Designing Advanced Alkaline Polymer Electrolytes for Fuel Cell Applications

Abstract

Although the polymer electrolyte fuel cell (PEFC) is a superior power source for electric vehicles, the high cost of this technology has served as the primary barrier to the large-scale commercialization. Over the last decade, researchers have pursued lower-cost next-generation materials for fuel cells, and alkaline polymer electrolytes (APEs) have emerged as an enabling material for platinum-free fuel cells. To fulfill the requirements of fuel cell applications, the APE must be as conductive and stable as its acidic counterpart, such as Nafion. This benchmark has proved challenging for APEs because the conductivity of OH(-) is intrinsically lower than that of H(+), and the stability of the cationic functional group in APEs, typically quaternary ammonia (-NR(3)(+)), is usually lower than that of the sulfonic functional group (-SO(3)(-)) in acidic polymer electrolytes. To improve the ionic conductivity, APEs are often designed to be of high ion-exchange capacity (IEC). This modification has caused unfavorable changes in the materials: these high IEC APEs absorb excessive amounts of water, leading to significant swelling and a decline in mechanical strength of the membrane. Cross-linking the polymer chains does not completely solve the problem because stable ionomer solutions would not be available for PEFC assembly. In this Account, we report our recent progress in the development of advanced APEs, which are highly resistant to swelling and show conductivities comparable with Nafion at typical temperatures for fuel-cell operation. We have proposed two strategies for improving the performance of APEs: self-cross-linking and self-aggregating designs. The self-cross-linking design builds on conventional cross-linking methods and works for APEs with high IEC. The self-aggregating design improves the effective mobility of OH(-) and boosts the ionic conductivity of APEs with low IEC. For APEs with high IEC, cross-linking is necessary to restrict the swelling of the membrane. In our self-cross-linking design, a short-range cross-linker, tertiary amino groups, is grafted onto the quaternary ammonia polysulfone (QAPS) so that the cross-linking process can only occur during membrane casting. Thus, we obtain both the stable ionomer solution and the cross-linked membrane. The self-cross-linked QAPS (xQAPS) possesses a tight-binding structure and is highly resistant to swelling: even at 80 °C, the membrane swells by less than 3%. For APEs with low IEC, the key is to design efficient OH(-) conducting channels. In our self-aggregating design, long alkyl side-chains are attached to the QAPS. Based on both the transmission electron microscopy (TEM) observations and the molecular dynamics (MD) simulations, these added hydrophobic groups effectively drive the microscopic phase separation of the hydrophilic and hydrophobic domains and produce enlarged and aggregated ionic channels. The ionic conductivity of the self-aggregated QAPS (aQAPS) is three-fold higher than that of the conventional QAPS and is comparable to that of Nafio...

Highly Stable, Anion Conductive, Comb-Shaped Copolymers for Alkaline Fuel Cells

“…Although the C16D40 ionomer-based alkaline fuel cell had better initial performance with higher peak power density, its durability (lifetime) in a single cell was only 12 h which could be attributed to the poor compatibility between long alkyl chain and polymer backbone causing mechanical failure of the electrode or the possible oxidative degradation the thin ionomer layer under fuel cell operating conditions where the steric shielding in the ionomer layer is not as effective as it is in the membrane. 13 On the contrary, the C6D60 ionomer kept 90% of its initial performance after 60 h testing, as shown in Figure 12. Since the C6D60 sample qualitatively had better mechanical properties in membrane samples than the C16D40 sample and the membrane degradation of these samples was not greatly different, we might conclude that mechanical properties of the ionomer in the fuel cell electrode may play a role in device lifetime.…”

“…7,8 Alkaline fuel cells (AFCs) have received significant interest in recent years relative to acidic fuel cells, 9−14 because of advantages when operating under alkaline conditions, which include enhancement of the electrode reaction kinetics, especially at the cathode, and the catalysts are not subjected to corrosion at high pH. 9,12,13 Consequently, non-noble metals or inexpensive metal oxides can be used as catalysts to greatly reduce the cost of the device. 13−16 In addition, high energy density liquids and gases such as ethanol, hydrazine, and ammonia can be adopted as fuels.…”

Two-dimensional NMR spectroscopy reveals cation-triggered backbone degradation in polysulfone-based anion exchange membranes

“…Approaches have included investigating new fixed cation chemistries and/or modification of the polymer electrolyte membrane network (e.g., cross-linking, spacer chain pendants, and block copolymers) (15-17). These efforts have facilitated an order of magnitude gain in hydroxide ion conductivity (roughly 10) (15,18,19). AEMs have traditionally exhibited poor chemical stability in alkaline environments.…”

Strategies for Developing New Anion Exchange Membranes and Electrode Ionomers

“…8 The work of Varcoe and Slade on new materials, along with reports of noble metal-free fuel cells and new membranes by Zhuang, et al, 9 generated tremendous interest across the globe and teams of electrochemical researchers and polymer chemists quickly formed. Today, radiation grafted membranes from Varcoe, 10 and Zhuang's membranes 11 and catalysts, 12 lead the field because of the breakthroughs and device insights made possible by the advent of these new materials.…”