CoNi Nanoparticles Supported on N-Doped Bifunctional Hollow Carbon Composites as High-Performance ORR/OER Catalysts for Rechargeable Zn-Air Batteries

Redox Activity of Co Species in the Active Sites of CoNx/CoOx Facilitates Oxygen Electrocatalysis for Zn-Air Batteries

Developing efficient bifunctional oxygen electrocatalysts is crucial for enhancing the performance of rechargeable Zn-air batteries (ZABs). In this study, cobalt/cobalt oxides embedded in N-doped carbon nanofibers (Co/CoOx/NCNFs) were synthesized through a combination of electrospinning and annealing processes. The resulting Co/CoOx/NCNFs catalysts feature abundant CoNx and CoOx active species, leveraging the large specific surface area of nanofibers to facilitate oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The optimized Co/CoOx/NCNFs-0.1 achieved a half-wave potential (vs. RHE) of 0.82 V and required only 429 mV to reach 10 mA cm-2 in a typical three-electrode system with 0.1 M KOH using an electrochemical workstation equipped with a pine instruments rotator, outperforming the Pt/C+RuO2. The assembled ZABs exhibited high specific capacity (771 mAh gZn -1), substantial power density (981.6 mWh gZn -1), and long-term stability (>325 h). In situ Raman spectroscopy confirmed that the electrocatalytic processes involve the redox activity of Co (II and III) species derived from abundant CoNx and CoOx, elaborating the origin of the catalysts' exceptional oxygen electrocatalysis performance. This work not only presents a straightforward and effective approach for producing bifunctional oxygen electrocatalysts in ZABs but also sheds light on the catalytic mechanisms underlying ORR and OER for CoNx/CoOx-based oxygen electrocatalysts.

A Photo-Assisted Zinc-Air Battery with MoS2/Oxygen Vacancies Rich TiO2 Heterojunction Photocathode

Converting solar energy into electrochemical energy is a sustainable strategy, but the design of photo-assisted zinc-air battery (ZAB) with efficient utilization of sunlight faces huge challenges. Herein, a photo-assisted ZAB of a three-electrode system using MoS2/oxygen vacancies-rich TiO2 heterojunction as charge cathode and Fe, N-doped carbon matrix (FeNC) as discharge cathode is constructed, where MoS2 is chosen as solar light-responsive catalytic material and TiO2 acts as electron transport layer and hole blocking layer, arising from a train of thought for efficient charging under sunlight irradiation and light-independent discharging. The introduction of oxygen vacancies in TiO2 facilitates the temporary trapping of carriers and triggers rapid carrier transfer at the interface of the heterojunction, which hinders the recombination of photogenerated holes, thereby facilitating their further participation in the oxygen evolution reaction. Moreover, FeNC exhibits superior oxygen reduction reaction performance due to strong d-π interactions. As a result, the well-built ZABs deliver a low charge voltage (0.71 V) under illumination at 0.1 mA cm-2, and a high power density (167.6 mW cm-2) in dark. This work paves a special way for the development of ZABs by directly harvesting solar energy in charging and efficiently discharging regardless of lighting conditions.

Enhancing activity and stability of FeNC catalysts through co incorporation for oxygen reduction reaction

FeNC single atom catalysts (SACs) have attracted great interest due to their highly active FeN4 sites. However, the pyrolysis treatment often leads to inevitable metal migration and aggregation, which reduces the catalytic activity. Moreover, due to the Fenton reaction caused by FeNC in alkaline and acidic solutions, the presence of Fe and peroxide in electrodes may generate free radicals, resulting in serious degradation of the organic ionomer and the membrane. Herein, we report an original strategy of introducing Co single atoms into FeNC catalysts, forming atomically dispersed bimetallic active sites (FeCoNC) and improving the activity and stability of the catalyst. Benefiting from this strategy, FeCoNC catalyst exhibits excellent oxygen reduction reaction (ORR) activity in alkaline media (E1/2 = 0.88 V) and in acidic media (E1/2 = 0.77 V). As the cathode of Zn-air battery (ZAB), FeCoNC shows an excellent peak power density of 142.8 mW cm-2 and a specific capacity of 806.6 mAh/gZn. This work provides a novel avenue to optimize and enhance the ORR performance of atomic dispersed FeNC catalysts.

