Air Cathode Research Articles

Interfacial engineering-induced Janus heterostructures with enhanced electronic regulation for efficient oxygen electrocatalysis in rechargeable Zn-air batteries

Construction of efficient non-noble metal electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are extremely important for zinc-air batteries (ZABs). Herein, a novel Janus heterostructure, coupling CeO2 with bimetallic metal sulfides (ZnCoS) with precise heterointerfaces, was designed through an interfacial engineering-induced strategy. Benefiting from the interfacial synergistic effect, the asymmetric Janus heterostructure of CeO2/ZnCoS (J−CeO2/ZCS) exhibits satisfactory ORR and OER activity, in terms of E1/2 of 0.87 V for ORR and overpotential of 290 mV for OER. Theoretical calculation further demonstrates that the heterointerface effectively modulates the electronic structure and charge redistribution, thus improving redox reactivity of active sites and the absorption/desorption of reaction intermediates. When the J-CeO2/ZCS applied in liquid and flexible ZABs as air cathodes, the devices deliver excellent power density and cycling stability. This Janus structure of composite catalysts affords a new opportunity for designing efficient oxygen electrocatalysts with heterostructures.

Enhanced degradation of refractory organics in ORR-EO system with a blue TiO2 nanotube array modified Ti-based Ni-Sb co-doped SnO2 anode

Recently, a novel 2-electron oxygen reduction reaction (ORR) based electro-oxidation (EO) system was developed, which utilizes a H2O2 generation cathode instead of H2 evolution cathode. A Ti-based Ni-Sb co-doped SnO2 (Ti/NATO) anode was selected for efficient degradation of refractory organics and O3 production. The synergistic reaction of O3/H2O2 further accelerated the generation of hydroxyl radicals (•OH) in the ORR-EO system. However, the catalytic activity and long-term effectiveness of the Ti/NATO anode limited the large-scale application of the ORR-EO process. In this study, a blue TiO2 nanotube array (blue-TiO2-NTA) inter-layer was introduced into the fabrication process between the Ti substrate and NATO catalyst layer. Compared to the Ti/NATO anode, the Ti/blue-TiO2-NTA/NATO anode achieved higher efficiency of organic removal and O3 generation. Additionally, the accelerated lifetime of the Ti/blue-TiO2-NTA/NATO anode was increased by 7 times compared to the Ti/NATO anode. When combined with CNTs-C/PTFE air cathode in ORR-EO system, all anodic oxidation and O3/H2O2 processes achieved higher •OH production. Over 92% of TOC in leachate bio-effluent was effectively eliminated with a relatively low energy cost of 45 kWh/t.

A Nanofiber-Based Gas Diffusion Layer for Improved Performance in Air Cathode Microbial Fuel Cells.

This work investigates a new nanostructured gas diffusion layer (nano-GDL) to improve the performance of air cathode single-chamber microbial fuel cells (a-SCMFCs). The new nano-GDLs improve the direct oxygen reduction reaction by exploiting the best qualities of nanofibers from electrospinning in terms of high surface-area-to-volume ratio, high porosity, and laser-based processing to promote adhesion. By electrospinning, nano-GDLs were fabricated directly by collecting two nanofiber mats on the same carbon-based electrode, acting as the substrate. Each layer was designed with a specific function: water-resistant, oxygen-permeable polyvinylidene-difluoride (PVDF) nanofibers served as a barrier to prevent water-based electrolyte leakage, while an inner layer of cellulose nanofibers was added to promote oxygen diffusion towards the catalytic sites. The maximum current density obtained for a-SCMFCs with the new nano-GDLs is 132.2 ± 10.8 mA m-2, and it doubles the current density obtained with standard PTFE-based GDL (58.5 ± 2.4 mA m-2) used as reference material. The energy recovery (EF) factor, i.e., the ratio of the power output to the inner volume of the device, was then used to evaluate the overall performance of a-SCMFCs. a-SCMFCs with nano-GDL provided an EF value of 60.83 mJ m-3, one order of magnitude higher than the value of 3.92 mJ m-3 obtained with standard GDL.

