The compelling nature of the highly adapted functional surface structures found in biological systems accompanied by delicately tuned chemical processes, has inspired the design of materials with varied wetting properties and a vast range of applications. Identifying relations between surface structure, chemistry and wettability, is pivotal towards the mechanistic understanding of wetting phenomena. Here we demonstrate how electrochemically driven hydrogen adsorption/absorption induces, irreversibly, a superhydrophilic state in an amorphous CrFeNi-based multi-principal element alloy thin film with close to equimolar composition, i.e., in the class of medium/high entropy alloys. By employing films with sub-nanometer roughness to exclude the influence of geometry on wetting, we show that both the extent of wetting and its dynamics are governed by the rate of the underlying electrochemical reactions. The absorption of hydrogen into the matrix of the amorphous films as proved by thermal desorption spectroscopy, is proposed to partially protonate the electrochemically resilient Ta and Cr surface oxides through a hydrogen spillover phenomenon initiated by the adsorption of hydrogen on the electrochemically reduced Fe sites. Furthermore, atom probe tomography measurements reveal Cr segregation at the outermost surface layers of the film following cathodic treatment. The above processes strongly influence surface energetics resulting in the transition from a mildly hydrophilic state to an ultra-high substrate surface energy regime. Our work establishes a previously unknown physicochemical link between multi-principal element-alloys - hydrogen interactions and surface wettability, that is of significance in frontline research areas spanning electrochemical energy conversion/storage and catalysis to materials degradation and liquids actuation.

The nanoscale form of the Chevrel phase, Mo6S8, is demonstrated to be a highly efficient zinc-free anode in aqueous zinc ion hybrid supercapacitors (ZIHSCs). The unique morphological characteristics of the material when its dimensions approach the nanoscale result in fast zinc intercalation kinetics that surpass the ion transport rate reported for some of the most promising materials, such as TiS2 and TiSe2. In situ Raman spectroscopy, post-mortem X-ray diffraction, Hard X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations were combined to understand the overall mechanism of the zinc ion (de)intercalation process. The previously unknown formation of the sulfur-deficient Zn2.9Mo15S19 (Zn1.6Mo6S7.6) phase is identified, leading to a re-evaluation of the mechanism of the (de)intercalation process. A full cell comprised of an activated carbon (YEC-8A) positive electrode delivers a cell capacity of 38 mA h g-1 and an energy density of 43.8 W h kg-1 at a specific current density of 0.2 A g-1. The excellent cycling stability of the device is demonstrated for up to 8000 cycles at 3 A g-1 with a coulombic efficiency close to 100%. Post-mortem microscopic studies reveal the absence of dendrite formation at the nanosized Mo6S8 anode, in stark contrast to the state-of-the-art zinc electrode. Nanoscale tailoring of Chevrel phase Mo6S8 unveils its properties as a host for zinc ions. Rapid ion transport, coupled with the emergence of a previously unknown phase, transform the understanding of zinc ion (de)intercalation in these materials.

The wetting properties of carbon surfaces are important for a number of applications, including in electrochemistry. An under-studied area is the electrowetting properties of carbon materials, namely the sensitivity of wetting to an applied potential. In this work we explore the electrowetting behaviour of glassy carbon substrates and compare and contrast the observed response with our previous work using highly oriented pyrolytic graphite. As with the graphite substrate, "water-in-salt" electrolytes are found to suppress faradaic processes, thereby enlarging the electrochemical potential window. A notable difference in response to positive and negative polarity was seen for the graphite and glassy carbon substrates. Moreover, whereas graphite has previously been shown to give a reversible electrowetting response over many cycles, an irreversible wetting was observed for glassy carbon. Similarly, the timescales of the wetting process were much faster on the graphitic substrate. Reasons underlying these marked changes in behaviour on the different carbon surfaces are suggested.

