Oxidizing Vacancies in Nitrogen-Doped Carbon Enhance Air-Cathode Activity

N-Decorated Main-Group MgAl2O4 Spinel: Unlocking Exceptional Oxygen Reduction Activity for Zn-Air Batteries

The development of economical and efficient oxygen reduction reaction (ORR) catalysts is crucial to accelerate the widespread application rhythm of aqueous rechargeable zinc-air batteries (ZABs). Here, a strategy is reported that the modification of the binding energy for reaction intermediates by the axial N-group converts the inactive spinel MgAl2O4 into the active motif of MgAl2O4-N. It is found that the introduction of N species can effectively optimize the electronic configuration of MgAl2O4, thereby significantly reducing the adsorption strength of *OH and boosting the reaction process. This main-group MgAl2O4-N catalyst exhibits a high ORR activity in a broad pH range from acidic and alkaline environments. The aqueous ZABs assembled with MgAl2O4-N shows a peak power density of 158.5 mW cm-2, the long-term cyclability over 2000 h and the high stability in the temperature range from -10 to 50 °C, outperforming the commercial Pt/C in terms of activity and stability. This work not only serves as a significant candidate for the robust ORR electrocatalysts of aqueous ZABs, but also paves a new route for the effective reutilization of waste Mg alloys.

Regulating d-Orbital Hybridization of Subgroup-IVB Single Atoms for Efficient Oxygen Reduction Reaction

Highly active single-atom electrocatalysts for the oxygen reduction reaction are crucial for improving the energy conversion efficiency, but they suffer from a limited choice of metal centers and unsatisfactory stabilities. Here, this work reports that optimization of the binding energies for reaction intermediates by tuning the d-orbital hybridization with axial groups converts inactive subgroup-IVB (Ti, Zr, Hf) moieties (MN4) into active motifs (MN4O), as confirmed with theoretical calculations. The competition between metal-ligand covalency and metal-intermediate covalency affects the d-p orbital hybridization between the metal site and the intermediates, converting the metal centers into active sites. Subsequently, dispersed single-atom M sites coordinated by nitrogen/oxygen groups have been prepared on graphene (s-M-N/O-C) catalysts on a large-scale with high-energy milling and pyrolysis. Impressively, the s-Hf-N/O-C catalyst with 5.08 wt% Hf exhibits a half-wave potential of 0.920 V and encouraging performance in a zinc-air battery with an extraordinary cycling life of over 1600 h and a large peak power-density of 256.9 mW cm-2. This work provides promising single-atom electrocatalysts and principles for preparing other catalysts for the oxygen reduction reaction.

Efficient and Selective Adsorption of Cationic Dye Malachite Green by Kiwi-Peel-Based Biosorbents

In this study, pristine kiwi peel (KP) and nitric acid modified kiwi peel (NA-KP) based adsorbents were prepared and evaluated for selective removal of cationic dye. The morphology and chemical structure of KP and NA-KP were fully characterized and compared, and results showed nitric acid modification introduced more functional groups. Moreover, the adsorption kinetics and isotherms of malachite green (MG) by KP and NA-KP were investigated and discussed. The results showed that the adsorption process of MG onto KP followed a pseudo-second-order kinetic model and the Langmuir isotherm model, while the adsorption process of MG onto NA-KP followed a pseudo-first-order kinetic model and the Freundlich isotherm model. Notably, the Langmuir maximum adsorption capacity of NA-KP was 580.61 mg g^-1, which was superior to that of KP (297.15 mg g^-1). Furthermore, thermodynamic studies demonstrated the feasible, spontaneous, and endothermic nature of the adsorption process of MG by NA-KP. Importantly, NA-KP showed superior selectivity to KP towards cationic dye MG against anionic dye methyl orange (MO). When the molar ratio of MG/MO was 1:1, the separation factor (α_MG/MO) of NA-KP was 698.10, which was 5.93 times of KP. In addition, hydrogen bonding, π-π interactions, and electrostatic interaction played important roles during the MG adsorption process by NA-KP. This work provided a low-cost, eco-friendly, and efficient option for the selective removal of cationic dye from dyeing wastewater.

