Electrochemical Quartz Crystal Microbalance Response Research Articles

ALC_EQCM: Automated stoichiometric resolution in electrochemistry through Density Functional Theory aided, Electrochemical Quartz Crystal Microbalance

ALC_EQCM is a software for post-processing Electrochemical Quartz Crystal Microbalance (EQCM) data. The implemented functionalities allow qualitative and quantitative characterization of the EQCM response, which is used to derive information of stoichiometric changes during electrochemical reactions and automatically generate atomistic models compatible with EQCM data. Through extra user-defined directives, ALC_EQCM also provides input files to execute Density Functional Theory simulations of the generated models on high-performance computing facilities. The possibility of linking EQCM data with simulation capabilities offers a novel strategy to explore configurations only compatible with experiments and support the research of complex reactions in electro-active materials.

Fast and slow EQCM response of zwitterionic weak electrolytes to changes in the electrode potential: a pH-mediated mechanism.

Using a fast electrochemical quartz crystal microbalance (EQCM), zwitterionic electrolytes were studied with regard to changes of resonance frequency and resonance bandwidth after the electrode potential was switched. In addition to a fast change of frequency (within milliseconds), a further, slower process with opposite direction is observed. Both the fast and the slow process change sign when the pH is varied across the isoelectric point (pI). The fast process can be attributed to double layer recharging. Its characteristic time is slightly larger than the charge response time (the RC-time) as inferred from electrochemical impedance spectroscopy (EIS). With regard to the slow process, amino acids with moderate concentration behave markedly different from concentrated solutions of proteins. For amino acids, the slow process is larger in amplitude than the fast process and the QCM response is Sauerbrey-like. The shift in half bandwidth is smaller than the shift in frequency and the overtone-normalized frequency shifts agree between overtones (-Δf/n ≈ const. with n the overtone order). This can be explained with a viscosity change in the diffuse double layer. Independent measurements show that the viscosities of these electrolytes are higher than the average in a pH range around the pI. Presumably, the slow process reflects a rearrangement of molecules after the net charge on the molecule has increased or decreased, changing the degree of dipolar coupling and, in consequence, the viscosity. For concentrated solutions of bovine serum albumin (BSA), the QCM response does not follow Sauerbrey behaviour, which can be explained with viscoelasticity and viscoelastic dispersion. The slow process lets the frequency and the bandwidth relax towards a baseline, which is the same for jumps to more positive and to more negative potentials. Presumably, the slow process in this case is caused by a reorientation of molecules inside the Helmholtz layer, such that they screen the electric field more efficiently than immediately after the voltage jump.

Charge Storage Properties of Nanostructured Poly (3,4-ethylenedioxythiophene) Electrodes Revealed by Advanced Electrogravimetry.

PEDOT nanowires (NWs) directly grown on the conducting electrode of quartz resonators enable an advanced electrogravimetric analysis of their charge storage behavior. Electrochemical quartz crystal microbalance (EQCM) and its coupling with electrochemical impedance spectroscopy (ac–electrogravimetry or AC–EG) were used complementarily and reveal that TBA+, BF4− and ACN participate in the charge compensation process with different kinetics and quantity. BF4− anions were dominant in terms of concentration over TBA+ cations and the anion transfer results in the exclusion of the solvent molecules. TBA+ concentration variation in the electrode was small compared to that of the BF4− counterpart. However, Mw of TBA+ is much higher than BF4− (242.3 vs. 86.6 g·mol−1). Thus, TBA+ cations’ gravimetric contribution to the EQCM response was more significant than that of BF4−. Additional contribution of ACN with an opposite flux direction compared with BF4−, led to a net mass gain/lost during a negative/positive potential scan, masking partially the anion response. Such subtleties of the interfacial ion transfer processes were disentangled due to the complementarity of the EQCM and AC–EG methodologies, which were applied here for the characterization of electrochemical processes at the PEDOT NW electrode/organic electrolyte interface.

Electrochemically Reduced Graphene Oxide‐Sheltered ZnO Nanostructures Showing Enhanced Electrochemical Performance Revealed by an In Situ Electrogravimetric Study

AbstractThe present work is on the synthesis and characterization of vertically aligned ZnO nanostructures sheltered by electrochemically reduced graphene oxide (ERGO), i.e., ZnO@ERGO, which are directly generated on quartz resonators of microbalance sensors. The vertical orientation of the ZnO nanorods is achieved by a two‐step synthesis method involving an electrochemically grown seed layer and a subsequent hydrothermal growth. Deposited ERGO thin layer turns out to be highly effective to enhance the electrochemical performances of vertically oriented ZnO nanorods as supercapacitor electrodes. The interfacial charge storage mechanism of ZnO@ERGO electrodes with unique architecture is first studied by classical electrochemical quartz crystal microbalance (EQCM), showing a global cation‐exchange behavior in Na2SO4 electrolyte. A complementary technique, electrogravimetric impedance spectroscopy, is then used to deconvolute the EQCM response into individual contributions from Na+·H2O, SO42−, and H2O molecules, offering a quantitative picture of each participant in the charge balance process with their transfer kinetics.

