Impedance Analysis of Capacitive and Faradaic Processes in the Pt/[Dema][TfO] Interface.

The electrochemical reaction kinetics, especially the oxygen reduction reaction (ORR) at the cathode, is crucial for the performance of a fuel cell. In this study, the electrochemical processes on a polycrystalline Pt electrode in the presence of protic ionic liquid (PIL) electrolyte diethylmethylammonium triflate [Dema][TfO] are investigated by means of cyclic voltammetry and electrochemical impedance spectroscopy. Since water is continually produced during fuel cell operation, the effect of the water content in the PIL has been intensively analyzed. In order to reveal the dependence of the interfacial reaction characteristics on the electrode potential, the impedance spectra were simulated by an equivalent circuit whose parameters can be related to both Faradaic and capacitive processes. Two interfacial resistances were identified, which differ by about 3 orders of magnitude. The larger one is a charge transfer resistance that can be associated with slow Faradaic processes like the ORR and platinum oxidation/oxide reduction. The smaller resistance is probably linked with fast processes that involve water molecules, such as hydrogen deposition and oxidation. The high- and midfrequency capacitive processes are attributed to "classical" double layer and pseudocapacitive behavior, similar to those identified under nitrogen atmosphere.

Revealing Interfacial Reactions on Pt Electrodes in Ionic Liquids by In Situ Fourier-Transform Infrared Spectroscopy.

In situ monitoring of the electrolyte/electrode interfacial processes, such as the oxygen reduction reaction (ORR), is crucial for the design of electrolytes for fuel cells. In this study, we investigate the electrochemical behavior of platinum electrodes in protic ionic liquids (PILs) by means of in situ Fourier-transform infrared spectroscopy coupled with cyclic voltammetry. The result provides direct evidence of the change of water at the Pt electrode surface due to Pt oxide formation and reduction. A decrease in the interfacial water was observed in the spectra upon the formation of the Pt oxide. Conversely, the local water concentration at the electrode surface increases if the Pt oxide is reduced and the ORR takes place. At the same time, more cations replace anions on the electrode. The ORR kinetics in the [TFSI]-based PILs is slower than in the [TfO]-based ones, which could result from a blockage of catalytic sites by the adsorbed [TFSI] anions. It suggests that reducing the anion adsorption on the platinum surface could provide an opportunity to enhance the ORR activity.

The Structure of the Electric Double Layer of the Protic Ionic Liquid [Dema][TfO] Analyzed by Atomic Force Spectroscopy.

Protic ionic liquids are promising electrolytes for fuel cell applications. They would allow for an increase in operation temperatures to more than 100 °C, facilitating water and heat management and, thus, increasing overall efficiency. As ionic liquids consist of bulky charged molecules, the structure of the electric double layer significantly differs from that of aqueous electrolytes. In order to elucidate the nanoscale structure of the electrolyte-electrode interface, we employ atomic force spectroscopy, in conjunction with theoretical modeling using molecular dynamics. Investigations of the low-acidic protic ionic liquid diethylmethylammonium triflate, in contact with a platinum (100) single crystal, reveal a layered structure consisting of alternating anion and cation layers at the interface, as already described for aprotic ionic liquids. The structured double layer depends on the applied electrode potential and extends several nanometers into the liquid, whereby the stiffness decreases with increasing distance from the interface. The presence of water distorts the layering, which, in turn, significantly changes the system's electrochemical performance. Our results indicate that for low-acidic ionic liquids, a careful adjustment of the water content is needed in order to enhance the proton transport to and from the catalytic electrode.

Acidic Ionic Liquids Enabling Intermediate Temperature Operation Fuel Cells.