Metal-organic framework-derived advanced oxygen electrocatalysts as air-cathodes for Zn-air batteries: recent trends and future perspectives

Electrochemical energy storage devices with stable performance, high power output, and energy density are urgently needed to meet the global energy demand. Among the different electrochemical energy storage devices, batteries have become the most promising energy technologies and ranked as a highly investigated research subject. Recently, metal-air batteries especially Zn-air batteries (ZABs) have attracted enormous scientific interest in the electrochemical community due to their ease of operation, sustainability, environmental friendliness, and high efficiency. The oxygen electrocatalytic reactions [oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)] are the two fundamental reactions for the development of ZABs. Noble metal-based electrocatalysts are widely considered as the benchmark for oxygen electrocatalysis, but their practical application in rechargeable ZAB is hindered due to several shortcomings. Thus, to replace noble metal-based catalysts, a wide range of transition-metal-based materials and heteroatom-doped metal-free carbon materials has been extensively investigated as oxygen electrocatalysts for ZABs. Recently, metal-organic frameworks (MOFs) with unique structural flexibility and uniformly dispersed active sites have become attractive precursors for the synthesis of a large variety of advanced functional materials. Herein, we summarize the recent progress of MOF-derived oxygen electrocatalysts (MOF-derived carbon nanomaterials, MOF-derived alloys/nanoparticles, and MOF-derived single-atom electrocatalysts) for ZABs. Specifically, we highlight MOF-derived single-atom electrocatalysts owing to the wide exploration of these emerging materials in electrocatalysis. The influence of the active sites, structural/compositional design, and porosity of MOF-derived advanced materials on the oxygen electrocatalytic performances is also discussed. Finally, the existing challenges and prospects of MOF-derived electrocatalysts in ZABs are briefly highlighted.

Coordinated single-molecule micelles: a self-template approach for preparing mesoporous doped carbons

Porous carbons prepared using a self-template approach inherit the pore features of template, but they exhibit almost no evenly dispersed mesopores, which is significant for diffusion-limited applications. Herein, N-doped hierarchically porous carbons (NHPCs) with uniform mesopores are prepared using a self-template method. The spherical single-molecule micelle of polystyrene-b-poly(4-vinyl pyridine) (PS-b-P4VP) is turned into a Zn²⁺-coordinated PS-b-P4VP micelle (CPM) by coordination of Zn²⁺ with the P4VP shell. Then, the self-template of the CPM is carbonized into a hollow carbon nanosphere. During carbonization, the PS core is decomposed to generate the central mesopore, whereas the Zn²⁺-coordinated P4VP shell is transformed into a carbonaceous shell. These even hollow carbon nanospheres aggregate to form uniformly mesoporous carbon lumps. Simultaneously, the coordinated Zn²⁺ of the CPM is reduced to metal zinc at high temperatures and then it is evaporated, thus creating numerous micropores in the carbonaceous shell. These NHPCs with uniform mesopores display a high specific surface area. As a demonstration in diffusion-limited applications, their catalytic performances for the oxygen reduction reaction (ORR) are investigated. Strikingly, NHPCs exhibit outstanding catalytic performances for the ORR. This self-template method paves a facile approach for preparing mesoporous carbons with high performances.

N, S, P-Codoped Graphene-Supported Ag-MnFe2O4 Heterojunction Nanoparticles as Bifunctional Oxygen Electrocatalyst with High Efficiency

Due to slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during discharging and charging processes, it is essential to rationally design and synthesize non-precious metal bifunctional electrocatalysts with good performance for metal-air batteries. Herein, Ag-MnFe2O4 heterojunction nanoparticles supported on N, S, P-codoped graphene (NSPG) are developed with enhanced ORR and OER bifunctional electrocatalytic activities and stability. In contrast, S, P-doped graphene (SPG) and N, P-doped graphene (NPG) show less stabilization for the heterojunction particles. For example, under alkaline conditions, the ORR half-wave potential of Ag-MnFe2O4/NSPG can reach 0.831 V, and the over potential for OER is 0.56 V at the current density 10 mA·cm−2. Furthermore, Ag-MnFe2O4/NSPG shows better methanol resistance and durability than Pt/C catalysts.

