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. Thermodynamics of Counterion release is Critical for Anion Exchange Membrane Conductivity. J. Am. Chem. Soc. 2018, 140, 7961–7969. Cation Hydration

Low Hydration High Hydration

H O H2O H2O H O 2 H2O 2 H O H O 2 H O 2 H2O 2 H2O Increasing Hydration H2O H2O H O H2O H2O H O 2 H O 2 H2O 2 H2O H2O H O H2O H O 2 H2O 2

Higher ion pair association strength Lower ion pair association strength Larger +ΔH from ITC Smaller +ΔH from ITC

R N Me Me Bis(terpy)Co < Bis(terpy)Ru < Bis(terpy)Ni N < Cl N Cl N Cl Low Spin Low Spin High Spin < N Cl Me Me Me Me Me Me Me Me Me Me • Larger, delocalized cations have less • High spin octahedral complexes have • Stabilizing effect of the benzyl group hydration; water molecules interact more charge localization on the metal allows counterion to dissociate more more with each other than the cation center, more hydration

Tew et al. Thermodynamics of Counterion release is Critical for Anion Exchange Membrane Conductivity. J. Am. Chem. Soc. 2018, 140, 7961–7969. Membrane Morphology and Ionic Conductivity

100–X X 100–X X TiCl 3 1) NMe3 Me Me Me 9 Br Et2AlCl 2) NaOH OH Me Me Me Br NMe3 4-methyl-1-pentene PMP-TMA-X PMP PMP-TMA-4 PMP-TMA-20 PMP-TMA-41

Calculated IEC (mequiv/g) 0.44 1.76 2.84

Measured IEC (mequiv/g) 0.41 1.47 1.92

Water Uptake (%) 4.7 21.8 29.2

Bulk Hydration (λ) 6.4 8.6 8.4

Ionic Conductivity (mS/cm) 5.1 20.3 43.2

Zhang, M.; Shan, C.; Liu, L.; Liao, J.; Chen, Q.; Zhu, M.; Wang, Y.; An, L.; Li, N. Facilitating Anion Transport in Polyolefin-Based Anion Exchange Membranes via Bulky Side Chains. ACS Appl. Mater. Interfaces 2016, 8, 23321−23330. Anion Exchange Membranes Applications in Fuel Cells

(1) Hydroxide Conductivity

(2) Membrane Stability

(3) State of the Art

(4) Research Update Alkaline Resiliency

US DOE Fuel Cell Technology Office’s 2018 Funding Opportunity Announcement specified AEMFC stability: 2 <10% voltage degradation from an initial performance of a current density of >600 mA/cm in H2/O2 at 1.5 atm and T > 60ºC over 2000 hr with a 50 cm2 MEA

Nucleophilic Substitution Deprotonation

H OH

OH N N N N OH Me Me Me Me Me Me Me Me Me Me Me

Rearrangement Elimination Me Me Me OH N NMe2 Me OH

N N N H Me Me Me Me Me Me Me Stevens Sommelet-Hauser

Arges et al. Anion Exchange Membranes’ Evolution toward High Hydroxide Ion Conductivity and Alkaline Resiliency. ACS Appl. Energy Mater. 2018, 1, 2991−3012. Alkaline Stability of Quarternary Ammonium Cations

Decomposition as a Function of Solvent Decomposition as a Function of [NaOH]

Water, 4 M NaOH

1:1 Water:Glycol, 2 M NaOH Me N Glycol, 2 M NaOH Me Me Me

N Me Me Me

• Different conditions are employed by different groups to assess stability • More in situ fuel testing and standardized stability assessments are a priority of the field

Marino, M. G.; Kreuer, K.D. Alkaline Stability of Quarternary Ammonium Cations for Alkaline Fuel Cell Membranes and Ionic Liquids. ChemSusChem. 2015, 8, 513–523. Half-Life at 160ºC in 6 M NaOH

Me Me N 61.9 hr N N Me Me Me 87.3 hr 13.5 hr Me 2.8 hr Me N N Me Me Me Me Bn Me 1 N 4.2 hr N Me Me N Me Me 33.2 hr Me N 1.4 hr Me 7.3 hr N Bn Bn Me Me Bn Me Me N 0.68 hr N Et Et 20.7 hr Me Me Me Me Et N N N/A (160ºC) Me Me

