Surface Redox Processes Research Articles

Construction of sulfur modified oxygen-deficient NiO/Ni nanoflakes as an effective electrode material for energy storage application

The inferior electronic conductivity and deficient number of electrochemical active sites of the transition metal oxides severely restrict electrode kinetics and the utilization of redox actives sites on the electrode surface, thus leading to ineffective electrochemical performance. Herein, we exhibit an effective strategy to obtain NiO/Ni nanoflakes with oxygen vacancies and sulfur dopants (S-NiO1-x/Ni) for serving as supercapacitor electrode. The incorporation of O vacancies and S dopants into NiO/Ni in concert to modulate the electronic structure and generates more accessible active sites, which improve the electrochemical conductivity and facilitates surface redox processes. Experimental and theoretical studies provide insight into the location of the introduction of O vacancy and S atom and their effects on the electrical properties of S-NiO1-x/Ni. Meanwhile, S atoms incorporation into S-NiO1-x/Ni not only involve in the redox reactions but also act as active sites. The as-obtained S-NiO1-x/Ni delivers a remarkable electrochemical behavior with a high specific capacitance of 648.4F g−1 at the current density of 1 A/g. Additionally, an asymmetric supercapacitor outfitted with the as-prepared S-NiO1-x/Ni as positive electrode and active carbon as negative electrode gives rise to an excellent energy density of 28.8 Wh kg−1 at the power density of 800 W kg−1, together with a long-term stability.

Facet-Dependent Oxygen Evolution Reaction Activity of IrO2 from Quantum Mechanics and Experiments.

The diversity of chemical environments present on unique crystallographic facets can drive dramatic differences in catalytic activity and the reaction mechanism. By coupling experimental investigations of five different IrO2 facets and theory, we characterize the detailed elemental steps of the surface redox processes and the rate-limiting processes for the oxygen evolution reaction (OER). The predicted complex evolution of surface adsorbates and the associated charge transfer as a function of applied potential matches well with the distinct redox features observed experimentally for the five facets. Our microkinetic model from grand canonical quantum mechanics (GC-QM) calculations demonstrates mechanistic differences between nucleophilic attack and O-O coupling across facets, providing the rates as a function of applied potential. These GC-QM calculations explain the higher OER activity observed on the (100), (001), and (110) facets and the lower activity observed for the (101) and (111) facets. This combined study with theory and experiment brings new insights into the structural features that either promote or hinder the OER activity of IrO2, which are expected to provide parallels in structural effects on other oxide surfaces.

Size Effect of RuO2-Based 2D Conductive Binder

In an electrode mixture MnO2 is one of the most well studied pseudocapacitive material for aqueous supercapacitors owing to the surface and near surface-confined redox processes. A large amount of conducting additive, generally 20-30 wt% acetylene black, needs to be added to achieve high-rate charge storage. Since the conducting additive does not contribute much to the specific capacitance, this leads to decrease in total energy density of the cell in terms of both mass and volume. Polymeric binders such as PTFE and PVdF are also necessary to fabricate rigid electrode with good contact. As an alternative to these typical conductive binders and polymer binders, we have studied the possibility of using RuO2 nanosheets as a novel inorganic binder with polymeric properties, high electronic conductivity and excellent pseudocapacitive properties. For example, by adding only 10-20 mol% of RuO2 nanosheets to particulate MnO2, a large synergetic effect is observed leading to a high utilization of MnO2 pseudocapacitance.[1] The optimized amount of RuO2 in the composite was 13 wt%, which is much lower than the amount generally used for conductive carbon additives despite the 10 times higher formula weight of RuO2 compared to carbon. In this work, we report the electrochemical properties of nanosheet composite electrodes composed of highly conductive RuO2 nanosheets (RuO2(ns)) and poorly conductive MnO2 nanosheets (MnO2(ns)) with various composition and nanosheet size.

Lignin-Derived Quinone Redox Moieties for Bio-Based Supercapacitors.

Because of their rapid charging and discharging, high power densities, and excellent cycling life stabilities, supercapacitors have great potential for use in electric vehicles, portable electronics, and for grid frequency modulation. The growing need for supercapacitors that are both efficient and ecologically friendly has generated curiosity in developing sustainable biomass-based electrode materials and electrolytes. Lignin, an aromatic polymer with remarkable electroactive redox characteristics and a large number of active functional groups, is one such candidate for use in renewable supercapacitors. Because its chemical structure features an abundance of quinone groups, lignin undergoes various surface redox processes, storing and releasing both electrons and protons. Accordingly, lignin and its derivatives have been tested as electroactive materials in supercapacitors. This review discusses recent examples of supercapacitors incorporating electrode materials and electrolytes derived from lignin, focusing on the pseudocapacitance provided by the quinone moieties, with the goal of encouraging the use of lignin as a raw material for high-value applications. Employing lignin and its derivatives as active materials in supercapacitor electrodes and as a redox additive in electrolytes has the potential to minimize environmental pollution and energy scarcity while also providing economic benefits.

