Showing papers on "Electrode potential published in 2021"

Mechanically driving supersaturated Fe–Mg solid solution for bone implant: Preparation, solubility and degradation

Abstract: Fe has attracted great attention as biodegradable bone implant. Unfortunately, its degradation rate is too slow for bone repair. It is well known that the lower the electrode potential is, the faster the metals degrade. In the case of dissolving low electrode potential species (Mg for instance) into Fe lattice, the corrosion potential of the matrix can be negatively shifted and the overall degradation rate can be accelerated. However, Fe and Mg are immiscible in equilibrium due to their large differences in melting and boiling temperature. In this study, a supersaturated Fe–Mg solid solution was developed in solid-state by mechanical alloying (MA). In detail, Fe particles experienced cycled mechanical impact and friction during MA, which produced a large number of dislocations and defects. In this condition, Mg atoms were forcibly diffused into Fe lattice along with the motion of dislocations and defects. The results showed that about 8.5 at.% Mg was dissolved in Fe (0.02 at.% in equilibrium). Subsequently, the MA powders were fabricated into bone implants by selective laser melting (SLM), in which the fast laser scanning speed resulted in a rapid cooling rate. The molten liquid passed the immiscible zone quickly, thereby avoiding the separation of Mg solute. SLMed implants exhibited a lowered electrode potential (−0.93 V) comparing with Fe. Additionally, the implants also had good cytocompatibility and promoted cell proliferation.

Electrogenerated Chemiluminescence of Luminol Mediated by Carbonate Electrochemical Oxidation at a Boron-Doped Diamond.

Abstract: The electrogenerated chemiluminescence of luminol is a process by which light generation is triggered by adding hydrogen peroxide and then applying a suitable electrode potential. Here, we take this phenomenon one step forward by avoiding the addition of hydrogen peroxide using a smart combination of a boron-doped diamond electrode and a carbonate electrolyte to generate the hydrogen peroxide directly in situ. The reaction occurs because of the carbonate electrochemical oxidation to peroxydicarbonate and the following hydrolysis to hydrogen peroxide, which triggers the emission from luminol. The electrogenerated chemiluminescence emission has been optimized by an investigation of the applied potentials, the carbonate concentration, and the pH. Furthermore, these results have been used to shine a light on the reaction mechanisms. Because this method does not require the addition of hydrogen peroxide, it might find application in efforts to avoid instability of hydrogen peroxide or its interference with the analytes of interest.

Correlating Li-Ion Solvation Structures and Electrode Potential Temperature Coefficients.

Abstract: Temperature coefficients (TCs) for either electrochemical cell voltages or potentials of individual electrodes have been widely utilized to study the thermal safety and cathode/anode phase changes of lithium (Li)-ion batteries. However, the fundamental significance of single electrode potential TCs is little known. In this work, we discover that the Li-ion desolvation process during Li deposition/intercalation is accompanied by considerable entropy change, which significantly contributes to the measured Li/Li+ electrode potential TCs. To explore this phenomenon, we compare the Li/Li+ electrode potential TCs in a series of electrolyte formulations, where the interaction between Li-ion and solvent molecules occurs at varying strength as a function of both solvent and anion species as well as salt concentrations. As a result, we establish correlations between electrode potential TCs and Li-ion solvation structures and further verify them by ab initio molecular dynamics simulations. We show that measurements of Li/Li+ electrode potential TCs provide valuable knowledge regarding the Li-ion solvation environments and could serve as a screening tool when designing future electrolytes for Li-ion/Li metal batteries.

Effects of applied voltage on water at a gold electrode interface from ab initio molecular dynamics

Abstract: Electrode–water interfaces under voltage bias demonstrate anomalous electrostatic and structural properties that are influential in their catalytic and technological applications. Mean-field and empirical models of the electrical double layer (EDL) that forms in response to an applied potential do not capture the heterogeneity that polarizable, liquid-phase water molecules engender. To illustrate the inhomogeneous nature of the electrochemical interface, Born–Oppenheimer ab initio molecular dynamics calculations of electrified Au(111) slabs interfaced with liquid water were performed using a combined explicit–implicit solvent approach. The excess charges localized on the model electrode were held constant and the electrode potentials were computed at frequent simulation times. The electrode potential in each trajectory fluctuated with changes in the atomic structure, and the trajectory-averaged potentials converged and yielded a physically reasonable differential capacitance for the system. The effects of the average applied voltages, both positive and negative, on the structural, hydrogen bonding, dynamical, and vibrational properties of water were characterized and compared to literature where applicable. Controlled-potential simulations of the interfacial solvent dynamics provide a framework for further investigation of more complex or reactive species in the EDL and broadly for understanding electrochemical interfaces in situ.

