Solid-state Electrochemical Cells Research Articles

Electrochemical Technique to Characterize the High Temperature Oxidation Behavior of Materials

This work demonstrates an approach using solid state electrochemical cells to study the long-term oxidation of materials at 800 °C. The capability of zirconia-based cells to control the oxygen partial pressure was first evaluated using an empty chamber. For most voltages applied to the pump cell, the steady state sensor voltage matches the pump voltage, leakage rates are low, and response times are short, allowing precise and prompt control over the chamber atmosphere. The technique was validated by measuring the oxidation of niobium and nickel. Niobium was oxidized at pump voltages ranging from 0 mV to +500 mV; decreasing the oxygen partial pressure around the specimen reduces the oxidation rate. Comparing the integrated oxidation rate with the weighed mass gain showed good agreement. Measured oxidation rates for nickel were of order 1 μg h−1, illustrating the sensitivity of this technique. For higher oxidation rates, a depression in oxygen partial pressure was observed around the specimen. Improved control over the oxidation potential was achieved by using a sensor cell to dynamically tune the pump voltage. Rates for both metals are compared to literature reports using other techniques.

In-Situ Raman Spectroscopy of Defined Copper Oxide Surfaces Formed by Electrochemically Controlled High-Temperature Oxidation

The oxidation processes of copper are of great importance since its occurrence leads to various problems, both in further processing and in operation of electronic devices and other technical applications. Extensive research on the thermal oxidation of copper has demonstrated that temperature and oxygen partial pressure affect the formation of oxides, oxidation rates and the chemical composition.In this work powerful electrochemical methods for controlled oxidation are presented, where the oxygen partial pressure on the metal surface can be varied over a range of 10-25 to 100 bar in a single experiment. A specific arrangement of the setup extends its capabilities to enable in-situ Raman spectroscopy measurements of the produced oxide. Controllable oxidation states are achieved in a solid-state electrochemical cell by polarizing a copper sample in contact with a single crystal of yttria-stabilized zirconia, which is conductive for oxygen ions at high temperatures. Optical transparency of the single crystal allows the Raman laser to pass through the crystal to measure the copper oxide formed on the copper-electrolyte interface. Cyclic voltammograms contain kinetic and thermodynamic information about the oxidation processes. Thus, the various oxidation reactions are controlled by the potential that corresponds to the oxygen partial pressure, while the current measures the rate of layer growth as well as the thickness of the oxide layer. In situ Raman measurements allow more detailed analyses of the oxides. The combined data will be used to build a model for oxide layer growth, with particular attention to the defect chemistry of the copper oxide.

