A comparison of the tape casting of α- and β-alumina

This work concerns the manufacture of planar cell configurations for molten sodium batteries in energy storage devices such as vehicle batteries and stationary storage cells. Tape casting of beta-alumina electrolyte could provide a low-cost mass production route but intriguingly there are only a few reports of tape casting using beta-alumina directly as the raw powder. We first compared tape casting of α-alumina and beta-alumina using polyvinyl butyral (PVB) in conventional formulations. While it is relatively easy to obtain homogeneous α-alumina tape cast sheet, beta-alumina resulted in adhesion to the substrate and cracking. These problems were shown to be attributable to particle characteristics. When the binder was changed to polymethylmethacrylate (PMMA), tape casting of beta-alumina and three different α-alumina powders was facilitated, producing a formulation that was more tolerant to different powder types. Screening of several commercial dispersants provided two which were effective with PMMA and a conventional MEK/ethanol dual solvent.     

Sodium-sulphur cells

The sodium-sulphur battery is reviewed and experimental results are presented. The experimental cells were capable of delivering steady state power densities as high as 50 W.kg^?1 and energy densities as high as 250 W.h.kg^?1. The theoretical open circuit voltage of 2.08 V was observed in all cases. Cell polarization was limited by cell internal resistance, which was attributed largely to the resistance of the β-alumina electrolyte. The “asymmetry effect” is explained in terms of an electrode blockage mechanism involving polysulphides. The sodium-sulphur battery appears to be technically feasible as the power source for an all-electric vehicle.     

The application of NASICON (Na2Zr2Si2PO12) to the thermodynamic study of beta-alumina

The solid-state galvanic cell with NASICON as an auxiliary electrolyte was built. The standard free enthalpy, entropy and enthalpy of beta-alumina formation as well as the Na₂O-activity in the alpha-alumina—beta-alumina coexistence region were determined in the temperature range 970–1220 K. The limiting temperature of spontaneous decomposition of beta-alumina was also estimated. The results were compared with those obtained by other authors.     

The failure of beta-alumina electrolyte by a dendritic penetration mechanism

A modified theory of beta-alumina breakdown is presented which takes into account factors not previously considered. Calculations show that whereas the population of critical surface flaws in a beta-alumina tube is likely to be initially zero, very small increases in the charge-transfer resistance at the sodium-electrolyte interface will result in previously sub-critical cracks attaining criticality. It is also shown that the time required for full penetration of the electrolyte tube is measured in minutes from the instant a crack achieves criticality. Thus electrolyte life is seen not as a continual process of degradation, but rather as an induction period, during which time changes are occurring at the sodium-electrolyte interface which ultimately lead to criticality and rapid electrolyte failure.     

Dynamic simulation of the charge-discharge characteristics of the sodium-sulphur cell

A dynamic model of the porous sulphur electrode including diffusion of the reactant is developed to simulate charge-discharge characteristics of the sodium-sulphur cell and transient behaviour of the cell after termination of operation. From the results of investigations the following are clarified: (1) sodium ion concentration on the solid electrolyte side is low during charge, on the other hand, it is high during discharge; (2) an insulating film of sulphur is formed at the surface of the solid electrolyte in the deeply charged stage. This results in an increase of the cell resistance and thus restricts charge acceptance; (3) the resistance in the single phase region is larger than that in the two phase region; and (4) open circuit voltage in the single phase region, observed after termination of operation, is not equal to the equilibrium value but gradually approaches it according to the concentration relaxation. These characteristics agree well with experimental results.     

Recent advances in the development of sodium-sulphur batteries for load levelling and motive power applications

This paper describes some recent progress in the design and development of sodium/sulphur batteries for motive power applications. The historical development of cell designs over the last 10 yr is reviewed, and includes a discussion of development problems such as the durability of beta-alumina, and corrosion of the sulphur electrode current collector. The tubular electrolyte is now widely adopted as the preferred development option, and a computer analysis shows that a volumetric energy density of about 200 Wh/litre can be obtained in an optimum design of a battery including thermal insulation. An advanced cell design has been adopted in which sodium is fed into the anodic reaction zone by capillary wick. This design is well suited to the demands of motive power applications in which the stored energy has to be supplied at about the 2 h rate. The design also permits the use of large electrolyte tubes, at least 30 mm dia × 500 mm in length, and this reduces the battery manufacturing costs. A large number of experimental cells have been tested, and over 85% utilization of the sodium and sulphur active materials has been achieved for repeated cycling of developed electrode constructions.     

