Triple-phase boundary and power density enhancement in thin solid oxide fuel cells by controlled etching of the nickel anode
© Ebrahim et al.; licensee Springer. 2014
Received: 3 April 2014
Accepted: 20 May 2014
Published: 9 June 2014
Fabrication of microporous structures for the anode of a thin film solid oxide fuel cell (SOFC(s)) using controlled etching process has led us to increased power density and increased cell robustness. Micropores were etched in the nickel anode by both wet and electrochemical etching processes. The samples etched electrochemically showed incomplete etching of the nickel leaving linked nickel islands inside the pores. Samples which were wet- etched showed clean pores with no nickel island residues. Moreover, the sample with linked nickel islands in the anode pores showed higher output power density as compared to the sample with clean pores. This enhancement is related to the enlargement of the surface of contact between the fuel-anode-electrolyte (the triple-phase boundary).
KeywordsThin film Solid oxide Fuel cell Triple-phase boundaries YSZ LSCO
The world's extensive use of petroleum increased drastically in the last decades causing not only a sharp drop in the world reserves but also resultant environmental concerns. Natural gas and other high hydrogen content fuels are better replacement candidates because of their lower environmental effects [1–3]. The major shortcomings of these types of fuels are their lower combustion efficiency and the larger volumes needed for machines that convert the fuel to electrical energy. This opens the field for more research on the development of low-volume and high-efficiency generators in order to use these fuels in a wide range. Extensive research has been held on fuel cells, which are one of the promising candidates. A number of hydrogen-oxygen-operated fuel cell designs already exist; solid oxide fuel cells (SOFCs) are one of the most attractive fuel cell types due to their high energy efficiency and environmental friendliness . Thick solid oxide fuel cells exhibited 0.2 to 1 W/cm2 with 60% to 70% reported efficiency but at undesired high operating temperatures >800°C [5, 6]. To avoid the high operating temperature of the SOFCs, it has been proposed to reduce electrolyte thickness and/or use a higher ion conducting electrolyte material. The fabrication of ultra-thin film SOFCs (10- to 15-μm cell thickness) built on microporous thin metallic foil substrates has already shown considerable reduction of the operating temperatures to 450°C to 550°C and also a reduction of cell volume. However, the cell was somewhat structurally weak, and cell output power density was low as compared to known SOFCs . In the present work, the enhancement of both cell physical structure and the output power density have been achieved by controlling the chemical etching process of the thin metal anode micropores which in turn enhanced the surface of contact between fuel-anode-electrolyte (the triple-phase boundary) [5, 8].
The crystalline structures of the successive layers of the fabricated fuel cells were characterized by X-ray diffraction (XRD) measurements which were carried out using a Siemens D-5000 spectrometer (Erlangen, Germany). The XRD scans were done in the standard θ-2θ configuration, using the Cu Kα radiation of wavelength 1.54 Å at scan steps of 0.05°. SEM analysis was carried out using a JEOL (JSM 5410, Akishima, Tokyo, Japan) scanning electron microscope. A computerized testing setup was used to test the fuel cells fuel-air performance (I-V and power output characteristics) as a function of operating temperature.
Results and discussion
Thin film solid oxide fuel cells were fabricated on porous nickel foils using PLD. Micropore openings were etched into the nickel foils for hydrogen fuel flow by wet and electrochemical etching so as to allow them to act as anodes. The electrochemical etching process showed incomplete etching leaving nickel islands linked to the pore frames. These islands lead to more surface area of contact between the nickel, fuel, and electrolyte - enhancement of the triple-phase boundary. The sample with the greater triple-phase boundary surface exhibits better performance and higher output power.
Dr. RE is a senior research scientist at the Center for Advanced Materials and the Physics Department at the University of Houston. His research is focused on advanced oxide materials and also involved in materials science in the energy arena where he has contributed to work on thin film solid oxide fuel cells and to safely store the hydrogen needed for fuel cells to operate. Mr. MY is a promising research assistant at the Kazakhstan Institute for Physics and Technology and also at the Center for Advanced Materials; during his Master work, he was focusing on the development of thin film solid oxide fuel cells. Dr. ArI is the associate director of the Kazakhstan Institute for Physics and Technology and has been involved in the field of materials science for the past 10 years with focus on silicon semiconductor technology. Prof. ST is the director of the Kazakhstan Institute for Physics and Technology and is an innovator in new energy materials stemming from the application of microelectronics technologies. Besides his work in fuel cells, he also has significant efforts in novel solar cells. Prof. AxI is the director of the Center for Advanced Materials at the University of Houston where he has research programs in energy materials, computational logic materials, and materials at the physical-biological interface. He has effectively applied thin film technologies to current problems in energy including increased efficiency and reduced cost for electrochemical energy conversion.
The authors wish to acknowledge the partial support for this work from the Institute of Physics and Technology, Almaty, Kazakhstan and the State of Texas through the Center for Advanced Materials, USA.
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