Günter Schiller

Development and characterization of vacuum plasma sprayed thin film solid oxide fuel cells

The vacuum plasma spraying (VPS) process allows the production of thin solid oxide fuel cells (SOFCs) with low internal resistances. This enables the reduction of the cell operating temperature without a significant decrease in power density. Consequently, the long-term stability of the cells can be improved and low-cost materials can be used.Different material combinations and spray parameter variations were applied to develop thin-film SOFCs, which were plasma sprayed in a consecutive deposition process onto different porous metallic substrates. The use of Laval nozzles, which were developed at the German Aerospace Center (DLR), and the use of conical F4V standard nozzles enable the fabrication of thin gas tight yttria- and scandia-stabilized ZrO2 (YSZ and ScSZ) electrolyte layers and of porous electrode layers with high material deposition rates. The optimization of the VPS parameters has been supported by laser doppler anemometry (LDA) investigations.The development of the plasma-sprayed cells with a total thickness of approximately 100 µm requires an overall electrical and electrochemical characterization process of the single layers and of the completely plasma-sprayed cell assembly. The plasma-sprayed cell layers reveal high electrical conductivities. The plasma-sprayed cells show very good electrochemical performance and low internal resistances. Power densities of 300 to 400 mW/cm2 at low operating temperatures of 750 to 800 °C were achieved. These cells can be assembled to high performance SOFC stacks with active cell areas up to 400 cm2, which can be operated at reduced temperatures and good long-term stability.

Development of vacuum plasma sprayed thin-film SOFC for reduced operating temperature

The reduction of the operating temperature of planar solid oxide fuel cells (SOFCs) to an intermediate temperature regime of 650–800°C is an important objective in current development activities worldwide, in order to reduce production costs and improve long-term stability. In addition, existing conventional SOFC production processes must be further developed towards automated production lines, and new manufacturing processes with the potential for mass production must be established to meet the strict cost targets for the successful introduction of SOFCs in the strongly competitive energy market. DLR the German aerospace research centre has developed a novel planar thin-film SOFC concept which is based on advanced plasma spray processes. These manufacturing techniques allow the subsequent deposition of the entire membrane-electrode assembly (MEA) onto a porous metallic substrate within a very short process time, and has the potential to be developed into an automated continuous production process. By applying the plasma spray technology single cells were fabricated and characterised as having high performance in the temperature range 750–900°C and good long-term behaviour. Further development work will concentrate on the assembly of stacks with several large-scale cells with dimensions of 10 × 10 and 20 × 20 cm 2 , in order to obtain stack performance and durability results.

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Preparation of perovskite powders and coatings by radio frequency suspension plasma spraying

Perovskite-type LaMnO3 powders and coatings have been prepared by a novel technique: reactive suspension plasma spraying (SPS) using an inductively coupled plasma of approximately 40 kW plate power and an oxygen plasma sheath gas. Suitable precursor mixtures were found on the basis of solid state reactions, solubility, and the phases obtained during the spray process. Best results were achieved by spraying a suspension of fine MnO2 powder in a saturated ethanol solution of LaCl3 with a 1 to 1 molar ratio of lanthanum and manganese. A low reactor pressure was helpful in diminishing the amount of corrosive chlorine compounds in the reactor. As-sprayed coatings and collected powders showed perovskite contents of 70 to 90%. After a posttreatment with an 80% oxygen plasma, an almost pure LaMnO3 deposit was achieved in the center of the incident plasma jet.

Thermal Plasma Chemical Vapor Deposition of Si-Based Ceramic Coatings from Liquid Precursors

In this paper a process based on the use of rf inductively coupled plasma is applied for the synthesis and deposition of Si-base ceramic materials (i.e., SiC, Si3N4, SiO2). The starting materials are low-cost liquid disilanes. The atomization process is first investigated and the structure of the resulting coatings is characterized by means of X-ray diffraction, scanning electron microscopy as well as with transmission electron microscopy. Results of the influence of some processing parameters (i.e., chamber pressure, spray distance, substrate cooling, plasma gas nature and composition, precursor composition and atomization parameters) on the phase and microstructure of the coating is reported. Control of the microstructure (or nanostructure) as well as the phase content, namely the α/β ratio of the phases for SiC and Si3N4, can be achieved with such a synthesis and deposition technique.

