Formation Of Electrical Double Layer Research Articles

AC-Modulated XPS Enables to Externally Control the Electrical Field Distributions on Metal Electrode/Ionic Liquid Devices.

X-Ray Photoelectron Spectroscopy (XPS) has been utilized to extract local electrical potential profiles by recording core-level binding energy shifts upon application of the AC [square-wave (SQW)] bias with different frequencies. An electrochemical system consisting of a coplanar capacitor with a polyethylene membrane (PEM) coated with the Ionic Liquid (IL) N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) as the electrolyte is investigated. Analyses are carried out in operando, such that XPS measurements are recorded simultaneously with current measurements. ILs have complex charging/discharging processes, in addition to the formation of Electrical Double Layers (EDL) at the interfaces, and certain properties of these processes can be captured using AC modulation within appropriate time windows of observation. Herein, we select two frequencies, namely, 10 kHz and 0.1 Hz, to separate effects of the fast polarization and slow migratory motions, respectively. Moreover, the local potential developments after adding two equivalent series resistors at three different physical positions of the device have been carefully evaluated from the binding energy shifts in the F 1s peak representing the anion of the IL. This circuit modification allows us to quantify the AC currents passing through the device, as well as the system's impedance, in addition to revealing the potential variations due the IR drops. The complex AC-modulated local XPS data recorded can also be faithfully reproduced using the unmodulated F 1s spectrum and by convoluting it with electrical circuit output provided by the LT-Spice software. The outcome of these efforts is a more realistic equivalent circuit model, which can be related to chemical/physical makeup of the electrochemical system. An important finding of this methodology emerges as the possibility to induce additional local electrical field developments within the device, the directions of which can be reversed controllably.

Dynamic Effect of Overscreening on Electrochemical and Thermal Properties of EDLC Electrodes during Charging/Discharging Cycles

Overscreening was commonly observed during charging/discharging cycles of electric double layer capacitors(EDLCs). This study presented a two-dimensional continuum electrochemical thermal model with a distribution function for concentration considering overscreening to investigate how overscreening affected the dynamic formation of electric double layer (EDL) near electrodes in macroscale. The results indicated that ion distribution was co-affected by EDL formation and overscreening near the electrode. Correspondingly, layered space charge density distribution resulted in a thicker diffuse layer, whose thickness was determined by the sum of co-ion and counter-ion diameters. Additionally, the diffuse layer capacitance was proportional to the reciprocal of counter-ion diameter, while the total capacitance was proportional to a linear combination of counter-ion and the larger ion diameters. Moreover, the shift from endothermic to exothermic for the total heat generation rate was dominated by heat of mixing, and was caused by the local ion mixing due to overscreening, resulting in local entropy increase. The results of this study can be used to further investigate the effect of overscreening on ion transient transport dynamic and heat generation rates.

Ionic Rectification by Dynamic Regulation of the Electric Double Layer at the Hydrogel Interface.

Hydrogels play a pivotal role in the realm of iontronics, contributing to the realization of futuristic human-machine interactions. The electric double layer (EDL) between the hydrogel and electrode provides an essential ionic-electronic coupling interface. While prior investigations primarily delved into elucidating the formation mechanism of the EDL, our study shifts the focus to showcasing the current generation through the mechanical modulation of the EDL at the hydrogel-metal interfaces. The dynamic EDL was constructed by the mechano-driven contact-separation process between the polyacrylamide (PAAm) hydrogel and Au. Influencing factors on the dynamic regulation of the EDL such as ion concentration, types of salt, contact-separation frequency, and deformation degree were investigated. Dehydration usually limits the practical applications of hydrogels, and it is a long-standing and difficult problem. However, it seemed to be able to slow the EDL formation process here, resulting in a sustained continuous direct current signal output. Such hydrogel iontronics could rectify the displacement electronic current of a triboelectric nanogenerator by the ionic current. The directional migration of ions could be further enhanced by using charge-collecting metals with different work functions, for example, Au and Al. It offers a paradigm to enable ionic rectification that could be seamlessly incorporated into electronic systems, ushering in a new era for efficient energy harvesting and biomimetic nervous systems.

