Fan the Flame with Water: Operation of PEM Fuel Cells with Dry Feeds

Transcription

1 H O O O + H O Fan the Flame with Water: Operation of PEM Fuel Cells with Dry Feeds Jay Benziger Department of Chemical Engineering Princeton University

2 H Basic O O O + H O Operation of Fuel Cells Fuel and oxidant are separated. Ions conducted through electrolyte, electrons carried through external circuit Fuel cells classified by electrolyte Anode : Cathode : O Overall : H 4H H O H 4e e H H O O

3 H Types O O O + H O of Fuel Cells Alkaline Fuel Cells hydroxide ions transported through 85% KOH solution Phosphoric Acid Fuel Cells protons transported through 100% H3PO4 Solid Oxide Fuel Cells oxygen anions transported through stabilized zirconia Molten Carbonate Fuel Cells carbonate ions conducted through a molten salt Fuel Cell protons conducted through an sulfonic acid derivatized polymer

4 Key Elements to All H O O O + H O Fuel Cells Good ion conduction in electrolyte Efficient electrocatalysts at the and High 3 phase interfacial contact (gas diffusion electrode)

5 Polymer Elec trolyte H O O O + H O Membrane Fuel Cell hydrogen in e- Sulfonic acid ionomer serves as proton conductor Anode (Pt supported on Carbon) Cathode (Pt supported on Carbon) oxygen in anod e H catho de O O O + H O

6 H O O O + H O s in Fuel Cells (e.g. Nafion) F F F F ( C C ) ( C C ) a b O CF Teflon backbone forms continuous phase giving structural integrity, a/b~6 FC O F C F C SO 3- CF 3 Ionic groups permit water and proton transport Water sorbs into the hydrophilic regimes swelling the membrane creating proton conduction pathways 6

7 H Why O O O + H O PEM Fuel Cells Operate at near ambient conditions Easy startup and shutdown Tolerant of multiple startup and shutdown Can start up at room temperature High Energy output on both volume and weight basis

8 H Key O O O + H O Elements to Fuel Cells Chemical Factors Good ion conduction in electrolyte Efficient electrocatalysts at the and High 3 phase interfacial contact (gas diffusion electrode) Engineering Factors Efficient Mass Transport to three phase interface Distribution of reactants/ removal of products Operation over broad range of operating conditions (temperature, reaction rate, etc.)

9 H What O O O + H O will we cover? The Fuel Cell as a Chemical Reactor Part I. Fan the Flame with Water. Ignition/Extinction and Front Propagation Part II. Drop, Slugs and Flooding in Gas Flow Channels Part III. Operation of PEM fuel cells with dry feeds Part IV. Simplifying Fuel Cell Control for following

10 H Fuel Cell as Simplified Reactor O O O + H O O feed and water H Feed and water Unreacted H and water Unreacted O and water Equivalent electrical circuit I R L i R M V R L V Membrane water content affects R M

11 Specific Conductivity (S/cm) H O O O + H O Part I. Fan the Flame with How much water is enough? Autohumidify use dry feeds eliminate the need for humidification system Dynamic vs. steady state operation How do you start up a fuel cell with dry feeds? How does the fuel cell respond to changes in operating parameters? Water 1.E+00 1.E-01 1.E-0 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 1.E C 100 C 10 C 140 C Empirical Fit Water Activity Too little water high membrane resistance H gas Too much water liquid condensation H gas Polymer Membrane Pt H--> + e- e- Polymer Membrane Pt condensed water Carbon Carbon

12 Delivery of Reactants H O O O + H O Flow Plates Typical serpentine flow plate for a fuel cell test station

13 Examples of Flow Plate H O O O + H O Designs for PEM Fuel Cells

14 H O O O + H O Simplifying the Fuel Cell to Understand how it works Local current density (A/cm ) Serpentine flow channels Anode Gradients are transverse to the membrane Membrane Cathode x No longitudinal gradients J. Van Zee, USC Center for Fuel Cell Research (

15 H Differential O O O + H O Fuel Cell Relative Humidity Sensor Hydrogen in Anode H H e Silicone gasket Cathode O 4 H 4 e H O Graphite Plate Nafion Membrane Oxygen In Relief Valve RL I V Thermocouple Residence time > Diffusion time V/Q L /D Benziger et al. AIChE J. 50 (004)

16 Current (ma/cm ) Current (ma/cm ) H Fuel O O O + H O cell ignition 1000 cw1.5 mg/cm ignition 1000 a 100 cw1.0 mg/cm 100 b c cw0.5 mg/cm d Time (s) Ignition by pre-injection of dry feeds 5sccmH /cm,.5 sccmo /cm extinction Time (s) (a) Base case, (b) increased resistance, (c) increased flow rate, (d) increased T Moxley, Tulyani and Benziger, Chem. Eng. Sci. (003) 58:

17 Current (A) H The O O O + H O Water Match! ml water injected Current took 3 days to stablize Current ignition Time (s) Hogarth and Benziger, J. Electrochem. Soc. (006) 153(11).

