Combination Of Cyclic Voltammetry Research Articles

In situ real-time investigation of ruthenium electrodeposition behavior in HCl solution using EQCM

Industrial waste solutions are rich in precious metals, which are valuable secondary resources. In order to realize the separation and recovery of ruthenium by electrodeposition, this work used a combination of cyclic voltammetry (CV), chronoamperometry (I-t), and electrochemical quartz crystal microbalance (EQCM) to study the electrochemical behavior and electrodeposition mechanism of ruthenium in HCl solution. The results show that the reduction of ruthenium in HCl solution is influenced by hydrogen evolution reaction (HER) and chlorine evolution reaction, and the reduction process of ruthenium varies at different potentials. The reduction currents of Ru(VIII)/Ru(0) and Ru(IV)/Ru(III) are observed at 0.88 V and 0.6 V. The reduction of Ru(III) to Ru(0) at negative potentials starts at −0.2 V, with peak cathodic currents between −0.2 V and −0.5 V accompanied by a significant mass increase, which is confirmed to be the electrodeposition of metallic ruthenium by SEM and EDS. The microscopic morphology of the ruthenium coating is spherical particles, and the nucleation mode of electrodeposition is progressive nucleation. The HER leads to a macroscopic morphology of a cleaved lamellar structure, which is not tightly adhered to the substrate. The metallic ruthenium starts to oxidize and dissolve rapidly after 1.12 V, leading to a rapid degradation of the electrode surface quality.

Monitoring the Electrochemical Failure of Indium Tin Oxide Electrodes via Operando Ellipsometry Complemented by Electron Microscopy and Spectroscopy.

Transparent conductive oxides such as indium tin oxide (ITO) are standards for thin film electrodes, providing a synergy of high optical transparency and electrical conductivity. In an electrolytic environment, the determination of an inert electrochemical potential window is crucial to maintain a stable material performance during device operation. We introduce operando ellipsometry, combining cyclic voltammetry (CV) with spectroscopic ellipsometry, as a versatile tool to monitor the evolution of both complete optical (i.e., complex refractive index) and electrical properties under wet electrochemical operational conditions. In particular, we trace the degradation of ITO electrodes caused by electrochemical reduction in a pH-neutral, water-based electrolyte environment during electrochemical cycling. With the onset of hydrogen evolution at negative bias voltages, indium and tin are irreversibly reduced to the metallic state, causing an advancing darkening, i.e., a gradual loss of transparency, with every CV cycle, while the conductivity is mostly conserved over multiple CV cycles. Post-operando analysis reveals the reductive (loss of oxygen) formation of metallic nanodroplets on the surface. The reductive disruption of the ITO electrode happens at the solid-liquid interface and proceeds gradually from the surface to the bottom of the layer, which is evidenced by cross-sectional transmission electron microscopy imaging and complemented by energy-dispersive X-ray spectroscopy mapping. As long as a continuous part of the ITO layer remains at the bottom, the conductivity is largely retained, allowing repeated CV cycling. We consider operando ellipsometry a sensitive and nondestructive tool to monitor early stage material and property changes, either by tracing failure points, controlling intentional processes, or for sensing purposes, making it suitable for various research fields involving solid-liquid interfaces and electrochemical activity.

Subsurface Hydride Formation Leads to Slow Surface Adsorption Processes on a Pd(111) Single-Crystal Electrode in Acidic Electrolytes.

Palladium is one of the most important catalysts due to its widespread use in heterogeneous catalysis and electrochemistry. However, an understanding of the electrochemical processes and interfacial phenomena at Pd single-crystal electrodes/electrolytes is still scarce. In this work, the electrochemical behavior of the Pd(111) electrode was studied by the combination of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in different acidic electrolytes, namely, sulfuric acid, perchlorate acid, methane sulfonic acid, and hydrofluoric acid. An analysis of CV profiles shows the strong adsorption of all anions at low electrode potential, partially overlapping with underpotential deposited hydrogen (UPD-H), leading to the appearance of a pair of sharp peaks in what would be considered the "hydrogen region". All anions studied (HSO4-, ClO4-, CH3SO3-, and F-) adsorb specifically and interact with (or effectively block) the surface-adsorbed hydroxyl phase formed on the Pd(111) terrace at higher potentials. Strikingly, the scan rate-dependent results show that the process of anion adsorption and desorption is a kinetically rather slow step. EIS measurements show that the exact mechanism of this slow anion ad/desorption process actually stems from (sub)surface phenomena: the direct hydrogen insertion into Pd lattice (hydrogen subsurface absorption) commences from ca. 0.40 V and leads to the formation of (subsurface) Pd hydrides (PdHx). We argue that the subsurface hydrogen phase significantly alters the work function and thereby the kinetics of the anion adsorption and desorption processes, leading to irreversible peaks in the voltammetry. This precise understanding is important in guiding further fundamental work on Pd single crystals and will be crucial to advancing the eventual design of optimized Pd electrocatalysts.