Highly conductive, conformable ionic laser-induced graphene electrodes for flexible iontronic devices

Iontronic devices, recognized for user-friendly soft electronics, establish an electrical double layer (EDL) at the interface between ion gels and electrodes, significantly influencing device performance. Despite extensive research on ion gels and diverse electrode materials, achieving a stable interfacial formation remains a persistent challenge. In this work, we report a solution to address this challenge by employing CO2 irradiation as a bottom-up methodology to directly fabricate highly conductive, conformable laser-induced graphene (LIG) electrodes on a polyimide (PI)-based ion gel. The PI ion gel exhibits exceptional EDL formation at the electrode interface, primarily attributable to efficient ion migration. Particularly, ionic laser-induced graphene (i-LIG) electrodes, derived from the PI ion gel as a precursor, yield high-quality graphene with enhanced crystallinity and an expanded porous structure in the upward direction. This outcome is achieved through a pronounced thermal transfer effect and intercalation phenomenon between graphene layers, facilitated by the presence of ionic liquids (ILs) within the PI ion gel. Ultimately, in comparison to alternative soft electrode-based vertical capacitors, the utilization of i-LIGs and PI ion gels in the vertical capacitor demonstrates reduced interfacial resistance and increased EDL capacitance, emphasizing the extensive potential of iontronic devices. These results not only highlight these features but also introduce a new perspective for advancing next-generation iontronic devices.

In Situ Analysis of the Effect of Ultrasonic Cavitation on Electrochemical Polishing of Additively Manufactured Metal Surfaces

Abstract The effects of a hybrid process that combines ultrasonic cavitation and electrochemical polishing on the electrochemical behavior and the resulting surface characteristics of additively manufactured 316-L stainless steel were investigated. In situ potentiodynamic scans and electrochemical impedance spectroscopy (EIS) were conducted to gain a fundamental understanding of the effect of ultrasonic cavitation on the electrochemical processes involved, considering the influence of electrolyte temperature at 60 and 70 °C. The potentiodynamic scans revealed that increasing the ultrasonic excitation amplitude from 20 to 80 µm at 20 µm intervals and temperature from 60 to 70 °C led to reduced polishing resistance, and elevated passivation current density at equivalent applied potentials, thus leading to an increased polishing rate. These findings are attributed to intensified cavitation near the material surface, which promoted anodic dissolution reactions and accelerated the polishing rate. In situ EIS measurements provided valuable information on the charge transfer resistance and double-layer capacitance and their influence on the hybrid process. Specifically, higher ultrasonic amplitudes and elevated temperatures contributed to enhanced electrical double-layer formation and ion adsorption, resulting in a faster rate of polishing, indicating the efficacy of the hybrid process. These findings enhance our understanding of the complex interactions between ultrasonic cavitation and electrochemical dissolution processes that occur during ultrasonic cavitation-assisted electrochemical polishing. The research provides valuable insights for optimizing the process and its potential application in the post-processing of metal additive manufactured parts.

Electrical Double Layer Formation at Intercalation Cathode–Organic Electrolyte Interfaces During Initial Lithium‐Ion Battery Reactions (Adv. Mater. Interfaces 5/2024)

Electrical Double Layer Formation at Intercalation Cathode–Organic Electrolyte Interfaces During Initial Lithium‐Ion Battery Reactions (Adv. Mater. Interfaces 5/2024)

The effect of surface hydrophobicity and hydrophilicity on ion-ion interactions at water-solid interfaces.

