Nanoscale Carbon Modified α-MnO2 Nanowires: Highly Active and Stable Oxygen Reduction Electrocatalysts with Low Carbon Content

Wide-Spectrum Modulated Electrochromic Smart Windows Based on MnO2/PB Films

Inorganic materials have been extensively studied for visible electrochromism in the past few decades. However, the single inorganic electrochromic (EC) material commonly exhibits a single color change, leading to a narrow spectrum of modulation, which offsets or limits the maximally energy-saving ability. Here, we present a wide-spectrum modulated EC device designed by combining the complementary EC nanocomposite of manganese dioxide (MnO₂) and Prussian blue (PB) for enhanced energy savings. Porous MnO₂ nanostructures serve as host frameworks for the templated growth of PB, resulting in MnO₂/PB nanocomposites. The complementary optical modulation ranges of MnO₂ and PB enable a widen-spectrum modulation across the solar region with the development of the MnO₂/PB nanocomposite. The colored MnO₂/PB device exhibited an optical modulation of 32.1% in the wide solar spectrum range of 320-1100 nm and blocked 72.0% of the solar irradiance. Furthermore, fast switching responses (2.7 s for coloration and 2.1 s for bleaching) and a high coloration efficiency (83.1 cm²·C^-1) of the MnO₂/PB EC device are also achieved. The high EC performance of the MnO₂/PB nanocomposite device provides a new strategy for the design of high-performance energy-saving EC smart windows.

MnO2 Synergized with N/S Codoped Graphene as a Flexible Cathode Efficient Electrocatalyst for Advanced Honeycomb-Shaped Stretchable Aluminum-Air Batteries

Aluminum-air batteries possess high theoretical specific capacities and energy densities. However, the desired application performance in the field of flexible electronics is limited by the rigid battery structure and slow kinetics of the oxygen reduction reaction (ORR). To address these issues, flexible, stretchable, and customizable aluminum-air batteries with a reference to honeycomb shape are composed of multilayer single battery units to achieve large scalability and start-stop control. The single aluminum-air battery combines MnO₂ with N/S codoped graphene to improve the electrocatalytic activity. Benefiting from an efficient electrocatalyst and reasonable structural design, the single aluminum-air battery exhibits excellent electrochemical characteristics under deformation conditions with a high specific capacity and energy density (1203.2 mAh g^-1 Al and 1630.1 mWh g^-1 Al). Furthermore, the obtained honeycomb-shaped stretchable aluminum-air batteries maintain a stable output voltage over the 2500% stretching. More interestingly, the stretchable honeycomb structure not only can solve the start-stop control problem but also has the potential to reduce the self-corrosion in disposable metal-air batteries. In addition, owing to the customizable shapes and sizes, the honeycomb-shaped stretchable aluminum-air batteries facilitate the integrated application of flexible batteries in wearables.

Ultrasound-assisted heterogeneous Fenton-like process for bisphenol A removal at neutral pH using hierarchically structured manganese dioxide/biochar nanocomposites as catalysts

Bisphenol A (BPA) is an important emerging contaminant with endocrine-disrupting potential that has frequently been detected in aquatic environments. In this study, two types of hierarchically structured manganese dioxide/biochar nanocomposites (MnO₂/BCs) were prepared for the first time via facile hydrothermal synthesis. The hydrothermal reaction was maintained at 100 °C for 6 h or 12 h, after which an ultrasound-assisted heterogeneous Fenton-like process was used to catalyze the removal of BPA under neutral pH condition. The characterization results indicated that MnO₂ nanoparticles were successfully formed on the nanocomposite surfaces and had flower-like (δ-MnO₂, 6 h) and urchin-like (α-MnO₂, 12 h) morphology. This enabled a significant improvement in the catalytic activity of BPA removal by the reversible redox reaction. A series of experiments confirmed that the crystalline properties of the nanocomposites affected their catalytic activity. In particular, the α-MnO₂/BCs exhibited catalytic activity in the ultrasound-assisted heterogeneous Fenton-like process and completely removed BPA within 20 min under the following conditions: [BPA]₀ = 100 μM; [H₂O₂]₀ = 10 mM; [catalyst]₀ = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C. This efficiency may have been due to the synergistic effect of ultrasound and α-MnO₂/BCs, which simultaneously induce the effective generation of reactive free radicals and increase the mass transfer rate at the solid-liquid interface. Overall, these results demonstrated that hierarchical urchin-like α-MnO₂/BCs have significant potential as an efficient and low-cost catalyst in ultrasound-assisted heterogeneous Fenton-like systems.

