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Ionic liquid gel polymer electrolytes (IL-GPEs) have attracted wide interest in the field of electrochemical energy storage devices, particularly for their use in flexible supercapacitors (FSCs). IL-GPEs integrate ionic liquids (ILs) as either a solvent or as a polymerizable unit. They possess several attractive properties including high ionic conductivity, wide operating potential range, good thermal stability, tunable chemical functionalities and self-healing capabilities. In this short review, an overview of IL-GPEs is provided, first describing their classification, fundamental properties, and chemical design, then detailing some of the most recent essential advances in the field along with their limitations.

Ionic liquids (ILs) are highly promising, tuneable materials that have the potential to replace volatile electrolytes in amperometric gas sensors in a ‘membrane-free’ sensor design. However, the drawback of removing the membrane is that the liquid ILs can readily leak or flow from the sensor device when moved/agitated in different orientations. A strategy to overcome the flowing nature of ILs is to mix them with polymers to stabilise them on the surface in the form of membranes. In this research, the room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]), has been mixed with the poly(ionic liquid) (poly(IL), poly(diallyldimethylammonium bis(trifluoromethylsulfonyl)imide), poly[DADMA][NTf2]) to form stable membranes on miniaturised, planar electrode devices. Different mixing ratios of the IL/poly(IL) have been explored to find the optimum membrane that gives both high robustness (non-flowing material) and adequate conductivity for measuring redox currents, with the IL/poly(IL) 60/40 wt% proving to give the best responses. After assessing the blank potential windows on both platinum and gold electrodes, followed by the kinetics of the cobaltocenium/cobaltocene redox couple, the voltammetry of oxygen, sulfur dioxide and ammonia gases have been studied. Not only were the membranes highly robust and non-flowing, but the analytical responses towards the gases were excellent and highly reproducible. The presence of the poly(IL) negatively affected the sensitivity, however the electron transfer kinetics and the limit of detection were actually improved for O2 and SO2, combined with the poly(IL) experiencing less reference potential shifting. These promising results show that membranes containing conductive poly(IL)s mixed with ionic liquids could be used as new ‘designer’ gas sensor materials in robust membrane free amperometric gas sensor devices.

Herein we demonstrate that ionic liquids can form long-lived double layers, generating electric fields detectable by straightforward open circuit potential (OCP) measurements. In imidazolium-based ionic liquids an external negative voltage pulse leads to an exceedingly stable near-surface dipolar layer, whose field manifests as long-lived (∼1-100 h) discrete plateaus in OCP versus time traces. These plateaus occur within an ionic liquid-specific and sharp potential window, defining a simple experimental method to probe the onset of interfacial ordering phenomena, such as overscreening and crowding. Molecular dynamics modeling reveals that the OCP arises from the alignment of the individual ion dipoles to the external electric field pulse, with the magnitude of the resulting OCP correlating with the product of the projected dipole moment of the cation and the ratio between the cation diffusion coefficient and its volume. Our findings also reveal that a stable overscreened structure is more likely to form if the interface is first forced through crowding, possibly accounting for the scattered literature data on relaxation kinetics of near-surface structures in ionic liquids.

Hydrogen is considered an attractive alternative to fossil fuels, but only a small amount of it is produced from renewable energy, making it not such a clean energy carrier after all. Producing hydrogen through water electrolysis is promising, but using a cost-effective and high-performing catalyst that has long-term stability is still a challenge. This study exploits, for the first time, the potential of zinc oxide nanoparticles with diverse morphologies as catalysts for the electrocatalytic production of hydrogen from water. The morphology of the nanoparticles (wires, cuboids, spheres) was easily regulated by changing the concentration of sodium hydroxide, used as the shape controlling agent, during the synthesis. The spherical morphology exhibited the highest electrocatalytic activity at the lowest potential voltage. These spherical nanoparticles had the highest number of oxygen vacancies and lowest particle size compared to the other two morphologies, features directly linked to high catalytic activity. However, the nanowires were much more stable with repeated scans. Density-functional theory showed that the presence of oxygen vacancies in all three morphologies led to diminished band gaps, which is of catalytic interest.

