Phase transition and proton transport characteristics in CsH2PO4/SiO2 composites

The stabilization of superprotonic phase in neat CsH₂PO₄ and CsH₂PO₄/SiO₂ composites as well as the anomalous phase transformation with a large hysteresis was investigated through proton conductivity, thermal analysis and Raman spectroscopy. The reversibility of the transition to the superprotonic phase and the phase transformation between monoclinic phase and cubic phase in neat CsH₂PO₄ at around T_c = 230 °C was confirmed under humidified and sealed conditions. In CsH₂PO₄/SiO₂ composites, a large asymmetric thermal hysteresis in the conductivity appeared, i.e. significant supercooling in the superprotonic phase was induced in silica matrices. A differential thermal analysis revealed that the temperature of a reverse transition from the cubic phase (superprotonic phase) to the monoclinic phase decreased in the composites. This effect became significant with an increase in silica volume fraction. The stabilization of superprotonic phase (cubic phase) in the composites will be induced by shear elastic forces at the interface between CsH₂PO₄ and silica particles. The main origin of the anomalous asymmetric thermal hysteresis in proton conductivity is the phase stability arising from the shear elastic forces and a proton-conducting network in silica matrices.     

Composite effects of silicon pyrophosphate as a supporting matrix for CsH5(PO4)2 electrolytes at intermediate temperatures

The proton-conductive electrolytes of CsH₅(PO₄)₂/SiP₂O₇ composites were synthesized, and composite effects of silicon pyrophosphate as a supporting matrix at intermediate temperatures were investigated by comparing the properties of CsH₅(PO₄)₂/SiO₂ composite. Although both composites showed similar thermal stability, the temperature dependence of the conductivity was quite different each other; the conductivity of the composite electrolyte of CsH₅(PO₄)₂/SiP₂O₇ was about one-order magnitude higher at every temperature investigated and the maximum conductivity achieved was 116 mS cm⁻¹ at 230 °C. These results suggested that the interfacial interaction between the proton-conductor phase of CsH₅(PO₄)₂ and the matrix of SiP₂O₇ played an important role in the proton conduction mechanism.     

Electrochemical performance for the electro-oxidation of ethylene glycol on a carbon-supported platinum catalyst at intermediate temperature

To determine the kinetic performance of the electro-oxidation of a polyalcohol operating at relatively high temperatures, direct electrochemical oxidation of ethylene glycol on a carbon supported platinum catalyst (Pt/C) was investigated at intermediate temperatures (235–255 °C) using a single cell fabricated with a proton-conducting solid electrolyte, CsH₂PO₄, which has high proton conductivity (>10⁻² S cm⁻¹) in the intermediate temperature region. A high oxidation current density was observed, comparable to that for methanol electro-oxidation and also higher than that for ethanol electro-oxidation. The main products of ethylene glycol electro-oxidation were H₂, CO₂, CO and a small amount of CH₄ formation was also observed. On the other hand, the amounts of C₂ products such as acetaldehyde, acetic acid and glycolaldehyde were quite small and were lower by about two orders of magnitude than the gaseous reaction products. This clearly shows that C–C bond dissociation proceeds almost to completion at intermediate temperatures and the dissociation ratio reached a value above 95%. The present observations and kinetic analysis suggest the effective application of direct alcohol fuel cells operating at intermediate temperatures and indicate the possibility of total oxidation of alcohol fuels.     

Applying cold sintering process to a proton electrolyte material: CsH2PO4

The Cold Sintering Process (CSP) was applied to CsH₂PO₄, a proton conducting electrolyte. The powders were sintered between 120 °C and 200 °C under 300 MPa for 1 h and the ceramics investigated for their respective structure-property-process relations. The microstructure development of the CsH₂PO₄ materials was characterized for grain size, and dihedral angle distributions for a variety of the CSP processing conditions. The electrical properties of the cold sintered CsH₂PO₄ were characterized through electrochemical impedance spectroscopy over a temperature range between 120 °C and 200 °C. The data suggests that the proton conductivity is 2.30 × 10⁻⁴ Scm^-1 at 200 °C with good repeatability during cycled measurements.     

