Simulations employing finite element method at liquid|liquid interfaces

Simulated curves compared to recorded data have provided a breadth of insight into mechanisms and kinetic aspects of charge transfer at the liquid|liquid interface (LLI). This is often performed with software employing finite element methods (FEMs). The advent and application of this asset to soft interfacial chemistry has allowed a more facile exploration of geometric considerations, the role of interfacial size (from macro to nano), while simultaneously expanding to include homo/heterogeneous reactions such as electrocatalytic, photochemical, nanoparticle interactions, etc. This article provides insight into the status of the field of LLI FEM studies as well as a perspective as to what role simulations and numerical analysis will play in the future

Electrocatalysis at the polarised interface between two immiscible electrolyte solutions

Electrocatalysis at the interface between two immiscible elec?trolyte solutions (ITIES) is an emerging field of research, which allows the separation of reactants according to their lipophilicity. Electrocatalysts of various natures (noble metals, carbon-based and inorganic nanomaterials, enzymes, and supramolecular ensembles) are assembled at the ITIES, either spontaneously or following the application of an interfacial Galvani potential difference. While primarily used for the electrocatalysis of the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), recent work has focused on the electrocatalysis of the oxygen evolution reaction (OER) and the electrocatalytic oxidation of elemental sulfur (S8) and an organosulfur compound. Protocols to compare electrocatalytic performances at the ITIES call for careful data analysis and a detailed knowledge of the catalyst’s morphological parameters (e.g., active surface area and catalyst loading). However, standardisation of such protocols at the ITIES has yet to be implemented and is required to allow better comparison of the results from individual biphasic systems.</p

Gold nanofilms at liquid-liquid interfaces: an emerging platform for redox electrocatalysis, nanoplasmonic sensors, and electrovariable optics

The functionality of liquid-liquid interfaces formed between two immiscible electrolyte solutions (ITIES) can be markedly enhanced by modification with supramolecular assemblies or solid nanomaterials. The focus of this review is recent progress involving ITIES modified with floating assemblies of gold nanoparticles or “nanofilms”. Experimental methods to controllably modify liquid-liquid interfaces with gold nanofilms are detailed. Also, we outline an array of techniques to characterise these gold nanofilms in terms of their physiochemical properties (such as reflectivity, conductivity, catalytic activity or plasmonic properties) and physical interfacial properties (for example, interparticle spacing and immersion depth at the interface). The ability of floating gold nanofilms to impact a diverse range of fields is demonstrated: in particular redox electrocatalysis, surface-enhanced Raman spectroscopy (SERS) or surface plasmon resonance (SPR) based sensors, and electrovariable optical devices. Finally, perspectives on applications beyond the state-of-the-art are provided

Gold nanofilm redox catalysis for oxygen reduction at soft interfaces

.Functionalization of a soft or liquid-liquid interface by a one gold nanoparticle thick “nanofilm” provides a conductive pathway to facilitate interfacial electron transfer from a lipophilic electron donor to a hydrophilic electron acceptor in a process known as interfacial redox catalysis. The gold nanoparticles in the nanofilm are charged by Fermi level equilibration with the lipophilic electron donor and act as an interfacial reservoir of electrons. Additional thermodynamic driving force can be provided by electrochemically polarising the interface. Using these principles, the biphasic reduction of oxygen by a lipophilic electron donor, decamethylferrocene, dissolved in α,α,α-trifluorotoluene was catalysed at a gold nanoparticle nanofilm modified water-oil interface. A recently developed microinjection technique was utilised to modify the interface reproducibly with the mirror-like gold nanoparticle nanofilm, while the oxidised electron donor species and the reduction product, hydrogen peroxide, were detected by ion transfer voltammetry and UV/vis spectroscopy, respectively. Metallization of the soft interface allowed the biphasic oxygen reduction reaction to proceed via an alternative mechanism with enhanced kinetics and at a significantly lower overpotential in comparison to a bare soft interface. Weaker lipophilic reductants, such as ferrocene, were capable of charging the interfacial gold nanoparticle nanofilm but did not have sufficient thermodynamic driving force to significantly elicit biphasic oxygen reductio