Ionic liquids (ILs) have become highly popular solvents over the last two decades in a range of fields, especially in electrochemistry. Their intrinsic properties include high chemical and thermal stability, wide electrochemical windows, good conductivity, high polarity, tunability, and good solvation properties, making them ideal as solvents for different electrochemical applications. At charged surfaces such as electrodes, an electrical double layer (EDL) forms when exposed to a fluid. IL ions form denser EDL structures compared to conventional solvent/electrolyte systems, which can cause differences in the behavior for electrochemical applications. This Perspective discusses some recent work (over the last three years) where the structure of the EDL in ILs has been examined and found to influence the behavior of supercapacitors, batteries, sensors, and lubrication systems that employ IL solvents. More fundamental work is expected to continue in this area, which will inform the design of solvents for use in these applications and beyond.

Microelectrodes are popular in electroanalysis because radial diffusion to the electrodes results in high current density. The current can then be multiplied by increasing the number of electrodes in an array configuration, allowing for low concentrations of analyte species to be detected. Microelectrode arrays are usually designed so that individual microelectrodes (in a hexagonal arrangement) are sufficiently spaced, ensuring that diffusion layers do not overlap during electrochemical experiments, but are not too far separated so that space is wasted. In this study, the effect of microelectrode spacing has been investigated for platinum deposition into the microholes of commercially available microarray devices. The microarrays have 91 recessed microelectrodes, 10 µm in diameter, 3.3 µm depth, but with four different centre-to-centre spacings of 80, 60, 40 and 20 µm (8, 6, 4 and 2 times the diameter). A 300 s deposition time in an aqueous hexachloroplatanic acid solution was used to deposit three-dimensional Pt structures into the array. The size of the deposits systematically decreased as the electrode spacing became smaller, as a result of overlapped diffusion layers during the deposition process. The modified microarrays were then used for the sensing of a model analyte (oxygen) in a room temperature ionic liquid, with the larger deposits (with larger surface areas) giving higher current responses. However, current densities were found to be quite comparable for all spacings. The 2 times diameter separation can theoretically fit 16 times the number of electrodes into the same area of the underlying Au electrode compared to the 8 times separation. Therefore, it should be possible to design devices that have significantly higher electrode density, which can maximise the overall current and lead to better analytical performances. This work shows that it is important to consider both the geometry and electrode separation for microarrays when used in electrodepositi

Gas solubilities of non-polar (hydrogen and oxygen) and polar (sulphur dioxide) gases in a set of functionalized alkyl imidazolium ionic liquids with the bis(trifluoromethylsulfonyl)imide ([NTFf2]−) anion are reported between 303 and 333 K at 1 bar. The alkyl side-chains in the imidazolium cations include different functional groups, such as –OH, –CN and benzyl; their effects on gas solubilities were studied. The solubility decreases with temperature for all gases, as expected for an exothermic dissolution. Sulphur dioxide is by far the most soluble gas, with mole fractions between 0.29 and 0.41 in the ionic liquids at 313 K and 1 bar, approximately 2–3 orders of magnitude higher than the two other gases studied. Oxygen is generally more soluble in the ionic liquids than hydrogen with mole fractions ranging from 9 × 10−4 to 21 × 10−4 and 5 × 10−4 to 15 × 10−4 at 313 K and 1 bar for oxygen and hydrogen, respectively. In the case of hydrogen, the solubility increases when the molar volume of the ionic liquid increases, whereas for oxygen, the presence of polar groups in the cation causes a reduction in the solubility. None of the three gases is chemically absorbed in the ionic liquids.

In this mini review, we highlight some key work from the last 2 years where ionic polymers have been used as a catalyst to convert CO2 into cyclic carbonates. Emerging ionic polymers reported for this catalytic application include materials such as poly(ionic liquid)s (PILs), ionic porous organic polymers (iPOPs) or ionic covalent organic frameworks (iCOFs) among others. All these organic materials share in common the ionic moiety cations such as imidazolium, pyridinium, viologen, ammonium, phosphonium, and guanidinium, and anions such as halides, [BF4]-, [PF6]-, and [Tf2N]-. The mechanistic aspects and efficiency of the CO2 conversion reaction and the polymer design including functional groups and porosity are discussed in detail. This review should provide valuable information for researchers to design new polymers for important catalysis applications.

