Nigel Sammes

Li∕Polymer Electrolyte∕Water Stable Lithium-Conducting Glass Ceramics Composite for Lithium–Air Secondary Batteries with an Aqueous Electrolyte

A water-stable Li metal anode with water-stable lithium-conducting glass ceramics, Li 1+x+y Ti 2-x Al x Si y P 3-y O 12 (LTAP), and a lithium-conducting polymer electrolyte, PEO 18 Li(CF 4 SO 2 ) 2 N (PEO 18 LiTFSI), was proposed as the lithium anode for lithium-air batteries with an aqueous solution at the air electrode. LTAP was unstable when in direct contact with Li metal, and the cell resistance of Li/LTAP/Li rapidly increased as a function of the contact time. The Li/PEO 18 LiTFSI/LTAP/PEO 18 LiTFSI/Li symmetrical cell showed no change in the total resistance (around 800 Ω cm 2 at 60°C) over a period of 1 month. The PEO 18 LiTFSI membrane served as a protective interlayer to suppress the reaction between the water-stable glass ceramics LTAP and Li metal effectively. The Li/PEO 18 LiTFSI/LTAP/aqueous LiCl/Pt air cell showed a stable open-circuit voltage of 3.70 V at 60°C for 2 months. The open-circuit voltage was comparable with that calculated from the cell reaction of 2Li + 1/2O 2 + H 2 O = 2LiOH. The cell exhibited a favorable discharge and charge performance at 0.25 mA cm -2 and 60°C.

Stability of a Water-Stable Lithium Metal Anode for a Lithium–Air Battery with Acetic Acid–Water Solutions

The stability of water-stable lithium metal in aqueous acetic acid solution was examined as an anode in a lithium-air rechargeable battery. The water-stable lithium anode consisted of a water-stable glass-ceramic Li 1+x+y Ti 2-x Al x Si y P 3-y O 12 (LTAP), a poly(ethylene oxide) (PEO)-based electrolyte with Li(CF 3 SO 2 ) 2 N (LiTFSI), and lithium metal. The LTAP immersed in CH 3 COOH (HAc)-H 2 O-saturated CH 3 COOLi (LiAc) solutions at 50°C for several weeks showed no change in the X-ray diffraction pattern and showed a slight decrease in the electrical conductivity. The water-stable lithium anode, Li/PEO 18 LiTFSI/LTAP, showed a total resistance of 164 Ω cm 2 at 60°C after being immersed in HAc (90 vol %)-H 2 O (10 vol %)-saturated LiAc for 1 week. The Li/PEO 18 LiTFSI/LTAP/HAc-H 2 O-LiAc/Pt black air cell had a low polarization for lithium dissolution and deposition at a current density of 1 mA cm -2 . A prototype lithium-air cell using a carbon air electrode with a platinum catalyst showed a good charge and discharge cycle performance, and about 30% of acetic acid in the cell was consumed and recovered in the charge and discharge process.

Water-Stable Lithium Anode with the Three-Layer Construction for Aqueous Lithium–Air Secondary Batteries

A water-stable multilayer Li-metal electrode consisting of a lithium metal, a PEO 18 LiN(SO 2 CF 3 ) 2 ―BaTO 3 composite polymer, and a lithium-conducting glass ceramic Li 1.3S Ti 1.7S Al 0.25 P 0.9 Si 0.1 O 12 (LTAP) was proposed as the lithium anode for aqueous lithium―air secondary batteries. The addition of finely dispersed nanosize BaTiO 3 in the polymer electrolyte greatly reduced the interfacial resistance between the Li anode and the polymer electrolyte. A Li/PEO 18 LiN(SO 2 CF 3 ) 2 ―10 wt % BaTiO 3 /LTAP electrode showed a total resistance of 175 Ω cm 2 in a 1 M aqueous LiCl solution at 60°C, with no change in the electrode resistance over a month. The Li/PEO 18 LiN(SO 2 CF 3 ) 2 ―10 wt % BaTiO 3 /LTAP/aqueous 1 M LiCl/Pt air cell had a stable open-circuit voltage of 3.80 V, which was equivalent to that calculated from the cell reaction of 2Li + 1/2O 2 + H 2 O = 2LiOH. The cell exhibited a stable and reversible discharge/charge performance of 0.5 mA cm ―2 at 60°C, suggesting excellent reversibility of the lithium oxidation reduction reaction for the Li/PEO 18 LiN(SO 2 CF 3 ) 2 ―10 wt % BaTiO 3 /LTAP electrode.