Phosphoric acid doped polybenzimidazole membranes: Physiochemical characterization and fuel cell applications

A polymer electrolyte membrane fuel cell operational at temperatures around 150–200 °C is desirable for fast electrode kinetics and high tolerance to fuel impurities. For this purpose polybenzimidazole (PBI) membranes have been prepared and H₃PO₄-doped in a doping range from 300 to 1600 mol %. Physiochemical properties of the membrane electrolyte have been investigated by measurements of water uptake, acid doping level, electric conductivity, mechanical strength and water drag coefficient. Electrical conductivity is found to be insensitive to humidity but dependent on the acid doping level. At 160 °C a conductivity as high as 0.13 S cm^–1 is obtained for membranes of high doping levels. Mechanical strength measurements show, however, that a high acid doping level results in poor mechanical properties. At operational temperatures up to 190 °C, fuel cells based on this polymer membrane have been tested with both hydrogen and hydrogen containing carbon monoxide.     

Proton conductors of cerium pyrophosphate for intermediate temperature fuel cell

The crystal structure and proton conductivity of cerium pyrophosphate are investigated to explore its potential electrolyte applications for intermediate temperature fuel cell. Among the CeP₂O₇ thin plates, which are sintered at 300–900 °C, the 450 °C CeP₂O₇ sample exhibits superior proton conductivity under humidified conditions. Its conductivity, measured with impedance spectroscopy, is higher than 10⁻² S cm⁻¹ in the intermediate temperature range, with a maximum value 3.0 × 10⁻² S cm⁻¹ at 180 °C. When 10 mol% Mg is doped on the Ce site of CeP₂O₇, the maximum conductivity is raised to 4.0 × 10⁻² S cm⁻¹ at 200 °C. The Mg doping not only raises the conductivity, but also shifts and widens its temperature window for electrolyte applications. Ce_0.9Mg_0.1P₂O₇ is considered a more appropriate composition, with conductivity >10⁻² S cm⁻¹ between 160 and 280 °C. Accordingly, a hydrogen–air cell is built with the Ce_0.9Mg_0.1P₂O₇ electrolyte and its performance is measured. The fuel cell generates electricity up to 122 mA cm⁻² at 0.33 V using 50% H₂ at 240 °C.     

A polymer electrolyte membrane for high temperature fuel cells to fit vehicle applications

Poly(tetrafluoroethylene) PTFE/PBI composite membranes doped with H₃PO₄ were fabricated to improve the performance of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC). The composite membranes were fabricated by immobilising polybenzimidazole (PBI) solution into a hydrophobic porous PTFE membrane. The mechanical strength of the membrane was good exhibiting a maximum load of 35.19 MPa. After doping with the phosphoric acid, the composite membrane had a larger proton conductivity than that of PBI doped with phosphoric acid. The PTFE/PBI membrane conductivity was greater than 0.3 S cm⁻¹ at a relative humidity 8.4% and temperature of 180 °C with a 300% H₃PO₄ doping level. Use of the membrane in a fuel cell with oxygen, at 1 bar overpressure gave a peak power density of 1.2 W cm⁻² at cell voltages >0.4 V and current densities of 3.0 A cm⁻². The PTFE/PBI/H₃PO₄ composite membrane did not exhibit significant degradation after 50 h of intermittent operation at 150 °C. These results indicate that the composite membrane is a promising material for vehicles driven by high temperature PEMFCs.     

PBI-based polymer electrolyte membranes fuel cells: Temperature effects on cell performance and catalyst stability

In this work, it has been shown that the temperature (ranging from 100 to 175 °C) greatly influences the performance of H₃PO₄-doped polybenzimidazole-based high-temperature polymer electrolyte membrane fuel cells by several and complex processes. The temperature, by itself, increases H₃PO₄-doped PBI conductivity and enhances the electrodic reactions as it rises. Nevertheless, high temperatures reduce the level of hydration of the membrane, above 130-140 °C accelerate the self-dehydration of H₃PO₄, and they may boost the process of catalyst particle agglomeration that takes place in strongly acidic H₃PO₄ medium (as checked by multi-cycling sweep voltammetry), reducing the overall electrochemical active surface. The first process seems to have a rapid response to changes in the temperature and controls the cell performance immediately after them. The second process seems to develop slower, and influences the cell performance in the “long-term”. The predominant processes, at each moment and temperature, determine the effect of the temperature on the cell performance, as potentiostatic curves display. “Long-term” polarization curves grow up to 150 °C and decrease at 175 °C. “Short-term” ones continuously increase as the temperature does after “conditioning” the cell at 125 °C. On the contrary, when compared the polarization curves at 175 °C a continuous decrease is observed with the “conditioning” temperature. A discussion of the observed trends is proposed in this work.     