Abstract Wetting phenomena underpin a plethora of key processes occurring in nature and artificial systems. Subtle control over wetting can boost the performance of devices with applications in diverse technologies, spanning electrochemical energy conversion, storage, and capacitive deionization to variable micro‐/nano‐fluidics platforms and electro(nano)tribology. A common characteristic of these systems is their operation under application of an electric field that induces a series of electrochemical processes that can impact wetting. The dependence of wetting on the applied potential bias is referred to as electrowetting. Carbon‐based materials hold a central role as both active electrodes and additives in such systems due to their distinct physicochemical properties and versatile characteristics. Deciphering the mechanisms of electrowetting on carbon in contact with electrolytes of varied types and compositions requires the fundamental understanding of the underlying processes taking place at the carbon/electrolyte interface. Simultaneously, a systematic investigation of electrowetting can often provide significant insights into the properties of the interface. In this concept article, we discuss the latest advances in the area along with the current challenges and future directions of this recently revived research topic.

The physical electrochemistry of the carbon/ionic liquids interface underpins the processes occurring in a vast range of applications spanning electrochemical energy storage, iontronic devices, and lubrication. Elucidating the charge storage mechanisms at the carbon/electrolyte interface will lead to a better understanding of the operational principles of such systems. Herein, we probe the charge stored at the electrochemical double layer formed between model carbon systems, ranging from single-layer graphene to graphite and the ionic liquid 1-ethyl-3-methyl-imidazolium bis(trifluoro-methylsulfonyl)imide (EMIM-TFSI). The effect of the number of graphene layers on the overall capacitance of the interface is investigated. We demonstrate that in pure EMIM-TFSI and at moderate potential biases, the electronic properties of graphene and graphite govern the overall capacitance of the interface, while the electrolyte contribution to the latter is less significant. In mixtures of EMIM-TFSI with solvents of varying relative permittivity, the complex interplay between electrolyte ions and solvent molecules is shown to influence the charge stored at the interface, which under certain conditions overcomes the effects of relative permittivity. This work provides additional experimental insights into the continuously advancing topic of electrochemical double-layer structure at the interface between room temperature ionic liquids and carbon materials.

The effect of potential bias on the wettability of graphite using immiscible electrolyte solutions (ITIES) is investigated. The mechanism of the phenomenon is elucidated by decoupling the effects of the electrochemical processes, i.e., double layer charging/discharging, adsorption of ions and charge/ion transfer reactions occurring at the graphite|aqueous electrolyte, graphite|non-aqueous electrolyte and liquid|liquid interfaces, on the electrowetting response. The derived findings are interpreted in terms of potential-induced changes in the total free energy of the system. On this basis, a complete phenomenological model is presented for electrowetting directly on conductors using an ITIES that combines the established interfacial theories of the solid|liquid and liquid|liquid interfaces. Our work should complement the existing, relatively scarce literature in the area and contribute to the design of practical systems, such as low voltage optofluidic devices, that exploit the properties of the ITIES.

Abstract High‐entropy (HE) metal chalcogenides are a class of materials that have great potential in applications such as thermoelectrics and electrocatalysis. Layered 2D transition‐metal dichalcogenides (TMDCs) are a sub‐class of high entropy metal chalcogenides that have received little attention to date as their preparation currently involves complicated, energy‐intensive, or hazardous synthetic steps. To address this, a low‐temperature (500 °C) and rapid (1 h) single source precursor approach is successfully adopted to synthesize the hexernary high‐entropy metal disulfide (MoWReMnCr)S2. (MoWReMnCr)S2 powders are characterized by powder X‐ray diffraction (pXRD) and Raman spectroscopy, which confirmed that the material is comprised predominantly of a hexagonal phase. The surface oxidation states and elemental compositions are studied by X‐ray photoelectron spectroscopy (XPS) whilst the bulk morphology and elemental stoichiometry with spatial distribution is determined by scanning electron microscopy (SEM) with elemental mapping information acquired from energy‐dispersive X‐ray (EDX) spectroscopy. The bulk, layered material is subsequently exfoliated to ultra‐thin, several‐layer 2D nanosheets by liquid‐phase exfoliation (LPE). The resulting few‐layer HE (MoWReMnCr)S2 nanosheets are found to contain a homogeneous elemental distribution of metals at the nanoscale by high angle annular dark field‐scanning transmission electron microscopy (HAADF‐STEM) with EDX mapping. Finally, (MoWReMnCr)S2 is demonstrated as a hydrogen evolution electrocatalyst and compared to 2H‐MoS2 synthesized using the molecular precursor approach. (MoWReMnCr)S2 with 20% w/w of high‐conductivity carbon black displays a low overpotential of 229 mV in 0.5 M H2SO4 to reach a current density of 10 mA cm−2, which is much lower than the overpotential of 362 mV for MoS2. From density functional theory calculations, it is hypothesised that the enhanced catalytic activity is due to activation of the basal plane upon incorporation of other elements into the 2H‐MoS2 structure, in particular, the first row TMs Cr and Mn.

