High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC)

Metal supported cells as developed according to the DLR SOFC concept by applying plasma deposition technologies were investigated for use as solid oxide electrolyser cells (SOEC) for high temperature steam electrolysis. Cells consisting of a porous ferritic steel support, a diffusion barrier layer, a Ni/YSZ hydrogen electrode, a YSZ electrolyte and a LSCF oxygen electrode were electrochemically characterised by means of i-V characteristics and electrochemical impedance spectroscopy measurements including a long-term test over 2000 h. The cell voltage during electrolysis operation at a current density of −1.0 A cm⁻² was 1.28 V at an operating temperature of 850 °C and 1.4 V at 800 °C. A long-term test run over 2000 h with a steam content of 43% at 800 °C and a current density of −0.3 A cm⁻² showed a degradation rate of 3.2% per 1000 h. The impedance spectra revealed a significantly enhanced polarisation resistance during electrolysis operation compared to fuel cell operation which was mainly attributed to the hydrogen electrode.     

High ultraviolet transparent conducting electrodes formed using tantalum oxide/Ag multilayer

We investigated the optical and electrical properties of Ta₂O₅/Ag/Ta₂O₅ films as functions of the thicknesses of the Ta₂O₅ and Ag layers. It was found that with an increase in the thicknesses of the Ta₂O₅ and Ag layers from 10 to 40 nm and from 12 to 24 nm, respectively, the sheet resistance, carrier concentration, electron mobility, and resistivity of the Ta₂O₅/Ag/Ta₂O₅ film varied from 2.02 to 8.95 Ω/sq, 5.74 × 10²¹ to 2.92 × 10²² cm^–3, from 13.21 to 24.07 cm²/V·s, and from 8.89 × 10^-6 to 8.24 × 10^-5 Ω cm, respectively. The average transmittance (T_av) of the multilayer samples ranged from 57.18% to 93.99%, and it depended on the Ta₂O₅ and Ag layer thicknesses. The highest T_av of 93.99% was observed for the film with 35 nm thick Ta₂O₅ and 18 nm thick Ag layers, and the peak Haacke's figure of merit (157.04 × 10^–3 Ω^–1) was obtained for 20 nm thick Ta₂O₅ and 21 nm thick Ag layers. Ta₂O₅ (100 nm) and Ta₂O₅/Ag/Ta₂O₅ (20 nm/21 nm/20 nm) samples had optical bandgaps of 4.70 and 4.45 eV, respectively. Film Wizard simulations were conducted to understand the dependence of the transmittance of the multilayer on the thicknesses of the Ta₂O₅ and Ag layers, and phasor analyses were performed to determine how the transmittance of the Ta₂O₅/Ag/Ta₂O₅ (20 nm/21 nm/20 nm) film depended on the Ta₂O₅ layer's thickness.     

High performance copper oxide cathodes for high temperature solid-oxide fuel cells

The cathodic polarization characteristics of CuO and YBa₂Cu₃O_7- electrodes were studied in the temperature range 600 to 800°C and at oxygen partial pressures ranging from 10^–4 to 0.21 atm. The activity of oxygen reduction on a CuO electrode is closely related to the electronic conductivity and the oxygen ion vacancy density in the surface layer of the electrode. The oxygen ion vacancies created in CuO by doping with Li and the modification of the electronic conductivity by adding Ag provide a new way of enhancing the activity of an oxide electrode for oxygen reduction. It is demonstrated that the rate limiting steps for oxygen reduction at high overpotential and low overpotential are oxygen adsorption and charge transfer on CuO, respectively.List of symbols F Faraday constant - f F/RT - i current - i₀ exchange current - k ⁰ intrinsic rate constant of charge transfer - N() electron density with an energy level E - n number of electrons - R gas constant T temperature Greek letters transfer coefficient - conductivity - overpotential - energy level     

Electrochemical performance of all-solid-state lithium secondary batteries with Li-Ni-Co-Mn oxide positive electrodes

