Electrochemical Transport Research Articles

Solutions to the Challenges of Lithium–Sulfur Batteries by Carbon Nanotubes

AbstractThe continuously increasing demands for energy storage devices for portable electronics and electric vehicles have aroused massive research interest in developing lithium–sulfur batteries (LSBs) with high energy density and long‐term stability. Carbon nanotubes (CNTs), possessing numerous superior properties, are integrated into various components of LSBs for performance improvement. Nevertheless, a systematic and insightful issue‐based study of their inherent roles in addressing the practical challenges of LSBs is lacking. There is a growing consensus that CNTs do not directly contribute to the specific capacity (i.e., being involved in the redox reactions with electron loss/gain), while their auxiliary roles, such as providing a conductive and mechanically reinforced framework for active materials, are of prime significance in regulating the electrochemical reaction, charge transport, and mass transfer in the system. In this paper, after briefly introducing the working principles of LSBs and the promising applicability of CNTs, current challenges in various components of LSBs are discussed with the corresponding CNT‐based solutions, followed by an evaluation of the potential for commercializing CNT‐involved LSBs. Finally, some future research directions are provided to improve the device performance further.

Multiscale Structural Engineering of Sulfur/Carbon Cathodes Enables High Performance All-Solid-State LiS Batteries.

Constructing all-solid-state lithium-sulfur batteries (ASSLSBs) cathodes with efficient charge transport and mechanical flexibility is challenging but critical for the practical applications of ASSLSBs. Herein, a multiscale structural engineering of sulfur/carbon composites is reported, where ultrasmall sulfur nanocrystals are homogeneously anchored on the two sides of graphene layers with strong SC bonds (denoted as S@EG) in chunky expanded graphite particles via vapor deposition method. After mixing with Li9.54 Si1.74 P1.44 S11.7 Cl0.3 (LSPSCL) solid electrolytes (SEs), the fabricated S@EG-LSPSCL cathode with interconnected "Bacon and cheese sandwich" feature can simultaneously enhance electrochemical reactivity, charge transport, and chemomechanical stability due to the synergistic atomic, nanoscopic and microscopic structural engineering. The assembled InLi/LSPSCL/S@EG-LSPSCL ASSLSBs demonstrate ultralong cycling stability over 2400 cycles with 100% capacity retention at 1 C, and a record-high areal capacity of 14.0mAhcm-2 at a record-breaking sulfur loading of 8.9mgcm-2 at room temperature as well as high capacities with capacity retentions of ≈100% after 600 cycles at 0 and 60°C. Multiscale structural engineered sulfur/carbon cathode has great potential to enable high-performance ASSLSBs for energy storage applications.

Preparation of sulfur vacancy modified NiCo2S4@NiCoS2 core-shell electrode material and its application in asymmetric supercapacitors

Nickel-cobalt based bimetallic sulfide has attracted much attention as positive electrode materials for supercapacitors due to its microstructure designability. However, charge storage occurs mostly on the surface of the material, which limits the full utilization of the active material inside. Here, rich sulfur vacancies modified NiCo2S4@NiCoS2 (Vs-NiCo2S4@NiCoS2) core-shell structures are grown tightly on a nickel foam substrate and prepared as positive material for supercapacitors. The rich sulfur vacancies in the Vs-NiCo2S4 "core" increase the electrochemical active sites and electron transport rates, while the amorphous NiCoS2 "shell" facilitates the rapid diffusion of ions. The results show that Vs-NiCo2S4@NiCoS2 core-shell structure has a specific capacitance of 3200 F g−1, achieving the effect of "1 + 1>2". The excellent electrochemical performance is attributed to the surface/interface modification of rich sulfur vacancies and the synergistic effect induced by heterogeneous core-shell boundaries. Moreover, the asymmetric supercapacitor Vs-NiCo2S4@NiCoS2//AC exhibits an energy density of 46.3 Wh kg−1 and a power density of 799.8 W kg−1, as well as excellent cyclic stability (nearly 80% capacity retention after 10,000 cycles). This work provides a new idea and strategy for core-shell structure design of nickel-cobalt sulfide electrode materials combined with surface/interface modification.