Unifying Concepts in Electro- and Thermocatalysis toward Hydrogen Peroxide Production.

Abstract: We examine relationships between H2O2 and H2O formation on metal nanoparticles by the electrochemical oxygen reduction reaction (ORR) and the thermochemical direct synthesis of H2O2. The similar mechanisms of such reactions suggest that these catalysts should exhibit similar reaction rates and selectivities at equivalent electrochemical potentials (μi), determined by reactant activities, electrode potential, and temperature. We quantitatively compare the kinetic parameters for 12 nanoparticle catalysts obtained in a thermocatalytic fixed-bed reactor and a ring-disk electrode cell. Koutecky-Levich and Butler-Volmer analyses yield electrochemical rate constants and transfer coefficients, which informed mixed-potential models that treat each nanoparticle as a short-circuited electrochemical cell. These models require that the hydrogen oxidation reaction (HOR) and ORR occur at equal rates to conserve the charge on nanoparticles. These kinetic relationships predict that nanoparticle catalysts operate at potentials that depend on reactant activities (H2, O2), H2O2 selectivity, and rate constants for the HOR and ORR, as confirmed by measurements of the operating potential during the direct synthesis of H2O2. The selectivities and rates of H2O2 formation during thermocatalysis and electrocatalysis correlate across all catalysts when operating at equivalent μi values. This analysis provides quantitative relationships that guide the optimization of H2O2 formation rates and selectivities. Catalysts achieve the greatest H2O2 selectivities when they operate at high H atom coverages, low temperatures, and potentials that maximize electron transfer toward stable OOH* and H2O2* while preventing excessive occupation of O-O antibonding states that lead to H2O formation. These findings guide the design and operation of catalysts that maximize H2O2 formation, and these concepts may inform other liquid-phase chemistries.

Investigation of Lithium Ion Diffusion of Graphite Anode by the Galvanostatic Intermittent Titration Technique.

Abstract: Graphite is used as a state-of-the-art anode in commercial lithium-ion batteries (LIBs) due to its highly reversible lithium-ion storage capability and low electrode potential. However, graphite anodes exhibit sluggish diffusion kinetics for lithium-ion intercalation/deintercalation, thus limiting the rate capability of commercial LIBs. In order to determine the lithium-ion diffusion coefficient of commercial graphite anodes, we employed a galvanostatic intermittent titration technique (GITT) to quantify the quasi-equilibrium open circuit potential and diffusion coefficient as a function of lithium-ion concentration and potential for a commercial graphite electrode. Three plateaus are observed in the quasi-equilibrium open circuit potential curves, which are indicative of a mixed phase upon lithium-ion intercalation/deintercalation. The obtained diffusion coefficients tend to increase with increasing lithium concentration and exhibit an insignificant difference between charge and discharge conditions. This study reveals that the diffusion coefficient of graphite obtained with the GITT (1 × 10−11 cm2/s to 4 × 10−10 cm2/s) is in reasonable agreement with literature values obtained from electrochemical impedance spectroscopy. The GITT is comparatively simple and direct and therefore enables systematic measurements of ion intercalation/deintercalation diffusion coefficients for secondary ion battery materials.

Understanding the Dynamic Potential Distribution at the Electrode Interface by Stochastic Collision Electrochemistry.

Abstract: The potential distribution at the electrode interface is a core factor in electrochemistry, and it is usually treated by the classic Gouy-Chapman-Stern (G-C-S) model. Yet the G-C-S model is not applicable to nanosized particles collision electrochemistry as it describes steady-state electrode potential distribution. Additionally, the effect of single nanoparticles (NPs) on potential should not be neglected because the size of a NP is comparable to that of an electrode. Herein, a theoretical model termed as Metal-Solution-Metal Nanoparticle (M-S-MNP) is proposed to reveal the dynamic electrode potential distribution at the single-nanoparticle level. An explicit equation is provided to describe the size/distance-dependent potential distribution in single NPs stochastic collision electrochemistry, showing the potential distribution is influenced by the NPs. Agreement between experiments and simulations indicates the potential roles of the M-S-MNP model in understanding the charge transfer process at the nanoscale.

Building Ab Initio Interface Pourbaix diagrams to Investigate Electrolyte Stability in the Electrochemical Double Layer: Application to Magnesium Batteries.

