Faradaic Response Research Articles

Nanostructured MoS2 grafted by anthraquinone for energy storage

Two-dimensional materials such as molybdenum disulfide (MoS2) can be employed as an electrode material in energy systems due its good electrical conductivity and high reachable capacity/capacitance. We demonstrate the concept of a covalent modification of nanostructured MoS2 with anthraquinone (AQ) molecules through diazonium salt chemistry for electrochemical capacitors and lithium-ion storage. AQ molecules are chemically bonded to MoS2 which was proved experimentally by spectroscopic techniques. Here, AQ is grafted (ca. 20 wt%) to the outer surface of nanostructured MoS2, but also confined within the MoS2 interlayer spacing. Modified AQ-MoS2 exhibited faradaic response which improved the overall charge stored from 191 F g−1 (461 F cm−3) to 263 F g−1 (631 F cm−3) at 0.2 A g−1 in 1 M H2SO4. The process of intercalation of ions within interlayer spacing of AQ-MoS2 was not hindered by the presence of confined AQ molecules. Impedance spectroscopy and voltammetry analysis revealed comparable non-diffusion limited response of MoS2 before and after modification. The MoS2 modification was likely conducted on the defect sites limiting the catalytic activity of MoS2 towards hydrogen evolution improving stability of electrodes. In organic medium, the pseudocapacitive response of MoS2, enhanced by additional redox reactions of grafted AQ was observed during lithium-ion storage.

The impact of oxygen-clustering on the transformation of electrochemically-derived graphite oxide framework

Oxygen-clustering in graphene oxide is the activated process of diffusion of oxygen functionalities: hydroxyls and epoxides, to form clusters and to trigger sp2 percolation, a key element for electrical conductivity. Impact of oxygen-clustering is thoroughly analyzed in electrochemically-derived graphite oxide (EGO), amine-functionalized frameworks (EGOF), and their pyrolyzed forms. Amine pillars within EGOF were suspected to amplify oxygen-clustering in EGOF as onset temperature of thermal decomposition was smaller in EGOF than in EGO. Thermochemical analysis proves that pillars within EGOF significantly augment oxygen-clustering aiding the reduction mechanism as inferred by the activation energy of thermal decomposition. Defective oxygen-clustering mechanism is evidenced using multiple techniques, including XRD, and is harnessed to enhance nitrogen doping. The pyrolytic morphological transformation from lamellae of EGOF into ravioli is revealed through SEM. A labyrinthic network of array prismatic dislocations provides stability to pyrolyzed EGOF ravioli whilst confining long chains of electron delocalization (localized π-orbitals), as unprecedently confirmed by HR-TEM/EELS and conductive AFM (C-AFM). Oxygen-clustering not only improves nitrogen doping in ravioli, but also improves density of array prismatic dislocations, which ultimately boosts Faradaic redox activity as observed in cyclic voltammetry (CV). Results show that Faradaic response originating from redox-active nitrogen groups predominantly relies on abundance of extended localized π-orbitals, which are geometrically and chemically confined within ravioli array prismatic dislocations. Our first-of-a-kind HR-TEM/EELS, C-AFM, and CV comparative and correlative analysis of pyrolyzed forms of EGO, EGOF, and their oxygen-clustered derivatives, confirms emergent role of confined electron delocalizations in amplifying Faradaic redox activity.

Synthesis strategies of iron nitrides at carbon cloth as battery-like electrode for hybrid supercapacitors

In recent years, hybrid supercapacitors (HSCs) or supercapatteries which combine a capacitor-type electrode with an electrode based on materials exhibiting a Faradaic (battery-like) response have been intensively investigated for next-generation energy storage applications. HSCs attracted great attention due to a significant increase of maximum energy density stored while providing stable long-term performance and good rate capability. However, the electrochemical performance of the device is closely related to the inherent properties of the electrode material, including morphology and structure. In this paper, we present synthesis protocols for iron oxide/hydrophilic carbon cloth (Fe2O3@hCC) composite electrodes and their electrochemical performance as a negative electrode operating in an alkaline electrolyte. Two environmentally friendly, scalable and facile synthesis approaches were applied, including hydrothermal treatment and direct electrodeposition. Next, the Fe2O3@hCC electrodes were treated to convert iron oxide to iron nitride (Fe2N). The results showed that the synthesis of the precursor for iron nitride has a direct impact on morphology, crystalline structure and electrochemical performance. Furthermore, the amorphous Fe2N obtained from electrodeposition exhibited significantly better Faradaic behavior, achieving a specific capacity up to 186 mAh g-1, 66% higher than the composite electrode with Fe2N from the hydrothermal approach.