Interfacial redox catalysis on gold nanofilms at soft interfaces

Soft or “liquid–liquid” interfaces were functionalized by roughly half a monolayer of mirror-like nanofilms of gold nanoparticles using a precise interfacial microinjection method. The surface coverage of the nanofilm was characterized by ion transfer voltammetry. These gold nanoparticle films represent an ideal model system for studying both the thermodynamic and kinetic aspects of interfacial redox catalysis. The electric polarization of these soft interfaces is easily controllable, and thus the Fermi level of the electrons in the interfacial gold nanoparticle film can be easily manipulated. Here, we study interfacial redox catalysis between two redox couples located in adjacent immiscible phases and highlight the catalytic properties of a gold nanoparticle film toward heterogeneous electron transfer reactions

Membraneless energy conversion and storage using immiscible electrolyte solutions

Breakthrough alternative technologies are urgently required to alleviate the critical need to decarbonise our energy supply. We showcase non-conventional approaches to battery and solar energy conversion and storage (ECS) system designs that harness key attributes of immiscible electrolyte solutions, especially the membraneless separation of redox active species and ability to electrify certain liquid–liquid interfaces. We critically evaluate the recent development of membraneless redox flow batteries based on biphasic systems, where one redox couple is confined to an immiscible ionic liquid or organic solvent phase, and the other couple to an aqueous phase. Common to all solar ECS devices are the abilities to harvest light, leading to photo-induced charge carrier separation, and separate the products of the photo-reaction, minimising recombination. We summarise recent progress towards achieving this accepted solar ECS design using immiscible electrolyte solutions in photo-ionic cells, to generate redox fuels, and biphasic “batch” water splitting, to generate solar fuels

Closed bipolar electrochemistry in a four-electrode configuration

Closed bipolar electrochemistry in a 4-electrode configuration is a highly versatile, but under-utilized, technique with major potential to emerge as a powerful methodology impacting areas as diverse as spectro-electroanalysis, energy storage, electrocatalysis and electrodeposition. In this perspective, we provide the thermodynamic framework for understanding all such future applications of closed bipolar electrochemistry in a 4-electrode configuration. We distinguish the differences between open and closed bipolar electrochemical cells. In particular, the use of the 4-electrode configuration in both open and closed bipolar electrochemical cells with immiscible aqueous-organic solutions is outlined. A comprehensive overview of the influence of external bias on the thermodynamics underpinning electron transfer from an organic redox couple to an aqueous redox couple, or vice versa, by electrons flowing along a conducting bipolar electrode serving as an electronic bridge is provided. Fermi level equilibration between redox species at opposite poles of a bipolar electrode under external bias is discussed. The concept of the Line of Zero Overpotential (LZO) on the bipolar electrode at steady-state conditions under an external bias is introduced. The influence of a series of experimental variables (redox potential of each redox couple, rate constant of electron transfer at each pole, an excess bulk concentration of one redox couple over the other, and areas of the poles of the bipolar electrode in contact with each electrolyte solution) on the final position of the LZO on the bipolar electrode is highlighted. A cyclic voltammogram obtained using a closed bipolar electrochemical cell in a 4-electrode configuration with immiscible aqueous-organic electrolyte solutions is explained using the thermodynamic theory detailed throughout the perspective. The theory presented herein is equally applicable to a closed bipolar electrochemical cell in a 4-electrode configuration with aqueous electrolyte solutions, each containing redox active species, in both compartments connected by a bipolar electrode