RTILs as Electrolytes in Electrochemical Gas Sensors for O2 and Other Gases

Oxygen is the third-most abundant element on this planet and is essential to life, the lifestyle of humans and virtually all sentient life. We need O2 to live and additionally use it for both heating and transportation. Since O2 is so essential in our everyday life, there are several applications where the oxygen levels need to be controlled or maintained within a certain range. To make sure that the O2 level is within the necessary range, devices are needed that can monitor and feedback/communicate O2 levels continuously. There are several types of chemical gas sensors for O2 which include chemoresistive gas sensors and electrochemical gas sensors. The Clark-type electrochemical sensor is the oldest and probably the most used sensor for oxygen especially in medical applications. One of the most well-known industrial applications for gas-phase oxygen sensors is probably the control of oxygen levels in combustion engines with the help of a Lambda probe, a solid-state electrochemical cell operated in amperometric and potentiometric modes.Electrochemical sensors have evolved over the past decades. By finding the suitable combination of electrolyte, electrode material, cell design and operation conditions, electrochemical sensors are tunable to meet the specific requirements of the many different applications. For example, by changing the electrode material from gold to platinum while keeping everything else the same, you can reduce the response to CO by orders of magnitude improving the selectivity in low level measurements. Further, by adjusting the thermodynamic voltage of the sensing electrode, the electrochemical reactivity is adjusted to maximize the sensor’s selectivity for the target analyte. While these developments have occurred over time, recent work implements room temperature ionic liquids (RTILs) as electrolytes. This idea has some serious merit to it for multiple reasons. Firstly, many electrochemical sensors have aqueous electrolytes which freeze at low temperatures and rapidly evaporate at elevated temperatures. Additionally, aqueous electrolytes can change in real time with variations in background humidity levels. To be more exact, aqueous electrolytes dry out when operated in low humidity backgrounds and significantly increase in volume (swell) in high humidity background levels. While drying out is an issue because this leads to poor ionic conductivities, swelling is an issue because the electrolyte needs expansion room in the sensor, typically an extensive cavity within the cell body. RTILs have shown promise because they can extend the operating temperature range as well as lead to new sensor designs operatable over a wider range of environmental conditions. The implementation of RTILs might even allow for smaller sensor designs due to a smaller volume change of the electrolyte in dependence of changing humidity levels.Several research groups have investigated the oxygen sensing capabilities of RTILs in electrochemical devices. Silvester et al. investigated the oxygen solubility in several different imidazolium cations combined with [bis(trifluoromethylsulfonyl)imide] ([NTf2]-) anions and found a high oxygen solubility in 3-butyl-1-methyl imidazolium [NTf2]- [1]. They further investigated the influence of humidity on the oxygen reduction reaction and found the most stable behavior for [3-etyhl-1-methyl imidiazolium] [NTf2]- over a broad range of different humidity levels [2]. Yin et al. investigated 1-butyl-1-methylpyrrolidinium [NTf2]- as an electrolyte in oxygen sensors and found good linear correlation of the sensor output to the oxygen concentration [3].In this work, we investigated different RTILs, including the before mentioned 1-butyl-1-methylpyrrolidinium [NTf2]- and 3-butyl-1-methyl imidazolium [NTf2]-, regarding their suitability for thin layer electrochemical oxygen sensors in a background of mostly nitrogen. The performance was investigated using CV (cyclic voltammetry) and other time dependent voltametric techniques. The ability to detect oxygen of sensors with different RTILs as electrolyte was compared to that of aqueous H2SO4.[1] S. Doblinger, D.S. Silvester, M. Costa Gomes, Functionalized Imidazolium Bis(trifluoromethylsulfonyl)-imide Ionic Liquids for Gas Sensors: Solubility of H2, O2 and SO2, Fluid Phase Equilib. 549 (2021). https://doi.org/10.1016/j.fluid.2021.113211.[2] S. Doblinger, J. Lee, D.S. Silvester, Effect of Ionic Liquid Structure on the Oxygen Reduction Reaction under Humidified Conditions, Journal of Physical Chemistry C. 123 (2019) 10727–10737. https://doi.org/10.1021/acs.jpcc.8b12123.[3] H. Yin, H. Wan, L. Lin, X. Zeng, A.J. Mason, Miniaturized Planar RTIL-based Electrochemical Gas Sensor for Real-Time Point-of-Exposure Monitoring, 2016 IEEE Healthcare Innovation Point-Of-Care Technologies Conference (HI-POCT). (2016) 85–88. https://doi.org/10.1109/HIC.2016.7797703.

Regulating energy gap in Ir-based ionic complexes to generate near-infrared emissions: Application in solid-state light-emitting electrochemical cells

Solid-state light-emitting electrochemical cells (LECs) exhibit high potential for commercial electronics owing to their simple solution-processable device architectures, low-voltage operation, and compatibility with inert metal electrodes. However, the low device efficiency of most deep-red and near-infrared (NIR) LECs hinders their application (external quantum efficiency (EQE) < 1.00%). In this study, we demonstrate a simple method to tune the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of iridium-based ionic transition metal complexes (iTMCs) to generate NIR emissions. We investigate a series of cationic iridium complexes with small energy gaps, with 2,2′-bibenzo[d]thiazole fixed as N^N ligand moiety mainly controlling the LUMO energy level, changing a series of C^N ligands. All complexes exhibited deep red/NIR phosphorescence, and these combined devices provided emission peaks at 690–730 nm and were applied as components in LECs, exhibiting a maximum EQE of 1.78% in electroluminescence devices. Using a host–guest emission system with the iridium complex YIr as the host and complex TBBI as the guest, the highest EQE of LECs could be further enhanced to>2.34%, which is the highest value reported for NIR LECs.

