A Comparison of the Higher Order Harmonic Components Derived from Large-Amplitude Fourier Transformed ac Voltammetry of Myoglobin and Heme in DDAB Films at a Pyrolytic Graphite Electrode

A debate as to whether heme remains bound or is released in myoglobin molecules incorporated into a didodecyldimethylammonium bromide (DDAB) film adhered to a pyrolytic graphite electrode has prompted a comparison of their electrochemistry by the highly sensitive large-amplitude Fourier transformed ac voltammetric method. The accessibility of third, fourth, and higher harmonic components that are devoid of background current and the enhanced resolution relative to that available in dc voltammetry have allowed a detailed comparison of the FeIII/FeII and FeII/FeI redox processes of myoglobin and heme molecules to be undertaken as a function of buffer composition and pH and in the presence and absence of NaBr in the buffer and/or film. Under most conditions examined, only very subtle differences, in the FeIII/FeII process were found, implying this process cannot be used to indicate the intactness or otherwise of myoglobin in myoglobin-DDAB films. In contrast, higher order ac harmonics obtained from myoglobin-DDAB and heme-DDAB films reveal pH dependent differences with respect to the FeII/FeI couple. Analysis of the ac harmonics, and with the hypothesis that the FeII/FeI process reflects the myoglobin state, suggests that the majority of the iron heme is released from myoglobin-DDAB (pH 5.0, no NaBr) films in contact with pH 5.0 (0.1 M sodium acetate) buffer solution devoid of or containing NaBr. However, myoglobin films prepared with pH 5.0 buffer containing NaBr shows significant difference in the higher harmonic shapes and midpoint potentials in the FeII/FeI process relative to the case when heme molecules are used, although as noted in other studies, a significant fraction of the Mb is rendered electroinactive in the presence of NaBr. The voltammetric responses of myoglobin and heme-DDAB (pH 5.0) films in contact with pH 7.0 (0.1 M) phosphate buffer solution also exhibit significant differences in the FeII/FeI redox couple in the higher harmonics in contrast to a report [de Groot, M.T.; Merkx, M.; Koper, M. T. M. J. Am. Chem. Soc. 2005, 127, 16224] that claimed identical midpoint potentials apply to both films under conditions of dc cyclic voltammetry. The FT-ac voltammetric data therefore suggest that a substantial fraction of myoglobin in myoglobin-DDAB (pH 5.0) films in contact with pH 7.0 phosphate buffer solution remains intact. No evidence of a catalytic effect that enhanced the released of heme from myglobin was found at the pyrolytic graphite electrode surface. In summary, higher harmonic ac voltammetric data indicate that the FeII/FeI process but not the FeIII/FeII reflects the state of myoglobin in DDAB films. On this basis, films prepared at pH 5.0 should include NaBr, or else films should be prepared at neutral pH to achieve films with myoglobin remains in its intact near native state when a myoglobin-DDAB film is confined to a graphite electrode surface. Otherwise, the release of heme in myoglobin molecules incorporated into a DDAB film is likely to be a dominant reaction pathway

Evaluation of Levels of Defect Sites Present in Highly Ordered Pyrolytic Graphite Electrodes Using Capacitive and Faradaic Current Components Derived Simultaneously from Large-Amplitude Fourier Transformed ac Voltammetric Experiments

The level of edge plane defect sites present in highly ordered pyrolytic graphite (HOPG) electrodes has been evaluated via analysis of dc, ac fundamental, and higher-order ac harmonics available from a single large-amplitude Fourier transformed (FT) ac voltammetric experiment. Deliberate introduction of a low level of edge plane defect was achieved by polishing, with a higher level being introduced via electrochemical pretreatment. Kinetics regimes associated with fast electron transfer on the edge plane defect sites and slow electron transfer on the basal plane surface are resolved under ac conditions when using the surface-sensitive [Fe(CN)6]3−/4− redox probe. However, because of their insensitivity to slow electron transfer, higher-order ac faradaic harmonics almost exclusively detect only the much faster processes that emanate from edge plane defect sites. Thus, detection of fourth- and higher-order ac Faradaic harmonic components that are devoid of background capacitive current is possible at freshly cleaved HOPG in the region near the reversible potential for the [Fe(CN)6]3−/4− process. Under these circumstances, dc cyclic voltammograms exhibit only reduction and oxidation peaks separated by more than 1 V. The fundamental ac harmonic provides detailed information on the capacitive current, which increases with the level of edge plane defect sites. Apparent charge transfer rate constants also can be derived from peak-to-peak separations obtained from the dc aperiodic component. Estimates of the percentage of edge plane defect sites based on ac higher harmonics, capacitance, and dc aperiodic component that are available from a single experiment have been compared. The edge plane defect levels deduced from capacitance (fundamental harmonic ac component) and higher harmonic Faradaic currents are considered to be more reliable than estimations based on apparent rate constants derived from the dc aperiodic component or conventional dc cyclic voltammogram

