High-Density Atomic Fe–N4/C in Tubular,Biomass-Derived,Nitrogen-Rich Porous Carbon as Air-Electrodes for Flexible Zn–Air Batteries

Developing low-cost single-atom catalysts (SACs) with high-density active sites for oxygen reduction/evolution reactions (ORR/OER) are desirable to promote the performance and application of metal–air batteries. Herein, the Fe nanoparticles are precisely regulated to Fe single atoms supported on the waste biomass corn silk (CS) based porous carbon for ORR and OER. The distinct hierarchical porous structure and hollow tube morphology are critical for boosting ORR/OER performance through exposing more accessible active sites, providing facile electron conductivity, and facilitating the mass transfer of reactant. Moreover, the enhanced intrinsic activity is mainly ascribed to the high Fe single-atom (4.3 wt.%) loading content in the as-synthesized catalyst.Moreover, the ultra-high N doping (10 wt.%) can compensate the insufficient OER performance of conventional Fe N C catalysts. When as-prepared catalysts are assembled as air-electrodes in flexible Zn–air batteries, they perform a high peak power density of 101 mW cm⁻², a stable discharge–charge voltage gap of 0.73 V for >44 h, which shows a great potential for Zinc–air battery. This work provides an avenue to transform the renewable low-cost biomass materials into bifunctional electrocatalysts with high-density single-atom active sites and hierarchical porous structure.     

Sulfur Mismatch Substitution in Layered Double Hydroxides as Efficient Oxygen Electrocatalysts for Flexible Zinc–Air Batteries

Although layered double hydroxides (LDHs) are extensively investigated for oxygen electrocatalysis, their development is hampered by their limited active sites and sluggish reaction kinetics. Here, sulfur mismatch substitution of NiFe–LDH (S–LDH) is demonstrated, which are in-situ deposited on nitrogen-doped graphene (S–LDH/NG). This atomic-level sulfur incorporation leads to the construction of the tailored topological microstructure and the modulated electronic structure for the improved catalytic activity and durability of bifunctional electrocatalysts. The combined computational and experimental results clarify that the electron transfer between the sulfur anion and Fe³⁺ generates the high-valence Fe⁴⁺ species, while the mismatch substitution of the sulfur anion induces the metallic conductivity, an increased carrier density, and the reduced reaction barrier. Consequently, the as-fabricated Zn–air battery achieves a high power density of 165 mW cm^-2, a large energy density of 772 Wh kg_Zn^-1 at 5 mA cm^-2, and long cycle stability for 120 h, demonstrating its real-life operation.     

Dual-Phasic Carbon with Co Single Atoms and Nanoparticles as a Bifunctional Oxygen Electrocatalyst for Rechargeable Zn–Air Batteries

The great interest in rechargeable Zn–air batteries (ZABs) arouses extensive research on low-cost, high-active, and durable bifunctional electrocatalysts to boost the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). It remains a great challenge to simultaneously host high-active and independent ORR and OER sites in a single catalyst. Herein a dual-phasic carbon nanoarchitecture consisting of a single-atom phase for the ORR and nanosized phase for the OER is proposed. Specifically, single Co atoms supported on carbon nanotubes (single-atom phase) and nanosized Co encapsulated in zeolitic-imidazole-framework-derived carbon polyhedron (nanosized phase) are integrated together via carbon nanotube bridges. The obtained dual-phasic carbon catalyst shows a small overpotential difference of 0.74 V between OER potential at 10 mA cm⁻² and ORR half-wave potential. The ZAB based on the bifunctional catalyst demonstrates a large power density of 172 mW cm⁻². Furthermore, it shows a small charge-discharge potential gap of 0.51 V at 5 mA cm⁻² and outstanding discharge-charge cycling durability. This study provides a feasible design concept to achieve multifunctional catalysts and promotes the development of rechargeable ZABs.     

