Mechanical and Transport Properties of Anion Exchange Membranes for Electrochemical Applications

Energy from limited and unevenly concentrated fossil fuel reserves is not perpetual and in fact, it has been a cause of political and economic strife. Global warming caused by the use of such fossil fuels and other anthropogenic CO₂ emission has been catastrophic in several areas. With the increase in population, the demand for the energy is increasing more than never before¹ and the only way to mitigate this problem is by moving the energy sector to renewable/carbon neutral technologies. The use of fuel cells for wide range of applications such as automotive and stationary power generation can reduce the emission of CO₂ to environment. In this work, we have focused on developing a mechanically and chemically robust polymer membrane for such applications.   

Two membranes poly(2,6-dimethyl-1,4-phenylene-oxide)-b-poly(vinylbenzyltrimethylammonium)(PPO-b-PVBTMA) membranes with IEC of 2.5 (AEM-1) and 1.5 mmol/gm (AEM-2) were prepared by melt pressing and used for mechanical and electro-chemical properties. The PPO, the hydrophobic group, was chosen for stable thermal, chemical and mechanical properties, while the PVB chloride, hydrophilic block, was chosen for stable electrochemical properties.

The AEM-1, the membrane containing ~ 70 - 75 wt% of PVBC as starting polymer, was highly conductive for OH^- while the AEM-2, the membrane containing ~60 - 70 wt% of PPO as starting polymer, was mechanically robust. A difference in ratio of hydrophilic and hydrophobic block in the diblock polymer membranes provided the trade-off in properties between ionic conductivity and high mechanical stability. Finding a balance between PPO and PVBC in the membrane for optimum chemical and mechanical properties is critical for a good robust membrane for practical applications. Understanding the ion transport, and the environmental (temperature and humidity) effects on such transport is crucial for the development of high performance membranes for many applications.

 Several groups have been able to show a very high OH^- conductivity under the fully hydrated conditions² but a very few groups has actually measured the OH^- conductivity at low humidity under a CO₂ free environment.^3,4 When a membrane in OH^- is exposed to air, the OH^- reacts with the atmospheric CO₂ (400 ppm) giving the mixture of bi/carbonates and some residual OH^-.^3,5 The ionic conductivity of the membrane is compromised in addition to chemical stability of the membrane in alkaline form. The AEM-1 and AEM-2 membranes have been mechanically and chemically stable in OH^- at least several weeks. The ionic conductivity of OH^- >100 mS/cm at 60°C and 95%RH make the membrane a strong candidate for many electrochemical application such as low temperature AEM fuel cells. The membrane had a reduced dimensional swelling and a lower water uptake due to cross-linking during melt pressing at 240°C.

In conclusion, we have designed, synthesized, and melt pressed an AEM that is mechanically and chemically stable, and is highly conduct in OH^-.

Acknowledgement

The authors would like to thank the US Army Research Office for the funding under the MURI #W911NF-10-1-0520 and Yating Yang and Daniel M. Knauss for providing the polymer for the study.

References

(1)           The Outlook for Energy: A View to 2040; 2013.

(2)           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.

(3)           Pandey, T. P.; Maes, A. M.; Sarode, H. N.; Peters, B. D.; Lavinia, S.; Vezzu’, K.; Yang, Y.; Poynton, S.; Varcoe, J. R.; Seifert, S.; et al. Interplay between Water Uptake, Ion Interactions, and Conductivity in an E-Beam Grafted Poly(ethylene-Co-Tetrafluoroethylene) Anion Exchange Membrane. Phys. Chem. Chem. Phys. 2014.

(4)           Marino, M. G.; Melchior, J. P.; Wohlfarth, A.; Kreuer, K. D. Hydroxide, Halide and Water Transport in a Model Anion Exchange Membrane. J. Memb. Sci. 2014, 464, 61–71.

(5)           Maes, A. M.; Pandey, T. P.; Vandiver, M. A.; Lundquist, L. K.; Yang, Y.; Horan, J. L.; Krosovsky, A.; Liberatore, M. W.; Seifert, S.; Herring, A. M. Preparation and Characterization of an Alkaline Anion Exchange Membrane from Chlorinated Poly(propylene) Aminated with Branched Poly(ethyleneimine). Electrochim. Acta 2013.