Wetting properties of the sulphur electrolyte in sodium — sulphur batteries

In the sulphur electrode of sodium-sulphur cells, carbon surfaces (generally as yarns or felts) are used as current collectors. During the charging cycle, oxidation of the molten polysulphides results in sulphur as a second (immiscible) phase. If sulphur has preferential wetting properties, an insulating sulphur layer may be established. A technique for wetting measurements with the molten sulphur electrolyte at 330° C is described. The wetting properties of molten sulphur and molten polysulphides were investigated with both untreated and metal oxide treated carbon surfaces as substrates. Results are reported for felts with Fe, Ni, Co, Cr, Al (as oxides), and with H₂0, NaOH, Na₂SO₄ and Na₂S₂O₃ as impurities in the fluid electrolyte.Presented at the 30th meeting of the International Society of Electrochemistry at Trondheim, Norway, August 1979.     

Comparative e.m.f. study of CaF2 and β-alumina cells with Ni/NiF2 and Fe/FeF2 or Cr/CrF2 electrodes

The reliability of employing beta-alumina as electrolyte for fluorine potential measurement is examined by measuring the e.m.f.s of the galvanic cells with metal/metal fluoride electrodes and comparing with those obtained by using CaF₂ as electrolyte under identical conditions. The results from both types of galvanic cell can be superimposed to give the following standard Gibbs energy of formation, ΔG _f ⁰ , of FeF₂ and CrF₂ over extended ranges of temperature: $$\begin{gathered} \Delta G_f^0 (FeF_2 ) = - 702.0 + 0.125 20T (K) ( \pm 0.70) kJ mol^{ - 1} (506 - 1063K) \hfill \\ \Delta G_f^0 (CrF_2 ) = - 732.8 + 0.087 90T (K) ( \pm 0.64) kJ mol^{ - 1} (497 - 1063K) \hfill \\ \end{gathered} $$ The absence of significant temperature-dependent errors in both these measurements are verified by a third law treatment of the data yielding values of ?716.8 and ?777.4 kJ mol^?1 for ΔH _f.298 ⁰ of FeF₂ and CrF₂, respectively. The feasibility of using beta-alumina electrolyte cells for e.m.f. measurements on other metal/metal fluoride systems is discussed in the light of the existence of a useful potential domain of beta-alumina. High sodium potential in the electrode system can lead to sodium depletion. Likewise, low sodium potential may result in oxidation of the metals in the electrodes. Both these limiting factors are also examined.     

Electro-mechanical degradation of polycrystalline beta-alumina

Deposition of metallic sodium under conditions of current flow of sodium ions through beta-alumina ceramics is an important cause of their breakdown when the solid electrolyte is used as a separator in various electrochemical devices. Cells of different combinations: Na/Beta-Alumina/Na amalgam, Na/Beta-Alumina/Cu(probe), and Na/Beta-Alumina/Na, etc have been investigated for sodium deposition at room temperature and at 350°C. A method is presented by which the progress of sodium deposition may be followed. A mechanism of formation of sodium deposition centers at impurity rich spots and other defect sites has been proposed.     

Reversible and irreversible heat effects in ZEBRA cells

When a sodium-nickel chloride cell is discharged reversibly, heat is liberated. The amount of heat involved has been found from measured values of the open circuit voltage of the cell over a range of temperatures. The thermal capacity of a 40 A h cell has also been measured, at various states of discharge, and is found to be a linear function of the temperature and of the degree of discharge. The information has been combined to find the temperature changes which occur when a fully charged cell undergoes a reversible discharge-charge cycle. Complete discharge of the cell involves a temperature rise of some 30–35 K, depending on the starting temperature; the effect is reversed on charge. The techniques employed in these calculations could be applied to other cells in which all reactants and products of the cell reaction are pure phases, such as the sodium-sulphur cell or ZEBRA cells involving transition metals other than nickel.     

Refining of gallium using gallium beta-alumina

The preparation of polycrystalline gallium beta-alumina samples using molten gallium and gallium dichloride is described. Gallium was selectively electrochemically transferred through a gallium beta alumina membrane from a gallium tin alloy.     