Me N 37.1 hr 7 Me 4 Me Me N N N 16.6 hr Me N/A (60ºC) Me Me 31.9 hr MeO Me N Me Me Me Bn Me N N Me N/A (25ºC) 6 N Me Me 12.7 hr N Me 110 hr Bn Bn 0.66 hr N O2N Me Me Me N/A (160ºC) 8 N N N N Me Me Me Me 4.4 hr Me Me Me Me

Ph Bn Me N 28.4 hr N 14 N 0.14 hr N/A (25ºC) N Me Me Me Me 1.9 hr Me Me

Marino, M. G.; Kreuer, K.D. Alkaline Stability of Quarternary Ammonium Cations for Alkaline Fuel Cell Membranes and Ionic Liquids. ChemSusChem. 2015, 8, 513–523. Hofmann Elimination

Strategies to reduce elimination: OH H H (1) Remove β hydrogens (2) Conformationally restrict hydrogens NR3 from achieving antiperiplanar alignment Antiperiplanar alignment between the σ and σ* orbitals necessary for elimination N N N Me Me

Me T = 87 hr N N 1/2 Me Me N N H Replace Me groups H 109.5º Major Product Me Half-Life of 110 hr despite the presence N 90º Me 120º H of 4 antiperiplanar hydrogens! NMe H H 2 H Not Observed HO

Marino, M. G.; Kreuer, K.D. Alkaline Stability of Quarternary Ammonium Cations for Alkaline Fuel Cell Membranes and Ionic Liquids. ChemSusChem. 2015, 8, 513–523. Spirocyclic Ammoniums

Structures Proposed by Marino & Kreuer

X N X N

Synthesized Spirocyclic Ammoniums

0,1,2

N N N

O O O S N N O

N N O

0,1,2 0,1,2 Me Pham, T. H.; Jannasch, P. Aromatic Polymers Incorporating Bis-N-spirocyclic Quaternary Ammonium Moieties for Anion-Exchange Membranes. ACS Macro Lett. 2015, 4, 1370−1375. Olsson, J. S.; Pham, T. H.; Jannasch, P. Poly(N,N-diallylazacycloalkane)s for Anion-Exchange Membranes Functionalized with N-Spirocyclic Quaternary Ammonium Cations. Macromolecules 2017, 50, 2784−2793. Pham, T. H.;Olsson, J. S.; Jannasch, P. N-Spirocyclic Quaternary Ammonium Ionenes for Anion-Exchange Membranes. J. Am. Chem. Soc. 2017, 139, 2888−2891. Chen, N.; Long, C.; Li, Y.; Lu, C.; Zhu, H. Ultrastable and High Ion-Conducting Polyelectrolyte Based on Six Membered N-Spirocyclic Ammonium for Hydroxide Exchange Membrane Fuel Cell Applications. ACS Appl. Mater. Interfaces 2018, 10, 15720−15732. Imidazolium Cation Stability

R2 R1 N 1 M KOH % Imidazolium cation N R3 CD3OH, 80ºC remaining after 30 days R5 R4

Bn N Me Me Me

Bn N N Me

Me Bn N N Me

Me Bn N N Me Ph Ph

Coates et al. Imidazolium Cations with Exceptional Alkaline Stability: A Systematic Study of Structure-Stability Relationships. J. Am. Chem. Soc. 2015, 137, 8730–8737. Imidazolium Cation Stability

R2 R1 N 1 M KOH % Imidazolium cation N R3 CD3OH, 80ºC remaining after 30 days R5 Me R4 Bn N N Me Ph Ph Me Me Bn N N Me Ph Ph Ph Bn N N Me Ph Ph

Bn Me N N Me Ph Ph Coates et al. Imidazolium Cations with Exceptional Alkaline Stability: A Systematic Study of Structure-Stability Relationships. J. Am. Chem. Soc. 2015, 137, 8730–8737. Imidazolium Cation Stability

R2 R1 N 1 M KOH % Imidazolium cation N R3 CD3OH, 80ºC remaining after 30 days R5 Xyl R4 Bn N N Me Ph Ph Xyl Bn N N Et Ph Ph Xyl Bn N N Pr Ph Ph Xyl Pr N N Pr Ph Ph

Coates et al. Imidazolium Cations with Exceptional Alkaline Stability: A Systematic Study of Structure-Stability Relationships. J. Am. Chem. Soc. 2015, 137, 8730–8737. Polymer Backbone Stability

Me3N

O O O O S O O S O O Base stable polymer backbone Alkaline stable cation

When functionalized with TMA, the poly(sulfone) becomes unstable to hydroxide. Why?