MWCNT buckypaper disc films as alternative to the drop casting method to modify electrode surfaces

We report the comparison of the modification of glassy carbon electrode (GCE) with multiwalled carbon nanotubes (MWCNTs) and nitroaromatic compounds using two different methodologies, the traditional drop casting (DC) method and an alternative buckypaper disc film (BP) method. The methodology for the construction of the BP disc film electrodes consists of a simple filtration of MWCNT and polystyrene (PS) dispersion in 1,3-dioxolane through a nylon membrane filter and then disc-shaped cutting. The adhesion of the disc film to the GCE is achieved by a direct STAMP process.The electrode surfaces were characterized by Raman spectroscopy (RS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and scanning electrochemical microscopy (SECM) while the electrochemical responses were followed by voltammetry and electrochemical impedance spectroscopy (EIS).The results showed an important enhancement in the reproducibility of the electrochemical response of the electrodes with coefficient of variation (c.v.) values for the faradaic response of 3.82 and 11.42% for the BP and DC methods, respectively. In addition, c.v. values for the capacitive response of 2.2 and 19.2% for the BP and DC methods, respectively. The preparation of BP disc film electrodes improves homogenization in MWCNT deposits causing this increase in the reproducibility of the electrodes.

Direct electrochemical determination of environmentally harmful pharmaceutical ciprofloxacin in 3D printed flow-through cell

Rising amounts of antibiotic residues in wastewater cause serious problems including increased bacterial resistance. Wastewater treatment plants (WWTPs) do not, in the case of new, modern pharmaceuticals, ensure their complete removal. Ciprofloxacin (CIP) is one of many micropollutants that partially pass through WWTPs, implying that its monitoring is essential for the assessment of the water quality. In real sewage systems, the determination of CIP needs to be performed under flowing conditions, which calls for the deployment of inexpensive, robust, and easily integrable approaches such as electrochemical techniques. However, to the best of our knowledge, there is no report on the electrochemical determination of CIP in a flowing matrix. To bridge this gap, we perform here cyclic and square-wave voltammetric sensing study of CIP employing boron-doped diamond screen printed electrodes in a custom-made 3D printed flow-through cell to mimic conditions in real sewage systems. An irreversible two-step oxidation of CIP is demonstrated, with the first step providing clear Faradaic response as analytically relevant signal. This response was found to scale with the sample flow rate according to the prediction given by Levich equation. Our work provides an in-depth inspection of the electrochemical response of CIP under controlled-convection conditions, which is an essential prerequisite for monitoring this antibiotic in real flowing sewage systems.

Fabrication of ionic liquid stabilized MXene interface for electrochemical dopamine detection.

Development of MXene (Ti3C2Cl2)-based sensing platforms by exploiting their inherent active electrochemistry is highly challenging due to their characteristic poor stability in air and water. Herein, we reporta cost-effective methodology to deposit MXene on aconductive graphitic pencil electrode (GPE). MXenes can provide active surface area due to their clever morphology of accordion-like sheets; however, the disposition to stack together limits their potential applications. A task-specific ionic liquid (1-methyl imidazolium acetate) is utilized as a multiplex host material to engineer MXene interface via π-π interactions as well as to act as a selective binding site for biomolecules. The resulting IL-MXene/GPE interface proved to be a highly stable interface owing to good interactions between MXene and IL that inhibited electrode leaching and boosted electron transfer at the electrode-electrolyte interface. It resulted in robust dopamine (DA) oxidation with amplified faradaic response and enhanced sensitivity (9.61 µA µM-1cm-2) for DA detection. This fabricated sensor demonstrated large linear range (10µM - 2000µM), low detection limit (702nM), high reproducibility, and good selectivity. We anticipate that such platform will pave the way for the development of stable and economically viable MXene-based sensors without sacrificing their inherent properties. Scheme 1 Schematic illustration of the IL-MXene/GPE fabrication and oxidative process towards non-enzymatic dopamine sensor.

