Novel Polyamide Proton Exchange Membranes with Bi-Functional Sulfonimide Bridges for Fuel Cell Applications

Abstract

We design and successfully synthesize non-fluorinated polyamides with controlled crosslinking using sulfonimide as a bi-functional linker to interconnect polymer backbones and as a bridge for proton conduction. We show that the bi-functional linkers are highly beneficial not only for mechanical enforcement of the proton exchange membranes but also for enhancement of water retention capacity. With an appropriate degree of crosslinking, higher water retention capacity than that of commercial Nafion membranes can be obtained. The maximum proton conductivity of the membranes is found to be as high as 0.139 S cm⁻¹ at 80 °C, almost the same as that of a Nafion 117 membrane. Excellent performance with the bi-functional polymer membranes in an air-breathing direct methanol fuel cell prototype, comparable to the performance of a Nafion 117 membrane, is demonstrated.

Introduction

Proton exchange membrane (PEM) fuel cell is one of the most efficient energy conversion devices and has become a subject of intense research and development activities worldwide [1], [2], [3], [4], [5], [6]. As the heart of PEM fuel cells, PEMs with high durability and low costs are the focal point of the fuel cell technology development since the high material cost and low proton conductivity at low humidity, arising from poor water retention capacity, of perfluorosulfonic acid ionomers, associated with the commercially most successful PEM, Nafion, still prevent PEM fuel cells from a large scale applications [7], [8], [9], [10], [11], [12]. Furthermore, water-swelling, which gives rise to serious dimensional change and thus leads to significant shortening of membrane lifespan, is another challenge inherently associated with the proton conduction process of Nafion [13], [14]. Significant effort has been made to modify Nafion membranes by heteropolyacids [15], [16], [17],silicon-based compounds [18], [19], [20], [21], [22] and other organic/inorganic materials [23], [24], [25], [26], [27] to physically block water inside the membrane and suppress the membrane dimensional swelling. Another effective method to minimize water swelling is to develop crosslinked polymer proton conductors [28], [29], [30] as alternatives to Nafion. It is deemed beneficial for mechanical enforcement of membranes by crosslinking polymer chains functionalized with sulfonated groups via a van der Waals interaction [31], [32], [33] or covalent bonds [34], [35], [36], [37], [38], [39]. Phu et al. [36] synthesized crosslinked sulfonated poly(phenylene sulfide sulfone nitrile) with considerable reduction of water swelling dimensional change from 43% without crosslinking to 23% at 90 °C. A recent study on photo-induced crosslinked sulfonated poly(arylene ether sulfone) [37] also indicated that dimensional change of a membrane caused by water swelling can be reduced by increasing the degree of crosslinking via irradiation time control. Unfortunately, the significant suppression of water swelling dimensional change comes at the price of smaller water retention capacity, leading to considerably reduced proton conductivity.

In this paper, we report a successful synthesis of novel non-fluorinated crosslinked polymers with sulfonimide group functionalized hydrophilic chains that act as bi-functional bridges to link sulfonated backbones. The sulfonimide bi-functional bridges, of which the effective proton conduction has been demonstrated [40], [41], [42], [43], [44], can not only reinforce the mechanical strength and suppress the dimensional changes of the polymer membrane, but also chemically retain water molecules and support network build-up of proton conducting channels in the polymer matrix as illustrated in Scheme 1a. The synthesized sulfonimide bi-functional crosslinked (BXL) polyamide PEMs exhibited great dimensional stability as expected. More importantly, the BXL PEMs displayed better water retention capacity than those of non-crosslinked (NXL) polyamide PEMs and a Nafion 117 membrane. With a 20% degree of crosslinking of the BXL PEM, the proton conductivity of 0.139 S cm⁻¹, comparable to that of a commercial Nafion 117 membrane, was obtained at 80 °C. An air-breathing passive direct methanol fuel cell (DMFC) prototype using a BXL PEM as electrolyte was fabricated with a comparable performance to that using a commercial Nafion 117 membrane.