Proton-conducting Solid Electrolyte Research Articles

Complete Mechanistic Elucidation of Current–Voltage Characteristics of Grain Boundaries in a Proton-Conducting Solid Electrolyte

Ionic current in proton-conducting polycrystalline ceramics is often hampered a great deal by the grain boundaries, limiting their prospective applications as solid electrolytes for next-generation solid oxide fuel cells. To elucidate the conduction mechanism at the grain boundaries, we use a linear diffusion model and impedance spectroscopy to report complete current–voltage (I–V) characteristics of the grain boundaries in 0.5 mol % Sr-doped LaNbO4. We provide the first experimental evidence of complete annihilation of the space charge-induced proton depletion at the grain boundaries upon applying moderate bias voltages. We also show that it is possible to distinguish between the grain boundary resistance caused by the space charge and other sources by analyzing the I–V characteristics. The analysis is equally valid for other solid ionic conductors such that it can serve as an important tool to guide further optimization of the conductivity of polycrystalline solid electrolytes.

Confinement of poly(allylamine) in Preyssler-type polyoxometalate and potassium ion framework for enhanced proton conductivity

Polyoxometalate based solids are promising candidates of proton-conducting solid electrolytes. In this work, a Preyssler-type polyoxometalate is crystallized with potassium ions and poly(allylamine), which is also a good proton conductor, from aqueous solutions. Here we show that the hygroscopicity induced low durability of polyoxometalate and poly(allylamine) can be circumvented by the electrostatic interaction between the polyoxometalate and protonated amine moieties in the solid state. Crystalline compounds are synthesized with poly(allylamine) of different average molecular weights, and all compounds achieve proton conductivities of 10−2 S cm−1 under mild-humidity and low-temperature conditions. Spectroscopic studies reveal that the side-chain mobility of poly(allylamine) and hydrogen-bonding network rearrangement contribute to the proton conduction of compounds with poly(allylamine) of low and high average molecular weights, respectively. While numbers of proton-conducting amorphous polyoxometalate-polymer composites are reported previously, these results show both structure-property relationship and high functionality in crystalline composites.

Synthesis and Properties of La0.95Sr0.05ScO3–δ Protic Electrolyte Films on Porous Cathode Material

Fundamental aspects of the phase formation and morphology of films of the proton-conducting solid electrolyte La0.95Sr0.05ScO3–δ, produced by wet deposition from inorganic salts on a porous cathode substrate La0.6Sr0.4MnO3–δ. The conditions in which gas-tight films are deposited onto substrates with porosity of up to 25% were determined. The electrical conductivity of La0.95Sr0.05ScO3–δ films was examined in relation to temperature and gas-phase composition (рН2О, рО2). The effect of the architecture of the electrolyte membrane on its transport properties was analyzed. The results of the study are of fundamental importance for development of proton-ceramic fuel cells with a supporting electrode.

Study of proton-conducting polymer electrolyte based on K-carrageenan and NH4SCN for electrochemical devices

A proton-conducting solid biopolymer electrolyte (SBE) that consists of kappa-carrageenan (K-carrageenan) incorporated with ammonium thiocyanate (NH4SCN) has been prepared by solution-casting technique. The prepared membranes are subjected to XRD, FTIR, and AC impedance analysis to study their structural, optical, and electrical properties of the electrolytes, respectively. The amorphous nature of the membrane has been inferred from the results of XRD analysis. The complexation between salt and host polymer matrix has been confirmed by FTIR. From AC impedance analysis, the polymer incorporated with 0.5% of NH4SCN achieved a maximum ionic conductivity of the order of 6.83 × 10−4 Scm−1. The dielectric spectra and modulus spectra reveal the non-Debye behavior of the polymer membrane. A fuel cell has been constructed using the prepared SBE, and its open circuit voltage has been measured.