Improving Plasma-Sprayed Yttria-Stabilized Zirconia Coatings for Solid Oxide Fuel Cell Electrolytes

Using a D-optimal design of experiments, the influences of feedstock powder and plasma gases on deposition efficiency, gas tightness, and the electrochemical behavior of vacuum plasma-sprayed yttria-stabilized zirconia for solid oxide fuel cell electrolytes were examined. In-flight particle temperature and velocity, measured by online particle diagnostics, were correlated with plasma and deposit properties. Electrochemical testing of cells was performed to determine the influence of gas tightness and microstructure of electrolyte deposit on cell behavior.

Near-Net-Shape Forming of Metallic Bipolar Plates for Planar Solid Oxide Fuel Cells by Induction Plasma Spraying

In one of the present designs of solid oxide fuel cells (SOFC), metallic bipolar plates with gas channels on the surface are used, which consist of a chromium alloy and are manufactured by a time consuming and costly multistep process. To reduce the production time and costs, attempts were made to develop an alternative near-net-shape production method based on RF-induction plasma spray technology. With this process raw powders, as applied for the “conventional” sintering route as well as recycled powders from used bipolar plates, have been applied. The process parameters were adapted to both powders, and the obtained products were qualified. The near-net-shape production requires the formation of a gas channel structure already with the spray process using structured substrates. Therefore, different spray angles occur during the deposition process. The influence of the spray angle on the microstructure of the free-standing parts was investigated. The required gas tightness for grooved profiles with relatively large channel depths and widths can only be achieved using spray angles between 90° and approximately 60°. Then a tilting of the substrate and an adapted design of the gas channel profiles are needed to fulfill the structural requirements for the bipolar plates.

Evaluation of the Effect of Sulfur on the Performance of Nickel/Gadolinium-Doped Ceria Based Solid Oxide Fuel Cell Anodes.

Abstract The focus of this study is the measurement and understanding of the sulfur poisoning phenomena of Ni/gadolinium‐doped ceria (CGO) based solid oxide fuel cells (SOFC). Cells with Ni/CGO10 and NiCu5/CGO40 anodes were characterized by using impedance spectroscopy at different temperatures and H2/H2O fuel ratios. The short‐term sulfur poisoning behavior was investigated systematically at temperatures of 800–950 °C, current densities of 0–0.75 A cm−2, and H2S concentrations of 1–20 ppm. A sulfur poisoning mitigation effect was observed at high current loads and temperatures. The poisoning behavior was reversible for short exposure times. It was observed that the sulfur‐affected processes exhibited significantly different relaxation times that depend on the Gd content in the CGO phase. Moreover, it was demonstrated that the capacitance of Ni/CGO10 anodes is strongly dependent on the temperature and gas‐phase composition, which reflects a changing Ce3+/Ce4+ ratio.

Investigation of Locally Resolved SOFC Characteristics along the Flow Path

Solid oxide fuel cells (SOFCs) are characterized locally in a segmented cell arrangement. The tests were performed in counter flow operation for hydrogen concentrations from 2% to 100% to identify concentration limitations and to optimize fuel utilization. Cell characterisations were obtained for different cell concepts by voltage/current density U(i) and temperature measurements as well as gas chromatography measurements at 16 distinct points across the cell. The results show a substantial variation of current density and voltage distribution along the flow path with varying hydrogen content and fuel utilization. Especially at high fuel utilizations approaching 100% a dramatic dependence of power density along the flow path occurs which is associated with increasing degradation. The observation of inhomogeneous temperature distribution emphasizes the importance of thermal management adapted to the cell operation characteristics, particularly when the stack itself only has a low thermal mass.

Diagnostic Tools for in-situ and ex-situ investigations of fuel cells and components at the German Aerospace Center

The analysis of the processes in fuel cells like the degradation and the alteration during start-up processes and the analysis of fuel cell components especially of the electrodes and their components is a main topic at the DLR. For this purpose physical and electroche¬mical methods are used individually and combined. In this paper different example of applications of studies performed at DLR using different methods in combination will be presented. The examples in the field of low temperature fuel cells - alkaline fuel cells, polymer electrolyte fuel cells and direct methanol fuel cells – are mainly focused on the alteration of the electrodes due to electrochemical loading; for SOFC an example for the investi¬ga¬tion of the local operating conditions is given.