Self-healing gold mirrors and filters at liquid-liquid interfaces

The optical and morphological properties of lustrous metal self-healing liquid-like nanofilms were systematically studied towards different applications (e.g., optical mirrors or filters). These nanofilms were formed by a one-step self-assembly methodology of gold nanoparticles (AuNPs) at immiscible water-oil interfaces, previously reported by our group. We investigated a host of experimental variables and report their influence on the optical properties of nanofilms: AuNP mean diameter, interfacial AuNP surface coverage, nature of the organic solvent, and nature of the lipophilic organic molecule that caps the AuNPs in the interfacial nanofilm. To probe the interfacial gold nanofilms we used both in situ (UV-vis-NIR spectroscopy and optical microscopy) as well as ex situ (SEM and TEM of interfacial gold nanofilms transferred to silicon substrates) techniques. The interfacial AuNP surface coverage strongly influenced the morphology of the interfacial nanofilms, and in turn their maximum reflectance and absorbance. We observed three distinct morphological regimes; (i) smooth 2D monolayers of "floating islands"Â of AuNPs at low surface coverages, (ii) a mixed 2D/3D regime with the beginnings of 3D nanostructures consisting of small piles of adsorbed AuNPs even at sub-full-monolayer conditions and, finally, (iii) a 3D regime characterised by the 2D full-monolayer being covered in significant piles of adsorbed AuNPs. A maximal value of reflectance reached 58% in comparison to a solid gold mirror, when 38 nm mean diameter AuNPs were used at a water-nitrobenzene interface. Meanwhile, interfacial gold nanofilms prepared with 12 nm mean diameter AuNPs exhibited the highest extinction intensities at ca. 690 nm and absorb around 90% of the incident light, making them an attractive candidate for filtering applications. Furthermore, the interparticle spacing, and resulting interparticle plasmon coupling derived optical properties, varied significantly on replacing tetrathiafulvalene with neocuproine as the AuNP capping ligand in the nanofilm. These interfacial nanofilms formed with neocuproine and 38 nm mean diameter AuNPs, at monolayer surface coverages and above, were black due aggregation and broadband absorbance

Photo-Ionic Cells: Two Solutions to Store Solar Energy and Generate Electricity on Demand

A solar energy conversion concept based on the photoinduced separation of a pair of redox species in a biphasic liquid cell is presented. The redox pair is subsequently discharged in an electrochemical flow cell to generate electricity. To illustrate this generic concept, we have revisited the thionine/cobalt EDTA system where, upon light excitation, the excited thionine dye is quenched in the aqueous solution by the [Co­(II)­EDTA]^2– complex to form both [Co­(III)­EDTA]⁻ and reduced thionine, namely leucothionine, that partitions to the organic phase. As a result, solar energy is converted to a redox pair, leucothionine/[Co­(III)­EDTA]⁻. The two immiscible liquid phases are separated, and the redox energy is stored in the respective electrolyte solutions. These two solutions can then be electrochemically discharged in a flow cell to generate electricity on demand. The electrode reactions involved are the reoxidation of leucothionine to thionine in the organic solvent and the reduction of the Co­(III) complex in water

Single Organic Droplet Collision Voltammogram via Electron Transfer Coupled Ion Transfer

Single-emulsion toluene oil droplets (femtoliter) containing a hydrophobic redox probe that are dispersed in water stochastically collide with an ultramicroelectrode (UME). The fast-scan cyclic voltammetry (FSCV) or Fourier-transformed sinusoidal voltammetry (FTSV) is applied: the UME was scanned with a fast, repetitive triangular, or sinusoidal potential, and its current in time/frequency domains were monitored. The electron transfer at the UME/oil interface is coupled with ion transfer at the oil/water interface. Thus, the obtained transient voltammograms of a myriad of ions were used to estimate thermodynamics of ion transfer at the toluene/water interface. Additionally, the single-droplet voltammogram combined with finite element simulations reveal the droplet’s size and shape distributions. Four collision mechanisms with new physical insights were also uncovered via comprehensive analysis of phase angle in the frequency domain, time domain FSCVs, and finite element simulations