Engineering Crystallinity and Oxygen Vacancies of Co(II) Oxide Nanosheets for High Performance and Robust Rechargeable Zn–Air Batteries

Efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes highly rely on the rational design and synthesis of high-performance electrocatalysts. Herein, comprehensive characterizations and density functional theory (DFT) calculations are combined to verify the important roles of the crystallinity and oxygen vacancy levels of Co(II) oxide (CoO) on ORR and OER activities. A facile and controllable vacuum-calcination strategy is utilized to convert Co(OH)₂ into oxygen-defective amorphous-crystalline CoO (namely ODAC-CoO) nanosheets. With the carefully controlled crystallinity and oxygen vacancy levels, the optimal ODAC-CoO sample exhibits dramatically enhanced ORR and OER electrocatalytic activities compared with the pure crystalline CoO counterpart. The assembled liquid and quasi-solid-state Zn–air batteries with ODAC-CoO as cathode material achieve remarkable specific capacity, power density, and excellent cycling stability, outperforming the benchmark Pt/C + IrO₂ catalysts. This study theoretically proposes and experimentally demonstrates that the simultaneous introduction of amorphous structures and oxygen vacancies could be an effective avenue towards high-performance electrocatalytic ORR and OER.     

Bifunctional Covalent Organic Framework-Derived Electrocatalysts with Modulated p-Band Centers for Rechargeable Zn–Air Batteries

Fine control over the physicochemical structures of carbon electrocatalysts is important for improving the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable Zn–air batteries. Covalent organic frameworks (COFs) are considered good candidate carbon materials because their structures can be precisely controlled. However, it remains a challenge to impart bifunctional electrocatalytic activities for both the ORR and OER to COFs. Herein, a pyridine-linked triazine covalent organic framework (PTCOF) with well-defined active sites and pores is readily prepared under mild conditions, and its electronic structure is modulated by incorporating Co nanoparticles (CoNP-PTCOF) to induce bifunctional electrocatalytic activities for the ORR and OER. The CoNP-PTCOF exhibits lower overpotentials for both ORR and OER with outstanding stability. Computational simulations find that the p-band center of CoNP-PTCOF down-shifted by charge transfer, compared to pristine PTCOF, facilitate the adsorption and desorption of oxygen intermediates on the pyridinic carbon active sites during the reactions. The Zn–air battery assembled with bifunctional CoNP-PTCOF exhibits a small voltage gap of 0.83 V and superior durability for 720 cycles as compared with a battery containing commercial Pt/C and RuO₂. This strategy for modulating COF electrocatalytic activities can be extended for designing diverse carbon electrocatalysts.     

A Low-Cost,Durable Bifunctional Electrocatalyst Containing Atomic Co and Pt Species for Flow Alkali-Al/Acid Hybrid Fuel Cell and Zn–Air Battery

Transition metal single atoms anchored on nitrogen-doped carbon (M-N-C) matrix with M-N-C active sites have shown to be promising catalysts for both hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Herein, a hybrid catalyst with low-level loading of atomic Pt and Co species encapsulated in nitrogen-doped graphene (Pt@CoN₄-G) is developed. The Pt@CoN₄-G shows low overpotential for HER in wide-pH electrolyte and manifests improved mass activity with almost eight times greater than that of Pt/C at an overpotential of 50 mV. The Pt@CoN₄-G also exhibits a top-level ORR activity (half-wave potential, E_1/2 = 0.893 V) and robust stability (>200 h) in alkaline medium. Using theoretical calculations and comprehensive characterizations , the strong metal–support interactions between Pt species and CoN₄-G support and synergistical cooperation of multiple active sites are clarified. A flow alkali-Al/acid hybrid fuel cell using Pt@CoN₄-G as cathode catalyst delivers a large power density of 222 mW cm⁻² with excellent stability to achieve simultaneously hydrogen evolution and electricity generation. In addition, Pt@CoN₄-G endows a flow Zn-air battery with high power density (316 mW cm⁻²), good stability under large current density (>100 h at 100 mA cm⁻²), and long cycle life (over 600 h at 5 mA cm⁻²).     