Defect and Doping Co-Engineered Non-Metal Nanocarbon ORR Electrocatalyst

Exploring low-cost and earth-abundant oxygen reduction reaction (ORR) electrocatalyst is essential for fuel cells and metal-air batteries. Among them, non-metal nanocarbon with multiple advantages of low cost, abundance, high conductivity, good durability, and competitive activity has attracted intense interest in recent years. The enhanced ORR activities of the nanocarbons are normally thought to originate from heteroatom (e.g., N, B, P, or S) doping or various induced defects. However, in practice, carbon-based materials usually contain both dopants and defects. In this regard, in terms of the co-engineering of heteroatom doping and defect inducing, we present an overview of recent advances in developing non-metal carbon-based electrocatalysts for the ORR. The characteristics, ORR performance, and the related mechanism of these functionalized nanocarbons by heteroatom doping, defect inducing, and in particular their synergistic promotion effect are emphatically analyzed and discussed. Finally, the current issues and perspectives in developing carbon-based electrocatalysts from both of heteroatom doping and defect engineering are proposed. This review will be beneficial for the rational design and manufacturing of highly efficient carbon-based materials for electrocatalysis.

Ni/NiO heterostructures encapsulated in oxygen-doped graphene as multifunctional electrocatalysts for the HER, UOR and HMF oxidation reaction

A controlled scalable arc-discharge method was developed to produce metal/metal oxide nanoparticles encapsulated in graphene as excellent catalysts for multiple reactions, including HER, UOR, and the HMF oxidation reaction.

Enhanced utilization of active sites of Fe/N/C catalysts by pore-in-pore structures for ultrahigh mass activity

Carbon material doped with nitrogen and transition metal is a kind of promising candidate of the platinum for oxygen reduction reaction (ORR) process due to its low cost, efficiency and stability. Here we demonstrate an original type of Fe/N/C catalyst based on pore-in-pore structures (P-P Fe/N/C), showing one of the highest oxygen reduction reaction performances among all reported Fe/N/C-type catalysts (onset potential of 0.995 V, half-wave potential of 0.881 V vs. RHE with a relatively low mass loading of 0.32 mg cm^-2 and long-term durability (97% relative current in 60 000 s operation) in alkaline media. Such outstanding performances can be ascribed to the efficient active sites activated by the encapsulated atomic and subnanoscale iron, and great exposure of these active sites due to the unique pore-in-pore hierarchical construction. Once assembled in lithium-O₂ batteries, a specific capacity of 7250 mA h g^-1 at 70 mA g^-1 can be obtained by the P-P Fe/N/C catalyst. Moreover, upon cycling, the P-P Fe/N/C electrode can be cycled 150 times with no capacity loss, which is much longer than six cycles of pure Super P air electrode. These results evidently reveal the developed Fe/N/C catalyst holds great promise to serve as an alternative to the conventional Pt-based noble metal catalysts.

3D Rosa centifolia-like CeO2 encapsulated with N-doped carbon as an enhanced electrocatalyst for Zn-air batteries

Reasonable design and synthesis of high-efficiency rare earth oxides-based materials as alternatives to noble-metal catalysts are of great significance for oxygen electrocatalysis. Herein, we report three-dimension (3D) Rosa centifolia-like CeO₂ encapsulated with N-doped carbon (NC) composites (CeO₂@NC) for enhancing oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities. This synthetic method allows CeO₂ to tune the oxygen vacancy concentration and electronic structure of a series of CeO₂@NC catalysts due to its large oxygen-storage-capacity (OSC) property. Moreover, benefiting from the exposed active sites in NC as well as the synergy between CeO₂ and NC, among as-prepared samples, the resultant CeO₂@NC-900 delivers a half-wave potential (E_1/2) of 0.854 V, which is more positive compared with counterpart of NC-900 (0.806 V) and even comparable to that of commercial Pt/C catalyst (0.855 V). This indicates that the ORR electrocatalytic activity of CeO₂@NC-900 is significantly improved. Meanwhile, CeO₂@NC-900 exhibits satisfactory performance toward OER. For practical application, the CeO₂@NC-900 involved rechargeable Zn-air battery possesses excellent energy efficiency, superior stability, and large energy density (666.1 Wh kg_Zn^-1 at 5 mA cm^-2). This approach provides a valid way to develop advanced rare earth oxides-based materials for energy applications.