Revealing the causes of degradation of an asymmetric ionic liquid supercapacitor with a composite polyaniline/carbon electrode

The extended window of electrochemical stability of ionic liquids and the high specific capacitance values of activated carbon (AC)/conductive polymer (polyaniline, Pani) composite materials are promising approaches to improve the performance of supercapacitors (SC).In this work, we studied the stability of an asymmetric AC//Pani-C cell with 1 M solution of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) in acetonitrile as an electrolyte. Pani-C (56 wt% of Pani) composite was prepared by oxidative polymerization of aniline in the presence of high surface area AC (SBET = 2300 m2‧g−1). Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission and scanning electron microscopies data confirmed the formation of Pani-C composite. Combination of cyclic voltammetry, galvanostatic charge/discharge method, and electrochemical impedance spectroscopy in 2- and 3-electrode cell configurations gave information about anodic (1 V vs. Ti) and cathodic (−1.8 V vs. Ti) potential limits of electrochemical stability, and potential of zero charge of Pani-C and AC.From these data, optimal mass ratio of Pani-C and AC electrodes was obtained for asymmetric SC assembly. It was found that after 500 charge/discharge cycles, the capacitance of the asymmetric AC//Pani-C SC decreased by 15% from 46.8 to 39.9 F·g−1 as a result of Pani-C degradation. The specific capacitance of AC remained virtually unchanged (decreased from 144 to 141 F·g−1), while that of Pani-C decreased by 8.7% from 230 to 210 F·g−1. Electrochemical measurements showed that degradation is accompanied by an increase in the working potential window of Pani-C (from 0.85 V to 0.97 V) and a shift in Emin…Emax working potentials from 0.15 … 1 to 0.52 … 1.49 V vs. Ti in the beginning and in the end of stability test, respectively. The last region is related to the potential for non-conductive pernigraniline formation (1.3 V vs. Ti). It was shown that the main reason of AC//Pani-C SC capacitance decay is a gradual increase in the operating potential window with a constantly shifts towards anodic values of positive Pani-C electrode, which resulted in accumulation of nonconductive polymer form that does not participate in charge accumulation.

Towards a Fully-Quantitative Picture of SEI Chemical Composition and Its Native Li+ Exchange in Liquid Electrolytes

