The formation of substoichiometric mixed zirconium-yttrium oxides by electroreduction of cubic 9.5YSZ is investigated. A strongly oxygen depleted phase with the stoichiometry (Zr,Y)[Formula: see text]O is observed, forming belt-shaped features below the sample surface in the heavily reduced region close to the cathode. It is embedded in another oxygen depleted phase with the stoichiometry (Zr,Y)[Formula: see text]O. The electroreduction is performed by drawing a DC current through a single crystal with circular platinum electrodes. The new phases, which are possibly metastable and not yet reported in literature, are identified by STEM investigations and EDX, which is used to measure the composition. The found composition indicates Zr and Y with an oxidation state of +I, respectively, between +I and 0. The (Zr,Y)[Formula: see text]O phase has a significant distortion of the original cubic symmetry. The (Zr,Y)[Formula: see text]O phase exhibits a slight, the (Zr,Y)[Formula: see text]O phase a strong decrease of the molar volume compared to unreduced YSZ. A chequerboard-like structure on the surface of the single crystal can be most probably explained by strain relaxation due to dislocation gliding. Misfit dislocations can be found in the interface between the substoichiometric phases. The induced strain due to the volume contraction may also be responsible for the known deterioration of the mechanical properties after reduction.

The phenomenon of electroreduction, or electrocoloration, in yttria-stabilised zirconia (YSZ) has garnered significant attention due to its dual role as a possible degradation mechanism in solid oxide electrolysers and as a beneficial effect during the flash sintering of ceramics with tailored properties. Despite extensive investigation over several decades, the precise mechanisms underlying the transformation of the transparent, purely ionic conductor YSZ into a black, mixed ionic-electronic conductor, and eventually into a metallic state, remain inadequately understood. In this study, we present a comprehensive analysis that integrates electrical characterisation during electroreduction with in situ microscopy and ex situ spectroscopy techniques. Our findings enable us to delineate three primary stages: a reversible electrocoloration associated with the development of blackening fingers, an accelerated electroreduction facilitated by the formation of mixed ionic-electronic conducting pathways between the anode and cathode, and a runaway-type process that induces morphological changes and filamentary phase transformations in the surface region.

Metal–organic frameworks (MOFs) are promising host materials for studying and controlling the dynamics of molecules and ions, including lithium ions. In this study, a lithium-ion-containing ionic liquid (IL) was successfully incorporated into the MOF ZIF-8, and its phase behavior and ionic conductivity were investigated. Using a capillary action method, lithium bis(trifluoromethanesulfonyl)imide (Li[TFSI])-containing 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][TFSI]), denoted as Li0.2[Emim]0.8[TFSI], was introduced into the pores of ZIF-8. The impact of varying Li0.2[Emim]0.8[TFSI] concentrations on the crystal structure, bond vibrations, and ionic conductivity was systematically analysed. Structural analysis revealed an increase in the lattice constant of ZIF-8 upon Li0.2[Emim]0.8[TFSI] encapsulation, accompanied by a weakening of the N–Zn–N bond as the cage structure expanded. Additionally, the migration of Li[TFSI] ion pairs into the cages disrupts ion pair interactions, leading to a reduction in the O=S=O bond strength within the [TFSI]− anion. When the ZIF-8 pores were fully saturated with Li0.2[Emim]0.8[TFSI], the composite exhibited an ionic conductivity of 0.36(2) mS·cm−1 at 80 °C, with the activation energy of 0.26(1) eV. Its ionic conductivity meets the performance criteria required for use in lithium-ion battery electrolytes. However, it has the low lithium transference number. And the LiCoO2||Li coin cells with LEI1.2@ZIF pellet has a capacity retention of 74.9% only after 100 cycles at 0.2C at room temperature. Compared with the pure lithium-salt-containing ionic liquid, Li0.2[Emim]0.8[TFSI], the quasi-solid-state electrolyte LEI1.2@ZIF, prepared by incorporating the lithium-salt-containing ionic liquid into the ZIF-8 framework, exhibits inferior electrochemical performance in lithium batteries. This is primarily due to the ionic conductivity that is two orders of magnitude lower and a higher interfacial impedance of the quasi-solid-state electrolyte LEI1.2@ZIF. These findings highlight the potential of MOF-based composites as quasi-solid-state or solid-state electrolytes for lithium-ion battery applications.

