Poly(aryl-co-aryl piperidinium) copolymers for anion exchange membrane fuel cells

Abstract

Anion exchange membrane fuel cells (AEMFCs) have seen increasing attention in recent years as a possible low-cost energy conversion device. Anion exchange membranes (AEMs) as one of the key parts of AEMFC are required with high ionic conductivity, durable chemical stability, and tough mechanical properties, which highly depend on the polymer backbone and cationic groups. Aromatic polymers with ether bonds have been proved to be susceptible to the attack of OH-. Thus, polymers with ether-free main-chain are highly recommended for durable AEM applications. Additionally, highly stable cationic groups are considered to be the guarantee of AEMFC with longevity. In chapter 2, a series of ether-free poly(diphenyl-terphenyl piperidinium) (PDTP) copolymers were synthesized for high-performance AEMFC application. PDTP based AEMs with ether-free backbone and durable piperidinium group exhibited excellent alkaline stability. The introduction of aliphatic chain-containing monomers in the polymer increases the flexibility of AEM and decreases the phenyl adsorption of ionomer in the catalyst. The present AEMFCs reach outstanding peak power densities of 2.58 W cm-2 (> 7.6 A cm-2 current density) and 1.38 W cm-2 at 80 oC in H2-O2 and H2-air, respectively. Currently, many reported AEMs possessed high ion conductivity (>100 mS cm-1) as well as satisfactory ex-situ alkaline stability (stable in 1 M NaOH at 80 oC for 500 h) and dimensional stability, while displayed unsatisfactory cell power density and in-situ durability. That is, the majority of AEMFCs may suffer from undiagnosed performance loss, such as inappropriate operating conditions and irrational cell design. In chapter 3, we systematically explored the effect of composition and morphology of catalyst layer, operation conditions (relative humidity, backpressure, gas flowrate) on fuel cell performance based on poly(aryl-co-aryl piperidinium) (c-PAP) copolymers. c-PAP-based AEMFCs with optimum catalyst composition achieve a peak power density (PPD) of 2.70 W cm-2 at 80 oC in H2-O2. Moreover, these AEMFCs can be operated under a 0.2 A cm-2 current density at 60 oC for over 300 h with a voltage decay rate of ~300 V h-1. On the basis of the optimized operating conditions in chapter 3, the effects of aliphatic chain length in c-PAP backbone on the fuel cell performance were explored in chapter 4. Poly(aryl-co-terphenyl piperidinium)-x (PDnTP-x) AEMs with different alkyl spacers (n=0, 1, 2, 6, 10) in the backbone were successfully synthesized. AEMs with long alkyl chains (n=6 or 10) shows superiority in dimensional stability and gas tightness (H2 permeability <10 Barrer, 1 Barrer=10−10 cm3 (STP) cm cm−2 s−1 cm Hg−1), while short chain type AEMs possesses higher ion conductivities (>150 mS cm-1 at 80 oC). Short-chain-type PD0TP-x based AEMFC reached a power density of 2.67 W cm-2 at 80 oC, which is much higher than that of long-chain-type fuel cells (1.7 W cm-2). Importantly, the short-chain type PDnTP-x AEM can operate stably under a 0.4 A cm-2 current density for 220 h with a low voltage decay rate of ~77 V h-1. The longevity of AEMFC is highly related to the alkaline stability and mechanical durability of AEMs. In chapter 5, we demonstrated an in-situ crosslinking strategy for improving the chemical and mechanical properties of c-PAP based AEMs (x-PAP-PS). Specifically, x-PAP-PS AEMs simultaneously possess high ion conductivity (>150 mS cm-1), tensile strength (>80 MPa), and chemical and mechanical durability. Due to the internal cross-linked networks and smaller ionic channels, crosslinked AEMs possess excellent water retention capacity and the corresponding AEMFCs obtained PPDs over 1 W cm-2 based on A/C Pt/C at 30/30% RH at 90 oC. Different from the crosslinking method to enhance mechanical properties in chapter 5, reinforced strategy was applied in chapter 6 for developing robust and durable membranes for AEMFC. Herein, c-PAP solution was filled into microporous polyethylene support for fabrication high quality reinforced composited membranes (RCMs). The prepared RCMs simultaneously possessed ultrahigh tensile strength (~114 MPa) and elongation at break (~159%) along with excellent gas tightness and mechanical durability. The RCM based AEMFC displayed excellent chemical and mechanical durability after running the cells at a current density of 0.6 A cm-2 at 70 oC for ~360 h. Lastly, in chapter 7, the future studies of AEMs were discussed. Currently, in-situ stability of AEMFC is a thorny problem, which hinders the development of AEMFC. The key issue is the imbalanced water environment in AEMFC. Thus, developing an effective method or designing new materials which can well tackle the water management issue is our future work.