, potential sweeps in some electrochemical systems depict current profile whose shape repeats in every n cycle (with n > 1), receiving the name of high order voltammograms (HOV). HOV have been observed during electrooxidation of organic molecules catalyzed mainly by Pt. The present study shows HOV behavior during ethanol electrooxidation reaction onto gold in alkaline media. Firstly, a strong dependence of HOV with minimum potential is remarkable in this system and has not been considered in previous numeric models. Moreover, due to high sensibility of HOV to surface conditions, it was possible to follow effects caused by cation interactions with adsorbed oxygenated species.
Keywords: high order voltammograms, ethanol electrooxidation, gold catalyst, cation effects
IntroductionIn a conventional cyclic voltammetry experiment the applied potential is continuously changed at a fixed rate between two limits (E min and E up ) while the current is registered. 1 The characteristic current-potential plot is called cyclic voltammogram and the current value at a given potential and scan direction can depict three behaviors: it becomes unchanged along the cycles; increase or decrease monotonically along the cycles; repeat its profile every set of cycles, that can be periodic, i.e., observed in every n cycle (n > 1) or aperiodic, in which n admits more than one value along the experiment. These voltammograms, periodic or aperiodic, are called high order voltammograms (HOV) and in contrast to the other cases, they are much less explored in the electrochemical field.Examples of HOV can be found for several small organic molecules electrooxidation reactions catalyzed by Pt in both alkaline and acid media. [2][3][4][5][6][7][8][9][10][11] Varela and Krischer 12,13 also found HOV during electrooxidation of hydrogen in acid media. In any case, two conditions are necessary to observe HOV: the reaction should contain an inhibition region at high overpotential, in general connected with oxide formation, giving origin to a peak current; an expressive ohmic drop, given by the product of current (I) and resistance (R), provided by low concentration supporting electrolyte or an external resistance.Ohmic drop allows the separation of applied potential (E) and the potential at interphase (ϕ) once E = ϕ + IR.14 When E up is set slightly more negative than the potential of peak current, the first cycles have almost the same current during positive and negative going scans, (small hysteresis) being classified as S cycles, but along the cycles, the current decreases and ϕ becomes higher and higher until reaching the region of current inhibition. At this point, ϕ quickly increases and oxides cover the surface. This condition makes the current in the negative going scan very distinct from that in the positive, i.e., with a large hysteresis and the cycle being classified as L. If the cycle after the L is the S, the initial condition is restored and the HOV is observed. 15 Indeed, to observe HOV the system should contain some pa...
