MOF-derived low Ru-loaded high entropy alloy as an efficient and durable self-supporting electrode in rechargeable liquid/flexible Zn-air batteries.

Exploiting the high-entropy alloy (HEA) electrocatalysts with the synergistic effect of multi-metal components is an effective approach to address the slow kinetics and undesirable stability of the oxygen evolution reaction (OER) in Zn-air batteries (ZABs), but still faces many challenges. In this study, a multimetallic Metal-organic framework (MOF)-derived HEA catalyst was successfully fabricated on carbon fiber as a flexible self-supporting electrode (denoted as CC@FeCoNiMoRu-HEA/C) for high-performance liquid/flexible ZABs using a facile and cost-effective strategy. The three-dimensional (3D) highly open network framework and hierarchical porous structure accelerate the mass transport of OH-/O2 and charge transfer. The electronic structure adjustment, lattice defects and high entropy effects enable the CC@FeCoNiMoRu-HEA/C catalysts to perform high OER catalytic activity and strong durability while reducing the Ru content and lowering the economic cost. In situ Raman spectra and XPS results reveal the generation of metal-OOH intermediates on the HEA surface during the OER process. In a practical demonstration, the liquid ZAB assembled with CC@FeCoNiMoRu-HEA/C + Pt/C as the air electrode offers stable open-circuit voltage, large power density, excellent specific capacity and satisfactory cycle life, outperforming the commercial RuO2 + Pt/C-based reference ZAB. More attractively, the flexible solid-state ZAB also achieves fast dynamic response, high peak power density, robust cycling stability as well as favorable mechanical flexibility, indicating a promising application prospect in future flexible electronics and wearable devices. This work provides a viable pathway to develop low precious metal-loaded HEAs as advanced OER self-supporting electrocatalysts and realize high-performance flexible energy storage devices.

Atomic-Level Tailoring of the Electronic Metal-Support Interaction Between Pt-Co3O4 Interfaces for High Hydrogen Evolution Performance.

Atomic-level modulation of the metal-oxide interface is considered an effective approach to optimize the electronic structure and catalytic activity of metal catalysts but remains highly challenging. Here, we employ the atomic layer deposition (ALD) technique together with a heteroatom doping strategy to effectively tailor the electronic metal-support interaction (EMSI) at the metal-oxide interface on the atomic level, thereby achieving high hydrogen evolution performance and Pt utilization. Theoretical calculations reveal that the doping of N atoms in Co3O4 significantly adjusts the EMSI between Pt-Co3O4 interfaces and, consequently, alters the d-band center of Pt and optimizes the adsorption/desorption of reaction intermediates. This work sheds light on the atomic-level regulation and mechanistic understanding of the EMSI in metal-oxide, while providing guidance for the development of advanced EMSI electrocatalysts for various future energy applications.

Architecting N-doped Carbon Nanotube-Rich Carbon Nanofibers with Biomimetic Vine-Leaf-Whisker Structure as Robust Bifunctional Electrocatalysts for Rechargeable Zn-Air Batteries.

Efficient and durable bifunctional catalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are urgently desirable but challenging for rechargeable Zn-air batteries (ZABs), especially flexible wearable ZABs. Inspired by the vine-leaf-whisker structure in nature, we proposed a three-dimensional (3D) hierarchical bifunctional catalyst (denoted as Co-Fe-Zn@N-CNT/CNF) consisting of N-doped carbon nanotubes embedded with abundant CoFe alloy nanoparticles, leaf-shaped N-doped carbon nanoflakes, and porous carbon fibers for rechargeable ZABs. The special biomimetic structure provides a large specific surface area, allowing for high exposure of the active site and ensuring fast mass transport/charge transfer. The close combination of CoFe bimetallic alloys and N-doped carbon nanotubes delivers high electrocatalytic activity, while the coexistence of various active sites such as metal nanoparticles (NPs), metal-Nx, doped N species, and their synergistic interactions endows the catalysts with more active sites. As such, the Co-Fe-Zn@N-CNT/CNF catalyst achieves superior bifunctional catalytic activities for the ORR (a half-wave potential of 0.84 V) and the OER (an overpotential of 326 mV at 10 mA cm-2) in alkaline media, comparable to commercial Pt/C and RuO2. Remarkably, both aqueous and solid-state ZABs assembled with Co-Fe-Zn@N-CNT/CNF catalysts as air electrodes demonstrate excellent charging/discharging performance, high peak power density, and robust long-term cycling stability. More interestingly, the flexible ZAB performs well even under bending conditions, displaying satisfactory device stability and mechanical flexibility. This study presents a new collective morphological-composition-structural engineering strategy for exploiting the efficient bifunctional oxygen electrocatalysts, which is of great significance for high-performance rechargeable ZABs and wearable energy storage devices.