Condensation and arrangement of ions at water-solid interfaces are of great importance in the formation of electrical double layers (EDL) and the transport of ions under a confined geometry. So far, the microscopic understanding of interfacial ion configurations is still far from complete, especially when the local ion concentration is high and ion-ion interactions become prominent. In this study, we directly visualized alkali metal cations within the hydrogen-bonding network of water on graphite and Cu(111)-supported graphene surfaces, using qPlus-based noncontact atomic force microscopy (NC-AFM). We found that the codeposition of the alkali cations and water molecules on the hydrophobic graphite surface leads to the formation of an ion-doped bilayer hexagonal ice (BHI) structure, where the ions are repelled from each other and scattered in a disordered distribution. In contrast, the hydrated alkali cations aggregate in one dimension on the more hydrophilic graphene/Cu(111) surface, forming a nematic state with a long-range order. Such a nematic state arises from the delicate interplay between water-ion and water-water interactions under surface confinement. These results reveal the high sensitivity of ion-ion interactions and ionic ordering to the surface hydrophobicity and hydrophilicity.

Exploring how cation entropy influences electric double layer formation and electrochemical reactivity.

Electric double layers are crucial to energy storage and electrocatalytic device performance. While double layer formation originates in electrostatic interactions, electric double layer properties are governed by a balance of both electrostatic and entropic driving forces. Favorable ion-surface electrostatic interactions attract counterions to charged surfaces to compensate, or "screen," potentials, but the confinement of these same ions from a bulk reservoir to the interface incurs an entropic penalty. Here, we use a dicationic imidazolium ionic liquid and its monovalent analogue to explore how cation valence and entropy influence double layer formation and electrochemical reactivity using CO2 electroreduction as a model reaction. We find that divalent and monovalent cations display similar CO2 reduction kinetics but differ vastly in steady-state reactivity due to rapid electrochemically induced precipitation of insulating dicationic (bi)carbonate films. Using in situ surface-enhanced Raman scattering spectroscopy, we find that potential-dependent cation reorientation occurs at similar potentials between the two ionic liquids, but the introduction of a covalent link in the divalent cation imparts a more ordered double layer structure that favors (bi)carbonate precipitation. In mixed monovalent-divalent electrolytes, we find that the divalent cations dominate interfacial properties by preferentially accumulating at surfaces even at very low relative concentrations. Our findings confirm that ion entropy plays a key role in modulating local electrochemical environments. Furthermore, we highlight how double layer properties are sensitive to the properties of counterions that pay the lowest entropic penalty to accumulate at interfaces. Overall, we illustrate that ion entropy provides a new knob to tune reaction microenvironments and unveil how entropy plays a major role in modulating electrochemical reactivity in mixed ion electrolytes.

(Invited) Development and Scale-up of Membrane Capacitive Deionization (MCDI) for Desalination and Water Reuse

Desalination and water reuse are the key solutions to address global water scarcity. It is important to recognize the water-energy interactions and develop energy-efficient technologies to separate ions from brackish water, seawater and reclaimed wastewater. Most recently, membrane capacitive deionization (MCDI), inspired by energy storage devices (e.g., supercapacitors), is a promising desalination technology with several advantages of energy efficiency (TDS < 4000 ppm), high water recovery, less chemical additive, and environmental friendliness. In MCDI, ions are electrostatically captured within highly porous carbon electrodes through the formation of electrical double layer during the charging step, and then released into the solution by discharging the electrodes. By incorporating ion-exchange membranes (IEMs) in front of each electrode, the desalination performance can be significantly improved, achieving high salt adsorption capacity and charge efficiency. Over the past few years, we have made efforts to scale up the MCDI stacks for commercialization purposes, which are composed of 32 pairs of 400 mm × 200 mm activated carbon electrodes. Note that a stop-flow operation is applied in the discharge process to increase the water recovery (> 75%). A pilot-scale study of an MCDI system was performed to reclaim the secondary effluents from a municipal wastewater treatment plant. In addition, MCDI prototypes offer promising engineering opportunities for water recycling in high-tech industries, including reverse osmosis reject and wet scrubbing. Furthermore, we will discuss the challenges and perspectives for the future of MCDI as a cost-efficient, low-energy approach for desalination and resource recovery.

