Excellent Electrochemical Performance Research Articles

Pristine Metal‐Organic Frameworks for Sodium‐Ion Batteries: Past, Present, and Future

AbstractOwing to their adjustable redox‐active sites, designable structures high porosity, and fully activated organic ligands, pristine metal‐organic frameworks (MOFs) have been widely utilized as advanced electrode materials (i. e., both anodes and cathodes) for sodium‐ion batteries (SIBs) to satisfied the insertion/extraction larger size and mass of Na+ cations, achieving significant progresses with excellent electrochemical performance in electrochemical energy storage devices. Here, the recent advances on pristine MOFs as anodes and cathodes for SIBs are summarized. A thorough investigation delves into the detailed characteristics, energy storage mechanisms, and electrochemical performance of diverse pristine MOFs for SIBs are also clarified. Furthermore, the outlooks on pristine MOF electrodes in SIBs are also provided.

Synergistic engineering of structural and electronic regulation in necklace-like UiO-66/ACNT structure toward lithium-sulfur batteries with fast polysulfide redox

Although UiO-66-based metal-organic framework (MOF) composites have been widely applied in the entrapment and catalytic effect toward lithium polysulfides (LiPSs), the synergistic engineering of structural and electronic regulation strategy has not been carefully considered. Herein, we optimize the internal cluster structure, rational regulate the size of UiO-66, and design the necklace-like structure composed of UiO-66 nanoparticles and acid-treated carbon nanotube (UiO-66/ACNT) through simple hydrothermal reaction and thermal treatment strategy. According to experiments and density universal function theory calculations, the improved UiO-66/ACNT architecture effectively provides more polarity to capture polysulfides via exposing more active sites and effectively delivers electrocatalytic activity of polysulfide conversion. Furthermore, systematic electrocatalytic kinetics, in situ Raman spectroscopy and in-situ impedance analysis indicate the functional separator can inhibit the shuttling effect, and catalyze the conversion of lithium polysulfide. As a result, Li-S batteries with functional separators exhibit excellent electrochemical performance (low fading rate over 1000 cycles of 0.06 % per cycle at 1C, high areal capacity of 4.01 mAh cm−2 at 0.2C). This work offers insight into the rational design of UiO-66-based metal-organic frameworks (MOFs) through synergistic engineering for high-performance Li-S batteries.

Communication — The Impact of Co-solvent Selection for Dimethyl-2,5-dioxahexane carboxylate in Sodium Ion Batteries

Traditional linear carbonates including dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) were investigated as co-solvents for the dimethyl-2,5-dioxahexane carboxylate (DMOHC)-based electrolyte in Na0.97Ca0.03[Mn0.39Fe0.31Ni0.22Zn0.08]O2 (NCMFNZO)/hard carbon (HC) pouch cells. The EMC-containing cell displays excellent electrochemical performance, exhibiting only a 1.6 mAh irreversible capacity loss during 500 h of storage at 4 V and 40 °C, and maintaining over 80% capacity retention after 200 cycles up to 4 V at 40 °C. Severe gas evolution and Na plating issues are present in all the tested systems.

In Situ Characterizations of the Dynamics of Cathode Electrolyte Interfaces at Different Current Densities.

LiNi0.8Mn0.1Co0.1O2 with high nickel content plays a critical role in enabling lithium metal batteries (LMBs) to achieve high specific energy density, making them a prominent choice for electric vehicles (EVs). However, ensuring the long-term cycling stability of the cathode electrolyte interfaces (CEIs), particularly at fast-charge conditions, remains an unsolved challenge. The decay mechanism associated with CEIs and electrolytes in LMB at high current densities is still not fully understood. To address this issue, in situ Fourier transform infrared (FTIR) is employed to observe the dynamic process of formation/disappearance/regeneration of CEIs during charge and discharge cycles. These dynamic processes further exacerbate the instability of CEIs as current density increases, leading to rupture and dissolution of CEIs and subsequent deterioration in battery performance because of continuous electrolyte reactions. Additionally, the dynamic changes occurring within individual components of CEIs at different cycling stages and various current densities are also discussed. The results demonstrate that excellent capacity retention at small current density is attributed to enrichment of inorganic compounds (Li2CO3, LiF, etc.) and rendering better stability and smaller expansion of CEIs. The key to achieving excellent electrochemical performance at high current densities lies on protecting CEIs, mainly inorganic components.

Tailoring of High-Valent Sn-Doped Porous Na3V2(PO4)3/C Nanoarchitechtonics: An Ultra High-Rate Cathode for Sodium-Ion Batteries.

