Search results

The aim of this paper is to find the electrical representation of a solid oxide fuel cell (SOFC) that enables the application of typical exploitation characteristics of fuel cells for estimation of fuel cell parameters (for example, exchange current) and easy analysis of phenomena occurred during the fuel cell operation.

Three-layer structure of an SOFC, where a thin semi-conducting layer of electrolyte separates the anode from the cathode, shows a strong similarity to typical semiconductor devices built on the basis of P-N junctions, like diodes or transistors. Current–voltage (I-V) characteristics of a fuel cell can be described by the same mathematical functions as I-V plots of semiconductor devices. On the basis of this similarity and analysis of impedance spectra of a real fuel cell, two electrical representations of the SOFC have been created.

The simplified electrical representation of SOFC consists of a voltage source connected in series with a diode, which symbolizes a voltage drop on a cell cathode, and two resistors. This model is based on the similarity of Butler-Volmer to Shockley equation. The advanced representation comprises a voltage source connected in series with a bipolar transistor in close to saturation mode and two resistors. The base-emitter junction of the transistor represents voltage drop on the cell cathode, and the base-collector junction represents voltage drop on the cell anode. This model is based on the similarity of Butler-Volmer equation to Ebers-Moll equation.

The proposed approach based on the Shockley and Ebers-Moll formulas enables the more accurate estimation of the ion exchange current and other fuel cell parameters than the approach based on the Butler-Volmer and Tafel formulas. The usability of semiconductor models for analysis of SOFC operation was proved. The models were successively applied in a new design of a planar ceramic fuel cell, which features by reduced thermal capacity, short start-up time and limited number of metal components and which has become the basis for the SOFC stack design.

The purpose of this paper is to present the embossing as a replication technique for structuring fuel channels in the anode-supported solid oxide fuel cells (SOFCs).

The anode laminate, which was exposed to embossing, consisted of eight layers of isostatic pressed Ni-yttria-stabilized zirconia tapes (Ni-YSZ). Preheating the mould and soaking the laminate in the plasticizer was applied before structuring of the channels to soften the laminate. Compressing tests were carried out to investigate the embossing ability of the Ni-YSZ laminate and to find the optimal processing parameters. Embossing was carried out with a testing machine, and the samples were exposed to embossing forces in the range up 20 kN with a certain speed of compression and a dwell time.

The embossing parameters (force of compression, speed of compression and dwell time) suitable for proper fuel channels formation were established. Shapes, profiles and dimensions of the embossed channels were analysed and compared to the micromachined channels. The model of the SOFC with embossed fuel channels was completed, and its current-voltage characteristic was measured.

Experiments proved that embossing can be the alternative method for patterning fuel microchannels in SOFCs. Embossing simplifies the process of fuel channel formation and reduces the time of SOFC fabrication. It is necessary to improve the geometry and quality of the mould formation to make the fuel channels deeper and of better surface finish.

The paper aims to present the innovative design of a planar multilayered high temperature solid oxide fuel cell (SOFC), which is easy to manufacture, and features high resistance to rapid temperature changes. Temperature resistance was accomplished thanks to easy to heat, thin flat ceramic structure of the cell and elimination of metallic interconnections.

The ceramic fuel cell consists of the anode core made of six to eight layers of nickel/yttria-stabilized zirconia tapes (Ni/YSZ) isostatically pressed into a laminate. Two networks of fuel distribution microchannels are engraved on both sides of the anode laminate. The microchannels are subsequently covered with a thin layer of the functional anode tape made of Ni/YSZ and a solid electrolyte tape made of YSZ.

The single planar double-sided ceramic SOFC of dimensions 19 × 60 × 1.2 mm3 provides 3.2 Watts of electric power. The prototype of the battery which consists of four SOFCs provides an output power of > 12 W. Tests show that the stack is resistant to the rapid temperature change. If inserted into a chamber preheated to 800°C, the stack provides the full power within 5 minutes. Multiple cycling does not destroy the stack.

This anode-supported fuel cell structure is provided with thin anode functional layers suspended on a large number of fine beams. The whole anode structure is made with the same ceramic material, so the mechanical stress is minimized during the cell operation.