© The Royal Society of Chemistry 2020. Gas sensors are important devices used to monitor the type and amount of gas present. Amperometric gas sensors-based on measuring the current upon an applied potential-have been progressing towards miniaturised designs that are smaller, lower cost, faster responding and more robust compared to commercially available sensors. In this work, a planar thin-film electrode device is employed for gas sensing with a thin layer of gel polymer electrolyte (GPE). The GPE consists of a room temperature ionic liquid (RTIL, with two different imidazolium cations and the tetrafluoroborate [BF4]- anion) mixed with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). The polymer acts as a scaffold, with the RTIL ions able to flow within the porous percolated channels, resulting in a highly robust gel with high conductivity. The chemical nature of the polymer allows thin-films (ca. 6 μm) to be evenly dropcast onto planar electrode devices, using minimal amounts of material. Remarkably, no significant effect of resistance was observed in the voltammetric response with such thin films. Oxygen (O2) and ammonia (NH3) gases were detected in the concentration ranges 1-20% O2 and 1-10 ppm NH3 in the two GPEs using both linear sweep voltammetry (LSV) and long-term chronoamperometry (LTCA). LTCA was the preferred detection method for both gases due to the steady-state current response compared to the sloping current response from LSV. The thin nature of the film gave fast response times for both gases-less than 10 seconds for O2 and ca. 40 seconds for NH3-easily rivaling the commercially available porous electrode designs and allowing for continuous monitoring of gas concentrations. These materials appear to be highly promising candidates as gas detection electrolytes in miniaturised devices, with accurate and fast responses in both the cathodic and anodic potential regions.

Replacing conventional aqueous-based electrolytes with room-temperature ionic liquids (RTILs) for electrochemical applications is a major research focus. However, in applications where RTILs are exposed to real-world environments, their hygroscopic nature affects their promising physicochemical properties, such as broad electrochemical windows (EWs) and high chemical stability. In this study, the electrochemical windows of nine commercially available RTILs have been determined on platinum thin-film electrodes in "dry"conditions (4.3-6.5 V) via cyclic voltammetry, and a systematic study over a wide humidity range (relative humidity (RH) between <1 and >95%) has been carried out. A significant reduction in the EW occurs even at low moisture contents (<10 RH%), which is especially evident for the most electrochemically stable ions in the study (i.e., [C4mpyrr]+, [FAP]-, and [NTf2]-). At saturated water levels, the electrochemical windows come close to that of water (approximately 2 V) regardless of the cation or anion structure, where the electrolyte behavior changes from "water-in-RTIL"to "RTIL-in-water."Additionally, the appearance of redox peaks from dissolved impurities inherent to the RTIL becomes more obvious with increasing water content. The effect of moisture on the electrochemical response of two model species where the presence of water does not alter the electrochemical mechanism, i.e., decamethylferrocene and ammonia, was also studied. For ammonia, the increase in current is not only caused by a change in the transport properties of the electrolyte (lower viscosity) but also by a shift in the anodic limit of the electrochemical window. This is believed to be the most detailed study of the effect of water on RTILs over a wide humidity range and emphasizes the importance of understanding the effect of water on voltammetric responses of dissolved species in RTILs under different environmental conditions.

