Change In Double Layer Capacitance Research Articles

One-Step Synthesis and Operando Electrochemical Impedance Spectroscopic Characterization of Heterostructured MoP-Mo2N Electrocatalysts for Stable Hydrogen Evolution Reaction.

This study presents a novel synthesis of self-standing MoP and Mo2N heterostructured electrocatalysts with enhanced stability and catalytic performance. Facilitated by the controlled phase and interfacial microstructure, the seamless structures of these catalysts minimize internal resistivity and prevent local corrosion, contributing to increased stability. The chemical synthesis proceeds with an etching step to activate the surface, followed by phosphor-nitriding in a chemical vapor deposition chamber to produce MoP-Mo2N@Mo heterostructured electrocatalysts. X-ray diffraction analyses confirmed the presence of MoP, Mo2N, and Mo phases in the electrocatalyst. Morphology studies using scanning electron microscopy characterize the hierarchical growth of structures, indicating successful formation of the heterostructure. X-ray photoelectron spectroscopy (XPS) analyses of the as-synthesized and postcatalytic activity samples reveal the chemical shift in terms of the binding energy (BE) of the Mo 3d XPS peak, especially after catalytic activity. The XPS BE shifts are attributed to changes in the oxidation state, electron transfer, and surface reconstruction during catalysis. Electrochemical evaluation of the catalysts indicates the superior performance of the MoP-Mo2N@Mo heterostructured catalyst in hydrogen evolution reactions (HER), with lower overpotentials and enhanced Tafel slopes. The stability tests reveal changes in double layer capacitance over time, suggesting surface reconstruction and an increased active surface area during catalysis. Operando electrochemical impedance spectroscopy (EIS) further elucidates the dynamic changes in resistance and charge transfer during HER. Overall, a comprehensive understanding of the synthesis, characterization, and electrochemical behavior of the developed MoP-Mo2N@Mo heterostructured electrocatalyst, as presented in this work, highlights its potential utilization in sustainable energy applications.

Changes in double layer capacitance of the positive electrode in Li-O2 battery during discharge: Effect of active material and aprotic electrolyte solvent

Changes in the double-layer capacitance of a positive electrode (CDL) are investigated during the discharge of a lithium-oxygen battery (LOB) having various carbon materials (CM), including XC-72, Super P, EC-300 J, and CNT, and electrolytes based on tetraglyme and DMSO, containing 0.1 M LiClO4. It is shown that the initial value of CDL increases proportionally to the external surface area of the carbon material in both the tetraglyme- and DMSO-based electrolyte. The difference between the CDL values in these electrolytes can be explained by the different wetting abilities of the solvents, as well as the ability of the DMSO medium to dissolve the binder (PVDF). During the LOB discharge, a decrease in CDL is observed, which is associated with increasing passivation of the positive electrode with lithium peroxide (Li2O2)). The highest (Li2O2) value (0.5–0.7) is observed for XC-72 and EC-300 J carbon black having the lowest average pore size (9 and 8 nm, respectively) among the studied materials. Nonetheless, no pronounced dependence of (Li2O2) on the nature of a solvent was observed under identical measurement conditions. Along with blocking the surface with lithium peroxide, oxygen transport limitations have a significant impact on the discharge process, which is most pronounced for large-pore CMs (Super P and CNT). Even at a relatively low (Li2O2) (from 0.15 to 0.24), the voltage of such LOBs decreases to 2 V. The results demonstrate that the initial parameter CDL of the active material can be used as a new criterion for pre-selection the electrolyte for LOB.