Dynamic Resolution of Ion Transfer in Electrochemically Reduced Graphene Oxides Revealed by Electrogravimetric Impedance

To accompany the search for optimal materials in electrochemical supercapacitors, appropriate characterization tools to assess key parameters of newly developed electrodes are required. Here we demonstrate the capabilities of ac-mode electrogravimetry to study in details the capacitive charge storage mechanisms in electrochemically reduced graphene oxide (ERGO) thin film electrodes. The coupling of electrochemical impedance spectroscopy (EIS) with fast quartz crystal microbalance (QCM) complements classical electrochemical quartz crystal microbalance (EQCM) by capturing here the dynamics of the electroadsorption process, identifying charged moieties and detecting solvation effects. We evidenced the coelectroadsorption of two types of cationic species (fully and partially hydrated cations) in the potential range studied and the indirect intervention of free solvent molecules. Further kinetic information on electroadsorption is also obtained which leads to a full deconvolution of the global EQCM response at...

Analysis of the electrochemical quartz crystal microbalance response during oxidation of carbon oxides adsorption products on platinum group metals and alloys

The analysis of the electrochemical quartz crystal microbalance (EQCM) signal is presented for the case of oxidation of carbon oxides adsorption products on Pt, Rh, and their alloys. It is demonstrated that the EQCM response can be roughly approximated by the mass balance involving adsorption/desorption of various species (carbon oxides, oxygen, anions, and water molecules) and metal dissolution. The results obtained by the EQCM are in good agreement with the electrochemical data and confirm the domination of CO radicals among the products of CO2 reduction and CO adsorption on Pt-rich electrodes. In the case of Rh-rich electrodes, the existence of additional species (CHO or COH), more reduced than CO, is suggested.

Electrochemical study on the adsorption of carbon oxides and oxidation of their adsorption products on platinum group metals and alloys

CO(2) reduction and CO adsorption on noble metals (Pt, Rh, Pd) and their alloys (Pt-Rh, Pd-Pt, Pd-Rh, Pd-Pt-Rh) prepared as thin rough deposits have been studied by chronoamperometry (CA), cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). The influence of alloy surface composition on the values of surface coverage, eps (electron per site) and potential of the oxidation of CO(2) reduction and CO adsorption products is shown. The oxidation of the adsorbate on Pt-Rh alloys proceeds more easily (at lower potentials) than on pure metals. On the other hand, in the case of Pd-Pt and Pd-Rh alloys the adsorbate oxidation is more difficult and requires higher potentials than on Pt or Rh. The analysis of the EQCM signal is presented for the case of adsorption and oxidation of carbon oxide adsorption products on the electrodes studied. The comparison of adsorption parameters and the EQCM response obtained for platinum group metals and alloys leads to the conclusion that reduced CO(2) cannot be totally identified with adsorbed CO.

Ion and solvent transfers and trapping phenomena during n-doping of PEDOT films

Electrochemical quartz crystal microbalance (EQCM) measurements were made during the n-doping of PEDOT films exposed to a range of electrolytes (TEABF 4, TPABF 4 and TEATos) in CH 3CN solvent. The films were acoustically thin (“rigid”), so that the EQCM response could be interpreted gravimetrically. Correlation of the current with the time differential of the mass response allowed the determination of dopant ion and solvent fluxes as functions of applied potential during cyclic voltammetric experiments. This interpretation required the use of Faraday's law, the electroneutrality condition and the assumption (supported by variation of co-ion) that the film is permselective. The ion and solvent fluxes are in opposing directions for TEA + insertion, but in the same direction for TPA + insertion. As a result, the net mass change is much smaller in TEABF 4 electrolyte. In both the doping or undoping half-cycles, the ion and solvent fluxes normalise with potential scan rate, indicating that they are not subject to kinetic or transport limitations on the moderate time scale of the experiment. However, there is considerable hysteresis between the responses in the two scan directions, indicating the presence of slow polymer-based processes. When the potential limits are extended to encompass p- and n-doping regimes sequentially, additional response components on the rising edges of the two doping processes signal the participation of “trapping” phenomena. The amounts of trapped dopant charge are independent of potential scan rate for both p- and n-doping. The fraction of injected charge that is trapped is larger by an order of magnitude for cation, but the absolute amounts of trapped ion are similar for cation and anion.

