Air cathode of zinc-air batteries: a highly efficient and durable aerogel catalyst for oxygen reduction

Co-Adjusting d-Band Center of Fe to Accelerate Proton Coupling for Efficient Oxygen Electrocatalysis

The problem in d-band center modulation of transition metal-based catalysts for the rate-determining steps of oxygen conversion is an obstacle to boost the electrocatalytic activity by accelerating proton coupling. Herein, the Co doping to FeP is adopted to modify the d-band center of Fe. Optimized Fe sites accelerate the proton coupling of oxygen reduction reaction (ORR) on N-doped wood-derived carbon through promoting water dissociation. In situ generated Fe sites optimize the adsorption of oxygen-related intermediates of oxygen evolution reaction (OER) on CoFeP NPs. Superior catalytic activity toward ORR (half-wave potential of 0.88 V) and OER (overpotential of 300 mV at 10 mA cm-2) express an unprecedented level in carbon-based transition metal-phosphide catalysts. The liquid zinc-air battery presents an outstanding cycling stability of 800 h (2400 cycles). This research offers a newfangled perception on designing highly efficient carbon-based bifunctional catalysts for ORR and OER.

A Review of Nitrogen-Doped Graphene Aerogel in Electromagnetic Wave Absorption

Graphene aerogels (GAs) possess a remarkable capability to absorb electromagnetic waves (EMWs) due to their favorable dielectric characteristics and unique porous structure. Nevertheless, the introduction of nitrogen atoms into graphene aerogels can result in improved impedance matching. In recent years, nitrogen-doped graphene aerogels (NGAs) have emerged as promising materials, particularly when combined with magnetic metals, magnetic oxides, carbon nanotubes, and polymers, forming innovative composite systems with excellent multi-functional and broadband absorption properties. This paper provides a comprehensive summary of the synthesis methods and the EMW absorption mechanism of NGAs, along with an overview of the absorption properties of nitrogen-doped graphene-based aerogels. Furthermore, this study sheds light on the potential challenges that NGAs may encounter. By highlighting the substantial contribution of NGAs in the field of EMW absorption, this study aims to facilitate the innovative development of NGAs toward achieving broadband absorption, lightweight characteristics, and multifunctionality.

Lewis Acid Site Assisted Bifunctional Activity of Tin Doped Gallium Oxide and Its Application in Rechargeable Zn-Air Batteries

The enhanced safety, superior energy, and power density of rechargeable metal-air batteries make them ideal energy storage systems for application in energy grids and electric vehicles. However, the absence of a cost-effective and stable bifunctional catalyst that can replace expensive platinum (Pt)-based catalyst to promote oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air cathode hinders their broader adaptation. Here, it is demonstrated that Tin (Sn) doped β-gallium oxide (β-Ga₂ O₃ ) in the bulk form can efficiently catalyze ORR and OER and, hence, be applied as the cathode in Zn-air batteries. The Sn-doped β-Ga₂ O₃ sample with 15% Sn (Sn_{x =0.15} -Ga₂ O₃ ) displayed exceptional catalytic activity for a bulk, non-noble metal-based catalyst. When used as a cathode, the excellent electrocatalytic bifunctional activity of Sn_{x =0.15} -Ga₂ O₃ leads to a prototype Zn-air battery with a high-power density of 138 mW cm^-2 and improved cycling stability compared to devices with benchmark Pt-based cathode. The combined experimental and theoretical exploration revealed that the Lewis acid sites in β-Ga₂ O₃ aid in regulating the electron density distribution on the Sn-doped sites, optimize the adsorption energies of reaction intermediates, and facilitate the formation of critical reaction intermediate (O*), leading to enhanced electrocatalytic activity.