Microwave-Assisted Scalable Synthesis of Pt/C: Impact of the Microwave Irradiation and Carrier Solution Polarity on Nanoparticle Formation and Aging of the Support Carbon

Microwave-assisted (MW-assisted) synthesis of nanomaterials is a facile method for material development studies involving small-scale synthesis. The method is being explored further toward industrial scale production of nanoparticle catalyst. Here, optimization of the process parameters for a scalable synthesis of Pt/C through a MW-assisted polyol route using (NH4)2PtCl6 as the Pt precursor has been presented. Further, the impact of MW-irradiation on the carbon support surface structure and its effect on the electrochemical stability of the synthesized Pt/C have been explored. Pt/C electrocatalysts were synthesized using a MW synthesizer in an open system configuration. Effects of different process parameters, namely, solvent composition and hence solvent polarity, reaction time, and Pt precursor concentration on the structure and the electrochemical performance of the synthesized Pt/C catalysts were studied. A mixture of water and ethylene glycol with a water content of 30% v/v was found appropriate for achieving Pt nanoparticles with a particle size of ∼2 nm, suitable for a high electrochemically active surface area (ECSA) of ∼75 m2/gPt. Pt concentrations higher than 5 mM were found unsuitable due to incomplete reduction of the Pt precursor. The minimum reaction time for complete reduction of the Pt precursor was observed to be 400 s. However, increasing the MW treatment time beyond 400 s leads to growth of the Pt nanoparticles and hence reduction in ECSA. Porosity and surface area analysis suggests decreased pore size and surface area of the carbon support by the MW-irradiation during Pt/C synthesis. Hence, the initial electrochemical double layer capacitance (DLC) decreases with increasing MW-irradiation time, leading to the increased surface corrosion and the decreased bulk corrosion of the support carbon, monitored respectively through the relative change in DLC and the evolution of the quinone–hydroquinone redox peak during an accelerated stress test meant for durability assessment of the electrocatalysts.

Insights on platinum-carbon catalyst degradation mechanism for oxygen reduction reaction in acidic and alkaline media

Developing durable electrocatalyst for oxygen reduction reaction (ORR) is essential for fuel cell commercialization. Herein, we perform a study of platinum-carbon (Pt/C) degradation mechanisms using potential cycling of accelerated durability testing protocols in acidic and alkaline media. Physicochemical results indicate that carbon surface oxides are formed after high-potential cycling in acid causing an increase in the double-layer capacitance and severe ORR activity loss due to Pt poisoning. Whereas, low-potential cycling in acid shows less ORR activity loss, mainly caused by Pt Ostwald ripening, and does not lead to a significant change in double-layer capacitance. In alkaline, the Pt/C catalyst after high-potential cycling shows a decrease of double-layer capacitance over time because of carbon layer dissolution. TEM images reveal larger Pt agglomerates in alkaline, due to high Pt mobility. These findings provide new insights into the role of catalyst and carbon support interface in developing mitigation strategies for stable fuel cell operation.

Probing Electrochemical Structure-Property Relationships at Non-Porous Monolayer Electrodes of Exfoliated Graphene and MoS2 Single Layers

The electrochemically available surface area and porosity of 2D materials is difficult to control due to the strong aggregation and restacking phenomena that occur when single layers are processed into thin or thick film electrodes. Furthermore, the distributed resistance, the potential for pore-size related effects and the uncertainty in relating estimated surface area and porosity data to the actual electrochemically accessible surface area make measurements of the intrinsic electrochemical performance of these materials difficult to assess. Ideally, electrochemical measurements are carried out on flat, polished or atomically flat electrode surfaces for the evaluation of intrinsic double-layer capacitance and electrochemical rate constants. In this talk, we will discuss our efforts to create large-area monolayers of solution exfoliated 2D materials for electrochemical evaluation in aqueous and non-aqueous electrolytes. Using a modified Langmuir-Blodgett technique, we demonstrate the ability to create uniform and large area monolayers of graphene oxide, thermally reduced graphene oxide as well as 1T and 2H molybdenum disulfide which can be transferred to atomically flat electrode substrates such as highly oriented pyrolytic graphite. These films are found to be stable during electrochemical cycling in various electrolytes and allow us to probe electrochemical properties as a function of 2D material structure without porosity-related artifacts. This model system has been used to probe the changes in double-layer capacitance in both aqueous and non-aqueous electrolytes for graphene-based materials as a function of defect density. It has allowed us to estimate the theoretical limits of graphene-based supercapacitors as a function of graphene “type”. More recently, this analysis has been extended to probe the intrinsic capacitance of the metallic 1T and semiconducting 2H polymorphs of MoS2 as a function of layer number to probe the charging mechanisms at play. The system also offers the ability to probe the capacitance of graphene/MoS2 heterostructures. These and future studies are expected to provide valuable insight into how we might build more energy dense supercapacitors from the optimal combination of 2D materials.

