Hollow carbon cavities decorated with highly dispersed manganese-nitrogen moieties for efficient metal-air batteries

A High-Rate and Ultrastable Ammonium Ion-Air Battery Enabled by the Synergy of ORR and NH4 + Storage

Ammonium ion batteries (AIBs) offer cost-effectiveness, nontoxicity, and eco-friendly attributes in energy storage technology. However, the constrained capacity and poor stability of conventional cathode materials have impeded their widespread adoption. Herein, a synergistic approach is introduced to overcome these challenges, by enhancing the air cathode with NH4 + and simultaneously leveraging atmospheric oxygen as a reservoir for NH4 + storage. Notably, NH4 + significantly enhances the oxygen reduction reaction (ORR) performance in neutral environments. Through in situ Raman spectroscopy and quantum density functional theory calculations, it is elucidated how NH4 + can act as a proton donor, replacing H2O in neutral media and reducing energy barriers in the protonation of *O2 - and *O, thereby accelerating ORR kinetics. The resulting ammonium ion-air battery, comprising an air cathode and a polymer (PNP) anode, showcases impressive metrics: high energy density of 78 Wh kg-1 and power density of 9369 W kg-1 at 1 A g-1, an initial capacity of 94.3 mAh g-1 and exceptional cycling stability (70.4% capacity retention after 12 500 cycles) at 10 A g-1. This pioneering research highlights the synergistic relationship between ORR and NH4 + storage and opens up new avenues for the design and advancement of innovative, sustainable, and environment-friendly AIBs.

Mn Doping at High-Activity Octahedral Vacancies of γ-Fe2O3 for Oxygen Reduction Reaction Electrocatalysis in Metal-Air Batteries

γ-Fe2O3 with the intrinsic cation vacancies is an ideal substrate for heteroatom doping into the highly active octahedral sites in spinel oxide catalysts. However, it is still a challenge to confirm the vacancy location of γ-Fe2O3 through experiments and obtain enhanced catalytic performance by preferential occupation of octahedral sites for heteroatom doping. Here, a Mn-doped γ-Fe2O3 incorporated with carbon nanotubes catalyst was developed to successfully achieve preferential doping into highly active octahedral sites by employing γ-Fe2O3 as the precursor. Further, the vacancy in γ-Fe2O3 was only located on octahedral sites rather than tetrahedral ones, which was first proved by direct experimental evidence through the clarification doping sites of Mn. Notably, the catalyst shows outstanding activity towards oxygen reduction reaction with the half-wave potential of 0.82 V and 0.64 V vs. reversible hydrogen electrode in alkaline and neutral electrolytes, respectively, as well as the maximum power density of 179 mWcm-2 and 403 mWcm-2 for Mg-air batteries and Al-air batteries, respectively. It could be attributed to the synergistic effect of the doping Mn on octahedral sites and the substrate γ-Fe2O3 along with the modification of the adsorption/desorption properties for oxygen-containing intermediates as well as the optimization of the reaction energy barriers.

Synthesis of lignin-based carbon/graphene oxide foam and its application as sensors for ammonia gas detection

The present study highlights the integration of lignin with graphene oxide (GO) and its reduced form (rGO) as a significant advancement within the bio-based products industry. Lignin-phenol-formaldehyde (LPF) resin is used as a carbon source in polyurethane foams, with the addition of 1 %, 2 %, and 4 % of GO and rGO to produce carbon structures thus producing carbon foams (CFs). Two conversion routes are assessed: (i) direct addition with rGO solution, and (ii) GO reduction by heat treatment. Carbon foams are characterized by thermal, structural, and morphological analysis, alongside an assessment of their electrochemical behavior. The thermal decomposition of samples with GO is like those having rGO, indicating the effective removal of oxygen groups in GO by carbonization. The addition of GO and rGO significantly improved the electrochemical properties of CF, with the GO2% sensors displaying 39 % and 62 % larger electroactive area than control and rGO2% sensors, respectively. Furthermore, there is a significant electron transfer improvement in GO sensors, demonstrating a promising potential for ammonia detection. Detailed structural and performance analysis highlights the significant enhancement in electrochemical properties, paving the way for the development of advanced sensors for gas detection, particularly ammonia, with the prospective market demands for durable, simple, cost-effective, and efficient devices.