© 2020 CSIRO. Room temperature ionic liquids (RTILs) have been widely investigated as alternative electrochemical solvents for a range of dissolved species over the past two decades. However, the behaviour of neutral radicals dissolved in RTILs is relatively unexplored. In this work, the electrochemistry of a stable verdazyl radical-1,5-dimethyl-3-phenyl-6-oxoverdazyl (MPV)-has been studied on a platinum thin-film electrode using cyclic voltammetry and chronoamperometry in 10 different RTILs. The organic solvent propylene carbonate is also employed as a comparison. The nature of the solvent system was found to have a large effect on the electrochemical behaviour, particularly on the reduction reaction of the verdazyl radical. Chronoamperometry on a microdisk electrode was used to calculate diffusion coefficients (D), and plots of D versus the inverse of viscosity were linear, suggesting typical hydrodynamic diffusional characteristics of the radical, in line with the behaviour of dissolved neutral and charged compounds (e.g. ferrocene and cobaltocenium) in RTILs. Overall, this study demonstrates that different RTILs have a significant influence on the electrochemistry of MPV, and therefore careful selection of the solvent system for electrochemical applications is advised.

The synthesis of thin films of metal organic frameworks (MOFs) is a rapidly growing area owing to the use of these highly functional nanomaterials for various applications. In this study, a thin layer of a typical MOF, copper benzene tricarboxylate (HKUST–1), was synthesized by electrodeposition on a glassy carbon (GC) electrode using a potential-step chronoamperometric technique at room temperature. Various characterization techniques including Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used to verify the successful deposition of the MOF film and its structure. The electrodeposited MOF crystals showed cuboctahedral morphology with macropores. The MOF modified electrode was applied for hydrogen gas sensing in a room-temperature ionic liquid (RTIL) for the first time. A 4-fold increase in current was observed compared to a precious metal, that is, platinum, and the electrode exhibited a significant catalytic activity compared to the bare GC electrode, making it a very promising low cost material for hydrogen gas sensing.

A simple and robust, low-cost electrochemical device is proposed for the combined sampling and detection of the trace solid explosive 2,4,6-trinitrotoluene (TNT) from a non-porous surface. Four different substrates were investigated to collect explosive residue – a bare thin-film electrode, glass microfiber filter paper, a gel-polymer electrolyte (GPE), and a GPE-filter paper composite. The GPE contained the hydrophobic room temperature ionic liquid (RTIL) trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P14,6,6,6][NTf2]) and the polymer poly(methyl methacrylate) (PMMA). A simple “swabbing” technique was used to sample explosive residue on all substrates. Square wave voltammetry was performed to determine the effects of oxygen and moisture on the current response. The most robust method for use in the field – a GPE drop-casted on a TFE – was applied in real environments using a hand-held portable potentiostat. The prototype device was able to detect TNT with a 30 min development time in different ambient environmental conditions. The portability, ease of use and low-cost of the sensor device makes this a viable platform for the rapid onsite detection of explosives.

© 2020 American Chemical Society. Transition metal phosphides (TMPs) have gained increased attention in energy storage due to their potential applications for optimizing electrochemical performances. However, their preparation routes usually require highly toxic and flammable phosphorus sources with strict reaction conditions. The existence of multiple energetically favorable stoichiometries also makes it a challenge to achieve phase control of metal phosphides. In this work, we have successfully realized the phase-controllable framework of cobalt phosphide from Co2P to CoP by employing a semi-interpenetrating network (semi-IPN) hydrogel as a precursor. Interestingly, the semi-IPN hydrogel could serve as a self-assembly/sacrificing template to accomplish 3D space confinement, where poly(vinylphosphonic acid) (PVPA) was identified as a prominent phosphorus source due to its strong metal complexation ability and high thermal stability. Furthermore, this route is successfully extended to the synthesis of other TMPs, including Fe2P, Ni2P, and Cu3P. The specific structure of cobalt phosphides gives rise to superior lithium storage performance, showing superior cycling stability (495.2 mAh g-1 after 1000 cycles at 2.0 A g-1). This approach envisions a new outlook on exploitation of essential functional hydrogels for the creation of inorganic materials toward sustainable energy development.