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks.

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

White Light-Emitting Electrochemical Cells Employing Phosphor-Sensitized Thermally Activated Delayed Fluorescence to Approach All-Phosphorescent Device Efficiencies.

Solid-state light-emitting electrochemical cells (LECs) show promising advantages of simple device architecture, low operation voltage, and insensitivity to the electrode work functions such that they have high potential in low-cost display and lighting applications. In this work, novel white LECs based on phosphor-sensitized thermally activated delayed fluorescence (TADF) are proposed. The emissive layer of these white LECs is composed of a blue-green phosphorescent host doped with a deep-red TADF guest. Efficient singlet-to-triplet intersystem crossing (ISC) on the phosphorescent host and the subsequent Förster energy transfer from the host triplet excitons to guest singlet excitons can make use of both singlet and triplet excitons on the host. With the good spectral overlap between the host emission and the guest absorption, 0.075 wt.% guest doping is sufficient to cause substantial energy transfer efficiency (ca. 40 %). In addition, such a low guest concentration also reduces the self-quenching effect and a high photoluminescence quantum yield of up to 84 % ensures high device efficiency. The phosphor-sensitized TADF white LECs indeed show a high external quantum efficiency of 9.6 %, which is comparable with all-phosphorescent white LECs. By employing diffusive substrates to extract the light trapped in the substrate, the device efficiency can be further improved by ca. 50 %. In the meantime, the intrinsic EL spectrum and device lifetime of the white LECs recover since the microcavity effect is destroyed. This work successfully demonstrates that the phosphor-sensitized TADF white LECs are potential candidates for efficient white light-emitting devices.

Electrical treeing and luminescence in a polymer p-n junction under extreme reverse bias

A polymer p-n junction was subjected to an extreme reverse bias, up to 1000 V, to investigate its reverse bias characteristics and possible breakdown. A full breakdown did not take place even after more than 2 h of continuous stress. Electrical treeing, a pre-breakdown phenomenon commonly encountered in insulating polymers, was instead observed. The electrical treeing was accompanied by intense light emission, and incurred significant material loss in the polymer film, the latter which was attributed to hot electron bombardment. The impact by hot electrons also caused light emission from the luminescent conjugated polymer. The polymer junction was realized via in situ redox doping reactions in a planar solid-state light-emitting electrochemical cell made from a polymer mixed conductor. The cell was activated with a forward bias and subsequently cooled to freeze the p-n junction. The reverse bias of a doped polymer junction brings interesting opportunities to study the electrical treeing phenomenon, the luminescent conjugated polymer and the still lacking reverse-bias characteristics of an organic p-n junction.

Halogen Redox Shuttle Explains Voltage-Induced Halide Redistribution in Mixed-Halide Perovskite Devices

Voltage-induced halide segregation greatly limits the optoelectronic applications of mixed-halide perovskite devices, but a mechanistic explanation behind this phenomenon remains unclear. In this work, we use electron microscopy and elemental mapping to directly measure the halide redistribution in mixed-halide perovskite solar cells with quasi-ion-impermeable contact layers under different bias polarities to find iodide and bromide accumulation at the cathode and anode, respectively. This is consistent with a mechanism based on preferential iodide oxidation at the anode, leading to unbalanced Ii+, IX−, and BrX− fluxes. Importantly, switching the anode from “inert” Au to “active” Ag prevents segregation because Ag oxidation precludes the oxidation of lattice iodide, which suggests employing redox-active additives as a general strategy to suppress halide segregation. Overall, these results show that halide perovskite devices operate as solid-state electrochemical cells when threshold voltages are exceeded, providing fresh insight to understand the impacts of voltage bias on halide perovskite devices.