Conductance and spectroscopy of protonic beta and beta″-aluminas

Five protonic beta and beta″-aluminas; viz hydrated sodium beta-alumina (1,24Na₂O·11Al₂O₃), hydronium beta-alumina (1.24H₂O·11Al₂O₃·2.6H₂O), partially dehydrated hydronium beta-alumina (1.24H₂O·11Al₂O₃·1.3H₂O), hydrogen beta-alumina (1.24H₂O·11Al₂O₃) and hydronium beta″-alumina (0.84H₂O·0.8MgO·5Al₂O₃·2.8H₂O) were examined by broad band nuclear magnetic resonance from ?196°C to 200°C. The spectra of hydronium beta-alumina and hydronium beta″-alumina are consistent with a mixed composition of H₂O, H₃O⁺ and H⁺ species in the conducting plane. Hydrogen beta-alumina and partially dehydrated hydronium beta-alumina appear to contain only relatively isolated (2.6–2.7Å) protons; no evidence of molecular water or hydronium ions is found. Water molecules intercalated into the conduction plane of sodium beta-alumina do not appear to be in rapid motion, even at 167°C, but are relatively stationary. The onset of motional narrowing in hydronium beta″-alumina occurs at ?40°C but not until +30°C in hydronium beta-alumina. This is consistent with the higher conductivity reported for hydronium beta″-alumina, 10^?3–10^?5 (ohm-cm)^?1 at 25°C, in comparison to 10^?10–10^?11 (ohm-cm)^?1 for hydronium beta-alumina at 25°C.     

Effects of electrolyte pattern on mechanical and electrochemical properties of solid oxide fuel cells

In order to enhance the electrochemical performance and reduce the operation temperature of a conventional electrolyte supported solid oxide fuel cell (SOFC), a three layered electrolyte with various geometry is designed and fabricated. Novel three layered electrolytes comprise a dense and thin scandia alumina stabilized zirconia (ScAlSZ) electrolyte layer sandwiched between two hallow ScAlSZ electrolyte layers each having the same thickness as the support but machined into a filter like architecture in the active region with circular, rectangular and triangular cut off patterns. The percent of thin electrolyte layer in the active region is kept constant as 30% for all designs in order to investigate the effect of pattern geometry on the mechanical properties and the performance of the electrolytes. Single cells based on novel electrolytes are manufactured and electrochemical properties are evaluated. A standard electrolyte and electrolyte supported cell are also fabricated as a base case for comparison. Although the electrolyte having triangular patterns has the highest peak power at all operation temperatures considered, it exhibits the lowest flexural strength.     

Influence of the Teflon loading in the gas diffusion layer of PBI-based PEM fuel cells

The influence of the PTFE content in commercial Toray graphite paper gas diffusion layer (GDL) on the performance of a PBI-based polymer electrolyte membrane fuel cell (PEMFC) has been studied. These materials have been characterised by evaluating the porosity, pore size distribution, SEM micrographs, hydrophobicity, air permeability and electrical resistance. Fuel cell results show that the lower the Teflon content, the better the cell performance and the lower the losses when oxygen was replaced by air. These results led to non-Teflonized carbon paper to be postulated as the most suitable candidate, provided that its mechanical integrity can be maintained throughout the whole process of preparation and testing of the MEA. However, some practical problems with this type of commercial non-Teflonized carbon paper were experienced in this work and led to damage of the support. The detrimental effects are described and discussed. As conclusion, the use of a minimally PTFE-loaded (10%) carbon paper is suggested because the inclusion of this level of Teflon improved properly the mechanical properties of the carbon support and only caused a very small drop in the performance.     

On the influence of electrolyte concentration,pH and temperature on surface brightening of nickel under ECM conditions

High rate anodic dissolution of nickel in sodium nitrate electrolytes was studied under controlled hydrodynamic conditions in order to investigate the influence of electrolyte concentration, pH and temperature on surface brightening under conditions comparable to electrochemical machining. Dissolution experiments were performed on a rectangular flow channel cell through which the electrolyte was pumped at a constant linear velocity of 1000 cm s^–1. Results show that the onset of surface brightening depends on concentration and temperature of the electrolyte but is little affected by pH. The data are consistent with a previously described salt precipitation model for surface brightening.     