The formation of substoichiometric mixed zirconium-yttrium oxides by electroreduction of cubic 9.5YSZ is investigated. A strongly oxygen depleted phase with the stoichiometry (Zr,Y)(8.6)O is observed, forming belt-shaped features below the sample surface in the heavily reduced region close to the cathode. It is embedded in another oxygen depleted phase with the stoichiometry (Zr,Y)(2)O. The electroreduction is performed by drawing a DC current through a single crystal with circular platinum electrodes. The new phases, which are possibly metastable and not yet reported in literature, are identified by STEM investigations and EDX, which is used to measure the composition. The found composition indicates Zr and Y with an oxidation state of +I, respectively, between +I and 0. The (Zr,Y)(8.6)O phase has a significant distortion of the original cubic symmetry. The (Zr,Y)(2)O phase exhibits a slight, the (Zr,Y)(8.6)O phase a strong decrease of the molar volume compared to unreduced YSZ. A chequerboard-like structure on the surface of the single crystal can be most probably explained by strain relaxation due to dislocation gliding. Misfit dislocations can be found in the interface between the substoichiometric phases. The induced strain due to the volume contraction may also be responsible for the known deterioration of the mechanical properties after reduction.

Joo and East have recently published a Comment on our article (F. Parisi et al., Phys. Chem. Chem. Phys., 2024, 26, 28037, https://doi.org/10.1039/D3CP06047K). The Comment is based on the wrong assumption that we misassigned the infrared spectrum of liquid diethylmethylammonium triflate [DEMA][TfO]. The authors incorrectly claim that our hypothesis was that the two bands are due to the NH stretch mode in two different ion-pair structural types. We clarify here that our original analysis did not invoke two separate, static ion-pair structures, but rather a continuum of dynamically evolving hydrogen-bonding environments that naturally produce a broadened, bimodal band shape. The results presented in our paper are aligned with the ones presented in the Comment. The Comment brings up the concept of Fermi resonance, which indeed gives a plausible explanation of the features seen in the experimental absorption spectra.

Metal–organic frameworks (MOFs) are three-dimensional crystalline materials composed of metal nodes and organic ligands, forming porous coordination networks with high surface areas and tunable internal cavities. These structural features enable MOFs to act as hosts for small guest molecules, such as ionic liquids (ILs), facilitating host–guest interactions that can modify physical properties and enhance performance in applications such as fuel cells and batteries. In this study, two imidazolium-based aprotic ILs with different anions, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim][TfO]) and 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([Emim][TFSI]), were successfully encapsulated within the pores of the MOF ZIF-8. The structural and vibrational impacts of these ILs on the ZIF-8 framework were systematically investigated using powder X-ray diffraction (PXRD) and Fourier-transform infrared spectroscopy (FT-IR). The results revealed two distinct mechanisms for the uptake of an IL, leading to an expansion of the MOF cage structure: [Emim][TfO]@ZIF-8 exhibited a phase transition characterized by the coexistence of two lattice constants, while [Emim][TFSI]@ZIF-8 maintained a single-phase structure with continuous lattice expansion upon increased loading. These behaviors are attributed to the swing motion of 2-methylimidazole linkers and the elongation of Zn–N bonds induced by internal pressure from the confined ILs. Additionally, confinement within the ZIF-8 cages led to the strengthening of –SO₃ and S–N–S bonds in the ILs, as evidenced by the blue-shifted and intensified IR vibrational modes. This study provides fundamental insights into the structural evolution and host–guest interactions in IL@MOF systems, offering guidance for tailoring MOF properties through ILs encapsulation.

