C. P. Bankston

Kinetics and Transport at AMTEC Electrodes I . The Interfacial Impedance Model

Mixed mass-transport and kinetic control of sodium ion reduction at porous inert electrodes on sodium beta-double-prime alumina solid electrolyte (BASE) ceramic in a high-temperature electrochemical cell has been observed and modeled. The high ionic conductivity of BASE and the reversibility of the liquid sodium/BASE anodic half-cell led to assignment of potential-dependent (nonohmic) resistances to kinetic and mass-transport processes associated with the porous electrode. The morphology of these electrodes and typical sodium gas pressures are consistent with Knudsen, or free-molecular, flow through the electrode.

Kinetics and Transport at AMTEC Electrodes II . Temperature Dependence of the Interfacial Impedance of Na(g)/Porous Mo/Na‐Beta′ Alumina

The exchange current, transfer coefficient, mass-transport parameters, and electrode capacitance at the Na(g)/porous Mo/Na-Beta-double prime alumina solid electrolyte (BASE) phase boundary have been evaluated from 740 to 1220 K. The transfer coefficient exhibits a value close to 0.5 and the exchange current is dominated by collision frequency, with no significant activation energy. Since the porous Mp-electrode adopts a fairly regular microstructure on the BASE surface, the magnitude of the exchange current of mature electrodes directly depends on the actual contact zone of the porous metal film with the BASE ceramic, and decreases slightly as grain growth occurs. The exchange currents and the mass-transport parameters derived for very porous, thin Mo electrodes indicate that the charge-transfer reaction occurs at a small fraction of the interface. High-frequency limiting capacitance and resistance values due to the interface show potential dependence and a value on the order of 1 F/sq m and 0.1-1.0 Ohm-sq cm.

Performance and impedance studies of thin, porous molybdenum and tungsten electrodes for the alkali metal thermoelectric converter

Columnar, porous, magnetron-sputtered molybdenum and tungsten films show optinum performance as AMTEC electrodes at thicknesses less than 1.0 μm when used with molybdenum or nickel current collector grids. Power densities of 0.40 W cm−2 for 0.5 μm molybdenum films at 1200 K and 0.35 W cm−2 for 0.5 μm tungsten films at 1180 K were obtained at electrode maturity after 40–90 h. Sheet resistances of magnetron sputter deposited films on sodium beta″-alumina solid electrolyte (BASE) substrates were found to increase very steeply as thickness is decreased below about 0.3–0.4 μm. The a.c. impedance data for these electrodes have been interpreted in terms of contributions from the bulk BASE and the porous electrode/BASE interface. Voltage profiles of operating electrodes show that the total electrode area, of electrodes with thickness <2.0 μm, is not utilized efficiently unless a fairly fine (∼1×1mm) current collector grid is employed.

Organic cathode materials in sodium batteries

In order to circumvent the corrosion problems prevalent in many existing electrochemical couples using the Na/β″-alumina half cell, a new class of high energy density organic materials was studied as cathode materials. In particular, one material tetracyanoethylene (TCNE), has favourable electrochemical characteristics with a potential >3.0 V against Na+/Na and energy density ∼620 Wh kg−1. Adopting a cell designed to permit sealing the anode half cell, the performance of TCNE was evaluated under various experimental conditions, that is, at different concentrations of TCNE in the catholyte and with different current collectors. The electrochemical behaviour of the TCNE cathode and the kinetics of TCNE reduction were examined. The kinetic parameters, exchange current density and diffusion coefficient, were determined from different a.c. and d.c. electrochemical techniques and evaluated with respect to the changes in TCNE concentrations in the catholyte. A chemical transformation occurring in the cell operating conditions which does not reduce the electrochemical activity of TCNE was identified from FTIR spectra. Finally, possible approaches to the use of TCNE or other organic materials in sodium or lithium rechargeable batteries are outlined.

Kinetics of Intercalation of Lithium into NbSe3 and TiS2 Cathodes

Titanium disulfide and niobium triselenide are two well-studied candidate materials for positive electrodes in rechargeable lithium cells. A comparative study of the kinetics of intercalation of lithium in both the cathodes is made here based on various electrochemical techniques, i.e., linear polarization, potentiodynamic polarization, and ac impedance under different experimental conditions such as prismatic or disk configuration of fresh, partially discharged, or cycled electrode. Further, the diffusion coefficients of lithium ions in these cathodes are estimated under these conditions using conventional techniques, i.e., ac impedance, chronocoulometry, chronoamperometry, and current pulse relaxation. Based on the values of the diffusion coefficients, the applicability of these methods for the determination of diffusion coefficients is discussed.

Electrochemical studies on niobium triselenide cathode material for lithium rechargeable cells

NbSe3 exhibits superior characteristics such as high capacity, high volumetric and gravimetric energy densities, and high discharge rate capability, as compared to other intercalating cathodes. This paper reports the preparation, characterization, and performance of NbSe3. Several electrochemical techniques, such as cyclic voltammetry, constant-current/constant-potential discharges, dc potentiodynamic scans, ac impedance, and ac voltammetry, have been used to give insight to the mechanisms of intercalation of three lithiums with NbSe3 and also into the rate determining process in the reduction of NbSe3.

Advanced rechargeable sodium batteries with novel cathodes

Abstract Various high energy density rechargeable batteries are being considered for future space applications. Of these, the sodium-sulfur battery is one of the leading candidates. The primary advantage is the high energy density (760 W h kg −1 theoretical). Energy densities in excess of 180 W h kg −1 have been realized in practical batteries. More recently, cathodes other than sulfur are being evaluated. We, at JPL, are evaluating various new cathode materials for use in high energy density sodium batteries for advanced space applications. Our approach is to carry out basic electrochemical studies of these materials in a sodium cell configuration in order to understand their fundamental behaviors. Thus far, our studies have focussed on alternative metal chlorides such as CuCl 2 and organic cathode materials such as TCNE.

A.c. impedance of niobium triselenide cathode in secondary lithium cells

Niobium triselenide is one of the cathode materials being evaluated at JPL for ambient temperature secondary lithium batteries for space applications. The mechanism of reduction of NbSe3 involves two steps, 1 mole of Li intercalating in the first step and 2 moles of Li intercalating at lower potentials in the second step. The a.c. impedance behaviour of the NbSe3 cathode under various conditions, i.e. at different reduction potentials, after discharges at various d.c. potentials and after charge/discharge cycling is discussed.

Morphology and dielectric properties of uniaxially and biaxially-oriented polycarbonate capacitor films

A reflective X-ray diffraction method developed to determine the absolute X/sub c/ (crystallinity) of UX and BX (uniaxially and biaxially oriented) PC (polycarbonate) film is described. Conditions for achieving optimum properties for producing BX high-X/sub c/ and isotropic PC film were found. For BX PC film of identical thickness to UX PC film the electric breakdown strength was found to be proportional to X/sub c/. The thermal and mechanical properties as well as the direct-current electric breakdown strengths of the newly developed isotropic high-X/sub c/ BX PC film are compared with those of the anisotropic commercial PC capacitor film. Capacitors made from the high-X/sub c/ isotropic BX PC film when subjected to a 1000-hour 100 degrees C, 42-V DC life test showed no change in dissipation factor and no change in capacitance, and they met 100 degrees C insulation resistance test requirements. In general, it is concluded that this novel BX PC film is much superior to the traditional UX commercial capacitor PC film.<<ETX>>