Mechanistic Insights into the Formation of CO and C2 Products in Electrochemical CO2 Reduction─The Role of Sequential Charge Transfer and Chemical Reactions

The electrochemical reduction of CO2 presents an attractive opportunity to not only valorize CO2 as a feedstock for chemical products but also to provide a means to effectively store renewable electricity in the form of chemical bonds. The recent surge of experimental and computational studies of electrochemical CO2 reduction (ECR) has brought about significant scientific and technological advances. Yet, considerable gaps in our understanding of and control over the reaction mechanism persist, in particular for the formation of products. Moreover, while theoretical and computational studies have proposed many candidate reaction pathways, comprehensively reconciling these models with experimental observations remains challenging and elusive. The conventional electrochemical analysis of catalyst activity and selectivity generally relies on steady-state measurements. In a departure from this convention, we show in this study that time-resolved measurements (i.e., chronoamperometry) provide a powerful diagnostic tool to gain valuable insights into the complex interplay of electrochemical reactions, chemical reactions, and mass transport. We show that the initial stages of the ECR reaction show signatures of an electrochemical reaction followed by a homogeneous chemical reaction. These signatures have important mechanistic implications and inform dominant reaction pathways, specifically for the sequential electron and proton transfer steps leading to the formation of formate intermediates (*COOH–). We hope that the methods and insights presented in this work will inspire future studies to exploit chronoamperometric analysis to resolve outstanding questions in ECR and other multi-step electrochemical reaction pathways.

Partial selenium surface modulation of metal organic framework assisted cobalt sulfide hollow spheres for high performance bifunctional oxygen electrocatalysis and rechargeable zinc-air batteries

There is still a significant technological barrier in the development of high-performance electrocatalysts with synergistic unreliable functions and morphological integrity that improves reversible electrochemical activity, electrical conductivity, and mass transport properties. Metal-organic compound networks are envisioned as a defect-rich porous framework that provides mesoporous hollow carbon nanostructures composed primarily of an in situ-grown N-doped graphitic carbon matrix and embedded selenium-doped CoS2 hollow spheres as efficient, highly reactive, and long-lasting chemical energy conversion functions of the system. This method enables hitherto inaccessible synthesis approaches to produce a highly porous conductive network at the microscopic level while exposing rich unsaturated reactive sites at the atomic level without losing electrical or structural integrity. Because of their inherent increased electrochemical surface area, and electron transfer, the porous framework, doping motifs, and tailored structural defects provide outstanding bifunctional oxygen electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reactions (OER). Moreover, using this selenium-doped MOF CoS2 hollow spheres electrode as an air-cathode, a rechargeable zinc-air battery with excellent discharge-charge performance and mechanical stability is successfully constructed. This study provides a feasible and universal technique for constructing diverse functional interconnected metal-organic coordinated compounds that may be employed for a wide variety of energy storage, conversion (e.g., fuel cells and metal-air batteries), and environmental applications.

Dynamic Multi-Dimensional Numerical Transport Study of Lithium-Ion Battery Active Material Microstructures for Automotive Applications

In support of GM’s traction battery efforts, we derived and implemented a method to describe the electrochemical performance of a battery cell considering the nuances of the electrode microstructure at the anode and the cathode and the corresponding impact on the electrochemical transport in the solid and liquid phases. To assess the capability of the method, we compared model results from the microstructure framework with the commonly used continuum-level porous electrode model, commonly referred to as the pseudo-2-dimensional model, or the Newman Model. The microstructure modeling framework was applied to simulate the electrochemical and transport processes within the battery cell to predict the concentration gradients, local state-of-charge distributions, reaction distributions, and the overall terminal voltage of the system. In this report, we provide a commentary on the validity and practicality of the microstructure approach to drive battery cell design.