Conductive Supramolecular Architecture Constructed from Polyoxovanadate Cluster and Heterocyclic Surfactant

Proton-conductive solid electrolytes are significant for fuel-cell battery technology. Especially for use in motor vehicles, proton conductors which work at intermediate temperatures (373–673 K) under an anhydrous atmosphere are desired to improve the fuel cell stability and efficiency. Inorganic–organic hybrid supramolecular architectures are a promising option for the realization of highly conductive proton conductors. Here, a hybrid layered crystal was synthesized for the first time by using an proton-containing decavanadate (V10) anion and a heterocyclic surfactant cation. A simple ion-exchange reaction led to the formation of an inorganic–organic hybrid of V10 by using dodecylpyridazinium (C12pda) as the heterocyclic surfactant. Single crystal X-ray analyses revealed that four C12pda cations were associated with one V10 anion, which was a diprotonated species forming a one-dimensional infinite chain structure through hydrogen bonds. Anhydrous proton conductivity was investigated by alternating current (AC) impedance spectroscopy in the range of 313–393 K, exhibiting a maximum value of 1.7 × 10−5 S cm−1 at 373 K.

Graphene oxide-anchored reactive sulfonated copolymer via simple one pot condensation polymerization: proton-conducting solid electrolytes

A rapid and efficient post-polymerization functionalization of poly(urea-co-urethane) (PUU) onto the graphene oxide (GO) nanosheets has been developed to produce super-acidic polymer/GO hybrid nanosheets. Thus, the surface of GO nanosheets were functionalized with 3-(triethoxysilyl)propyl isocyanate (TESPIC) from hydroxyl groups to yield isocyanate functionalized graphene oxide nanosheets. Then, sulfonated polymer/GO hybrid nanosheets were prepared by condensation polymerization of isocyanate-terminated pre-polyurea onto isocyanate functionalized graphene oxide nanosheets through the formation of carbamate bonds. FTIR and TGA results indicated that TESPIC modifier agent and poly(urea-co-urethane) were successfully grafted onto the GO nanosheets. The grafting efficiency of poly(urea-co-urethane) polymer onto the GO nanosheets was estimated from TGA thermograms to be 205.9%. Also, sulfonated polymer/GO hybrid nanosheets showed a proton conductivity as high as 3.7 mS cm−1. Modification and morphology of GO nanosheets before and after modification processes were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD).

Electrochemical Acceleration of Ammonia Synthesis on Fe-Based Alkali-Promoted Electrocatalyst with Proton Conducting Solid Electrolyte

To accelerate ammonia synthesis, the effect of the electrode potential on the kinetics of ammonia synthesis was investigated with a proton-conducting solid electrolyte, BaCe0.9Y0.1O3 (BCY), at temperatures between 500–650 °C. Ammonia synthesis was conducted using a double chamber electrochemical setup with an electrolyte-supported Pt|BCY|K,Al-modified Fe-BCY cell. Although slow ammonia formation kinetics by cathodic polarization was observed when pure N2 was supplied to the cathode side, obvious acceleration of the ammonia formation rate by cathodic polarization was observed following addition of 15% H2 to the cathode side. The ammonia formation rate increased more than 20 times at −1.5 V relative to that at the open circuit voltage, which was not observed by anodic polarization. Notably, the acceleration at the cathodic potential was observed over 610 °C. These results indicate that the enhancement of ammonia formation occurs because of promotion of nitrogen dissociation by cathodic polarization and a ch...

Incorporation of NH4NO3 into MC-PVA blend-based polymer to prepare proton-conducting polymer electrolyte films

Proton-conducting solid polymer blend electrolytes based on methylcellulose-polyvinyl alcohol:ammonium nitrate (MC-PVA:NH4NO3) were prepared by the solution cast technique. The structural and electrical properties of the samples were examined by X-ray diffraction (XRD), Fourier transform infrared (FTIR), and electrical impedance (EI) spectroscopy. The shifting and change in the intensity of FTIR bands of the electrolyte samples confirm the complex formation between the MC-PVA polymer blend and the NH4NO3 added salt. The observed broadening in the XRD pattern of the doped samples reveals the increase of the amorphous fraction of polymer electrolyte samples. The increase in electrical conductivity of polymer electrolyte samples with increasing salt concentration attributed to the formation of charge-transfer complexes, and to increase in the amorphous domains. A maximum ionic conductivity of about 7.39 × 10−5 S cm−1 was achieved at room temperature for the sample incorporating 20 wt% of NH4NO3. The DC conductivity of the present polymer system exhibits Arrhenius-type dependence with temperature. The decrease in the values of activation energies with increasing salt concentration indicates the ease mobility of ions. The decrease in dielectric constant with increasing frequency was observed at all temperatures. Optical properties such as absorption edge, optical band gap, and tail of localized state were estimated for polymer blend and their electrolyte films. It was found that the optical band gap values shifted towards lower photon energy from 6.06 to 4.75 eV by altering the NH4NO3 salt content.

