Molecular-Level Control over Ionic Conduction and Ionic Current Direction by Designing Macrocycle-Based Ionomers

Abstract

Poor ionic conductivity of the catalyst-binding, sub-micrometer-thick ionomer layers in energy conversion and storage devices is a huge challenge. However, ionomers are rarely designed keeping in mind the specific issues associated with nanoconfinement. Here, we designed nature-inspired ionomers (calix-2) having hollow, macrocyclic, calix[4]arene-based repeat units with precise, sub-nanometer diameter. In ≤100 nm-thick films, the in-plane proton conductivity of calix-2 was up to 8 times higher than the current benchmark ionomer Nafion at 85% relative humidity (RH), while it was 1–2 orders of magnitude higher than Nafion at 20–25% RH. Confocal laser scanning microscopy and other synthetic techniques allowed us to demonstrate the role of macrocyclic cavities in boosting the proton conductivity. The systematic self-assembly of calix-2 chains into ellipsoids in thin films was evidenced from atomic force microscopy and grazing incidence small-angle X-ray scattering measurements. Moreover, the likelihood of alignment and stacking of macrocyclic units, the presence of one-dimensional water wires across this macrocycle stacks, and thus the formation of long-range proton conduction pathways were suggested by atomistic simulations. We not only did see an unprecedented improvement in thin-film proton conductivity but also saw an improvement in proton conductivity of bulk membranes when calix-2 was added to the Nafion matrices. Nafion–calix-2 composite membranes also took advantage of the asymmetric charge distribution across calix[4]arene repeat units collectively and exhibited voltage-gating behavior. The inclusion of molecular macrocyclic cavities into the ionomer chemical structure can thus emerge as a promising design concept for highly efficient ion-conducting and ion-permselective materials for sustainable energy applications.

Ion transport on self-assembled block copolymer electrolytes with different side chain chemistries

“…15 Alternative electric circuit equivalent models have been used for thin film Nafion, PFSA, and lithium salt imbibed PEO conductivity. 21,[30][31][32] Figure 3a shows the Nyquist plot obtained from EIS measurements for the xLPSbP2VP/alkoxy BCE when interfaced with DI water along with the multivariable equivalent model circuit. In this figure, R film is the resistance and CPE film the dielectric behavior of the polymeric film between the electrodes; CPE dl models the ion depletion double-layer at the polymer-electrode interface (accumulation of charge at the electrode); R e models the electrode resistance (e.g., electrical contact).…”

“…15 Alternative electric circuit equivalent models have been used for thin film Nafion, PFSA, and lithium salt imbibed PEO conductivity. 21,[30][31][32] Fig. 3a shows the Nyquist plot obtained from EIS measurements for the xLPSbP2VP/alkoxy BCE when interfaced with DI water along with the multivariable equivalent model circuit.…”

Engineering Ionomer–Substrate Interface to Improve Thin-Film Proton Conductivity in Proton Exchange Membrane Fuel Cells

“…To address these interfacial weak ion and gas transport issues within ionomer thin films, attempts have been made to alter the structure and chemistry of catalysts ,, and ionomers. ,,, Interfacial restructuring is emerging as a promising approach to negate interfacial chain pinning and its detrimental effects (catalyst poisoning and slow ORR). For example, modifying the electrocatalysts with an inert skin layer, rendering substrates with hydrophobic ,− /electrostatically repulsive moieties, or favorably orienting the lamellar structure , weakened the specific adsorption of −SO 3 – anions of Nafion chains onto the substrate , and increased the overall proton conductivity as well as ORR activity .…”

In-Situ Ionomer-Free Catalyst-Coated Membranes for Anion Exchange Membrane Water Electrolyzers

“…9−13 AEIs act as the physical binder to immobilize the active CLs on the surface of AEMs or electrode substrates, effectively maximizing catalyst utilization and reducing the interfacial resistance (i.e., ohmic resistance of the electrolyzer system). 14,15 In addition, AEIs create a hydroxide ion (OH − ) transfer pathway at the AEM−electrode interface for the transport of the reaction intermediates. 11,16,17 However, there are several drawbacks to the extensive use of AEIs in conventional MEA fabrication methods, including a catalystcoated substrate (c-CCS) and a catalyst-coated membrane (c-CCM).…”

“…MEAs are the active components of the AEMWEs, consisting of catalyst layers (CLs), gas diffusion layers (GDLs), anion exchange membranes (AEMs), and anion exchange ionomers (AEIs), which affect the efficiency and durability of the AEMWEs. Among them, AEIs play a prominent role in the advancement of MEA fabrication but are considered a performance-limiting factor of AEMWEs. − AEIs act as the physical binder to immobilize the active CLs on the surface of AEMs or electrode substrates, effectively maximizing catalyst utilization and reducing the interfacial resistance (i.e., ohmic resistance of the electrolyzer system). , In addition, AEIs create a hydroxide ion (OH – ) transfer pathway at the AEM–electrode interface for the transport of the reaction intermediates. ,, However, there are several drawbacks to the extensive use of AEIs in conventional MEA fabrication methods, including a catalyst-coated substrate ( c -CCS) and a catalyst-coated membrane ( c -CCM). For example, the use of AEIs in the MEA fabrication process limits the exposure of catalyst active sites for electrochemical reactions and the mass transport of gaseous products (O 2 and H 2 ). − Moreover, most AEIs consist of an unstable phenylic backbone that undergoes oxidation at high operating cell voltages (>2.1 V).…”