Confronting Sulfur Electrode Passivation and Li Metal Electrode Degradation in Lithium-Sulfur Batteries Using Thiocyanate Anion.

Salt anions with a high donor number (DN) enable high sulfur utilization in lithium-sulfur (Li-S) batteries by inducing three-dimensional (3D) Li2 S growth. However, their insufficient compatibility with Li metal electrodes limits their cycling stability. Herein, a new class of salt anion, thiocyanate (SCN- ), is presented, which features a Janus character of electron donor and acceptor. Due to a strong Li+ coordination by SCN- and the direct interaction of SCN- with polysulfide anions, the LiSCN electrolyte has a remarkably high lithium polysulfide solubility. This electrolyte induces 3D Li2 S formation and ameliorates cathode passivation, even more than Br- , a typical high DN anion. Moreover, SCN- forms a Li3 N-enriched stable SEI layer at the surface of the Li metal electrode, enhancing cycling stability. A Li-S battery with the LiSCN electrolyte shows high current density operation (2.54 mA cm⁻2 ) with high discharge capacity (1133 mAh g⁻1 ) and prolonged cycle life (100 cycles). This work demonstrates that the cathode and anode performance in a Li-S battery can be simply and concurrently enhanced by the single salt anion.

Modulation of Solvation Structure and Electrode Work Function by an Ultrathin Layer of Polymer of Intrinsic Microporosity in Zinc Ion Batteries.

Zinc ion batteries are promising candidates for large-scale energy storage systems. However, they suffer from the critical problems of insufficient cycling stability due to internal short-circuiting by zinc dendrites and zinc metal orphaning. In this work, a polymer of intrinsic microporosity (PIM-1) is reported as an ion regulating layer and an interface modulator, which promotes a uniform Zn plating and stripping process. According to spectroscopic analyses and computational calculations, PIM-1 enhances the reaction kinetics of a Zn metal electrode by altering the solvation structure of Zn2+ ions and increasing the work function of the Zn surface. As a result, the PIM-1 coating significantly improves the cyclability (1700 h at 0.5 mA cm-2 ) and Coulombic efficiency (99.6% at 3 mA cm-2 ) of the Zn/Zn2+ redox reaction. Moreover, the PIM-1 coated Zn operates for more than 200 h at 70% Zn utilization even under 10 mA cm-2 and 110 h at 95% Zn utilization of the Zn metal electrode. A Zn||V2 O5 full cell employing the PIM-1 layer exhibits seven times longer cycle life compared to the cell using bare Zn. The findings in this report demonstrate the potential of microporous materials as a key ingredient in the design of reversible Zn electrodes.

Hierarchical Wrinkle-Structured Catalyst Layer/Membrane Interface for Ultralow Pt-Loading Polymer Electrolyte Membrane Fuel Cells (PEMFCs).

The optimal architecture of three-dimensional (3D) interface between a polymer electrolyte membrane (PEM) and catalyst layer (CL) is one of the most important issues to improve PEM fuel cells' (PEMFCs) performance. Here, we report the fabrication of hierarchical wrinkled PEM/CL interface over a large area. We fabricated the hierarchical wrinkles on a multiscale from nanometers to micrometers by bottom-up-based facile, scalable, and simple method. Notably, it allows one to go beyond the limit of the catalyst utilization by extremely enlarged interfacial area. The resulting hierarchical wrinkled PEM/CL displays a dramatically increased electrochemically active surface area (ECSA) and power performance by the enhancement factors of 89% and 67% compared with those of flat interface, which is one of the best enhancements compared to previous PEMFCs. We believe the scalability of hierarchical wrinkled interface can be exploited to design advanced 3D interfaces for high-performance PEMFCs even with ultralow Pt-loading.

Dispersion-Solvent Control of Ionomer Aggregation in a Polymer Electrolyte Membrane Fuel Cell.