(Invited) Understanding and Mitigating Chemo-Mechanical Degradation in Solid Oxide Cells

Solid-state electrochemical cells may undergo significant chemical potential changes in the electrolyte region near the electrodes, depending on operating conditions. Lithium dendrite growth, which may eventually induce mechanical stresses and short circuit in all-solid-state batteries (SSBs) in the charging process, is one of the well-known examples representing chemo-mechanical failure in solid-state electrochemical cells [1, 2]. A similar phenomenon has been observed in solid oxide cells (SOCs) operated in electrolysis mode and also in fuel cell mode with a negative voltage [3, 4]. Contrary to lithium precipitation in SSBs, high partial pressure of oxygen is developed in SOCs, resulting in electrode/electrolyte delamination. In the present study, we investigated the mechanism of delamination growth in SOCs with respect to operating conditions, based on which we determined a critical factor correlated to mechanical failure. This study provides basic understanding and critical insights into the chemo-mechanical failure mechanism of SOCs with respect to operating conditions.

Systematic Studies of Phase Relation and Thermodynamic Stability of Ln-Rh-O System for Catalysis and Electrochemical Devices

Ln-Rh-O systems show potential interest in different catalytic applications. Phase diagram of these systems revealed that only LnRhO3, an orthorhombic perovskite structure, has been found as stable ternary oxide. Employed solid-state electrochemical cell to determine the thermodynamic parameters of LnRhO3 compounds from their binary oxides Rh2O3 and Ln2O3. Calcia stabilized and yttria stabilized zirconia has been used as solid electrolytes in these cells. Ternary phase relation for Ln-Rh-O systems is established using isothermal equilibration technique. Phase diagram of few Ln-Rh-O systems are computed with the help of supplementary data for other binary phases (Ln-Rh) from the literature and thermodynamic data for LnRhO3 from experiment at high temperature. At 298.15 K, 〖∆H〗_f^0and S^0for LnRhO3are calculated from its constituting elements. Thermodynamic data has been used to compute an oxygen potential ‒ composition and three-dimensional chemical potential diagram for the system Ln-Rh-O.

Cationic Ir(III) Complexes Featuring Phenylimidazole-type Cyclometalated Ligands: Fluorine-Free Blue Phosphorescent Emitters for Light-Emitting Devices.

The development of blue emissive cationic Ir(III) complexes with no fluorine substitutions but with sufficient blue color purity and high phosphorescence efficiency has remained challenging. Here, fluorine-free cyan to deep blue emissive cationic Ir(III) complexes with phenylimidazole-type cyclometalated ligands (C∧N) are reported, which are [Ir(dphim)2(dmapzpy)]PF6 (1), [Ir(ipr-dphim)2(dmapzpy)]PF6 (2), [Ir(ipr-dphim)2(bipz)]PF6 (3), and [Ir(ipr-dphim)2(bicb)]PF6 (4). 1,2-Diphenyl-1H-imidazole (dphim) and 1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole (ipr-dphim) are the phenylimidazole-type C∧N ligands, and 4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine (dmapzpy), di(1H-pyrazol-1-yl)methane (bipz), and 3,3'-methylenebis(1-methyl-1H-imidazol-3-ium-2-ide) (bicb) are the neutral ancillary ligands (A∧A). In both solution and diluted films, complex 1 shows a cyan emission with the emission maxima at ∼472 and 495 nm, and complexes 2-4 provide a deep blue emission with the emission maxima at ∼460 and 480 nm. While the complexes exhibit low to moderate phosphorescence efficiencies (0.05-0.35) in a degassed CH3CN solution, they exhibit high phosphorescence efficiencies (up to 0.82) in diluted films. Theoretical calculations revealed that the mixed 3π-π* (C∧N-centered)/3MLCT (Ir → C∧N) states are responsible for the emission afforded by complexes 1-4, which undergo nonradiative deactivations induced by different types of metal-centered states. Organic light-emitting diodes with complexes 1-4 as phosphorescent dopants are fabricated by a solution process, which affords a blue-green to blue emission with the emission maxima at ∼460 and 490 nm for the blue devices and a high current efficiency at 28.1 cd A-1 for the blue-green device. Solid-state light-emitting electrochemical cells are also fabricated with complexes 1-2 as phosphorescent dopants, which provide green-blue to blue emission with a high luminance (up to 840 cd m-2) and current-efficiency (up to 16.8 cd A-1) under a constant-current driving. The work reveals that, by using phenylimidazole-type C∧N ligands and optimized A∧A ligands, blue emissive cationic Ir(III) complexes with no fluorine substitutions but with sufficient blue-color purity and a high phosphorescence efficiency can be developed.