Enhancing Electrocatalytic Kinetics via Synergy of Co Nanoparticles and Co/Ni-N4-C- and N-Doped Porous Carbon

Transition-metal species embedded in carbon have sparked intense interest in the fields of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, improvement of the electrocatalytic kinetics remains a challenge caused by the synergistic assembly. Here, we propose a biochemical strategy to fabricate the Co nanoparticles (NPs) and Co/Ni-N4-C co-embedded N-doped porous carbon (CoNPs&Co/Ni-N4-C@NC) catalysts via constructing the zeolitic imidazolate framework (ZIF)@yeast precursor. The rich amino groups provide the possibility for the anchorage of Co2+/Ni2+ ions as well as the construction of Co/Ni-ZIF@yeast through the yeast cell biomineralization effect. The functional design induces the formation of CoNPs and Co/Ni-N4-C sites in N-doped carbon as well as regulates the porosity for exposing such sites. Synergy of CoNPs, Co/Ni-N4-C, and porous N-doped carbon delivered excellent electrocatalytic kinetics (the ORR Tafel slope of 76.3 mV dec-1 and the OER Tafel slope of 80.4 mV dec-1) and a high voltage of 1.15 V at 10 mA cm-2 for the discharge process in zinc air batteries. It provides an effective strategy to fabricate high-performance catalysts.

Movable-type printing method to fabricate ternary FeCoNi alloys confined in porous carbon towards oxygen electrocatalysts for rechargeable Zn-air batteries

Transition metal-based carbon catalysts are a promising class of electrocatalysts to enhance the efficiency of energy conversion and storage devices. However, it remains a challenging task to develop multi-metal alloy catalysts. Herein, ternary FeCoNi alloy nanoparticles (NPs) confined in nitrogen-doped carbon (NC) catalysts were fabricated via a facile movable-type printing method, where a range of transition metals confined in NC catalysts was prepared using the same technique except for the adjustment of the metal precursors. Due to the unique electronic structure and significant active sites of the medium-entropy alloy, the FeCoNi-NC catalysts demonstrated highly efficient bifunctional electrocatalytic activities for the oxygen reduction (E1/2 = 0.838 V) and evolution (Eoverpotential = 330 mV, 10 mA cm-2) reactions, which were comparable to those of Pt/C and RuO2. Moreover, the FeCoNi-NC-based liquid rechargeable ZABs displayed a substantial power density of 231.2 mW cm-2, and the homemade flexible ZABs also exhibited outstanding activity and cycling durability. Thus, this movable-type printing method is suitable for constructing a variety of multi-metal-based catalysts for metal air batteries.

Tetraiodo Fe/Ni phthalocyanine-based molecular catalysts for highly efficient oxygen reduction reaction and oxygen evolution reaction: Constructing a built-in electric field with iodine groups

In this paper, we report on the preparation and catalysis of a bifunctional molecular catalyst (Fe[Pc(I)4]+Ni[Pc(I)4]@NCPDI) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable Zn-air batteries. This catalyst is prepared by self-assembling tetraiodo metal phthalocyanines (Fe[Pc(I)4] and Ni[Pc(I)4]) on a 2D N-doped carbon material (NCPDI) through π-π interactions. The introduction of iodine groups in the edge of phthalocyanines controls the density of electron cloud and electrostatic potential around Fe-N/Ni-N sites and constructs a built-in electric field that facilitates directional transport of charges, enhancing the catalytic activity of the catalyst. Density functional theory (DFT) calculations support this mechanism by showing a reduced energy barrier for the ORR rate-determining step (RDS). The Fe[Pc(I)4]+Ni[Pc(I)4]@NCPDI exhibits excellent performance outperforming 20 wt% Pt/C and single-molecule self-assembled Fe[Pc(I)4]@NCPDI and Ni[Pc(I)4]@NCPDI, with a half-wave potential of E1/2 = 0.940 V in the ORR process under alkaline condition. During the OER process, Fe[Pc(I)4]+Ni[Pc(I)4]@NCPDI exhibited a low overpotential of 298 mV at 10 mA cm-2 under the alkaline condition, which is much better than RuO2, Fe[Pc(I)4]@NCPDI and Ni[Pc(I)4]@NCPDI. The catalyst also demonstrates excellent catalysis and durability in rechargeable Zn-air batteries. This work provides a simple and specific method to develop efficient multifunctional molecular electrocatalysts.