Proton conductive YSZ-phosphate composite electrolyte for H2S SOFC

A new oxide-salt composite electrolyte, YSZ–K₃PO₄–Ca₃(PO₄)₂, shows proton conductivity in the order of 10^?2 S/cm at 700 °C. The proton transport number, determined using a hydrogen concentration cell, rises from more than 93% at 550 °C to 99% at 700 °C. The composite electrolyte is chemically stable in H₂S containing atmosphere, and so is a good candidate electrolyte material for H₂S solid oxide fuel cells applications.     

Crystallization Kinetics of Superionic Conductive Al(B,La)‐ Incorporated LiTi2(PO4)3 Glass‐Ceramics

For the purpose of developing high‐performance glass‐ceramic superionic conductor, the controllable precipitation of LiTi₂(PO₄)₃‐like superionic conducting phase in the Li₂O–TiO₂–P₂O₅ glass system was studied. Al with B or La co‐incorporated LiTi₂(PO₄)₃‐based glass‐ceramics were prepared by the crystallization of the corresponding original glasses. Compared with the sole Al‐incorporated LiTi₂(PO₄)₃‐based glass‐ceramics, the ionic conductivity shows an increase for the boron co‐incorporated one and a decrease for the lanthanum co‐incorporated one. Through the further in‐depth analysis based on the methods of DSC and X‐ray diffractive technique, this opposite change in ion conductivity was ascribed to the alterations of crystallization mechanism together with quantity of crystal phases within the glass‐ceramics.. The boron addition promoted the precipitation of LiTi₂(PO₄)₃ phase and restrained the precipitation of second phase. The highest ionic conductivity 1.3 × 10^?3 S/cm at 25°C was obtained through the heat treatment of B and Al co‐incorporated glassy samples at 900°C for 12 h. These inorganic solid electrolytes have a potential application in lithium batteries or other electrochemical ionic devices.     

Assessment of structurally stable cubic Bi12TiO20 as intermediate temperature solid oxide fuels electrolyte

Colloid processing and subsequent pressure filtration were used to prepare 14.3 mol% TiO₂ doped Bi₂O₃ (Bi₁₂TiO₂₀, 14BTO) as solid oxide fuel cell electrolyte. Materials characterization and electrical behaviors of 14BTO samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and two-point probe DC conductivity. A pure 14BTO with a cubic sillenite single phase was prepared at the sintering process of 850 °C with a high relative sintered density of 96.82%. In situ and batch-type long-term conductivity measurements at 600 °C were carried out to verify the possible reason of degradation. Additional reduction-oxidation tests under CH₄ atmosphere by thermogravimetric analysis (TGA) revealed possible application temperature of 14BTO electrolytes below 700 °C.     

Fabrication and characterization of composite electrolyte for intermediate-temperature SOFC

In this study, a ceria-based composite electrolyte was investigated for intermediate-temperature solid oxide fuel cells (SOFCs) based on SDC-25 wt.% K₂CO₃. Sodium carbonate co-precipitation process by which SDC powder was adopted and sound cubic fluorite structure was formed after SDC powders were sintered at 750 °C for 3 h. The crystallite size of the particle was 21 nm in diameter as calculated from data obtained through X-ray diffraction. The conductivity of the composite electrolyte proposed in this study was much higher than that of pure SDC at the comparable temperature of 550-700 °C. The transition of the ionic conductivity occurred at 650 °C. Based on this type of composite electrolyte, single cell with the electrolyte thickness of 0.3 mm were fabricated using dry pressing, with nickel oxide adopted as anode and SSC as cathode. The single cell was then tested at 550-700 °C on home-made equipment in this study, using hydrogen/air. The maximum power density and open circuit voltage (OCV) achieved 600 mW cm⁻² and 1.05 V at 700 °C, respectively.     

Anhydrous proton conducting polymer electrolytes based on poly(vinylphosphonic acid)-heterocycle composite material

The development of anhydrous proton conducting membrane is important for the operation of polymer electrolyte membrane fuel cell (PEMFC) at intermediate temperature (100-200 °C). In this study, we have investigated the acid-base hybrid materials by mixing of strong phosphonic acid polymer of poly(vinylphosphonic acid) (PVPA) with the high proton-exchange capacity and organic base of heterocycle, such as imidazole (Im), pyrazole (Py), or 1-methylimidazole (MeIm). As a result, PVPA-heterocycle composite material showed the high proton conductivity of approximately 10⁻³ S cm⁻¹ at 150 °C under anhydrous condition. In particular, PVPA-89 mol% Im composite material showed the highest proton conductivity of 7×10⁻³ S cm⁻¹ at 150 °C under anhydrous condition. Additionally, the fuel cell test of PVPA-89 mol% Im composite material using a dry H₂/O₂ showed the power density of approximately 10 mW cm⁻² at 80 °C under anhydrous conditions. These acid-base anhydrous proton conducting materials without the existence of water molecules might be possibly used for a polymer electrolyte membrane at intermediate temperature operations under anhydrous or extremely low humidity conditions.     

