Step Potential Electrochemical Spectroscopy Research Articles

Fundamentals and Implication of Point of Zero Charge (PZC) Determination for Activated Carbons in Aqueous Electrolytes.

The point of zero charge (PZC) is a crucial parameter for investigating the charge storage mechanisms in energy storage systems at the molecular level. This paper presents findings from three different electrochemical techniques, compared for the first time: cyclic voltammetry (CV), staircase potentio electrochemical impedance spectroscopy (SPEIS), and step potential electrochemical spectroscopy (SPECS), for two activated carbons (ACs) with 0.1molL-1 aqueous solution of LiNO3, Li2SO4, and KI. The charging process of AC operating in aqueous electrolytes appears as a complex phenomenon - all ionic species take an active part in electric double-layer formation and the ion-mixing zone covers a wide potential region. Therefore, the so-called PZC should not be considered as an absolute one-point potential value, but rather as a range of zero charge (RZC). SPECS technique is found to be a universal and fast method for determining RZC, as applied here together with the EQCM. In most cases, the RZC covers a potential range from ≈100 to ≈200mV and the correlation of the range with the carbon microtexture is clear, highlighting the role of the ion-sieving effect. It is postulated that PZC for porous materials in aqueous electrolytic solutions should be considered instead as RZC.

The Polarity of Co-solvents Regulates the Charge Storage Mechanisms in Supercapacitors with Concentrated Electrolytes.

Developing better energy storage devices depends on comprehending the underlying mechanisms involved in charge storage. With the continuous conception of new electrolytes, this task becomes progressively more urgent and complex. An example is the utilization of co-solvated concentrated solutions. While these show promising electrochemical responses, their dynamic properties (especially under confinement) and their relationships with performance are not fully understood. Here, we combined modified step potential electrochemical spectroscopy and quasielastic neutron scattering to investigate systems composed of activated mesoporous carbon (AMC) and concentrated solutions of lithium bis(trifluoromethanesulfonyl)imide in acetonitrile co-solvated with either toluene or acetone. We report that acetone does not impair surface-controlled mechanisms, contrary to the case with toluene, which competes with charged species to populate the AMC's pores without contributing to charge storage. In turn, toluene promotes a greater overall capacitance owing to Faradaic processes, which may be related to changes in the solvation structures under confinement.

Three-dimensional step potential electrochemical spectroscopy (SPECS) simulations of porous pseudocapacitive electrodes

This study validates the step potential electrochemical spectroscopy (SPECS) method and refines the associated analysis for differentiating the contributions of electrical double layer (EDL) formation and Faradaic reactions to the total charge storage in three-dimensional porous pseudocapacitive electrodes. The modified Poisson–Nernst–Planck (MPNP) model coupled with the Frumkin–Butler–Volmer theory were used to numerically reproduce experimental data obtained from the SPECS method accounting for interfacial, transport, and electrochemical phenomena in porous electrodes consisting of monodisperse spherical nanoparticles ordered in face-centered cubic (FCC) packing. The fitting analysis of the SPECS method was modified for the Faradaic current. The new model can accurately predict the individual contributions of EDL formation and Faradaic reactions to the total current. Moreover, the contributions of EDL formation at the electrode surface or at the electrode/electrolyte interface within the porous electrode can be identified. Similarly, the Faradaic reactions due to surface-controlled or diffusion-controlled mechanisms can be distinguished. Furthermore, the capacitance associated with EDL formation obtained from SPECS was in good agreement with that obtained from cyclic voltammetry. Finally, cyclic voltammograms were reconstructed using the multiple potential step chronoamperometry (MUSCA) method, and the integral capacitance associated with each charge storage mechanism was calculated for a range of scan rates.

Exploring Impact of Various Operating Parameters on the Specific Capacitance at the Glassy Carbon/H2SO4 Interface: A Comparative Analysis Using Electrochemical Characterization

AbstractThe aim of this study is to investigate the influence of scanning rate, electrolyte concentration, temperature and electrode rotation speed on the capacitance of glassy carbon electrode in sulfuric acid solution using cyclic voltammetry (CV). The results show that they have a significant influence on the capacitance with varying experimental conditions. In addition, the electrochemical impedance spectroscopy (EIS) method was employed to conduct a more thorough analysis of the ways in which hydrodynamic circumstances resulting from a rotating electrode affect the electrode capacity and the charge storage process. Step potential electrochemical spectroscopy (SPECS) was used to identify the various charge storage mechanisms involved. It was shown that the voltammograms measured by the CV method and determined from the SPECS method have a quasi‐rectangular shape, revealing a predominantly capacitive behavior and that capacitance decreases with increasing scanning rate. Furthermore, both methods confirm that the contribution of the diffusion process is the main factor influencing the charge storage mechanism when the sweep rate is taken into account. However, the SPECS approach provides more accurate and detailed information regarding the kinetic characteristics of the electrode.