Structural and transport properties of lanthanum tungstate with high La/W ratio: Suitability for proton-conducting solid oxide fuel cells operating at intermediate temperature

Systematic characterization of lanthanum tungstate (LWO) with a high La/W ratio was conducted to evaluate its suitability as a proton-conducting solid electrolyte for fuel cells operating at intermediate temperature. In this study, the crystal structure, ionic and electronic transport properties of LWO with a high La/W ratio (La/W=6.7: LWO67), which had high proton conductivity, were investigated. Rietveld analysis using synchrotron powder X-ray diffraction confirmed the single-phase formation of LWO67 having a cubic crystal structure with the Fm3¯m space group. The coefficient of thermal expansion of LWO67 was also evaluated from Rietveld analysis at high temperatures. Thermogravimetric analysis indicated a good chemical stability of LWO67 under wet CO2 conditions. The observed electrical properties indicated that the proton conductivity was dominant under wet conditions below 873K. The dependence of the electrical conductivity on oxygen partial pressure suggested that the electron and hole conductivities were suppressed sufficiently below 973K. Based on the observed transport properties, the superiority of LWO67 applied to a proton-conducting solid electrolyte fuel cells (p-ITFCs) was also discussed. The optimized electrolyte thickness of LWO67 was several tens of micrometers, which is more suitable than perovskite-type proton conductors such as BaZrO3 and BaCeO3 for electrode-supported fuel cells, when they are compared in terms of power generation efficiency.

High-temperature and composite proton-conducting electrolytes

This paper presents a continuation of a review of research on proton-conducting solid electrolytes. We consider high-temperature proton-conducting materials based on mixed oxides of polyvalent elements whose structure contains a high concentration of oxygen vacancies. Their interaction with water vapor leads to the formation of two OH groups with a highly mobile proton. Conduction in such materials is not unipolar: proton conduction in them is usually accompanied by oxygen-ion conductivity and, often, by electron transport. The proton conductivity of acid salts can be raised using their composites with fine particles of polyvalent element oxides. Their large surface area and its high activity lead to an increase in defect density in the ionic salt near interfaces owing to sorption processes. A similar approach, involving the formation of hybrid systems through the incorporation of oxide nanoparticles, is used to increase the conductivity of ion exchange membranes, but the effect is achieved due to a transformation of the system of pores and channels.

Low- and intermediate-temperature proton-conducting electrolytes

We consider the main achievements and problems in the field of proton-conducting solid electrolytes. Because of the extremely small proton radius, mechanisms of proton transport differ drastically from those for other ions. This review examines general aspects of proton transport in low- and intermediate-temperature proton-conducting solid electrolytes, such as hydrated acids, acid salts, and bases and anhydrous acid salts. Proton transport in these materials is due to a sequence of rotations of proton-containing groups and proton hops between them.

Changes of coordination modes of Cu-based coordination complexes as tuneable proton-conducting solid electrolytes

We have synthesised three proton-conducting coordination complexes as solid proton-conducting electrolytes. Changes of sulfonate group coordination modes and different packing modes, as well as introduction of halide ions may remarkably enhance the proton conductivity of coordination compounds.