Abstract: Insights into the electrochemical processes occurring at the electrode-electrolyte interface are a crucial step in most electrochemistry domains and in particular in the optimization of the battery technology. However, studying potential-dependent processes at the interface is one of the biggest challenges, both for theoreticians and experimentalists. The challenge is pushed further when stable species also depend on the concentration of specific ligands in the electrolyte, such as chlorides. Herein, we present a general theoretical ab initio methodology to compute a Pourbaix-like diagram of complex electrolytes as a function of electrode potential and anion's chemical potential, that is, concentration. This approach is developed not only for the bulk properties of the electrolytes but also for electrode-electrolyte interfaces. In the case of chlorinated magnesium complexes in dimethoxyethane, we show that the stability domains of the different species are strongly shifted at the interface compared to the bulk of the electrolyte because of the strong local electric fields and charges occurring in the double layer. Thus, as the interfacial stability domains are strongly modified, this approach is necessary to investigate all interface properties that often govern the reaction kinetics, such as solvent degradation at the electrode. Interface Pourbaix diagram is used to give some insights into the improved stability at the Mg anode induced by the addition of chloride. Because of its far-reaching insights, transferability, and wide applicability, the methodology presented herein should serve as a valuable tool not only for the battery community but also for the wider electrochemical one.

Synergistic effects between electrocatalyst and electrolyte in the electrocatalytic reduction of lignin model compounds in a stirred slurry reactor

Abstract: Valorization of biomass-derived substrates via electrocatalytic hydrogenation-hydrogenolysis (ECH) is an attractive approach for selective production of organic chemicals. The electrocatalytic activity is strongly dependent on the surface coverage of adsorbed hydrogen radicals, which is a complex function of the catalytically active surface sites, electrolyte (pH and composition) and electrode potential. The performance of carbon-supported catalysts (Pt/C, Ru/C, Pd/C) was explored in the ECH of phenol and guaiacol in a stirred slurry electrochemical reactor where the cathode and anode compartments were separated by a Nafion® 117 membrane. Acid (H2SO4) and neutral (NaCl) catholytes were used. Pt/C showed superior activity in the acid-acid electrolyte pair, while the activity of Ru/C and Pd/C were significantly improved in the neutral-acid catholyte-anolyte pairs. By pairing neutral catholyte and acid anolyte, the anodic protons transported through the membrane can be effectively utilized for ECH reactions. In terms of reaction pathways for guaiacol ECH, ring saturation leading to 2-methoxycyclohexanol was generally the dominant pathway. However, for Pt/C in either 0.2 or 0.5 M NaCl catholyte paired with 0.5 M H2SO4 anolyte the demethoxylation–ring saturation pathway producing cyclohexanol and cyclohexanone was equally competitive at a constant superficial current density of -109 mA cm−2 and 50 0C. Efficient reductive upgrading of lignin model compounds can be achieved under mild conditions via electrocatalysis in the slurry reactor by exploiting synergistic effects between the catalyst and electrolyte.

Properties of the Pt(111)/electrolyte electrochemical interface studied with a hybrid DFT-solvation approach.

Abstract: Self-consistent modeling of the interface between solid metal electrode and liquid electrolyte is a crucial challenge in computational electrochemistry. In this contribution, we adopt the effective screening medium reference interaction site method (ESM-RISM) to study the charged interface between a Pt(111) surface that is partially covered with chemisorbed oxygen and an aqueous acidic electrolyte. This method proves to be well suited to describe the chemisorption and charging state of the interface at controlled electrode potential. We present an in-depth assessment of the ESM-RISM parameterization and of the importance of computing near-surface water molecules explicitly at the quantum mechanical level. We found that ESM-RISM is able to reproduce some key interface properties, including the peculiar, non-monotonic charging relation of the Pt(111)/electrolyte interface. The comparison with independent theoretical models and explicit simulations of the interface reveals strengths and limitations of ESM-RISM for modeling electrochemical interfaces.

Recent progress on multiscale modeling of electrochemistry

Abstract: Computational electrochemistry, an important branch of electrochemistry, has shown its advantages in studying electrode/electrolyte interfaces, such as the structures of electric double layers. However, modeling electrochemical systems is still a challenge, especially in interface electrochemistry, because not only solvation effects and ion distribution in electrolyte solutions should be considered, but also the treatment of the electrode potential and the response of electrolytes to applied potentials. Here, we review the latest development in the field of computational electrochemistry. We first introduce various energy models used in simulating electrolytes and electrodes at multiple scales. Then, to better explain and compare between different methods, we discuss the calculation methods of solution electrochemistry and interface electrochemistry in separate. At last, we introduce the methods to electrify the interfaces in various multiscale models. This review aims to help understand various levels of methods in simulations of different scenarios in electrochemistry, and summarizes a set of schemes covering multiple scales.