Understanding Oxygen Anion Transport in Nb-Doped La0.7Sr0.3CoO3 Perovskite Materials

Simple perovskite structured electrode materials of the series (LaxSr1-xCoO3- δ) are commonly utilised for high-temperature solid-state electrochemical cells and Li-air batteries with some limitations observed on prolong operations due to the structural instability associated with the segregation of strontium ions. In order to overcome this structural incompatibility, doping of heterovalent dopant at the B-site is proposed which could possibly improve upon the stability by controlling the segregation of surface cations. In addition, the doped structure is likely to provide faster oxygen anion transport through the vacancies in the structure, thereby improving its electrochemical performance. Herein, oxygen anion diffusion in pure La0.7Sr0.3CoO3-- δ (LSC) and Nb-doped La0.7Sr0.3Co0.95Nb0.05O3- δ (LCSN) perovskite structure is calculated using molecular dynamics (MD) simulations. Self diffusion coefficient of oxygen ions in the LSCN structure is observed to increase by an order of magnitude (1.356×10-8 cm2/s) on Nb doping as compared to the undoped LSC structure (6.499×10-9 cm2/s) at 1073 K. On varying the Nb concentration from 0.05 to 0.15 and optimum Nb concentration is estimated to be 0.05 corresponding to a maximum oxygen anion diffusivity in the structure (1.356×10-8 cm2/s) at 1073 K.

Green to blue-green-emitting cationic iridium complexes with a CF3-substituted phenyl-triazole type cyclometalating ligand: synthesis, characterization and their use for efficient light-emitting electrochemical cells.

Green to blue-green-emitting cationic iridium complexes free of sp2 C-F bonds, namely [Ir(CF3-dPhTAZ)2(bpy)]PF6 (1), [Ir(CF3-dPhTAZ)2(dmebpy)]PF6 (2) and [Ir(CF3-dPhTAZ)2(phpyim)]PF6 (3), have been designed and synthesized with 3,4-diphenyl-5-(trifluoromethyl)-4H-1,2,4-triazole (CF3-dPhTAZ) as the cyclometalating ligand (C^N) and 2,2'-bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (dmebpy) or 2-(1-phenyl-1H-imidazol-2-yl)pyridine (phpyim) as the ancillary ligand (N^N). In CH3CN solution, complexes 1-3 afford green to blue-green emission centered at 521, 508 and 498 nm, respectively. The electron-withdrawing CF3 group attached at the triazole ring in CF3-dPhTAZ largely blue-shifts (by over 20 nm) the emission of the complex through stabilizing the highest occupied molecular orbital. In doped films, the complexes afford sky-blue emission with near-unity phosphorescent efficiencies. In neat films, the complexes show largely suppressed phosphorescence concentration-quenching, with phosphorescent efficiencies of up to 0.66. Theoretical calculations reveal that the emission of the complexes can arise from either charge-transfer (Ir → C^N/C^N → N^N) or C^N/N^N-centered 3π-π* states, depending on the local environment of the complexes. Solid-state light-emitting electrochemical cells (LECs) based on the complexes afford green to blue-green electroluminescence centered at 525, 517 and 509 nm, respectively, with high current efficiencies of up to 35.1 cd A-1. The work reveals that CF3-dPhTAZ is a promising C^N ligand free of sp2 C-F bonds for constructing efficient cationic iridium complexes with blue-shifted emission.

Optimizing carrier balance of a red quantum-dot light-emitting electrochemical cell with a carrier injection layer of cationic Ir(III) complex