In situ formation of Ru nanoparticles on La1−x Sr x TiO3-based mixed conducting electrodes and their application in electrochemical synthesis of ammonia using a proton-conducting solid electrolyte

In situ formation of highly dispersed 2–5 nm Ru nanoparticles was successfully achieved in highly Ru-doped (40% Ru) perovskite cathode material, Ru-doped La0.3Sr0.6TiO3 (LSTR), used for electrochemical ammonia synthesis in a proton-conducting fuel cell (PCFC). Highly dispersed Ru nanoparticles were observed by exsolution of doped Ru from the B-site in the LSTR perovskite lattice under PCFCs operating conditions, that is, a reducing atmosphere, without agglomeration, which is difficult to achieve with a conventional impregnation method. The particle size and dispersion were successfully controlled by reduction temperature and Ru doping amount. The optimized microstructure was finally adopted for the electrochemical synthesis of ammonia with BaCe0.9Y0.1O3 solid electrolyte at 500 °C. Ammonia formation using a 40% Ru-doped LSTR (LSTR40) cathode was observed by applying cathodic overpotentials. The maximum ammonia formation rate using the LSTR40 cathode was 3.8 × 10−12 mol s−1 cm−2 at −0.1 V. The results suggest that Ru nanoparticles exsoluted from the LSTR perovskite act as a active site effectively for the electrochemical reaction.

Mixed Proton/Electron Conductivity in Nano-Grained Bulk Body of TiO2

Background Proton-conducting solid electrolytes are paid much attention from various applications such as SOFC and electrochemical catalyst. As a new class of materials other than bulk conductors, low temperature proton conductivity on the surface of nano-grained oxides like YSZ [1] and CeO2 is studied by many researchers in these days. The previous study by the authors has suggested that the dominant mechanism for proton conduction depends on the temperature, and three types of mechanism are suggested for corresponding temperature ranges; a proton hopping via hydroxyl(M-OH−) on the surface of metal oxide at the highest temperature range above 250, that via hydrogen-bonded H2O molecules by the Grotthuss mechanism at temperatures between 60 ºC and 250 ºC, and the Vehicle mechanism, using H3O+s as a vehicle. H3O+s are delocalized from the surface to diffuse into the hydrated multilayer above the surface below 60 ºC. Possible and unique applications of this surface proton conductivity are electrochemical catalysts and reactors by combining surface proton conductivity with variety of bulk functionalities like electronic conductivity and photocatalytic activity. To promote the catalytic reaction, i.e.COOH+3H++3e-→CH3OH+1/2O2, both protons and electrons are to be migrated to the active sites for anodic and cathodic reactions. As TiO2 in anatase form is a bulk n-type semiconductor, nano-grain anatase is a candidate to achieve both proton and electron conduction simultaneously. In this research, electrical conductivity of nano-grained TiO2 is analyzed to examine a possible application of TiO2 for such an electrochemical and photochemical catalysts. Method TiO2 nanoparticle in anatase form was obtained by hydrolysis of alkoxide (Ti(OC4H9)4). The obtained nano-grained powder with its grain size about ~20nm, estimated from XRD and SEM images, was pressed at room temperature (RT), using cubic anvil at 4GPa with pyrophylite as a pressure media, resulting in a compact body of anatase. After shaping into rectangular plate, Au electrodes on both sides of rectangular face ware fabricated, and the sample was subjected to subsequent electrical conductivity measurement. Two-probe ac impedance measurements ware conducted under controlled temperature and gas atmospheres. The gas mixtures employed are categorized as reducing (1%H2-99%Ar) and oxidizing atmosphere (100%O2) with and without CO2. All the gas mixtures are humidified by either H2O or D2O, to examine isotope effect. The measurements were made from RT to 400 ºC FT-IR is used to characterize surface adsorbates at the same atmosphere and temperatures with electrical measurements. Result Figure1 shows the electrical conductivity under oxidizing and reducing conditions with and without CO2. In oxidized atmosphere, the measured conductivity above 250 ºC increases with the increase of temperature, while the conductivity increases with the decrease of temperature below 250 ºC which is typical behavior of surface proton conductivity of nano-grained oxide materials. In addition, H-D isotope effect on the conductivity is observed in entire temperature range measured, suggesting the predominant proton conductivity. An inversion of the temperature dependence of the electrical conductivity around 250 ºC suggests the presence of two different proton conductivity mechanisms in the nano-grained TiO2 system. In reducing atmosphere (1%H2−99%Ar), the conductivities above 150 ºC are higher than those in oxidized atmosphere by several orders of magnitude, in contrast to the similar values between oxidizing and reducing atmosphere below 150 ºC In addition, the conductivity is approximately proportional to one half power of P H2(partial pressure of H2 gas). This dependency can be explained by the following formation reaction of interstitial Ti3+, which is considered to be the dominant defect in bulk TiO2. TiTi x+2OO x+2H2→Tii •••+3e’+2H2O This consistency with the bulk defect chemical reaction suggests that the electronic conduction in bulk is dominant in reduced atmosphere above 150 ºC The influence of the CO2 adsorption is limited to the low temperature range below 175 ºC, probably because of desorption of CO2 above this temperature. The decrease of conductivity by CO2 adsorption observed agrees with the decrease observed in nano-grained YSZs reported earlier, suggesting similar mechanism is seemingly working behind. In summary, we could successfully observe both electronic and ionic conductivities for nano-grained TiO2 bulk body fabricated by ultra-high pressure pressing at room temperature. Reference [1] S. Miyoshi et al. Chem. Mater., 26 (2014) 5194.[2] R. Sato et al. J. Phys. Chem. C, 119 (2015) 28925. Figure 1