Bimetallic-Coordinated Covalent Triazine Framework-Derived FeNi Alloy Nanoparticle-Decorated Coral-Like Nanocarbons for Oxygen Electrocatalysis

It is highly desirable to fabricate transition bimetallic alloy-embedded porous nanocarbons with a unique nanoarchitecture for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in rechargeable zinc-air batteries. In this work, we introduce a template-assisted in situ alloying synthesis of FeNi alloy nanoparticle-decorated coral-like nanocarbons (FeNi-CNCs) as efficient OER/ORR dual-functional electrocatalysts. The present materials are produced through polycondensation of a covalent triazine framework (CTF), the coordination of Ni and Fe ions, and sequential pyrolytic treatment. Through the pyrolysis process, the nanolamellar FeNi-CTF precursors can be facilely converted into FeNi alloy nanoparticle-decorated nanocarbons. These nanocarbons possess a distinctive three-dimensional (3D) coral-like nanostructure, which is favorable for the transport of oxygen and the diffusion of electrolyte. As a result, FeNi-CNC-800 with the highest efficiency exhibited remarkable electrocatalytic performance and great durability. Additionally, it also can be assembled into rechargeable zinc-air batteries that can be assembled in both liquid and solid forms, offering a superior peak power density, large specific capacity, and outstanding reusability during charging/discharging cycles (e.g., 5160 charging-and-discharging cycles at 10 mA cm-2 for the liquid forms). These traits make it a highly promising option in the burgeoning field of wearable energy conversion.

Confined covalent organic framework anchored Fe sites derived highly uniform electrocatalysts for rechargeable aqueous and solid-state Zn-air batteries

Exploiting clean, highly efficient energy storage and conversion device like Zn-air battery is of significance for alleviating the energy and environmental crises of this society. Metal organic coordination polymers/frameworks have been regarded as ideal templates to synthesize non-noble metal catalysts for a long time. However, the high density of metal nodes inevitably leads to the heavy aggregation of metal nanoparticles during thermolysis transformation process, which greatly hinders the maximizing of electrochemical performances. Herein, covalent organic framework (COF) has been employed to anchor the quantificational Fe ions (COF-Fe) and then confined into the macropores of g-C3N4 to improve the dispersion of metal active sites and avoid severe aggregation during high temperature pyrolysis. After calcination, the metal nanoparticles highly dispersed Fe-CFN catalysts can be obtained. The optimal Fe-CFN-800 catalysts exhibit excellent ORR and OER performances with the potential difference between ORR and OER of merely 0.723 V. Moreover, experimental way and DFT theoretical calculations are also employed to disclose the reaction mechanism. Finally, the all-solid-state and aqueous Zn-air batteries assembled with the optimized Fe-CFN-800 as cathode present excellent performances with high peak power density, flexible rate performance, strong discharge stability and long-term charge-discharge cycling performance.