Electrochemical microelectrode degradation monitoring: in situ investigation of platinum corrosion at neutral pH

Objective. The stability of platinum and other noble metal electrodes is critical for neural implants, electrochemical sensors, and energy sources. Beyond the acidic or alkaline environment found in most electrochemical studies, the investigation of electrode corrosion in neutral pH and chloride containing electrolytes is essential, particularly regarding the long-term stability of neural interfaces, such as brain stimulation electrodes or cochlear implants. In addition, the increased use of microfabricated devices demands the investigation of thin-film electrode stability in combination with electrode performance. Approach. We developed a procedure of electrochemical methods for continuous tracking of electrode degradation in situ over the complete life cycle of platinum thin-film microelectrodes in a unique combination with simultaneous chemical sensing. We used chronoamperometry and cyclic voltammetry to measure electrode surface and analyte redox processes, together with accelerated electrochemical degradation. Main results. We compared degradation between thin-film microelectrodes and bulk electrodes, neutral to acidic pH, different pulsing schemes, and the presence of the redox active species oxygen and hydrogen peroxide. Results were confirmed by electrochemical impedance spectroscopy, as well as mechanical profilometry and microscopy to determine material changes on a nanometer scale. We found that electrode degradation is mainly driven by repeated formation and removal of the platinum surface oxide, also within the electrochemical stability window of water. There was no considerable difference between thin-film micro- and macroscopic bulk electrodes or in the presence of reactive species, whereas acidic pH or extending the potential window led to increased degradation. Significance. Our results provide valuable fundamental information on platinum microelectrode degradation under conditions found in biomedical applications. For the first time, we employed a unified method to report quantitative data on electrode degradation up to a defined endpoint. Our method is a widely applicable framework for comparative long-term studies of electrode micro-/nanomaterial, sensor and neural interface stability.

Redox‐mediated proton transport of two‐dimensional polyaniline‐based nanochannels for fast capacitive performance

AbstractTwo‐dimensional (2D) materials are being increasingly exploited for ion transport and storage under nanoconfinement. Here we demonstrate distinct ion transport behavior upon potential‐induced redox reaction in 2D tungstate anion cross‐linked polyaniline (TALP) electrode. It is found that, in the neutral electrolyte, SO42− ion serves as the main charge carrier in 2D polyaniline backbone when the electrical double layer charging dominates. While in an acidic electrolyte, proton transport in TALP turns to be the dominant ion behavior that is associated with the proton‐promoting surface redox processes. Moreover, higher capacitance along with better capacitive retention at a high rate is also demonstrated for TALP electrode in acidic electrolyte compared with that in the neutral environment. The results show that 2D nanoionics can be manipulated by applying redox‐active materials to build the nanochannels that allow the regulation of surface charge and chemistry with potential‐specific redox reactions.

Hybrid aqueous supercapacitors based on mesoporous spinel-analogous Zn-Ni-Co-O nanorods: Effect of Ni content on the structure and energy storage

The structure and the electrochemical performance of mesoporous spinel-analogous Zn-Ni-Co-O (Zn1−xNixCo2O4; 0.2 ≤ x ≤ 0.8) nanorods are investigated. Cyclic voltammetry gives strong indication that the charge storage mechanism is governed by pore surface redox processes without significant contribution from the sub-surface charge storage. Since the size of the Zn1−xNixCo2O4 rods decreases with increasing Ni content x, the BET surface and thus the charge storage capacity increases with increasing x, namely from 266 C g−1 to 463 C g−1 at 3.12 A g−1 when increasing x from 0.2 to 0.8. Hybrid aqueous supercapacitors (HSC) are fabricated with Zn1−xNixCo2O4, activated carbon (AC) and 6 M KOH as positive electrode, negative electrode and electrolyte, respectively. The specific energy densities of HSC are in the range of 27–2 Wh kg−1 at the specific power densities range of 780–2240 W kg−1. Real-world usage of the aqueous HSC device is demonstrated by enlightening a red light-emitting diode. These outcomes show that the mesoporous spinel-analogous Zn-Ni-Co-O materials are potential candidates for next-generation electrochemical energy storage applications.