Electrowetting is a simple way to induce the spreading and retraction of electrolyte droplets. This method is widely used in "device" applications, where a dielectric layer is applied between the electrolyte and the conducting substrate. Recent work, including contributions from our own laboratory, have shown that reversible electrowetting can be achieved directly on conductors. We have shown that graphite surfaces, in particular when combined with highly concentrated electrolyte solutions, show a strong wetting effect. The process is driven by the interactions between the electrolyte ions and the surface, hence models of double-layer capacitance are able to explain changes in the equilibrium contact angles. Herein, we extend the approach to the investigation of electrowetting on graphene samples of varying thickness, prepared by chemical vapor deposition. We show that the use of highly concentrated aqueous electrolytes induces a clear yet subtle electrowetting response due to the adsorption of ions and the suppression of the negative effect introduced by the surface impurities accumulating during the transfer process. The latter have been previously reported to fully hinder electrowetting at lower electrolyte concentrations. An amplified wetting response is recorded in the presence of strongly adsorbed/intercalated anions in both aqueous and non-aqueous electrolytes. The phenomenon is interpreted based on the anion-graphene interactions and their influence on the energetics of the interface. By monitoring the dynamics of wetting, an irreversible behaviour is identified in all cases as a consequence of the irreversibility of anion adsorption and/or intercalation. Finally, the effect of the underlying reactions on the timescales of wetting is also examined.

Deciphering the mechanisms of charge storage on carbon-based materials is pivotal for the development of next generation electrochemical energy storage systems. Graphene, the building block of graphitic electrodes, is an ideal model for probing such processes on a fundamental level. Herein, we investigate the thermodynamics of the graphene/aqueous electrolyte interface by utilizing a multiscale quantum mechanics – classical molecular dynamics (QM/MD) approach to provide insights into the effect of alkali metal ion (Li+) concentration on the interfacial tension (γSL) of the charged graphene/electrolyte interface. We demonstrate that the dependence of γSL on the applied surface charge exhibits an asymmetric behaviour relative to the neutral surface. At the positively charged graphene sheet, the electrowetting response is amplified by electrolyte concentration, resulting in a strongly hydrophilic surface. On the contrary, at negative potential bias, γSL shows a weaker response to the charging of the electrode. Changes in γSL greatly affect the total areal capacitance predicted by the Young-Lippmann equation but have negligible impact on the simulated total areal capacitance indicating that the EDL structure is not directly correlated with the wettability of the surface and different interfacial mechanisms drive the two phenomena. The proposed model is validated experimentally by studying the electrowetting response of highly oriented pyrolytic graphite over a wide range of electrolyte concentrations. Our work presents the first combined theoretical and experimental study on electrowetting using carbon surfaces, introducing new conceptual routes for the investigation of wetting phenomena under potential bias.

Control of wetting on conducting surfaces using external stimuli such as electricity, underlies the operation of devices in a broad range of technologies including micro-/nano-fluidics, energy conversion/storage and filtration systems. Electrowetting is the change in contact angle of a liquid relative to its equilibrium value upon application of a potential bias. The phenomenon, being identified almost a century ago, is fundamentally an electrochemical process. However, the majority of the most recent research in this area focuses on electrowetting from the "applications" perspective. Device-based electrowetting uses substrates with insulating overlayers to eliminate charge transfer at the solid|liquid interface and hence suppress the electrochemical character of the overall process. In this short review, we focus on electrowetting directly on conductors and discuss the purely electrochemical aspects of the phenomenon along with the open questions related to this rejuvenated topic.