Comparative EQCM study on electrooxidation of carbon oxides adsorption products on noble metals and their alloys. Polycrystalline Pd-based systems

CO2 reduction and CO adsorption on Pd alloys with Pt and Rh (Pd–Pt, Pd–Rh, Pd–Pt–Rh) have been studied by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). The values of surface coverage, eps (electron per site) and potential of the oxidation of CO2/CO adsorption products depend on alloy surface composition. The oxidation of reduced CO2 on Pd-poor Pd–Pt–Rh alloys proceeds more easily (at lower potentials) than on Pt or Rh. On the other hand, in the case of Pd–Pt and Pd–Rh alloys the adsorbate oxidation is more difficult and requires higher potentials than on pure metals. The EQCM response in CO2/CO adsorption experiments is affected not only by the effects connected with the changes in mass attached to the resonator but also by the non-mass effects including changes in metal–solution interactions and variation of solution density and viscosity in the vicinity of the electrode. The comparison of adsorption parameters suggests that the products of CO2 reduction and CO adsorption on Pd alloys are not identical.

Charge Transfer Property of Self-Assembled Viologen Derivative by Electrochemical Quartz Crystal Microbalance Response

Viologen modified electrodes have been extensively investigated with quartz crystal microbalance (QCM), which has been known as a nano-gram order mass detector, because of their highly reversible electrochemical properties, especially the first reduction-oxidation cycle of V2+ <--> V*+. The purpose of this work was to study the charge transfer characteristics of self-assembled monolayer (SAM) by changing electrolyte solutions where the cations and anions are different. The redox peak currents were nearly equal charges during redox processes and showed an excellent linear interrelation between the scan rates and second redox peak currents. The charge transfer of self-assembled viologen monolayer was determined by the mass change during the cyclic voltammetry (CV). The total frequency change was about 17.8 Hz, 19.6 Hz, 9.5 Hz, and 8.4 Hz. From this data, we could know the transferred mass was about 19.0 ng, 20.9 ng, 10.2 ng, and 9.0 ng. Finally, the electrochemical quartz crystal microbalance (EQCM) has been employed to monitor the electrochemically induced adsorption of self-assembled monolayer.

Charge Transfer Property of Self-Assembled Viologen Derivative by Electrochemical Quartz Crystal Microbalance Response

Viologen modified electrodes have been extensively investigated with quartz crystal microbalance (QCM), which has been known as a nano-gram order mass detector, because of their highly reversible electrochemical properties, especially the first reduction-oxidation cycle of V2+ ↔ V•+. The purpose of this work was to study the charge transfer characteristics of self-assembled monolayer (SAM) by changing electrolyte solutions where the cations and anions are different. The redox peak currents were nearly equal charges during redox processes and showed an excellent linear interrelation between the scan rates and second redox peak currents. The charge transfer of self-assembled viologen monolayer was determined by the mass change during the cyclic voltammetry (CV). The total frequency change was about 17.8 Hz, 19.6 Hz, 9.5 Hz, and 8.4 Hz. From this data, we could know the transferred mass was about 19.0 ng, 20.9 ng, 10.2 ng, and 9.0 ng. Finally, the electrochemical quartz crystal microbalance (EQCM) has been employed to monitor the electrochemically induced adsorption of self-assembled monolayer.

Study on the Electrodeposition of Hydroxides in Hydrated Perchlorate + Organic Solvent Systems Using EQCM

The electrochemical quartz crystal microbalance (EQCM) method was used to quantitatively examine the precipitation of LiOH (or NaOH) onto a gold electrode at potentials negative to ca −0.8 V ( vs SCE) during the cathodic sweep reduction of dissolved oxygen and coexisting water in acetone, DMF, DMSO, C 2H 5OH, or CH 3OH containing hydrated perchlorate, as a result of the poor solubility of electrogenerated hydroxide in the nonaqueous medium. In contrast, the EQCM response indicative of precipitate adherence was negligibly small, when tetrabutyl ammonium bromide was used as the supporting electrolyte. Effects of electrolyte and its concentration, solvent, and water content on the electrodeposition of hydroxide were discussed, and the electrode-collection efficiency for the precipitate was evaluated.