Synergetic Nanostructure Engineering and Electronic Modulation of a 3D Hollow Heterostructured NiCo2O4@NiFe-LDH Self-Supporting Electrode for Rechargeable Zn-Air Batteries.

Developing electrocatalysts that integrate the merits of the hollow structure and heterojunction is an attractive but still challenging strategy for addressing the sluggish kinetics of oxygen evolution reaction (OER) in many renewable energy technologies. Herein, a 3D hierarchically flexible self-supporting electrode with a hollow heterostructure is intentionally constructed by assembling thin NiFe layered double hydroxide (LDH) nanosheets on the surface of metal-organic framework-derived hollow NiCo2O4 nanoflake arrays (NiCo2O4@NiFe-LDH) for rechargeable Zn-air batteries (ZABs). Theoretical calculations demonstrate that the interfacial electron transfer from NiFe-LDH to NiCo2O4 induces the electronic modulation, improves the conductivity, and lowers the reaction energy barriers during OER, ensuring high catalytic activity. Meanwhile, the 3D hierarchically hollow nanoarray architecture can afford plentiful catalytic active sites and short mass-/charge-transfer pathways. As a result, the obtained catalyst exhibits remarkable OER electrocatalytic performance, showing low overpotentials (only 231 mV at 10 mA cm-2, 300 mV at 50 mA cm-2) and robust stability. When assembling liquid and flexible solid-state ZABs with NiCo2O4@NiFe-LDH as the OER catalyst, the ZABs achieve excellent power density, high specific capacity, superior cycle durability, and good bending flexibility, exceeding the RuO2 + Pt/C benchmarks and other previously reported self-supporting catalysts. This work not only constructs an advanced hollow heterostructured catalyst for sustainable energy systems and wearable electronic devices but also provides insights into the role of interfacial electron modulation in catalytic performance enhancement.

Construction of 3D Hierarchical Co3O4@CoFe-LDH Heterostructures with Effective Interfacial Charge Redistribution for Rechargeable Liquid/Solid Zn-Air Batteries.

Constructing three-dimensional (3D) hierarchical heterostructures is an appealing but challenging strategy to improve the performance of catalysts for electrical energy devices. Here, an efficient and robust flexible self-supporting catalyst, interface coupling of ultrathin CoFe-LDH nanosheets and Co3O4 nanowire arrays on the carbon cloth (CC/Co3O4@CoFe-LDH), was proposed for boosting oxygen evolution reaction (OER) in rechargeable liquid/solid Zn-air batteries (ZABs). The strong interfacial interaction between the CoFe-LDH and Co3O4 heterostructures stimulated the charge redistribution in their coupling regions, which improved the electron conductivity and optimized the adsorption free energy of OER intermediates, ultimately boosting the intrinsic OER performance. Besides, the 3D hierarchical nanoarray structure facilitated the exposure of catalytically active centers and rapid electron/mass transfer during the OER process. As such, the CC/Co3O4@CoFe-LDH catalyst achieved excellent OER catalytic activity in alkaline medium, with a small overpotential of 237 mV at 10 mA cm-2, a low Tafel slope of 35.43 mV dec-1, and long-term durability of up to 48 h, significantly outperforming the commercial RuO2 catalyst. More impressively, the liquid and flexible solid-state ZABs assembled by the CC/Co3O4@CoFe-LDH hybrid catalyst as the OER catalyst presented a stable open circuit voltage, large power density, superb cycling life, and satisfactory flexibility, indicating great potential applications in energy technology. This work provides a good guidance for the development of advanced electrocatalysts with heterostructures and an in-depth understanding of electronic modulation at the heterogeneous interface.

A comparison between bridges and force-chains in photoelastic disk packing.

In a dense granular material, force-chains form naturally as a backbone to support an external load. Due to the limitation of experimental techniques, the determination of the force-chain network in 3D packing is rather difficult, requiring a precise measurement of contact forces between particles. For 3D packing under gravity, it has long been proposed that bridges form as load-bearing structures. Identification of a bridge is relatively easy, requiring only the geometric information of particles. Although both concepts have existed for years, their relationship remains elusive. In this study, we aim towards resolving such a connection by comparing bridges and force-chains in packing of photoelastic disks. No direct correspondence between bridges and force-chains is observed. In addition, when a bridge grows longer, its probability of being a force-chain or part of the force-chain network decreases exponentially. Nonetheless, both are consistent in distinguishing certain properties of packing of strong hysteresis. To test the load-bearing assumption of bridges, stresses of particles in/out of bridges are analysed and compared and only minor differences are found.