The PEM Fuel Cells

Frick Laboratory, Princeton University Catalyst Layer Pt/C with Proton Conducting Polymer Proton Conducting Membrane H2H2 Pt C H 2 2H + + 2e - Pt C 4H + + 4e - + O 2 2H 2 O O2O2 Anode Cathode Gas Diffusion Layer Proton Exchange Membrane (PEM) Fuel Cell

Frick Laboratory, Princeton University Fuel Cell Thermodynamics

Frick Laboratory, Princeton University Current Density / mA cm Cell Potential / V Thermodynamic Reversible Potential (E) Cell Potential losses due to activation overpotential Linear drop due to resistance of membrane Mass transport losses causes E to go to zero The ideal cell-potential relation Performance Characteristic of a Fuel Cell

Frick Laboratory, Princeton University  = symmetry factor~0.5

Frick Laboratory, Princeton University What are the Challenges? The fuel cell must work on a real world fuels. The first generation hydrogen source will be reformate. Steam Reforming H 2 +CO 2 +CO Simple Steam reforming yields ~1% CO Today’s PEM Fuel Cells are poisoned by ~10ppm CO Thermal management using low grade heat Present cell temperature range: 60-80C Water management

Frick Laboratory, Princeton University Increased CO tolerance Above ~120˚C H 2 sorption on Pt starts to compete with carbon monoxide chemisorption  Improved water and thermal management -More efficient waste heat rejection New Setback: Dehydration of electrolyte i.e. Nafion 115 A Solution: Elevated Temperature PEMFC

Frick Laboratory, Princeton University Preparation of Nafion/Metal Oxide Composites Sol-Gel Processing Si-(OC 2 H 5 ) 4 + H 2 O  SiO 2 /-OH/-OEt Colloidal Solution Recasting Oxide Nano-Particle + Nafion Recasting Suspension

Frick Laboratory, Princeton University Scanning Electron Microscopy Unmodified Membranes Silicon oxide/Composite Membranes (O left, Si right) (O left, Si right) SEM data confirms an even and uniform distribution of Si and O across the cross-section of the membrane

Frick Laboratory, Princeton University Nafion o C 130 o C Silicon Oxide/Aciplex o C Cell Potential / V Current Density / mA cm 2 Nafion Thermal Behavior

Frick Laboratory, Princeton University §The composite typically contains 3-6 wt% metal oxide. §TGA indicates the same water content and dehydration temperature for pure Nafion and the composite. §The conductivity of the composite measured in a mechanically unconstrained environment is the same or slightly worse than the conductivity of pure Nafion.  The metal oxide is not simply providing a water retentive or hydrated interface. Is the Metal Oxide Phase Water Retentive?

Frick Laboratory, Princeton University If it’s not a question of direct dehydration, then what is occurring? First, we will seek a molecular picture. Then, we will attempt to make connections between our understanding of the molecular structure and bulk materials properties.

Frick Laboratory, Princeton University o C (Degussa-Huls) TiO 2 ; 21nm; 50 m 2 /g (R ) SiO 2 ; 20nm; 90 m 2 /g (R ) Al 2 O 3 ; 13 nm; 100 m 2 /g (R ) Recast Nafion Control (R - 0.5) Cell Potential / V Current Density / mA cm -2 Effect of Metal Oxide Identity on Membrane Performance

Frick Laboratory, Princeton University TiO 2 (AA)/Recast Nafion; 130˚C unmodified (R ) silylated (R ) H 2 SO 4, HNO 3, "degreased" (R ) Cell Potential / V Current Density / mA cm -2 Interfacial Chemistry is Critical

Frick Laboratory, Princeton University The Effect of Relative Humidity on Recast Nafion

Frick Laboratory, Princeton University 75% Relative Humidity 125µ Film 3 atm pressure 40 ml/min

Frick Laboratory, Princeton University ----CF 2 -CF OH Metal Oxide -----CF----- O=S=O OH Metal Oxide O Ti Metal Oxide HO -----CF----- SO 3 - Potential Chemical Interactions

Frick Laboratory, Princeton University Thermal decomposition of Nafion SO 2 1st step CFO + 2nd step C3F5+C3F5+ H2OH2O H2OH2O H2OH2OSO 2 C 3 F 5 + CFO + - Temperature Programmed Decomposition (TG- MS) of Nafion 117

Frick Laboratory, Princeton University Thermal decomposition of Nafion SO 2 1st step CFO + 3rd step C3F5+C3F5+ H2OH2O H2OH2O SO 2 C 3 F 5 + CFO + 2nd step - HO TiO 2 TG-MS Profile of Nafion/TiO 2 Composite Membranes