Early Stage Anodic Instability of Glassy Carbon Electrodes in Propylene Carbonate Solvent Containing Lithium Hexafluorophosphate.

Irreversible changes to the morphology of glassy carbon (GC) electrodes at potentials between 3.5 and 4.5 V vs Li/Li+ in propylene carbonate (PC) solvent containing lithium hexafluorophosphate (LiPF6) are reported. Analysis of cyclic voltammetry (CV) experiments in the range of 3.0 to 6.0 V shows that the capacitance of the electrochemical double-layer increased irreversibly beginning at potentials as low as 3.5 V. These changes resulted from nonfaradaic interactions, and were not due to oxidative electrochemical decomposition of the electrode and electrolyte, anion intercalation, nor caused by the presence of water, a common impurity in organic electrolyte solutions. Atomic force microscopy (AFM) images revealed that increasing the potential of a bare GC surface from 3.0 to 4.5 V resulted in a 6× increase in roughness, in good agreement with the changes in double-layer capacitance. Treating the GC surface via exposure to trichloromethylsilane vapors resulted in a stable double-layer capacitance between 3.0 and 4.5 V, and this treatment also correlated with less roughening. These results inform future efforts aimed at controlling surface composition and morphology of carbon electrodes.

Bulk Aging of Graphite-Polypropylene Current Collectors Induced by Electrochemical Cycling in the Positive Electrolyte of Vanadium Redox Flow Batteries

Carbon-based bipolar plates (BPP) are essential components of the vanadium redox flow battery (VRFB) stack guaranteeing electrical conductivity and physical separation of adjacent cells. They are usually made of composite materials with complex structures containing conductive carbon components and polymer binder materials. In order to investigate their aging by simulation of repetitive charging/discharging conditions in areas with sluggish electrolyte flow, commercially available graphite-polypropylene BPPs were subjected to galvanodynamic treatment under diffusion limitation conditions in a three electrode electrochemical cell with positive electrolyte (VO2+/VO2+) using different states of charge (SOC). The current was swept between 100 and −100 mA/cm2. After aging, cyclic voltammetry (CV) in diluted H2SO4 was used to analyze the change in double layer capacitance and redox activity of the BPPs. Morphological investigations of the surface texture and bulk material were performed by means of scanning electron and confocal microscopies as well as by X-ray computed microtomography. After 3000 sweeping cycles in the positive electrolyte at high SOC the BPP showed an increase in double layer capacitance as well as higher roughness and increase in open pore volume in depths up to 100 μm. The aging effects are ascribed to oxidation and corrosion of the surface.

RuO2–SiO2 mixed oxides as corrosion-resistant catalyst supports for polymer electrolyte fuel cells

The objective of this study was to evaluate the suitability of mixed oxides of silica (SiO2) and ruthenia (RuO2) for use as electrocatalyst supports in polymer electrolyte fuel cells (PEFCs). The RuO2:SiO2 oxides were synthesized in three different molar ratios (0.1:0.9, 0.3:0.7 and 0.5:0.5). Of these formulations, RuO2–SiO2 (0.5:0.5) displayed high BET surface area (∼290m2g−1), high electronic conductivity (>5Scm−1), and outstanding electrochemical stability (change in double layer capacitance of <10% compared to 360% for Vulcan XC72 carbon upon rigorous accelerated potential cycling tests that mimic automotive drive cycles). The RuO2–SiO2 (0.5:0.5) support material was subsequently catalyzed by deposition of 24wt% of platinum nanoparticles. The resultant electrocatalyst yielded a mass activity of 50mAmgPt−1 and an area specific activity of 156.3μAcm−2 at 0.9V vs. RHE when tested in a fuel cell operating at 80°C and 75% relative humidity.