18 Current (ma) Current Density (A/cm) Current (ma) H Ignition O O O + H O increase Proton Current i in a PEM fuel cell out A in A out C in C F P,, 0.5 ' A w F P A w in FC Pw FC Pw in V Critical condition for ignition RT Rmembrane R Time (s) Water Production decrease Membrane Resistance R M ignition extinction increase Membrane Hydration a w Water removed Water produced Water Activity (Pw/Po) extinguished state ignited state Water Activity (Pw/Po) RL10 RL4 water removal RL8 RL15 RL0 Benziger et al. AIChE Journal (004) 50: RL6 Load

19 H What O O O + H O happens when we consider ignition with flow channels? Plug Flow Reactor Fuel Cell Gas flow channel Split permits local current measurement along the gas flow channel Benziger et al. J.Electrochem.Soc., (007) 154(8) B835-B844

20 H Co-current O O O + H O Ignition Current ignition front Reduce temperature to 5ºC, and resistance to 0.5 W 8 sccm H, 4 sccm O

21 Time (min) Current (ma) Time (min) Current (ma) H O O O + H O Ignition Fronts in PEM Fuel Cells Ignition in co-current flow Ignition in counter current flow H Feed H Feed Anode Cathode Membrane Anode Cathode Membrane O Feed Benziger et al Journal of Physical Chemistry C, (007) 111: O Feed

22 H O O O + H O Autothermal (Exothermic Reaction) and Autohumidified PEM Fuel Cell Balance of Heat Generation and Heat Removal Temperature ignition and extinction Increasing flow extinguishes flame Flame fronts propagate by heat conduction Balance of Water Generated and Water Removed Current ignition and extinction Increased flow extinguished current Current density fronts propagate by water diffusion

23 H O O O + H O Part II. Drop, Slugs and Flooding in PEM Fuel Cells How does water get from the catalyst membrane interface to the gas flow channel? Capillary Condensation? Hydraulic Pressure? small pores (<0 mm) Large pores (~100 mm) large pores (~100 mm) small pores (<0 mm) Gas Diffusion Layer Gas Diffusion Layer Catalyst Layer Catalyst Layer Membrane Membrane CY Wang, Acct. Chem Res. (004) 104:477. Pasaogullari and Wang, JES (004) 151: A399 Benziger et al. J. Mem. Sci. (005) 61:98

24 H O O O + H O Measuring Flow Through the Gas Diffusion Layer water reservoir Liquid drop Gas Diffusion Media Pgh Camera water collection

25 Flow rate (g/s) e- H O O O + H O Water Flow Through GDL Media Minimum Pressure to Penetrate GDL P min Gw r pore b c Pressure (Pa) a

26 H Carbon O O O + H O Cloth Pore Sizes 100 mm E-tek Carbon Cloth, 60x

27 H O O O + H O Pore Penetration and Water Flow Penetration, Applied Pressure>Surface Tension P cos water r pore P<P min no water penetration into pore Detachment, Surface Tension<Gravity 4 3 wrpore rdrop water g 3 r Liquid Flow Qpore 8m 4 pore water P L pore

28 H O O O + H O Combined flow visualization and local current measurement

29 H O O O + H O Cathode Down Drops detached by gravity Role of Gravity Cathode Up Drops detached by shear flow 0 % Stoichiometric Excess 100 % Stoichiometric Excess 0 % 100 % Stoichiometric Excess Stoichiometric Excess Benziger et al, J.Electrochem. Soc. (007) 154:B835-B844.

30 H O O O + H O gravity Combining visual observations at with current measurements: Vertical Orientation Cathode inlet down Oxygen flow Kimball et al., AIChE Journal (008) 54:

31 H O O O + H O (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) 1

32 H Conclusions O O O + H O on Flooding Liquid water is pushed through the largest pores of the GDL forming drops. The drops detach by gravity or shear forces and coalesce forming slugs that are pushed along the channel. The slugs deprived certain areas of the flow channel of reactants and result in local current fluctuations. Stable operation occurs with excess flow and gravity assisting the draining of the liquid.