The most important key challenge for 5 V-class LiNi0.5Mn1.5O4 (LNMO) lithium metal batteries is to design electrolytes that are stable above 4.5 V while enabling fast Li+ transport. A promising approach is ionic liquid bases electrolyte, composed of 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr14FSI) and lithium bis(fluorosul-fonyl)imide (LiFSI), where both the Li+ concentration and the amount of a fluoroether-based cosolvent (BTFE, bis(trifluoroethyl)ether) are independently adjusted to achieve a multidimensional control over the local Li+ solvation environment. The Li+ solvation sheath can be reshaped to a compact, anion-rich configuration, characterized by full-contact ion pairs (FCIPs) and enhanced Li+-FSI- coordination by adjusting the LiFSI and BTFE ratio. The ionic conductivity is markedly enhanced from 0.49 to 1.34 mS cm- 1, and the anodic stability window is extended up to 5.4 V vs. Li/Li+. Most crucially, a rapid formation of a dense, oxidationresistant cathode-electrolyte interphase (CEI) during the initial charge cycle can be observed for systems with anion-rich solvation sheath, thereby suppressing further electrolyte oxidation and mitigating impedance growth. As the results, LNMO cells employing the optimized electrolytes retain 84.1 % of their initial capacity after 100 cycles, compared to only 30.68 % after 70 cycles for carbonate based electrolytes. This work provides a reference for the effect of ether co-solvents on the coordination structure of Li+ in ionic liquid electrolytes and explores the use of such electrolyte in 5 V high-voltage lithium secondary battery systems with LNMO as the cathode.

Controlling the morphology and exposed facets of nano/microparticles is crucial for enhancing material properties in electrochemical reactions, photoreactions, and biosensors. However, Prussian Blue (PB), a type of metal–organic framework (MOF), rapidly and stably forms a cubic structure, making shape control difficult and leaving its formation process unclear. Here, the octahedral formation of PB particles in glycerol is discovered, which differs from the cubic particles formed in water. Glycerol slows down the crystallization‐growth process of PB, allowing direct observation of a stepwise growth in which cubic seeds initially form and subsequently self‐assemble into octahedra. Molecular dynamics (MD) simulation suggests that glycerol molecules stabilize the (111) surfaces of PB crystals over the (100) planes, inducing the self‐assembly of PB particles into an octahedral shape. It is also observed that the water‐to‐glycerol ratio influences PB's surface charge and affects the assembly behavior of PB crystals, resulting in the formation of isolated nanocubes (edge length ≈200 nm), microcubes (edge length ≈1 µm), and octahedra (edge length ≈1 µm). As an active material for sodium‐ion capacitors, octahedron‐shaped PB, with its significantly higher surface area, exhibits outstanding cycling performance, surpassing cubic PB.

Prussian blue (PB) is a promising low‐cost cathode material for sodium‐ion batteries (SIBs), but the impact of crystal water on performance degradation remains unclear. This study explores how PB's crystal water interacts with different electrolyte salts—NaClO 4 and NaTFSI—affecting solvation structure and interfacial stability. Based on the Hofmeister series, it is demonstrated that the strong hydration of ClO 4 – sustains water reactivity, promoting Fe oxidation and solvent decomposition at high voltages. In contrast, the weakly hydrated TFSI – suppresses water‐induced side reactions and facilitates the formation of stable interphases on both cathode and anode. Electrochemical analysis at 4.0 V and 4.2 V revealed that NaTFSI consistently improves reversibility, particularly at 4.2 V, achieving 77.1% capacity retention over 500 cycles—56.8% for NaClO 4 . The results highlight the crucial role of electrolyte‐dependent water coordination in determining PB electrode stability, offering valuable insights for designing electrolytes and interphases for long‐life PB‐based SIBs.