Me N 3 OH

O O O S O

Me3N

O O OH HO S O

Arges, C. G.; Ramani, V. Two-dimensional NMR spectroscopy reveals cation-triggered backbone degradation in polysulfone-based anion exchange membranes. PNAS 2013, 110, 2490–2495. Mohanty, A. D.; Tignor, S. E.; Krause, J. A.; Choe, Y.-K.; Bae, C. Systematic Alkaline Stability Study of Polymer Backbones for Anion Exchange Membrane Applications. Macromolecules, 2016, 40, 3361–3372. Oxidative Stability

• Membranes exposed to alkaline solutions can become more brittle • Loss of side chain cation does not explain loss in mechanical integrity Question: could other mechanisms of degradation be resposible?

O2 O2 OH OH

NMe3 NMe3 PMe3 6 N N N O O O Me Me Me Me Me Me Me Me Me Me Me Me Parrondo, J. Wang, Z.; Jung, M.-S. J.; Ramani, V. Reactive oxygen species accelerate degradation of anion exchange membranes based on polyphenylene oxide in alkaline environments. Phys. Chem. Chem. Phys. 2016, 18, 19705--19712. Anion Exchange Membranes Applications in Fuel Cells

(1) Hydroxide Conductivity

(2) Membrane Stability

(3) State of the Art

(4) Research Update Properties of Leading AEMs

Me Me CF3 O

O O CF3 O Me Me Me Me 5 N Hex N Me

Me Me Me N N Hex Me 5 N 5 σOH– = 140 mS/cm No change in IEC, 7% drop in σOH– after 41 days σOH– = 98 mS/cm 6% change in IEC, 7% drop in σOH– after 25 days Me Hex Me Hex Me N N Me

NMe2

σ = 137 mS/cm σ = 124 mS/cm OH– OH– 5–18% change in IEC after 20 days 5% change in IEC after 30 days

Arges et al. Anion Exchange Membranes’ Evolution toward High Hydroxide Ion Conductivity and Alkaline Resiliency. ACS Appl. Energy Mater. 2018, 1, 2991−3012. Properties of Leading AEMs

N N

F2 F2 C F C σ = 115 mS/cm C C OH– F2 x No changes in 1H NMR @ T=120ºC over 75 days O CF2 O O F2 CF C S F3C O C NMe F2 y NMe3 Me2 Me2 Me2 Me2 N N N N Me Me 5 5 5 O O σOH– = 122 mS/cm 10% change in IEC after 14 days

σOH– = 110 mS/cm 22% change in IEC, 25% drop in σOH– after 30 days

F3C

Me2 NMe3 N NMe3 Me O

σOH– = 112 mS/cm σOH– = 96 mS/cm 2% change in IEC after 60 days 9% change in IEC, 10% drop in σOH– after 30 days

Arges et al. Anion Exchange Membranes’ Evolution toward High Hydroxide Ion Conductivity and Alkaline Resiliency. ACS Appl. Energy Mater. 2018, 1, 2991−3012. Properties of Leading AEMs

NMe3

N CF3

CF3 CF3

σOH– = 101 mS/cm 1 σOH– = 120 mS/cm Minor changes in H NMR in 2 M KOD @ 5% change in IEC, no drop in σOH– after 30 days T=120ºC over 14 days

Me3N NMe3 F2 H2 C C O O CN CN C C F H O O O 2 2

O O Me F3C CF3 Me3N NMe3 N

σ σOH– = 116 mS/cm OH– = 159 mS/cm 5% change in IEC after 20 days 15% change in IEC after 28 days

Arges et al. Anion Exchange Membranes’ Evolution toward High Hydroxide Ion Conductivity and Alkaline Resiliency. ACS Appl. Energy Mater. 2018, 1, 2991−3012. ROMP Route to AEMs

NMe3 Grubbs Gen II cat.