Electrospinning Synthesis of Self-Standing Cobalt/Nanocarbon Hybrid Membrane for Long-Life Rechargeable Zinc–Air Batteries

Integrating high-efficiency oxygen electrocatalyst directly into air electrodes is vital for zinc–air batteries to achieve higher electrochemical performance. Herein, a self-standing membrane composed of hierarchical cobalt/nanocarbon nanofibers is fabricated by the electrospinning technique. This hybrid membrane can be directly employed as the bifunctional air electrode in zinc–air batteries and can achieve a high peak power density of 304 mW cm⁻² with a long service life of 1500 h at 5 mA cm⁻². Its assembled solid-state zinc–air battery also delivers a promising power density of 176 mW cm⁻² with decent flexibility. The impressive rechargeable battery performance would be attributed to the self-standing membrane architecture integrated by oxygen electrocatalysts with abundant cobalt–nitrogen–carbon active species in the hierarchical electrode. This study may provide effective electrospinning solutions in integrating efficient electrocatalyst and electrode for energy storage and conversion technologies.     

Assembling Amorphous Metal–Organic Frameworks onto Heteroatom-Doped Carbon Spheres for Remarkable Bifunctional Oxygen Electrocatalysis

Multifunctional electrocatalysts play an increasingly crucial role in various practical electrochemical energy conversion devices. Especially, on the air cathode of rechargeable zinc–air batteries (ZABs), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), requiring efficient bifunctional electrocatalysts, are switched during discharging and charging process. Here, supported by the theoretical computations, a facile strategy for the in situ assembly of NiFe-MOFs nanosheets on heteroatoms-doped porous activated carbon spheres is developed. The newly designed electrocatalyst (NP-ACSs@NiFe-MOFs) shows excellent performance toward bifunctional oxygen electrocatalysis. Specifically, a remarkable low value of potential gap (ΔE = 0.61 V), which is the difference between the potential to reach an OER current density of 10 mA cm⁻² and ORR half-wave potential, is achieved in 0.1 m KOH. Notably, the aqueous ZAB based on NP-ACSs@NiFe-MOFs shows super cycle stability with small voltage gap of only 0.79 V when cycled for 450 h at 10 mA cm⁻². Also, the quasi-solid-state ZAB indicates excellent flexibility and cycling stability. This study presents a facile strategy for the rational integration of different catalytically active components, and can be extended to prepare other strongly competitive multifunctional electrocatalysts.     

Bidirectional Atomic Iron Catalysis of Sulfur Redox Conversion in High-Energy Flexible Zn?S Battery

To achieve the full theoretical potential of high energy Zn S electrochemistry, the incomplete and sluggish conversion during battery discharging and high reactivation energy barrier during battery recharging associated with the sulfur cathodes must be overcome. Herein, the atomically dispersed Fe sites with Fe N₄ coordination are experimentally and theoretically predicted as bidirectional electrocatalytic hotspots to simultaneously manipulate the complete sulfur conversion and minimize the energy barrier of ZnS decomposition. It is discovered that the Fe sites were favorable for strong sulfur and possible zinc polysulfide intermediate adsorption, and ensure nearly complete sulfur to ZnS conversion during discharge. For the following recharging process, the electrodeposited ZnS can be readily reversible charged back to S without a noticeable activation overpotential around Fe N₄ moieties comparing to pure carbon matrixes. As expected, the freestanding iron embedded carbon fiber cloth supported sulfur cathode delivers a high specific capacity of 1143 mAh g⁻¹ and a lower voltage hysteresis of 0.61 V. As elaborated by postmortem analysis, the degradation mechanism of Zn S cell is the accumulation of inactive ZnS crystals on the cathode side rather than the Zn metallic depletion. More encouragingly, a flexible solid-state Zn S battery with a high discharge capacity and stable reversibility is also demonstrated.     