Energy density requirements for next-generation batteries make the lithium (Li) metal anode a key candidate to replace graphite in lithium-ion batteries due to Li’s improved capacity (3,860 mAh/g vs. 372 mAh/g). However, Li anodes display lower Coulombic efficiency (CE, <99.9%) than graphite (>99.95%),1 resulting in faster loss of reversible Li over cycling. This shortfall derives from parasitic Li-electrolyte reactions that lead to accumulation of inactive Li0 and electrolyte-derived byproducts, which result in the formation of a native solid electrolyte interphase (SEI) on Li. In addition to contributing to capacity loss, the SEI also has the important role of regulating Li+ exchange between Li and the electrolyte, which may affect Li plating morphology and reversibility.2 Despite significant advances, the framework on which the understanding of SEI composition and function was built has largely been qualitative, often due to experimental limitations and/or lack of self-consistent techniques.To bridge this gap, my thesis work focuses on advancing experimental techniques and methodologies to precisely quantify the chemical composition and rates of Li+ exchange in native SEIs. Using a combination of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), self-consistent rates of Li+ exchange are reported across a wide range of electrolytes.3 Notably, we found that native SEIs formed in high CE electrolytes displayed consistently higher rates of Li+exchange. The improved transport in high-CE electrolytes was found to emerge after an SEI formation cycle, and amplified over cycling, indicating that fast Li+ exchange is beneficial to Li reversibility.In order to identify which SEI phases grant high-CE electrolytes their beneficial functional properties, we also developed and expanded upon a series of titration experiments based on GC, ICP-AES and NMR that can track an extended array of key SEI phases: semicarbonates (ROCO2Li), lithium carbide (Li2C2), olefins (RLi), LiF, P-containing phases, and total Li loss.4 In the 1 M LiPF6 EC/DEC electrolyte, we demonstrate chemical resolution up to 71% of Li loss and 33% of SEI loss. Among the quantifiable SEI phases, ROCO2Li was consistently the major phase, but its proportions were invariant with CE. Instead, Li2C2, a minor phase, exhibited clear inverse correlation with CE. We also demonstrate further quantitative chemical resolution can be achieved with online GC, which can differentiate between the different types of semicarbonate (i.e., which “R” in ROCO2Li) and alkoxide phases that form on Li.5 Altogether, these results add further nuance to the current understanding of SEI composition and their function, which we hope will allow more quantitative electrolyte design. Hobold, G. M.; Lopez, J.; Guo, R.; Minafra, N.; Banerjee, A.; Shirley Meng, Y.; Shao-Horn, Y.; Gallant, B. M., Moving Beyond 99.9% Coulombic Efficiency for Lithium Anodes in Liquid Electrolytes. Nat. Energy 2021, 6 (10), 951-960. Zheng, J.; Kim, M. S.; Tu, Z.; Choudhury, S.; Tang, T.; Archer, L. A., Regulating Electrodeposition Morphology of Lithium: Towards Commercially Relevant Secondary Li Metal Batteries. Chem. Soc. Rev. 2020, 49 (9), 2701-2750. Hobold, G. M.; Kim, K.-H.; Gallant, B. M., Beneficial Vs. Inhibiting Passivation by the Native Lithium Solid Electrolyte Interphase Revealed by Electrochemical Li+ Exchange. 2022. Hobold, G. M.; Gallant, B. M., Quantifying Capacity Loss Mechanisms of Li Metal Anodes Beyond Inactive Li0. ACS Energy Lett. 2022, 7 (10), 3458-3466. Hobold, G. M.; Khurram, A.; Gallant, B. M., Operando Gas Monitoring of Solid Electrolyte Interphase Reactions on Lithium. Chem. Mater. 2020, 32 (6), 2341-2352.

Investigation of copper underpotential deposition on polycrystalline gold electrodes

A combination of cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry techniques has been used to study the underpotential deposition (UPD) of copper on polycrystalline gold (pc Au) electrodes. The experiments were conducted in an electrolyte composed of CuSO4 (5 mM) and H2SO4 (0.5 M). The results revealed that the concentration of copper (II) ions and the applied voltage exerted a synergistic effect on the position, shape, and range of spanning of the curves describing the redox potential and current density. The charge transfer resistance (Rct) corresponding to the UPD of Cu decreased monotonically with a decrease in the applied voltage, and the phase angle shifted toward high-frequency regions with a negative shift in the potential. The process of UPD of Cu on pc Au can be divided into three stages: Langmuir-type adsorption, growth of Cu monolayers on pc Au following the process of two-dimensional instantaneous nucleation, and the continuous two-dimensional nucleation of grains on the Cu layer. UPD on single crystal substrates and the single-layer and multi-layer structures associated with UPD have been extensively studied worldwide. However, the UPD of polycrystalline electrodes with various crystal faces, grain boundaries, and defects is complex, and the associated mechanism has rarely been studied. In recent years, the process of UPD has been widely used in various fields, such as fields associated with electrochemical sensing, fabrication of fuel cells, electrochemical CO2 reduction, and fabrication of solar cells.

Studies of Benzotriazole on and into the Copper Electrodeposited Layer by Cyclic Voltammetry, Time-of-Flight Secondary-Ion Mass Spectrometry, Atomic Force Microscopy, and Surface Enhanced Raman Spectroscopy.