The unique layered structure of graphite with its tunable interlayer distance establishes almost ideal conditions for the accommodation of ions into its structure. The smooth and chemically inert nature of the graphite surface also means that it is an ideal substrate for electrowetting. Here, we combine these two unique properties of this material by demonstrating the significant effect of anion intercalation on the electrowetting response of graphitic surfaces in contact with concentrated aqueous and organic electrolytes as well as ionic liquids. The structural changes during intercalation/deintercalation were probed using in situ Raman spectroscopy, and the results were used to provide insights into the influence of intercalation staging on the rate and reversibility of electrowetting. We show, by tuning the size of the intercalant and the stage of intercalation, that a fully reversible electrowetting response can be attained. The approach is extended to the development of biphasic (oil/water) systems that exhibit a fully reproducible electrowetting response with a near-zero voltage threshold and unprecedented contact angle variations of more than 120° within a potential window of less than 2 V.

Multicomponent as well as high-entropy-based nitrides have received increasing interest in the field of materials science and engineering. The structural characteristics of these compounds result in a mix of covalent, metallic, and ionic bonds that give rise to a number of attractive properties including high hardness, electrical and thermal conductivities as well as chemical stability. These properties render these materials promising candidates for various industrial applications involving harsh operating conditions. Herein, the corrosion resistances of dc magnetron sputtered nitrogen-containing TiZrTaNby thin films with Nb content ranging from 8.0 to 24.5 at% have been investigated to provide insights regarding the corrosion resistances of multicomponent systems containing more than one passive element. The corrosion resistances and anodic behavior of the films were examined by electrochemical means in 0.1 M H2SO4 and 0.1 M HCl solutions. The results demonstrate that despite the significant differences in the concentration of one of the two main passive elements in the films i.e., Nb, the corrosion resistance did not differ significantly between the films. To provide insights into this phenomenon, the surface chemical state and composition of the prepared films were probed using X-ray photoelectron spectroscopy. It was shown that all samples exhibited Ta-rich surfaces after positive polarization up to 3.0 V vs. Ag/AgCl (3 M NaCl) as a result of the anodic dissolution of Zr and Ti. The thickness of the oxide layer formed upon different anodic polarization was studied using transmission electron microscopy, while complementary electrochemical impedance studies were performed. The extent of Nb dissolution from the surface of the films was, on the other hand, found to be small. These findings highlight the dominant role of Ta in the passivation of the films and demonstrate the minor effect of Nb concentration on the corrosion resistances of the films. However, it was demonstrated that the presence of Nb was still important for the corrosion resistance of the films above 1.4 V vs. Ag/AgCl (3 M NaCl), when replacing Nb with Cr, due to transpassive dissolution of Cr. These results facilitate the design of highly corrosion resistant multicomponent nitrides containing more than one passive element.(c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Wetting of carbon surfaces is one of the most widespread, yet poorly understood, physical phenomena. Control over wetting properties underpins the operation of aqueous energy-storage devices and carbon-based filtration systems. Electrowetting, the variation in the contact angle with an applied potential, is the most straightforward way of introducing control over wetting. Here, we study electrowetting directly on graphitic surfaces with the use of aqueous electrolytes to show that reversible control of wetting can be achieved and quantitatively understood using models of the interfacial capacitance. We manifest that the use of highly concentrated aqueous electrolytes induces a fully symmetric and reversible wetting behavior without degradation of the substrate within the unprecedented potential window of 2.8 V. We demonstrate where the classical "Young-Lippmann" models apply, and break down, and discuss reasons for the latter, establishing relations among the applied bias, the electrolyte concentration, and the resultant contact angle. The approach is extended to electrowetting at the liquid|liquid interface, where a concentrated aqueous electrolyte drives reversibly the electrowetting response of an insulating organic phase with a significantly decreased potential threshold. In summary, this study highlights the beneficial effect of highly concentrated aqueous electrolytes on the electrowettability of carbon surfaces, being directly related to the performance of carbon-based aqueous energy-storage systems and electronic and microfluidic devices.