Conceptual design for critically-high power generation efficiency by using multi-stage solid oxide fuel cells (SOFCs) or proton-conducting ceramic fuel cells (PCFCs)

Recently we have developed the conceptual design for realizing a critically-high electrical efficiency with solid oxide fuel cells (SOFCs). SOFCs are promising electrochemical devices that enable the highest fuel-to-electricity conversion efficiencies under high operating temperatures. The concept of multi-stage electrochemical oxidation using SOFCs has been proposed and studied over the past several decades for further improving the electrical efficiency. However, the improvement is limited by fuel dilution downstream of the fuel flow. Recently we developed and reported a conceptual design that has a potential to realize a critically-high fuel-to-electricity conversion efficiency of up to as high as 85% (LHV, gross DC), in which a high-temperature multi-stage solid oxide fuel cells (SOFCs) is combined with a proton-conducting solid oxide electrolyte. Switching a solid electrolyte material from a conventional oxide-ion conducting material to a proton-conducting material under the high-temperature multi-stage electrochemical oxidation mechanism has proven to be highly advantageous for the electrical efficiency. The DC efficiency of 85% (LHV) corresponds to a net AC efficiency of approximately 77% (LHV), where the net AC efficiency refers to the transmission-end AC efficiency. This evolved concept will yield a considerably higher efficiency with a much smaller generation capacity than the state-of-the-art several tens-of-MW-class most advanced combined cycle (MACC). In the conceptual design the proton-conducting electrolyte was assumed to have a protonic transport number of 1. However, the protonic transport number of the proton-conducting solid oxide electrolyte depends on the material and operating conditions such as temperature, oxygen partial pressure, kinds of fuel and so on, and would affect the electrical efficiency. Therefore, to realize the critically-high electrical efficiency by using the multi-stage electrochemical oxidation, proton-conducting solid oxide having a high ionic transport number should be developed.

Novel Proton Conducting Solid Bio-polymer Electrolytes Based on Carboxymethyl Cellulose Doped with Oleic Acid and Plasticized with Glycerol.

The plasticized solid bio-polymer electrolytes (SBEs) system has been formed by introducing glycerol (Gly) as the plasticizer into the carboxymethyl cellulose (CMC) doped with oleic acid (OA) via solution casting techniques. The ionic conductivity of the plasticized SBEs has been studied using Electrical Impedance Spectroscopy. The highest conductivity achieved is 1.64 × 10−4 S cm−1 for system containing 40 wt. % of glycerol. FTIR deconvolution technique had shown that the conductivity of CMC-OA-Gly SBEs is primarily influenced by the number density of mobile ions. Transference number measurement has shown that the cation diffusion coefficient and ionic mobility is higher than anion which proved the plasticized polymer system is a proton conductor.

