N-Heterocyclic Carbene Ligands’ Electronic Effects on Metallopolymer Anion Exchange Membranes

Abstract

Metallopolymers are intriguing prospective materials for use as anion exchange membranes (AEMs) in fuel cells and water electrolysis applications. Metallopolymers potentially offer high ion conductivity due to the multivalent nature of metal cations; however, similar to organic cations, the durability of the organic component can be compromised by the alkaline environment. To develop AEMs with long operational lifetimes, the fundamental relationship between the metal−ligand pair and alkaline stability needs to be understood. Here, we synthesize metallopolymers with different N-heterocyclic carbene ligand side chains that are connected to gold cations to study the ligand electronic effects on the AEM relevant properties. Interestingly, when tested under the same conditions, the different metallopolymers present markedly different alkaline stabilities, ion conductivities, and water uptake values. Explanation for such discrepancies is provided based on data from organometallic complexes. Designing optimized complexes with the correct electronic parameters can advance the development of practical AEMs for alkaline fuel cells and water electrolyzers.

Polymer Backbone Chemistry Shapes the Alkaline Stability of Metallopolymer Anion‐Exchange Membranes

“…The precursor haloalkyl polymers (P−X) were prepared using different strategies. Functionalized PS was made by copolymerization of styrene with VBC [14,19] . PPO was directly functionalized using N‐bromosuccinimide (NBS) [21] .…”

“…To understand the number of NHC ligands per gold center, the Au to N ratio was used [19] . If a single NHC ligand per gold atom is present, an Au to N mass ratio 7.04 is expected.…”

“…Functionalized PS was made by copolymerization of styrene with VBC. [14,19] PPO was directly functionalized using N-bromosuccinimide (NBS). [21] PBPAÀ Br was synthesized by acid-catalyzed Friedel-Crafts polycondensation.…”

“…In previous studies, we have tested the alkaline stability of numerous metallopolymers based on metals with low oxophilicity, [18] and polymers having N‐heterocyclic carbene (NHC) coordinated gold ions were demonstrated to be the most stable amongst the different low oxophilicity tested metal cations [14] . We also studied the effect of the heterocycle structure in the NHC ligands, and found that imidazole, which forms the shortest C−Au bond, was found to better withstand the harsh alkaline environment applied to the AEM, leading to lower gold leaching upon exposure to a strong base [19] . In this study, we look at the effect of the polymer chemistry, typically a bystander in the reaction between hydroxide and cation.…”

“…[14] We also studied the effect of the heterocycle structure in the NHC ligands, and found that imidazole, which forms the shortest CÀ Au bond, was found to better withstand the harsh alkaline environment applied to the AEM, leading to lower gold leaching upon exposure to a strong base. [19] In this study, we look at the effect of the polymer chemistry, typically a bystander in the reaction between hydroxide and cation. For that, different polymers are prepared, all typically used as AEMs, and all having the same NHC precursor in the side chains.…”