The development of intrinsically safe and environmentally sustainable energy storage devices is a significant challenge. Recent advances in aqueous rechargeable lithium-ion batteries (ARLIBs) have made considerable steps in this direction. In parallel to the ongoing progress in the design of aqueous electrolytes that expand the electrochemically stable potential window, the design of negative electrode materials exhibiting large capacity and low intercalation potential attracts great research interest. Herein, we report the synthesis of high purity nanoscale Chevrel Phase (CP) Mo6S8via a simple, efficient and controllable molecular precursor approach with significantly decreased energy consumption compared to the conventional approaches. Physical characterization of the obtained product confirms the successful formation of CP-Mo6S8 and reveals that it is crystalline nanostructured in nature. Due to their unique structural characteristics, the Mo6S8 nanocubes exhibit fast kinetics in a 21 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte as a result of the shorter Li+ ion diffusion distance. Full battery cells comprised of Mo6S8 and LiMn2O4 as negative and positive electrode materials, respectively, operate at 2.23 V delivering a high energy density of 85 W h kg-1 (calculated on the total mass of active materials) under 0.2 C-rate. At 4 C, the coulombic efficiency (CE) is determined to be 99% increasing to near 100% at certain cycles. Post-mortem physical characterization demonstrates that the Mo6S8 anode maintained its crystallinity, thereby exhibiting outstanding cycling stability. The cell outperforms the commonly used vanadium-based (VO2 (B), V2O5) or (NASICON)-type LiTi2(PO4)3 anodes, highlighting the promising character of the nanoscale CP-Mo6S8 as a highly efficient anode material. In summary, the proposed synthetic strategy is expected to stimulate novel research towards the widespread application of CP-based materials in various aqueous and non-aqueous energy storage systems.

The structure of the electrical-double layer of the graphene/aqueous–electrolyte interface is challenging to model and characterize experimentally. Here we review the published data and discuss their interpretation in the context of the EDL theory.

The scarcity and cost of noble metals used in commercial electrolyzers limit the sustainability and scalability of water electrolysis for green hydrogen production. Herein, we report the ultralow loading of Au nanoparticles onto MoS2 electrodes by the spontaneous process of galvanic deposition. AuNP@MoS2 electrode synthesis was optimized, and electrodes containing the smallest Au nanoparticle diameter (2.9 nm) and the lowest Au loading (0.044 mu g cm(-2)) exhibited the best overall and intrinsic electrocatalytic performance. This enhancement is attributed to an increased Au-MoS2 interaction with smaller nanoparticles, making the MoS2 electrode more n-type. DC electrochemical characterization for the AuNP@MoS2 electrodes showed an exchange current density of 7.28 mu A cm(-2) and an overpotential at 10 mA cm(-2) of -323 mV. These values are 4.5 times higher and 100 mV lower than those of the unmodified MoS2 electrode, respectively. Electrochemical AC experiments were used to evaluate the electrodes' intrinsic catalytic activity, and it was shown that the AuNP@MoS2 electrodes exhibited an enhanced activity by as much as 3.5 times compared with MoS2. Additionally, the turnover frequency as estimated by the reciprocal of the RctCdl product, the latter calculated from the AC data, is estimated to be 58.8 s(-1) and is among one of the highest reported for composite MoS2 materials.

Developing inexpensive, earth abundant materials for electrocatalytic and photoelectrochemical water splitting is critical to the decarburization of our energy system. Aerosol-assisted chemical vapor deposition (AACVD) is a potentially scalable, one-step synthesis technique that can produce large area, homogenous edge-aligned films of transition metal dichalcogenides including MoS2. Herein, we report a hydrogen evolution reaction (HER) study on such electrode films under acidic conditions. Using iR compensated near-steady state DC current-potential measurements and linear sweep voltammetry, the HER onset potential is recorded at -175 mV vs. RHE (current density 0.1 mA cm(-2)), while a current density of 10 mA cm(-2) per nominal area is reached at an overpotential of -430 mV vs. RHE. A Tafel slope of 113 mV dec(-1) suggests the existence of increased edge-active sites on the surface of 2H-MoS2. The electrodes offered high stability over 5000 cycles, which continued to improve with cycling, with the current density at -350 mV improved by a factor exceeding 20 between the 1st and 5000th cycle. X-ray photoelectron spectroscopy, contact angle measurements and electrochemical impedance spectroscopy were employed to provide thorough insights on the changes in the intrinsic properties of the material, which led to the observed HER activity enhancement with cycling. It is shown that a reduction in the surface oxide layer and increased electrowetting are the reasons for the improved performance. Electrodes of differing thicknesses were prepared and their performance was compared in association with the surface morphology. The best performing electrode had the lowest mass loading of MoS2, which is attributed to improved faradaic efficiency through the resistive 2H-MoS2 electrode. (C) 2021 Elsevier Ltd. All rights reserved.