Revelation of Multiple Underlying RuO₂ Redox Processes Associated with Pseudocapacitance and Electrocatalysis

Advances in basic knowledge relevant to the pseudocapacitive and electrocatalytic properties of RuO2 materials require a detailed understanding of the redox chemistry that occurs at the electrode interface. Although several redox processes have been identified via dc cyclic voltammograms derived from surface-confined RuO2 materials, mechanistic details remain limited because the faradaic signals of interest are heavily masked by the background current. Here, it is shown that the underlying electron transfer reactions associated with the VI to II oxidation states of surface-confined RuO2 materials in acidic medium are far more accessible in the background current free fourth and higher harmonic components available via large-amplitude Fourier transformed ac voltammetry. Enhanced resolution and sensitivity to both electron transfer and protonation processes and discrimination against solvent and background capacitance are achieved so that the Ru(V) to Ru(VI) process can be studied for the first time. Thus, kinetic and thermodynamic information relevant to each ruthenium redox level is readily deduced. The relative rate of electron transfer and the impact of protonation associated with Ru(VI) to Ru(II) redox processes are found to depend on the nature of the RuO2 materials (extent of crystallinity and hydration) and concentration of sulfuric acid electrolyte. In the electrocatalytic oxidation of glucose in alkaline medium, access to the underlying electron transfer processes allows ready detection of the redox couple associated with the catalysis. Thus, application of an advanced ac electroanalytical technique is shown to provide the methodology for enhancing our understanding of the charge transfer processes of RuO2, relevant to pseudocapacitance and electrocatalysis

A Non-Noble Metal Catalyst-Based Electrolyzer for Efficient CO₂‑to-Formate Conversion

Electrochemical CO2 reduction offers a promising approach to alleviate environmental and climate impacts attributed to increasing atmospheric CO2. Intensive research work has been performed over the years on catalysts, membranes, and other associated components related to the development of CO2 electrolyzers. Herein, we assembled a full cell comprising a Bi nanoparticle (NP)-based cathode for reducing CO2 to formate and the earth-abundant NiFe layered double hydroxide (LDH)-based anode for oxygen evolution. The electrolyte used was 1 M KOH, and an anion exchange membrane separator was employed. A formate conversion Faradaic efficiency (FEformate) of 90 ± 2% was obtained at the cell voltage of 2.12 V. This full cell system operating at 2.12 V was found to perform well over 10 h, as the FEformate remained above 85% with ∼82% retention of current. This is among the best performing CO2-to-formate conversion systems based on all non-precious metal catalysts. The low water oxidation overpotential of NiFe LDH, coupled with the highly efficient Bi NPs CO2 reduction catalyst, and the use of KOH electrolyte operated under flow cell configuration that maximizes the reactant/product mass transfer all contribute to this high-performance electrolyzer

Effects of Coupled Homogeneous Chemical Reactions on the Response of Large-Amplitude AC Voltammetry: Extraction of Kinetic and Mechanistic Information by Fourier Transform Analysis of Higher Harmonic Data

Large-amplitude ac voltammograms contain a wealth of kinetic information concerning electrode processes and can provide unique mechanistic insights compared to other techniques. This paper describes the effects homogeneous chemical processes have on ac voltammetry in general and provides experimental examples using two well-known chemical systems: one simple and one complex. Oxidation of [Cp*Fe(CO)2]2 (Cp* = η5-pentamethylcyclopentadienyl) in noncoordinating media is a reversible one-electron process; in the presence of nucleophiles, however, the resulting ligand-induced disproportionation changes the process to a multiple step regeneration. The chemical kinetic parameters of the regeneration mechanism were discerned via analysis of the third and higher harmonics of Fourier-transformed ac voltammetry data. Comparison of experimental data to digital simulations provides clear evidence that the reaction proceeds via a rapid pre-equilibrium between the electrogenerated monocation and the coordinating ligand; simultaneous fitting of the first nine harmonics indicates that kf = 7500 M−1 s−1 and kr = 100 s−1, and that the unimolecular decomposition of the corresponding intermediate occurs with a rate constant of 2.2 s−1. The rapid cis+ → trans+ isomerization of the electrogenerated cis-[W(CO)2(dpe)2]+, where dpe = 1,2-diphenylphosphinoethane, was examined to illustrate the effects of a simpler EC mechanism on the higher harmonics; a rate constant of 280 s−1 was determined. These results not only shed new light on the chemistry of these systems, but provide a clear demonstration that the higher harmonics of ac voltammetry provide mechanistic insights into coupled homogeneous processes far more detailed than those that are readily accessible with dc techniques