The thin-film rotating disk electrode (TF-RDE) technique is widely used for catalyst screening in oxygen evolution reaction (OER) studies due to its ability to separate mass transport from redox kinetics and its low catalyst requirement. However, the manual drop-casting method frequently yields unsatisfactory reproducibility, primarily due to the "coffee-ring" effect, caused by nonuniform catalyst distribution, leading to inconsistent measurements. This study proposes an automated spray-coating method for depositing commercial iridium oxide (IrO2) nanoparticles on RDEs as a reliable alternative. Spray-coating ensures uniform catalyst layers, significantly improving the reproducibility in electrochemical measurements. A comparison of the morphology and the electrochemical properties of the catalyst layers prepared by both methods identifies the sources of the performance variations. Requiring minimal catalyst loading (0.10 mg cm(-2)), spray-coating establishes a standardized approach for OER catalyst evaluation, offering improved consistency and accuracy in electrochemical characterization.

To explore the impact of the aspect ratio of the channels in the flow fields of solid oxide electrolysis cells on the performance of the cell, we developed three-dimensional models for cells with varying aspect ratios. Our findings revealed that channels with low and high aspect ratios exhibit higher maximum pressure drops, whereas those with medium aspect ratios have the lowest pressure drops. Additionally, the mole fraction of the hydrogen decreases as the channel’s aspect ratio increases. We also computed the polarization curves for SOEC operating under three distinct aspect ratio channels. Our results suggest that structures with low aspect ratios exhibit the poorest electrochemical performance, suitable only for brief operations at low current densities; medium aspect ratio structures exhibit a balanced performance, making them suitable for various operating conditions; and high aspect ratio structures are best suited for operations at high current densities. This study on selecting different aspect ratios aids in determining the optimal channel parameters for different operating conditions, ultimately enhancing the performance of solid oxide electrolysis cells.

The geometry and configuration of the flow field significantly influence the overall performance of solid oxide electrolysis cells (SOECs). Among the commonly employed flow field designs, parallel and serpentine channels are most prevalent. The serpentine flow field is widely favored due to its superior mass transport characteristics and more uniform current distribution. However, its application is limited by inherent drawbacks such as a high pressure drop and increased manufacturing complexity. In this study, a three-dimensional numerical model of a hybrid flow field is developed to explore an alternative flow field configuration. Simulation results indicate that the hybrid flow field can reduce the overall pressure drop by nearly 50 % while preserving efficient mass transport. Moreover, it exhibits a higher reaction extent and improved electrochemical performance. These findings provide theoretical guidance for the design of advanced high-performance SOECs and offer a promising solution for balancing flow uniformity, mass transfer, and system efficiency.

Medium-temperature proton exchange membrane fuel cells (PEMFCs) operating between 120–160°C offer significant advantages for water and heat management, reduce platinum sensitivity to impurities, and enhance overall fuel cell efficiency [1,2]. Thanks to wide electrochemical window, high chemical and thermal stability, good proton conductivity, small vapour pressures, and therefore low flammability, Protic ionic liquids (PILs) have emerged as a promising class of electrolytes for these systems [3]. With the aid of molecular simulations we have investigated the critical role of water in PIL-water mixtures. The focus of our studies is on proton transfer mechanisms, water structuring, and the clustering/percolation behavior within the system. We will discuss the effects of varying water content on the formation of hydrogen-bond networks, the dynamic characteristics of the PIL-water system, and the interplay between vehicular and grotthuss-type proton transfer mechanisms. Our results reveal the critical water content required to activate distinct proton transfer pathways. We will show how different PIL acidities and anion types influence the clustering and percolation of water molecules. The results demonstrate that due to the strong electrostatic interactions, high acidity of the cations screen the movement of molecules and hydrogen carriers. On the other hand, by increasing the water content, the chance to activate a grotthus-type mechanism for more efficient proton conduction and mobility in less acidic systems increases. Our findings provide valuable insights for optimization of PIL-based electrolytes for medium-temperature PEMFCs. [1] Q. Li , R. He , J. Jensen and N. Bjerrum , Chem. Mater., 2003, 15 , 4896 [2] S. Peighambardoust , S. Rowshanzamir and M. Amjadi , Int. J. Hydrogen Energy, 2010, 35 , 9349 [3] A. Noda , M. A. B. Hasan Susan , K. Kudo , S. Mitsushima , K. Hayamizu and M. Watanabe , J. Phys. Chem. B, 2003, 107 , 4024 —4033