Nanocoral CuCo2S4 thiospinels: Oxygen evolution reaction via redox interaction of metal ions

Understanding the electrocatalytic activity and redox mechanism in mixed metal sulfides has steered the present work. Mixed metal sulfide, CuCo2S4, synthesised by hydrothermal method, reveals the presence of highly crystalline nanocoral sheets of CuCo2S4 with mixed oxidation states of Co and Cu ions. Electrochemical measurements show superior oxygen evolution reaction (OER) activity and high exchange current density over CuCo2S4 in comparison with pristine (Co3S4 and Cu2S) and sintered CuCo2S4. In 0.5 M KOH, the nanocoral CuCo2S4 catalyst and its sintered form show a current density of 10 mA cm−2 at overpotential of 368 mV and 511 mV, respectively. A close inspection of the electrocatalytic performance in correlation with XPS spectra reveals the synergistic effect of Co and Cu ions through a redox interaction. Density Functional Theory calculations also indicate strong dependence of redox activity on redox couples as accompanied with changes in the partial charges of different constituent ions in the catalyst upon adsorption of hydroxyl ions which is a key step in OER. The study not only details sulfide-based catalytic compound for OER but also provides conclusive insights into the surface redox processes thereby providing an alternative direction to the search on novel OER catalysts.

Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries.

Layered lithium nickel, manganese, and cobalt oxides (NMC) are among the most promising commercial positive electrodes in the past decades. Understanding the detailed surface and bulk redox processes of Ni-rich NMC can provide useful insights into material design options to boost reversible capacity and cycle life. Both hard X-ray absorption (XAS) of metal K-edges and soft XAS of metal L-edges collected from charged LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) showed that the charge capacity up to removing ∼0.7 Li/f.u. was accompanied with Ni oxidation in bulk and near the surface (up to 100 nm). Of significance to note is that nickel oxidation is primarily responsible for the charge capacity of NMC622 and 811 up to similar lithium removal (∼0.7 Li/f.u.) albeit charged to different potentials, beyond which was followed by Ni reduction near the surface (up to 100 nm) due to oxygen release and electrolyte parasitic reactions. This observation points toward several new strategies to enhance reversible redox capacities of Ni-rich and/or Co-free electrodes for high-energy Li-ion batteries.

Surface-induced synthesis of hybrid N, P functionalized hierarchically porous carbon nanosheets for lithium-ion batteries

The design and fabrication of two-dimension (2D) nanostructure carbon with desired lithium-ion storage properties is highly demanded for substitution graphite. Herein, the hierarchically porous carbon nanosheets with hybrid N, P functionalization (NPHC) via a layered magadiite surface-induced strategy are reported, followed by thermal annealing at high temperature. The negative silicon hydroxyls on the magadiite surface bond with aniline via an electrostatic force with phytic acid as molecular level dopant and surfactant, yielding hybrid homogenous N, P functionalization and ultrathin and porous geometry. When applied as an anode for Li-ion batteries, the obtained carbonsheets deliver a high discharge capacity of 763.4 mA h g−1 at 100 mA g−1, and superior rate capability of 146.6mA h g−1 at 8000 mA g−1. The impressive performance is mainly attributed to the synergistic effects from the hierarchical porous structure and homogenous N, P functionalization which increase the electrode-electrolyte contact area and active storage sites, further are beneficial for electrical double-layer or surface redox processes contributing to the overall charge storage.

Preparation and electrochemical performance of modified Ti3C2Tx/polypyrrole composites

ABSTRACTMXenes with a large surface area have been widely studied to improve the pseudocapacitance of electrode materials by combining conductive polymer materials. In this article, a superficial strategy to enhance the electrochemical properties by in situ polymerization of a pyrrole monomer between the Ti3C2Tx layers modified with 1,5‐naphthalene disulfonic acid (NA) and cetyltrimethylammonium bromide (CTAB) was investigated. It is found that polypyrrole (PPy) and Ti3C2Tx can be combined through strong interactions between each other, and the specific capacitance of the modified Ti3C2Tx/PPy composite was increased to a maximum value of 437 F g−1, which was more than thrice higher than that of pure PPy. The composite also exhibited good cycling performance (76% capacitance retention after 1000 cycles). Moreover, owing to the synergistic effect between the PPy and Ti3C2Tx layers, the composite provided better electron or ion transfer and surface redox processes than that of pure PPy, which indicated that this composite can be used as a promising electrode material for supercapacitors. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47003.

Innovative Ti1- xNb xN-Ag Films Inducing Bacterial Disinfection by Visible Light/Thermal Treatment.