A functional layer for direct use of hydrocarbon fuel in low temperature solid-oxide fuel cells

Solid-oxide fuel cells (SOFCs), which consist of ceramic components, directly convert the chemical energy of a fuel into electrical energy with the highest efficiency among various kinds of fuel cells. Because SOFCs are operated at high temperatures, typically in excess of 700 °C, direct use of hydrocarbon fuel becomes possible, which minimizes the system size as well as reducing the cost. It is, however, difficult to utilize direct reforming of hydrocarbon fuel when the operating temperature is below 600 °C, which is the target for intermediate temperature SOFCs. Here, we report a new concept of an SOFC utilizing a functional layer on the surface of an anode, for the direct reformation of a hydrocarbon fuel using a micro-tubular design. Preparation of the functional layer is cost-effective and the cell with a pure-ceria (CeO2) functional layer was successfully fabricated. The cell displays practical cell performance below 500 °C using methane–water mixture as the fuel gas, and shows enhanced performance compared to systems without a functional layer.

Computational Thermal-Fluid Analysis of a Microtubular Solid Oxide Fuel Cell

A computational fluid dynamics model is developed to study the steady-state behavior of a microtubular solid oxide fuel cell (SOFC). The model incorporates mass, momentum, species, and heat balances along with ionic and electronic charge transfers. The anode-supported SOFC studied in this work consists of a ceria-based electrolyte which is known as an electronic conductor in reducing atmospheres, letting electrons leak through the electrolyte. Related internal leakage currents are calculated implicitly in the model to incorporate the performance losses. Moreover, to have a more realistic approach while cutting down the computational effort, in this study a fuel cell test furnace is also modeled separately to evaluate the distribution of the oxygen concentration and temperature field inside the furnace. Results from the furnace model are used as boundary conditions for the fuel cell model. Fuel cell model results are compared with the experimental data which shows good agreement.

Performance Degradation of Microtubular SOFCs Operating in the Intermediate-Temperature Range

Decreasing the operating temperature of solid oxide fuel cells (SOFCs) can reduce degradation of cell and stack materials and can reduce the cost of these materials as well through the use of metallic materials. These qualities have motivated the development of microtubular SOFCs for operation in the intermediate temperature range between 450 and 500°C. In this study microtubular SOFCs are fabricated and tested for analysis of start-up behavior and electrochemical properties under various conditions in the intermediate temperature range. Start-up behavior, performance at varying flow rates of fuel, and performance during load cycling is investigated. The impedance spectra and current-voltage performance from individual 1.8 mm diameter, 1.2 cm length microtubular SOFCs are obtained for the intermediate temperature range. The microtubular SOFC investigated is anode-supported, consisting of a NiO and Gd 0.2 Ce 0.8 O 2-x (GDC) cermet anode, thin GDC electrolyte, and a La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-y and GDC cermet cathode.

