Alkaline Stability and Fuel Cell Performance of Piperidinium-Based Anion Exchange Membranes

Abstract

There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cells (AMFCs) and in other energy conversion and storage systems such as redox flow batteries (RFBs), alkaline water electrolyzers (AWEs) and reverse electrodialysis (RED) cells. The most commonly used cation group in AEMs is the benzyl trimethylammonium cation. However, it had been shown that quaternary ammonium-based AEMs are sensitive towards Hofmann elimination [1] and direct nucleophilic elimination reactions [2] that result in loss of ion exchange capacity (IEC) and ionic conductivity. To solve the alkaline stability issue inherent to quaternary-ammonium-group-containing AEMs, alternative cations such as piperidinium-based cations has been proposed and investigated. The piperidinium-based AEM was able to maintain its ca. 90% of its initial IEC after immersion into 1M KOH at 80 ºC for 30 days. Such piperidinium-based AEMs showed higher alkaline stability than benzyl-trimethylammonium-based AEM (ca. 20% degradation in 1M KOH at 60 ºC for 30 days) [3]. The improved alkaline stability was mainly attributed to the avoidance of attaching quaternary ammonium onto benzylic position. The planar structure of piperidinium-based cation slows down the β-elimination phenomenon. Also, there is no ether linkage with the polymer backbone and hence, the stability of polymer backbone is superior. The chloride ion conductivity of the piperidinium-based AEM was 65 mS/cm at 80 ºC under an IEC of 2.26 mmol/g. An AEMFC was assembled using our piperidinium-based AEM with Pt/C catalysts used for both anode and cathode. A peak power density of 700 mW/cm2 with 2 A/cm2 current density was obtained at 70 ºC without any backpressure.