Facile strategy of using conductive additive supported NaMnPO4 nanoparticles for delivering high performance Na-ion supercapacitors

A novel strategy of incorporating conducting additive to enhance the performance of NaMnPO4-based Na-ion supercapacitors is presented. It is observed that forming composite with a conducting polymer, leads to nearly two-fold increment in the specific capacitance of a Na-ion based system. The higher conductivity, fast Faradaic response alongside with greater number of ion transport channels lead to this improvement. Here, the best performance is obtained at an optimized polyaniline (PANi) percentage of 30 wt%, where a specific capacitance of 201 F g−1 at 1 A g−1 current density was observed. This is more than twice the value obtained by pure NaMnPO4. The fabricated supercapacitors also deliver 26.5 W h kg−1 specific energy and high cycling stability.

Nanosilver‐Promoted Trimetallic Ni–Co–Mn Perovskite Fluorides for Advanced Aqueous Supercabatteries with Pseudocapacitive Multielectrons Phase Conversion Mechanisms

AbstractAqueous supercapacitors (ASCs) and batteries (ABs) have drawn great attention as promising energy storage devices. However, the key issues of limited energy density of ASCs and inferior power density/poor cycling life of ABs discourage their further application. Herein, a new concept of advanced aqueous supercabatteries (ASCBs) realized by nanosilver‐promoted trimetallic Ni–Co–Mn perovskite fluorides (K1.0Ni0.4Co0.2Mn0.4F3.2 (KNCMF‐10#)/Ag(37%), denoted as 10#/Ag(37%)) electrode materials is proposed, integrating with the respective superior specific power/cycling behavior and energy density of ASCs and ABs. A pseudocapacitance‐dominated multielectrons phase conversion mechanism of the 10#/Ag(37%) electrode materials can be deduced by ex situ characterizations and electrochemical techniques. The constructed ASCBs by matching 10#/Ag(37%) cathode with activated carbon (AC)/Bi(17%) anode achieve great energy density without sacrifice of power density and cycling life in wide temperatures, benefiting from the synergistic energy storage superiority of ASCs and ABs containing capacitive, pseudocapacitive, and Faradaic response in electrochemical processes. Overall, this work highlights the new idea of nano‐Ag‐promoted trimetallic Ni–Co–Mn perovskite fluorides with a pseudocapacitive multielectrons phase conversion mechanism as a new pop star for advanced ASCBs, showing a great significance in the context of designing advanced electrode materials and in‐depth understanding of their complicated charge storage mechanisms for aqueous electrochemical energy storage systems.

Insights into the Voltammetry of Cavity Microelectrodes Filled with Metal Powders: The Value of Square Wave Voltammetry

AbstractCavity microelectrodes (CMEs) offer a valuable platform to evaluate the electrocatalytic performance of micro‐ and nanoparticulate materials. The technical factors and physicochemical processes affecting the electrochemical response at CMEs are to be recognized; specifically, the accessibility of redox species to the electrocatalyst surface. With this aim, the voltammetric response of CMEs is investigated through a joint experimental and theoretical approach including a comparative study of cyclic and square wave voltammetry (SWV). Experiments reveal a capacitive distortion of the response that increases with the powder surface area, but with a faradaic response analogous to that of recessed or inlaid microdisks, that is, with electrochemical reactions occurring essentially on the first layer of the powder load. We show that SWV is well suited to discriminate faradaic processes at CMEs and we present accurate mathematical expressions to describe it. These results provide guidelines for the design and analysis of CME voltammetric measurements.

Quantitative Analysis of Redox-Inactive Ions by AC Voltammetry at a Polarized Interface between Two Immiscible Electrolyte Solutions.

