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Anion Exchange Membranes Applications in Fuel Cells

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Anion Exchange Membranes Applications in Fuel Cells Anion Exchange Membranes Applications in Fuel Cells Jake Ganley Group Meeting January 17, 2019 Fossil Fuels and the Promise of Hydrogen CnH2n+2 + O2 CO2 + H2O H2 + O2 H2O Jan 2003: President Bush’s Hydrogen Fuel Initiative commits a total of $1.7 billion to fuel cell R&D “…consumers will have the practical option of Sketch of William Grove’s 1839 fuel cell purchasing a competitively priced hydrogen published in The London and Edinburgh powered vehicle, and be able to refuel it near Philosophical Magazine and Journal of Science their homes and places of work, by 2020.” -Secretary Abraham, Apr 2003 Paster, M. President’s Hydrogen Fuel Initiative. US Department of Energy, August 2005. Basics of a Hydrogen Fuel Cell Load + – – – Anode: H2 2H + 2e 4e 4e 2H2 O2 (Ea = 0 V vs SHE at 1 bar, 298.15 K) 0.5 O – + H O Cathode: 2 + 2e + 2H 2 4H+ (E = 1.23 V vs SHE at 1 bar, 298.15 K) c Anode Cathode Overall: H + 0.5 O H O 2 2 2 2H2O (Ecell = 1.23 V vs SHE at 1 bar, 298.15 K) Proton-Exchange Membrane • Common catalysts: Platinum and Platinum Group Metals (PGMs) • Common proton exchange membrane: Nafion Varcoe, J. R.; Slade, R. C. T. Prospects for alkaline anionexchange membranes in low temperature fuel cells. Fuel Cells 2005, 5, 187−200. Nafion: The Most Common PEM Nafion General Structure • Separated hydrophilic/hydrophobic microdomains facilitate ion transport F2 F2 C F C • Excessive water uptake reduces mechanical integrity C C F2 x • Nafion PEM’s have conductivities commonly >100 mS/cm, O CF2 O O depending on relative humidity F2 CF C S F3C O C O F y 2 Protons from on the sulfonic acid “hop” from one acidic site to the next • Introduced by DuPont in the 1960’s • Structurally based on Teflon (PTFE) Common traits of an effective Ion Exchange Membrane: (1) Good ionic conductivity, but low electronic conductivity (2) Low gas permeability (3) Resistant to swelling (4) High mechanical strength and integrity (5) High chemical stability Dicks, A. L. (2011). PEM fuel cells - Applications. Comprehensive renewable energy. Amsterdam, Netherlands: Elsevier. 203-245. Why Aren’t We Driving Hydrogen Powered Cars? Two Main Challenges Facing the Implementation of Fuel Cells: (1) Infrastructure surrounding a Hydrogen economy (production, storage, distribution) (2) The currently prohibitively high cost of proton exchange membrane fuel cells Load Load 4e– 4e– 4e– 4e– 2H O, O 2H2 O2 2H2 2 2 4H+ 4OH– Anode Anode Cathode Cathode 4H O 2H2O 2 Proton-Exchange Membrane Anion-Exchange Membrane • High loadings of expensive catalysts required at • Kinetics of ORR in high pH environments cathode due to slow kinetics of oxygen reduction significantly faster, allowing for less active reaction (cheaper) catalysts Anion Exchange Membrane Fuel Cell Load Anode: 2H + 4OH– + – 4e– 4e– 2 4H2O 4e 2H O, O 2H2 2 2 (Ea = 0.83 V vs SHE at 1 bar, 298.15 K) O + + – – Cathode: 2 2H2O 4e 4OH 4OH– (E = 0.40 V vs SHE at 1 bar, 298.15 K) c Anode Cathode O Overall: 2H2 + 2 2H2O 4H2O (Ecell = 1.23 V vs SHE at 1 bar, 298.15 K) Anion-Exchange Membrane • Able to use cheaper Ag and Ni based catalysts • Development of new AEMs an area of active research and will be the focus of this talk Varcoe, J. R.; Slade, R. C. T. Prospects for alkaline anionexchange membranes in low temperature fuel cells. Fuel Cells 2005, 5, 187−200. Common AEM Components Polymer Backbones O O O S O Poly(phenylene) Poly(ethylene) Poly(arylene ether sulfone) Me Me N Me O N N Me N Poly(benzimidazole) Poly(phenylene oxide) Poly(styrene) Cationic Component Me R R N N X N N X R N R P R S 3 X Me Me 3 2 X N N X X R R Me Me Ammonium Imidazolium Guanidinium Pyridinium Phosphonium Sulfonium Anion Exchange Membranes Applications in Fuel Cells (1) Hydroxide Conductivity (2) Membrane Stability (3) State of the Art (4) Research Update Transport Mechanisms in AEMs Purpose of Anion Exchange Membrane: to conduct hydroxyl ions at very high rates from the cathode to the anode Grotthuss Mechanism Diffusion Convection H H H H O O H O H H H O H O H O H H H H H O O H H H O H H O O H H O H O H O O H O H H H H H H H H O O O O O H O H H H H O H H O O H H H H O O H H H O H H H H H O O O ν Surface Site Hopping Δ COH– H O NMe3 NMe3 NMe3 NMe3 Merle, G.; Wessling, M.; Nijmeijer, K. Anion exchange membranes for alkaline fuel cells: A review. J. Membrane Science 2011, 377, 1–35. Measuring Hydroxide Conductivity The resistance of a solution and/or material can be calculated using Ohm’s Law ( V = R x I). Conductance is defined as the reciprocal of the resistance between the two electrodes. where: R = V/I V = voltage (volts) I = current (amperes) –1 G = R R = resistance of the solution (ohms) G = conductance (Siemens) Cell constant is the ratio of the distance between the electrodes and the area of the elctrodes. This number is held constant between measurements of different materials. Hydroxide conductivity is then the product of the cell constant and the measured conductance. where: K = d/a K = cell constant (cm–1) a = effective area of the electrodes (cm2) σOH– = G x K d = distance between the electrodes (cm) σOH– = conductivity (S/cm) Li et al. Highly stable anion exchange membranes based on quarternized polypropylene. J. Mater. Chem. A 2015, 3, 12284–12296. Hydroxide Conductivity: Associated Challenges • OH– ionic mobility is less than that of H+: H+ = 4.76 and OH– = 2.69 relative to K+ “Given what is known about the empirical upper limit of proton conductivity in practical PEMs…the prospect of meeting or exceeding this perceived limit for hydroxyl anion conduction is small given functionalized polymer approach for OH– conduction.” -Hibbs et al. 2008 Me3N O O O O S O O S O O • High Ion Exchange Capacaty (IEC) required for any appreciable conductivity (30 mS/cm @ 30ºC) • Ion Exchange Capacity: number of ion exchange sites; quantity of ions that can be taken up by a specific volume of of polymer (reported as mmol/g) • High IECs typically result in dramatically increased water uptake and degradation of mechanical properties Cornelius et al. Transport Properties of Hydroxide and Proton Conducting Membranes. J. Chem. Mater. 2008, 20, 2566–2573. Hydroxide Conductivity: Associated Challenges Carbonation of AEMs • Difficult to directly compare conductivities due to varying concentrations of hydroxide, bicarbonate, and CO + – – 2 OH HCO3 carbonate anions • Two strategies to cope with this challenge: use CO + – 2– 2 2OH CO3 rigorously CO2 free glovebox conditions or measure CO2 concentration in air: 400 ppm the conductivity of more stable anions Conductivity Under Rigorously CO2 Free Conditions Anion Ionic Conductivities –5 –2 –1 –1 OH– 197.6 x 10 cm V s OH– – –5 –2 –1 –1 HCO3 46.4 x 10 cm V s – –5 –2 –1 –1 Cl 76.3 x 10 cm V s – – Br , HCO3 • Ratio strategy does not take into account structure- related steric effects, hydration level, and ion pairing Arges, C. G.; Zhang, L. Anion Exchange Membranes’ Evolution toward High Hydroxide Ion Conductivity and Alkaline Resiliency. ACS Appl. Energy Mater. 2018, 1, 2991−3012. Janarthanan, R.; Horan, J. L.; Caire, B. R.; Ziegler, Z. C.; Yang, Y.; Zuo, X.; Liberatore, M. W.; Hibbs, M. R.; Herring, A. M. Understanding anion transport in an aminated trimethyl polyphenylene with high anionic conductivity. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1743−1750. Thermodynamics of Counterion Release Identifying parameters related to cation identity that impact conductivity is crucial for developing highly conductive AEMs Ammonium Imidazolium Guanidinium Pyridinium Phosphonium Bis(terpy)Metal 2+ Me N X R R R3N N N N R3P X X X X Me Me N N N R R N N Me Me M N N N Conductivity IEC Polymer Cation Condition (mS/cm) (mmol/g) Polyethylene Ammonium 40 1.29 20ºC, liquid water Phosphonium 22 0.67 22ºC, liquid water Polysulfone Ammonium 55 1.20 80ºC, liquid water Phosphonium 16 1.37 80ºC, 100% RH Tew et al. Thermodynamics of Counterion release is Critical for Anion Exchange Membrane Conductivity. J. Am. Chem. Soc. 2018, 140, 7961–7969. Polymer Synthesis and Characterization Sample Water Uptake (%) IEC (mmol/g) Ea (kJ/mol) 0.36 169 ± 53 0.89 15.6 RuPEO 0.55 152 ± 25 1.12 16.2 RuPEO 0.36 NiPEO 162 ± 27 0.99 16.3 0.55 NiPEO 193 ± 46 1.26 16.5 0.36 CoPEO 154 ± 12 0.99 15.1 0.55 CoPEO 152 ± 29 1.26 Tew et al. Thermodynamics of Counterion release is Critical for Anion Exchange Membrane Conductivity. J. Am. Chem. Soc. 2018, 140, 7961–7969. Bulk Hydration and Ion Concentration mhyd – mdry 1000 x WU ρ x IEC Water Uptake = λ = c = mhyd MH2O x IEC 1 + WU 0.55 Neither IEC, bulk hydration, nor ion concentration were sufficient to explain high conductivity of NiPEO Tew et al. Thermodynamics of Counterion release is Critical for Anion Exchange Membrane Conductivity. J. Am. Chem. Soc. 2018, 140, 7961–7969. Isothermal Titration Calorimetry 2Cl– 2+ 2Cl– 2+ 2Cl– 2+ N N N N N N N N N Me Co Ru Ni N N N N N N N Me N N N Cl Cl N N N Cl Me Me Me Me Me Me Me – – – – – – – – – – – – +2HCO3 –2Cl +2HCO3 –2Cl +2HCO3 –2Cl +HCO3 –Cl +HCO3 –Cl +HCO3 –Cl – – – 2HCO3 2+ 2HCO3 2+ 2HCO3 2+ Me Me HCO3 N N N N N Me Me Me HCO N N N N N N 3 N Co Ru Ni N ΔH=0.35 kcal/mol Me Me Me N N N N N N HCO3 Me N N N ΔH=0.21 kcal/mol ΔH=0.48 kcal/mol ΔH=1.12 kcal/mol ΔH=1.0 kcal/mol ΔH=0.59 kcal/mol Increasing Thermodynamic Driving Force Tew et al.
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