溶胶-凝胶法制备双掺杂的CeO2基电解质粉末

目前对CeO2电解质的研究已经成为中低温燃料电池的一个热点,无论从过去的单掺杂以及发展到如今的双掺杂,国内外都有了很多的研究。对ZrO2掺杂的CeO2虽已被证实对其机械性能有很大的改善,但作为电解质的研究还不是很多,尤其对电导率的影响还未形成较一致的共识。本研究在此基础上引进了Sc2O3,并用溶胶-凝胶法制备了此电解质粉末,并对其微观性质和热过程进行了分析,得到了溶胶-凝胶法的最优条件及对比固相法的优点。     

Mechanical and electrochemical behavior of novel electrolytes based on partially stabilized zirconia for solid oxide fuel cells

In this study, 3 mol% yttria stabilized zirconia (3YSZ) is investigated as a SOFC electrolyte alternative to 8 mol% yttria stabilized zirconia (8YSZ). The mechanical and electrochemical properties of both materials are compared. The mechanical tests indicate that the thickness of 3YSZ can be reduced to half without sacrificing the strength compared to 8YSZ. By reducing the thickness of 3YSZ from 150 µm to 75 µm, the peak power density is shown to increase by around 80%. The performance is further enhanced by around 22% by designing of novel electrode structure with regular cut-off patterns previously optimized. However, the cell with novel designed 3YSZ electrolyte exhibits 30% lower maximum power density than that of the cell with 150 µm-thick standard 8YSZ electrolyte. Nevertheless, the loss in the performance may be tolerated by decreasing the fabrication cost revealing that 3YSZ electrolyte with cut-off patterns can be employed as SOFC electrolyte alternative to 8YSZ.     

Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers

This paper presents an overview of the synthesis, chemical and electrochemical properties, and polymer electrolyte fuel cell applications of new proton-conducting polymer electrolyte membranes based on hydrocarbon polymers. Due to their chemical stability, high degree of proton conductivity, and remarkable mechanical properties, perfluorinated polymer electrolytes such as Nafion^®, Aciplex^®, Flemion^®, and Dow membranes are some of the most promising electrolyte membranes for polymer electrolyte fuel cells. A number of reviews on the synthesis, electrochemical properties, and fuel cell applications of perfluorinated polymer electrolytes have also appeared during this period. While perfluorinated polymer electrolytes have satisfactory properties for a successful fuel cell electrolyte membrane, the major drawbacks to large-scale commercial use involve cost and low proton-conductivities at high temperatures and low humidities. Presently, one of the most promising ways to obtain high performance proton-conducting polymer electrolyte membranes is the use of hydrocarbon polymers for the polymer backbone. The present review attempts for the first time to summarize the synthesis, chemical and electrochemical properties, and fuel cell applications of new proton-conducting polymer electrolytes based on hydrocarbon polymers that have been made during the past decade.     

Measurements of a.c. impedance of sodium-sulphur cells with symmetric and asymmetric resistance behaviour

Measurements of a.c. impedance in the frequency range 10^–2 Hz f 10⁴ Hz were performed on laboratory sodium-sulphur battery cells with symmetric and asymmetric internal resistance behaviour. The impedance spectra of the symmetric cells show only resistive behaviour in the whole frequency range studied. The resistance determined from these spectra is equal to the bulk resistance of the solid electrolyte. In contrast, the impedance of the asymmetric cells is characterized by different high-frequency capacitive and low-frequency inductive behaviour depending on the value of the d.c. current applied. In this case the impedance spectra are very similar to those often observed at metal passivation processes in electrolyte solutions. It is shown that the results obtained can be qualitatively interpreted by a model including the existence of a passivating film on the-Al₂O₃/molten sodium interface. The modification of this film during the deposition or dissolution of the sodium can be assumed as a cause for the asymmetric behaviour of the internal cell resistance observed.     

Recent progress in direct ethanol proton exchange membrane fuel cells (DE-PEMFCs)

Direct ethanol fuel cells (DEFCs) belong to the family of proton exchange membrane fuel cells (PEMFCs), in which ethanol is directly used as the fuel. In the present work, the main aspects related to DEFCs such as electrocatalysts, membrane electrode assembly (MEA) preparation and their corresponding effects on the total cell performance are summarized and discussed. Furthermore, the issues about the disadvantages such as ethanol crossover and the electrolyte membrane's thermal and mechanical stability, as well as the challenges for DEFC's rapid development and commercialization are addressed.