In situ growth of surface-reconstructed aluminum fluoride nanoparticles on N, F codoped hierarchical porous carbon nanofibers as efficient ORR/OER bifunctional electrocatalysts for rechargeable zinc-air batteries

Modified porous carbon fibers have emerged as crucial electrocatalytic materials for zinc-air battery (ZAB) systems. However, most methods for preparing porous carbon fibers are complex and exhibit single functionality and poor catalytic activity, which hinders the development of ZABs. In this study, we design and synthesize a novel type of N, F codoped hierarchical porous carbon fiber with in situ growth of aluminum fluoride nanoparticles (AlF3@HPCNFs) through electrospinning and high-temperature carbonization. The N, F codoping effectively adjusts the charge density of neighboring carbon atoms and introduces additional active sites. Furthermore, the catalytic process induces surface reconstruction of AlF3 nanoparticles, allowing for their full exposure to the liquid electrolyte and accelerated catalytic reactions. Additionally, this interconnected hierarchical porous structure accelerates mass transfer at the oxygen/carbon-based substrate/electrolyte three-phase interfaces, thereby enhancing reaction kinetics and the accessibility of catalytic active sites, ultimately improving the utilization efficiency of these sites. Consequently, the AlF3@HPCNFs catalyst exhibits excellent bifunctional performance with a narrow potential difference (△E = 0.67 V). Moreover, the obtained bifunctional electrocatalyst is utilized for rechargeable ZABs, surpassing commercially available Pt/C + RuO2 cells in terms of high specific capacity (796 mAh gzn-1) and outstanding cycling stability (over 500 h). This research demonstrates the potential of AlF3@HPCNFs as a bifunctional electrocatalyst and introduces a simplified and effective method for the fabrication of metal fluoride-modified and hierarchically porous carbon nanofibers for rechargeable ZABs.

CoNi Alloys Encapsulated in N-Doped Carbon Nanotubes for Stabilizing Oxygen Electrocatalysis in Zinc-Air Battery

Alloy-based catalysts with high corrosion resistance and less self-aggregation are essential for oxygen reduction/evolution reactions (ORR/OER). Here, via an in situ growth strategy, NiCo alloy-inserted nitrogen-doped carbon nanotubes were assembled on a three-dimensional hollow nanosphere (NiCo@NCNTs/HN) using dicyandiamide. NiCo@NCNTs/HN exhibited better ORR activity (half-wave potential (E_1/2) of 0.87 V) and stability (E_1/2 shift of only -13 mV after 5000 cycles) than commercial Pt/C. NiCo@NCNTs/HN displayed a lower OER overpotential (330 mV) than RuO₂ (390 mV). The NiCo@NCNTs/HN-assembled zinc-air battery exhibited high specific-capacity (847.01 mA h g^-1) and cycling-stability (291 h). Synergies between NiCo alloys and NCNTs facilitated the charge transfer to promote 4e^- ORR/OER kinetics. The carbon skeleton inhibited the corrosion of NiCo alloys from surface to subsurface, while inner cavities of CNTs confined particle growth and the aggregation of NiCo alloys to stabilize bifunctional activity. This provides a viable strategy for the design of alloy-based catalysts with confined grain-size and good structural/catalytic stabilities in oxygen electrocatalysis.

Synthesis of hierarchically structured Fe3C/CNTs composites in a FeNC matrix for use as efficient ORR electrocatalysts

Fe-N-C has a high number of FeN _x active sites and has thus been regarded as a high-performance oxygen reduction reaction (ORR) catalyst, and combining Fe₃C with Fe-N-C typically boosts ORR activity. However, the catalytic mechanism remains unknown, limiting further research and development. In this study, a precipitation-solvothermal process was used in conjunction with pyrolysis to produce a series of Fe-N-C catalysts derived from a zeolitic imidazolate framework (ZIF) that was composited with Fe₃C. The prepared catalysts had a multiscale structure of ZIF-like carbon particles and rod-like structures, as well as bamboo-like carbon nanotubes (CNTs) and carbon layers wrapped with Fe₃C particles while a series of studies revealed the origin of the rod-like structures and Fe₃C phase. The hierarchical structure was beneficial to the enhanced electrocatalytic performance of catalysts for ORR. The optimal sample had the highest half-wave potential of 0.878 V vs. RHE, which was higher than that of commercial Pt/C (0.861 V vs. RHE). The ECSA of the optimal sample was 1.08 cm² μg^-1, with an electron transfer number close to 4, and functioning kinetics. The optimal sample exhibited high durability and methanol tolerance for the ORR. Finally, blocking different Fe active sites with coordination ions demonstrated that Fe(ii) was the main active site, indicating that Fe₃C primarily served as a cocatalyst to optimize the electron structure of Fe-N-C, thereby synergistically improving the ORR activity.