Water in Gel Electrolyte for Zinc Ion Battery.

Gel electrolyte is being intensively explored for aqueous rechargeable zinc ion batteries, especially towards high performance and multi-functionalities. Water plays its central role on the fundamental properties, interface reaction/interaction, and performance of the gel-type zinc electrolyte. In this minireview, the influence of water on the physiochemical properties of gel electrolytes is focused on. The correlation between water activity and the fundamental properties of zinc electrolytes is presented. Current approaches and challenges in manipulating water activity and the consequent influence on the electrochemical stability, transport, and interface kinetics of gel electrolytes are summarized. An outlook on approaches to tuning and investigating water activity is provided to shed light on the design of advanced gel electrolytes.

Simulation of the magnetic field assisted electrochemical machining

Electrochemical machining (ECM) is an unconventional manufacturing method based on the anodic dissolution of the metal workpiece material. A superimposed magnetic field causes additional body forces within the working gap influencing the electrolyte convection and consequently afecting the electrochemical kinetics and gas transport. In this study, a multiphysics simulation model was built to analyze the interactions between the magnetic field characteristics, the convective heat- and charge-transport in the working gap and the electrochemical workpiece shaping. Therefore, the Lorentz force and Kelvin force were considered. In addition, machined geometries and surface properties were measured and related to the simulation findings.

Spherical Particles Growth with Dynamic Oscillation during Lithium Electrodeposition

In this paper, the spherical particles growth during lithium electrodeposition was investigated by directly solving the governing equations based on the Landau transformation method. The basic growth kinetic characteristics of a spherical particle during electrodeposition was studied. Predicted results show that the dynamic oscillation of the growth velocity occurs during the spherical particle growth. It was found from numerical simulations that applied electrical potential difference, electrolyte concentration, and diffusion coefficient are 3 main factors influencing the spherical growth and the existence of the dynamic oscillation state of the growth velocity during electrodeposition. The increase in both the applied electrical potential difference and the electrolyte concentration can lead to the increase of the growth velocity of the spherical particle, while the growth velocity is independent of the diffusion coefficient. Moreover, it was found that the wavelength and the amplitude of the dynamic oscillation of the growth velocity can be influenced by the applied electrical potential difference, the electrolyte concentration, and the diffusion coefficient. We determined that the dynamic competition between electrochemical reactions and ion transport in the electrodeposition is the reason for the existence of the oscillation of the growth velocity.

Construct NiSe/NiO Heterostructures on NiSe Anode to Induce Fast Kinetics for Sodium-Ion Batteries

It is of great significance to design and innovate electrode materials with unique structures to effectively optimize the electrochemical properties of the secondary battery. Herein, inspired by neuron networks, an ingenious synthesis is proposed to fabricate NiSe with multidimensional micro-nano structures, followed by in situ construction of NiSe/NiO heterostructures via a temporary calcination. The major structure of bulk NiSe synthesized by the solvothermal method is 3-dimensional micron cluster spherical particles interwoven by uniform one-dimensional nanofibers. Such structures possess the synergistic advantages of nano and micro materials. After a temporary calcination in air, NiSe/NiO heterostructures should be formed in the bulk NiSe, which provides a built-in electric field to enhance diffusion kinetics of sodium ions. This special neural-like network and heterojunction structures ensure the excellent structural stability combined with rapid kinetics of the electrode, releasing 310.9 mAh g −1 reversible capacity after 2,000 cycles at 10 A g −1 . Furthermore, the electrochemical storage and ion transport mechanisms are elaborated by electrochemical analysis and theoretical calculation in more detail.