Suppress moisture sensitivity of Ni-rich cathode materials by bioinspired self-assembly hydrophobic layer

Ni-rich layered oxides have shown great potential as lithium-ion batteries in the field of powered vehicles. However, the chemical instability at the cathode–air interface leads to high sensitivity in humid air and easily produces residual lithium, posing challenges for interfacial stability. In this work, we used alkyl phosphoric (AP) material as a bioinspired coating on the surface of LiNi0.83Mn0.11Co0.06O2 (N83) for molecular self-assembly to form a strongly robust cathode–air interface with improved chemical environmental stability. Results showed that the bioinspired self-assembled hydrophobic coating (BSHC) inhibits the formation of residual lithium due to Li+/H+ ion exchange on the high nickel surface and improves the stability of the cathode–air interface. Moreover, the lithium phosphate generated by surface conversion remarkably enhances the migration rate of Li+, alleviates irreversible gas release, and effectively inhibits electrolyte erosion. Given the synergistic effect, the Ni-rich material coated with 16 carbon chains of alkyl phosphoric (N83@16C) provide higher capacity (151.3 mAh/g) and capacity retention (91.38%) after 300 cycles, exhibiting excellent chemical and cycling stability after exposure. This work helps improve the environmental chemical stability of Ni-rich materials and provides guidance for the development of other cathode materials with sensitive cathode–air interfaces.

Super‐fast Charging Biohybrid Batteries through a Power‐to‐formate‐to‐bioelectricity Process by Combining Microbial Electrochemistry and CO2 Electrolysis

AbstractExtensive study on renewable energy storage has been sparked by the growing worries regarding global warming. In this study, incorporating the latest advancements in microbial electrochemistry and electrochemical CO2 reduction, a super‐fast charging biohybrid battery was introduced by using pure formic acid as an energy carrier. CO2 electrolyser with a slim‐catholyte layer and a solid electrolyte layer was built, which made it possible to use affordable anion exchange membranes and electrocatalysts that are readily accessible. The biohybrid battery only required a 3‐minute charging to accomplish an astounding 25‐hour discharging phase. In the power‐to‐formate‐to‐bioelectricity process, bioconversion played a vital role in restricting both the overall Faradaic efficiency and Energy efficiency. The CO2 electrolyser was able to operate continuously for an impressive total duration of 164 hours under Gas Stand‐By model, by storing N2 gas in the extraction chamber during stand‐by periods. Additionally, the electric signal generated during the discharging phase was utilized for monitoring water biotoxicity. Functional genes related to formate metabolism were identified in the bioanode and electrochemically active bacteria were discovered. On the other hand, Paracoccus was predominantly found in the used air cathode. These results advance our current knowledge of exploiting biohybrid technology.

Super-fast Charging Biohybrid Batteries through a Power-to-formate-to-bioelectricity Process by Combining Microbial Electrochemistry and CO2 Electrolysis.

Extensive study on renewable energy storage has been sparked by the growing worries regarding global warming. In this study, incorporating the latest advancements in microbial electrochemistry and electrochemical CO2 reduction, a super-fast charging biohybrid battery was introduced by using pure formic acid as an energy carrier. CO2 electrolyser with a slim-catholyte layer and a solid electrolyte layer was built, which made it possible to use affordable anion exchange membranes and electrocatalysts that are readily accessible. The biohybrid battery only required a 3-minute charging to accomplish an astounding 25-hour discharging phase. In the power-to-formate-to-bioelectricity process, bioconversion played a vital role in restricting both the overall Faradaic efficiency and Energy efficiency. The CO2 electrolyser was able to operate continuously for an impressive total duration of 164 hours under Gas Stand-By model, by storing N2 gas in the extraction chamber during stand-by periods. Additionally, the electric signal generated during the discharging phase was utilized for monitoring water biotoxicity. Functional genes related to formate metabolism were identified in the bioanode and electrochemically active bacteria were discovered. On the other hand, Paracoccus was predominantly found in the used air cathode. These results advance our current knowledge of exploiting biohybrid technology.

Nickel Phosphonate MOF Derived N-Doped Carbon-Coated Phosphorus-Vacancies-Rich Ni2 P Particles as Efficient Bifunctional Oxygen Electrocatalyst.

