Asif Ansar

Progress in the Metal Supported Solid Oxide Fuel Cells and Stacks for APU

DLR is one of the pioneer groups to introduce metal supported solid oxide fuel cells (MS-SOFC) in mid 90s and it is continuing the development of the concept towards a reliable light weight 1 kW stack for APU in a funded consortium with ElringKlinger, Plansee and Sulzer Metco. The joint work is an integrated approach incorporating improved materials for functional layers, advanced industrial scale manufacturing and an evolved stack design to define a pilot production set-up for large volume manufacturing. At laboratory scale, the latest generation button cells (12 to 15 cm² effective area) exhibited more than 750 and 520 mW/cm² respectively with hydrogen and simulated reformate gas as fuel at 800°C. 2000 hours tests were performed with degradation rate of less than 1.5%/kh. Redox stability of these cells was demonstrated for 20 cycles during which the anodes were fully oxidized. Up-scaling to 85 cm² and 100 cm² effective area cells was accomplished and the cells showed 400 mW/cm² power density using simulated reformate gas at a fuel utilisation of 32%. Further improvements consisting of developing large scale industrial processes for the fabrication of functional layers including low pressure plasma spraying (LPPS) and atmospheric plasma spraying (APS) with TriplexPro are in progress. The aim is to get higher productivity and reproducibility. In addition, a new alloy for interconnects and substrates will be incorporated to further enhance the durability of cells.

IDEAL-Cell, Innovative Dual mEmbrAne fueL-Cell: Fabrication and Electrochemical Testing of First Prototypes

The IDEAL-Cell concept is based on the junction between a PCFC anode/electrolyte section and a SOFC cathode/electrolyte section, through a mixed proton and oxide ion conducting porous ceramic membrane, operating in the temperature range 600-700°C. Recombination of ions takes place within the junction, or central membrane (CM), and water vapor is evacuated through open porosity. The first results on the fabrication of multilayered samples reproducing the IDEAL-Cell structure are reported here, together with electrochemical tests carried out on selected samples in order to evaluate the performances of the chosen materials and to demonstrate the feasibility of this innovative concept of fuel cell.

Improving Stochiometery and Processing of LSCF Oxygen Electrode for SOFC

New stochiometries of La1-xSrxCo0.2Fe0.8O3-a (LSCF) for the oxygen electrode of SOFC were developed. The functional layers were manufactured by atmospheric plasma spraying (APS) using TriplexPro gun. Processing of LSCF was optimized for high gas permeability and low decomposition of the powder with focus on high production rates. Studies on microstructure using optical microscopy, scanning electron microscopy (SEM) and x-ray diffraction (XRD) were linked to plasma spray parameters. As a critical factor for decomposition the dwell time of the particles inside the plasma was identified. Functional layers had an improved porosity of about 20 vol%. At 800°C, power densities of above 640 mW/cm² at 0.7 V were recorded with H2/air

Nanostructured Composite Cathodes by Suspension Plasma Spraying for SOFC Applications

In the metal supported SOFCs (MS-SOFCs), conventional sintering route for cathode layer appears problematic due to excessive oxidation of metal support in air at sintering temperatures and degradation of perovskites during sintering in low oxygen partial pressures. The novel process of suspension plasma spraying (SPS), capable of producing a wide variety of coating structures and compositions without any post-deposition sintering, is developed and evaluated in detail for the fabrication of cathode layers for MS-SOFCs. Composite cathodes of LSCF and CGO were developed and the influence of different cathode microstructures on fuel cell performance was examined. Four exemplary cathode coatings have been created, ranging from partially dense with small pores to highly porous layers preserving submicronic agglomerated structures. These cathode coatings have been tested on equal half-cells made of the same anode and electrolyte. The best performing microstructure had a power density of 798 mW/cm2 at 800°C. Microstructural differences could be attributed to different behavior and performance of the coatings.

High Temperature Water Electrolysis Using Metal Supported Solid Oxide Electrolyser Cells (SOEC)

Metal supported cells as developed at DLR for use as solid oxide fuel cells by applying plasma deposition technologies were investigated in operation of high temperature steam electrolysis. The cells consisted of a porous ferritic steel support, a diffusion barrier layer, a Ni/YSZ fuel electrode, a YSZ electrolyte and a LSCF oxygen electrode. During fuel cell and electrolysis operation the cells were electrochemically characterised by means of i-V characteristics and electrochemical impedance spectroscopy measurements including a long-term test over 2000 hours. The results of electrochemical performance and long-term durability tests of both single cells and single repeating units (cell including metallic interconnect) are reported. During electrolysis operation at an operating temperature of 850 °C a cell voltage of 1.28 V was achieved at a current density of -1.0 A cm-2; at 800 °C the cell voltage was 1.40 V at the same operating conditions. 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. During a long-term test run of a single cell over 2000 hours a degradation rate of 3.2% per 1000 hours was observed for operation with steam content of 43% at 800 °C and a current density of -0.3 Acm-2. Testing of a single repeating unit proved that a good contacting of cell and metallic interconnect is of major importance to achieve good performance. A test run over nearly 1000 hours showed a remarkably low degradation rate.

Towards Understanding the Dual Membrane Fuel Cell (IDEAL-Cell) Using a Metallic Central Membrane

This work presents results of a measurement setup which was constructed for understanding the performance contributions of the individual compartments and reactions taking place in the patented concept of IDEAL-Cell (described in detail in(1)). By inserting a third measurement point at the central membrane of an ideal cell it was possible to measure the cell in three different modes corresponding to hydrogen, oxygen compartment and the full cell. Recorded maximum power densities of (~4, ~7, 11, 16) mW/cm² at (600, 650, 700, 750) °C in H2-O2 atmosphere respectively were observed which were similar to the values of proof of concept ideal cell samples tested so far. Moreover in this setup it was possible to record higher maximum power densities for the hydrogen (proton conducting) compartment which was (~14, 22, 33*, ~38*) mW/cm² at (600, 650, 700, 750) °C.