Variationally consistent homogenization of electrochemical ion transport in a porous structural battery electrolyte

In this paper, we develop a multi-scale modeling framework for a multiphysics problem characterized by electro-chemically coupled ion transport in a Structural Battery Electrolyte (SBE). The governing equations of the problem are established by coupling Gauss law with mass conservation for each mobile species. By utilizing variationally consistent homogenization, we are able to establish a two-scale model where both the macro-scale and sub-scale equations are deduced from a single-scale problem. Investigations of the sub-scale RVE problem show that the transient effects are negligible for the length scales relevant to the studied application, which motivates the assumption of micro-stationarity. In the special case of linear constitutive response, we get a numerically efficient solution scheme for the macro-scale problem that is based on a priori upscaling. As a final step, we demonstrate the numerically efficient solution scheme by solving a 2D macro-scale problem using upscaled constitutive quantities based on a 3D RVE.

電気化学技術とTransport Phenomena（輸送現象論）：恩師に導かれて

電気化学技術とTransport Phenomena（輸送現象論）：恩師に導かれて

Designing graded fuel cell electrodes for proton exchange membrane (PEM) fuel cells with recurrent neural network (RNN) approaches

The graded distribution of Pt loading in the catalyst layer (CL) and the porosity of the gas diffusion layer (GDL) significantly affect the spatial distributions of electrochemical reaction and mass transport rates, thus influencing the cell performance and durability. A sophisticated physics-based model is established to study the influence of graded Pt loading and GDL porosity at the cathode, with their distribution function obeying the elliptic equation along the in-plane and through-plane directions, on the current density and its uniformity at a given cell voltage. To reduce the computational time and resources, an RNN algorithm-based data-driven surrogate model is developed to assist in the identification of the relationship between the design parameters and the objective functions. Latin hypercube sampling (LHS) method is implemented for sampling and then the initial data acquisition is conducted for training and testing the surrogate model. Results show that the machine learning (ML) algorithm could effectively assist the optimal design of the functionally graded electrode, and the surrogate model achieves > 97.9 % prediction accuracy for current density and less than 0.13 root mean square error (RMSE) for current homogeneity. Both the individual variation of Pt loading and GDL porosity and their interaction are respectively analysed. Results also indicate that the inhomogeneous Pt distribution improves the current density. On the contrary, GDL porosity has a greater impact on the cell performance since current density monotonically increases with the homogeneous GDL porosity. When both the inhomogeneous distributions of Pt loading and GDL porosity are simultaneously considered, the homogeneity of current density is improved. However, the improvement of the homogeneity of current density (increases by 54 %) sacrifices the maximum current density (reduces by 22 %).

4]‐Cyclo‐2,7‐Carbazole as Host Material in High‐Efficiency Phosphorescent OLEDs: A New Perspective for Nanohoops in Organic Electronics

AbstractIn the last ten years, the development of π‐conjugated nanohoops has been considerable owing to their remarkable properties. However, to date, their incorporation in organic electronic devices remains very scarce. In this work, the first high‐performance organic electronic device (i.e., phosphorescent organic light‐emitting diode, PhOLED) incorporating a nanohoop ([4]‐cyclo‐N‐butyl‐2,7‐carbazole, [4]C‐Bu‐Cbz) is reported, revealing the potential of nanohoops in electronics. Thus, using the red phosphor Ir(MDQ)2(acac), the [4]C‐Bu‐Cbz‐based PhOLED displays a high external quantum efficiency (EQE) of 17.0%, a current efficiency (CE) of 20.6 cd A−1 and a power efficiency (PE) of 25.8 lm W−1 demonstrating that the charge injection, transport, and recombination are particularly efficient. This performance is significantly higher than that of its linear counterpart, N‐butyl‐2,7‐quartercarbazole ([4]L‐Bu‐Cbz), which presents an EQE of 11.1%, a CE of 13.0 cd A−1 and a PE of 15.7 lm W−1. The significant difference, in terms of device performance, between cyclic and acyclic compounds provides a new basis to construct high‐performance electronic devices. This study, which includes optical, electrochemical, morphological, and charge transport properties, shows that nanohoops can be efficiently used as organic semiconductors in electronics and opens the way to their practical uses in high‐performance optoelectronic devices, which is now the next stage of their evolution.