The design of non-noble metal bifunctional electrocatalysts with outstanding performance and remarkable stability for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is one of the most essential issues to the realization of rechargeable zinc-air battery, and transition metal phosphides (TMPs) have emerged as robust candidates for oxygen electrocatalysts. Herein, N-doped carbon-coated phosphorus-vacancies-rich Ni2 P particles (Vp -Ni2 P@NC) is proposed via simple carbonization and following Ar plasma treatment from a single nickel phosphonate metal-organic framework (MOF) without extra phosphine and nitrogen sources. The facile and rapid plasma treatment can achieve phosphorus vacancies which could modulate the electronic structure to enhance the inherent active and electrical conductivity. Meanwhile, the pyridine-N and graphitized-N produced during calcination also could provide more active sites and increase the electrical conductivity. The resultant Vp -Ni2 P@NC catalyst shows excellent bifunctional electrocatalytic activity (OER/ORR) based on synergistic effect of introducing P vacancies into Ni2 P and N-doped carbon. Vp -Ni2 P@NC catalyst shows more advantageous ΔE value (0.70 V) compared to Pt/C+RuO2 (0.73 V) and most reported catalysts. Additionally, the zinc-air bbatterie (ZAB) employing Vp -Ni2 P@NC as air cathode shows excellent performance. The maximum power density of 203.48 mW cm-2 , the cycling stability of more than 150 h at 10 mA cm-2 .

A conceptual, mathematical and quantitative reassessment of the Thin-film Flooded Agglomerate Model for air cathodes

Design, fabrication, scale-up, operation and degradation analysis of GDEs electrode are complex operations, requiring a precise knowledge-base and quantitative guidelines. Mathematical tools are strongly needed for this purpose, but often they exhibit conceptual, formal and computational complexities that - in practice - are manageable only by modelling experts. Consequently, models are, in fact, not sufficiently exploited to support systematically metal-air battery research, development and control, as they would have the potentialities to do. Sometimes, experiments are essentially used just to validate models, rather than using models to direct experimental work. In order to enable the concrete integration of mathematical modelling with experimental work on GDEs for ORR, in this paper we present a conceptual restatement of a state-of-the-art model for ORR GDEs, in view of physico-chemical and mathematical perspicuity. In addition, we provide a simplified analytical solution, that enables straightforward and physically transparent parametric analysis, and a detailed description of the numerical solution of the full problem.

All-Solid-State Mg-Air Battery Enhanced with Free-Standing N-Doped 3D Nanoporous Graphene.

Nitrogen (N) doping of graphene with a three-dimensional (3D) porous structure, high flexibility, and low cost exhibits potential for developing metal-air batteries to power electric/electronic devices. The optimization of N-doping into graphene and the design of interconnected and monolithic graphene-based 3D porous structures are crucial for mass/ion diffusion and the final oxygen reduction reaction (ORR)/battery performance. Aqueous-type and all-solid-state primary Mg-air batteries using N-doped nanoporous graphene as air cathodes are assembled. N-doped nanoporous graphene with 50-150nm pores and ≈99% porosity is found to exhibit a Pt-comparable ORR performance, along with satisfactory durability in both neutral and alkaline media. Remarkably, the all-solid-state battery exhibits a peak power density of 72.1mW cm-2 ; this value is higher than that of a battery using Pt/carbon cathodes (54.3mW cm-2 ) owing to the enhanced catalytic activity induced by N-doping and rapid air breathing in the 3D porous structure. Additionally, the all-solid-state battery demonstrates better performances than the aqueous-type battery owing to slow corrosion of the Mg anode by solid electrolytes. This study sheds light on the design of free-standing and catalytically active 3D nanoporous graphene that enhances the performance of both Mg-air batteries and various carbon-neutral-technologies using neutral electrolytes.

Vermiculite nanosheet augmented novel proton exchange membrane for microbial fuel cell

Microbial fuel cell (MFC) is promoted as an eco-friendly technology for wastewater treatment. However, the use of synthetic polymeric membrane separators raises concerns about the sustainability of MFC, mandating efforts to develop sustainable polymeric membrane with high protonic transfer for MFC assembly. Low-cost and eco-friendly polymer matrix of agar-polyvinyl alcohol (AP) augmented with exfoliated vermiculite nanosheets (Vm NSs) are used to prepare membrane separators for MFC. A simple and facile method for synthesis of AP membrane without and with Vm NSs concentration of 0.25 mg/mL (0.25 Vm), 0.5 mg/mL (0.5 Vm) and 1 mg/mL (1.0 Vm) is adopted in aqueous phase. The proton mass transfer coefficients of prepared membranes (AP, 0.25 Vm, 0.5 Vm and 1.0 Vm) are respectively 94.4, 134.46, 216.26 and 197.69 % higher than commercially available Nafion membrane. These membranes are tested in three different configurations of MFC: aerated cathode, air cathode and anaerobic dye cathode MFC. Highest current densities of 1515 and 949 mA/m3 are obtained with 1.0 Vm membrane in aerated and air cathode-MFC respectively while Nafion membrane could produce 623 and 405 mA/m3. In anaerobic dye cathode MFC, 0.5 Vm membrane is able to achieve the highest current density of 935 mA/m3 along with 71 % methyl red decolorization while Nafion membrane produced highest current density of 104 mA/m3 with 59 % decolorization. Vm NSs augmentation enhances the properties of base AP membrane which can be conveniently used as an alternative to expensive Nafion membrane in the desired configuration of MFC.