Ni Single Atoms on Hollow Nanosheet Assembled Carbon Flowers Optimizing Polysulfides Conversion for Li−S Batteries

The sluggish conversion kinetics and shuttling behavior of lithium polysulfides (LiPSs) seriously deteriorate the practical application of lithium–sulfur (Li–S) batteries. Herein, Ni single atoms on hollow carbon nanosheet-assembled flowers (Ni-NC) are synthesized via a facile pyrolysis-adsorption process to address these challenges. The as-designed Ni-NC with enhanced mesoporosity and accessible surface area can expose more catalytic sites and facilitate electron/ion transfer. These advantages enable the Ni-NC-modified separator to exhibit both enhanced confinement-catalysis ability and suppressed shuttling of LiPSs. Consequently, the Li−S battery with Ni-NC-modified separator shows an initial capacity of 1167 mAh g⁻¹ with a low capacity decay ratio (0.033% per cycle) over 700 cycles at 1 C. Even at the sulfur loading of 6.17 mg cm⁻², a high areal capacity of 5.17 mAh cm⁻² is realized at 0.1 C, together with superior cycling stability over 300 cycles. This work provides a facile catalyst design strategy for the development of high-performance Li−S batteries.     

Tuning the Bonding Behavior of d-p Orbitals to Enhance Oxygen Reduction through Push–Pull Electronic Effects

The regulation of electronic structure is intricately linked to the intrinsic activity of oxygen reduction. Herein, a strategy for electronic structure modulation induced by bimetallic push–pull electronic effects in dual-atom catalysts (Fe,Ni/N-C@NG) is developed. Experiments and theoretical analysis reveal that Fe sites exhibit favorable bonding behaviors (Fe–O: d_xz-p, d_yz-p, and d_z²-p) and spin configurations, which can enable rapid desorption of *OH and thus enhance the intrinsic activity of oxygen reduction. In situ monitoring techniques and Gibbs free energy diagram further demonstrate that the adjacent Ni could serve as second active center to participate in oxygen reduction. The Fe,Ni/N-C@NG exhibits enhanced oxygen reduction reaction activity and excellent stability. Meanwhile, the assembled Zn–air battery maintains stability for over 300 h with a small voltage gap. This study provides multiple insights into the orbital scale laws of oxygen reduction.     

High-Loading Co Single Atoms and Clusters Active Sites toward Enhanced Electrocatalysis of Oxygen Reduction Reaction for High-Performance Zn–Air Battery

The development of precious-metal alternative electrocatalysts for oxygen reduction reaction (ORR) is highly desired for a variety of fuel cells, and single atom catalysts (SACs) have been envisaged to be the promising choice. However, there remains challenges in the synthesis of high metal loading SACs (>5 wt.%), thus limiting their electrocatalytic performance. Herein, a facile self-sacrificing template strategy is developed for fabricating Co single atoms along with Co atomic clusters co-anchored on porous-rich nitrogen-doped graphene (Co SAs/AC@NG), which is implemented by the pyrolysis of dicyandiamide with the formation of layered g-C₃N₄ as sacrificed templates, providing rich anchoring sites to achieve high Co loading up to 14.0 wt.% in Co SAs/AC@NG. Experiments combined with density functional theory calculations reveal that the co-existence of Co single atoms and clusters with underlying nitrogen doped carbon in the optimized Co₄₀SAs/AC@NG synergistically contributes to the enhanced electrocatalysis for ORR, which outperforms the state-of-the-art Pt/C catalysts with presenting a high half-wave potential (E_1/2 = 0.890 V) and robust long-term stability. Moreover, the Co₄₀SAs/AC@NG presents excellent performance in Zn–air battery with a high-peak power density (221 mW cm⁻²) and strong cycling stability, demonstrating great potential for energy storage applications.