Interfacial electrochemistry study of particle–wall interaction using electrochemical impedance spectroscopy

Particulate fouling by corrosion products reduces the efficiency of heat transport systems. This paper examines particle–wall interactions during the course of deposition. Electrochemical impedance spectroscopy was employed to evaluate the interaction of submicron magnetite and hematite particles with a platinum electrode at acidic, near-neutral and alkaline conditions. It was observed that changes in double-layer capacitance on the platinum surface could be used to detect deposition at different conditions. It was observed from impedance data that double layer capacitance at pH=3 did not change with particle mass loading. Modeling suggests that strong electrostatic repulsion at this pH keeps particles above the surface at equilibrium distances large enough that EIS cannot detect them. Significant changes in the double layer capacitance at pH=10 were ascribed to surface blockage by particles. At pH=5, a cathodic potential of −1V vs. Ag/AgCl was utilized for particle detection.

Double-Layer-Capacitance Titration of Self-Assembled Monolayers of ω-Functionalized Alkanethiols on Au(111) Surface

The acid−base equilibrium of ω-functionalized alkanethiol monolayers on Au(111) has been studied using the change in double-layer capacitance accompanied by the protonation−deprotonation of ω-terminals. The pK of ω-carboxyl alkanethiols on Au(111) increases by four pH units. The shift becomes greater as the alkyl chain length increases. The same magnitude of the pK shift to the acidic side occurs in the monolayers of aminoethane thiol. The surface pK varies little with the electrode potential, whereas the increase in the supporting electrolyte concentration slightly diminishes the pK shift. The drawn-out shape of the titration curves is consistent with the mean-field model taking account of the repulsive interaction between adsorbed molecules, indicating the significance of the strong electrostatic repulsion between adsorbed thiol molecules in its charged states. The magnitude of the pK shift is, more than that predicted by the mean-field model, however, and suggests the considerable contribution from oth...

Instrumental development of an inexpensive double-layer capacitance meter for studying surface adsorption phenomena

The design, development, and calibration of an inexpensive double-layer capacitance meter is reported. The instrument measures the impedance response from a small-amplitude sinusoidal voltage applied across the electrical double layer. The two elements of the signal, the in-phase and the quadrature, are distinguished by two phase-sensitive detectors. The relationship between the quadrature voltage and the double-layer capacitance is established by using standard capacitances. Once calibrated, this capacitance meter was used to measure the change in double-layer capacitance as the protein lysozyme is adsorbed on a metal/solution interface.

Applications of electrosorptive detection in ion chromatography

Illustrative applications to simulated and real-world samples are presented for electrosorptive detection in ion chromatography. Detection is demonstrated at both mercury and silver surfaces utilizing changes in double-layer capacitance as the primary means of detection. Strategies for dealing with the gradual fouling of the solid electrode are briefly discussed.

Thermodynamics and kinetics of underpotential deposition of metal monolayers on polycrystalline substrates

The equilibrium and dynamic properties of underpotential deposits are discussed, as are some problems of methodology in estimating these properties. In particular, there is a special emphasis on interpreting the ring—disc electrode response during the underpotential deposition, UPD, of silver on polycrystalline gold. The underpotential-coverage relationship as a function of solution composition provides an insight into particle—substrate, particle—particle and particle—electric field interactions. Some of the features that emerge from the analysis of equilibrium UPD behaviour are: (a) the predominant influence of the particle—substrate interaction through work function changes, (b) the absence of discreteness of charge effects due to any partially charged UPD species over wide coverage ranges, and (c) the importance of particle—particle interactions in causing multiple energy states. The dynamic charge flux during the UPD process is analysed in terms of dynamic changes in double layer capacitance and the potential of zero charge, and other parallel processes through the formalism of a dynamic electrosorption valency, which includes both charge separation and charge-transfer components. The dynamic material flux of the UPD species is found to be under mixed control by solution mass transport, surface adsorption and charge-transfer processes. The phenomenological rate parameters and double layer capacitance changes suggest the formation of ordered structures at higher UPD coverages and random aggregates at lower coverages.