33 H O O O + H O Part III. Operation of PEM Fuel Cells with Dry Feeds Why are we interested in dry feeds? Advantages Simplicity of operation Reduction of balance of plant (no humidifiers) Problems Low current densities Limited to low temperature operation?? Start-up

34 H O O O + H O Is autohumidification limited to low temperature operation? This leads to the important conclusion that extra humidification of the reactant gases is essential in PEM fuel cells operating above about 60 C. This has been confirmed by the general experience of PEM fuel cell users. {Larminie and Dicks, Fuel Cell Systems Explained (003)}.

35 Gravity H Exploiting O O O + H O Self Draining PEM Fuel Cell gravity Hogarth and Benziger, The channel-less self-draining PEM fuel cell, provisional patent, 005

36 H Self-draining O O O + H O PEM Fuel Cell gravity Liquid drops in the gas plenum fall by gravity to the outlet reduces flooding Liquid drops coalesce at the corners and block gas flow - flooding

37 Voltage (V) H O O O + H O Steady-state Autohumidified Operation Fully humidified feed Current Control Voltage Control Ext. Load control Constant Water I-V Extinguished Steady-state Ignited steady-state Critical current required to sustain ignited steady-state Current (A) Hogarth and Benziger, J. Electrochem. Soc. (006) 153(11), A139-A146

38 H O O O + H O Autohumidified Channel-less vs. Humidified Serpentine Steady state iv curves, H /O operation with stoichiometry 1.5, and bar total pressue Serpentine flow gave ZERO current with dry feeds at 80ºC Hogarth and Benziger, Journal of Power Sources 159 (006)

39 H O O O + H O Why conventional fuel cell design is improper for autohumidification Parallel Flow Re-enterant Without humidification flow of the gas streams a parallel flow system will always kill off the fuel cell Serpentine Flow water content in membrane Interleaved Flow increasing time distance from feed Countercurrent Flow

40 H O O O + H O Dry feeds with Parallel Flow Channel Q 6 ml/min Lower T less water convection Q3mL/min Ignited section is pushed downstream with increasing flow Trade-off between convection in flow channel and diffusion in the membrane Increased flow and increased temperature extinguish current

41 Requirement for operation H O O O + H O with dry feeds Water removed Water Produced > by convection i F i 4F Q P o out out w A QC PT in QA Q Pw 1 PT in C o Dry operation At 3 bar feasible Dry operation At 3 bar infeasible

42 H O O O + H O Requirements for autohumidification in PEMFC with flow channels Net water convection < Water production + Net water diffusion o o P i P PT 4F PT L w w w, mem 1/ Q A QC Dw, mem Achannel tmembrane c ζ effective mixing length diffusion convection Q A QC RT 1 D w P T A channel

43 Fuel Cell Design H O O O + H O for Dry Feeds Maximize mixing of water in the fuel cell with the entering feed gases, equivalent to maximizing diffusion (which mixes the gases) compared to convection Peclet Number (Pe) convection diffusion vl D Pe Pe STR, Dh J channel RT F P membrane serpentine, Dhchannel F P wchannel A J RT A membrane

44 H Characteristic O O O + H O Operation 0.5 A/cm current density, Stoich, STR 1 cm x 1 cm x 0.1 cm plenum Pe 0.67 System well mixed uniform composition Serpentine 10 cm x 0.1 cm x 0.1 cm channel Pe 67 Plug flow Little axial dispersion Autohumidifed Operation Pe serpentine, Pe STR, v serpentine v STR L L serpentine STR Humidified Feeds Required D D membrane channel Mixing becomes a greater problem with size for serpentine flow channel fuel cells, but mixing is nearly independent of size for STR type fuel cells A w

45 H Advantages O O O + H O of an STR PEM Fuel Cell Back-mixing of reactants greatly improves uniformity of water concentration critical for autohumidified operation Flow regime scaling is independent of fuel cell size with STR plenum design Orientation of plenum to permit selfdraining of liquid by gravity reduces mass transport limitations at high currents

46 H Honda O O O + H O has it half right! Honda Demonstrates the FCX Concept Vehicle - Fully functional Next-Generation Fuel Cell makes its driving debut - To meet Honda objectives for significant gains in both environmental and driving performance, the FCX Concept is equipped with a V Flow1 fuel cell platform consisting of a compact, high-efficiency fuel cell stack arranged in an innovative center-tunnel layout /406095FCXConcept/ While with previous fuel cell stacks the hydrogen and the water formed in electricity generation flowed horizontally, the new FCX Concept features vertical-flow design. This allows gravity to assist in discharging the water that is produced, resulting in a major improvement in water drainage, key to high-efficiency fuel stack performance. The result is stable power generation under a broad range of conditions, and higher output from a smaller package.

47 H Toyota O O O + H O seems to have it right! Toyota has major patent positions on the channelless fuel cell, which shows superior performance with dry feeds. (They have not published in the open literature!)