Interconnected Hollow Porous Polyacrylonitrile-Based Electrolyte Membrane for a Quasi-Solid-State Flexible Zinc-Air Battery with Ultralong Lifetime

Quasi-solid-state flexible zinc-air batteries (FZABs) have received enormous attention due to their low cost and high safety. However, the constraints in lifetime resulting from the lack of stable quasi-solid-state electrolyte membranes and efficient bifunctional electrocatalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) hinder the large-scale manufacture and commercialization of FZABs to power electric devices. Herein, a polyacrylonitrile (PAN)-based membrane (HPPANP) fabricated via facile coaxial electrospinning, water dissolution, lyophilization, and KOH preimmersion method was utilized as the quasi-solid-state electrolyte membrane. The interconnected hollow porous structure based on PAN nanofibers endows HPPANP with outstanding electrolyte-uptake/retention capabilities for high ionic conductivity and nanolevel wetted electrolyte/anode interface for uniform Zn dissolution/deposition, thus prolonging the lifespan of the FZABs. In addition, the in situ alkaline hydrolysis of KOH solution supplies HPPANP with abundant oxygen-containing groups, which also improves its ionic conductivity. Additionally, we synthesized a Co/N-doped hollow carbon sphere (CoN-CS) electrocatalyst that exhibits superior ORR and OER electrocatalytic activities with a low potential difference (ΔE) of 0.73 V. Such favorable ORR and OER performances can be mainly attributed to the hierarchical hollow micro/nanostructures with abundant active sites, long-term stability, and favorable electron/ion diffusion pathway. As a result, the assembled FZAB equipped with the CoN-CS catalyst and HPPANP displays high power density (123.8 mW cm^-2) and preferable long-term cycling performance (more than 50 h at 3 mA cm^-2).

Efficient iron-cobalt oxide bifunctional electrode catalysts in rechargeable high current density zinc-air batteries

Iron-cobalt (FeCo) oxides dispersed on reduced graphene oxide (rGO) were synthesized from nitrate precursors at loading levels from 10 wt% to 60 wt%. These catalysts were tested in lab-scale zinc-air batteries (ZABs) at a high current density of 100 mA cm^-2 of the cathode area for the first time, cycling between 60 min of discharging and 60 min of charging. The optimum loading level for the best ZAB cycling performance was found to be 40 wt%, at which CoFe₂O₄ and CoO nanocrystals were detected. A discharge capacity of at least 90% was maintained for about 60 cycles with FeCo 40 wt%, demonstrating superior stability over amorphous FeCo oxides with FeCo 10 wt% despite similar performance at electrochemical tests. At a high current density of 100 mA cm^-2, OER catalytic activity was found to be the limiting factor in ZAB's cyclability. The discrepancies between the ORR/OER catalytic activities by electrochemical and battery cycling test results highlight the role and importance of rGO in improving electrical conductivity and activation of metal oxide electrocatalysts under high current density conditions. The difference of battery cycling test results from traditional electrochemical test results suggests that electrochemical tests conducted at low current densities may be inadequate in predicting practical battery cycling performance.

High-ammonia selective metal–organic framework–derived Co-doped Fe/Fe2O3 catalysts for electrochemical nitrate reduction

Electrochemical reduction of nitrate pollution in water into value-added ammonium is essential for modern agriculture and industry and represents a potentially sustainable strategy to replace the traditional Haber–Bosch process. However, the nitrate reduction reaction (NO₃⁻RR) process under ambient conditions often suffers from low selectivity. Here, we developed a strategy of tuning an electronic structure for preparing cobalt-doped Fe@Fe₂O₃ electrocatalysts. Cobalt doping tunes the Fe d-band center, thereby modulating the adsorption energies of intermediates and suppressing hydrogen production. Therefore, the electrocatalysts exhibit superior NO₃⁻RR activity with a high nitrate removal capacity (100.8 mg N g_cat⁻¹ h⁻¹), NH₃ selectivity (99.0 ± 0.1%), and faradaic efficiency (85.2 ± 0.6%). This strategy provides an approach to design advanced materials for NO₃⁻RR.

Ammonia (NH₃) is an ideal carbon-free power source in the future sustainable hydrogen economy for growing energy demand. The electrochemical nitrate reduction reaction (NO₃⁻RR) is a promising approach for nitrate removal and NH₃ production at ambient conditions, but efficient electrocatalysts are lacking. Here, we present a metal–organic framework (MOF)–derived cobalt-doped Fe@Fe₂O₃ (Co-Fe@Fe₂O₃) NO₃⁻RR catalyst for electrochemical energy production. This catalyst has a nitrate removal capacity of 100.8 mg N g_cat⁻¹ h⁻¹ and an ammonium selectivity of 99.0 ± 0.1%, which was the highest among all reported research. In addition, NH₃ was produced at a rate of 1,505.9 μg h⁻¹ cm⁻², and the maximum faradaic efficiency was 85.2 ± 0.6%. Experimental and computational results reveal that the high performance of Co-Fe@Fe₂O₃ results from cobalt doping, which tunes the Fe d-band center, enabling the adsorption energies for intermediates to be modulated and suppressing hydrogen production. Thus, this study provides a strategy in the design of electrocatalysts in electrochemical nitrate reduction.

