Hybrid Carbon Sphere Chain-MnO2 Nanorods as Bifunctional Oxygen Electrocatalysts for Rechargeable Zinc-Air Batteries

Crystalline MnCO3@Amorphous MnOx Composite as Cathode Material for High-Performance Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (RAZIBs) have received extensive attention because of their advantages of low cost, high safety, and nontoxicity. However, problems such as dissolution of the active cathode material, dendrites/passivation of the zinc anode, and slow reaction kinetics hindered their further applications. In this work, a crystalline/amorphous composite-type material composed of crystalline MnCO3 and amorphous MnOx was prepared and used as the cathode material for RAZIBs. The MnCO3@amorphous MnOx (MnCO3@A-MnOx) composite possesses the merits of both the pure crystalline phase of MnCO3 and the amorphous phase of MnOx, which can deliver better electrochemical performance than the corresponding single component in repeated cycles. In addition, crystalline MnCO3 undergoes a complex phase transition to the active MnO2 during the first charge process, providing the composite with a stable structure and additional electrochemical capacity. The electrochemical measurement results indicate that the MnCO3@A-MnOx electrode can display high reversible discharge capacity at 0.1 A g-1, excellent rate performance at 5.0 A g-1, and long cycling stability over 2000 cycles, showing great potential as a cathode material for high-performance RAZIBs.

The impact of support electronegativity on the electrochemical properties of platinum

Modulating the electronic structure of platinum (Pt) through a support is an important strategy for enhancing its electrocatalytic properties. In this work, to explore the impact of support electronegativity on Pt's catalytic activity for hydrogen evolution, we chose diverse metals with varying electronegativities that are stable in acidic solutions, such as titanium (Ti), molybdenum (Mo), and tungsten (W), as supports. Ti is the optimal support according to density functional theory (DFT) calculations. As expected, the Pt@Ti catalyst demonstrated remarkable efficiency in the hydrogen evolution reaction (HER), displaying a minimal overpotential of 13 mV at -10 mA cm-2, a Tafel slope of 34.5 mV dec-1, and sustained durability over 110 h in a 0.5 M H2SO4 solution. To unravel the metal-support interaction (MSI) between Pt and Ti, a comprehensive exploration encompassing both experimental investigations and DFT calculations was undertaken. The results elucidate that the outstanding HER performance of Pt@Ti stems from robust synergies forged between Pt and Ti atoms within the Ti support. This work not only furnishes a technique for producing electrocatalysts with superior efficiency and stability but also streamlines the process of choosing the most appropriate metal support. Moreover, it enhances comprehension of the interaction between Pt and the metal support.

Porous carbonaceous materials simultaneously dispersing N, Fe and Co as bifunctional catalysts for the ORR and OER: electrochemical performance in a prototype of a Zn-air battery

Infiltration of the mesoporous structure of SBA-15 silica as a hard template with phenanthroline complexes of Fe3+ and Co2+ allowed the simultaneous dispersion of nitrogen, iron and cobalt species on the surface of the obtained carbonaceous CMK-3 silica replica, with potential as bifunctional heterogeneous catalysts for the cathodic oxygen reduction and evolution reactions (ORR and OER). The textural properties and mesopore structure depended on the composition of the material. The carbonaceous FeCoNCMK-3 (1/1), obtained with an Fe/Co molar ratio of 1/1, exhibited an ordered cylindrical mesoporous structure with a high mesopore volume, a rather homogeneous composition in terms of total and surface concentrations of iron and cobalt, and a balanced presence of pyridinic-, pyrrolic- and graphitic-N species. FeCoNCMK-3 (1/1) could improve the ORR kinetics by adsorption and reduction of O2 through the 4-electron mechanism with a current density of -17.37 mA cm-2, Eonset of 1.13 V vs. RHE and E1/2 of 0.75 V when compared to metal-free, monometallic or bimetallic electrocatalysts with a higher amount of cobalt than that of iron. In addition, FeCoNCMK-3 (1/1) exhibited activity for the OER, presenting lower values of Eonset (1.52 V), Ej10 (1.78 V) and the Tafel slope (76.3 mV dec-1) with respect to other catalysts. When evaluated as a cathode in a prototype of a Zn-air battery, FeCoNCMK-3 (1/1) exhibited a high open circuit voltage of 1.41 V, a peak power density of 66.84 mW cm-2, a large specific capacity of 818.88 mA h gZn-1, and cycling for 20 h but with deactivation upon cycling.

Investigation of Highly Active Carbon-, Cobalt-, and Noble Metal-Free MnO2/NiO/Ni-Based Bifunctional Air Electrodes for Metal-Air Batteries with an Alkaline Electrolyte

Compared to other battery technologies, metal-air batteries offer high specific capacities because the active material at the cathode side is supplied by ambient atmosphere. To secure and further extend this advantage, the development of highly active and stable bifunctional air electrodes is currently the main challenge that needs to be resolved. Herein, a highly active carbon-, cobalt-, and noble-metal-free MnO₂/NiO-based bifunctional air electrode is presented for metal-air batteries in alkaline electrolytes. Notably, while electrodes without MnO₂ reveal stable current densities over 100 cyclic voltammetry cycles, MnO₂ containing samples show a superior initial activity and an elevated open circuit potential. Along this line, the partial substitution of MnO₂ by NiO drastically increases the cycling stability of the electrode. X-ray diffractograms, scanning electron microscopy images, and energy-dispersive X-ray spectra are obtained before and after cycling to investigate structural changes of the hot-pressed electrodes. XRD results suggest that MnO₂ is dissolved or transformed into an amorphous phase during cycling. Furthermore, SEM micrographs show that the porous structure of a MnO₂ and NiO containing electrode is not maintained during cycling.

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.