Step Potential Electrochemical Spectroscopy (SPECS) Technique to Investigate the Ageing of Carbon Electrodes in Organic-Based Electrochemical Capacitors

Current research on electrode degradation in electrochemical capacitors (ECs) mainly focuses on the application of new materials and the establishment of their stability on the basis of various standards and qualitative tests. However, there exists limited research that focuses on the elucidation and determination of the degradation mechanisms and pathways of carbon electrodes. The proposed study uses an innovative approach with the use of the Step Potential Electrochemical Spectroscopy (SPECS) technique, which allows one to quantify the contribution of different charge-storing mechanisms, and successfully track any changes that take place after accelerated ageing, including structural changes of the electrode in the organic medium. Additionally, the role of oxygen content on the degradation of ECs was explored.In this report carbon material, namely binder-free Activated Carbon Cloth (ACC) Kynol® 507-20 was prepared in a furnace under a nitrogen atmosphere at different temperatures, namely 120, 500, and 1000ᵒC. This is so that the effect of the presence of different oxygen functional groups on the surface of carbon could be investigated. The material was selected as a self-standing electrode to eliminate any influence the binder would have on the electrode porosity. Subsequently, these materials were prepared as electrode materials and assembled in a symmetric two-electrode setup with 1M TEABF4/AN (tetraethylammonium tetrafluoroborate in Acetonitrile solvent). In each cell, the potential range of both electrodes was first determined in a 2-electrode Swagelok® cell with RE using galvanostatic charge/discharge (1 A g-1). The further procedure involved 3 CV cycles (1 mV s-1) at a predetermined potential for each electrode in a 3-electrode set-up, followed by SPECS measurement where 5-minute potentiostatic hold with 10 mV potential steps was applied. The obtained current vs. time plots were used to calculate the corresponding capacitance, diffusion and residual parameters. Floating tests at 2.7 V were then carried out for each system. According to the international standard (IEC 62391-1), system failure is reported when the initial capacitance drops below 80% of its initial value; thus floating experiments were halted after that value was reached for each system or when the resistance of the cell doubled in value. The electrochemical investigation of the individual aged electrodes was then repeated in the same manner.Our study allowed for a thorough examination of individual electrodes prior to and after floating experiments, which helped to elucidate the degradation causes and pathways in organic medium.

In-Depth Investigation of Manganese Dioxide as Pseudocapacitive Electrode in Lithium- and Sodium-Doped Ionic Liquids

We investigate the contribution of pseudocapacitance to the overall capacitance of MnO2 electrodes in pure and alkaline-doped ionic liquids via two spectroscopic methods: step potential electrochemical spectroscopy (SPECS) and in situ Raman spectroscopy. For both characterization methods, thin-film electrodes of birnessite-like amorphous MnO2 were cycled in Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, pure or doped with lithium or sodium. SPECS allows determination of the influence of the electrolyte composition on the electrochemical behavior of the MnO2 electrodes. Pseudocapacitive charge storage can account for over half of the total capacitance with alkaline-doped ionic liquids. In situ Raman spectroscopy provided insight into the reversible ion intercalation in the MnO2 structure, which appears to be controlled by EMIm+ cations. These findings are supported by density functional theory (DFT) calculations, which further help unveil the charge storage mechanism in birnessite-like amorphous MnO2 thin films operated in pure and alkaline-doped ionic liquids.

Monitoring the ion population at a carbon electrode/aqueous electrolyte interface at various pHs using electrochemical dilatometry

Changes in the electrode thickness in electrochemical capacitors depend strongly on the electrolyte pH. Hence, different ions (or ions with various solvation shells) participate in electrical double-layer formation. In this study, operando electrochemical dilatometry experiments incorporating various electrochemical techniques, including cyclic voltammetry, large amplitude sinusoidal voltammetry, and step potential electrochemical spectroscopy, were performed on ECs operating in 1 mol L−1 Li2SO4 with different pHs (6, 9, and 12) and 1 mol L-1 LiOH (for comparative purposes). A microporous carbon (Kuraray YP-80F) was used as the electrode material. Comparable trends for the cell behaviour were observed for electrolytes with pHs of 6 and 9; however, the cell containing the pH 12 solution exhibited similar behaviour to that containing the LiOH solution, suggesting that during positive polarization, the primary role was played by OH−, because the same salt concentration was used in all cases. This result raises an essential concern regarding the actual ion population at the electrode/electrolyte interface, which affects the water decomposition potentials and triggers various parasitic reactions impacting long-term electrochemical capacitor performance.