© 2020 Elsevier B.V. Sulfur dioxide (SO2) is a toxic gas at low parts-per-million (ppm) concentrations, with a permissible exposure limit (PEL) of 2 ppm. Its detection is mandatory, particularly in the fields of occupational health and safety, and environmental pollution. In this work, ppm concentration detection of sulfur dioxide was performed in six room temperature ionic liquids (RTILs), as well as on two different electrode materials – platinum and gold – and with two different electrode geometries, i.e. macro thin-film electrodes (TFEs) and microarray thin-film electrodes (MATFEs). Calibration curves were established for 10–200 ppm SO2 using cyclic voltammetry to determine the optimum combination of RTIL, electrode surface and geometry for the sensing. The RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonium)imide ([C4mpyrr][NTf2]) with a platinum thin-film electrode was found to give the best response due to the relatively low viscosity of the RTIL combined with the high sensitivity and a clean blank response. On MATFEs, deposited sulfur particles – confirmed using scanning electron microscopy (SEM) coupled to an energy dispersive spectrometer – were found to passivate and block some of the microholes, leading to unstable long-term chronoamperometric responses. To the best of our knowledge, this is the first observation of sulfur deposition from SO2 reduction in aprotic ionic liquids. Consecutive additions of 2 ppm SO2 were studied in [C4mpyrr][NTf2] on a TFE using long-term chronoamperometry, showing excellent reproducibility upon successive additions. This demonstrates that small volumes of RTILs can be combined with miniaturized, low-cost TFEs and applied for the reliable and continuous monitoring of sulfur dioxide gas at concentrations lower than the permissible exposure limit of 2 ppm.

© 2019 Elsevier B.V. From a safety perspective, it is vital to have fast responding gas sensors for toxic and explosive gases in the event of a gas leak. Amperometric gas sensors have been developed for such a purpose, but their response times are often relatively slow – on the order of 50 seconds or more. In this work, we have developed sensors for hydrogen gas that demonstrate ultra-fast response times. The sensor consists of an array of gold microchannel electrodes, electrodeposited with platinum nanoparticles (PtNPs)to enable hydrogen electroactivity. Very thin layers (∼9 μm)of room temperature ionic liquids (RTILs)result in an extremely fast response time of only 2 s, significantly faster than the other conventional electrodes examined (unmodified Pt electrode, and PtNP modified Au electrode). The RTIL layer in the microchannels is much thinner than the channel length, showing an interesting yet complex diffusion pattern and characteristic thin-layer behavior. At short times (e.g. on the timescale of cyclic voltammetry), the oxidation current is smaller and steady-state in nature, compared to macrodisk electrodes. At longer times (e.g. using long-term chronoamperometry), the diffusion layer is large for all surfaces and extends to the liquid/gas phase boundary, where the gas is continuously replenished from the flowing gas stream. Thus, the current response is the largest on the microchannel electrode, resulting in the highest sensitivity and lowest limit of detection for hydrogen. These microchannel electrodes appear to be highly promising surfaces for the ultrafast detection of hydrogen gas, particularly at relevant concentrations close to, or below, the lower explosive limit of 4 vol-% H2.

Solvated lithium closo-dodecaborate, Li2B12H12 with tetrahydrofuran and acetonitrile, show unexpected melting below 150 °C. This feature has been explored to melt-infiltrate Li2B12H12 in a nanoporous SiO2 scaffold. The ionic conductivity of Li2B12H12·xACN reaches 0.08 mS cm-1 in the liquid state at 150 °C making them suitable as battery electrolytes.

Microelectrodes offer higher current density and lower ohmic drop due to increased radial diffusion. They are beneficial for electroanalytical applications, particularly for the detection of analytes at trace concentrations. Microelectrodes can be fabricated as arrays to improve the current response, but are presently only commercially available with gold or platinum electrode surfaces, thus limiting the sensing of analytes that are more electroactive on other surfaces. In this work, gold (Au), copper (Cu), and palladium (Pd) are electrodeposited at two different potentials into the recessed holes of commercial microelectrode arrays to produce 3-dimensional (3D) spiky, dendritic or coral-like structures. The rough fractal structures that are produced afford enhanced electroactive surface area and increased radial diffusion due to the 3D nature, which drastically improves the sensitivity. 2,4,6-trinitrotoluene (TNT), carbon dioxide gas (CO2), and hydrogen gas (H2) were chosen as model analytes in room temperature ionic liquid solvents, to demonstrate improvements in the sensitivity of the modified microelectrode arrays, and, in some cases (e.g., for CO2 and H2), enhancements in the electrocatalytic ability. With the deposition of different materials, we have demonstrated enhanced sensitivity and electrocatalytic behaviour towards the chosen analytes.

