Title: Scalable and Cost-Effective Barrier Layer Coating to Improve Stability and Performance of SOFC Cathode

During this project, a dense GDC barrier layer was deposited on YSZ substrate to prevent chemical reaction between LSCF and YSZ by a cost-effective EPD method. PPy was used as the agent to make YSZ conductive. Highly compact GDC green layer was obtained by the EPD process in an ethanol-based suspension. The deposition rate on PPy coated YSZ is slower than that on graphite because the charge transfer process is the rate-determining step. The main resistance is from the ion accumulation/depletion zone. The H⁺ ions reduced at the cathode consist of free H⁺ ions and absorbed H⁺ ions corresponding to the unavoidable electrochemical reaction and deposition, respectively. A factor f was introduced to reflects EPD efficiency and describe the competitive relationship between deposition and electrochemical reaction during EPD process. f is in the range from 0 to 0.5 which is determined by the amount ration between free H⁺ ions and absorbed H⁺ ions at the suspension/deposit interface and the conductivity of the cathode. f decreases with the increase of current density or applied voltage and the conductivity of the cathode. GDC thin layers formed by DC-EPD in a thickness range of 5-8 µm were successfully densified at as low as 1300 °C and the adhesion between GDC and YSZ was good. The densification temperature can be further reduced to 1250 °C when AC-EPD was used to replace DC-EPD. The deposition rate increases with the increase of duty cycle and voltage ratio. The deposition rate increases with the increase of frequency from 10 Hz to 500 Hz, while decrease when the frequency increases from 500 Hz to 5000 Hz. Desorption of absorbed charge carrier is easy and irreversible. The deposition rate is independent on the conductivity of the cathode when the electrochemical reaction can be ignored with a high frequency. Inter-diffusion of cation ions between the electrolyte and barrier layer has been systematically characterized via 3-D atom probe, TEM, and other techniques. No measurable Sr transport occurs across the GDC layer with a thickness of 1.5 – 6 μm at a LSCF processing temperature of 1000 °C. However, Sr transport does occur across GDC layer with a thickness of 1.5 or 3 μm at a higher LSCF sintering temperature (1000 °C or 1200 °C). The primary pathway are open pores or grain boundaries in the GDC. The observed zirconate layer is discontinuous even for the most extreme conditions (1.5 μm GDC, 1200 °C LSCF sintering temperature). Increasing the thickness of GDC to 4.5 μm can effectively suppress any zirconate formation measurable by SEM-EDX. Cobalt is observed at grain boundaries within the GDC, suggesting some depletion of Co from the LSCF layer that may ultimately impact electrode performance.