Herein we show that protic ionic liquids (PILs) are promising electrolytes for fuel cells operating in the temperature range 100-120 °C. N,N-Diethyl-N-methyl-3-sulfopropan-1-ammonium hydrogen sulfate ([DEMSPA][HSA]), N,N-diethyl-N-methyl-3-sulfopropan-1-ammonium triflate ([DEMSPA][TfO]), N,N-diethyl-3-sulfopropan-1-ammonium hydrogen sulfate ([DESPA][HSA]), and N,N-diethyl-3-sulfopropan-1-ammonium triflate ([DESPA][TfO]) are investigated in this study with regard to their specific conductivity, thermal stability, viscosity, and electrochemical properties. The [DEMSPA][TfO] and [DESPA][TfO] electrolytes offer high limiting current densities for the oxygen reduction reaction (ORR) on platinum electrodes, that is, about 1 order of magnitude larger than 98% H3PO4. This is explained by the minor poisoning of the Pt catalyst and the significantly larger product of the oxygen self-diffusion coefficient and concentration in these two PILs.

Mapping the conducting channels formed along extended defects in SrTiO3 by means of scanning near-field optical microscopy.

Mixed ionic-electronic-conducting perovskites such as SrTiO3 are promising materials to be employed in efficient energy conversion or information processing. These materials exhibit a self-doping effect related to the formation of oxygen vacancies and electronic charge carriers upon reduction. It has been found that dislocations play a prominent role in this self-doping process, serving as easy reduction sites, which result in the formation of conducting filaments along the dislocations. While this effect has been investigated in detail with theoretical calculations and direct observations using local-conductivity atomic force microscopy, the present work highlights the optical properties of dislocations in SrTiO3 single crystals. Using the change in optical absorption upon reduction as an indicator, two well-defined arrangements of dislocations, namely a bicrystal boundary and a slip band induced by mechanical deformation, are investigated by means of scanning near-field optical microscopy. In both cases, the regions with enhanced dislocation density can be clearly identified as regions with higher optical absorption. Assisted by ab initio calculations, confirming that the agglomeration of oxygen vacancies significantly change the local dielectric constants of the material, the results provide direct evidence that reduced dislocations can be classified as alien matter embedded in the SrTiO3 matrix.

Correction: Influence of residual water and cation acidity on the ionic transport mechanism in proton-conducting ionic liquids.

Correction for 'Influence of residual water and cation acidity on the ionic transport mechanism in proton-conducting ionic liquids' by Jingjing Lin et al., Phys. Chem. Chem. Phys., 2020, 22, 1145-1153.

Influence of the acid-base stoichiometry and residual water on the transport mechanism in a highly-Brønsted-acidic proton-conducting ionic liquid.

In this study, Brønsted-acidic proton conducting ionic liquids are considered as potential new electrolytes for polymer membrane fuel cells with operating temperatures above 100 °C. N-Methyltaurine and trifluoromethanesulfonic acid (TfOH) were mixed at various stoichiometric ratios in order to investigate the influence of an acid or base excess. The proton conductivity and self-diffusion of the "neat" and with 6 wt% water samples were investigated by following electrochemical and NMR methods. The composition change in the complete species and the relative proton transport mechanism based on the NMR results are discussed in detail. During fuel cell operation, the presence of significant amounts of residual water is unavoidable. In PEFC electrolytes, the predominating proton transfer process depends on the cooperative mechanism, when PILs are fixed on the polymer matrix within the membrane. Due to the comparable acidity of the cation [2-Sema]+ and the hydroxonium cation, with excess N-methyltaurine or H2O in the compositions, fast proton exchange reactions between the protonated [2-Sema]+ cation, N-methyltaurine and H2O can be envisaged. Thus, an increasing ratio of cooperative proton transport could be observed. Therefore, for polymer membrane fuel cells operating at elevated temperatures, the highly acidic PILs with excess bases are promising candidates for future use as electrolytes.

Influence of residual water and cation acidity on the ionic transport mechanism in proton-conducting ionic liquids.