Hierarchical Porous Molybdenum Carbide Based Nanomaterials for Electrocatalytic Hydrogen Production

The electrocatalytic hydrogen evolution reaction (HER) for the preparation of hydrogen fuel is a very promising technology to solve the shortage of hydrogen storage. However, in practical applications, HER catalysts with excellent performance and moderate price are very rare. Molybdenum carbide (Mo_xC) has attracted extensive attention due to its electronic structure and natural abundance. Here, a comprehensive review of the preparation and performance control of hierarchical porous molybdenum carbide (HP-Mo_xC) based catalysts is summarized. The methods for preparing hierarchical porous materials and the regulation of their HER performance are mainly described. Briefly, the HP-Mo_xC based catalysts were prepared by template method, morphology-conserved transformations method, and secondary conversion method of an organic-inorganic hybrid material. The intrinsic HER kinetics are enhanced by the introduction of a carbon-based support, heteroatom doping, and the construction of a heterostructure. Finally, the future development of HP-Mo_xC based catalysts is prospected in this review.

Wiping off oxygen bonding to maximize heteroatom-induced improvement in oxygen reaction activity of metal site for high-performance zinc-air battery

Heteroatom doping has recently been utilized to improve the catalytic performance of transition metal-based electrocatalysts. However, the doping process is inevitably accompanied by the introduction of oxygen, influencing the heteroatom-induced asymmetric spin density over the active sites and leading to inconspicuous promotion in the property. Herein, by wiping off the undesired heteroatom-oxygen bonding, we maximize the heteroatom-induced improvement in oxygen reaction activity of metal site, providing descendant energy barrier and favorable reaction efficiency for zinc-air batteries. The proof-of-concept material delivers a superior half-wave potential of 0.88 V versus reversible hydrogen electrode for oxygen reduction reaction, a small overpotential of 410 mV at the current density of 10 mA cm^-2 for oxygen evolution reaction, and a reversible oxygen electrode index of 0.76 V in electrochemical measurements. Aqueous zinc-air battery with such catalysts delivers an excellent power density of 162.3 mW cm^-2 and superior durability over 635 cycles. Moreover, in consideration of high safety and flexibility of solid-state batteries, all-solid-state zinc-air battery adopting gel electrolyte is assembled and used to illumine an LED wristband, showing great promises for the next-generation energy system.

Atomic Metal Vacancy Modulation of Single-Atom Dispersed Co/N/C for Highly Efficient and Stable Air Cathode

Metal-air batteries have received great attention as a new energy supply for next-generation electronic devices. However, their widespread application is still hindered by several challenges including sluggish kinetics of the cathodic reactions and undesirable stability of the air cathode due to the possible deposition of the discharge product. Herein, we propose an atomic metal vacancy modulation of a single-atom dispersed Co/N/C cathode to provide the zinc-air battery with both reduced overpotential and enhanced stability. As illustrated by theoretical calculations and electrochemical measurements, deliberate introduction of metal vacancies would modulate the electronic structure and contribute to enhanced catalytic activity, affording the catalyst with a half-wave potential of 0.89 V versus reversible hydrogen electrode and an overall oxygen electrode potential gap of 0.72 V. Moreover, abundant pyridinic-N groups are exposed due to the removal of metal centers, generating strong Lewis basicity to effectively prevent the access of negatively charged zincate ions and realize the nondeposition of ZnO on the air cathode. Rechargeable zinc-air battery assembled with such an air cathode delivers superior cyclic performance with low discharge/charge overpotential and negligible plateau gap increase of only 0.05 V for 1000 cycles. Flexible all-solid-state battery demonstrates robust durability of over 35 h and excellent flexibility to light-up a light-emitting diode (LED) rose, indicating its potential feasibility as a flexible and safe power source for modern life.

Understanding the electrochemistry of armchair graphene nanoribbons containing nitrogen and oxygen functional groups: DFT calculations