Impact of lithium bis(fluorosulfonyl)imide (LiFSI) concentration on lithium intercalation into graphite monitored with Step Potential ElectroChemical Spectroscopy (SPECS)

Impact of lithium bis(fluorosulfonyl)imide (LiFSI) concentration on lithium intercalation into graphite monitored with Step Potential ElectroChemical Spectroscopy (SPECS)

Machine-learning-assisted material discovery of oxygen-rich highly porous carbon active materials for aqueous supercapacitors

Porous carbons are the active materials of choice for supercapacitor applications because of their power capability, long-term cycle stability, and wide operating temperatures. However, the development of carbon active materials with improved physicochemical and electrochemical properties is generally carried out via time-consuming and cost-ineffective experimental processes. In this regard, machine-learning technology provides a data-driven approach to examine previously reported research works to find the critical features for developing ideal carbon materials for supercapacitors. Here, we report the design of a machine-learning-derived activation strategy that uses sodium amide and cross-linked polymer precursors to synthesize highly porous carbons (i.e., with specific surface areas > 4000 m2/g). Tuning the pore size and oxygen content of the carbonaceous materials, we report a highly porous carbon-base electrode with 0.7 mg/cm2 of electrode mass loading that exhibits a high specific capacitance of 610 F/g in 1 M H2SO4. This result approaches the specific capacitance of a porous carbon electrode predicted by the machine learning approach. We also investigate the charge storage mechanism and electrolyte transport properties via step potential electrochemical spectroscopy and quasielastic neutron scattering measurements.

Electrochemical oxidation of glycerol to hydroxypyruvic acid on cobalt (oxy)hydroxide by high-valent cobalt redox centers

Transforming glycerol to value-added chemicals by mild electrochemical oxidation on earth-abundant, low-cost transition metal electrocatalysts is both environmentally and economically beneficial. Herein, substoichiometric cobalt (oxy)hydroxide CoOxHy has been demonstrated as an effective electrocatalyst for selective glycerol oxidation to hydroxypyruvic acid (HPA), an important chemical intermediate with three carbon atoms bearing three different functional groups. The Faradaic efficiency for HPA production can reach 43.2%, with an average production rate of 679.2 µmol min−1 mgeo−2. Through systematic investigation using cyclic voltammetry, step potential electrochemical spectroscopy, and in situ Raman spectroscopy characterizations, we have revealed that the high-valent cobalt center is responsible for the enhanced activity and selectivity, where electrochemically more stable CoOx exhibits much lower activity and Faradaic efficiency in comparison. This redox-mediated glycerol oxidation mechanism offers a new perspective for future electrocatalyst development toward selective glycerol valorizations.

Identifying the Potential of Zero Charge and Faradaic Reactions By Step Potential Electrochemical Spectroscopy