Frick Laboratory, Princeton University TPD-MS profiles of Nafion/Inorganic composite membranes - SO 2 (m/z 64) H 2 O (m/z 18) CFO (m/z 47) C 3 F 5 (m/z 131)

Frick Laboratory, Princeton University Crosslinking controls the mechanical properties of the polymer Glass transition temperature Bulk rigidity – better water retention under stress load Molecular Model

Frick Laboratory, Princeton University Dependence of Nafion Glass Transition on Metal Oxide

Frick Laboratory, Princeton University SAXS Studies

Frick Laboratory, Princeton University Heat Self Assembled Disordered Crystalline Order-Disorder Transition

Frick Laboratory, Princeton University Ionic inclusions swell with water uptake, requiring the membrane to push the electrodes apart. Membrane Mechanical Properties Affect Cell Response

Frick Laboratory, Princeton University x x x x x x x x x x x10 6 Metal Oxide Composite Nafion 112 Stress (N/m 2 ) Strain(%) Stress-Strain Response

Frick Laboratory, Princeton University Too Much of a Good Thing is Bad

Frick Laboratory, Princeton University Membrane Swelling (c)Additional pressure further increases the membrane/catalyst contact. However, the larger pressure forces water out of the membrane. (a)The membrane is in contact with the catalyst support particles. (b)Applied pressure enhances the membrane/catalyst contact. membrane Carbon support

Frick Laboratory, Princeton University Hydrogen Crossover Open Circuit Voltage (mV) Crossover Current (mA/cm 2 ) 125µm 40µm 40µm Composite Membrane

Frick Laboratory, Princeton University Increased Tg allows maintenance of hydrated proton conduction paths at elevated temperatures. Improved mechanical rigidity allows for dimensional stability under conditions where water content of the membrane may be changing. Maintains good catalyst contact on deswelling Eliminates water loss on swelling. What Role Does the Metal Oxide Play?

Frick Laboratory, Princeton University Nafion o C Pt Anode w/o CO w/100 ppm CO TiO o C Pt/Ru Anode w/100 ppm CO w/500 ppm CO Cell Potential / V Current Density / mA cm -2 Carbon Monoxide Tolerance

Frick Laboratory, Princeton University Summary High Temperature Nafion Based PEM Fuel Cells overcome several limitations associated with current cell design Addition of a metal oxide phase affects the mechanical properties of the membrane: Increased Tg Improved gas barrier Mitigation of swelling/deswelling effects

Bonus Material (It’s not electrochemistry, but it is interesting) So, How Does One Store Hydrogen on the Run?

Frick Laboratory, Princeton University Storage Issues Safety For mobile applications range & power should be maintained. 5-10Kg of H 2 needed for a 65-75kW engine. H 2 feed rate is ~1000 liters/minute Weight Effective Density of Hydrogen Volume Requirements Size Geometry Refill Availability Recharge rate. Cost

Frick Laboratory, Princeton University Hydrogen Storage Phases

Frick Laboratory, Princeton University Storage Options Standard steel tanks ( psi) Known technology. Good Safety Record Subject to hydrogen imbrittlement Forms projectiles if structure is breached Tanks are challenging to fill because hydrogen heats upon expansion Heavy Storage capacity is only 0.5-1% by weight Poor volumetric storage due to non-ideality of hydrogen:

Frick Laboratory, Princeton University Storage Options Composite Tanks (~10,000psi) High storage capacity: Light weight Can store 7% H 2 by weight! Does not fragment upon failure Cost

Frick Laboratory, Princeton University Storage Options Generation on the fly: in-situ or ex-situ reforming of hydrocarbon fuels using an on-site reformer. Energy density of gasoline Easy access to fuel (gasoline stations) Systems integration is poor No carbon mitigation. Solid-state storage by intercalation (metal hydrides, carbons) Safe Heavy Expensive Chemical thermodynamics and kinetics are difficult Significant heating is required to release the hydrogen ∆H losses up to 30% are typical with operating temperatures of C. Tank filling is very exothermic Chemical kinetics are a difficult to handle

Frick Laboratory, Princeton University Hydride Storage Capacity

Frick Laboratory, Princeton University Storage Options Chemical Hydrides “Hydrogen on Demand” (Sodium Borohydride) Not flammable High Effective hydrogen pressure (~7000psi) Low Volume Simple system Chemical Safety Recyclable Cost??