The surface and edge chemistry are vital points to assess a specific application of graphene and other carbon nanomaterials. Based on first-principles density functional theory, we investigate twenty-four chemical functional groups containing oxygen and nitrogen atoms anchored to the edges of armchair graphene nanoribbons (AGNRs). Results for the band structures, formation energy, band gaps, electronic charge deficit, oxidation energy, reduction energy, and global hydrophilicity index are analyzed. Among the oxygen functional groups, carbonyl, anhydride, quinone, lactone, phenol, ethyl-ester, carboxyl, α-ester-methyl, and methoxy act as electron-withdrawing groups and, conversely, pyrane, pyrone, and ethoxy act as electron-donating groups. In the case of nitrogen-functional groups, amine, N-p-toluidine, ethylamine, pyridine-N-oxide, pyridone, lactam, and pyridinium transfer electrons to the AGNRs. Nitro, amide, and N-ethylamine act as electron-withdrawing groups. The carbonyl and pyridinium group-AGNRs show metallic behavior. The formation energy calculations revealed that AGNRs with pyridinium, amine, pyrane, carbonyl, and phenol are the most stable structures. In terms of the global hydrophilicity index, the quinone and N-ethylamine groups showed the most significant values, suggesting that they are highly efficient in accepting electrons from other chemical species. The oxidation and reduction energies as a function of the ribbon's width are discussed for AGNRs with quinone, hydroquinone, nitro, and nitro + 2H. Besides, we discuss the effect of nitrogen-doping in AGNRs on the oxidation and reduction energies for the quinone and hydroquinone functional groups.

Identifying the Lewis Base Chemistry in Preventing the Deposition of Metal Oxides on Ketone-Enriched Carbon Cathodes for Highly Durable Metal-Air Batteries

Metal-air batteries have exhibited unlimited potential and economic value because of their considerably high energy density. However, under repeated cycling, air cathodes undergo a well-known problem, the deposition of metal oxide, clogging the active surface and ultimately leading to the severe degradation of the cyclic performance. Herein, we address this challenge in a zinc-air battery by introducing ketone as the Lewis base into the air catalyst. As illustrated by in situ X-ray diffraction observations, the ketone-enriched material could generate an ultrahigh negative potential to prevent the access of negatively charged zincate ions and thus enable the nondeposition of zinc oxide on the air cathode because of the strong electrostatic repulsion. Using this strategy, we demonstrate 650 highly stable cycles of a zinc-air battery under a high rate (25 mA cm^-2). Such a Lewis-base-assisted method opens up new avenues to prevent air cathodes from being poisoned for highly durable metal-air batteries.

Updating the Intrinsic Activity of a Single-Atom Site with a P-O Bond for a Rechargeable Zn-Air Battery

Rechargeable Zn-air batteries have drawn great attention over the past decade, but their further development will require efficient bifunctional electrocatalysts to drive the sluggish cathodic reactions. Although a single-atom catalyst with maximum utilization per metal atom shows great promise, its catalytic performance is still far from satisfactory. Here we tackle this challenge by introducing a P-O bond to update the intrinsic activity of a single-atom site and thus reduce the reaction overpotential of the Zn-air battery. The critical role of the P-O bond in producing a favorable surface electronic environment of the single-atom metal site and improving its catalytic activity is identified with density functional theory simulations. The P-O-doped, atomically dispersed catalyst is shown experimentally to deliver excellent bifunctional performance, with a remarkable half-wave potential of 0.89 V versus reversible hydrogen electrode (vs RHE) for oxygen reduction reaction and a reversible oxygen electrode index of 0.74 V, exceeding those of most reported nonprecious metal catalysts. When subjected to practical application, both aqueous and all-solid-state Zn-air batteries illustrate superior power density and robust cyclic performance, confirming their potential feasibility in next-generation electronic devices.

Plasma engraved Bi0.1(Ba0.5Sr0.5)0.9Co0.8Fe0.2O3-δ perovskite for highly active and durable oxygen evolution

The development of highly active and cost-effective catalysts based on noble metal free oxygen electro-catalysis is critical to energy storage and conversion devices. Herein, we highlight a plasma-treated Bi0.1(Ba0.5Sr0.5)0.9Co0.8Fe0.2O3-δ perovskite (denoted as P-Bi0.1BSCF) as a promising catalyst for oxygen evolution reaction (OER) in alkaline media. H2/Ar plasma engraving could significantly increase electrochemically active O22-/O- concentration and tune the electronic structure of Co ions efficiently, and consequently tailor the intrinsic electrocatalytic ability for OER. Of note, P-Bi0.1BSCF, with unique crystalline core/amorphous shell structure, exhibits an enhanced intrinsic OER activity and higher stability than the noble metal IrO2 catalyst, which outperforms most of the reported perovskite catalysts. The present work provides new insights into exploring efficient catalysts for OER, and it suggests that, in addition to the extensively applied for surface treatment of various catalysts such as carbons and metal oxides, the plasma engraved perovskite materials also exhibits great potential as precious metal-free catalysts.