© 2019 Elsevier B.V. Gas detection is an essential part of everyday life; for some applications, using sensors for toxic and hazardous gases can literally mean the difference between life and death. In this minireview, recent progress in amperometric gas sensing using miniaturised electrodes and devices is described. The focus is on the use of nonvolatile room-temperature ionic liquids (RTILs) as electrolytes, which possess inherent advantages such as wide electrochemical windows, high thermal and chemical stability, intrinsic conductivity and good solvating properties. Various different gases, electrodes and RTILs have been investigated in the strive towards new materials for improved gas sensors. The most recent developments using porous membrane electrodes, planar devices (e.g. screen-printed, thin-film, microarray and interdigitated electrodes) and the modification of these surfaces for improved sensitivity are described. RTILs have great potential to be used as electrolytes in amperometric gas sensors, with improved lifespan of the sensor in hot/dry environments and allowing miniaturisation of devices. However, it is clear that more understanding of their long-term operation and utility in real environments (e.g. background air, varying temperatures and humidity levels) is needed before their realisation in successful commercial devices.

The oxygen reduction reaction (ORR) is widely studied in room-temperature ionic liquids (RTILs) but typically in dry environments. Because water is known to affect diffusion coefficients and reaction outcomes, the influence of water on the ORR is expected to be significant. We have therefore studied the effect of RTIL structure on the ORR at different relative humidity (RH) levels using cyclic voltammetry. A broad range of cations including imidazolium, ammonium, pyrrolidinium, pyridinium, sulfonium, and phosphonium, and anions such as [BF4]-, [PF6]-, [NTf2]-, and [FAP]- were employed. The cation was found to have a large effect on the reduction current of oxygen, even at low humidity levels (<40 RH %), whereas the anion mainly influenced the current at higher humidity levels (>65 RH%). Consequently, the choice of cation needs to be carefully considered when selecting a suitable RTIL solvent for oxygen reduction in humidified environments. The size, structure, and hydrophobicity of the ions were found to dictate the degree at which the RTIL is susceptible to changes in humidity. The physical characteristics of the RTIL electric double layer on platinum electrode surfaces were further investigated by atomic force microscopy force-curve studies in three selected RTILs. The results suggest that there is a significant amount of water incorporated at the electrode-RTIL interface in [C2mim][NTf2] and [N4,1,1,1][NTf2] but not in the more hydrophobic [P14,6,6,6][NTf2]. The presence of moisture has a significant impact on ORR currents in [C2mim][NTf2], even at extremely low humidity levels, which was verified by the higher level of water incorporation in [C2mim][NTf2] compared with [N4,1,1,1][NTf2] and [P14,6,6,6][NTf2]. Hydrophobic and large RTIL cations and anions (e.g., [P14,6,6,6]+ and [FAP]-) are recommended for applications where a stable ORR current response is required under humidified conditions.

Copyright © 2019 American Chemical Society. The electrochemical properties of gas molecules are of great interest for both fundamental and applied research. In this study, we introduce a novel concept to systematically alter the electrochemical behavior and, in particular, the redox potential of neutral gas molecules. The concept is based on the use of an ion-binding agent, or "ionophore", to bind and stabilize the ionic electrochemical reaction product. We demonstrate that the ionophore-assisted electrochemical oxidation of hydrogen in a room-temperature ionic liquid electrolyte is shifted by almost 1 V toward more negative potentials in comparison to an ionophore-free electrolyte. The altered electrochemical response in the presence of the ionophore not only yields insights into the reaction mechanism but also can be used to determine the diffusion coefficient of the ionophore species. This ionophore-modulated electrochemistry of neutral gas molecules opens up new avenues for the development of highly selective electrochemical sensors.