Effect of titanium doping on chemical and structural stability and electrical properties of proton-conducting solid electrolyte BaCe0.8Sm0.2O3−δ

In this study, the effects of Ti substitution for Ce on chemical and structural stability and electrical properties of proton-conducting solid electrolyte membrane BaCe0.8−xTixSm0.2O3−δ (BCST) are investigated. Our results show that both chemical and structural stability of the BCST samples in CO2 containing and moist atmospheres are remarkably enhanced with increasing the content of Ti which is higher electronegative than Ce. However, the proton conductivity of the sample decreases with increasing Ti content, which may be due to the increasing difficulty of lattice hydration and smaller grain size (corresponding to more grain boundaries) caused by Ti doping. The activation energy for proton migration increases with Ti content, which is mainly related with the weaker hydrogen bond. The results show that the BaCe0.75Ti0.05Sm0.2O3−δ has a great electrical conductivity (1.03×10−2Scm−1 at 650°C) and it also demonstrates good chemical and structural stability in CO2 and water containing atmospheres. Therefore, BaCe0.75Ti0.05Sm0.2O3−δ can be considered as a promising proton conducting electrolyte for intermediate temperature solid oxide fuel cells.

A Sensitive and Self-Regenerable Particulate Matter Sensor with a H3PO4-Modified BaZr0.8Y0.2O3-δ Electrolyte and an IrO2-Catalyzed Sensing Electrode

There have been many attempts to develop solid-state sensor devices for detecting particulate matter (PM) in diesel exhaust; however, in most of these, the accumulated PM must be burned intermittently to allow subsequent sensing cycles. Here, we report a self-regenerable PM sensor using a proton conductive solid electrolyte and an active working electrode for PM oxidation. The withstanding voltage capability of BaZr0.8Y0.2O3-δ was greatly improved by the growth of a dense Zr1-xYxP2O7 film on the electrolyte surface. The reaction of PM with active oxygen under anodic polarization was further enhanced by the addition of IrO2 to the working electrode. As a result of these combined modifications, when the working electrode was anodically polarized, PM was oxidized to CO2 according to a four-electron reaction (C + 2H2O → CO2 + 4H+ + 4e−) while remaining below the self-ignition temperature. This amperometric sensor successfully produced a current signal corresponding to the quantity of PM in a gas stream at an operating temperature of 300°C. These results demonstrate that the sensor can carry out continuous monitoring of PM concentrations while self-regenerating.

A mechanochemically synthesized covalent organic framework as a proton-conducting solid electrolyte

Mechanochemically synthesized bipyridine based covalent organic framework showing high proton conductivity of 0.014 S cm−1 with improved performance over the solvothermal one giving a stable Open Circuit Voltage (0.93 V at 50 °C) on fabrication in PEM fuel cell.

Effect of proton-conduction in electrolyte on electric efficiency of multi-stage solid oxide fuel cells.

Solid oxide fuel cells (SOFCs) are promising electrochemical devices that enable the highest fuel-to-electricity conversion efficiencies under high operating temperatures. The concept of multi-stage electrochemical oxidation using SOFCs has been proposed and studied over the past several decades for further improving the electrical efficiency. However, the improvement is limited by fuel dilution downstream of the fuel flow. Therefore, evolved technologies are required to achieve considerably higher electrical efficiencies. Here we present an innovative concept for a critically-high fuel-to-electricity conversion efficiency of up to 85% based on the lower heating value (LHV), in which a high-temperature multi-stage electrochemical oxidation is combined with a proton-conducting solid electrolyte. Switching a solid electrolyte material from a conventional oxide-ion conducting material to a proton-conducting material under the high-temperature multi-stage electrochemical oxidation mechanism has proven to be highly advantageous for the electrical efficiency. The DC efficiency of 85% (LHV) corresponds to a net AC efficiency of approximately 76% (LHV), where the net AC efficiency refers to the transmission-end AC efficiency. This evolved concept will yield a considerably higher efficiency with a much smaller generation capacity than the state-of-the-art several tens-of-MW-class most advanced combined cycle (MACC).