Proton-conducting ionic liquids (PILs) are discussed herein as potential new electrolytes for polymer membrane fuel cells, suitable for operation temperatures above 100 °C. During fuel cell operation, the presence of significant amounts of residual water is unavoidable, even at these elevated temperatures. By using electrochemical and NMR methods, the impact of residual water on 2-sulfoethylmethylammonium triflate [2-Sema][TfO], 1-ethylimidazolium triflate [1-EIm][TfO] and diethylmethylammonium triflate [Dema][TfO] is analyzed. The cationic acidity of these PILs varies by over ten orders of magnitude. Appropriate amounts of the PIL and H2O were mixed at various molar ratios to obtain compositions, varying from the neat PIL to H2O-excess conditions. The conductivity of [2-Sema][TfO] exponentially increases depending on the H2O concentration. The results from 1H-NMR spectroscopy and self-diffusion coefficient measurements by 1H field-gradient NMR indicate a fast proton exchange process between [2-Sema]+ and H2O. Conversely, [1-EIm][TfO] and [Dema][TfO] show only very slow or non-significant proton exchange, respectively, with H2O during the time-scale relevant for transport. The proton conduction follows a combination of vehicle and cooperative mechanisms in highly acidic PIL, while a mostly vehicle mechanism in medium and low acidic PIL occurs. Therefore, highly acidic ionic liquids are promising new candidates for polymer electrolyte fuel cells at an elevated temperature.

Operando Raman Spectroscopy Measurements of a High-Voltage Cathode Material for Lithium-Ion Batteries.

As a high-voltage spinel, LiNi0.5Mn1.5O4 (LNMO) is a promising candidate for high energy density cathodes in lithium-ion batteries (LiBs). The material has not yet achieved any commercial success, as there remain problems with capacity fade after extended charge and discharge cycling. In order to enable improvements, it is necessary to understand the fundamental underlying processes in the material. In this experimental study, we present operando Raman measurements to investigate the potential-resolved structural evolution of ordered LNMO as a cathode material during the charging and discharging process. Using the method of Raman spectroscopy, only two phases can be unequivocally distinguished in the case of ordered LNMO, namely, LiNi0.5Mn1.5O4 and Ni0.5Mn1.5O4 (NMO). The half-delithiated phase, Li0.5Ni0.5Mn1.5O4, cannot be discriminated by using this spectroscopic method. The dynamics of the phase changes between LiNi0.5Mn1.5O4 and Ni0.5Mn1.5O4 differ for lithiation and delithiation. Long-term operando Raman measurements of half-cells prove that a decomposition of the solvent takes place and that the conductive salt LiPF6 is consumed, i.e., the concentration of PF6- is strongly decreasing. The solvent component ethylene carbonate (EC) is preferentially decomposed during the cycling process, and byproducts such as esters and alcohols can be detected.

Current channeling along extended defects during electroreduction of SrTiO3.

Electroreduction experiments on metal oxides are well established for investigating the nature of the material change in memresistive devices, whose basic working principle is an electrically-induced reduction. While numerous research studies on this topic have been conducted, the influence of extended defects such as dislocations has not been addressed in detail hitherto. Here, we show by employing thermal microscopy to detect local Joule heating effects in the first stage of electroreduction of SrTiO3 that the current is channelled along extended defects such as dislocations which were introduced mechanically by scratching or sawing. After prolonged degradation, the matrix of the crystal is also electroreduced and the influence of the initially present dislocations diminished. At this stage, a hotspot at the anode develops due to stoichiometry polarisation leading not only to the gliding of existing dislocations, but also to the evolution of new dislocations. Such a formation is caused by electrical and thermal stress showing dislocations may play a significant role in resistive switching effects.

On the interfacial charge transfer between solid and liquid Li+ electrolytes.