Synthesis and properties of imidazole-grafted hybrid inorganic-organic polymer membranes

Imidazole rings were grafted on alkoxysilane with a simple nucleophilic substitute reaction to form hybrid inorganic-organic polymers with imidazole rings. Proton exchange membranes (PEM) based on these hybrid inorganic-organic polymers and H₃PO₄ exhibit high proton conductivity and high thermal stability in an atmosphere of low relative humidity. The grafted imidazole rings improved the proton conductivity of the membranes in the high temperature range. It is found that the proton conductivities increase with H₃PO₄ content and temperature, reaching 3.2 × 10⁻³ S/cm at 110 °C in a dry atmosphere for a membrane with 1 mole of imidazole ring and 7 moles of H₃PO₄. The proton conductivity increases with relative humidity (RH) as well, reaching 4.3 × 10⁻² S/cm at 110 °C when the RH is increased to about 20%. Thermogravimetric analysis (TGA) indicates that these membranes are thermally stable up to 250 °C in dry air, implying that they have a good potential to be used as the membranes for high-temperature PEM fuel cells.     

High-temperature C1-gas fuel cells using proton-conducting solid electrolytes

With proton-conducting solid electrolytes based on SrCeO₃ or BaCeO₃, high-temperature fuel cells were constructed and cell performances were examined. Mixtures of water vapor and some C₁ gases such as methanol vapor or methane were used as fuel by internal reforming to liberate hydrogen in the anode compartment. These fuel cells worked stably above 900°C. Addition of water vapor to the fuel was necessary to prevent carbon deposition at the anode. The performances of the cells were limited mainly by ohmic resistance of the solid electrolyte.     

Microwave-assisted synthesis of gadolinia-doped ceria powders for solid oxide fuel cells

Gadolinia doped ceria (GDC) is an attractive electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs) for its high ionic conductivity at low temperature (500-700 °C). A number of different methods are currently used to prepare nano-sized doped-ceria powder. Among the others, precipitation in solution remains the best method to obtain well-dispersed particles of controlled properties. In this work, nanocrystalline Ce_1−xGd_xO_2−δ (GDC) particles were produced by polyol microwave assisted method in very mild conditions (170 °C, 2 h, 1 atm). The as-synthesized powder showed good sinterability and ionic conductivity comparable to the ones of the corresponding nanometric commercial GDC.     

Fabrication of YSZ electrolyte using electrostatic spray deposition (ESD):I – a comprehensive parametric study

Electrostatic spray deposition (ESD) was applied to fabricate a thin-layer (3 m thickness) yttria-stabilized zirconia (YSZ) electrolyte on a solid oxide fuel cell (SOFC) anode substrate consisting of nickel-YSZ cermet. Reducing the thickness of a state-of-the-art electrolyte, and thereby reducing the cell internal IR drop, is a promising strategy to make the intermediate temperature SOFC (ITSOFC) operating at 600–800 °C possible. About 8 mol% YSZ colloidal solution in ethanol was sprayed onto the substrate anode surface at 250–300 °C by ESD. After sintering the deposited layer at 1250–1400 °C for 17–6 h, the cathode layer, consisting of lanthanum strontium manganate (LSM), was sprayed or brush coated onto the electrolyte layer. Performance tests on the cell were carried out at 800 °C to evaluate the electrolyte layer formed by ESD. With a 97 H₂/3 H₂O mixture and air as fuel and oxidant gas, respectively, open circuit voltage (OCV) was found to be close to the theoretical value.     