Abstract Solid-state light-emitting electrochemical cells (LECs) with promising features of solution processability, low-voltage operation and compatibility with inert cathode metals have shown great potential in display and lighting applications in recent years. Among the reported emissive materials for LECs, ionic transition metal complexes (iTMCs) have relatively higher electroluminescence (EL) efficiencies due to their phosphorescent property. However, the red iTMCs generally exhibit moderate color saturation and low emission efficiency, limiting their display applications. To improve color saturation and device efficiency of red LECs, efficient quantum dots (QDs) with narrow emission bandwidth are good alternative emissive materials. In this work, efficient and saturated red QD LECs employing iTMC carrier injection layers to provide in situ electrochemical doping are demonstrated. The thicknesses of iTMC and red-QD layers are systematically adjusted to achieve the best carrier balance. In the optimized device, the iTMC carrier injection layer facilitates hole injection into the red-QD layer while electrons are injected from the cathode into the red-QD layer directly since the electron injection barrier is low. The Commission Internationale de I'Eclairage (CIE) coordinates of the EL spectra approach the red standard point of National Television System Committee (NTSC). High external quantum efficiency and current efficiency reaching 9.7% and 16.1 cd A−1, respectively. These results confirm superior carrier balance in such a simple iTMC/QD bilayer device structure. Furthermore, compared with iTMC LECs, less degree of device efficiency roll-off upon increasing device current is observed in QD LECs since a shorter excited-state lifetime of fluorescent QDs reduces the probability of collision exciton quenching. Saturated and efficient red EL with mitigated efficiency roll-off from red-QD LECs employing iTMC carrier injection layers confirms that they are good candidates of saturated light sources for displays.

Electrogenerated Chemiluminescence and Electroluminescence of N-Doped Graphene Quantum Dots Fabricated from an Electrochemical Exfoliation Process in Nitrogen-Containing Electrolytes.

Artificial lighting sources are one of the most important technological developments for our modern lives; the search for cost-effective and efficient luminophores is therefore crucial to a sustainable future. Graphene quantum dots (GQDs) are carbon-based nanomaterials that exhibit exceptional optical and electronic properties, making them a prime candidate for a luminophore in a light-emitting device. Nitrogen-doped GQDs fabricated from a facile top-down electrochemical exfoliation process with a nitrogen-containing electrolyte in this report showed strong photoluminescent emission at 450 nm, and electrogenerated chemiluminescence at 660 nm in the presence of benzoyl peroxide as a coreactant. When introduced into solid-state light-emitting electrochemical cells, for the first time, the GQDs displayed a broad white emission centered at 610 nm, corresponding to Commision Internationale de l'eclairage (CIE) colour coordinates of (0.38, 0.36).

Real-time measurement of tumour hypoxia using an implantable microfabricated oxygen sensor

Hypoxia commonly occurs within tumours and is a major cause of radiotherapy resistance. Clinical outcomes could be improved by locating and selectively increasing the dose delivered to hypoxic regions. Here we describe a miniature implantable sensor for real-time monitoring of tissue oxygenation that could enable this novel treatment approach to be implemented. The sensor uses a solid-state electrochemical cell that was microfabricated at wafer level on a silicon substrate, and includes an integrated reference electrode and electrolyte membrane. It gave a linear response to oxygen concentration, and was unaffected by sterilisation and irradiation, but showed susceptibility to biofouling. Oxygen selectivity was also evaluated against various clinically relevant electroactive compounds. We investigated its robustness and functionality under realistic clinical conditions using a sheep model of lung cancer. The sensor remained functional following CT-guided tumour implantation, and was sufficiently sensitive to track acute changes in oxygenation within tumour tissue.

In Situ Raman Characterization of SOFC Materials in Operational Conditions: A Doped Ceria Study.

The particular operational conditions of electrochemical cells make the simultaneous characterization of both structural and transport properties challenging. The rapidity and flexibility of the acquisition of Raman spectra places this technique as a good candidate to measure operating properties and changes. Raman spectroscopy has been applied to well-known lanthanide ceria materials and the structural dependence on the dopant has been extracted. The evolution of Pr-doped ceria with temperature has been recorded by means of a commercial cell showing a clear increment in oxygen vacancies concentration. To elucidate the changes undergone by the electrolyte or membrane material in cell operation, the detailed construction of a homemade Raman cell is reported. The cell can be electrified, sealed and different gases can be fed into the cell chambers, so that the material behavior in the reaction surface and species evolved can be tracked. The results show that the Raman technique is a feasible and rather simple experimental option for operating characterization of solid-state electrochemical cell materials, although the treatment of the extracted data is not straightforward.

