Micro-SOFCs for portable power generation Paul D. Ronney Department of Aerospace and Mechanical Engineering University of Southern California, Los Angeles, CA USA Presented at the Institute for Nuclear Energy Research, Jhong-Li, Taiwan October 4, 2005

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Paul Ronney  B.S. Mechanical Engineering, UC Berkeley  M.S. Aeronautics, Caltech  Ph.D. in Aeronautics & Astronautics, MIT  Postdocs: NASA Glenn, Cleveland; US Naval Research Lab, Washington DC  Assistant Professor, Princeton University  Associate/Full Professor, USC  Research interests  Microscale combustion and power generation (10/4, INER; 10/5 NCKU)  Microgravity combustion and fluid mechanics (10/4, NCU)  Turbulent combustion (10/7, NTHU)  Internal combustion engines  Ignition, flammability, extinction limits of flames (10/3, NCU)  Flame spread over solid fuel beds  Biophysics and biofilms (10/6, NCKU)

Paul Ronney

Swiss roll  Energy storage density of hydrocarbon fuels (e.g. propane, 46.4 MJ/kg) >> batteries (≈ 0.5 MJ/kg for Li-ion)  Mesoscale or microscale fuel  electrical power conversion device would provide much higher energy/weight than batteries for low power applications, even with very low efficiency  Problems at micro-scales  Heat losses to walls - quenching, efficiency loss  Friction losses in devices with moving parts  Precision manufacturing and assembly difficult Micro-scale power generation - Why?

1/2 O 2 + 2e -  O = Conventional dual chamber SOFC fueloxidant CH 4 + 4O =  CO 2 + 2H 2 O +8e - seals Why solid oxide fuel cells ?  Advantages  Uses hydrocarbons (Propane: 12.9 kWh/kg (other HCs similar); methanol 2.3x lower; formic acid 8.4x lower )  No CO poisoning  High power (≈ 400 mW/cm 2 vs ≈ 100 mW/cm 2 for DMFCs)  Disadvantages  Not thought to be suitable for micropower generation because of high temperature needed (thermal management difficult)  Sealing / thermal cycling problems  Coking  Need to pump & meter 2 separate streams (fuel & air)

1D counterflow heat exchanger and reactor Linear device rolled up into 2D “Swiss roll” reactor (Weinberg, 1970’s) Reaction zone Reactants Products Products Reactants Solution to thermal management  Transfer heat from exhaust to incoming gases in “Swiss roll” to minimize heat losses and quenching  React in center of spiral counter-current “Swiss roll” heat exchanger  Operates effectively over wide range of Re and equivalence ratio   Reduces heat losses, sustain high core temperatures with low surface & exhaust temperatures, even at small scales

Solution to thermal cycling & coking  Single chamber solid oxide fuel cell - Hibino et al. Science (2000)  Fuel & oxidant mixed - no sealing issues, no coking problems  “Reforming” done directly on anode  Highly selective anode & cathode catalysts essential since fuel & oxidant exposed to both anode & cathode CH O 2  CO + 2H 2 H 2 + O =  H 2 O + 2e - CO + O =  CO 2 + 2e -.5 O 2 + 2e -  O = anodecathode O=O= C x H y + O 2 O2O2 H 2 O + CO 2 e-e- e-e- electrolyte

Objectives  Assess the feasibility of using a single chamber solid oxide fuel cell in a Swiss roll heat exchanger for power generation at small scales  Test using scaled-up devices operated at low to moderate Re

Swiss roll designs  Baseline: titanium (low thermal expansion & conductivity), EDM- cut & welded  Also: DuPont Vespel SP-1 polyimide (25x lower thermal conductivity), CNC milling (world’s first all polymer combustor?) 5.5 cm

Single-Chamber Fuel Cell development ComponentMaterial Electrolyte Sm-CeO 2 [SDC] Anode SDC-NiO [SDC-Ni] Cathode Many types  Both anode-supported (Caltech) & cathode supported (LBL) fuel cells examined; anode-supported somewhat better, probably due to increased area for reforming Spray cathode NiO + SDC SDC Dual dry press Sinter, 1350 o C 5h 600 o C 5h, 15%H 2 Porous anode Calcine, 950 o C 5h, inert gas cathode electrolyte anode Anode supported

Self-sustaining SOFCs in Swiss-roll reactors 7 cm 1.3 cm 0.71 cm 2

Experimental approach  SOFC  Anode: Nickel + Samarium-doped ceria (SDC)  Electrolyte: SDC  Cathode: transition metal perovskites (several types developed at Caltech) 1.3 cm 0.71 cm 2 5 cm Catalyst region  3 turn 2-D rectangular ceramic (k = 2.7 W/m˚C) Swiss roll  3 mm channel width, 0.6 mm wall thickness, 3 cm tall  Bare metal Pt catalyst in central region

Implementation of experiments Mass Flow Controllers Air PC with LabView Fuel Flashback arrestor NI-DAQ board Thermocouples Incoming reactants PC with LabView NI-DAQ board Fuelcell V A  V A  Keithley 2420 sourcemeter

Operation limits in Swiss roll Operation limits in Swiss roll  Determine parameters providing optimal operating conditions (T, mixture, residence time) for SCFC  NH 3 -conditioned catalyst very beneficial at very low Re  Lean limit can be richer than stoichiometric (!) (catalytic only)  Near stoichiometric, higher Re: reaction zone not centered SCFC target conditions fuel lean fuel rich propane-air mixtures Re = VD/ l l V = Velocity l l D = Channel width = kinematic viscosity Calculated at burner inlet