Tailoring the MOF structure via ligand optimization afforded a dandelion flower like CoS/Co-Nx/CoNi/NiS catalyst to enhance the ORR/OER in zinc-air batteries

Due to their affordability and good catalytic activity for oxygen reactions, MOF-derived carbon composites containing metal alloys have piqued interest. However, during synthesis, MOFs have the disadvantage of causing significant carbon evaporation, resulting in a reduction of active sites and durability. This study proposes tailoring the molecular structure of MOFs by optimizing bipyridine and flexible 4-aminodiacetic terephthalic acid ligands, which have numerous coordination modes and framework structures, resulting in fascinating architectures. MOF frameworks having optimized N and O units are coordinated with Co and Ni ions to provide MOF precursors that are annealed at 700 °C in argon. The MOF-derived Co₉S₈/Co-N_x/CoNi/Ni₃S₂@CNS-4 catalyst exhibits excellent catalytic activity, revealing an ORR half-wave potential of 0.86 V and an overpotential (OER) of 196 mV at 10 mA cm^-2, a potential gap of 0.72 V and a Tafel slope of 79 mV dec^-1. The proposed strategy allows for the rational design of N-coordinated Co and CoNi alloys attached to ultrathin N, S co-doped graphitic carbon sheets to enhance bifunctional activity and sufficient active sites. Consequently, the zinc-air battery using the synthesized catalyst shows a high peak power density of 206.9 mW cm^-2 (Pt/C + RuO₂ 116.1 mW cm^-2), a small polarization voltage of 0.96 V after 370 h at 10 mA cm^-2, and an outstanding durability of over 2400 cycles (400 h). The key contributions to the superior performance are the synergetic effects of the CoNi alloys plus the N,S-incorporated carbon skeleton, due to the small charge transfer resistances and enhanced active sites of CoNi, metal-S, and pyridinic N.

Hierarchically hollow N-doped carbon-cobalt nanoparticle heterointerface for efficient bifunctional oxygen electrocatalysis

The rational design and fabrication of high-performance and durable bifunctional non-noble-metal electrocatalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are still a great challenge in the practical applications of rechargeable zinc-air (Zn-air) batteries. Herein, we report a simple yet robust route to synthesize cobalt nanoparticles rooted in the hierarchically hollow nitrogen-doped carbon frameworks (Co@HNCs). This strategy employs the pyrolysis of nanostructured hollow Co-based metal-organic framework (ZIF-67) precursors produced by selective linker cleaving with pyrazino(2,3-f)(1,10)phenanthroline-2,3-dicarboxylic acid molecules (H₂PPDA). The designed hierarchically architecture is favorable for the accessibility of the active sites in the catalyst, which affords enhanced bifunctional performance for ORR and OER. Moreover, when used as a cathode in liquid and all-solid-state Zn-air batteries, the resultant Co@HNCs delivers high efficiency and outstanding durability, even outperforming the benchmark Pt/C + RuO₂. This work provides a feasible design avenue to achieve advanced dual-phasic oxygen electrocatalyst and promotes the development of rechargeable Zn-air batteries.