48 H O O O + H O Part IV. Simplifying the Fuel Cell System for following Operate with dry feeds Eliminate humidification system and its requisite temperature control Develop a system to deal with water management that is insensitive to temperature Eliminate need for un-reacted fuel recovery and recycle

49 H Self-draining O O O + H O Fuel Cell Mass Flow controllers H O /Air Anode Cathode gas inlet Gas Plenum outlet out in in out out V P H Q A P A H Q A P H i RT t RT RT F Water reservoirs i F Q P in A RT in H

50 Voltage (V) Current (amp) H O O O + H O IV (polarization curves) Constant R L variable flow W 0.5 W W W W Hydrogen flow rate (ml/min) W W 1.5 W 1.0 W 0.8 W 0.5 W Current (A)

51 H O O O + H O Variable Area Fuel Cell Hydrogen Feed Anode P Water Reservoir Woo and Benziger, Chem Eng. Sci. (007) 6 pp

52 current (A) Control H O O O + H O by fuel feed regulation H flow stopped for 5s setpoint current changed from 0.3A to 0.4A Load reduced from 1.0 to 0.5 Ohm time (s) 5C 60C 80C Load following - Variable power fixed impedance, 100% fuel utilization, dry feeds, independent of temperature.

53 H Credit O O O + H O to Our Undergraduates

54 H Conclusions O O O + H O PEM Fuel Cells have analogous behavior to the classic chemical reactors They ignite with a critical water content Ignition fronts propagate due to reaction/diffusion coupling They develop wet spots compared to exothermic hot spots Idealized reactors have led to ways to simplify and improve PEM fuel cell design Channel-less self-draining STR design Autohumidified Operation to elevated tempertures Simplified Dynamic Models Simplified Control Systems

55 Are we asking the right H O O O + H O questions? Focus on overcoming critical technical hurdles at the component level to improve overall polymer electrolyte membrane fuel cell performance and durability while lowering costs, including: Proton-conducting membranes that operate at 10ºC (maximum) for transportation applications and greater than 10ºC for stationary applications Membranes that can operate at low relative humidity Cathodes with decreased precious metal ing Non-precious metal catalysts Bipolar plate materials and coatings with improved corrosion resistance U.S. DOE Hydrogen Posture Plan, December 006

56 DOE is continuing to search H O O O + H O for magic bullets Selected 4 new fuel cell projects, including: five projects which address critical fuel cell cost and durability issues for consumer electronics and other applications ($13 million over three years); 1 projects ($19 million over five years) for research on polymer electrolyte-type membranes with improved performance at higher temperatures and lower humidity; and 5 projects ($100 million over four years) for research in a range of fuel cell topic areas including fuel cell membranes, water transport within the stack, advanced catalysts and supports, cell hardware, innovative fuel cell concepts, effects of impurities on fuel cell performance and durability, and stationary fuel cell demonstration projects to help foster international and intergovernmental partnerships16 U.S. DOE Hydrogen Posture Plan, December 006

57 H O O O + H O Here is what VW believes Low temperature fuel cells are operated at a membrane temperature of approx. 80 degrees celsius. If the temperature greatly exceeds this value fuel cell performance breaks down and irreparable damage is done to the fuel cell. This is why vehicle prototypes with LT fuel cells have an extremely sophisticated and expensive cooling system. The cooling surface alone is approximately three times as large as for diesel engines. In addition, in an LT system the supply of hydrogen gas and air must be continuously humidified, because otherwise the production of energy will also break down, permanently damaging the fuel cell. This humidification of the water molecules stored in the membrane also adds unwelcome additional weight, eating up both space and money.

58 H O O O + H O The problem is not the membrane stupid. It s the reactor!

59 H O O O + H O Great science cannot compensate for bad engineering! We seek to solve problems we think are important, which are not necessarily the most important problems.

60 H O O O + H O Acknowledgements Joel Moxley and Sonia Tulyani for demonstrating steady state multiplicity in PEM fuel cells Joanne Chia and Yannick DeDecker for modeling the autohumidification PEM fuel cell and demonstrating the front propagation Warren Hogarth for showing the utility of the STR design Erin Kimball for showing how water droplets cause fluctuations of the local currents Tamara Whitaker for showing the liquid flow through the largest GDL pore Claire Woo for showing us wrong about power regulation of fuel cells Andy Bocarsly and S. Srinivasan for introducing me to PEM fuel cells Yannis Kevrekidis for encouragement in the face of adverse skepticism NSF Chemical and Thermal Sciences for partial support

61 H Princeton O O O + H O University

62 H Not O O O + H O like the rest of New Jersey New Jersey Turnpike The Jersey Shore