Virtual Issue: Designing Polymers for Use in Electrochemical Energy Storage Devices

Anion Exchange Membranes Based on Bis-Imidazolium and Imidazolium-Functionalized Poly(phenylene oxide) for Vanadium Redox Flow Battery Applications

Although the Nafion membrane has a high energy efficiency, long service life, and operational flexibility when applied for vanadium redox flow battery (VRFB) applications, its applications are limited due to its high vanadium permeability. In this study, anion exchange membranes (AEMs) based on poly(phenylene oxide) (PPO) with imidazolium and bis-imidazolium cations were prepared and used in VRFBs. PPO with long-pendant alkyl-side-chain bis-imidazolium cations (BImPPO) exhibits higher conductivity than the imidazolium-functionalized PPO with short chains (ImPPO). ImPPO and BImPPO have a lower vanadium permeability (3.2 × 10^-9 and 2.9 × 10^-9 cm² s^-1) than Nafion 212 (8.8 × 10^-9 cm² s^-1) because the imidazolium cations are susceptible to the Donnan effect. Furthermore, under the current density of 140 mA cm^-2, the VRFBs assembled with ImPPO- and BImPPO-based AEMs exhibited a Coulombic efficiency of 98.5% and 99.8%, respectively, both of which were higher than that of the Nafion212 membrane (95.8%). Bis-imidazolium cations with long-pendant alkyl side chains contribute to hydrophilic/hydrophobic phase separation in the membranes, thus improving the conductivity of membranes and the performance of VRFBs. The VRFB assembled with BImPPO exhibited a higher voltage efficiency (83.5%) at 140 mA cm^-2 than that of ImPPO (77.2%). These results of the present study suggest that the BImPPO membranes are suitable for VRFB applications.

Photoinduced Electron Transfer and Changes in Surface Free Energy in Polythiophene-Polyviologen Bilayered Thin Films

Bipyridiniums, also known as viologens, are well-documented electron acceptors that are generally easy to synthesize on a large scale and reversibly cycle between three oxidation states (V²⁺, V^•+, and V⁰). Accordingly, they have been explored in a number of applications that capitalize on their dynamic redox chemistry, such as redox-flow batteries and electrochromic devices. Viologens are also particularly useful in photoinduced electron transfer (PET) processes and therefore are of interest in photovoltaic applications that typically rely on electron-rich donors like polythiophene (PTh). However, the PET mechanism and relaxation dynamics between interfacing PTh and viologen-based thin films has not been well studied as a function of thickness of the acceptor layer. Here, a novel, bilayered thin film composite was fabricated by first spin-coating PTh onto glass slides, followed by spin-coating and curing polyviologen (PV)-based micron-sized films of variable thicknesses (0.5-11.3 μm) on top of the PTh layer. The electron-transfer mechanism and relaxation dynamics from the PTh sublayer into the upper PV film were investigated using femtosecond transient absorption (fTA) spectroscopy and electrochemistry to better understand how the charge-transfer/relaxation lifetimes could be extended using thicker PV acceptor films. The fTA experiments were performed under inert N₂ conditions as well as in ambient O₂. The latter shortened the lifetimes of the electrons in the PV layer, presumably due to O₂ triplet-based trap sites. Contact angle measurements using H₂O and MeI were also performed on top of the bilayered films to measure changes in surface free energy that would aid the assessment related to efficiency of the combined processes involving light penetration, photoexcitation, electron mobility, and relaxation from within the bilayered thin films. Insights gained from this work will support the development of future devices that employ viologen-based materials as an alternative electron-acceptor that is both easily processable and scalable.

Energy storage emerging: A perspective from the Joint Center for Energy Storage Research

Energy storage is an integral part of modern society. A contemporary example is the lithium (Li)-ion battery, which enabled the launch of the personal electronics revolution in 1991 and the first commercial electric vehicles in 2010. Most recently, Li-ion batteries have expanded into the electricity grid to firm variable renewable generation, increasing the efficiency and effectiveness of transmission and distribution. Important applications continue to emerge including decarbonization of heavy-duty vehicles, rail, maritime shipping, and aviation and the growth of renewable electricity and storage on the grid. This perspective compares energy storage needs and priorities in 2010 with those now and those emerging over the next few decades. The diversity of demands for energy storage requires a diversity of purpose-built batteries designed to meet disparate applications. Advances in the frontier of battery research to achieve transformative performance spanning energy and power density, capacity, charge/discharge times, cost, lifetime, and safety are highlighted, along with strategic research refinements made by the Joint Center for Energy Storage Research (JCESR) and the broader community to accommodate the changing storage needs and priorities. Innovative experimental tools with higher spatial and temporal resolution, in situ and operando characterization, first-principles simulation, high throughput computation, machine learning, and artificial intelligence work collectively to reveal the origins of the electrochemical phenomena that enable new means of energy storage. This knowledge allows a constructionist approach to materials, chemistries, and architectures, where each atom or molecule plays a prescribed role in realizing batteries with unique performance profiles suitable for emergent demands.

Influence of Host Polarity on Correlating Salt Concentration, Molecular Weight, and Molar Conductivity in Polymer Electrolytes

We use coarse-grained molecular dynamics simulations to study the effect of salt concentration and host polymer molecular weight on ion transport in polymer electrolytes. We find that increasing salt concentration or molecular weight similarly slows polymer dynamics across a wide range of host polarities, and that the resulting relaxation times display a correlation to the product of the salt concentration and polymer molecular weight. However, we find that molar conductivity only decreases with polymer dynamics at high polarities but is uncorrelated with the latter at low polarities. We attribute such differences to the variation in ionic aggregation between high and low polarity electrolytes. At low polarity, ionic dissociation significantly increases with molecular weight and salt concentration, offsetting the slowdown in polymer dynamics and yielding the observed insensitivity of molar conductivity. However, at high polarity, ions are mostly dissociated, independent of either molecular weight or salt concentration, thereby strongly coupling molar conductivity to polymer dynamics.