Solid Oxide Fuel Cell Based on Proton Conducting Ceramic Electrolyte* U. (Balu) Balachandran, T. H. Lee, L. Chen, B. Ma, and S. E. Dorris Energy Systems Division *Work supported by U.S. Department of Energy under Contract Number DE-AC02-06CH11357

2 cathode Fuel (H 2 ) H+H+ External Load e-e- e-e- proton conductor (electrolyte) anode Oxidant (air) H2OH2O Overall reaction: H 2 +1/2O 2 H 2 O SOFC based on Proton Conducting Oxides

3 SOFC based on Proton-Conducting Oxides Advantages: Lower (intermediate) temperature of operation –Overcomes material issues such as seals, inter-diffusion, interconnects, etc. No dilution of the fuel (water is not formed in the fuel side) Possibility for higher fuel efficiency Prevent anode coking by internal self-regulated methane reforming (chemical diffusion of water across the electrolyte -- protonic defects & oxide ion vacancies coexist as charge carriers)

4 Methane Reforming & Cell Reaction in SOFC Based on Proton-Conducting Oxides Ref: K. D. Kreuer, Annu. Rev. Mater. Res., 33, 333, 2003.

5 Why BaCe 0.8 Y 0.2 O 3 (BCY20) Electrolyte J. Guan, S. E. Dorris, U. Balachandran, and M. Liu Solid State Ionics 100 (1997) 45 4% H 2 BaCe(Y)O 3 : highest total proton conductivity among perovskite type oxides. 20 % Y doped barium cerate showed the highest total conductivity. BCY20 has been developed as a potential hydrogen separation membrane material.

6 BaCe 0.8 Y 0.2 O 3-  (BCY) Film Preparation Colloidal Spray Deposition (CSD)* BCY Colloid BCY powder (Praxair) was dispersed in isopropyl alcohol. Substrate (NiO/BCY composite) Green substrate (1” O.D.) was partially for 12 h. Film Green film was prepared by a colloidal spray deposition. Disk sintering Disks were for 5 h. *A. Pham, T.H. Lee, and R.S. Glass, Proc. 6th Int. Symp. on Solid Oxide Fuel Cells, Electrochem. Soc., PV (1999) 172. *A. Pham, R.S. Glass, and T.H. Lee, U.S. Patent No (2002).

7 NiO/BCY SEM Micrographs of Fracture Surface (Film & Substrate) Uniform film thickness. Film is well-bonded to the substrate.

8 (80% H 2 / balance He) Schematic of Experimental Setup

9 I-V and Power Density of a Cell (≈10-  m thick BCY electrolyte; H 2 /wet air)

10 Temp. ( o C) OCV(Volt, measured) OCV (Volt, theoretical) where, E o is the EMF at standard pressure. Open-circuit voltage of a BCY20-based fuel cell (H 2 /wet air)

11 Power Outputs of Laboratory Fuel Cells using Proton-Conductors Ref: K. D. Kreuer, Annu. Rev. Mater. Res., 33, 333, Compilation of laboratory fuel cells operating with proton-conducting oxides as electrolytes and their reported maximum power outputs 1.4 W/cm 600°C with 0.7 micron thick BCY electrolyte (N. Ito et al., J. Power Sources, 152, 200, mW/cm 2 at 800°C (present work)

12 Cell Voltage Vs. Current Density (≈10-  m thick BCY electrolyte; 800°C) Voltage drop across the electrolyte (IR drop) was calculated using the resistance of the electrolyte measured at open- circuit condition by impedance analyzer. Curvature of the I-V data indicates concentration polarization (diffusion overpotential) at high current density The voltage drop at 800°C is mainly due to electrode polarization 800°C

13 Cell Voltage Vs. Current Density (≈10-  m thick BCY electrolyte; 500°C) At 500°C, polarization loss is still dominant, but the contribution of IR loss increases (because of decrease of ionic conductivity). At low current densities, the drop is from the activation polarization as indicated by the curvature of the data. Concentration polarization dominates at high current densities.

14 Measured Resistance of Electrodes and Electrolyte (≈10-  m thick BCY electrolyte) Impedance measured under open circuit condition. Electrolyte & electrode resistances separated.

15 Power Densities of Cells with 10 & 13-  m thick Electrolyte The cell with thinner electrolyte gave higher current density & peak power approximately proportional to inverse of the electrolyte thickness.

16 SOFC based on Proton-Conducting Oxides Disadvantages of using cerates as electrolyte: Stability problem with respect to the decomposition into binary oxides Reacts with CO 2 and forms carbonates Form alkaline earth hydroxides at high water activities Solution: Doped Zirconates have higher grain conductivity and stability

17 Conductivities of Proton- & Oxide-ion Conductors Ref: K. D. Kreuer, Annu. Rev. Mater. Res., 33, 333, Bulk (grain) proton conductivity of Y:BaZrO 3 compared with the oxide ion conductivity of the best oxide ion ceramics

18 Summary Successfully prepared dense and crack-free thin films of BCY on NiO/BCY substrates using CSD method. Performance of the fuel cell was evaluated with hydrogen-air at °C. Peak power densities of ≈90 and ≈1500 mW/cm 2 were measured at 450 and 800°C, respectively. The main voltage drop in the fuel cell is due to electrode polarization (contribution from electrolyte resistance becomes significant as temperature decreases). Efforts should be focused on the electrode developments to decrease the electrode polarization. Fuel cell power density can be increased by decreasing the electrolyte thickness.

19 DOE year 2010 target for PEM electrolytes  >0.1 temperature  >0.07 temperature Conductivity (S/cm) 1000/T (1/K) Temperature (°C) Nafion-117  ≈0.08 S/cm at 80°C  <0.01 S/cm at 120°C Solid Acids (e.g. CsHSO 4 )  ≤10 -5 S/cm at 80°C  ≈0.008 S/cm above 140°C Sn 1-x In x P 2 O 7  >0.2 S/cm at 200°C in air (x=0.1) Anhydrous Proton-Conducting Ceramic Electrolyte Ref: M. Nagao, et al., Electrochem. Solid-State Lett., 9 (2006) J1

20