The kinetics of the electrochemically driven lithium ion (Li+) transfer from a liquid Li+ electrolyte to a solid (ceramic) Li+ electrolyte is investigated. A DC polarisation is applied to measure the current density i vs. the drop in the electrochemical potential ΔLi+ of Li+ ions at the interface. LLZO:Ta and LATP were chosen in this study as the two most promising oxide-ceramic electrolytes and combined with LiPF6 in EC/DMC (1 : 1) and LiBOB in THF/DME (1 : 1) as the most relevant liquid electrolytes. To determine the rate-limiting step of the Li+ transfer across the interface, the results were modelled using a combination of a constant ohmic resistance and a current-dependent, thermally activated Butler-Volmer-like ion transfer process. At low Li+ concentrations in the liquid electrolyte, the Butler-Volmer-like transfer process is rate limiting, while at high Li+ concentrations, the low-conductive surface layer on the solid electrolyte is rate limiting. The areal resistance of the low-conductivity surface layer is in the order of 600 Ω cm2 (25 °C) for LLZO:Ta, and thus about three times higher compared to that for LATP. The activation energy of the ionic transport in the low-conductivity surface layer is about twice that of the solid electrolytes LLZO:Ta and LATP. The exchange current density of the Butler-Volmer-like transfer process is in the order of 100-300 μA cm-2 (25 °C, 1 mol l-1 Li+). There is a symmetric transition state (α ≈ 1/2).

In this work, we present a study on the thermal/transport properties of a novel deep eutectic solvent (DES) obtained by using N-methyltrifluoroacetamide (FNMA) as the hydrogen bond donor (HBD) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the hydrogen bond acceptor (HBA). The binary phase diagram, thermal stability, flammability, viscosity and ionic conductivity of the as-prepared DESs were investigated at atmospheric pressure. The binary phase diagram shows a range of eutectic molar ratios (xLiTFSI = 0.2~0.33), with the lowest deep eutectic temperature of -84 °C. At xLiTFSI = 0.2 (i.e., FNMA:LiTFSI = 4:1 and denoted as DES-4:1). The as-prepared DES composition exhibits high thermal stability (onset temperature of weight loss = 78 °C), a low viscosity (η = 48.9 mPa s at 25 °C), relatively high ionic conductivity (σ = 0.86 mS cm-1 at 25 °C) and non-flammability. The transport properties, including ionic conductivity and viscosity, as a function of temperature are in accordance with the Vogel-Fulcher-Tammann (VFT) equations. With increasing molar ratio of HBD vs. HBA, the viscosity decreases, and the ionic conductivity increases at a given temperature between 25 °C and 80 °C. The roughly equal pseudo-activation energies for ion transport and viscous flow in each composition imply a strong coupling of ion transport and viscous flow. Walden plots indicate vehicular transport as the main ion transport mechanism for the DES-4:1 and DES-3:1 compositions; meanwhile, it was confirmed that the ionic conductivity and viscous flow are strictly coupled. The present work is expected to provide strategies for the development of wide-temperature-range and safer electrolytes with low salt concentrations.

The remarkable progress of atomic force microscopy (AFM) opens new horizons into the investigation of the electrochemical processes at liquid–solid interfaces on a molecular scale. In this chapter, we first present a brief description of the 3D interfacial structure between solid electrodes and liquid electrolytes. For an accurate understanding of structure–property relationships, the interface is divided into different domains and their related features are explained. This is followed by an introduction of the instrumentation and methodology of AFM. The advantages and limitations of different operations and imaging approaches are also discussed. Moreover, we provide an overview of the current state of research in the visualization of interfacial structures by means of electrochemical atomic force measurements, including representative examples in different research fields, e.g., electrocatalysts, batteries, corrosion, and lubricants.