LiNi_1/3Co_1/3Mn_1/3O₂ was applied as a promising material to the all-solid-state lithium cells using the 80Li₂S·19P₂S₅·1P₂O₅ (mol%) solid electrolyte. The cell showed the first discharge capacity of 115 mAh g⁻¹ at the current density of 0.064 mA cm⁻² and retained the reversible capacity of 110 mAh g⁻¹ after 10 cycles. The interfacial resistance was observed in the impedance spectrum of the all-solid-state cell charged to 4.4 V (vs. Li) and the transition metal elements were detected on the solid electrolyte in the vicinity of LiNi_1/3Co_1/3Mn_1/3O₂ by the TEM observations with EDX analyses. The electrochemical performance was improved by the coating of LiNi_1/3Co_1/3Mn_1/3O₂ particles with Li₄Ti₅O₁₂ film. The interfacial resistance was decreased and the discharge capacity was increased from 63 to 83 mAh g⁻¹ at 1.3 mA cm⁻² by the coating. The electrochemical performance of LiNi_1/3Co_1/3Mn_1/3O₂ was compared with that of LiCoO₂, LiMn₂O₄ and LiNiO₂ in the all-solid-state cells. The rate capability of LiNi_1/3Co_1/3Mn_1/3O₂ was lower than that of LiCoO₂. However, the reversible capacity of LiNi_1/3Co_1/3Mn_1/3O₂ at 0.064 mA cm⁻² was larger than that of LiCoO₂, LiMn₂O₄ and LiNiO₂.     

Cycling of lithium/metal oxide cells using composite electrolytes containing fumed silicas

The effect on cycle capacity is reported of cathode material (metal oxide, carbon, and current collector) in lithium/metal oxide cells cycled with fumed silica-based composite electrolytes. Three types of electrolytes are compared: filler-free electrolyte consisting of methyl-terminated poly(ethylene glycol) oligomer (PEGdm, M_w=250)+lithium bis(trifluromethylsufonyl)imide (LiTFSI) (Li:O=1:20), and two composite systems of the above baseline liquid electrolyte containing 10-wt% A200 (hydrophilic fumed silica) or R805 (hydrophobic fumed silica with octyl surface group). The composite electrolytes are solid-like gels. Three cathode active materials (LiCoO₂, V₆O₁₃, and Li_xMnO₂), four conducting carbons (graphite Timrex® SFG 15, SFG 44, carbon black Vulcan XC72R, and Ketjenblack EC-600JD), and three current collector materials (Al, Ni, and carbon fiber) were studied. Cells with composite electrolytes show higher capacity, reduced capacity fade, and less cell polarization than those with filler-free electrolyte. Among the three active materials studied, V₆O₁₃ cathodes deliver the highest capacity and Li_xMnO₂ cathodes render the best capacity retention. Discharge capacity of Li/LiCoO₂ cells is affected greatly by cathode carbon type, and the capacity decreases in the order of Ketjenblack>SFG 15>SFG 44>Vulcan. Current collector material also plays a significant role in cell cycling performance. Lithium/vanadium oxide (V₆O₁₃) cells deliver increased capacity using Ni foil and carbon fiber current collectors in comparison to an Al foil current collector.     

High temperature polarography in chloride melts with dropping electrodes

An experimental technique for polarographic studies with dropping electrodes at 1000°C in chloride melts was developed. Liquid silver and a gold-bismuth alloy were used as electrode materials for studying the reduction of Mn²⁺, Ag⁺ and Cu⁺ ions in molten KCl. Diffusion limited currents were observed for these ions but the halfwave potentials could not be determined because of corrosion reactions of the liquid metal electrode with the melt.     

High temperature voltammetry on gold electrodes in halide melts

The voltammetric behaviour of Fe²⁺, Mn²⁺ and Cr²⁺ in molten NaCl, NaF and Na₃AlF₆ has been studied using a gold wire as the working electrode. The metal ions studied behaved reversibly and the reduction product dissolved into the gold substrate. Diffusion coefficients were calculated from the voltammetric peak currents.     

High temperature-type proton conductive solid oxide fuel cells using various fuels

Using a high temperature-type proton conductive solid electrolyte based on SrCeO₃, the performances of various types of fuel cells were studied at 800 to 1000°C. Ethane, carbon monoxide and ethanol were used as the fuel. In the ethane-air fuel cell, the simultaneous generation of electricity and ethylene could be carried out without oxidizing ethylene into carbon dioxide. These cells worked stably and the major limitation of the cell system was the resistance of the solid electrolyte.     