Mathematical Model Based on Staircase Structure for Impedance Analysis of Non-Ideal and Non-Uniform Processes in Porous Electrodes

Electrochemical impedance spectroscopy analysis for porous electrodes in energy conversion and storage, described as resistance and capacitance, is widely used to understand their electrochemical processes. These processes include: (i) electronic resistance inside the electrode (R e), (ii) ionic resistance in the electrolyte bulk and inside the porous electrode (R ion), (iii) electrical double layer formation (C dl) and charge transfer resistance at the electrode electrolyte interface (R ct) , and (iv) diffusion for charge compensation.As one of the mathematical models in electrochemical impedance spectroscopy for porous electrodes, a transmission line model is known, in which the pore structure is described as a cylinder geometry.1 We have proposed an analytical method that combines the transmission line model with electrochemical impedance spectroscopy using a symmetric cell,2 and have investigated resistance separation in lithium-ion batteries and its effect on battery performances.3-5 Due to mathematical constraints, the transmission line model is limited to a representation of an ideal, uniform process model. Here we propose a mathematical model based on a network model consisting of a staircase structure for electrochemical impedance spectroscopy at porous electrodes that can describe not only ideal and uniform processes but also non-ideal and non-uniform processes (Fig. 1), and aims to help elucidate their borderline behavior.6 The proposed staircase model (Z SCM) gives a series equivalent circuit consisting of Z 0, which is a series circuit of electrode/electrolyte interface impedance (Z int0) and electronic resistance (R e0), and electrolyte resistance (R ion0) as an initial step without pores (Z SCM0 = R ion(0) + Z 0). The one-step model (Z SCM1) is then given a series equivalent circuit consisting of the parallel circuit of Z SCM0 and Z 1 and the electrolyte resistance R ion(1) in series (Z SCM1 = R ion(1) + (Z SCM0 −1+Z1 −1)−1). The staircase model is calculated by incorporating the 1 prior step (Fig. 2). Thus, the model with n steps (Z SCM(n)) is composed of a series equivalent circuit of the parallel circuit of Z SCM(n−1) and Z n and the electrolyte resistance R ion(n) (Z SCM(n) = R ion(n) + (Z SCM(n−1) −1 +Z n −1)−1).Fig. 3 shows Nyquist plots computed for each input parameter at 50 steps for non-Faradic and Faradic processes in the staircase model. The results are in good agreement with the profiles calculated for the transmission electric model.2 The presentation will further show examples of the calculated results using the staircase model for non-ideal and inhomogeneous processes and discuss the analysis of impedance behavior using real electrodes reflecting non-uniform processes. References R. de Levie, Electrochim. Acta, 9, 1231-1245 (1964). N. Ogihara, S. Kawauchi, C. Okuda, Y. Itou, Y. Takeuchi and Y. Ukyo, J. Electrochem. Soc., 159, A1034-A1039 (2012). N. Ogihara, Y. Itou, T. Sasaki and Y. Takeuchi, J. Phys. Chem. C, 119, 4612-4619 (2015). N. Ogihara, Y. Itou and S. Kawauchi, J. Phys. Chem. Lett., 10, 5013-5018 (2019). Y. Itou, N. Ogihara and S. Kawauchi, J. Phys. Chem. C, 124, 5559-5564 (2020). N. Ogihara and Y. Itou, Phys. Chem. Chem. Phys., 24, 21863-21871 (2022). Figure 1

Electrochemically Induced Breakdown of Integrated Microsupercapacitors Based on Lipon

