Fabrication of the Large Area Thin-Film Solid Oxide Fuel Cells with Anodized Aluminum Oxide
Authors: Hong Soonwook , Kim Young-Beom , Park Hun , Han Tae Hee 
Published: February 2020

Solid oxide fuel cells (SOFCs) have been developed toward lowering their operating temperature (800-1000oC) due to the thermal issues such as degradation of catalyst, material selection and sealing integrity. Thin film-solid oxide fuel cells (TF-SOFCs) using nano-scale thin film electrolyte are one of the promising research for lowering operating temperature SOFCs (LT-SOFCs) which is normally operated in 300-500oC. This TF-SOFC suggests the probability for compensation of low ionic conductivity in low temperature region by reducing the ohmic loss.

For the several groups researching the TF-SOFCs, they use only small active area of cathode which not exceeds a few millimeter squares because of the thin morphology of cathode for TF-SOFCs. The thin porous films of cathode are mainly due to the triple phase boundary (TPB) of cathode which is meeting point of gas phase of oxygen, catalyst material of cathode and solid state electrolyte. This porous thin nature of cathode has the high sheet resistance which is normally considered for dominant factor of increasing ohmic resistance for TF-SOFCs. It may mitigate the sheet resistance by depositing more thick thickness of cathode, but increased thickness of cathode is lost active TPBs on the cathode surface because of increased thickness may form the intact thin films which is detrimental to maintain the TPBs on the cathode surface. Thus the cathode of TF-SOFCs is considered to necessity of both porous morphology and as thin thickness of cathode but this thin thickness of porous cathode still deteriorates the cell performance with a high sheet resistance which still dominantly affects the drop of cell performance for enlarged cathode area of TF-SOFCs.

The multi-wall carbon nano-tube (MWCNT) which is considered the promising electronic conductor has the features of current collection for the TF-SOFCs due to their perfection bonding structure with a high strength and ability of electronic conduction. MWCNTs also suggest the permeability of oxygen fuel through the chain-bond structure of CNT, while their current path is preserved in elevating temperature. Therefore, the adopting MWCNT as a current collector on the cathode materials is possible to provide novel method of maintain the TPB density on the cathode with few nano-meters of thickness, as well as furnish the high electrical path-way with reduced sheet resistance of TF-SOFCs.

For fabrication of fuel cells, the anodized aluminum oxides are used as a porous substrate which is able to accommodate the thin film membrane electrode assembly (MEA). The simple symmetry structure of MEA, which platinum as both anode and cathode materials, is composed with electrolyte of yttria-stabilized zirconia (YSZ) deposited with sputtering process. The cathode area of these fuel cells is fabricated with the ~3cm2 active area with a sputtering. The diluted MWCNT with dimethylformamide (DMF) is consecutively coated on the cathode of fuel cells with a spray deposition method. A fuel cell with MWCNT current collector indicates higher open circuit potential of 1.01V and peak power density of 4.4mW/cm2 at 450oC, comparing with a fuel cell with pristine symmetry Pt/YSZ/Pt fuel cells in AAO (as shown in figure 1.). These high OCV and enhanced peak power density is considered that the MWCNT has perfect roll to current collector with decreased sheet resistance and to no blockage of oxygen fuel access to the cathode side as well.

Figure1. (a) Polarization curves and (b) Electrochemical impedance spectroscopy (EIS) analysis for pristine fuel cell of Pt/YSZ/Pt on AAO and MWCNT-Pt/YSZ/Pt

Figure 1