The interface between two immiscible electrolyte solutions (ITIES) is ideally suited to detect redox-inactive ions by their ion transfer. Such electroanalysis, based on the Nernst-Donnan equation, has been predominantly performed using amperometry, cyclic voltammetry, or differential pulse voltammetry. Here, we introduce a new electroanalytical method based on alternating-current (AC) voltammetry with inherent advantages over traditional approaches such as avoidance of positive feedback iR compensation, a major issue for liquid|liquid electrochemical cells containing resistive organic media and interfacial areas in the cm2 and mm2 range. A theoretical background outlining the generation of the analytical signal is provided and based on extracting the component that depends on the Warburg impedance from the total impedance. The quantitative detection of a series of model redox-inactive tetraalkylammonium cations is demonstrated, with evidence provided of the transient adsorption of these cations at the interface during the course of ion transfer. Since ion transfer is diffusion-limited, by changing the voltage excitation frequency during AC voltammetry, the intensity of the Faradaic response can be enhanced at low frequencies (1 Hz) or made to disappear completely at higher frequencies (99 Hz). The latter produces an AC voltammogram equivalent to a "blank" measurement in the absence of analyte and is ideal for background subtraction. Therefore, major opportunities exist for the sensitive detection of ionic analyte when a "blank" measurement in the absence of analyte is impossible. This approach is particularly useful to deconvolute signals related to reversible electrochemical reactions from those due to irreversible processes, which do not give AC signals.

Copper electroplating of 3D printed composite electrodes

The manufacture of electrodes by 3D printing approaches has recently been recognized as a fast, inexpensive and environmentally friendly alternative to traditional preparation techniques based on subtractive manufacturing tools. However, as-prepared 3D printed electrodes typically show a considerable intrinsic kinetic barrier for the electron transfer. In this work we employ fused deposition modelling 3D printing to fabricate electrodes from a polylactic acid/copper composite filament and subject them to the surface functionalization by the copper electroplating aiming at eliminating this kinetic barrier. Cyclic voltammetry employing [Ru(NH3)6]3+/2+ couple as the electroactive probe was employed to inspect electron transfer properties of the manufactured electrodes. We demonstrate that electrodes modified by an optimized electroplating procedure show virtually no kinetic barrier and generate faradaic response with the magnitude comparable to that obtained at conventional metallic and carbon-based electrodes. The obtained faradaic peak separation value (70–75 mV) is superior to all values reported for 3D printed electrodes in the literature.

The aggregation and micellization of ionic surfactants in aqueous solution detected using surface-confined redox and ion-pairing reactions

This article describes the use of a redox-induced ion-pairing reaction confined to an electrode surface to detect the micellar aggregation of ionic surfactants in water. Cyclic voltammetry characterizes the Faradaic response of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold in aqueous solutions of the sodium salts of the anionic surfactants n-decyl sulfate, n-decyl sulfonate, and taurodeoxycholate, the bromide salt of the cationic surfactant decyltrimethylammonium, and the sodium salt of the azo dye Orange II. These electrolytes present different aggregation behaviors in water. The cyclic voltammograms indicate that the surfactant and dye anions and the bromide counteranion of the cationic decyltrimethylammonium pair with the electrogenerated SAM-bound ferrocenium. The voltammetric signatures are sensitive to the concentration and aggregation state of the ions in solution. The concentration dependence of the apparent redox potential (E°’SAM) presents the same features observed by potentiometry using surfactant ion-selective or bromide electrodes, indicating that E°’SAM tracks the activity of the unaggregated or free surfactant, dye or bromide anions in solution. The formation of micellar aggregates above the critical micelle concentration (cmc) causes the free anion activity to deviate from the molar concentration of added surfactant, resulting in a break in the plot of E°’SAM versus the logarithm of the surfactant concentration. The quantity of surfactant anions associated to the oxidized SAM increases with the solute concentration and plateaus out at concentrations > cmc. These findings point to the potential application of ferrocene-terminated SAMs to the investigation of ionic micelle-forming systems and the determination of their cmc’s.

Hot-Tip Scanning Electrochemical Microscopy: Theory and Experiments Under Positive and Negative Feedback Conditions.