Lithium Phosphorous Oxynitride (LiPON) solid electrolyte has gathered a lot of attention due to its favorable ionic conductivity (10-6 Scm-1), good electronic insulation (10-14 Scm-1) and electrochemical stability[1]. Solid state microsupercapacitors based on LiPON have been demonstrated to meet demanding performances required for novel on-chip technologies[2,3]. The electric double layer (EDL) formation at the electrode interface results in the high capacitance and energy density of these devices. The EDL and to an extent the bulk of the electrolyte layer will be affected by the electric potential/field present during the operation. In this work, the electrochemical breakdown of microsupercapacitors were studied under different bias conditions. The influence of device geometry and test conditions on failure of LiPON based devices will provide the scope for potential applications as well.PVD depositions of thin LiPON films in MIM structure (20nm ,50nm,100nm) were fabricated for the study with Pt bottom electrode and Ti/TiN top electrodes. The devices were electrically characterised by Impedance spectroscopy and subjected to a voltage sweep from 0 to ±19V at various scan rates. LiPON was found to undergo electrochemical degradation followed by electrical breakdown as previously reported elsewhere[1]. Increasing the scan rate and thickness of LiPON, increased the breakdown voltage of LiPON. The electrode material influenced the degradation and the interface was found to evolve with successive voltage levels. The interface was found to drive the degradation using equivalent circuit analysis of Impedance spectroscopy results. Charge densities extracted for breakdown indicate possible electronic conduction and Li depletion, which were further verified with physico-chemical characterisation. A model of capacitive and resistive elements will be discussed to capture the individual contributions for electrochemically driven failure and thereby identify the parameters to tune or delay such mechanisms.This work identifies various factors which could modify electrochemical breakdown behaviour of LiPON. The modification of electrochemical breakdown voltage/strength and variation in charge density with scan rate has been newly identified. Further, resolving various contributions to different circuit elements will provide a way to understand and decorrelate their role in electrochemical breakdown.

Linking Electric Double Layer Formation to Electrocatalytic Activity

Linking Electric Double Layer Formation to Electrocatalytic Activity

Recent advances in the fixed-electrode capacitive deionization (CDI): Innovations in electrode materials and applications

Conventional capacitive deionization (CDI), originally developed as a promising desalination technique based on the formation of electric double layers (EDLs) at the surface of porous electrodes, has recently shown significant advancements. Notably, the utilization of pseudocapacitive electrodes has demonstrated the capability to enhance salt adsorption capacity (SAC) and selectivity of specific ions. This comprehensive review encapsulates the latest research endeavored in CDI, with an emphasis on electrode materials and their role in augmenting SAC. The development of electrode materials has broadened the scope of CDI applications on the water and wastewater treatment and resource recovery. Hence, we also evaluated the manifold applications of CDI, including resource recovery (e.g., lithium, phosphate and nitrate), anion removal (e.g., fluoride, sulfate, chromium and arsenic), water softening, and the removal of heavy metals (e.g., nickel, cesium, lead, copper, cadmium and chromium) in addition to the water desalination. Lastly, we provide a forward-looking perspective on forthcoming research directions and potential developments within the realm of CDI technology.

Electrical Double Layer Formation at Intercalation Cathode–Organic Electrolyte Interfaces During Initial Lithium‐Ion Battery Reactions

AbstractInformation on the cathode/organic–electrolyte interface structure provides clues regarding the rate and reversibility of lithium intercalation reactions in lithium‐ion batteries. Herein, structural changes within the LiCoO2 electrode, throughout the interphase region, and in the LiPF6/propylene carbonate electrolyte are observed concurrently by in situ neutron reflectometry. The formation of an electrical double layer (EDL) during the early stages of charging and discharging is investigated and compared with that at an intercalation‐inactive Nb:SrTiO3 electrode. At the intercalation‐inactive interface between Nb:SrTiO3 and the electrolyte, a voltage‐dependent ionic distribution corresponding to the EDL forms on the electrolyte side without the formation of a cathode/electrolyte interphase (CEI) layer. In contrast, at the intercalation‐active LiCoO2/electrolyte interface, a CEI layer forms immediately after cell construction, and the ionic distribution in the electrolyte formed outside the CEI layer scarcely changes upon voltage application. The CEI/electrolyte interface is shielded from potential changes by the electronically insulating CEI; therefore, structural changes in the EDL are restricted. This supports the prevailing understanding that the CEI layer defines the rates of solvation/de‐solvation and adsorption/desorption reactions of lithium ions.