Rotating Disk Electrode Study of Supported and Unsupported Catalysts for PEMFC Application

The electrochemical study of different supported and unsupported Pt-based catalysts used in polymer electrolyte fuel cells (PEMFCs) has been made by means of thin film rotating disk electrode (RDE) method. The comparison of electrochemical surface area (ESA) was made using cyclic voltammetry measurements in H2SO4 or HClO4 at room temperature and scanning rates of 20 and 100 mV/ s. Oxygen reduction activity for aerogel-supported and carbon-supported Pt catalysts was evaluated at various rotation speeds in the range of 0 – 2500 rpm and compared to the catalytic activity of unsupported Pt-black catalyst. The calculated values of Levich constant for the oxygen reduction reaction (ORR) indicate dependence on the applied voltage and either 4 or 2‐4 electron transfer mechanism. The hydrogen peroxide by-product formation was determined with a rotating ring disk electrode (RRDE) and was observed to take place mostly at voltages below 0.6 V due to the limitations from mass transport effects. The results obtained at different Pt loadings in the range of 10– 46.5 wt % Pt demonstrated that XC-72 and aerogel-based catalysts are foreseen to have higher ESA in comparison to other tested supported and unsupported commercially available catalysts. DOI: 10.1115/1.2349524

Chapter 4 – Electrolytes

This chapter discusses fundamental and practical features of fluorite structured electrolytes. The fluorite structure is a face-centered cubic arrangement of cations with anions occupying all the tetrahedral sites, leading to a large number of octahedral interstitial voids. Thus, this structure is a rather open one and rapid ion diffusion might be expected. At high temperatures, zirconia has the fluorite structure, stabilized by addition of divalent or trivalent (that is, aliovalent) cations such as Ca or Y at lower temperatures. Pure ceria also has the fluorite structure. Oxide ion conduction is provided by oxide ion vacancies and interstitial oxide ions. Intrinsic defects are fixed by thermodynamic equilibrium in pure compounds, while extrinsic defects are established by the presence of aliovalent dopants. To maintain electroneutrality, a soluble aliovalent ion in an ionic compound is compensated by an increase in the concentration of an ionic defect. In addition to fluorite structure electrolytes such as stabilized zirconia and ceria, there are many non-fluorite structure oxides which are potentially attractive for solid oxide fuel cells (SOFC) electrolyte application. These include perovskites like lanthanum gallate and to a lesser degree calcium titanate.

Yttrium-Doped Barium Zirconates as Ceramic Conductors in the Intermediate Temperature Range

A modified Pechini process has been used to synthesize perovskite-structured yttrium-doped barium zirconate of the form BaZr 0.85 Y 0.15 O 3-δ (BZY15). The X-ray diffraction pattern for the sintered material (5 h at 1500°C) confirms a cubic structure with unit cell parameter of 8.424 A. Scanning electron microscopy images reveal a dense electrolyte structure. Thermal analysis (TGA/DTA) was also conducted on the material to determine possible endothermic and exothermic peaks and phase transformations. Raman spectroscopy data indicated that the most intense band in the spectrum of BZY15 occurs at 616 cm -1 , which suggests that cubic zirconia (Zr-O) vibrations are present. No bands were observed for secondary oxides, such as BaO, which would have a characteristic sharp band at 949 cm -1 , or Y 2 Ο 3 , whose characteristic band occurs at 376 cm -1 . ACimpedance measurements at 600°C indicate that the conductivity of the sample is higher in humidified gases compared to dry ones and has the lowest value in humidified air.

Aerogel-Based PEMFC Catalysts Operating at Room Temperature

A carbon-aerogel-supported Pt catalyst with 22 nm pore size distribution and low Pt loading (0.1 mg/cm 2 ) has been tested in a proton exchange membrane fuel cell (PEMFC). The performance of the PEMFC and kinetic parameters of the catalyst at room temperature are discussed in terms of microstructure of the support and sulfonated tetrafluoroethylene (Nafion) distribution. The PEMFCs demonstrated power densities up to 0.5 mW/cm 2 at 0.6 V in air/hydrogen and 2 atm backpressure on both cathode and anode. Continuous cycling with upper potential sweep limits of 1.0 and 1.2 V leads to degradation effects that result in decreasing of the electrochemical surface area (ESA) of the catalyst. The comparison of an ESA decrease for a 1.0 and 1.2 V sweep limit after 1000 cycles indicated that the higher degradation effects are due to the oxidation of carbon support.