Hot-tip scanning electrochemical microscopy (HT-SECM) is a novel surface characterization technique utilizing an alternating current (ac) polarized disk microelectrode as a probe. A high-frequency (∼100 MHz) ac waveform applied between the tip and a counter electrode causes the resistive heating of the surrounding electrolyte solution that leads also to the electrothermal fluid flow (ETF). The effects of the temperature and the convection driven by the ETF result in the increased rate of mass transfer of the redox species. In this paper, HT-SECM was studied in positive and negative feedback modes, for which approach curves and cyclic voltammograms were recorded. The experimental data showed that the use of a hot tip leads to a more pronounced feedback compared to that at room temperature. Numerical simulations performed in COMSOL Multiphysics supported the experimental findings. Additional analytical approximations were developed that could be used to predict the faradaic response in HT-SECM experiments. Finally, a possible contribution to the current from the Soret effect was studied theoretically. A good understanding of HT-SECM was achieved, both experimentally and theoretically, suggesting that this methodology could be applied to investigate electrode kinetics under the conditions of elevated temperature and increased rate of mass transfer.

Pulse Electrodeposition of Copper Octahedrons: Investigations into the CO2RR on These Surfaces

A well-tamed energy conversion strategy for the valorization of CO2 is a necessity for maintaining a check on CO2 levels (~ 500ppm) and borrow enough time for the scientific community to perfect zero-emission fuel prospects. Mimicking photosynthetically fixed CO2 by plants in an electrochemical cell is both scientifically intriguing and sociologically promising- given more than half of world still runs on fossil fuel derived energy and standard of life are determined by the intake of energy. Catalysing CO2RR with economical, selective heterogeneous metal nano-structures proved a promising way to improve upon the status quo. We synthesised copper octahedrons by two electrode electrodeposition, to selectively reduce CO2RR into CO2+H2 (synthesis gas) to use it as starting material in well known Fischer-tropsch reaction. In a representative experiment, copper was dissolved from a sacrificial copper anode and collected as micro-octahedrons on the surface of a platinum foil. The structures were simply tapped off the surface, cleaned, dried and used as catalyst ink to reduce CO2 into value-added fuels. A set of experimental parameters were varied to study the effect of them on morphology, size and activity of the obtained copper powders. A pulse method involving anodic and cathodic waves passed through the copper anode was found to have a determining capability - be it shape, topology or size of the crystallites. In a catholyte composed of 0.2 M KHCO3, cyclo-voltammograms (CVs) were recorded in N2 saturated and CO2 catholyte in a 3-electrode, 2-chambered cell. From preliminary results, it was shown that CuOcta showed activity towards CO2RR as was evident in enhanced faradaic response in CO2 saturated catholyte. To confirm the path of fixing and valorization, a thorough gas chromatography (GC) analysis was done on the gases that were collected in a Tedlar bag. CO was found to be major reduction product in gaseous phase overcoming the inherit HER (hydrogen evolution reaction) competition on metal surfaces. A significant amount of CH4 was also measured. The array of obtained products signal role of morphology, the surface topography of catalytic structures on the mechanism and kinetics of CO2RR. Figure 1

In Situ Measurement of Stimulus Induced pH Changes Using ThinFilm Embedded IrOx pH Electrodes.

The high complexity of the biological response to implanted materials builds a serious barrier against implanted recording and stimulation electrode arrays to succeed in clinically relevant chronic studies. Some of the cell and molecular interactions and their contribution to inflammation and device failure are still unclear. The interrelated mechanisms leading to tissue damage and electrode array failure during simultaneous faradaic, electrochemical reactions and biological response under electrical stimulation are not understood sufficiently. One variable, with which inflammatory and electrode surface processes can be analyzed and assessed, is the pH change in the immediate environment of the material-tissue interface. Here, the greatest challenges are in the biocompatibility and in-vivo long-term stability of selected sensor materials, the measurement of small transient pH oscillations and positioning of the sensor at a defined and nearest possible distance in the micrometer range, to the site of activity without the pH sensing being affected by the material- issue interactions itself. This work represents the in-situ measurement of local and transient pH changes at apulsed electrode with an embedded in-vivo compatible pH sensor and therein differentiating from current approaches of pH sensing during electrical stimulation.