Benzotriazole (BTA) is an important compound that demonstrates the strongest anticorrosion properties of copper and plays a role as a leveler and an additive to the electroplating bath for control of the roughness and corrosion resistance of the electrodeposited copper layer. In this paper, we combined cyclic voltammetry (CV), time-of-flight secondary-ion mass spectrometry (TOF-SIMS), surface enhanced Raman spectroscopy (SERS), and atomic force microscopy (AFM) to study the interaction of BTA with copper surfaces at varied concentrations with and without the presence of chloride ions. We identified the most relevant molecular copper and its complex forms with BTA on the copper electrodeposited layer. BTA is adsorbed and incorporated into the copper surface in monomeric, dimeric, trimeric, tetrameric, and pentameric forms, inhibiting the copper electrodeposition. The addition of chloride ions diminishes the inhibiting properties of BTA. The Cu-BTA-Cl complexes were identified in the forms C12H8N6Cu2Cl- and C6H4N3CuCl-. Coadsorption of chloride ions and BTA molecules depends on their concentration and applied potential. Chloride ions are replaced by BTA molecules. BTA and chloride ions, depending on their concentration and applied potential, control the copper nucleation processes at the micro- and nanoscales. We compared the abilities and limitations of TOF-SIMS and SERS for studies of the interactions of benzotriazole with copper and chloride ions at the molecular level.

Electrochemical Reduction of 1H‐Thioxanthene‐1,4,9‐trione and 3‐Methyl‐1H‐thioxanthene‐1,4,9‐trione – Representatives of a New Class of Heterocycles Related to Thioxanthenone

AbstractA convenient synthetic route was proposed for the synthesis of 3‐methyl‐1H‐thioxanthene‐1,4,9‐trione. This compound and its 3‐H congener are representatives of a new class of thioxanthone‐like heterocycles, which can be used as building blocks for materials used in organic electronics. Electrochemical reduction (ECR) of 1H‐thioxanthene‐1,4,9‐trione and its 3‐methyl derivative in DMF and acetonitrile is a three‐stage process leading to the formation of the corresponding long‐lived radical anions and unstable dianions at the first and second stages, respectively. In a protolytic equilibrium, dianions are converted into 1‐oxy‐4‐hydroxy‐1H‐thioxanthene‐9‐one anions. Their one‐electron reversible ECR leads to the corresponding radical dianions, followed by the formation of (3‐methyl/H)‐1,4‐dihydroxy‐1H‐thioxantene‐9‐ones as the final products. The latter compounds can be converted into the initial (3‐methyl/H)‐1H‐thioxanthene‐1,4,9‐triones. The main intermediates were characterized using a combination of cyclic voltammetry, EPR spectroscopy, 3D spectroelectrochemistry, and DFT calculations at the (U)B3LYP/6‐31+G*/PCM level of theory. A general ECR scheme was proposed for both types of compounds.

Electrochemical Properties of Sputtered Ruthenium Oxide Neural Stimulation and Recording Electrodes

A chronically stable electrode material with a low impedance for recording neural activity, and a high charge-injection capacity for functional electro-stimulation is desirable for the fabrication of implantable microelectrode arrays that aim to restore impaired or lost neurological functions in humans. For this purpose, we have investigated the electrochemical properties of sputtered ruthenium oxide (RuOx) electrode coatings deposited on planar microelectrode arrays, using an inorganic model of interstitial fluid (model-ISF) at 37 °C as the electrolyte. Through a combination of cyclic voltammetry (CV) and an electrochemical impedance spectroscopy (EIS) modelling study, we have established the contribution of the faradaic reaction as the major charge-injection contributor within the safe neural stimulation potential window of ±0.6 V vs. Ag|AgCl. We have also established the reversibility of the charge-injection process for sputtered RuOx film, by applying constant charge-per-phase current stimulations at different pulse widths, and by comparing the magnitudes of the leading and trailing access voltages during voltage transient measurements. Finally, the impedance of the sputtered RuOx film was found to be reasonably comparable in both its oxidized and reduced states, although the electronic contribution from the capacitive double-layer was found to be slightly higher for the completely oxidized film around 0.6 V than for its reduced counterpart around −0.6 V.

