Best Practices for Investigating Anion Exchange Membrane Suitability for Alkaline Electrochemical Devices: Case Study Using Quaternary Ammonium Poly(2,6-dimethyl 1,4-phenylene)oxide Anion Exchange Membranes

Abstract

Alkaline fuel cells containing anion exchange membrane electrolytes (i.e., solid-state alkaline fuel cells -SAFCs) promise to provide high power densities without platinum group metal catalysts. In the past decade, SAFC performance has improved substantially due to improvements in electrode binders that facilitate good membrane-electrode contact and ionic conductivity in the electrode layer. However, the alkaline (in)stability of AEMs is a long-standing challenge that currently precludes commercialization of this technology. To date, there have not been any satisfactory strategies or approaches to adequately assess an AEM's suitability for SAFC applications. Here, we report an all-encompassing "best practices" approach to evaluate a leading AEM candidate (poly(2,6-dimethyl 1,4-phenylene) oxide (PPO) with quaternary ammonium groups) for SAFCs. Additionally, this work reports an excellent peak power density of 294 mW cm −2 when the fuel cell was operated with hydrogen-oxygen. This high fuel cell performance was attained by painting the electrodes directly onto the membrane to minimize membrane-electrode contact resistance losses.

Alkaline Stability of Poly(Phenylene Oxide) Based Anion Exchange Membranes Containing Imidazolium Cations

“…The details of the method can be found in our previous paper. 10 The IEC for P1 was very close to the theoretical IEC, confirming that the reaction yield was almost 100%. For P12, PC7 and PC12, the yields for the corresponding reactions were 95%, 54% and 70%, respectively (see Table S1).…”

“…Additional details of the application of the method can be found in our previous papers. 3,6,10,14,47 Results and Discussion Figure 1 shows the scheme for the synthesis of modified imidazolium cation groups: 1-heptyl-2-methyl-imidazole (2MI-C7) and 1-dodecyl-2-methyl-imidazole (2MI-C12). The formation of 2MI-C7 was confirmed by the 1 H-NMR and 2D NMR (Correlation Spectroscopy; COSY) spectra as shown in Figures S1 and S2 49 The protons initially at 3.4 ppm (in the spectrum of 1-bromoheptane) shifted, after reaction with 2-methylimidazole, to ca.…”

“…Details of the calculations employed to arrive at this value can be found in our previous papers. 10,47 The shift of the protons present initially in the bromomethyl group (peak "3", initially at 4.33 ppm) to higher frequencies (5.30 ppm) was attributed to the proximate presence of the positively charged cation and demonstrated the formation of the product (P1, see Figure S6). A small shift in the protons of type "6", from ca.…”

“…10,14,36,44 Hibbs 45 has shown that poly(phenylene)-based AEMs with quaternary ammonium cations affixed to the benzene rings in the backbone using a six-carbon alkyl chain spacer were considerably more stable than those prepared with benzyl trimethylammonium cations attached directly to the backbone. When these two types of AEMs were immersed in nitrogen-degassed 4 M KOH at 60…”

Stability of Poly(2,6-dimethyl 1,4-phenylene)Oxide-Based Anion Exchange Membrane Separator and Solubilized Electrode Binder in Solid-State Alkaline Water Electrolyzers

“…37,38 All these mechanisms involve the presence of a strong base and cause the formation of tertiary amines (or other leaving groups depending on the cation used) with the subsequent loss of ion exchange capacity and ionic conductivity. [39][40][41][42] Arges and coworkers, 40,42 recently proposed that polysulfone and polyphenylene oxide based AEMs could undergo backbone hydrolysis when immersed in alkaline solutions at 60…”