3D Hollow Hierarchical Porous Carbon with Fe-N4 -OH Single-Atom Sites for High-Performance Zn-Air Batteries

The development of innovative and efficient Fe-N-C catalysts is crucial for the widespread application of zinc-air batteries (ZABs), where the inherent oxygen reduction reaction (ORR) activity of Fe single-atom sites needs to be optimized to meet the practical application. Herein, a three-dimensional (3D) hollow hierarchical porous electrocatalyst (ZIF8@FePMPDA-920) rich in asymmetric Fe-N4 -OH moieties as the single atomic sites is reported. The Fe center is in a penta-coordinated geometry with four N atoms and one O atom to form Fe-N4 -OH configuration. Compared to conventional Fe-N4 configuration, this unique structure can weaken the adsorption of intermediates by reducing the electron density of the Fe center for oxygen binding, which decreases the energy barrier of the rate-determining steps (RDS) to accelerate the ORR and oxygen evolution reaction (OER) processes for ZABs. The rechargeable liquid ZABs (LZABs) equipped with ZIF8@FePMPDA-920 display a high power density of 123.11 mW cm-2 and a long cycle life (300 h). The relevant flexible all-solid-state ZABs (FASSZABs) also display outstanding foldability and cyclical stability. This work provides a new perspective for the structural design of single-atom catalysts in the energy conversion and storage areas.

Selected Kraft lignin fractions as precursor for carbon foam: Structure-performance correlation and electrochemical applications

The rapid exhaustion of fossil fuels brings to the fore the need to search for energy efficient strategies. The conversion of lignin into advanced functional carbon-based materials is considered one of the most promising solutions for environmental protection and the use of renewable resources. This study analyzed the structure-performance correlation of carbon foams (CF) when lignin-phenol-formaldehyde (LPF) resins produced with different fractions of kraft lignin (KL) were employed as carbon source, and polyurethane foam (PU) as sacrificial mold. The lignin fractions employed were KL, fraction of KL insoluble in ethyl acetate (LFIns) and fraction of KL soluble in ethyl acetate (LFSol). The produced CFs were characterized by thermogravimetric analysis (TGA), X-ray diffractometry (XRD), Raman spectroscopy, 2D HSQC Nuclear magnetic resonance (NMR) analysis, scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and electrochemical measurements. The results showed that when LFSol was employed as a partial substitute for phenol in LPF resin synthesis, the final performance of the produced CF was infinitely higher. The improved solubility parameters of LFSol along with the higher S/G ratio and β-O-4/α-OH content after fractionation were the key to produce CF with better carbon yields (54 %). The electrochemical measurements showed that LFSol presented the highest current density (2.11 × 10^-4 mA.cm^-2) and the lowest value of resistance to charge transfer (0.26 KΩ) in relation to the other samples, indicating that the process of electron transfer was faster in the sensor produced with LFSol. LFSol's potential for application as an electrochemical sensor was tested as a proof of concept and demonstrated excellent selectivity for the detection of hydroquinone in water.

In situ Self-Catalyzed Growth of Manganese-Embedded 3D Flakes-Coated Carbon Rod as an Efficient Oxygen-Reduction Reaction Catalyst of Zinc-Air Batteries

The in situ self-catalyzed growth of manganese-embedded 3D flakes-coated carbon rods (GFC) as an efficient oxygen-reduction reaction (ORR) catalyst of Zinc-air batteries is described for the first time. By optimizing the amount of Mn in the precursor, a series of 3D graphene-like flakes-coated carbon rods were synthesized. GFC with a doping amount of Mn of 10 % (GFC-10) exhibits excellent ORR performance with an onset potential of 0.94 V (vs. reversible hydrogen electrode). The Zinc-air battery is constructed with GFC-10 as the cathode catalyst, and it exhibits a peak power density of 128.9 mW cm-2 and a cycling stability of 75 h at a current density of 10 mA cm-2 , which are superior to the commercial 20 wt% Pt/C-based Zinc-air battery. Interestingly, the introduction of Mn facilitates the self-catalyzed growth of carbon rods, and the change of Mn amount can effectively regulate the morphology of materials. The improved ORR performance of the catalyst is ascribed to the synergistic effect of unique hierarchical porous structure, high-charge transport capacity, abundant carbon defects/edges and Mn-Nx sites. This research provides a new avenue to fabricating highly active Mn-based electrocatalysts for renewable energy systems.