Bypass Configurations of Membrane Humidifiers for Water Management in PEM Fuel Cells

Water management is an important criterion in the operation of proton-exchange membrane fuel cells to maintain the high performance and reliability of the system. The water content in the cathode air that is supplied to the cathode channel contributes to the membrane humidification and the transport of protons inside the membrane structure. In automotive applications, the supply air is typically driven through an external membrane humidifier to absorb more moisture from the recirculated cathode exhaust. In the literature, humidifiers and fuel cell stacks have been separately investigated without considering whole-system configurations for water management. This study investigates changes in the cathode air characteristics through a membrane humidifier and compares two configurations using a humidifier bypass of the supply flow and exhaust flow to adjust the cathode inlet air relative humidity. Each component in the system was modeled using mathematical relations and converted into blocks of inputs and outputs in MATLAB/Simulink for simulation. The bypass valve was demonstrated to effectively reduce the relative humidity of the supply air from the saturation rate to above 60%, with a bypass fraction of up to 0.6 in both configurations. These adjustments provide system flexibility to accommodate load changes and prevent flooding in the stack channels. Bypassing the supply air through the humidifier effectively maintained consistent cathode inlet humidity across a wide operational range. A 0.4 bypass fraction on the supply side sustained a relative humidity of around 80% for the whole range of operating flow rates. In contrast, the exhaust-side bypass had a smaller impact, and the relative humidity of the cathode air was reduced when the flow rate and bypass fraction increased. This study further supports the control system design to regulate the bypass fraction according to load transients.

ZIF-derived Fe,Co coordinated N/O-codoped three-dimensional tungsten–carbon matrix for the performance-enhanced zinc-air flow battery and water splitting

Constructing efficient and cost-effective electrocatalysts with oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) for the development of zinc-air flow batteries (ZAFBs) and overall water splitting (OWS) is significant, but still challenging. Herein, we have fabricated a heterojunction catalyst (Fe,CoZn9+9-NO/WC) featuring uniformly dispersed Fe and Co coordinated with riched-nitrogen/oxygen codoped into a three-dimensional highly porous tungsten–carbon matrix modified carbon nanotube by a simple stepwise construction route using water as the solvent. The resultant Fe,CoZn9+9-NO/WC exhibits a positive half-wave potential (E1/2) of 0.889 V for ORR, a low overpotential at 10 mA·cm−2 for OER (290 mV) and HER (84 mV), and excellent electrochemical durability in alkaline electrolyte, outperforming even benchmark catalysts. Moreover, Fe,CoZn9+9-NO/WC as a bifunctional air cathode for ZAFBs delivers higher open circuit voltage, peak power density (190 mW·cm−2), and specific capacity (814 mAh·g−1), lower charge/discharge voltage gap and prolonged operational cycles durability, largely exceeding those of the hybrid Pt/C+RuO2 bifunctional catalyst. The alkaline OWS electrolyzer is constructed by Fe,CoZn9+9-NO/WC with good stability and requires only 1.556 V cell voltage at 10 mA·cm−2. Impressively, the self-constructed Fe,CoZn9+9-NO/WC electrodes were applied to practical ZAFBs and OWS devices, demonstrating superior performance to the recently reported ZIF/MOF-derived catalysts in organic solvents. This work offers an economical synthetic strategy to prepare highly efficient multifunctional catalysts with practical potential for commercial production, promising to facilitate the development of clean energy storage and conversion devices.

Proposal for a practical MFC using vertical electrodes with improved air cathode function

In this study, we proposed a low-cost vertical electrode MFC device using Bokuju and smoked charcoal and evaluated its air cathode function by varying the cathode area. The vertical design of the device ensures contact between the cathode electrode and the air, easing the installation of the device and the replacement of the electrode. The output increase was also confirmed by the cathode, accompanied by the sterilizing effect of copper and reliable contact with air. Unfortunately, there was a slight improvement in air cathode function even when the area of the cathode exposed to air was wider than 100 mm2. However, when the device was inserted into a new soil of the same material after the first operation, the power density reached the same level as the first time. Thus, this device is nearing practical application by improving its long-term stability and increasing output power.

