Graphene-based Air Electrodes for Solid Oxide Electrochemical Cells April 18, 2014 Prof. Min Hwan Lee School of Engineering
Graphene for electrochemical devices Properties Applications Electron conducting High surface area Catalytic Batteries Supercapacitors Fuel Cells Sensors
What is a fuel cell? Electrochemical Energy Conversion Device Fuel : hydrogen, alcohols, hydrocarbons, etc. Chemical Energy Fuel Cell Electrical Energy Efficient and quiet
Fuel cell operation e- e- H 2 O 2- O 2 H 2 2H + + 2e - (HOR) ½ O 2 + 2e - O 2- (ORR) Overall: H 2 + ½ O 2 H 2 O + electricity
Driving force? Ans. The chemical potential difference between th e reactants (H 2 + O 2 ) and the product (H 2 O) Chemical Potential H 2 + O 2 H 2 O Catalyst
Cell losses H 2 O 2- O 2 (HOR) (ORR) Activation Loss for HOR Ohmic Loss Activation Loss for ORR
Solid Oxide Fuel Cell (SOFC) High Operating Temperature > 800 C Advantages à Fuel Flexibility à Simpler System (No humidity control, etc.) Disadvantages à Material/Part selection à Durability à Limited applicability Lower Operating Temperature! (< 400 C)
Reduction in T causes significant Losses! H 2 O 2- O 2 (HOR) (ORR) Activation Loss for HOR Ohmic Loss Activation Loss for ORR Both ohmic and activation loss exp(e a /kt)
To counteract the significant ohmic loss Thinning the electrolyte < 100 nm H. Huang et al. J. Electrochem. Soc., 154, B20, 2007 Y. B. Kim et al. Electrochem. Comm., 13, 403, 2011
To counteract the significant electrode loss Need a totally new material system because Conventional perovskite-based electrodes à Not active at low temperatures Pt-based electrode à Expensive à Fast degradation
Objective New air electrode (cathode) materials for LT-SOFCs Doped Graphene?
Why graphene as the cathode? Graphene (and its derivatives) Extraordinary thermal and electrical conductivities High specific surface area (theoretically 2630 m 2 /g for single-layer) Strong mechanical strength and flexibility Excellent catalytic activity (Doped Graphene) Qu et al. ACS Nano, 4, 1321, 2010
N-doped Graphene as an ORR catalyst in Fuel Cell Catalytic Activity Durability over cycle (200k cy.) Catalytically superior to Pt Highly durable Resistant to CO and methanol poisoning Qu et al. ACS Nano, 4, 1321, 2010
How N-doping catalyze ORR? N-doping Active site: C with high spin density or high charge density L. Zhang and Z. Xia, J Phys. Chem., 115, 11170, 2011
Solution & symmetric cell preparation procedure GO (Graphene Oxide) Flake graphite powder Expansion of graphite sheets Graphene oxide solution Oxidization using KMnO4 Filtration with PTFE filter Centrifuging to remove unexfoliated particles Sonication for better dispersion NrGO (Nitrogen Doped Reduced Graphene Oxide) GO solution + dicyandiamide(dcda) + D.I. water Teflon-lined auto-clave Hydrothermal rxn. @ 140, 180 C sonication in the suspension (90% water+10% MeOH)
Graphene-based electrode deposition rgo YSZ preparation Tape Masking Plasma cleaning Dropping electrode solution NrGO140 Drying and re-dropping Removing the mask A symmetric cell NrGO180 (a) (b) 500 nm 500 nm rgo NrGO180
Electrochemical Impedance Meas. Air R L R H 30 mhz Au mesh Electrode YSZ Electrode Au mesh current collection U%% / I - Im(Z ) [x10 3 Ω] 150 100 50 CPE L CPE H 4000 2000 0.7 MHz E xperimental Fitted 10 khz Air 0 0 0 2000 4000 0 100 200 Re(Z) [x10 3 Ω]
Oxygen Reaction Performance rgo NrGO140 NrGO180 Au Pt Y. Jee et al. submitted
Thermal Stability over Time ASR ct / ASR ct-initial R L / R L- Init. 9 6 3 Pt NrGO 0 0 3 6 T ime [h] Y. Jee et al. submitted
Thermal stability of Pt: accelerated agglomeration Agglomeration of Pt At 500 C for 5 hrs Ostwald ripening à Loss of active sites for ORR
Thermal stability: morphological (c) (d) 350 C 500nm as-deposited NrGO180 500nm annealed NrGO180 (e) as-deposited (f) annealed NrGO 350 C
Thermal stability: defects & stoichiometry XPS Raman 20 Oxygen Nitrogen 1.5 1.4 rg O NrGO180 Atomic ratio [%] 10 intens ity ratio (G /D ) 1.3 1.2 1.1 1.0 As-dep. 250 C 350 C 0.9 225 250 275 300 325 350 375 400 Annealing temperature [ ÆC] Y. Jee et al. submitted
Thermal stability: d-spacing as-dep. 250 C 300 C 350 C 400 C 20 25 30 35 2? [ ] (Co) NrGO180 (As - 350 C) : ~3.4Å Y. Jee et al. submitted
Cell Integration O 2 Nitrogen-doped graphene: ~ micron N-rGO Gadolinia-doped Ceria (GDC): 200 nm Yttria Stabilized Zirconia (YSZ): 80 nm Platinum (Pt): 300 nm Anodic Aluminum Oxide (AAO): 80 nm pore YSZ+GDC Pt AAO H 2 S. Ji et al. In preparation
Open Circuit Voltage OCV [V] 1.2 1 0.8 0.6 0.4 0.2 0 1 2 3 4 5 6 7 8 9 Cell # S. Ji et al. In preparation
Full Cell Performance 3 2 1 4 5 6 9 8 7 # 1 2 3 4 5 6 7 8 9 Peak Power Density (mw/cm 2 ) 0.1 0.25-0.08 0.08 0.02 0.04 0.13 0.09 Ohmic Resistance (Ωcm 2 ) 319 328-275 289 199-174 313 Most loss occurs at the electrolyte-nrgo interface
Summary Ø LT-SOFC needs new air electrodes (Perovskites: poor catalysis; Pt: expensive and limited) Ø N-doped graphene showed a great oxygen reaction performance and durability at < 400 C Ø Needs much engineering opportunity (interface w/ current collector; cheaper doping process, enhanced reaction sites, etc.) Ø Good potential to be an alternative cathode material for LT-SOFCs
Acknowledgement Dr. Youngseok Jee Andrew H. Moon Alireza Karimaghaloo Dr. Hidetaka Ishihara, Prof. Vincent C. Tung, UC Merced Sanghoon Ji, Prof. Suk-Won Cha, Seoul National Univ. Dr. Jin-Woo Han @ NASA Ames Dr. Jihwan An and Prof. Fritz Prinz @ Stanford
Thank you. Min Hwan Lee mlee49@ucmerced.edu