Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite (original) (raw)
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The Journal of Chemical Physics
Understanding the microscopic behavior of aqueous electrolyte solutions in contact with graphene and related carbon surfaces is important in electrochemical technologies, such as capacitive deionization or supercapacitors. In this work, we focus on preferential adsorption of ions in mixed alkali–halide electrolytes containing different fractions of Li+/Na+ or Li+/K+ and/or Na+/K+ cations with Cl− anions dissolved in water. We performed molecular dynamics simulations of the solutions in contact with both neutral and positively and negatively charged graphene surfaces under ambient conditions, using the effectively polarizable force field. The simulations show that large ions are often intuitively attracted to oppositely charged electrodes. In contrast, the adsorption behavior of small ions tends to be counterintuitive. In mixed-cation solutions, one of the cations always supports the adsorption of the other cation, while the other cation weakens the adsorption of the first cation. In...
The Journal of Physical Chemistry Letters
At an electrode, ions and solvent accumulate to screen charge, leading to a nanometer-scale electric double layer (EDL). The EDL guides electrode passivation in batteries, while in (super)capacitors, it determines charge storage capacity. Despite its importance, quantification of the nanometer-scale and potential-dependent EDL remains a challenging problem. Here, we directly probe changes in the EDL composition with potential using in situ vibrational spectroscopy and molecular dynamics simulations for a Li-ion battery electrolyte (LiClO 4 in dimethyl carbonate). The accumulation rate of Li + ions at the negative surface and ClO 4 − ions at the positive surface from vibrational spectroscopy compares well to that predicted by simulations using a polarizable APPLE&P force field. The ion solvation shell structure and ion-pairing within the EDL differs significantly from the bulk, especially at the negative electrode, suggesting that the common rationalization of interfacial electrochemical processes in terms of bulk ion solvation should be applied with caution. 50 depend on the availability of specific species at the surface and 51 their partitioning within the EDL. 52 Previous simulations 15−17 and experiments 18−22 revealed 53 strongly modulated electrolyte compositions next to the 54 electrode, which influence many ion-related processes, 55 including ion transport, desolvation, charge transfer, and 56 insertion into the electrode. For example, areas within the 57 EDL of increased local ion density will alter interfacial 58 desolvation rates. Further, during the initial steps of SEI 59 formation at the graphite−electrolyte interface, Li ions will 60 either intercalate with a partial solvation shell or shed their 61 solvation shell completely. 23 Both processes are strongly 62 influenced by the ion density and the Li + solvation shell 63 composition within the EDL. At the cathode−electrolyte 64 interface, preferential ion and solvent adsorption/desorption 65 were suggested as one of the factors responsible for the
The chemisorption of species from supporting electrolytes on electrode surfaces is ubiquitous in electrochemical systems and affects the dynamics and mechanism of various electrochemical reactions. The understanding of chemical structure and property of the resulting electrical double layer is vital but limited. In this work, we operando probed the electrochemical interface between a gold electrode surface and a common supporting electrolyte, phosphate buffer, using our newly developed in situ liquid secondary ion mass spectrometry during dynamic potential scanning. We surprisingly found that on the positively charged gold electrode surface sodium cations coexisted within the inner Helmholtz layer to form ion pairs with the accumulated phosphate anions, resulting in a strong and dense adsorption phase which was further revealed to retard the electro-oxidation reaction of ascorbate. This finding addressed one major gap in the fundamental science of the electrode-electrolyte interface...
Langmuir, 2010
Efforts were made to differentiate the contributions to the so-called "ion transfer" barrier at the electrolyte/graphite junction from two distinct processes: (1) desolvation of Li þ before it enters graphene interlayer and (2) the subsequent migration of bare Li þ through the ad hoc interphase. By leveraging a scenario where no substantial interphase was formed on Li þ intercalation hosts, we were able to quantify the distribution of "ion transfer" activation energy between these two interfacial processes and hence identify the desolvation process of Li þ as the major energy-consuming step. The result confirmed the earlier belief that the rate-determining step in the charging of a graphitic anode in Li þ intercalation chemistry relates to the stripping of solvation sheath of Li þ , which is closely interwoven with the interphasial resistance to Li þ migration.