Preparation and properties of flexible integrated cathode and electrolyte all-gel zinc-air batteries

Zinc-air batteries (ZABs) are of great interest in meeting the growing demand for flexible portable electronics due to their high energy density and low cost. In this paper, the interfacial dry crosslinking strategy is adopted, in which the dehydrated air cathode spontaneously absorbs water from the surface and inside of the electrolyte under the combined action of osmotic pressure and hydrophilic amide group, and the full gel integrating cathode and electrolyte is obtained by the interfacial dry crosslinking method of hydrogen bond adsorption. After water absorption, the conductive gel dry film electrode softens again, and a fusion interface is formed between the electrode and the electrolyte, and the integrated full gel is the best material for flexible batteries, which plays a vital role in the mechanical and electrochemical properties of the battery. In this paper, a polyacrylamide-based (PAM) all-in-one full gel with an optimal monomer content of 30 wt% is synthesized and used for bendable flexible packaging ZAB. The prepared ZAB exhibits good cycle stability of 177 h, excellent battery rate performance, stable discharge performance (high gram capacity of 785.16 mAh g−1), relatively high maximum power density (111.0 mW cm−2), and low interfacial contact impedance. With this 30 wt% all-in-one gel assembly soft pack ZAB can power a wide range of electronic devices and exhibit excellent flexibility in a wide range of bending conditions.

Solid Oxide Electrochemical Cells for Nitrogen and Oxygen Production

This work presents an efficient electrochemical process for N2 and O2 production based on a solid oxide electrochemical cell technology by oxygen extraction from air. Cells with a 53×53 mm2 size consist of ca.10 μm CGO electrolyte sandwiched between composite electrodes made from either LSCF-CGO, or LSF-CGO were produced by a conventional tape casting method. To boost performance at low temperature, the electrodes were infiltrated with Pr-oxide. The electrochemical performance of the cells were characterized by iV and EIS at different temperatures with air used as feed stock to the cathode compartment and air, or oxygen used as sweep gas to the anode compartment. A voltage below 200mV is achieved to drive a current of 1A/cm2 through the cell, which can provide a purity of 90% of N2 at 650 °C. The cells were demonstrated to operate for producing a >98% pure N2 stream without degradation over a 1500h period at cell voltages below 255mV.

Bifunctional Oxygen Electrocatalyst of Co4N and Nitrogen‐Doped Carbon Nanowalls/Diamond for High‐Performance Flexible Zinc–Air Batteries

AbstractRational design of heterogeneous catalysts with unique structural and electronic properties is one of the major challenges to improve the activity toward the reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), the bottleneck in the construction of air cathodes for the next‐generation flexible zinc–air batteries (ZABs). Herein, density functional theory calculations are combined with experimental attempts to exploit the roles of the electronic effects at the interface between Co4N nanoparticles and nitrogen‐doped carbon nanowalls/diamond (d‐NCNWs/D) toward the ORR and OER activities. The vacancy defect‐induced Co‐pyridinic N─C bond optimizes the electronic structure of Co 3d orbitals and balances the adsorption energies of intermediates along the reaction pathways. Consequently, as‐synthesized Co4N@d‐NCNWs/D composites exhibit superior bifunctional oxygen catalytic activity. The overpotential of the OER is as low as 340 mV at 10 mA cm−2 and the high half‐wave potential reaches 0.83 V for the ORR. As a binder‐free and flexible ZABs cathode, this composite exhibits an open circuit voltage of 1.41 V and excellent bendable stability, proving its promising potential for the assembly of wearable devices. This work offers theoretical evidence and a controllable strategy to design high‐performance ZAB cathodes for their application in smart electronic devices.

Development and Optimization of Air-Electrodes for Rechargeable Zn–Air Batteries

Rechargeable Zn–air batteries (ZABs) can play a significant role in the transition to a cleaner and more sustainable energy system due to their high theoretical energy density, high cell voltage, and environmental friendliness. ZAB’s air cathode is the principal determinant in predicting the battery’s overall performance, as it is responsible for catalyzing the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during the discharging and charging process, respectively. In this work, a detailed optimization study of the architecture of the air cathode was carried out using the benchmark bifunctional oxygen electrocatalyst (Pt/C-RuO2). The air cathode composition and architecture were optimized regarding the choice of the commercial gas diffusion layer (GDL), the effect of hot pressing the catalyst layer (CL), and the optimum pore size of the current collector. The best cathode from this study shows a maximum power density (PDmax) of 167 mW/cm2, with a round trip efficiency and a voltage gap (Egap) of 59.8% and 0.78 V, respectively, indicating the air cathodes preparation approach proposed in this work as a promising strategy for the improvement of the overall performance of ZABs.