Electrochemical grafting assisted modification of graphite surface by 3,4,5-trimethoxybenzenediazonium based monolayer

A conventional route to create such nanoscale electronic devices using the autonomous ordering and assembly of organic molecules on atomically well-defined surfaces has been proposed. However, these thin films are unstable in realistic environments due to weak interaction between the organic molecules and crystal surfaces. Therefore, enhancing the interfacial interaction between them is assigned as the key approach to extend the scope of application of these promising monolayer thin films. In this work, we demonstrate the formation of an organic monolayer, namely 3,4,5-trimethoxy diazonium (3,4,5-TMD), on a Highly Ordered Pyrolytic Graphite (HOPG) electrode by using the power of electrochemical method. The structural properties at the nano scale as well as the bonding nature between 3,4,5-TMD adlayer and HOPG electrode at the interface were investigated by a combination of cyclic voltammetry (CV), atomic force spectroscopy (AFM) and Raman spectroscopy. As a result, the 3,4,5-TMD molecules covalently bond to carbon atoms located at the HOPG interface for forming a monolayer with its thickness of 0.92 ± 0.02 nm. This finding opens a partway to apply the electrochemical grafting for covalent functionalization of 2D materials on HOPG and other metallic surfaces.

Guanine adsorption on gold electrodes studied by in situ surface-enhanced infrared reflection absorption spectroscopy

The adsorption of guanine on single-crystal and thin-film gold electrodes has been explored by combination of cyclic voltammetry and ATR-SEIRA ‘in situ’ spectroscopy at two pD values (8 and 11.6) employing D2O as a solvent. The experimental conditions are selected to study the adsorption / desorption of the two forms of guanine involved in the second acid-base equilibria and its tautomeric forms.The ATR-SEIRA spectra of adsorbed guanine were compared to the absorption FT-IR spectra in solution and interpreted in light of DFT calculations of the different acid-base and tautomeric forms with different solvation states of the CO group. Furthermore, the preponderance of each tautomeric form and the possible changes in orientation with the electric field have been determined by analyzing the two-dimensional correlation spectrum (2D COS) as well as the influence of the potential on the integrated intensities of the spectral signals in the 1800–1400 cm−1 region. It has been found that at pD 8 the adsorbed keto-amino tautomer of neutral guanine predominates in solution and in adsorbed state, and adsorbed guanine slightly rotate its molecular plane with the potential. At pD 11.6 the keto-amino tautomer of anionic guanine is the unique form detectable in the solution, while in adsorbed state the keto-imino tautomeric form is also present with its preponderance increasing with the electrode potential.

Optical and Optoelectronic Studies of Binary and Ternary Films of Poly(L-Tryptophane), Poly(5-hydroxy-L-tryptophane), and P(TER-CO-TRI) Doped with Sudan Dye

In this work, the optical properties and optoelectronics parameters of binary and ternary composite films made of two electron acceptors, poly(L-Tryptophane) and poly(5-hydroxy-L-Tryptophane), with an electron donor, P(TER-CO-TRI), doped with Sudan dyes, are comprehensively investigated. The films with different volumetric ratios of the components were deposited onto the glass substrates using spin coating technique. Results showed that with the help of dye doping into the binary systems of poly(L-Tryptophane):P(TRI-co-TER) (1:2) and poly(5-hydroxy-L-Tryptophane):P(TRI-co-TER)(1:2), the refractive index was increased from 2.01 to 2.32. The nature of the electronic transition in the studied films was found to be a direct allowed transition, which was derived from Tauc’s equation. The combination of Cyclic voltammetry (CV) technique and absorption spectroscopy was used to determine the molecular energy levels, HOMO and LUMO of the polymer samples. It was seen that the mixture of poly(L-Tryptophane):P(TRI-co-TER):Sudan dye (1:2:2) has led to increase the energy gap to 2.95 eV and the real optical conductivity ( ) to about 433.11 S.cm-1. According to the findings, the investigated polymers can have a great potential for semitransparent organic solar cells.