Exploring the Dominant Role of Atomic- and Nano-Ruthenium as Active Sites for Hydrogen Evolution Reaction in Both Acidic and Alkaline Media

Ru nanoparticles (NPs) and single atoms (SAs)-based materials have been investigated as alternative electrocatalysts to Pt/C for hydrogen evolution reaction (HER). Exploring the dominant role of atomic- and nano-ruthenium as active sites in acidic and alkaline media is very necessary for optimizing the performance. Herein, an electrocatalyst containing both Ru SAs and NPs anchored on defective carbon (RuSA+NP /DC) has been synthesized via a Ru-alginate metal-organic supramolecules conversion method. RuSA+NP /DC exhibits low overpotentials of 16.6 and 18.8 mV at 10 mA cm-2 in acidic and alkaline electrolytes, respectively. Notably, its mass activities are dramatically improved, which are about 1.1 and 2.4 times those of Pt/C at an overpotential of 50 mV in acidic and alkaline media, respectively. Theoretical calculations reveal that Ru SAs own the most appropriate H* adsorption strength and thus, plays a dominant role for HER in acid electrolyte, while Ru NPs facilitate the dissociation of H2 O that is the rate-determining step in alkaline electrolyte, leading to a remarkable HER activity.

A defect-rich ultrathin MoS2/rGO nanosheet electrocatalyst for the oxygen reduction reaction

The structural properties such as high specific surface area, good electrical conductivity, rich-defects of the catalyst surface guarantee outstanding catalytic performance and durability of oxygen reduction reaction (ORR) electrocatalysts. It is still a challenging task to construct ORR catalysts with excellent performance. Herein, we have reported column-like MoS2/rGO with defect-rich ultrathin nanosheets prepared by a convenient solvothermal method. The structure and composition of MoS2/rGO are systematically investigated. MoS2/rGO shows a remarkable electrocatalytic performance, which is characterized by an outstanding onset potential of 0.97 V, a half-wave potential of 0.83 V, noticeable methanol tolerance, and durability of 93.7% current retention, superior to commercial Pt/C. The ORR process occurring on MoS2/rGO is a typical four electron pathway. Therefore, this study achieves the design of a low-cost, highly efficient and stable nonprecious metal ORR electrocatalyst in alkaline media.

Carbon-nanotube-entangled Co,N-codoped carbon nanocomposite for oxygen reduction reaction

The design of highly efficient and stable electrocatalysts for oxygen reduction reaction (ORR) is still a great challenge. Herein, we prepared Co,N-codoped carbon nanocomposites (Co@NC-ZM) with entangled carbon nanotubes. The large Brunauer-Emmett-Teller surface area (604.7 m² g^-1), rich mesoporous feature, Co,N doping and synergetic effect between various species of Co@NC-ZM can expose more active sites and facilitate conductivity and mass transport. Benefiting from the above unique advantages, Co@NC-ZM exhibits excellent ORR performance with more positive onset potential (0.96 V) and half-wave potential (0.83 V) than those of commercial Pt/C (0.96 and 0.81 V, correspondingly). This work provides a new strategy for further exploring efficient non-precious-metal-based catalysts for ORR.

Surface-confinement assisted synthesis of nitrogen-rich single atom Fe-N/C electrocatalyst with dual nitrogen sources for enhanced oxygen reduction reaction