Tracking Ion Transport in Nanochannels via Transient Single‐Particle Imaging

AbstractThe transport behavior of ions in the nanopores has an important impact on the performance of the electrochemical devices. Although the classical Transmission‐Line (TL) model has long been used to describe ion transport in pores, the boundary conditions for the applicability of the TL model remain controversial. Here, we investigated the transport kinetics of different ions, within nanochannels of different lengths, by using transient single‐particle imaging with temporal resolution up to microseconds. We found that the ion transport kinetics within short nanochannels may deviate significantly from the TL model. The reason is that the ion transport under nanoconfinement is composed of multi basic stages, and the kinetics differ much under different stage domination. With the shortening of nanochannels, the electrical double layer (EDL) formation would become the “rate‐determining step” and dominate the apparent ion kinetics. Our results imply that using the TL model directly and treating the in‐pore mobility as an unchanged parameter to estimate the ion transport kinetics in short nanopores/nanochannels may lead to orders of magnitude bias. These findings may advance the understanding of the nanoconfined ion transport and promote the related applications.

Tracking Ion Transport in Nanochannels via Transient Single-Particle Imaging.

The transport behavior of ions in the nanopores has an important impact on the performance of the electrochemical devices. Although the classical Transmission-Line (TL) model has long been used to describe ion transport in pores, the boundary conditions for the applicability of the TL model remain controversial. Here, we investigated the transport kinetics of different ions, within nanochannels of different lengths, by using transient single-particle imaging with temporal resolution up to microseconds. We found that the ion transport kinetics within short nanochannels may deviate significantly from the TL model. The reason is that the ion transport under nanoconfinement is composed of multi basic stages, and the kinetics differ much under different stage domination. With the shortening of nanochannels, the electrical double layer (EDL) formation would become the "rate-determining step" and dominate the apparent ion kinetics. Our results imply that using the TL model directly and treating the in-pore mobility as an unchanged parameter to estimate the ion transport kinetics in short nanopores/nanochannels may lead to orders of magnitude bias. These findings may advance the understanding of the nanoconfined ion transport and promote the related applications.

Potential of electrolyte-gated transistors for anionic molecule detection: proof of concept using dye solution

The use of electrolyte-gated transistors (EGTs) as sensors can be an advantageous alternative for the detection of anionic molecules due to their capability to detect various ions in solution. In this study, we explore the potential of EGTs as analytical tools for detecting anionic molecules, utilizing a copper phthalocyanine-3,4′,4″,4‴-tetrasulfonic acid tetrasodium salt (CuTsPc) solution as a proof of concept. The results demonstrate the EGT’s capacity in detecting CuTsPc in an aqueous solution, which molecule dissociates into sodium ions (Na+) and CuPc(SO3 −)4 ions, leading to high ionic conductivity and the formation of electrical double layers (EDLs). Varying the concentration of the molecule induced alterations in the EDLs, exhibiting good linearity and sensitivity in the transconductance, and a detection limit of 6.0 × 10−8 mol l−1. Transistors employing the CuTsPc solution as electrolyte operated at low voltages and performed better than water-gated transistors (W-GTs). The transconductance (gm ) value for EGTs using CuTsPc solution reached 1.93 mS, while for W-GTs it was around 0.10 mS. Thus, the CuTsPc solution not only serves as a target-molecule in sensor measurements, but also demonstrates potential as an electrolyte in EGTs, thereby assuming a dual role within the device. The main advantage of the EGTs as an analytical tool is their use as a multiparameter device that enables the detection of the analytes using different phenomena that occur at the EDLs interface and which, consequently, changes the device’s performance.