Synthesis of poly(3,4-ethylenedioxythuophene) derivatives using three-armed conjugated cross-linker and its thermoelectric properties

Conducting polymer based thermoelectric systems are very effective at harvesting electricity from waste heat with low-temperature gradients relative to environmental temperature. However, although various studies concerning the doping effect exist, there are insufficient reports on the effect of the change in polymer chain structure on the thermoelectric properties. Here we demonstrate different poly(3,4-ethylenedioxythiophene) (PEDOT)-derivative chain structures (such as linear, two-dimensional, co-polymer), by varying the amount of three-armed conjugated cross-linker (1,3,5-tri(2-thienyl)-benzene (TTB)). At a small amount of TTB addition (until 0.5 mM), the electrochemical charge transport of the PEDOT films increased without shift of the UV–Vis absorbing property. However, from higher than 0.5 mM TTB, electrochemical charge transport decreased with blue shift of the UV–Vis absorbing curve. Moreover, the power factor of the PEDOT-derivative films without TTB to a small amount of TTB (0.5 mM) was improved from (27.03 to 49.43) µW m−1 K−2, and at higher than 0.5 mM TTB concentration, was diminished, respectively, because a small amount of TTB enhanced the π–π interaction of the polymer chains without disrupting the conjugation, while excess TTB molecules blocked conjugation of the polymer.

Polyhydroxy Urethanes and Carbonates Bearing Bio-Based Solid Electrolyte for Solid-State Lithium Batteries

With higher energy density, flexibility, safe operation at both room temperature as well as high temperature, solid-state batteries technology has been strongly looked upon as superior diversion from conventional liquid electrolytes-based lithium batteries. Among various other class of polymers, there is strong need to focus on sustainable and yet better alternatives other than most researched polyethylene oxide (PEO). Poly(hydroxyurethanes) (PHU) owing to their high degree of structural tunability can be interesting candidates for Solid polymer electrolyte (SPE). Here, we synthesized bio-based PHU networks by facile ring-opening polymerization of carbonated soybean oil (CSBO) without catalyst. The existence of polyether and polyester provides the PHU network with soft segments while urethane linkage with hard segments. The lower Tg with higher thermal stability widespread its application at elevated temperature without melting and significant degradation. PHU shows remarkable adhesive properties to the surface of metal speculating better interfacial contact and stability. The electrochemical investigations exhibited decent ionic conductivity (> 10-4 at 60o C), electrochemical stability window (ESW > 4.5 V vs. Li/Li+) and transport properties allowing it to be explored for high voltage cathodes. Lithium ferrophosphate (LiFePO4) cells with PHU also demonstrated promising charge-discharge (> 100 mAhg-1 at 60o C) with the li-metal anode. The attempt to develop bio-based batteries materials are much required added step toward green and clean energy for a sustainable future.

Modeling Electrochemical Transport within a Three-Electrode System Towards Fast Charge Applications

In support of GM's traction battery efforts, we derive and implement a method to describe the electrochemical performance of a battery cell through the combination of a modified Newman Pseudo 2-Dimensional model and a three-electrode experimental apparatus. To assess the capability of the method, we compare model results with experimental data for a lithiated graphite and lithium nickel manganese cobalt oxide system. The model is applied to simulate the electrochemical and transport processes within the battery cell to predict the negative electrode potential and positive electrode potential with respect to a lithium iron phosphate reference electrode, as well as the terminal voltage. We extend this discussion to cover applicability to fast charge considerations to avoid lithium plating at the anode during a fast charge event.

