Tunable OH– Transport and Alkaline Stability by Imidazolium-Based Groups of Poly(2,6-dimethyl-1,4-phenylene oxide) Anion Exchange Membranes: A Molecular Dynamics Simulation

Rational Understanding Hydroxide Diffusion Mechanism in Anion Exchange Membranes during Electrochemical Processes with RDAnalyzer

Enhancing the understanding of hydroxide transport mechanisms in anion exchange membranes (AEMs) is beneficial for the rational design of high-performance AEMs in the renewable energy system. However, the high complexity and lack of adequate analytic tools make it challenging to clarify different mechanisms unambiguously. Herein, we developed an in-house toolkit, the Reactive Diffusion Analyzer (RDAnalyzer), to conduct an effective analysis of hydroxide diffusion mechanisms from ReaxFF molecular dynamic simulations. Using the experimentally well-synthesized T20NC6NC5N as a model system, we successfully decoupled the hydroxide diffusion mechanisms into free Vehicular and free Grotthuss, as well as associated Vehicular and associated Grotthuss, which was not yet achieved previously. Meanwhile, RDAnalyzer managed to specifically identify the drift length of hydroxide species for each mechanism under the electric field, which worked as a useful variable for calculating the conductivity of AEMs. Our theoretically predicted conductivity for T20NC6NC5N agrees reasonably with experimental results, indicating the reliability of RDAnalyzer. This work not only provides a rational understanding of the complex hydroxide diffusion mechanisms in AEMs but also holds the potential to guide the rational design of high-performance AEMs with computations.

Biomass Solid-State Electrolyte with Abundant Ion and Water Channels for Flexible Zinc-Air Batteries

Flexible zinc-air batteries are the leading candidates as the next-generation power source for flexible/wearable electronics. However, constructing safe and high-performance solid-state electrolytes (SSEs) with intrinsic hydroxide ion (OH-) conduction remains a fundamental challenge. Herein, by adopting the natural and robust cellulose nanofibers (CNFs) as building blocks, the biomass SSEs with penetrating ion and water channels are constructed by knitting the OH--conductive CNFs and water-retentive CNFs together via an energy-efficient tape casting. Benefiting from the abundant ion and water channels with interconnected hydrated OH- wires for fast OH- conduction under a nanoconfined environment, the biomass SSEs reveal the high water-uptake, impressive OH- conductivity of 175 mS cm-1 and mechanical robustness simultaneously, which overcomes the commonly existed dilemma between ion conductivity and mechanical property. Remarkably, the flexible zinc-air batteries assemble with biomass SSEs deliver an exceptional cycle lifespan of 310 h and power density of 126 mW cm-2. The design methodology for water and ion channels opens a new avenue to design high-performance SSEs for batteries.

Electrostatic Potential of Functional Cations as a Predictor of Hydroxide Diffusion Pathways in Nanoconfined Environments of Anion Exchange Membranes

Nanoconfined anion exchange membranes (AEMs) play a vital role in emerging electrochemical technologies. The ability to control dominant hydroxide diffusion pathways is an important goal in the design of nanoconfined AEMs. Such control can shorten hydroxide transport pathways between electrodes, reduce transport resistance, and enhance device performance. In this work, we propose an electrostatic potential (ESP) approach to explore the effect of the polymer electrolyte cation spacing on hydroxide diffusion pathways from a molecular perspective. By exploring cation ESP energy surfaces and validating outcomes through prior ab initio molecular dynamics simulations of nanoconfined AEMs, we find that we can achieve control over preferred hydroxide diffusion pathways by adjusting the cation spacing. The results presented in this work provide a unique and straightforward approach to predict preferential hydroxide diffusion pathways, enabling efficient design of highly conductive nanoconfined AEM materials for electrochemical technologies.

Molecular Dynamics Study on the Effect of Cyclic Conducting Moieties on Poly(2,6-dimethyl-1,4-phenylene oxide) Anion Exchange Membranes

We investigate PPO quaternized with different azoles (five-membered heterocyclic compounds) with a different odd number of Nitrogen atoms (1N-pyrrole, 3N-1,2,3-triazole, and 5N-pentazole) to form pyrrolium-PPO(py-PPO), 1,2,3,-triazolium-PPO(tri-PPO) and pentazolium-PPO(pen-PPO) AEMs, using molecular dynamics (MD) simulations to compare and evaluate their OH- transport via the vehicular mechanism. OH- diffusivity at the hydration level λ = 12 is 3.10 × 10-10 m2/s, 1.92 × 10-10 m2/s m2/s, and 1.91 × 10-10 m2/s for py-PPO, tri-PPO, and pen-PPO, respectively. This trend is due to the shorter distance between adjacent groups of py-PPO (7.5 Å) leading to an efficient hydroxide transport than tri-PPO (7.8 Å) and pen-PPO (8.1 Å) at λ = 12. Also, this trend is justified by the smaller average number of clusters for py-PPO (1.2), smaller than tri-PPO(2.0), and pen-PPO (1.5) at λ = 12, which suggests better connectivity and hence better conductivity.

Molecular Modeling in Anion Exchange Membrane Research: A Brief Review of Recent Applications

Anion Exchange Membrane (AEM) fuel cells have attracted growing interest, due to their encouraging advantages, including high power density and relatively low cost. AEM is a polymer matrix, which conducts hydroxide (OH-) ions, prevents physical contact of electrodes, and has positively charged head groups (mainly quaternary ammonium (QA) groups), covalently bound to the polymer backbone. The chemical instability of the quaternary ammonium (QA)-based head groups, at alkaline pH and elevated temperature, is a significant threshold in AEMFC technology. This review work aims to introduce recent studies on the chemical stability of various QA-based head groups and transportation of OH- ions in AEMFC, via modeling and simulation techniques, at different scales. It starts by introducing the fundamental theories behind AEM-based fuel-cell technology. In the main body of this review, we present selected computational studies that deal with the effects of various parameters on AEMs, via a variety of multi-length and multi-time-scale modeling and simulation methods. Such methods include electronic structure calculations via the quantum Density Functional Theory (DFT), ab initio, classical all-atom Molecular Dynamics (MD) simulations, and coarse-grained MD simulations. The explored processing and structural parameters include temperature, hydration levels, several QA-based head groups, various types of QA-based head groups and backbones, etc. Nowadays, many methods and software packages for molecular and materials modeling are available. Applications of such methods may help to understand the transportation mechanisms of OH- ions, the chemical stability of functional head groups, and many other relevant properties, leading to a performance-based molecular and structure design as well as, ultimately, improved AEM-based fuel cell performances. This contribution aims to introduce those molecular modeling methods and their recent applications to the AEM-based fuel cells research community.