Polymers and copolymers of pyrrole and thiophene as electrodes in lithium cells

The performance of pyrrole and thiophene polymer electrodes in lithium cells has been examined in the lithium perchlorate–propylene carbonate electrolyte by cyclic voltammetry. Polypyrrole films were synthesized in 'wet' and 'dry' conditions; pyrrole and thiophene copolymers were prepared at different potentials and bilayers were prepared by sequential deposition of polythiophene (PTh) and polypyrrole (PPy) films. The polymers were cycled between 2.0V and 4.0V in the lithium cells. The effects of disconnecting the electrodes from the cell on the behaviour of the polymers regarding doping and coulombic efficiency were also studied. The cycling performance of the 'wet' PPy is better than 'dry' PPy, bilayer PTh/PPy and copolymers. No mixed behaviour was observed for a bilayer where the inner layer was polythiophene and the outer layer was polypyrrol with a thickness PPy/Pth ratio equal to ten. The copolymer prepared at 3.9V vs Li/Li+ showed the higher energy capacity in Whkg–1 calculated from the anodic charge.     

The electrochemical oxidation of organics using tungsten oxide based electrodes

High surface area tungsten oxide (WO_x) based electrodes containing centers of Pt, Sn or Ru were synthesized. The WO_x electrodes were found to display good capacitive behavior and relatively high specific capacitance values of up to 180 F g⁻¹. The oxidation behavior of particularly HCOOH and (COOH)₂, using the WO_x electrodes containing Pt and Sn centers (Pt/WO_x and Sn/WO_x, respectively), was studied in detail in aqueous solutions at high potentials, i.e. at which O₂ is evolved. Both HCOOH and (COOH)₂ appear to be oxidized following 1st order kinetics. The (COOH)₂ oxidation reaction is faster than the HCOOH reaction using otherwise the same experimental conditions. The reaction mechanism of both the HCOOH and (COOH)₂ oxidation was found to most likely involve the adsorptive interaction of the two organics with the anode surface. The WO_x based anodes appear to be promising catalysts for the anodic oxidation of both (COOH)₂ and HCOOH.     

Film percolation for composite electrodes of solid oxide fuel cells

A film percolation model is proposed for composite electrodes of solid oxide fuel cells (SOFCs). The model is developed to predict the percolation properties of 2D-infinite structures which represent the structural characteristics of composite electrodes of electrochemical devices such as SOFCs. The model can be used to estimate electrode properties, such as percolation probability, active three-phase boundary length and interfacial polarization resistance. Compared with the classic percolation theory, which is particularly applicable to 3D-infinite bulks, the model can explicitly capture the effects of thinly layered nature of composite electrodes, and describes a cross-over between 2D-infinite films and 3D-infinite bulks. It also permits the prediction within whole electrode composition range, and can be easily applied in SOFC modeling.     

Symmetrical solid oxide fuel cells based on titanate nanocomposite electrodes

Nanocomposite electrodes of (Sr_0.7Pr_0.3)_0.95TiO_3±δ?Ce_0.9Gd_0.1O_1.95 are directly prepared by spray-pyrolysis deposition on Zr_0.82Y_0.16O_1.92 electrolytes and their properties are compared with those obtained by the traditional screen-printing powder method. The structural, microstructural and electrical characteristics are investigated for their potential use as both cathode and anode in Solid Oxide Fuel Cells. The nanocomposite electrodes with reduced particle size ～30 nm achieved a polarization resistance at 700 ºC of 0.50 and 0.46 Ω cm² in air and pure H₂, respectively, outperforming those obtained for the analogous screen-printed electrodes with particle size of 450 nm, i.e. 4.8 and 3.9 Ω cm², respectively. An electrolyte-supported cell with symmetrical electrodes reached a maximum and stable power density of 354 mW cm^-2 at 800 ºC. These results demonstrate that the performance of electrode materials with modest electrochemical properties but high phase stability, such as doped-SrTiO₃, can be highly improved by preparing nanocomposite electrodes directly on the electrolyte surface.     