A simple, alternative, catalyst preparation method was proposed in order to combine the properties of photoactive and stable TiO2 with state-of-the art of IrO2 for the oxygen evolution reaction (OER). IrO2 nanoparticles were obtained in the process of photodeposition on the surface of TiO2 powder by UV illumination from appropriate Ir salt aqueous solution. Physicochemical characterization of the resulting IrO2/TiO2 composite, containing 25% w/w Ir, was carried out by transmission electron microscopy (TEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical behaviour of the prepared IrO2/TiO2 catalyst was compared with that of commercial unsupported IrO2. Cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry (CA) were performed to identify the surface electrochemistry and OER of the IrO2/TiO2 catalyst. Electrochemical impedance spectroscopy (EIS) was used to determine uncompensated and charge transfer resistance. CA experiments were carried out in order to evaluate the stability of the IrO2/TiO2 composite during OER in the dark and under UV light irradiation. The photodeposition method on a TiO2 support resulted in 1-2 nm Ir nanoparticles, very well dispersed on the surface and in their oxidized state (IrO2). Electrochemical results indicated that despite its lower conductivity, the IrO2/TiO2 composite exhibits comparable intrinsic electrocatalytic activity for OER with that of the commercial IrO2 catalyst. It was found that under UV light irradiation there has been improvement of the stability of the IrO2/TiO2 performance during oxygen evolution (presumably due to sustained activation of reactive species via photogenerated holes or OH radicals) as well as current enhancement (due to the interaction between the photogenerated holes in TiO2 support and IrO2 nanoparticles). These would be beneficial effects in prolonged water photo-electrolysis.

Developing a simple, safe, and efficient route for the preparation of nanoparticulate ternary Chevrel phases MxMo6S8 (CPs; where M = metal) is of great interest because of their applications in energy conversion and storage technologies. Currently, the wide use of these materials is restricted by the prolonged reaction time, the high energy demands required for their synthesis, the complexity of the preparation process, and the ambiguity in the size of the resultant particles. Herein, we report a simple, efficient, and controllable molecular precursor approach for the synthesis of nanoscale CPs without the use of hydrogen gas as a reducing agent. A mixture of precursors based on molybdenum and copper dithiocarbamate complexes was subjected to thermolysis in the presence of finely divided molybdenum to furnish the copper CP, Cu2Mo6S8. The successful formation of the Cu2Mo6S8 CP is confirmed by X-ray diffraction analysis and Raman spectroscopy, while the surface chemistry of the material was examined by X-ray photoelectron spectroscopy photon depth profiling via tunable synchrotron radiation. Microscopic characterization results demonstrate that the synthesized material has a homogeneous structure at the nanoscale, in contrast to the microparticles obtained from conventional approaches previously reported. The prepared CP was assessed as an electrocatalyst for the hydrogen evolution reaction in acidic media. Because of its unique nanoscale texturing, the Cu-leached CP, Mo6S8, exhibits a highly promising electrocatalytic activity toward hydrogen evolution with an overpotential required to reach a current density of 10 mA cm(-2) equal to 265 mV versus reversible hydrogen electrode. The overpotential reduces to 232 mV upon mixing of the catalyst with 20% w/w of high-conductivity carbon. It is expected that the proposed synthetic strategy, which represents a facile route to tailored CPs, can be extended to the preparation of versatile, easily tunable CP Mo6S8-based electrode materials for applications in electrocatalysis.