Effect of Surface Ionization of Doped MnO2 on Capacitive Deionization Efficiency

Associating MnO₂ with carbonaceous supports profoundly enhances capacitive deionization (CDI) efficiency. A fundamental question of how the surface chemistry of MnO₂ itself influences CDI efficiency is not yet fully understood. In this study, the effect of surface ionization on the CDI efficiencies of Fe-, Co-, and Ni-doped α-MnO₂ (<0.1 mol %) as a model cathode material was studied. A pattern that CDI efficiency decreased with increasing negative surface charge density resulting from surface deprotonation was noted. This is likely attributed to the appreciable co-ion expulsion occurring at a highly ionized surface in the mesopores of MnO₂. It is thus concluded that the combination of surface charge modification and a microporous environment would be important for CDI efficiency enhancement by minimizing co-ion exclusion effect. In the former case, structural stress adjustment by doping elements would be a practical route to regulate the p K_a1 and p K_a2 values and consequently the degree of surface ionization of MnO₂.

Achieving nitrogen-doped carbon/MnO2 nanocomposites for catalyzing the oxygen reduction reaction

N-Doped carbon/MnO₂ nanocomposites generated via pyrolyzing polypyrrole/MnO₂ show excellent catalytic performance towards the oxygen reduction reaction owing to the synergistic effect between N-doped carbon and MnO₂.

Insights into the spontaneous formation of hybrid PdO x /PEDOT films: electroless deposition and oxygen reduction activity

Hybrid palladium oxide/poly(3,4-ethylenedioxythiophene) (PdO_x/PEDOT) films were prepared through a spontaneous reaction between aqueous PdCl₄²⁻ ions and a nanostructured film of electropolymerized PEDOT. Spectroscopic and electrochemical characterization indicate the presence of mixed-valence Pd species as-deposited (19 ± 7 at% Pd⁰, 64 ± 3 at% Pd²⁺, and 18 ± 4 at% Pd⁴⁺ by X-ray photoelectron spectroscopy) and the formation of stable, electrochemically reversible Pd⁰/α-PdO_x active species in alkaline electrolyte and furthermore in the presence of oxygen. The elucidation of the Pd speciation as-deposited and in solution provides insight into the mechanism of electroless deposition in neutral aqueous conditions and the electrocatalytically active species during oxygen reduction in alkaline electrolyte. The PdO_x/PEDOT film catalyses 4e⁻ oxygen reduction (n = 3.97) in alkaline electrolyte at low overpotential (0.98 V vs. RHE, onset potential), with mass- and surface area-based specific activities competitive with, or superior to, commercial 20% Pt/C and state-of-the-art Pd- and PEDOT-based nanostructured catalysts. The high activity of the nanostructured hybrid PdO_x/PEDOT film is attributed to effective dispersion of accessible, stable Pd active sites in the PEDOT matrix.

Hybrid PdO_x/PEDOT films efficiently catalyse the direct 4e⁻ oxygen reduction reaction in alkaline electrolyte.

Fabrication and high ORR performance of MnOx nanopyramid layers with enriched oxygen vacancies

MnO_x nanopyramids with enriched OVs vertically grown on nitrogen doped carbon microlaminate arrays with narrow spacing yield a high ORR performance.