Specific Ion Effects on the Mediated Oxidation of NADH

AbstractThe electrochemical mediated oxidation of dihydronicotinamide adenine dinucleotide (NADH) by 2,2‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid) (ABTS) was examined in a range of electrolytes. Changes in the Faradaic response were observed that were dependent on the nature of the salt used. In the presence of 200 mM chloride salts, the Faradaic response followed the trend Na+>K+>Li+>Cs+>Gnd+ while in the presence of 200 mM sodium salts, the trend Cl−>Br−>F−>ClO4−>SCN− was observed. The observed trends varied when the concentration of the ions was altered. The effect of the cation also changed when the counterion was changed, indicating that specific ion pair effects were present. The results indicate that the nature of the electrolyte alters the Faradaic response.

Electrolyte effects on enzyme electrochemistry

Summary The electrolytes used in enzymatic biosensors or biofuel cells have always been considered to be inert. However, recent studies have demonstrated that this assumption is not correct and that the nature of the electrolyte needs to be considered. Ion-specific interactions can occur with the faradaic response observed in both direct and mediated electron transfer being modulated by the nature of the salt used in solution. Specific ion effects arise from the Hofmeister series, which is well established in studies of protein systems but not in electrochemical studies of redox enzymes. Recent experimental and theoretical work on explaining the Hofmeister effect is described.

Electrokinetic Manipulation of Silver and Platinum Nanoparticles and Their Stochastic Electrochemical Detection.

Electrokinetic phenomena such as dielectrophoresis and electrothermal fluid flow are used to increase the rate of mass transfer of silver and platinum nanoparticles and improve their stochastic electrochemical detection. These phenomena are induced by applying a high frequency alternating current (ac) waveform between a counter electrode and a working disk microelectrode. By recording chronoamperograms at room temperature and various ac powers, it is shown that the ac heating leads to an increase in the collision frequency of studied nanoparticles with working electrode surface by a factor of ∼101-103 as well as the increase in the magnitude of the measured faradaic response. It is suggested that the developed methodology could be used in the future to improve the detection of ultralow concentrations of various important bioanalytes.

Nanocomposites based on carbon nanotubes and redox-active polymers synthesized in a deep eutectic solvent as a new electrochemical sensing platform

The redox polymer poly(methylene blue) was synthesized by electropolymerization in ethaline deep eutectic solvent (PMBDES), and employed in a new nanostructured composite together with carbon nanotubes (CNT). The polymer was formed either before or after CNT deposition on glassy carbon electrodes (GCE) to obtain GCE/PMBDES/CNT and GCE/CNT/PMBDES modified electrodes, respectively. The morphology of the hybrid films was investigated by scanning electron microscopy, and their electrochemical performance by cyclic voltammetry and electrochemical impedance spectroscopy. The composite with the highest faradaic response and electronic conductivity was identified and applied as a platform for the determination of ascorbic acid by fixed potential amperometry at +0.2 V vs. Ag/AgCl, and acetaminophen by square wave voltammetry and differential pulse voltammetry at +0.67 V vs. Ag/AgCl. The limits of detection of the best sensor configurations are 1.7 μM for AA, and 1.6 μM for APAP, and respective electrochemical sensitivities are 2.2 μA cm-2 μM−1 and 68.7 μA cm-2μ​M-1. The applicability of the electrochemical sensors was demonstrated by the successful quantification of both these key analytes in complex pharmaceutical formulations.

Polypyrrole-based amperometric cation sensor with tunable sensitivity

This article presents a novel technique to quantify the kinetics of faradaic response in polypyrrole (doped with dodecylbenzenesulfonate) (PPy(DBS)) and its application in developing a cation sensor. This technique is based on the analysis of the impedance/admittance transfer function of PPy(DBS) and the representation of its reduction chronoameprometric response using system poles and residues. The results presented in this paper demonstrate that the pole of the transfer function is an intrinsic quantity and the residue corresponding to the system pole is an extrinsic quantity. The residue of the system pole for a redox-step potential is dependent on the concentration of cations and is observed to have linear dependence on electrolyte concentration ( R2 > 0.9). Thus, the residues can be used to accurately estimate the concentration of cations in an electrolyte solution and enables the use of PPy(DBS) as a high-precision cation sensor. The experimental investigation supporting this analysis demonstrates that the sensitivity of residue to concentration can be varied by equilibrating PPy(DBS) a priori in the cation containing solution of known concentration and by varying the morphology of PPy(DBS).