Mediator Enhanced Water Oxidation Using Rb₄[Ru^II(bpy)₃]₅[{Ru^III₄O₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₆)₂] Film Modified Electrodes

The water insoluble complex Rb4[RuII(bpy)3]5[{RuIII4O4(OH)2(H2O)4}­(γ-SiW10O36)2], ([RuIIbpy]5[RuIII4POM]), was synthesized from Rb8K2[{RuIV4O4(OH)2(H2O)4}­(γ-SiW10O36)2] and used for electrocatalytic water oxidation under both thin- and thick-film electrode conditions. Results demonstrate that the [RuIIbpy]5[RuIII4POM] modified electrode enables efficient water oxidation to be achieved at neutral pH using thin-film conditions, with [Ru­(bpy)3]3+([RuIIIbpy]) acting as the electron transfer mediator and [RuV4POM] as the species releasing O2. The rotating ring disc electrode (RRDE) method was used to quantitatively determine the turnover frequency (TOF) of the catalyst, and a value of 0.35 s–1 was obtained at a low overpotential of 0.49 V (1.10 V vs Ag/AgCl) at pH 7.0. The postulated mechanism for the mediator enhanced catalytic water process in a pH 7 buffer containing 0.1 M LiClO4 as an additional electrolyte includes the following reactions (ion transfer for maintaining charge neutrality is omitted for simplicity): [RuIIbpy]5[RuIII4POM] → [RuIIIbpy]5[RuV4POM] + 13 e– and [RuIIIbpy]5[RuV4POM] + 2H2O → [RuIIIbpy]5[RuIV4POM] + O2 + 4H+. The voltammetry of related water insoluble [RuIIbpy]2[S2M18O62] (M = W and Mo) and [FeIIPhen]x[RuIII4POM] materials has also been studied, and the lack of electrocatalytic water oxidation in these cases supports the hypothesis that [RuIIIbpy] is the electron transfer mediator and [RuV4POM] is the species responsible for oxygen evolution

Theoretical Analysis of the Two-Electron Transfer Reaction and Experimental Studies with Surface-Confined Cytochrome <i>c</i> Peroxidase Using Large-Amplitude Fourier Transformed AC Voltammetry

A detailed analysis of the cooperative two-electron transfer of surface-confined cytochrome <i>c</i> peroxidase (C<i>c</i>P) in contact with pH 6.0 phosphate buffer solution has been undertaken. This investigation is prompted by the prospect of achieving a richer understanding of this biologically important system via the employment of kinetically sensitive, but background devoid, higher harmonic components available in the large-amplitude Fourier transform ac voltammetric method. Data obtained from the conventional dc cyclic voltammetric method are also provided for comparison. Theoretical considerations based on both ac and dc approaches are presented for cases where reversible or quasi-reversible cooperative two-electron transfer involves variation in the separation of their reversible potentials, including potential inversion (as described previously for solution phase studies), and reversibility of the electrode processes. Comparison is also made with respect to the case of a simultaneous two-electron transfer process that is unlikely to occur in the physiological situation. Theoretical analysis confirms that the ac higher harmonic components provide greater sensitivity to the various mechanistic nuances that can arise in two-electron surface-confined processes. Experimentally, the ac perturbation with amplitude and frequency of 200 mV and 3.88 Hz, respectively, was employed to detect the electron transfer when C<i>c</i>P is confined to the surface of a graphite electrode. Simulations based on cooperative two-electron transfer with the employment of reversible potentials of 0.745 ± 0.010 V, heterogeneous electron transfer rate constants of between 3 and 10 s^–1 and charge transfer coefficients of 0.5 for both processes fitted experimental data for the fifth to eighth ac harmonics. Imperfections in theory-experiment comparison are consistent with kinetic and thermodynamic dispersion and other nonidealities not included in the theory used to model the voltammetry of surface-confined C<i>c</i>P