In this study, we examined the influence of the dispersion solvent in three dipropylene-glycol/water (DPG/water) mixtures, with DPG contents of 0, 50, and 100 wt%, on ionomer morphology and distribution, using dynamic light scattering (DLS) and molecular-dynamics (MD) simulation techniques. The DLS results reveal that Nafion-ionomer aggregation increases with decreasing DPG content of the solvent. Increasing the proportion of water in the solvent also led to a gradual decrease in the radius of gyration (Rg) of the Nafion ionomer due to its strong backbone hydrophobicity. Correspondingly, MD simulations predict Nafion-ionomer solvation energies of -147 ± 9 kcal/mol in water, -216 ± 21 kcal/mol in the DPG/water mixture, and -444 ± 9 kcal/mol in DPG. These results suggest that higher water contents in mixed DPG/water solvents result in increased Nafion-ionomer aggregation and the subsequent deterioration of its uniform dispersion in the solvent. Moreover, radial distribution functions (RDFs) reveal that the (-CF2CF2-) backbones of the Nafion ionomer are primarily enclosed by DPG molecules, whereas the sulfonate groups (SO3-) of its side chains mostly interact with water molecules.

Tuning the Ionomer Distribution in the Fuel Cell Catalyst Layer with Scaling the Ionomer Aggregate Size in Dispersion.

With the demands for better performance of polymer electrolyte membrane fuel cells, studies on controlling the distribution of ionomers have recently gained interest. Here, we present a tunable ionomer distribution in the catalyst layer (CL) with dipropylene glycol (DPG) and water mixtures as the ionomer dispersion medium. Dynamic light scattering and molecular dynamics simulation demonstrate that, by increasing the DPG content in the dispersion, the size of the ionomer aggregates in the dispersion is exponentially reduced because of the higher affinity of DPG for Nafion ionomers. The ionomer distribution of the resulting CLs dictates the dimensional feature of the ionomer dispersion. Although the ionomer distribution becomes more uniform with increasing the DPG content, an optimal power performance is obtained at a DPG content of 50 wt % regardless of feed humidity because of balanced proton and mass transports. As a guide for tuning the ionomer distribution, we suggest that the ionomer aggregates in the dispersion with a size close to that of the Pt/C aggregates form a highly connected ionomer network and maintain a porosity in the catalyst/ionomer aggregate, resulting in high power performance.

In-Plane Channel-Structured Catalyst Layer for Polymer Electrolyte Membrane Fuel Cells.

In this study, we present a novel catalyst layer (CL) with in-plane flow channels to enhance the mass transports in polymer electrolyte membrane fuel cells. The CL with in-plane channels on its surface is fabricated by coating a CL slurry onto a surface-treated substrate with the inverse line pattern and transferring the dried CL from the substrate to a membrane. The membrane electrode assembly with the in-plane channel-patterned CL has superior power performances in high current densities compared with an unpatterned, flat CL, demonstrating a significant enhancement of the mass-transport property by the in-plane channels carved in the CL. The performance gain is more pronounced when the channel direction is perpendicular to the flow field direction, indicating that the in-plane channels increase the utilization of the CL under the rib area. An oxygen-transport resistance analysis shows that both molecular and Knudsen diffusion can be facilitated with the introduction of the in-plane channels. The direct CL patterning technique provides a platform for the fabrication of advanced CL structures with a high structural fidelity and design flexibility and a rational guideline for designing high-performance CLs.

Ultrathin Nafion-filled porous membrane for zinc/bromine redox flow batteries.

In this work, we present a 16 µm-thick Nafion-filled porous membrane for Zn/Br redox flow batteries (ZBBs). By using molecular dynamics simulation and dynamic light scattering analysis, we rationally design Nafion solution for Nafion impregnation into a porous polypropylene (PP) separator. A void-free Nafion/PP membrane is successfully fabricated by using NMP as a solvent for the Nafion solution. The resulting membrane shows a smaller area specific resistance in comparison with 600 µm-thick, commercial SF-600 porous membrane. Due to its dense morphology, Br2 diffusivity of the Nafion/PP membrane is two orders of magnitude lower than that of SF-600, resulting in a comparable Br2 crossover in spite of 37.5 times smaller membrane thickness. As a result, the ZBB based on the Nafion/PP membrane exhibits a higher energy efficiency, demonstrating that ion exchange membrane can outperform the conventional porous membrane by reducing the membrane thickness with inexpensive porous substrate.

Three-Dimensional Interlocking Interface: Mechanical Nanofastener for High Interfacial Robustness of Polymer Electrolyte Membrane Fuel Cells.

A scalable nanofastener featuring a 3D interlocked interfacial structure between the hydrocarbon membrane and perfluorinated sulfonic acid based catalyst layer is presented to overcome the interfacial issue of hydrocarbon membrane based polymer electrolyte membrane fuel cells. The nanofastener-introduced membrane electrode assembly (MEA) withstands more than 3000 humidity cycles, which is 20 times higher durability than that of MEA without nanofastener.