Monitoring the ion population at a carbon electrode/aqueous electrolyte interface at various pHs using electrochemical dilatometry

Changes in the electrode thickness in electrochemical capacitors depend strongly on the electrolyte pH. Hence, different ions (or ions with various solvation shells) participate in electrical double-layer formation. In this study, operando electrochemical dilatometry experiments incorporating various electrochemical techniques, including cyclic voltammetry, large amplitude sinusoidal voltammetry, and step potential electrochemical spectroscopy, were performed on ECs operating in 1 mol L−1 Li2SO4 with different pHs (6, 9, and 12) and 1 mol L-1 LiOH (for comparative purposes). A microporous carbon (Kuraray YP-80F) was used as the electrode material. Comparable trends for the cell behaviour were observed for electrolytes with pHs of 6 and 9; however, the cell containing the pH 12 solution exhibited similar behaviour to that containing the LiOH solution, suggesting that during positive polarization, the primary role was played by OH−, because the same salt concentration was used in all cases. This result raises an essential concern regarding the actual ion population at the electrode/electrolyte interface, which affects the water decomposition potentials and triggers various parasitic reactions impacting long-term electrochemical capacitor performance.

Assessing Local Electrical Properties of Ionic Liquid/Metal Interfaces with Operando-XPS and by Incorporating Additional Circuit Elements.

X-ray photoelectron spectroscopy (XPS) has been utilized to record binding energy changes upon applying direct current (DC) and/or alternating current (AC) (square-wave) bias with different frequencies on a coplanar capacitor, having an ionic liquid (IL) film as the electrolyte. Electrical potential developments in numerous locations on the device are extracted from the variations in binding energy positions of the atomic core levels, which together with electrochemical measurements are used to extract local information before and after insertion of additional resistors in series. The presence of the IL introduces complex charging/discharging processes with a direct influence on the electrical double layer (EDL) formation, some of which can be untangled from each other via AC modulation by choosing appropriate time windows of observation. Accordingly, under 10 kHz modulation, fast processes are sampled, which are associated with electronic currents, and effects of slow migratory currents can be measured using 0.1 Hz. The addition of serial resistors allows us to quantify AC currents passing through, which reveals the magnitude of the system's impedance under different conditions. This process surprisingly reverses differences(s) in the voltage developments between the low and high frequencies over the electrified electrodes compared to those over the porous membrane in between. Our approach turns XPS into a powerful electrical and surface-sensitive tool for extracting localized electrochemical properties in a noninvasive and direct way. We expect that a wider utilization of the technique will lead to better identification of the obstacles for developing the next-generation sensing, energy harvesting, and storage systems as well as devices for iontronic/neuromorphic applications.

Preparation of manganese-doped cubic carbon electrode based on ZIF-8 and its capacitive deionization performance for fluoride removal

Fluoride pollution will cause serious harm to the human ecological environment. The capacitive deionization method for fluoride removal has the advantages of low cost, low energy consumption, and convenient operation. Electrode material is crucial for desalination performance. In this study, a novel manganese doped hollow cubic carbon material (MHCC) was prepared by constructing ZIF-8 with hollow cubic morphology as a precursor and carbonizing it with manganese acetate as a manganese source. Then, the derived MHCC material was characterized and tested for capacitive fluoride removal. The adsorption capacity of MHCC10 (the mass ratio of carbon material to manganese acetate tetrahydrate was 10:1) reached 6.2 mg/g and 25.5 mg/g at the initial NaF concentration of 30 mg/L and 150 mg/L respectively. It could maintain 85.5% of the initial capacity after 10 adsorption–desorption cycles. The CDI defluoridation mechanism of MHCC was analyzed. The hollow porous structure inherited from etched ZIF-8 can promote the formation of electric double layers. An appropriate amount of manganese doping has a pseudocapacitive effect, which helps to increase adsorption capacity and improve cycling performance. This work provides a new approach for the design and preparation of novel capacitive fluoride removal electrode materials.