Participation in Negative Capacitance of Diffusion-Controlled Voltammograms of Hemin

Abstract

Hemin in dimethyl sulfoxide solution has exhibited voltammograms controlled by diffusion at glassy carbon electrodes for slow scan rates, although it is adsorbed slightly. In contrast, voltammograms for high scan rates, v > 1 V s–1, were governed by some kinds of kinetics judging from the scan rate dependence of peaks. The kinetics is close to that of a ferrocenyl derivative, in which the currents include the capacitive component with negative values. The capacitive one can be identified with the proportionality to the scan rates. The variation of the peak currents with v yielded −200 μF cm–2. This negative value, being associated with the charge transfer reaction, makes cyclic voltammograms deviated downward from the diffusion-controlled behavior, resembling an irreversible reaction of the Butler–Volmer kinetics. Double layer capacitances are generally formed so that the applied electric field may be relaxed. The reduction of hemin forms a dipole coupled with a cation of the salt. The dipole orients from the electrode to the bulk, whereas the solvent dipoles orients in the opposite direction. Therefore, the capacitance is observed negatively. The capacitance determined by ac impedance took also negative values when the applied dc potential was only in the potential domain for the charge transfer. These complications can be avoided in electrocatalysis by use of such slow voltammetry as scan rates of 0.1 V s–1 and ac frequency of 0.2 Hz.

A Loss of Charge at Reduction of Hydrogen Ion by Fast Scan Voltammetry

“…The slope was consistent with the value of 0.465F 3/2 c*(D/RT) 1/2 for the diffusion current of the one-electron transfer reaction at D = 9.3 × 10 -5 cm 2 s −1 , as predicted from reaction 1 per H + . The lower deviation from the proportionality at v > 0.15 V s −1 may be ascribed to a contribution of the negative capacitance, 26,27…”

“…28 The lower deviation from the linearity can be attributed to the negative capacitance. 26,27 Then the peak current is represented as 26…”

“…It allows us to carry out fast scan voltammetry of H + without IR drop or floating capacitive currents, as predicted from previous work at scan rates over 1 V s −1 . 26,27 We carry out here the fast cyclic voltammetry of reduction of H + at a Pt wire electrode 30 μm in diameter for scan rates ranging from 0.01 to 8 V s −1 in solution with various concentrations of HCl less than 5 mM. A point is to identify adsorption species and evaluate its amount in the context of the negative capacitance.…”

The Difference in the Effects of IR-Drop from the Negative Capacitance of Fast Cyclic Voltammograms

“…Ferrocenyl compounds, however, are not satisfied with the condition of the rates; hence, item (iii) is unsuitable for explaining the present potential shifts. (iv) The negative capacitance was brought about through the following steps according to Figure 4: a charge-transferred redox species (Fc) was coupled with a counterion (Cl − ) for electric neutrality by responding to the externally applied field E ap to yield an electric dipole (Fc + -Cl − ), where Cl − came from the supporting electrolyte because of the highest concentration of anions: the dipole with the dipole moment p was oriented in the direction for enhancing the external field by -pc/ε 0 to yield the effective field E ef , which generated capacitance with a sign opposite to double-layer capacitance [16,34].…”

Enhancement of the Negative Capacitance Associated with the Dissolution of Silver by Salt Concentrations by Means of Anodic Stripping Voltammetry

“…For example, diffusioncontrolled currents by linear sweep voltammetry at the scan rate, v, are proportional to v 1/2 , whereas those by capacitance are to v. The observed current is represented as a sum of the two components. The graphical analysis enabled us to evaluate the capacitive current [32][33][34]. If adsorption occurs potentiostatically for a long time, the observed charge is purely the faradaic component without capacitive one.…”