Electrochemical quartz crystal microbalance study on hydrogen absorption and desorption into/from palladium and palladium–noble metal alloys

The processes of hydrogen absorption/desorption into/from Pd and Pd alloys with Pt and Rh have been studied using the electrochemical quartz crystal microbalance (EQCM). The frequency shift connected with hydrogen electrosorption exceeds the value predicted by Sauerbrey equation and can be attributed not only to the mass changes but also to the stresses generated inside the metal/alloy. The influence of stresses on the EQCM response depends on the amount of electrosorbed hydrogen and the kind of the system (pure Pd or an alloy of a given composition) as well as being sensitive to the effect of electrode ageing. The magnitude of stresses measured by EQCM correlates with the relative change in the lattice constant caused by hydrogen absorption. EQCM experiments indicate that the magnitude and distribution of stresses are different for absorption and desorption processes, which seems to confirm the important role of the stress effect in the phenomenon of absorption/desorption hysteresis.

Electrochemical quartz crystal microbalance study on carbon oxides adsorption in the presence of electrosorbed hydrogen on Pd alloys with Pt and Rh

CO 2 and CO adsorption on Pd–Pt and Pd–Rh alloys has been studied by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). Adsorbed CO 2 inhibits partially hydrogen adsorption on Pt and Rh surface atoms but does not block significantly hydrogen absorption into alloy bulk. In the presence of adsorbed CO both hydrogen adsorption and absorption are strongly suppressed. On electrodes covered with adsorbed CO the oxidation of previously absorbed hydrogen is significantly shifted into higher potentials. The EQCM response in CO 2/CO adsorption experiments is affected by both the effects connected with the changes in mass attached to the resonator and the non-mass effects including changes in metal–solution interactions and variation of solution density and viscosity in the vicinity of the electrode. Differences in the EQCM behavior suggest that the products of CO 2 and CO adsorption on the alloys studied are not totally identical.

Temporal resolution of ion and solvent transfers at nickel hydroxide films exposed to LiOH

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor mobile species transfers on short time scales following the application of a potential step to potentiostatically deposited α-nickel hydroxide films exposed to aqueous LiOH solution. Upon film oxidation, hydroxide ions enter the film and protons are deintercalated. However, the protons are not detected directly since, upon ejection from the film, they immediately combine with hydroxide ions from the solution to yield water. The relatively rigid film lattice (which allows gravimetric interpretation of the EQCM response) imposes volume constraints. As ions are driven into/out of the film in order to satisfy electroneutrality, this volume constraint causes solvent to be driven out of/into the film by internal pressure. Consequently, solvent transfer is forced into the same time scale as ion transfer and is no longer simply diffusionally controlled. Combining the EQCM/PBD-derived fluxes of individual mobile species with their molar volumes, one can estimate the implied changes in film volume driven by redox switching. These volume changes are relatively small and correspond to a monotonic expansion on film oxidation and a transient expansion then eventual contraction upon film re-reduction. These observations are in the opposite sense to those predicted on the basis of unit cell volumes for crystalline α-Ni(OH) 2 and γ-NiOOH, suggesting that the material studied is poorly represented by these idealised structures.

Admittance studies of the EQCM on rough surfaces. The double layer region

The admittance of the electrochemical quartz crystal microbalance (EQCM) was studied for gold surfaces of different morphologies, produced by electrodeposition of gold at current densities near the limiting current density. It was observed that at surfaces of certain morphology, the real and the imaginary parts of the EQCM response become very sensitive to changes of potential in the double-layer region, even when Faradaic reactions and specific adsorption do not take place. This effect masks smaller effects of mass and viscosity changes in the diffuse double layer, described in our previous publication. It was suggested that this rather large effect is a result of surface hydration, depending on electrode potential. Orientation of water molecules in the first layer attached to the surface results in a shift of the equilibrium between clusters of water of different sizes in the “transition” layer. The latter determines the viscosity of this layer.

Study of dissolution mechanisms of nickel in sulfuric acid solution by electrochemical quartz crystal microbalance

Investigations of nickel dissolution in sulfuric acid solution have been performed by using an electrochemical quartz crystal microbalance (EQCM). The anodic polarization curve of electrodeposited nickel shows distinct three dissolution regions, i.e. two active dissolution regions at low anodic overvoltages and the transpassive dissolution region at high anodic overvoltages. The comparison of EQCM response with transient current indicates that the final products in the two active dissolution regions are almost Ni(II) and that the Ni(ll) dissolution ratio to the total anodic charge in the transpassive dissolution region is smaller than that in the two active dissolution regions. On the basis of reaction parameters obtained experimentally, the active dissolution mechanism of nickel can be explained by the Bockris mechanism.