(Invited) Electrochemistry Meets Heterogeneous Catalysis: Solid Acid Based Electrochemical Cells Using Cu- and Pt-Based Electrodes in CO2 Containing Atmospheres

Electrochemical reactors operating at intermediate temperatures (200 – 400 °C) have the advantage that they can be thermally integrated into other chemical processes like synthesis of synthetic fuels and chemicals, such as methanol and ammonia. Electrochemical reduction of CO2 to chemical building blocks such as CO, CH3OH, and CH4 is a dream for electrochemists, and high faradaic efficiencies have been reported for liquid electrochemical cells operated at ambient temperature, using e.g. Cu electrodes. However, selectivity and electrochemical activity are far from being technically relevant, so that heterogeneous catalysis processes still are the matter of choice. In this contribution, solid state electrochemical cells under operating conditions close to the well-known catalytic synthesis of methanol. For CO2 reduction, cells based on CsH2PO4 as electrolyte and Cu based cathodes have been investigated towards their electrochemical activity in both H2/H2O and H2/H2O/CO2 containing atmospheres at elevated temperatures of 240 °C.

Potential Control of Oxygen Non-Stoichiometry in Cerium Oxide and Phase Transition Away from Equilibrium.

Cerium oxide (ceria, CeO2) is a technologically important material for energy conversion applications. Its activities strongly depend on redox states and oxygen vacancy concentration. Understanding the functionality of chemical active species and behavior of oxygen vacancy during operation, especially in high-temperature solid-state electrochemical cells, is the key to advance future material design. Herein, the structure evolution of ceria is spatially resolved using bulk-sensitive operando X-ray diffraction and spectroscopy techniques. During water electrolysis, ceria undergoes reduction, and its oxygen non-stoichiometry shows a dependence on the electrochemical current. Cerium local bonding environments vary concurrently to accommodate oxygen vacancy formation, resulting in changes in Ce-O coordination number and Ce3+/Ce4+ redox couple. When reduced enough, a crystallographic phase transition occurs from α to an α' phase with more oxygen vacancies. Nevertheless, the transition behavior is intriguingly different from the one predicted in the standard phase diagram of ceria. This paper demonstrates a feasible means to control oxygen non-stoichiometry in ceria via electrochemical potential. It also sheds light on the mechanism of phase transitions induced by electrochemical potential. For electrochemical systems, effects from a large-scale electrical environment should be taken into consideration, besides effective oxygen partial pressure and temperature.

The ruthenium pyrochlore Dy2Ru2O7(s): Stability and calorimetry

The 4d transition metal ruthenium pyrochlore Dy2Ru2O7(s) was synthesized by solid state reaction route. High temperature studies on the ternary oxide Dy2Ru2O7(s) an important ferroelectric and magnetic material, has been carried out using solid-state electrochemical cell, with calcia stabilized zirconia electrolyte and the reversible electromotive force were measured. The Gibbs energy of formation of Dy2Ru2O7(s) from elements in their standard state, is: $$ \begin{aligned} \{ \Delta_{\text{f}} G^{ \circ } ({\text{Dy}}_{2} {\text{Ru}}_{2} {\text{O}}_{7} ,{\text{s}}) /({\text{kJ mol}}^{ - 1} ) \pm 4.4\} & = - 2505.0 + 0.627 \cdot \left( {T/{\text{K}}} \right); & \quad (859 \le T / {\text{K}} \le 1136). \end{aligned} $$ The standard molar heat capacity C˚p,m(T) of Dy2Ru2O7(s), measured using a differential scanning calorimeter run in the heat flux mode as a function of temperature. The values fitted to a polynomial expression can be represented by: $$ C_{{{\text{p}},{\text{m}}}}^{ \circ } \,({\text{Dy}}_{2} {\text{Ru}}_{2} {\text{O}}_{7} ,{\text{s}},T)\,({\text{J K}}^{ - 1} \, {\text{mol}}^{ - 1} ) = 235.09 + 3.99 \times 10^{ - 2} T({\text{K}}) - 10.53 \times 10^{5} / {\text{T}}^{2} ({\text{K}}). $$ Stability of Dy2Ru2O7(s) and high temperature thermodynamic functions of this transition metal pyrochlore were determined for the first time, an important fundamental study.