Interface engineering of Ruddlesden–Popper perovskite/CeO2/carbon heterojunction for rechargeable zinc-air batteries

The development of noble metal-based bifunctional electrocatalysts is the key to driving the sluggish oxygen reduction/evolution reaction (ORR/OER) for rechargeable zinc-air battery applications. There is an urgent need to design and construct robust and cost-efficient bifunctional electrocatalysts. Herein, an interface engineering strategy of Ruddlesden–Popper (RP) perovskite/CeO2/carbon heterojunction with core–shell nanostructures is described. Ce-based metal–organic framework derived CeO2-C nanosheets are decorated on the surface of RP type perovskite Pr3Sr(Ni0.5Co0.5)3O10−δ (PSNC) nanofibers. Benefiting from the favorable conductivity, abundant oxygen vacancies and strong interfacial coupling, the hierarchical CeO2-C/PSNC electrode delivers a half-wave potential of 0.78 V (ORR), and an OER overpotential of 370 mV at 10 mA cm−2, respectively. A liquid rechargeable zinc-air battery (ZAB) assembled with CeO2-C/PSNC electrocatalysts as the air cathode exhibits a peak power density of 161 mW cm−2 and a long-term cycling life over 219 h. In addition, the CeO2-C/PSNC-based all-solid-state cable-type ZAB provides a high open-circuit voltage (∼1.44 V), good flexibility and durability. Our study opens a new insight into the design of efficient electrocatalysts for rechargeable ZABs by constructing hierarchical heterojunctions.

Rechargeable Zinc-Air Batteries: Advances, Challenges, and Prospects.

Rechargeable zinc-air batteries (Re-ZABs) are one of the most promising next-generation batteries that can hold more energy while being cost-effective and safer than existing devices. Nevertheless, zinc dendrites, non-portability, and limited charge-discharge cycles have long been obstacles to the commercialization of Re-ZABs. Over the past 30 years, milestone breakthroughs have been made in technical indicators (safety, high energy density, and long battery life), battery components (air cathode, zinc anode, and gas diffusion layer), and battery configurations (flexibility and portability), however, a comprehensive review on advanced design strategies for Re-ZABs system from multiple angles is still lacking. This review underscores the progress and strategies proposed so far to pursuit the high-efficiency Re-ZABs system, including the aspects of rechargeability (from primary to rechargeable), air cathode (from unifunctional to bifunctional), zinc anode (from dendritic to stable), electrolytes (from aqueous to non-aqueous), battery configurations (from non-portable to portable), and industrialization progress (from laboratorial to practical). Critical appraisals of the advanced modification approaches (such as surface/interface modulation, nanoconfinement catalysis, defect electrochemistry, synergistic electrocatalysis, etc.) are highlighted for cost-effective flexible Re-ZABs with good sustainability and high energy density. Finally, insights are further rendered properly for the future research directions of advanced zinc-air batteries.

Surface-optimized carbon nanocages with tailorable atomic Fe-N4 sites to boost oxygen reduction in long stable zinc-air battery

High-efficiency and durable precious-metals-free electrocatalysts for oxygen reduction (ORR) are crucial to Zn-air battery (ZAB). Herein, a molten salt-assisted pyrolysis is reported to construct fully open carbon nanocages with highly atomically dispersed Fe-N4 sites (Fe-SA/NC) as air cathode. The chemical etching in pyrolysis can induce such highly active surface structure with abundant surface Fe anchoring sites and regulate the uncoordinated pyridinic nitrogen. The theoretical investigations reveal that engineering the adjacent uncoordinated pyridinic nitrogen can efficiently modulate the charge distribution and lower d-band center position of FeN4, thus reducing the dissociation energy of intermediate OH*. The Fe-SA/NC exhibit outstanding catalytic activity with 0.905 V (vs. RHE) half-wave potential and excellent cycling stability, surpassing commercial Pt/C and most reported M-N-C catalysts. The ZAB with the Fe-SA/NC as air cathode delivers a high power density of 240 mW cm−2, large specific capacity (815 mAh g−1) and long-term durability over 1050 h. This work highlights the rational design and fabrication of efficient M-N-C single-atom catalysts from the point of view of both morphological engineering and local coordination chemistry.