Degradation of Anion Exchange Membranes (AEM) and Solubilized AEM Binders in Solid-State Alkaline Water Electrolyzers

Hydrogen production using alkaline membrane water electrolysis has recently attracted interest as an alternative to traditional liquid alkaline water electrolysis, proton exchange membrane water electrolysis, and solid-oxide water electrolysis (1).  Alkaline membrane electrolyzers provide an efficient, modular, and reliable method to produce hydrogen from water and renewable electricity sources. One advantage arises from the fact that the alkaline environment facilitates better oxygen evolution reaction (OER) kinetics and allows the use of non-platinum group catalysts for the OER (2) (Of course, one must acknowledge that this is countered by the more sluggish HER in alkaline media). In a solid-state alkaline membrane water electrolyzer, the anion exchange membrane (AEM) has two functions: 1) it acts as an impermeable barrier to the fuel and oxidant gases; and 2) conducts the hydroxide ions from the cathode, where they are generated by the electrochemical reduction of oxygen, to the anode.

In this work we will discuss our latest findings on AEM degradation under alkaline conditions encountered during AEM water electrolyzer operation. Despite the promising results reported in terms of hydroxide ion conductivities and electrolyzer performance, there is a general understanding that the AEM membranes and solubilized AEM binders do not yet have the necessary alkaline stability for practical applications. It is accepted that the mechanisms of AEM degradation under alkaline conditions are related to the common and well-reported modes under which the fixed cation groups degrade.  Quaternary ammonium groups can degrade under alkaline conditions through: 1) Hoffman elimination, where the quaternary cation is cleaved, leaving as tertiary amine, and resulting in the formation of an alkene at the carbon where the ammonium was bonded (requires the presence of alpha and beta hydrogen); 2) a direct nucleophilic reaction where the cation is completely cleaved, resulting in the formation of a tertiary amine and an alcohol at the carbon where the ammonium was bonded to the polymer backbone; and 3) through another nucleophilic substitution reaction where the adjacent organic moiety to the inorganic atom (usually a methyl or alkyl group) is cleaved resulting in the formation of a tertiary amine and an alcohol. There are other less frequently observed degradation pathways, that involve the formation of ylide intermediates, known as Sommelet–Hauser and Stevens rearrangements. All these mechanisms involve the presence of a strong base and cause the formation of tertiary amines with the subsequent loss of ion exchange capacity and ionic conductivity.

However, cation degradation alone cannot account for all the membrane deterioration encountered during AMFCs operation. It has been observed that the AEM membranes and solubilized AEM binders suffer degradation that affects the integrity of the polymer backbone. Postmortem inspection of the MEAs and probing via 1D and 2D NMR spectroscopy confirmed backbone hydrolysis.  The degradation is especially intense in the anode side of the electrolyzer. In an experiment with two AEM separators placed together, we observed preferential thinning of the membrane in contact with the anode.

We also investigated the prevailing hypothesis that the backbone hydrolysis was triggered by the presence of quaternary ammonium cations in close proximity to the aromatic rings (ether hydrolysis leading to chain scission) (3). We evaluated the alkaline stability of AEMs with six carbon pendant chains tethered to the polyphenylene backbone and  derivatized with trimethylamine (TMA) and quinuclidine (ABCO). We found that such AEMs underwent chemical degradation under alkaline conditions yielding similar products encountered in AEMs without pendant chains.

In an attempt to explain the preferential degradation and thinning of the membrane at the anode of an operating electrolyzer, we studied the effect of oxygen on AEM degradation in alkali. We compared the AEM degradation of PPO-based AEMs in oxygen-saturated 1M KOH and nitrogen-degassed 1M KOH. The presence of oxygen accelerated the degradation of the AEMs. It was found that the PPO-TMA membranes lost 50% of their ion exchange capacity after 30 days immersed in oxygen-saturated 1M KOH (at 60°C). When the membranes were kept in nitrogen-degassed 1M KOH they only lost 20% of their IEC under similar conditions. This is clear evidence that oxygen, and probably reactive oxygen species (superoxide), are actively involved in the alkaline degradation of the PPO-TMA⁺ AEMs.  NMR spectra of degraded AEMs as well as other evidence of preferential degradation in the presence of oxygen will be presented and discussed.

References

1.         A. Brisse, J. Schefold and M. Zahid, Int. J. Hydrogen Energy, 33, 5375 (2008).

2.         K. Zeng and D. Zhang, Progr. Energy Combust. Sci., 36, 307 (2010).

3.         C. G. Arges and V. Ramani, Proc. Natl. Acad. Sci. U. S. A., 110, 2490 (2013).