This study presents innovative Ti1- xNb xN-Ag films obtained by a suitable combination of low-energy and high-energy sputtering leading to bacterial inactivation. The bacterial inactivation kinetics by the TiNbN layers was drastically enhanced by the addition of 6-7% Ag and proceeded to completion within 3 h after the film autoclaving. By X-ray photoelectron spectroscopy (XPS), the samples after autoclaving presented in their upper layers TiO2, Nb2O5 and Ag2O with a surface composition of Ti0.81Nb0.19N0.99Ag0.068. Surface potential/pH changes in the Ti1- xNb xN-Ag films were monitored during bacterial inactivation. Surface redox processes during the bacterial inactivation were detected by XPS. The diffusion of Ag in the Ti1- xNb xN-Ag films was followed at 50 and 70 °C pointing. The beneficial thermal treatment points out to the bifunctional bacterial inactivation properties of these films and their potential application in healthcare facilities. Interfacial charge transfer (IFCT) under light irradiation between Ag2O, Nb2O5 and TiO2 is suggested consistent with the data found during the course of this study. The TiO2/Nb2O5 lattice mechanism is discussed in the framework of the Verwey's controlled valence model. The surface properties of the Ti1- xNb xN-Ag films were investigated by X-ray diffraction, atomic force microscopy, and scanning electron microscopy.

High‐Performance Ultrathin Flexible Solid‐State Supercapacitors Based on Solution Processable Mo1.33C MXene and PEDOT:PSS

AbstractMXenes, a young family of 2D transition metal carbides/nitrides, show great potential in electrochemical energy storage applications. Herein, a high performance ultrathin flexible solid‐state supercapacitor is demonstrated based on a Mo1.33C MXene with vacancy ordering in an aligned layer structure MXene/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) composite film posttreated with concentrated H2SO4. The flexible solid‐state supercapacitor delivers a maximum capacitance of 568 F cm−3, an ultrahigh energy density of 33.2 mWh cm−3 and a power density of 19 470 mW cm−3. The Mo1.33C MXene/PEDOT:PSS composite film shows a reduction in resistance upon H2SO4 treatment, a higher capacitance (1310 F cm−3) and improved rate capabilities than both pristine Mo1.33C MXene and the nontreated Mo1.33C/PEDOT:PSS composite films. The enhanced capacitance and stability are attributed to the synergistic effect of increased interlayer spacing between Mo1.33C MXene layers due to insertion of conductive PEDOT, and surface redox processes of the PEDOT and the MXene.

Visualizing atomic-scale redox dynamics in vanadium oxide-based catalysts

Surface redox processes involving oxygen atom exchange are fundamental in catalytic reactions mediated by metal oxides. These processes are often difficult to uncover due to changes in the surface stoichiometry and atomic arrangement. Here we employ high-resolution transmission electron microscopy to study vanadium oxide supported on titanium dioxide, which is of relevance as a catalyst in, e.g., nitrogen oxide emission abatement for environmental protection. The observations reveal a reversible transformation of the vanadium oxide surface between an ordered and disordered state, concomitant with a reversible change in the vanadium oxidation state, when alternating between oxidizing and reducing conditions. The transformation depends on the anatase titanium dioxide surface termination and the vanadium oxide layer thickness, suggesting that the properties of vanadium oxide are sensitive to the supporting oxide. These atomic-resolution observations offer a basis for rationalizing previous reports on shape-sensitive catalytic properties.

Effect of template post-annealing on Y(Dy)BaCuO nucleation on CeO2 buffered metallic tapes

Substrate engineering is very significant in the synthesis of the high-temperature superconductor (HTS) coated conductor. Here we design and synthesize several distinct and stable Cerium oxide (CeO2) surface reconstructions which are used to grow epitaxial films of the HTS YBa2Cu3O7-δ (YBCO). To identify the influence of annealing and post-annealing surroundings on the nature of nucleation centers, including Ar/5%H2, humid Ar/5%H2 and O2 in high temperature annealing process, we study the well-controlled structure, surface morphology, crystal constants and surface redox processes of the ceria buffers by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and field-emission scanning electronic microscopy (FE-SEM), respectively. The ceria film post-annealed under humid Ar/5%H2 gas shows the best buffer layer properties. Furthermore, the film absorbs more oxygen ions, which appears to contribute to oxygenation of superconductor film. The film is well-suited for ceria model studies as well as a perfect substitute for CeO2 bulk material.