Protic ionic liquids (PILs) are promising candidates as electrolytes for proton exchange polymer membrane fuel cells. In order to optimize their properties, a detailed understanding of the molecular interactions within the bulk and at the electrode-electrolyte interface is needed, which can be obtained by infrared spectra. A prerequisite for extracting information on the molecular structure and inter- or intramolecular interactions from an experimental spectrum is a reasonable interpretation of the observed spectral features. Here, we employed density functional theory to understand the vibration modes of PILs composed of ammonium cations and different counteranions. Different from the previous calculation methods performed on small cluster model systems consisting of isolated species, a periodically repeated system of four ion pairs was used in order to approximate the bulk liquid environment. The computed frequencies and IR intensity match well with the corresponding experimental spectra, allowing for its proper interpretation, especially the characteristic features of the interionic interaction. The presented approach enables accurate computation of a variety of ionic liquid systems in a highly efficient way.

A hydrogen-based energy system will be the backbone of a future energy grid using renewable energies. It is widely accepted that polymer electrolyte membrane fuel cells (PEMFCs) are promising converters of chemical energy stored as hydrogen into electrical energy. An increase of the operation temperature from below 80 degrees C to above about 160 degrees C is considered beneficial, as it would allow for much simpler water management and the use of waste heat. Here, we are investigating protic ionic liquids (PILs) immobilized in a polybenzimidazole polymer as electrolytes for high-temperature PEMFCs. Ionic liquids are promising for fuel cell applications as they provide high thermal and chemical stability and high proton conductivity. In contrast to aqueous electrolytes, ionic liquids form a dense layered structure at the electrode-electrolyte interface that depends on the potential and on the content of residual water in the electrolyte. We investigate how PILs interact with the host polymer of the membrane revealing that porous polymer structures can be formed by solution casting, which allows for an encapsulation of the ionic liquid within the pores. After doping the polymer with small amounts of phosphoric acid, the membranes showed reasonable conductivity and fuel cell performance.

The properties of SrTiO3 (STO), a well-known and often employed model material of transition metal oxides with perovskite structure, have been the subject of numerous studies over the last decades. However, even fundamental mechanisms such as the reaction of STO to changes in oxygen activity are still not yet fully understood. In this paper, we focus on the role of dislocations on reduction and chemical diffusion. We demonstrate that upon reduction of STO crystals, metallic filaments form along the dislocations. Using a bicrystal boundary as an indicator for dislocation-related properties, we provide direct evidence for fast chemical oxygen diffusion along dislocation networks in reduced STO using 18O isotope oxidation experiments. Consequently, it is possible to manipulate the global conductivity of a macroscopic crystal by means of oxidation in a low-temperature regime, in which classical bulk diffusion is not expected. We illustrate that the impact of dislocations is larger than previously assumed and should not be neglected when analyzing and modeling solid oxide materials with mixed electronic-ionic conductivity.

Protic ionic liquids (PILs) have a set of properties that make them interesting candidates for many applications including energy storage and conversion, gas absorption, biological applications and organic synthesis. Compared to stoichiometric (neat) PILs, non-stoichiometric PILs are formed by the combination of neat PILs with an excess of the (precursor) acids or bases, which is generally ignored by many researchers and there are only limited studies. Non-stoichiometric PILs have a variety of specific physicochemical properties, which are likely caused by the unique interaction between neat salts and excess acids or bases and in some cases by particular anion structures. In this first review of non-stoichiometric PILs, we briefly discuss the history of PIL development and how non-stoichiometric PILs differ in properties. An analysis of the physicochemical properties of non-stoichiometric PILs is presented and their application in various fields is discussed. Finally, we give a prospect on the development of non-stoichiometric PILs including conceivable future challenges.