The utilization of earth abundant iron and nitrogen doped carbon as a precious-metal-free electrocatalyst for oxygen reduction reaction (ORR) significantly depends on the rational design and construction of desired Fe-N_xmoieties on carbon substrates, which however remains an enormous challenge. Herein a typical nanoporous nitrogen-rich single atom Fe-N/C electrocatalyst on carbon nanotube (NR-CNT@FeN-PC) was successfully prepared by using CNT as carbon substrate, polyaniline (PANI) and dicyandiamine (DCD) as binary nitrogen sources and silica-confinement-assisted pyrolysis, which not only facilitate rich N-doping for the inhibition of the Fe agglomeration and the formation of single atom Fe-N_xsites in carbon matrix, but also generate more micropores for enlarging BET specific surface area (up to 1500 m²·g^-1). Benefiting from the advanced composition, nanoporous structure and surface hydrophilicity to guarantee the sufficient accessible active sites for ORR, the NR-CNT@FeN-PC catalyst under optimized conditions delivers prominent ORR performance with a half-wave potential (0.88 V versus RHE) surpass commercial Pt/C catalyst by 20 mV in alkaline electrolyte. When assembled in a home-made Zn-air battery device as cathodic catalyst, it achieved a maximum output power density of 246 mW·cm^-2and a specific capacity of 719 mA·h·g^-1_Znoutperformed commercial Pt/C catalyst, holding encouraging promise for the application in metal-air batteries.

Micelles of Mesoporous Silica with Inserted Iron Complexes as a Platform for Constructing Efficient Electrocatalysts for Oxygen Reduction

Iron, N-codoped carbon materials (Fe-N-C) are promising electrocatalysts toward oxygen reduction reactions due to their high atom utilization efficiency and intrinsic activity. Nanostructuring of the Fe-N-C materials, such as introducing porosity into the carbon structure, would be conducive to further increasing the exposure of active sites as well as improving the mass transfer. Herein, we explore the potential of iron complex-functionalized micelles of mesoporous SiO₂ as a platform for constructing porous Fe-N-C materials. The classical three-dimensional MCM-48 was selected as a proof-of-concept example, which was utilized as the hard template, and cetyltrimethylammonium bromide micelles inside it played the role of the main carbon source. Fe-N_x sites were derived from Fe-1,10-phenanthroline complexes in the micelles introduced by in situ incorporation of 1,10-phenanthroline and post Fe²⁺ insertion in an aqueous solution. After thermal annealing in a nitrogen atmosphere and subsequent removal of the MCM-48 framework, a carbon material that possesses porous structural features with uniformly dispersed Fe-N_x sites (MPC@PhFe) was obtained, which shows superior ORR activity in a 0.1 M KOH solution and great potential for Zn-air battery applications as well. This work demonstrates the feasibility as well as the effectiveness of turning micelles of mesoporous SiO₂ into porous carbon structures and might offer a universal strategy for manufacturing carbon materials for future application in energy storage and conversion.

Electronically interacted Co3O4/WS2 as superior oxygen electrode for rechargeable zinc-air batteries

The electronically interacted Co3O4/WS2 with a maximum power density of 174 mW cm-2, 2.3 fold better than Pt/C-IrO2, shows its superiority as an oxygen electrode for rechargeable zinc-air batteries.

A Composite Bifunctional Oxygen Electrocatalyst for High-Performance Rechargeable Zinc-Air Batteries

Rechargeable zinc-air batteries are considered as next-generation energy storage devices because of their ultrahigh theoretical energy density of 1086 Wh kg^-1 (including oxygen) and inherent safety originating from the use of aqueous electrolyte. However, the cathode processes regarding oxygen reduction and evolution are sluggish in terms of kinetics, which severely limit the practical battery performances. Developing high-performance bifunctional oxygen electrocatalysts is of great significance, yet to achieve better bifunctional electrocatalytic reactivity beyond the state-of-the-art noble-metal-based electrocatalysts remains a great challenge. Herein, a composite Co₃ O₄ @POF (POF=framework porphyrin) bifunctional oxygen electrocatalyst is proposed to construct advanced air cathodes for high-performance rechargeable zinc-air batteries. The as-obtained composite Co₃ O₄ @POF electrocatalyst exhibits a bifunctional electrocatalytic reactivity of ΔE=0.74 V, which is better than the noble-metal-based Pt/C+Ir/C electrocatalyst and most of the reported bifunctional ORR/OER electrocatalysts. When applied in rechargeable zinc-air batteries, the Co₃ O₄ @POF cathode exhibits a reduced discharge-charge voltage gap of 1.0 V at 5.0 mA cm^-2 , high power density of 222.2 mW cm^-2 , and impressive cycling stability for more than 2000 cycles at 5.0 mA cm^-2 .