Propane conversion over a Ru/CGO catalyst and its application in intermediate temperature solid oxide fuel cells

A Ru/CGO catalyst was investigated in combination with a Cu current collector for the direct electro-oxidation and internal reforming of propane in a solid oxide fuel cell. The electrochemical power densities for the direct oxidation were larger than in the internal reforming process at 750 °C. The electrochemical performance in the presence of propane was significantly affected by the polarization resistance which was about three times larger than that obtained for the SOFC fed with hydrogen at 750 °C. However, out-of-cell steam reforming tests showed a C₃H₈ conversion to syngas approaching 90% at 800 °C. Thus, significant enhancements may be achieved by properly optimizing the anode structure. No formation of carbon deposits was observed both upon operation of the anode in the direct oxidation and internal reforming processes at 750 °C.     

Combined X-ray study of lithium (tin) cobalt oxide matrix negative electrodes for Li-ion batteries

The lithium insertion behaviours of the oxides Co₃O₄ and Co₂SnO₄ were studied using a range of electrochemical, spectroscopic and diffraction techniques. Co K-edge EXAFS studies on the Co₃O₄ oxide showed that the reversible lithium insertion is coupled with changes in cobalt oxidation state. On lithium insertion, Co₃O₄ is reduced to yield Co(II) and yields only metallic cobalt species on complete reduction. On lithium removal an oxide of Co is formed, which from coulometry should be CoO, however, EXAFS indicates the short range structure is quite different to that of the rocksalt CoO. The long range structure of the matrix is amorphous according to XRD. The EXAFS and XRD data also revealed that both the metallic and oxide phases were disordered, having low co-ordination numbers and large shell spacings, and that there was an initial reduction to CoO before full reduction to metallic Co. The electrochemical behaviour of Co₂SnO₄ cells was more reminiscent of that of SnO₂ than that of Co₃O₄, but did exhibit significant differences due to the presence of cobalt. EXAFS on Co₂SnO₄ cells revealed that Co is reduced to metallic cobalt on the initial discharge, but that it does not convert back to an oxide on cycling even though the electrochemical treatment was the same as for Co₃O₄. Together the EXAFS and Mössbauer data show that the Co and Sn are reduced concurrently, and that some of the Sn remains in the oxidised form. In summary, we have a surprising result in that the presence of the tin dramatically changes the redox behaviour of the cobalt. In a matrix derived from a cobalt oxide spinel, cobalt undergoes redox cycling, whereas in a matrix derived from a cobalt tin oxide spinel, the cobalt does not cycle whilst the tin does.     

Degradation mechanisms of nickel oxide electrodes in zinc/nickel oxide cells with low-zinc-solubility electrolytes

Nickel oxide electrodes that suffered capacity degradation during extended cycling in zinc/nickel oxide cells were examined by a variety of chemical and physical techniques. Nickel hydroxyzincates, which have been speculated to cause such capacity degradation, were also examined. Powder X-ray diffraction experiments indicated that the intersheet distance between layers of turbostratic nickel hydroxide increased when zinc was incorporated. Photoelectron spectra (XPS) showed that this material is probably a mixture of NiOH)₂ and ZnO or Zn(OH)₂. Raman spectroscopy data also supported this conclusion. XPS indicated that the form of zinc in degraded nickel oxide electrodes is probably ZnO or Zn(OH)₂. Significant increases in resistivity were found in cycled nickel oxide electrodes, and optical microscopy provided visible evidence of mechanical damage during cycling. These results suggest that the observed capacity degradation was largely mechanical in nature, and not due to the formation of nickel-zinc double hydroxides, as had been reported by others. Cell-cycling experiments indicated that the mechanical degradation is largely irreversible.     

Low temperature sintering of CaZrO3 using lithium fluoride addition

The effects of lithium fluoride on the sinterability, the microstructure and the low frequency dielectric properties of acceptor/donor-doped CaZrO₃ ceramics were investigated. The acceptor(Mn)/donor(W) doping was used to avoid CaZrO₃ reduction. Lithium fluoride was selected as a liquid phase sintering aid to lower the sintering temperature. The dielectric properties of CaZrO₃ ceramics with LiF additions are strongly dependent on the densification, the microstructure, and the reaction with LiF. CaZrO₃ ceramics without LiF addition sintered at high temperature display a high permittivity, low losses and a good behavior under an electrical field as a function of temperature. Using LiF, this dielectric can be sintered at 1000 °C to achieve theoretical densities of 91%, ε_r values of 31 and losses close to 0.4%. This low sintering temperature allows co-sintering with base metal, nickel and copper, for which scanning electron microscopy and energy dispersive spectroscopy results are presented.