The Li+ ion transfer between a solid and a liquid Li+ electrolyte has been investigated by DC polarisation techniques. The current density i is measured as a function of the electrochemical potential drop Δ[small mu, Greek, tilde]Li+ at the interface, using a liquid electrolyte with different Li+ concentrations. The subject of this experimental study is the interface between the solid electrolyte Ta-substituted lithium lanthanum zirconate (Li6.6La3Zr1.6Ta0.4O12) and a liquid electrolyte consisting of LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (1 : 1). The functional course of i vs. Δ[small mu, Greek, tilde]Li+ can be described by a serial connection between a constant ohmic resistance Rslei and a current dependent thermally activated ion transfer process. For the present solid-liquid electrolyte interface the areal resistance Rslei of the surface layer is independent of the Li+ concentration in the liquid electrolyte. At room temperature a value of about 300 Ω cm2 is found. The constant ohmic resistance Rslei can be attributed to a surface layer on the solid electrolyte with a (relatively) low conductivity (solid-liquid electrolyte interphase). The low conducting surface layer is formed by degradation reactions with the liquid electrolyte. Rslei is considerably increased if a small amount (ppm) of water is added to the liquid electrolyte. The thermally activated ionic transfer process obeys a Butler-Volmer like behaviour, resulting in an exchange current density i0 depending on the Li+ concentration in the liquid electrolyte by a power-law. At a Li+ concentration of 1 mol l-1 a value of 53.1 µA cm-2 is found. A charge transfer coefficient α of ∼0.44 is measured. The finding of a superposed constant ohmic resistance due to a solid-liquid electrolyte interphase and a current dependent thermally activated ion transfer process is confirmed by the results of two former experimental studies from the literature, performing AC measurements/EIS.

Oxygen tracer diffusion along interfaces of strained Y2O3/YSZ multilayers.

Heterophase boundaries can offer fast transport paths in solid electrolyte materials. In recent studies an enhancement of the ionic conductivity was indeed observed in micro-/nanoscaled Y(2)O(3)-stabilised ZrO(2) (YSZ) composites and hetero multilayers of thin films. As space charge regions can be neglected due to high charger carrier concentrations, we assume that strain and microstructural changes at the heterophase boundaries are responsible for the observed conductivity effects. In order to obtain independent information on the role of heterophase boundaries for fast transport in strained solid electrolytes, systematic measurements of the (18)O-tracer diffusion coefficient in nanoscaled YSZ/Y(2)O(3) multilayers were performed. Multilayer samples were prepared by Pulsed Laser Deposition (PLD) on (0001) Al(2)O(3) substrates and characterised by X-Ray Diffraction (XRD), Scanning Electron Microscopy (HRSEM) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). To separate interface and bulk transport from the total oxygen diffusivity of the multilayer system, the (average) thickness of the YSZ-layers in the multilayers was varied from 45 nm to 12 nm. Upon decreasing the thickness of the YSZ layers, respectively increasing the density of parallel interfaces, the total diffusion coefficient of the multilayer system is increased by a factor of 2 compared to bulk YSZ. The experimental results agree well with formerly published data for ionic conductivity measurements. They also support a negligible contribution of partial electronic conductivity in the multilayer.

18O-tracer diffusion along nanoscaled Sc2O3/yttria stabilized zirconia (YSZ) multilayers: on the influence of strain.

The oxygen tracer diffusion coefficient describing transport along nano-/microscaled YSZ/Sc2O3 multilayers as a function of the thick-ness of the ion-conducting YSZ layers has been measured by isotope exchange depth profiling (IEDP), using secondary ion mass spec-trometry (SIMS). The multilayer samples were prepared by pulsed laser deposition (PLD) on (0001) Al2O3 single crystalline substrates. The values for the oxygen tracer diffusion coefficient were analyzed as a combination of contributions from bulk and interface contributions and compared with results from YSZ/Y2O3-multilayers with similar microstructure. Using the Nernst-Einstein equation as the relation between diffusivity and electrical conductivity we find very good agreement between conductivity and diffusion data, and we exclude substantial electronic conductivity in the multilayers. The effect of hetero-interface transport can be well explained by a simple interface strain model. As the multilayer samples consist of columnar film crystallites with a defined inter-face structure and texture, we also discuss the influence of this particular microstructure on the interfacial strain.

Raman study of the polybenzimidazole-phosphoric acid interactions in membranes for fuel cells.