The study of electrode processes at carbon electrodes has great relevance to a variety of electrochemical applications. The wide range of carbon materials, their possible combination, the variability of electrode design and fabrication techniques, the specific application purposes, all these elements show the richness and complexity of this research area in electrochemistry. In particular, Activated Carbon (AC) electrodes are widely used for charge storage in supercapacitors and capacitive deionization cells. For their porous structure and surface characteristics, this class of electrodes may pose serious challenges to the capacity of routine electrochemical methods to achieve an in-depth analysis and the deconvolution of different electrode processes. A relatively new and interesting method, the Step Potential Electrochemical Spectroscopy (SPECS), has been shown to be an effective way to separate the contributions of faradaic and non-faradaic processes to the total current at AC electrodes1. The technique consists in applying to the working electrode (WE) a series of small potential steps and recording the current decay during a time interval long enough to allow for equilibration. Relatively simple models are fitted to the transients, to derive parameters of capacitive charging and faradaic processes. In the present work, we explore further the capability of SPECS to characterize electrode processes at AC electrodes, having in mind two objectives in particular: the determination of the Potential of Zero Charge (EPZC ) and the separation and possible identification of faradaic processes, through a closer examination of experimental data.AC (YEC-8A) paste electrodes (80% AC, 10% carbon black and 10% PTFE) on graphite current collector (130 μm thick foil) were used as WE in 0.1 M NaClaq electrolyte. The EPZC was determined as the minimum of differential capacitance vs. potential plot (single-frequency EIS measurement at 5 mHz). SPECS was performed over the potential window from –0.25 to 0.75 VSCE both in the anodic and cathodic direction, using a step of 25 mV and equilibration time of 300 s (sampling time 0.1 s). SPECS current transients are simulated by a two-exponential decay function with a residual current.Normalized differential capacitance curves from EIS and SPECS are shown in Figure 1a. The value of EPZC is found at about 0.175 and 0.190 VSCE by EIS and SPECS, respectively. Additionally, a good agreement is found between the absolute capacitance minimum determined by either technique at electrolyte concentration from 0.01 to 1.0 M, see inset of Figure 1a. The plot of residual currents vs. potential from the analysis of SPECS data is shown in Figure 1b. The trend of the currents, in both scan directions, highlights two “inversion” potentials: at about 0 VSCE, during forward polarization, and at about 0.57 VSCE, during backward polarization; in other words, the residual current is positive on the anodic side and negative on the cathodic side vs. the inversion potential, thus recreating the typical shape of a polarization curve. With this premise, though speculative, we identify tentatively two different reactions in the anodic and in the cathodic scan of the SPECS measurements, corresponding to carbon oxidation and oxygen reduction, respectively. Notably, the equilibrium potential of these reactions2 is reasonably close to the inversion potential observed in Figure 1b.1. Dupont, M. F. & Donne, S. W. Separating Faradaic and Non-Faradaic Charge Storage Contributions in Activated Carbon Electrochemical Capacitors Using Electrochemical Methods I . Step Potential Electrochemical Spectroscopy. 162, 1246–1254 (2015).2. Zhang, C., He, D., Ma, J., Tang, W. & Waite, T. D. Faradaic reactions in capacitive deionization (CDI) - problems and possibilities: A review. Water Research 128, 314–330 (2018). Figure 1

Capacitive Charge Storage at the Glassy Carbon Electrode: Comparison Between Aqueous and Non-Aqueous Electrolytes

Herein the charge storage capabilities of a planar non-porous glassy carbon electrode (GCE) in electrolytes of aqueous 0.5 M Na2SO4 and 1 M tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile are compared using a combination of cyclic voltammetry (CV), step potential electrochemical spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS). In all techniques the resultant capacitance was substantially higher for the GCE in 1 M TEABF4 compared to 0.5 M Na2SO4, with the differences due to solvation of the adsorbing electrolyte ions. Adsorbing Na+ cations in the aqueous system have a substantial solvation sheath that increases the resistance associated with capacitive charge storage, decreases the capacitance, as well as inhibits the packing of similar electrolyte ions in the vicinity of the GCE. Conversely, the adsorbing BF4− ions in the acetonitrile-based electrolyte exhibit a low resistance to charge storage, a greatly enhanced capacitance, as well as exhibit an improved ability to pack at the electrode-electrolyte interface. The impact of this behaviour on electrode performance has also been discussed.

Combined step potential electrochemical spectroscopy and electrochemical impedance spectroscopy analysis of the glassy carbon electrode in an aqueous electrolyte

The nature of the electrified interface between a planar non-porous glassy carbon electrode and an aqueous electrolyte of 0.5 M Na2SO4 is examined using the combination of step potential electrochemical spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS) across the full potential window of stability. This combination of techniques provides insight into the dynamics of interface formation (SPECS), as well as interfacial structure (EIS), highlighting the variation in capacitance across the potential window. EIS frequency also plays a significant role in determining capacitance via the role that it plays in determining ionic and dipole movement. High frequencies stimulate electron cloud movement, followed by dipole rotation and ionic movement as the frequency progressively decreases. Outcomes further highlight the role that mass transport has on the amount of charge storage at the interface.

Thermodynamic and kinetic examination of the glassy carbon electrode in neutral aqueous electrolytes

Herein we report on the charge storage behaviour of a non-porous planar glassy carbon electrode (GCE) in an aqueous electrolyte of 0.5 M Na2SO4 at temperatures in the range 25 °C to 50 °C. Preliminary cyclic voltammetry (CV) data indicate a decrease in electrode performance at increasing temperatures, and this was supported by detailed analysis of step potential electrochemical spectroscopy (SPECS) data. Kinetic analysis of the deconvoluted SPECS data using the Arrhenius equation has indicated that diffusional charge storage is not thermally activated, instead being possibly influenced by thermal scattering of electrolyte ions. Entropimetric analysis of the SPECS data has generated entropy and enthalpy data for charge storage in this system. These results are discussed in terms of charge storage at this interface. • Entropy and enthalpy of double layer formation has been examined. • The electrified interface extends a significant distance into the electrolyte. • The implications on double layer formation have been discussed.