Me Me

Me3N crosslinks NMe Me CHO 3 steps 3 σ = 18 mS/cm @ 50ºC Me OH– Stability not reported

NMe3 (1) Grubbs Gen II cat. Me Me (2) H2, Crabtree’s cat. NMe3

NMe σ = 48 mS/cm @ 22ºC 8 steps 3 OH– Me Lengthy Synthesis

Clark, T. J.; Robertson, N. J.; Kostalik, H. A.; Lobkovsky, E. B.; Mutolo, P. F.; Abruña, H. D.; Coates, G. W. A Ring-Opening Metathesis Polymerization Route to Alkaline Anion Exchange Membranes: Development of Hydroxide-Conducting Thin Films from an Ammonium-Functionalized Monomer. J. Am. Chem. Soc. 2009, 131, 12888–12889. Kostalik, H. A.; Clark, T. J.; Robertson, N. J.; Mutolo, P. F.; Longo, J. M.; Abruña, H. D.; Coates, G. W. Solvent Processable Tetraalkylammonium- Functionalized Polyethylene for Use as an Alkaline Anion Exchange Membrane. Macromolecules 2010, 43, 7147–7150. ROMP Route to AEMs

(1) Grubbs Gen II cat. Me NMe2 Me2N Me Me

NMe2

8 steps Me2N NMe2 Me2N Me Me

σOH– = 69 mS/cm @ 22ºC Poor alkaline stability

Robertson, N. J.; Kostalik, H. A.; Clark, T. J.; Mutolo, P. F.; Abruña, H. D.; Coates, G. W. Tunable High Performance Cross-Linked Alkaline Anion Exchange Membranes for Fuel Cell Applications. J. Am. Chem. Soc. 2010, 132, 3400–3404. ROMP Route to AEMs

R (1) Grubbs Gen II cat. Me Me (2) H2, Crabtree’s cat. R

N N Et Cy Me N N Cy N Me N P N Ph Co N N Me Me Cy Et

9-step synthesis 6-step synthesis 5-step synthesis

σOH– = 22 mS/cm @ 22ºC σOH– = 20 mS/cm @ 22ºC σOH– = 37 mS/cm @ 22ºC Stable in 1 M NaOH @ Stable in 1 M NaOH @ Stable in 5 M NaOH @ 80ºC for 25 days 80ºC for 35 days 80ºC for 30 days

Noonan, K. J. T.; Hugar, K. M.; Kostalik, H. A.; Lobkovsky, E. B.; Abruña, H. D.; Coates, G. W. Phosphonium-Functionalized Polyethylene: A New Class of Base Stable Alkaline Anion Exchange Membranes. J. Am. Chem. Soc. 2012, 134, 18161−18164. Zhu, T.; Xu, S.; Rahman, A.; Dogdibegovic, E.; Yang, P.; Pageni, P.; Kabir, M. P.; Zhou, X.-d.; Tang, T. Cationic Matello-Polyelectrolytes for Robust Alkaline Anion-Exchange Membranes. Angew. Chem. Int. Ed. 2018, 57, 2388–2392. You, W.; Hugar, K. M. Coates, G. W. Synthesis of Alkaline Anion Exchange Membranes with Chemically Stable Imidazolium Cations: Unexpected Cross-Linked Macrocycles from Ring-Fused ROMP Monomers. Macromolecules 2018, 51, 3212−3218. ROMP Route to AEMs

R (1) Grubbs Gen II cat. Me Me (2) H2, Crabtree’s cat. R

N N Et Cy Me N N Cy N Me N P N Ph Co N N Me Me Cy Et

9-step synthesis 6-step synthesis 5-step synthesis

σOH– = 22 mS/cm @ 22ºC σOH– = 20 mS/cm @ 22ºC σOH– = 37 mS/cm @ 22ºC Stable in 1 M NaOH @ Stable in 1 M NaOH @ Stable in 5 M NaOH @ 80ºC for 25 days 80ºC for 35 days 80ºC for 30 days

• Challenging monomer synthesis and scalability concerns limit the practicality of these AEMs • An easily synthesized AEM with excellent alkaline conductivity and stability remains elusive

• Anion Exchange Membrane fuel cells will be a critical technology in the search for more environmentally friendly sources of energy

• Alkaline conductivity and stability historically have been challenges, but recent advances have made it such that AEM conductivity will most likely not be the limiting factor in AEMFC success

• Engineering challenges include: water management in the fuel cell, preventing carbonation, fuel cell dimensions, and the infrastructure of the hydrogen economy

• Future priorities of the field should include more in situ fuel cell testing and standardization of conductivity/stability testing

• Piperidinium Polyethylene is a promising AEM material, and is significantly easier to synthesize than materials with comparable performance