NASICON structured Na3V2(PO4)3 (NVP) has captured enormous attention as a potential cathode for next-generation sodium-ion batteries (SIBs), owing to its sturdy crystal structure and high theoretical capacity. Nonetheless, its poor intrinsic electronic conductivity has led to inferior electrochemical performance in terms of rate capability and long cycling performance. To address this problem, a combined strategy is adopted, such as (1) carbon coating and (2) high valent Sn4+ ion doping in the lattice site of vanadium in the NVP cathode. Carbon coating can effectively enhance the surface electronic conductivity, wherein high-valent Sn4+ improves the bulk intrinsic electronic conductivity of the materials. Moreover, Sn is a well-known alloying/dealloying type anode for SIBs; thus, doping of such metal in cathode materials will assume the role of structure stabilizing pillars and establishing high-performing cathode materials. Herein, Na3V2-xSnx(PO4)3/C (denoted as Sn(x)-NVP/C, where x = 0.00, 0.03, 0.05, 0.07, 0.1) were synthesized via sol-gel route, followed by calcination at 800 °C. XRD, Raman, XPS, and electron microscopy data confirmed the high purity of the synthesized cathode. The optimized Sn(0.07)-NVP/C exhibited excellent electrochemical performance in terms of high rate capability and long cycling performance, a high appreciable capacity of 98 mAh g-1 with capacity retention of 85% after 500 cycles. Similarly, at a high current of 20C, it is still able to deliver a stable capacity of 76 mAh g-1 with 85% capacity retention after 3000 cycles. The rate capability study indicates the high current tolerance of Sn(0.07)-NVP/C up to 70 C with a capacity delivery of 55 mAh g-1. It is worth mentioning that CV and EIS analysis for Sn(0.07)-NVP/C cathode displayed minimum voltage polarization and enhanced diffusion coefficient. Moreover, DFT calculation also proved that the electronic and ionic conductivity of NVP is promoted by Sn doping. Hence, the present results demonstrated that Sn(0.07)-NVP/C is considered a promising cathode for sodium-ion battery application.

Developing a novel polyurethane-based gel polymer electrolyte to enable high safety and remarkably cycling performance of sodium ion batteries

A novel gel polymer electrolyte (GPE) with polydopamine (PDA) modified thermoplastic polyurethane (TPU) porous membranes as the matrix skeleton and N-methyl-N-methoxyethyl pyrrolidine bis (fluorosulfonyl) imine (Pyr12O1FSI) ionic liquid (IL) as the plasticizer is successfully demonstrated to be suitable for sodium ion batteries (SIBs). The GPE exhibits excellent heat resistance due to the non-flammability of Pyr12O1FSI. Moreover, Density Functional Theory (DFT) and Fourier transform infrared spectroscopy (FTIR) results indicate that there is a strong intermolecular interaction between PDA, TPU and IL, thus greatly improving the structural stability and mechanical properties of this GPE. In addition, the functional groups (-OH and –NH–) in PDA have strong hydrogen bonds with TFSI− in sodium bis (trifluoromethanesulfonyl) imide (NaTFSI), which can immobilize TFSI− anions and promote Na+ selective transport. Therefore, the ionic conductivity and Na+ transference number of the GPE-based SIBs are significantly increased to 3.57 × 10−4 S cm−1 and 0.34 at 30 °C. As a consequence, Na|TPU@PDA-IL GPE|Na symmetric cells exhibit an ultra-long cycle life for 1200 h at 0.1 mA cm−2. This work provides an approach to develop GPE for high safety and excellent electrochemical performance SIBs.

Unveiling the Role of Cationic Pyridine Sites in Covalent Triazine Framework for Boosting Zinc-Iodine Batteries Performance.

Rechargeable Zinc-iodine batteries (ZIBs) are gaining attention as energy storage devices due to their high energy density, low-cost, and inherent safety. However, the poor cycling performance of these batteries always arises from the severe leakage and shuttle effect of polyiodides (I3 - and I5 -). Herein, a novel cationic pyridine-rich covalent triazine framework (CCTF-TPMB) is developed to capture and confine iodine (I2) species via strong electrostatic interaction, making it an attractive host for I2 in ZIBs. The as-fabricated ZIBs with I2 loaded CCTF-TPMB (I2@CCTF-TPMB) cathode achieve a large specific capacity of 243mAhg-1 at 0.2Ag-1 and an exceptionally stable cyclic performance, retaining 93.9% of its capacity over 30000 cycles at 5Ag-1. The excellent electrochemical performance of the ZIBs can be attributed to the pyridine-rich cationic sites of CCTF-TPMB, which effectively suppress the leakage and shuttle of polyiodides, while also accelerating the conversion reaction of I2 species. Combined in situ Raman and UV-vis analysis, along with theoretical calculations, clearly reveal the critical role played by pyridine-rich cationic sites in boosting the ZIBs performances. This work opens up a promising pathway for designing advanced I2 cathode materials toward next-generation ZIBs and beyond.

Stabilizing Zinc Hexacyanoferrate Cathode by Low Contents of Cs Cations for Aqueous Zn-Ion Batteries.