Dynamic Monitoring of the Structural Evolution of Au@Pd under Electrochemistry

Core–shell nanocatalysts have shown superior catalytic activity than monometallic catalysts. However, these metastable materials are susceptible to structural changes during catalysis. Comprehending the evolution of surface sites and their stability under different reaction conditions is crucial for designing durable and highly active core–shell nanocatalysts. Herein, structural transformation of the atomic layer thickness of Pd shells on Au nanocubes in different electrolytes at various electrochemical windows was investigated by a combination of cyclic voltammetry (CV), surface-enhanced Raman spectroscopy (SERS) of adsorbed probe molecules, and elemental analysis. Pd sites are stable under basic and neutral conditions but experience severe structure evolution under acidic conditions. Pd atoms that are directly coordinated by Au atoms, upon oxidation at evaluated potential, transform into Pd ions via the reaction with H+ which would also be adsorbed on the Au sites. These Pd ions are easily coreduced with the formed Au ions into surface alloys in the backward CV scan. In contrast, Pd atoms in the thick Pd overlayer (>1 monolayer) are likely to dissolute into the electrolyte solution and leach. SERS revealed that the change of Pd sites primarily occurred at contiguous Pd sites and isolated Pd sites were relatively stable. This evolution mechanism provides new insight into the rational design of efficient and stable catalysts and is expected to promote further application of core–shell nanocatalysts.

On the role of functional groups in the formation of diazonium based covalent attachments: dendritic vs. layer-by-layer growth.

Multilayered growth is often observed upon electrografting aryl diazonium derivatives on graphitic substrates due to the reactive nature of aryl radicals. The mechanism of the multilayer formation has been investigated either by measuring the thickness of the grafted layer, the charge transfer, or via simulations. Spectroscopy and in particular microscopy approaches are underrepresented. Herein, we demonstrate a comparative characterization of the multilayer growth of two diazonium derivatives on highly oriented pyrolytic graphite using a combination of cyclic voltammetry, atomic force microscopy, and scanning tunneling microscopy. While dendritic growth is observed for 4-nitro phenyl diazonium (4-NBD), 4-carboxy phenyl diazonium (4-CBD) shows layer-by-layer growth upon increasing the molecular concentration, revealing the impact of the functional groups on the growth mechanism.

Combining impedance and hydrodynamic methods in electrocatalysis. Characterization of Pt(pc), Pt5Gd, and nanostructured Pd for the hydrogen evolution reaction

Electrochemical hydrodynamic techniques typically involve electrodes that move relative to the solution. Historically, approaches involving rotating disc electrode (RDE) configurations have become very popular, as one can easily control the electroactive species’ mass transport in those cases. The combination of cyclic voltammetry and RDE is nowadays one of the standard characterization protocols in electrocatalysis. On the other hand, impedance spectroscopy is one of the most informative electrochemistry techniques, enabling the acquisition of information on the processes taking place simultaneously at the electrode/electrolyte interface. In this work, we investigated the hydrogen evolution reaction (HER) catalyzed by polycrystalline Pt (Pt(pc)) and Pt5Gd disc electrodes and characterized them using RDE and electrochemical impedance spectroscopy techniques simultaneously. Pt5Gd shows higher HER activities than Pt in acidic and alkaline media due to strain and ligand effects. The mechanistic study of the reaction showed that the rotation rates in acidic media do not affect the contribution of the Volmer–Heyrovsky and Volmer–Tafel pathways. However, the Volmer–Heyrovsky pathway dominates at lower rotation rates in alkaline media. Besides, the HER in acidic solutions depends more strongly on mass diffusion than in alkaline media. In addition to simple and clearly defined systems, the combined method of both techniques is applicable for systems with greater complexity, such as Pd/C nanostructured catalysts. Applying the above-presented approach, we found that the Volmer–Tafel pathway is the dominating mechanism of the HER for this catalytic system.

Effect of Molar Ratio Modulation of Cobalt Precursors on Supercapacitor Behavior of Engineered Co–Co3O4 Nanocomposite Electrodes

Nanostructured Co–Co3O4 composite thin films deposited on nickel foam (NF) using combined cyclic voltammetry (CV) and pulse reverse potential (CV PRP) modes of electrodeposition from an electrolyte composed of CoCl2 and Co(CH3COO)2 precursor salt solutions with varied molar ratios demonstrated a dependency of the supercapacitance performance over the precursor components’ molar ratios. The specific capacity and its retention were found to increase with increasing CoCl2 in the electrolyte with a starting value of 474.6 C/g (791 F/g) at an applied current load of 1 A/g for a molar ratio of Co(CH3COO)2:CoCl2 of 100:0. High specific capacity of 1548 C/g (2580 F/g) and large retention (90.5%) were obtained at a critical molar ratio of Co(CH3COO)2:CoCl2 of 20:80. However, with 100% CoCl2 in the electrolyte, the specific capacity lowered to 276 C/g (460 F/g) with poor retention of only 60%. Crystallinity and morphological features, driven by the electrolyte concentration with molar variations of the deposited films, have been found to influence the specific capacitance performance. The degree of crystallinity, and the presence of Co and Co3O4 phases for the different molar ratios have been revealed by X-ray diffraction (XRD) studies. The diverse morphological features obtained by scanning electron microscopy (SEM) and the varying quantities of Co and O in the Co–Co3O4 nanocomposite thin films as confirmed by energy-dispersive X-ray spectroscopy (EDX) spectra correlate well with the electrochemical performance. The phase composition was further confirmed by the presence of Co–O bonds via the attenuated total reflection–Fourier transform infrared (ATR-FTIR) spectra obtained on these films.