Extinction limit temperatures  Minimum operating temperatures reduced dramatically with Pt catalyst compared to gas-phase combustion - compatible with SCFCs, polymer combustors

 Best performance mW/cm 2 (propane fuel) - higher than PEM fuel cells using methanol or formic acid  Performance similar to stand-alone fuel cell in furnace SCFC in Swiss roll - performance 370 mW/cm 2

Effect of cell temperature and O 2 :fuel ratio  Performance not to sensitive to temperature - range of T within 20% of max. power ≈ ±50˚C  Performance sensitive to O 2 :fuel ratio - best results at lower O 2 :fuel ratio (more fuel-rich)

 Butane: slightly higher power density, but more excess fuel required to obtain higher power SCFC in Swiss roll - butane

 Best power: ≈ 570˚C, Fuel:O 2 ≈ 2 (3.5x stoichiometric!)  Need supplemental air after partial reaction for improved fuel utilization

Effect of cell orientation T = 480˚C, C 3 H 8 : O 2 = 1 : 2, and Re = 65  Better performance with cathode side facing the inner (hotter) wall  Cathode function:  Electrochemically react O 2 with e - to make O = ions (faster at higher temps)  Anode function:  Prefer lower temps to obtain partial but not complete oxidation of fuel

SCFC in Swiss roll - effects of temperature  Effect of temperature similar in propane & butane  Fuel cell temperature ≈ 100˚C higher than gas (small T rise compared to complete oxidation, ≈ 1500˚C)

SCFC Operation on Methane  Ni + SDC | SDC (20  m) | SDC + Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 (BSCF)  Haile et al., Nature, Sept. 9, 2004  Monotonic increase in power output with temperature  Higher power outputs than with propane (less fuel decomposition at cathode, higher “Octane number”) 730 mW/cm 2

Higher (liquid) hydrocarbons  Iso-octane (2, 2, 4 trimethylpentane) used as a surrogate for various hydrocarbon fuels including gasoline, diesel & JP-8  “1.5 chamber” fuel cell  Cathode: Ni-SDC, reactant air  Anode: LSCF-GDC, reactant fuel- rich (7% iso-octane in air) mixture  Electrolyte SDC  Enabling technology: “special catalyst layer” on anode (Barnett et al., Nature 2005)

Iso-octane / air SOFC  Power density ≈ 550 mW/cm 2 at 600˚C  Power density ≈ 250 mW/cm at 450˚C (temperature limit for polymer Swiss rolls)  Iso-octane power comparable to hydrogen  Cell stable over 60 hr test, no coking observed  Needs to be tested in single-chamber cells  Results should transfer well to other hydrocarbons…

Iso-octane / air SOFC  Catalyst layer greatly increases longevity

Automotive gasoline / air SOFC  Catalyst/Ni-YSZ/YSZ/LSCF-GDC cell  Power density ≈ 900 mW/cm 2 at 800˚C  No coking except at T < 650˚C  SEM-EDX measurements showed sulfur on the catalyst layer is responsible for degradation over time

Conclusions  (Probably) world’s smallest thermally self-sustaining solid oxide fuel cell  Maximum power density ≈ 420 mW/cm 2 at T ≈ 550 ˚C  Superior performance was obtained when the cathode side facing the hotter inner wall  Fuel cell performance is dependent on both temperature and mixture composition, but > 50% of peak performance is obtained over  T ≈ 200 ˚C (≈ 400 ˚C to 600 ˚C) and  ≈ 2 (  ≈ 1.5 to 3.5)

Future work  Potential complete micropower system  Polymer 3D Swiss roll  Hydrocarbon fuel  Single-chamber solid oxide fuel cell for power generation - direct utilization of hydrocarbons  Thermal transpiration pumping of fuel/air mixture - no moving parts, uses thermal energy, not electrical energy

Polymer combustors  Experimental & theoretical studies show importance of wall thermal conductivity on combustor performance (counterintuitive: lower is better)  Polymer Swiss rolls???  Low k ( W/m˚C)  Polyimides, polyetheretherketones, etc., rated to T > 400˚C, even in oxidizing atmosphere, suggesting SCFC operation possible  Inexpensive, durable, many fabrication options  Key issues  Survivability  Control of temperature, mixture & residence time for SCFC

Results - extinction limits  Sustained combustion as low as 2.9 W thermal (candle ≈ 50 W)  Extinction limit behavior similar to macroscale at Re > 20  Improved “lean” limit performance compared to inconel macroscale burner at 2.5 < Re < 20  Good performance under target conditions for SCFC  Sudden, as yet unexplained cutoff at Re ≈ 2.5 in polymer burner

Results - temperatures  Prolonged exposure at > 400˚C (high enough for single chamber SOFCs) with no apparent damage  Sustained combustion at T max = 72˚C (lowest T ever self-sustaining hydrocarbon combustion?)  If combustion can be sustained at 72˚C, with further improved thermal management could room temp. ignition be possible?

Thanks to…  Institute of Nuclear Energy Research  Prof. Shenqyang Shy  Combustion Institute (Bernard Lewis Lectureship)  DARPA, USAF (funding for this research)