Application of step potential electrochemical spectroscopy in pouch cell prototype capacitors

Herein we demonstrate the application of the step potential electrochemical spectroscopy (SPECS) technique to evaluate the electrochemical behaviour of large prototype capacitor devices for the first time. SPECS is generally applied for deconvoluting different charge storage mechanisms such as capacitive and diffusional processes. This study showed the effectiveness of SPECS technique to characterise the electrochemical performance of prototype pouch cell and to obtain invaluable information, including instability of the cell such as leakage current, degradation of the electrochemical cell, and a shortage of electrolyte species at highest potentials. This study also showed that the behaviour of capacitive and diffusional processes throughout different sweep rates is the same regardless of cell footprint and capacitance.

Mechanistic Insights of Pore Contributions in Carbon Supercapacitors by Modified Step Potential Electrochemical Spectroscopy

Porous carbons are widely used as electrode materials for electrical double layer (EDL)-type supercapacitors. Differentiation of capacitance contributions from micropores and mesopores is important to understand porous carbon capacitive behavior. However, this remains challenging to accomplish. Here, we present an improved step potential electrochemical spectroscopy method to investigate the contributions to capacitance from hierarchical pores and redox processes.

Electrical double layer formation on glassy carbon in aqueous solution

Herein we report on the use of step potential electrochemical spectroscopy (SPECS) to characterize the dynamics of charge storage on a planar non-porous glassy carbon electrode in an aqueous electrolyte. Comparison between the spatial requirements of solvated electrolyte ions and the amount of charge added to the electrode surface implies that electrolyte charge storage is diffuse in nature, extending a substantial distance into the electrolyte, even for a concentrated electrolyte. This is contrary to the expected parallel planes of charge expected for double layer formation at the interface. The implications of this on the use of porous materials is also discussed.

New insight into ion dynamics in nanoporous carbon materials: An application of the step potential electrochemical spectroscopy (SPECS) technique and electrochemical dilatometry

This paper reports on the charging mechanism of an activated carbon electrode in a symmetric electrochemical capacitor operating in an ionic liquid electrolyte (EMIm+TFSI−). With the application of step potential electrochemical spectroscopy (SPECS) and electrochemical dilatometry, it is determined that the electrical double-layer formation mechanism strongly depends on the porous structure of the electrode material. Furthermore, SPECS calculations allow us to separate and quantify the charge component responsible for the ionic movement.

Semiconductor Properties of Electrodeposited Manganese Dioxide for Electrochemical Capacitors: Mott-Schottky Analysis

Herein the semiconducting nature of electrodeposited manganese dioxide (γ-MnO2) is examined for its effects on pseudo-capacitive electrode behaviour in neutral electrolytes. Electrochemical analysis of the manganese dioxide electrode is achieved using a combination of cyclic voltammetry (CV), step potential electrochemical spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS), in particular the combination of SPECS and EIS in a single experiment, enabling the determination of electronic properties across the full potential window. After establishing stable cycling with CV, electrode performance is examined using SPECS. EIS data, recorded under quasi-equilibrium conditions at the end of each potential step rest period, is then analyzed using the Mott-Schottky equation, leading to the identification of both n-type and p-type behaviour for the manganese dioxide electrode. Flat band potentials are determined (Vfb,n = −0.34 V vs SCE and Vfb,p = 0.98 V vs SCE), and conclusions about the electrochemically active surface area being more closed related to the geometric surface area were reached.

(Invited) Demystifying the Electrode/Electrolyte Interface in Carbon-Based Electrochemical Capacitors with Specs Technique and Electrochemical Dilatometry

It is often said that the highly developed surface area of electrode materials ensures high capacitance, power and energy in electrochemical capacitors. However, the real ‘efficiency’ of the surface exploited in charge storage remains unknown. Recently, it turned out that the Step Potential Electrochemical Spectroscopy (SPECS) technique can be a useful tool for determining it.The SPECS technique allows for the comprehensive analysis of the total current output and evaluation of the capacitive, redox and other components. Together with the physicochemical data from nitrogen and CO2 adsorption techniques, we aim at the determination of the real electrode/electrolyte interface surface.Based on the knowledge gained to date, we assume that the efficiency of the interface ‘exploitation’ depends on many factors: pore size distribution, wettability of the electrode material by electrolyte, as well as the size of the ions. Thus, inappropriate selection of one of these parameters can aggravate the device performance, even if the excellent carbon material is applied as electrode component.The paper will provide the results from SPECS correlated with the physicochemical properties of carbon materials. It appears that the electrochemically accessible surface does not exceed 30% of the values estimated by N2 or CO2adsorption. The results will be supported by the recent findings from electrochemical dilatometry experiments.