Poly(2,5-benzimidazole) (AB-PBI) membranes are investigated by studying the FT-Raman signals due to the benzimidazole ring vibration together with the C-C and C-H out-of- and in-plane ring deformations. By immersion in aqueous ortho-phosphoric acid for different time periods, membranes with various doping degrees, i.e. different molar fractions of acid, are prepared. The chemical-physical interactions between polymer and acid are studied through band shifting and intensity change of diagnostic peaks in the 500-2000 cm(-1) spectral range. The formation of hydrogen bonding networks surrounding the polymer seems to be the main reason for the observed interactions. Only if the AB-PBI polymer is highly doped, the Raman spectra show an additional signal, which can be attributed to the presence of free phosphoric acid molecules in the polymer network. For low and intermediate doping degrees no evidence for free phosphoric acid molecules can be seen in the spectra. The extent of the polymer-phosphoric acid interactions in the doped membrane material is reinvestigated after a period of one month and the stability discussed. Our results provide insight into the role of phosphoric acid as a medium in the conductivity mechanism in polybenzimidazole.

On the influence of strain on ion transport: microstructure and ionic conductivity of nanoscale YSZ|Sc2O3 multilayers.

Multilayer samples of the type (YSZ|Sc2O3) × n with layer thicknesses between 8 nm (n=100) and 250 nm (n=5) were prepared on (0001) sapphire substrates by pulsed laser deposition (PLD). The samples were characterised using X-ray diffraction (XRD), scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM/HRTEM, SAED (selected-area electron diffraction) and quantitative EELS (electron energy-loss spectroscopy)). The polycrystalline layers show a columnar microstructure, which is typical for the used preparation technique. The layers are highly textured and only one axial orientation relation is found between yttria-stabilised zirconia (YSZ), scandium oxide and the substrate: (0001) Al2O3‖(111) Sc2O3‖(111) YSZ. A preferred orientation relationship also exists for the azimuthal rotation of the crystallites, which was demonstrated by SAED, XRD pole figure measurements and fast Fourier transformation (FFT) of HRTEM micrographs. The interfaces between YSZ, Sc2O3 and the substrate are sharp and do not contain diffuse transition regions. Dislocations appear not to be arranged in regular arrays. With increasing interface density (thinner individual layers in the multilayer), the conductivity of the multilayers decreases. We relate this to the negative nominal misfit present at the YSZ|Sc2O3 interfaces (compressive stress in YSZ at the phase boundaries). This observation agrees well with the previously investigated case of YSZ|Y2O3 (A. Peters et al., Phys. Chem. Chem. Phys., 2008, 10, 4623), where tensile misfit strain was present in YSZ at the phase boundaries, leading to a conductivity increase.

Influence of interface structure on mass transport in phase boundaries between different ionic materials: Experimental studies and formal considerations.

ABSTRACT: Internal and external interfaces in solids exhibit completely different transport properties compared to the bulk. Transport parallel to grain or phase boundaries is usually strongly enhanced. Transport perpendicular to an interface is usually blocked, i.e., transport across an interface is often much slower. Due to the high density of interfaces in modern micro- and nanoscaled devices, a severe influence on the total transport properties can be expected. In contrast to diffusion in metal grain boundaries, transport phenomena in boundaries of ionic materials are still less understood. The specific transport properties along metal grain boundaries are explained by structural factors like packing densities or dislocation densities in the interface region. In most studies dealing with ionic materials, the interfacial transport properties are merely explained by the influence of space charge regions. In this study the influence of the interface structure on the interfacial transport properties of ionic materials is discussed in analogy to metallic materials. A qualitative model based on the density of misfit dislocations and on interfacial strain is introduced for (untilted and untwisted) phase boundaries. For experimental verification, the interfacial ionic conductivity of different multilayer systems consisting of stabilised ZrO2 and an insulating oxide is investigated as a funtion of structural mismatch. As predicted by the model, the interfacial conductivity increases when the lattice mismatch is increased.