Iron-nickel alloy nanoparticles encapsulated in nitrogen-doped carbon nanotubes as efficient bifunctional electrocatalyst for rechargeable zinc-air batteries

In this work, we present an efficient bifunctional electrocatalyst comprising iron-nickel (FeNi) alloy nanoparticles confined in nitrogen-doped carbon nanotubes (denoted FeNi-NCNT) for zinc-air batteries. The FeNi-NCNT electrocatalyst is fabricated by anion-exchange of nickel(II) ion/zinc(II) ion-2,4,6-tris-(di(pyridin-2-yl)amino)-1,3,5-triazine complex with ferricyanide anion followed by mixing with melamine and then carbonization. The resultant FeNi-NCNT electrocatalyst displays excellent bifunctional activity with a low reversible oxygen electrode index of 0.725 V. The rechargeable aqueous zinc-air battery fabricated with FeNi-NCNT air cathode manifests both high specific capacity (819 mAh g^-1_Zn at 5.0 mA cm^-2) and long cycle life (1500 cycles/1000 h at 10 mA cm^-2, 600 cycles/400 h at 25 mA cm^-2, and 165 cycles/110 h at 50 mA cm^-2). Moreover, flexible solid-state zinc-air battery assembled with FeNi-NCNT air cathode can deliver a specific capacity of 765 mAh g^-1_Zn at 5.0 mA cm^-2, a power density of 84.8 mW cm^-2, and a cycle life of 110 h (330 cycles) at 2.0 mA cm^-2.

Electrostatic Interaction in Amino Protonated Chitosan-Metal Complex Anion Hydrogels: A Simple Approach to Porous Metal Carbides/N-Doped Carbon Aerogels for Energy Conversion

In the face of the increasingly serious rapid depletion of fossil fuels, exploring alternative energy conversion technologies may be a promising choice to alleviate this crisis. Transition metal carbides (TMCs)/carbon composites are considered as prospective electrocatalysts due to their high catalytic activities and structural stability. In this work, we report the simple synthesis of TMCs/N-doping carbon aerogels (TMCs/NCAs, including Fe₃C/NCA, Mo₃C₂/NCA, and Fe₃C-Mo₂C/NCA) for the oxygen reduction reaction (ORR) using protonated chitosan/metal complex anion-chelated aerogels. Among them, the Fe₃C/NCA composite possesses efficient ORR activity (similar to Pt/C), and the Fe₃C/NCA-assembled Zn-air battery exhibits high power densities of about 250 mW cm^-2. The density functional theory calculation reveals that the presence of graphite-N, pyridine-N, and carbon defects in the carbon framework effectively reduces the free energy of ORR occurring in Fe₃C. This work provides a simple and extensible strategy for the preparation of TMCs from chitosan, which is expected to be extended to other metal carbides.

The crystal structure and oxygen reduction reaction of Ni(II)-complex templated borate-sulfate and borate

Three nickel borate compounds, [Ni(1-MI)₆]·[B(OH)₃]₄·SO₄ (1-MI = 1-methylimidazole) (1), [Ni(H₂O)₃(1-MI)₃]·[B₅O₆(OH)₄]₂ (2) and [Ni(DMA3)]·[B₆O₇(OH)₆]·3.5H₂O (DMA3 = N,N-dimethylethylenediamine) (3), have been synthesized. It is noteworthy that the structures of 1 and 2 can be adjusted by varying the ratio of amine. Compound 3 has shown an unexpected example of unique water clusters in its structure. The three frameworks exhibit different interlinkage modes, resulting in channels varying in their size and shape. These compounds have been characterized by FTIR, UV-vis and PXRD. In addition, 1, 2, and 3 exhibited different wide band gaps (4.4 eV for 1, 4.5 eV for 2 and 4.4 eV for 3), and ORR activities with a half-wave potential of 0.78 V for 1, 0.74 V for 2 and 0.79 V for 3.

Cu/Co/CoS2 embedded in S,N doped carbon as highly-efficient oxygen reduction and evolution electrocatalyst for rechargeable zinc-air batteries

In order to improve the retarded oxygen reduction and evolution reaction (ORR/OER) in rechargeable metal-air cells in electrochemical energy conversion systems, constructing multiphase nanostructured catalysts is an alternative strategy, where...