Evaluation of an Electrodeposited Bimetallic Cu/Ag Nanostructured Screen Printed Electrode for Electrochemical Surface-Enhanced Raman Spectroscopy (EC-SERS) Investigations

The field of plasmonics has experienced rapid growth over the past decade with a host of emerging applications including single molecule sensing and plasmon-assisted catalysis. The vast majority of these applications use either silver or gold as the plasmonic metal, which are both high cost and face earth-abundance limitations in the next 100 years. Recent efforts have focused on taking advantage of the plasmonic properties of copper, a more abundant and low cost coinage metal as a sustainable route for plasmonic applications. In particular, there has been great interest in developing copper substrates capable of reliable and efficient enhancement of Raman signals for use in surface-enhanced Raman spectroscopy (SERS) sensing. Herein we describe a sequential electrodeposition technique whereby highly functional and robust Cu/Ag bimetallic SERS-active screen printed electrodes can be produced rapidly and at low cost, which display excellent plasmonic performance and are capable of supporting surface-plasmon assisted catalysis (SPAC). This modified screen printed electrode allows for the in situ spectroelectrochemical investigation of surface redox processes using a sustainable alternative to traditional monometallic electrodes.

Lateral Size Effects of Two-dimensional IrO<sub>2</sub> Nanosheets towards the Oxygen Evolution Reaction Activity

The oxygen evolution reaction of anode catalyst is an important reaction in polymer electrolyte membrane water electrolysis. Recent research has focused on understanding the factors governing the catalytic properties of nanostructured IrO2 in order to decrease the use of expensive and precious IrO2. We demonstrate here the equivalent diameter (De) effect (lateral size effect) of IrO2 nanosheets for the oxygen evolution reaction. IrO2 nanosheets with four different De from 110 to 260 nm and same thickness (∼1.5 nm) were prepared. The electrical double layer capacitance, which should be proportional to the electrochemically active surface area, of IrO2 nanosheets was independent of De. On the other hand, the pseudo-capacitance, which is related to surface redox processes, and the oxygen evolution reaction activity increased with decreasing De. The enhanced oxygen evolution reaction activity is discussed based on the electrochemically active surface area and edge effect.

Supercapacitor performance of perovskite La1-xSrxMnO3.

In this study, Sr-doped lanthanum manganite perovskites (La1-xSrxMnO3, LSM) as electrode materials for supercapacitors were prepared via an improved sol-gel method. Among LSM, the x = 0.15 sample shows superior electrochemical performance, delivering the highest specific capacitance of 102 F g-1 at a current density of 1 A g-1 and lower intrinsic resistance in 1 M KOH. The effective charge storage of LSM is due to the redox reaction of the anion intercalation corresponding to the surface redox processes of Mn2+/Mn3+ and Mn3+/Mn4+ occurring within the electroactive materials. The maximum energy density of 3.6 W h kg-1 was achieved at a power density of 120 W kg-1 for the symmetric supercapacitor with a long cycling life after 5000 charging and discharging cycles. With the increase of cycle times, the element leaching phenomenon leads to the decrease of electrochemical performance. Our work indicated that the perovskite manganite La0.85Sr0.15MnO3 shows potential applications in the field of pseudocapacitance electrode materials and is worthy of further investigation.

Electrochemical Oxidation of Amoxicillin in Its Commercial Formulation on Thermally Prepared RuO2/Ti

In this work, a ruthenium dioxide electrode has been prepared by thermal decomposition at 400 C then used for the oxidation of commercial amoxicillin. The physical characterization showed that RuO2 electrode presents a mud cracked structure. Its electrochemical characterization has revealed an increase of the voltammetric charge in acid electrolyte compared to neutral electrolyte indicating the importance of protons in its surface redox processes. The voltammetric study of the oxidation of amoxicillin has been investigated. It has been obtained that the oxidation of amoxicillin is controlled by both adsorption and diffusion processes. Moreover, the oxidation of amoxicillin occurs via direct and indirect processes in free or electrolyte containing chlorides. Through preparative electrolysis, enhancement of amoxicillin oxidation was observed in the presence of chloride where the amoxicillin degradation yield reached more than 50 % compared to less than 5 % in the absence of chlorides. Spectrophotometric investigations have revealed the degradation of intermediates absorbing at 350 nm.

A perfectly stoichiometric and flat CeO2(111) surface on a bulk-like ceria film.

In surface science and model catalysis, cerium oxide (ceria) is mostly grown as an ultra-thin film on a metal substrate in the ultra-high vacuum to understand fundamental mechanisms involved in diverse surface chemistry processes. However, such ultra-thin films do not have the contribution of a bulk ceria underneath, which is currently discussed to have a high impact on in particular surface redox processes. Here, we present a fully oxidized ceria thick film (180 nm) with a perfectly stoichiometric CeO2(111) surface exhibiting exceptionally large, atomically flat terraces. The film is well-suited for ceria model studies as well as a perfect substitute for CeO2 bulk material.