A dual-cathode study on Ag-Cu sequential CO2 electroreduction towards hydrocarbons

Abstract: Electroreduction of CO2 to high value-added products such as hydrocarbons provides a promising way to create a carbon neutral circular economy. Copper (Cu) is the most effective metal catalyst with selectivity towards hydrocarbons but Cu catalyst alone generally produces hydrocarbons at low selectivity and yield. Operando surface enhanced infrared absorption spectroscopic (SEIRAS) analysis reveals that the low hydrocarbon yield is attributed to the poor ability of Cu to produce carbon monoxide (CO), which is a crucial intermediate for hydrocarbon formation. In the current study, an Ag-Cu dual-cathode CO2RR device is developed, which allows separate control of electrode potential. This dual-cathode cell shows increased hydrocarbon yield (83 % for methane and 106 % for ethylene) and a 200 mV lower onset potential for C2H4, compared to electrolysis with a single Cu cathode. The CO produced by the Ag cathode transfers to Cu and increase its CO coverage, facilitating methane formation as well as promoting C C coupling to form ethylene. This study also reveals a mismatch of optimum electrode potential between Ag and Cu, which results in compromised performance of the two components in an Ag-Cu bimetallic electrocatalyst, but poses no challenge in the dual-cathode cell. To optimize the performance of the dual-cathode cell, we propose the use of a single-pass dual-gas-diffusion-cathode reactor with enhanced mass transfer for maximum hydrocarbon production. This work verifies that splitting the complex CO2RR process into two separate steps is a feasible method to improve the overall efficiency and rate of advanced products while reducing the energy input.

Design principle of MoS2/C heterostructure to enhance the quantum capacitance for supercapacitor application

Abstract: 1T Molybdenum disulfide (1T-MoS2) has been widely studied experimentally as an electrode for supercapacitors due to its excellent electrical and electrochemical properties. Whereas the capacitance value in MoS2 is limited due to the lower density of electrons near the Fermi level, and unable to fulfill the demand of industry i.e. quantum capacitance preferably higher than 300 μF/cm2. Here, we investigated the performance of 2H, 1T, and 1T′ phases of MoS2 in its pristine form and heterostructures with carbon-based structures as an electrode in the supercapacitors using density functional theory. Specifically, we reported that the underneath carbon nanotube (CNT) is responsible for the structural phase transition from 1T to 1T′ phase of MoS2 monolayer in 1T′-MoS2/CNT heterostructure. This is the main reason for a large density of states near Fermi level of 1T′-MoS2/CNT that exhibits high quantum capacitance (CQ) of 500 μF/cm2 at a potential of 0.6 V. Also, we observed that the nitrogen doping and defects in the underneath carbon surface amplify the CQ of heterostructure for a wider range of electrode potential. Therefore, the 1T′-MoS2/N doped CNT can be explored as an electrode for next-generation supercapacitors.

Effective Electrochemical Modulation of SERS Intensity Assisted by Core-Shell Nanoparticles.

Abstract: An effective and reversible tuning of the intensity of surface-enhanced Raman scattering (SERS) of nonelectroactive molecules at nonresonance conditions by electrochemical means has been developed on plasmonic molecular nanojunctions formed between Au@Ag core-shell nanoparticles (NPs) and a gold nanoelectrode (AuNE) modified with a self-assembled monolayer. The Au@Ag nanoparticle on nanoelectrode (NPoNE) structures are formed in situ by the electrochemical deposition of Ag on AuNPs adsorbed on the AuNE and can be monitored by both the electrochemical current and SERS signals. Instead of introducing molecular changes by the applied electrode potential, the highly effective SERS intensity tuning was achieved by the chemical composition transformation of the ultrathin Ag shell from metallic Ag to insulating AgCl. The electrode potential-induced electromagnetic enhancement (EME) tuning in the Au@Ag NPoNE structure has been confirmed by finite-difference time-domain simulations. Moreover, the specific Raman band associated with Ag-molecule interaction can also be tuned by the electrode potential. Therefore, we demonstrated that the electrode potential could effectively and reversibly modulate both EME and chemical enhancement in Au@Ag NPoNE structures.