Effect of sintering and annealing on electrochemical and mechanical characteristics of Na3Zr2Si2PO12 solid electrolyte

NASICON (Sodium superionic conductor) type Na₃Zr₂Si₂PO₁₂ (NZSP) has received a lot of interest as the solid electrolyte for all-solid-state sodium-ion batteries (ASSIBs). The electrolyte has superior interfacial characteristics, high thermal stability, and good ionic conductivity. Because of their higher energy density, improved mechanical stability, no liquid leakage problem, and higher operating voltages, All solid-state batteries are expected to replace liquid electrolyte-based batteries in many applications. The solid electrolyte also acts as a separator, and hence additional separator is not required for cell operations. Because of its 3D open architecture and continuous diffusion channels, NZSP is considered a better solid electrolyte. The NZSP solid electrolyte has been synthesized by spark plasma sintering (SPS) followed by annealing the sintered materials. The SPS method leads the material to have higher density and ionic conductivity. Conventional sintering of the materials requires a temperature as high as 1225°C; however, the temperature required for the SPS is as low as 1050°C. Moreover, conventional sintering yields samples of relative density up to 91%, while SPSed samples have achieved a maximum density of around 98%. The ionic conductivity of solid electrolyte SPSed at 1050°C for 10 min is found to be 3.5 × 10⁻⁴ S/cm with an activation energy of 0.27 eV. The annealing of the SPSed samples improves the ionic conductivity of the SPS1050-20mins sample to roughly double the value obtained from the as-prepared SPS sample because there are fewer secondary phases and a structural change from a rhombohedral to a monoclinic system. To ascertain the samples' crystal structure, particle shape, and ionic conductivity, materials were characterized using X-ray diffraction, scanning electron microscopy, and electrochemical impedance spectroscopy. The samples' mechanical characteristics, for example, the hardness and fracture toughness of the samples, were also determined.     

High Na+ conducting Na3Zr2Si2PO12/Na2Si2O5 composites as solid electrolytes for Na+ batteries

Sodium superionic conductor Na₃Zr₂Si₂PO₁₂ (NZSP) is a promising material as a solid electrolyte for sodium-ion batteries. The highest conductivity of ∼1.0 mS/cm at room temperature (RT) was reported for the compound with a Na content of approximately 3.3 per formula unit (f. u.) and when the material is synthesized with a final sintering temperature ≥1220°C. Herein, we propose a new synthesis method to enhance the conductivity of the NZSP by liquid-phase sintering with the optimum amount of additive of amorphous-Na₂Si₂O₅. In this regard, a series of composite materials were prepared by mixing Na₃Zr₂Si₂PO₁₂ with amorphous-Na₂Si₂O₅ (NZSP/NS-x wt.%; with x = 0.0, 2.5, 5.0, 7.5, 10.0) and sintering at a lower temperature of 1150°C. Enhanced conductivity of 1.7 mS/cm at RT has been achieved for the Na₃Zr₂Si₂PO₁₂/Na₂Si₂O₅-5.0 wt.% (NZSP/NS-5.0) composite. The effects of additives on the NZSP phase formation, microstructure, and ion conductivity have been investigated by XRD, MAS NMR, SEM, and impedance spectroscopy. Our study demonstrates that the higher conductivity of the NZSP/NS-5.0 composite is caused by the combined effect of increased Na content in the NZSP phase (by diffusion of Na⁺ ions from the liquid phase of NS to bare NZSP phase), higher density, and microstructures with lesser pores.     

Electrochemical behaviour of propane-fed solid oxide fuel cells based on low Ni content anode catalysts

Two different low Ni content (10 wt.%) anode catalysts were investigated for intermediate temperature (800 °C) operation in solid oxide fuel cells fed with dry propane. Both catalysts were prepared by the impregnation of a Ni-precursor on different oxide supports, i.e. gadolinia doped ceria (CGO) and La_0.6Sr_0.4Fe_0.8Co_0.2O₃ perovskite, and thermal treated at 1100 °C for 2 h. The Ni-modified perovskite catalyst was mixed with a CGO powder and deposited on a CGO electrolyte to form a composite catalytic layer with a proper triple-phase boundary. Anode reduction was carried out in-situ in H₂ at 800 °C for 2 h during cell conditioning. Electrochemical performance was recorded at different times during 100 h operation in dry propane. The Ni-modified perovskite showed significantly better performance than the Ni/CGO anode. A power density of about 300 mW cm⁻² was obtained for the electrolyte supported SOFC in dry propane at 800 °C. Structural investigation of the composite anode layer after SOFC operation indicated a modification of the perovskite structure and the occurrence of a La₂NiO₄ phase. The occurrence of metallic Ni in the Ni/CGO system caused catalyst deactivation due to the formation of carbon deposits.