“Bridging the gap between theory and experiment: which theoretical Leveraging Simulations approaches are best suited to solve real problems in nanotechnology and biology?” Leveraging Simulations to Gain Insights into Polymer Electrolyte Membrane F l C ll Fuel Cells Stanford University Dr. Lalitha Subramanian Sr. Director and Fellow 24 th February, 2010 Accelrys Inc.
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“Bridging the gap between theory and experiment: which theoretical
Leveraging Simulations
g g g p y papproaches are best suited to solve real problems in nanotechnology and biology?”
Leveraging Simulations to Gain Insights into
Polymer Electrolyte Membrane F l C llFuel Cells
Stanford University
Dr. Lalitha SubramanianSr. Director and Fellow
24th February, 2010
Accelrys Inc.
Outline
• PEM FC overview
• Rational PEM DesignRational PEM Design– Morphology of perfluorosulfonic acid
(i.e., Nafion®) membranes– Further PEM studies
• Proton transport mechanism• Chemical/mechanical durability• Alternate membrane materials
• Rational Electrocatalyst Design– High Throughput Screening
• Combines both experimental and• Combines both experimental and simulation/modeling insight
• Cost– The cost of fuel cell power systems must be reduced before they can be
competitive with conventional technologiescompetitive with conventional technologies
• Durability and Reliability– Match durability and reliability of current automotive engines [i.e., 5,000-
hour lifespan (150 000 miles)] and the ability to function over the fullhour lifespan (150,000 miles)] and the ability to function over the full range of vehicle operating conditions (40°C to 80°C). For stationary applications, more than 40,000 hours of reliable operation in a temperature at -35°C to 40°C will be required for market acceptancetemperature at -35 C to 40 C will be required for market acceptance
• System Size– The size and weight of current fuel cell systems must be further reduced
t t th k i i t f t bilto meet the packaging requirements for automobiles
• A good performance at a temperature of 120 ºC without the need to pressurize, i.e(RH) ≤ 40%. At this temperature, about 50 ( ) pppm CO can be tolerated without air bleed
• Conductivity σ = 0.1 -1 cm-1
• Hydrogen oxygen gas permeability <• Hydrogen-oxygen gas permeability < 1x10−12 (mol cm)/(cm2 s kPa))
• Limited swelling in water
• Mechanical properties better than Nafion®
• A chemical stability similar or superior to Nafion, i.e., a durability of around 40,000 h (≤ 1 μV/h)
• A cost target of ≤ $10/kW at 500,000 stacks/y (for automotive application)
– Characterizing the chemical features that affect performance– the chemical nature of protonation sites– local concentration of protons – and local level of hydration
– Characterizing the underlying polymer morphologyU d t di t di t ib ti l ti h d ti it– Understanding water distribution, percolation, hence conductivity
Morphology in hydrated perfluorosulfonic acid membranes
• Morphology of Nafion at the nanoscale?
• SAXS SANS:• SAXS, SANS: – Nanophase segregation into hydrophilic and hydrophobic domains, – Debate over the shape and structure of the ionic clusters: spherical, ellipsoid, or
lamellar?
• Observations of the surface morphology via TEM and AFM– Three-phase model consisting of spherical water clusters surrounded by sulfonic
acid interfaces.– Also observed the coalescence and growth of ionic clusters with an increasingAlso observed the coalescence and growth of ionic clusters with an increasing
water content using AFM.
• Use mesoscale modeling to compare and contrast with exp. Observations
Wescott, Qi, Subramanian and Capehart, J. Chem. Phys. 124, 134702 (2006) collaboration between Accelrys and General Motors
[1] J. Electrochem. Soc. 128, 1880 (1981)[2] J. Membr. Sci. 45, 261 (1989)
• Water cluster/fluorocarbon degree ofPhase separation increases with increasing water volume fraction
Three-phase Morphology at =8
W30nm
W F SWater clusters (~4nm) surrounded by sulfonic phase embedded in a hydrophobic PTFE matrixC i t t ith Y St k[1] thConsistent with Yeager-Steck[1] three
[1] J. Membr. Sci. 45, 261 (1989)[2] J. Elctrochem. Soc. 128, 1880 (1981)
Percolation of water domains/ Percolation for conductivity
%62 %208
• Simulated morphology consistent with the structural i f ti i f d f ll
Volume fraction of water
%6,2 %20,8information inferred from small-angle scattering• Simulated morphology at low gywater content produces spherical hydrophilic domains of reverse micelles - similar to
%11,4 %33,16of reverse micelles similar to model of Gierke• Simulated morphology at hi h t t thigher water content –domains deform into elliptical and barbell shapes – similar to
More reaction steps need to be added for electro-reduction
Eads1=E1-E0
Summary
• Ab initio High Throughput Approach offers valuable insight into factors defining catalytic activity of materials.
I t tl it ll t dd i lt l th bl f f d• Importantly it allows to address simultaneously the problems of surface and chemical reactivity.
• Our approach streamlines calculations of descriptors such as d-band centre position, atomic fraction of solute atoms near the surface and electron workposition, atomic fraction of solute atoms near the surface and electron work function.
• This opens the possibility of in silico cathode material optimisation complimentary to the experiment.