Study on the fabrication of diazonium molecular monolayer material on graphite substrate viaelectrochemical grafting approach

Surface denaturation of carbon surfaces with molecular monolayer is appointed as one of the most effective bottom-up techniques to enlarge the applicability of the carbon-based electrodes in the electrochemical sensor field. Diazonium molecules have been often employed to denature the surface of materials such as graphite and graphene through the formation of C-C chemical bonds between them. However, the layered formation of these molecules is not well controlled yet due to their intrinsic high reactivity. This report demonstrated a practical approach for covalent functionalisation of the highly oriented pyrolytic graphite (HOPG) surface, a model system of multilayered graphene, by using 3,5-bis-tert-butylbenzenediazonium molecules. Due to the substituents at the 3,5 positions the aryl radicals prefer bonding directly to the HOPG surface than attacking the grafted aryls towards the monolayered formation. The efficiency of this approach was determined by the combined cyclic voltammetry, atomic force microscopy and scanning tunnelling microscopy.

Enabling Aqueous Processing of Prussian White Cathodes for Sodium Ion Cells By Understanding the Effect of Structural Water

Sodium-ion batteries (SIBs) have been identified as one of the most promising alternative technologies for future stationary large-scale storage and light electromobility applications. Indeed, the potential cost decrease with respect to the lithium-ion technology, and the sustainability provided by using Earth abundant elements lead to lower economic and geopolitical impact.[1,2] Great achievements in terms of materials development have been reported in the recent years. However, further work is still needed to fully understand the structure-function correlation in several SIB materials. Easy-synthesis Prussian blue analogues (PBAs) have attracted much attention as potential cathode material in SIBs. Their open framework structure with large interstitial spaces allows a range of different compositions and enables fast Na+ insertion/extraction. However, a key factor affecting the electrochemical performance of PBAs is the crystal water in the structure arising from aqueous synthesis environments.[3] Thus, many have investigated improved synthesis methods aiming at reducing crystal water and associated structural defects.[4] Goodenough et al[5] firstly reported the improved electrochemical stability of PBAs through dehydration converting Na-rich monoclinic Na2MnFe(CN)6 to a rhombohedral structure. More recent studies, including Zhou et al[6] and Chou et al[7], further highlighted the negative impact of structural water on electrochemical performance. So far, the water sensitivity of PBAs has hindered aqueous electrode processing as demonstrated by the several studies conducted on N-methyl pyrrolidone-based electrodes. A more comprehensive understanding of the effect of crystal water and the induced degradation processes is necessary for obtaining high-performance electrode materials produced through environmentally friendly aqueous processing.Herein, we investigate the electrochemical behaviour of an aqueous-processed Fe-based Prussian white (PW) cathode material (Na2-x Fe[Fe(CN)6]1-y·zH2O).[8] The use of such material and processing matches the requirements for low toxicity, cost, and resource abundance in large-scale applications. We uniquely show that, by carefully tuning the water content in the PW structure, the electrochemical performance can be significantly improved. As structural water is removed by tuning the temperature and vacuum conditions during the electrode preparation, the crystallinity of the structure can be controlled, and optimal electrochemical behaviour can be achieved. Moreover, with the combination of cyclic voltammetry, impedance spectroscopy, X-Ray diffraction, X-ray absorption spectroscopy and scanning electron microscopy, we elucidate the activity of the low spin/ high spin Fe2+/3+redox reactions, the Na+ transport properties and the electrode’s structural and morphological changes. This allows to understand holistically the effect of crystal water on the electrochemical performance of PW cathodes. We then demonstrate that our approach is scalable and can be applied to different cell formats representing a step forward in the upscale of sustainable and low-cost sodium-ion cells.

