A circular membrane for nano thin film micro solid oxide fuel cells with enhanced mechanical stability

Abstract

Solid oxide fuel cells (SOFCs) utilizing nanoscale thin film technology has effectively decreased their operating temperatures from typically required to be in the 800°C to 1000°C range to below 500°C. By the help of silicon micromachining technology, such nanoscale thin film electrolytes were fabricated as a free-standing membrane. However, in order to minimize the internal resistance of the cell, the membrane is usually made to be only a few tens of nanometers in thickness, and subsequently its mechanical stability is poor. The lateral dimension of a fuel cell determines the total power output of this device, and for nano thin film electrolyte SOFCs, enlarging the electrolyte to obtain higher power output without cracking the membrane becomes virtually impossible. In this work, we demonstrated a fabrication method for a new nano thin film SOFC architecture that can provide upward membrane scalability to obtain higher device power, and at the same time enhance membrane mechanical stability for stable fuel cell operation.We demonstrate a new architecture for a low temperature solid oxide fuel cell to enlarge the lateral dimension of the fragile nano thin film electrolyte from micrometer to millimeter scale with greatly enhanced mechanical stability. The new structure was achieved by simple silicon micromachining processes to change the membrane shape from a square to a circle to reduce buckling-induced stress concentration that often caused membrane fracture. A tapered silicon membrane support with the thickest end to be 30 µm was introduced as an effective membrane stress absorber. The new architecture effectively suppressed membrane buckling and decreased the maximum principal stress by 30-40%. The largest lateral dimension of the stable membranes was 3 mm in diameter, and the survival rate was significantly improved over square membranes having the same lateral dimension. Fuel cells with 100 nm-thick electrolytes showed stable open circuit voltages of 1.12 V at 400 º C for more than 8 hours without any membrane failure observed, showing the superior mechanical stability of the new cell architecture that are promising in further the practical application of such devices.

Numerical Study on Electrochemical Performance of Low-Temperature Micro-Solid Oxide Fuel Cells with Submicron Platinum Electrodes

“…We adopted an MEA structure that was composed of Pt electrodes and yttria-stabilized zirconia (YSZ) electrolyte with thickness of 100 nm as a reference case. Based on the reference case, the effect of operating temperature and electrolyte thickness on electrochemical performance was investigated and compared with experimental data of literature [13]. Following assumptions were made in this study.…”

“…All of the boundary and operating conditions for the present study were determined from the experimental condition of the literature [13], as shown in Table 3. At the interface of ambient air/cathode, a mass fraction (O 2 :N 2 :H 2 O = 0.228:0.749:0.023) and electric potential (0.2~1.0 V) were applied as boundary conditions.…”

“…The model was calibrated with the experimental data in [13] to obtain the reference exchange current density, and the resulting parameters for calculating the reference exchange current density are summarized in Table 4. We confirm that estimated reference exchange current density by using this set of parameters was within the range of the literature [18,25], as shown in Table 5.…”

Role of carbonate phase in ceria-carbonate composite for low temperature solid oxide fuel cells: A review

“…As we have known, suitable temperature is required to bond electrode paste to electrolyte layer while to maintain enough porosity/composition integration for fast gas diffusion, facilitated electrode reaction and easy current collection, and to prevent from the undesirable reaction. The cathode fabrication temperatures (900–1100 °C) are much higher than those used in the ceria–carbonate‐based fuel cell device, except with some dedicated cell fabrication technologies, such as impregnation or vacuum cell component fabrication method . Therefore, it is believed that carbonate plays as the ‘glue’ to conglutinate the electrode and electrolyte, which ensures a low annealing temperature, while gives a well‐bonded interface for faster charge transportation (Figure b and c).…”

Micro solid oxide fuel cells: a new generation of micro-power sources for portable applications

“…Moreover, most of the self-supported membranes on silicon are square shaped due to the anisotropic wet etching of the substrate 7 . However, it has been demonstrated that circular shape provides enhanced mechanical stability to the membrane avoiding stresses created on the corners 14 . Thanks to the doped-silicon slabs the shape of the membrane is defined by the lithography step rather than the wet etching and circular membranes can be achieved.…”