Effects of Composite Electrode Structure on Performance of Intermediate-Temperature Solid Oxide Electrolysis Cell

The composite electrode structure plays an important role in the optimization of performance of the intermediate-temperature solid oxide electrolysis cell (IT-SOEC). However, the structural influence of the composite electrode on the performance of IT-SOEC is not clear. In this study, we developed a three-dimensional macroscale model coupled with the mesoscale model based on percolation theory. We describe the electrode structure on a mesoscopic scale, looking at the electrochemical reactions, flow, and mass transport inside an IT-SOEC unit with a composite electrode. The accuracy of this multi-scale model was verified by two groups of experimental data. We investigated the effects of operating pressure, volume fraction of the electrode phase, and particle diameter in the composite electrode on electrolysis reaction rate, overpotential, convection/diffusion flux, and hydrogen mole fraction. The results showed that the variation in the volume fraction of the electrode phase had opposite effects on the electrochemical reaction rate and multi-component diffusion inside the composite electrode. Meanwhile, an optimal range of 0.8–1 for the particle diameter ratio was favorable for hydrogen production. The analysis of IT-SOEC with composite electrodes using this multi-scale model enables the subsequent optimization of cell performance and composite electrode structure.

Enhancing the Backbone Coplanarity of n-Type Copolymers for Higher Electron Mobility and Stability in Organic Electrochemical Transistors.

Electron-transporting (n-type) conjugated polymers have recently been applied in numerous electrochemical applications, where both ion and electron transport are required. Despite continuous efforts to improve their performance and stability, n-type conjugated polymers with mixed conduction still lag behind their hole-transporting (p-type) counterparts, limiting the functions of electrochemical devices. In this work, we investigate the effect of enhanced backbone coplanarity on the electrochemical activity and mixed ionic-electronic conduction properties of n-type polymers during operation in aqueous media. Through substitution of the widely employed electron-deficient naphthalene diimide (NDI) unit for the core-extended naphthodithiophene diimide (NDTI) units, the resulting polymer shows a more planar backbone with closer packing, leading to an increase in the electron mobility in organic electrochemical transistors (OECTs) by more than two orders of magnitude. The NDTI-based polymer shows a deep-lying lowest unoccupied molecular orbital level, enabling operation of the OECT closer to 0 V vs Ag/AgCl, where fewer parasitic reactions with molecular oxygen occur. Enhancing the backbone coplanarity also leads to a lower affinity toward water uptake during cycling, resulting in improved stability during continuous electrochemical charging and ON–OFF switching relative to the NDI derivative. Furthermore, the NDTI-based polymer also demonstrates near-perfect shelf-life stability over a month-long test, exhibiting a negligible decrease in both the maximum on-current and transconductance. Our results highlight the importance of polymer backbone design for developing stable, high-performing n-type materials with mixed ionic-electronic conduction in aqueous media.

Electrochemical and Ion Transport Studies of Li+ Ion-Conducting MC-Based Biopolymer Blend Electrolytes.

A facile methodology system for synthesizing solid polymer electrolytes (SPEs) based on methylcellulose, dextran, lithium perchlorate (as ionic sources), and glycerol (such as a plasticizer) (MC:Dex:LiClO4:Glycerol) has been implemented. Fourier transform infrared spectroscopy (FTIR) and two imperative electrochemical techniques, including linear sweep voltammetry (LSV) and electrical impedance spectroscopy (EIS), were performed on the films to analyze their structural and electrical properties. The FTIR spectra verify the interactions between the electrolyte components. Following this, a further calculation was performed to determine free ions (FI) and contact ion pairs (CIP) from the deconvolution of the peak associated with the anion. It is verified that the electrolyte containing the highest amount of glycerol plasticizer (MDLG3) has shown a maximum conductivity of 1.45 × 10−3 S cm−1. Moreover, for other transport parameters, the mobility (μ), number density (n), and diffusion coefficient (D) of ions were enhanced effectively. The transference number measurement (TNM) of electrons (tel) was 0.024 and 0.976 corresponding to ions (tion). One of the prepared samples (MDLG3) had 3.0 V as the voltage stability of the electrolyte.