In situ growth of Ni-Co-S nanosheet arrays on rGO decorated Ni foam toward high-performance supercapacitors

Ni–Co–S nanosheet arrays were successfully anchored on graphene covered Ni foam (Ni–Co–S–rGO/NF-1) by consecutive electrochemical methods and high-performance positive electrode material for supercapacitors was obtained. • Ni–Co–S nanosheet arrays were grown on reduced graphene oxide covered Ni foam (rGO/NF). • Synchronous electroreduction and electrodeposition of graphene oxide resulted in rGO/NF. • Combination of cyclic voltammetry and potentiostatic techniques was adopted for Ni–Co–S arrays. • The as-prepared electrode revealed excellent rate performance and higher specific capacity. Electrodes for supercapacitors (SCs) with excellent capacity and rate performance have always been in high demand. Herein, Ni–Co–S nanosheet arrays were successfully anchored on reduced graphene oxide (rGO) covered Ni foam (denoted as Ni–Co–S–rGO/NF–1) by consecutive electrochemical methods and high-performance positive electrode material for SCs was obtained. It was worth mentioning that there were two innovations in this preparation: the first one was that synchronous electroreduction and electrodeposition of graphene oxide (GO) resulted in rGO tightly wrapped Ni foam (denoted as rGO/NF), which enhanced the electroconductibility of the substrate and achieved outstanding rate performance; the second one was that a two-step combination approach composed of cyclic voltammetry (CV) technique and potentiostatic technique was adopted to deposit Ni–Co–S nanosheet arrays, which ensured both higher specific capacity and relatively robust growth of Ni–Co–S on rGO/NF. On account of a synergistic effect of the Ni–Co–S and rGO as well as the unique thin sheet-like nanostructure, the as-prepared electrode revealed excellent supercapacitor properties, including higher specific capacity of 976 C g −1 at current density of 1 A g −1 , impressive rate capability with 86.4 % retention at 60 A g −1 , and long-term stability of 82.2 % capacity retention after 10,000 cycles. An asymmetric supercapacitor fabricated by the Ni–Co–S–rGO/NF–1 and activated carbon exhibited a maximum energy density of 27 Wh kg −1 at the power density of 801 W kg −1 , and a power density of 8115 W kg −1 at the energy density of 11.0 Wh kg −1 . The excellent performance of the Ni–Co–S–rGO/NF–1 electrode suggests that it could be the hopeful electrode materials for energy storage devices.

Deciphering the Molecular Mechanism of Substrate-Induced Assembly of Gold Nanocube Arrays toward an Accelerated Electrocatalytic Effect Employing Heterogeneous Diffusion Field Confinement.

The complex electrocatalytic performance of gold nanocubes (AuNCs) is the focus of this work. The faceted shapes of AuNCs and the individual assembly processes at the electrode surfaces define the heterogeneous conditions for the purpose of electrocatalytic processes. Topographic and electron imaging demonstrated slightly rounded AuNC (average of 38 nm) assemblies with sizes of ≤1 μm, where the dominating patterns are (111) and (200) crystallographic planes. The AuNCs significantly impact the electrochemical performance of the investigated electrode [indium–tin oxide (ITO), glassy carbon (GC), and bulk gold] systems driven by surface electrons promoting the catalytic effect. Cyclic voltammetry in combination with scanning electrochemical microscopy allowed us to decipher the molecular mechanism of substrate-induced electrostatic assembly of gold nanocube arrays, revealing that the accelerated electrocatalytic effect should be attributed to the confinement of the heterogeneous diffusion fields with tremendous electrochemically active surface area variations. AuNC drop-casting at ITO, GC, and Au led to various mechanisms of heterogeneous charge transfer; only in the case of GC did the decoration significantly increase the electrochemically active surface area (EASA) and ferrocyanide redox kinetics. For ITO and Au substrates, AuNC drop-casting decreases system dimensionality rather than increasing the EASA, where Au–Au self-diffusion was also observed. Interactions of the gold, ITO, and GC surfaces with themselves and with surfactant CTAB and ferrocyanide molecules were investigated using density functional theory.