Spontaneous adsorption of ions on graphene at the electrolyte–graphene interface
Applied Physics Letters, 2020
We report the spontaneous adsorption of ions on graphene at the interface with electrolytes through an investigation based on the electrolyte-gated field effect transistor configuration. It is found that the gate voltage at which the minimum conductivity occurs in these devices is highly sensitive to the type of ions and their concentrations in the electrolytes; yet the experimental results exhibit non-trivial deviations from the predictions based on the Gouy–Chapman–Stern (GCS) model, which only takes account of the electrostatic interactions among the charges in the system. By incorporating a Langmuir-type adsorption term into the GCS model, we achieve quantitative alignment with the experiments, thus demonstrating that these deviations originate from the spontaneous adsorption of ions onto graphene. Analysis of the transport characteristics in these devices indeed confirms the existence of the adsorbed ions.
Selective transport of water molecules through interlayer spaces in graphite
Nature Communications, 2022
Interlayer space in graphite is impermeable to ions and molecules, including protons. Its controlled expansion would find several applications in desalination, gas purification, high-density batteries, etc. In the past, metal intercalation has been used to modify graphitic interlayer spaces; however, resultant intercalation compounds are unstable in water. Here, we successfully expanded graphite interlayer spaces by intercalating aqueous KCl ions electrochemically. Our spectroscopy studies provide clear evidence for cation-π interactions explaining the stability of the devices, though weak anion-π interactions were also detectable. The water conductivity shows several orders of enhancement when compared to unintercalated graphite. Water evaporation experiments further confirm the high permeation rate. There is weak ion permeation through interlayer spaces, up to the highest chloride concentration of 1 M, an indication of sterically limited transport. In these very few transported io...
ACS Applied Materials & Interfaces, 2021
Dual-ion batteries (DIBs) generally operate beyond 4.7 V vs Li + /Li 0 and rely on the intercalation of both cations and anions in graphite electrodes. Major challenges facing the development of DIBs are linked to electrolyte decomposition at the cathode−electrolyte interface (CEI), graphite exfoliation, and corrosion of Al current collectors. In this work, X-ray photoelectron spectroscopy (XPS) is employed to gain a broad understanding of the nature and dynamics of the CEI built on anionintercalated graphite cycled both in highly concentrated electrolytes (HCEs) of common lithium salts (LiPF 6 , LiFSI, and LiTFSI) in carbonate solvents and in a typical ionic liquid. Though Al metal current collectors were adequately stable in all HCEs, the Coulombic efficiency was substantially higher for HCEs based on LiFSI and LiTFSI salts. Specific capacities ranging from 80 to 100 mAh g −1 were achieved with a Coulombic efficiency above 90% over extended cycling, but cells with LiPF 6-based electrolytes were characterized by <70% Coulombic efficiency and specific capacities of merely ca. 60 mAh g −1. The poor performance in LiPF 6-containing electrolytes is indicative of the continual buildup of decomposition products at the interface due to oxidation, forming a thick interfacial layer rich in Li x PF y , PO x F y , Li x PO y F z , and organic carbonates as evidenced by XPS. In contrast, insights from XPS analyses suggested that anion intercalation and deintercalation processes in the range from 3 to 5.1 V give rise to scant or extremely thin surface layers on graphite electrodes cycled in LiFSI-and LiTFSI-containing HCEs, even allowing for probing anions intercalated in the near-surface bulk. In addition, ex situ Raman, SEM and TEM characterizations revealed the presence of a thick coating on graphite particles cycled in LiPF 6-based electrolytes regardless of salt concentration, while hardly any surface film was observed in the case of concentrated LiFSI and LiTFSI electrolytes.
Unravelling the electrochemical double layer by direct probing of the solid/liquid interface
Nature communications, 2016
The electrochemical double layer plays a critical role in electrochemical processes. Whilst there have been many theoretical models predicting structural and electrical organization of the electrochemical double layer, the experimental verification of these models has been challenging due to the limitations of available experimental techniques. The induced potential drop in the electrolyte has never been directly observed and verified experimentally, to the best of our knowledge. In this study, we report the direct probing of the potential drop as well as the potential of zero charge by means of ambient pressure X-ray photoelectron spectroscopy performed under polarization conditions. By analyzing the spectra of the solvent (water) and a spectator neutral molecule with numerical simulations of the electric field, we discern the shape of the electrochemical double layer profile. In addition, we determine how the electrochemical double layer changes as a function of both the electroly...