Exploring cathode materials with excellent electrochemical performance is crucial for developing rechargeable aqueous zinc ion batteries (RAZIBs). Zinc hexacyanoferrate (ZnHCF), a promising candidate of cathode materials for RAZIBs, suffers from severe electrochemical instability issues. This work reports using low contents of alkaline metal cations as electrolyte additives to improve the cycle performance of ZnHCF. The cations with large sizes, particularly Cs+, changes the intercalation chemistry of ZnHCF in RAZIBs. During cycling, Cs+ cations co-inserted into ZnHCF stabilize the host structure. Meanwhile, a stable phase of CsZn[Fe(CN)6] forms on the ZnHCF cathode, suppressing the loss of active materials through dissolution. ZnHCF gradually converts to an electrochemically inert Zn-rich phase during long-term cycling in aqueous electrolyte, leading to irreversible capacity loss. Introducing Cs+ in the electrolyte inhibits this conversion reaction, resulting in the extended lifespan. Owing to these advantages, the capacity retention rate of ZnHCF/Zn full batteries increases from the original 7.0% to a high value of 54.6% in the electrolyte containing 0.03 M of Cs2SO4 after 300 cycles at 0.25 A∙g-1. This research provides an in-depth understanding of the electrochemical behavior of ZnHCF in aqueous zinc electrolyte, beneficial for further optimizing ZnHCF and other metal hexacyanoferrates.

Multi-Interface Engineering of MXenes for Self-Powered Wearable Devices.

Self-powered wearable devices with integrated energy supply module and sensitive sensors have significantly blossomed for continuous monitoring of human activity and the surrounding environment in healthcare sectors. The emerging of MXene-based materials has brought research upsurge in the fields of energy and electronics, owing to their excellent electrochemical performance, large surface area, superior mechanical performance, and tunable interfacial properties, where their performance can be further boosted via multi-interface engineering. Herein, a comprehensive review of recent progress in MXenes for self-powered wearable devices is discussed from the aspects of multi-interface engineering. The fundamental properties of MXenes including electronic, mechanical, optical, and thermal characteristics are discussed in detail. Different from previous review works on MXenes, multi-interface engineering of MXenes from termination regulation to surface modification and their impact on the performance of materials and energy storage/conversion devices are summarized. Based on the interfacial manipulation strategies, potential applications of MXene-based self-powered wearable devices are outlined. Finally, proposals and perspectives are provided on the current challenges and future directions in MXene-based self-powered wearable devices.

Polyoxomolybdate-Based Metal-Organic Framework-Derived Cu-Embedded Molybdenum Dioxide Hybrid Nanoparticles as Highly Efficient Electrocatalysts for Al-S Batteries.

High-performance rechargeable aluminum-sulfur batteries (RASB) have great potential for various applications owing to their high theoretical capacity, abundant sulfur resources, and good safety. Nevertheless, the practical application of RASB still faces several challenges, including the polysulfide shuttle phenomenon and low sulfur utilization efficiency. Here, we first developed a synergistic copper heterogeneous metal oxide MoO2 derived from polymolybdate-based metal-organic framework as an efficient catalyst for mitigating polysulfide diffusion. This composite enhances sulfur utilization and electrical conductivity of the cathode. DFT calculations and experimental results reveal the catalyst Cu/MoO2@C not only effectively anchors aluminum polysulfides (AlPSs) to mitigate the shuttle effect, but also significantly promotes the catalytic conversion of AlPSs on the sulfur cathode side during charging and discharging. The unique nanostructure contains abundant electrocatalytic active sites of oxide nanoparticles and Cu clusters, resulting in excellent electrochemical performance. Consequently, the established RASB exhibits an initial capacity of 875 mAh g-1 at 500 mA g-1 and maintains a capacity of 967 mAh g-1 even at a high temperature of 50 °C.

Regulating the “core-shell” microstructure of hard carbon through sodium hydroxide activation for achieving high-capacity SIBs anode

Pore structure engineering has been acknowledged as suitable approach to creating active sites and enhancing ion transport capabilities of hard carbon anodes. However, conventional porous carbon materials exhibit high BET and surface defects. Additionally, the sodium storage mechanism predominantly occurs in the slope region. This contradicts practical application requirements because the capacity of the plateau region is crucial for determining the actual capacity of batteries. In our work, we prepared a novel “core-shell” carbon framework (CNA1200). Introducing closed pores and carboxyl groups into coal-based carbon materials to enhance its sodium storage performance. The closed pore structure dominates in the “core” structure, which is attributed to the timely removal of sodium hydroxide (NaOH) to prevent further formation of active carbon structure. The presence of closed pores is beneficial for increasing sodium ion storage in the low-voltage plateau region. And the “shell” structure originates from coal tar pitch, it not only uniformly connects hard carbon particles together to improve cycling stability, but is also rich in carboxyl groups to enhance the reversible sodium storage performance in slope region. CNA1200 has excellent electrochemical performance, it exhibits a specific capacity of 335.2 mAh g−1 at a current density of 20 mA g−1 with ICE = 51.53 %. In addition, CNA1200 has outstanding cycling stability with a capacity retention of 91.8 % even when cycling over 200 times. When CNA1200 is used as anode paired with Na3V2(PO4)3 cathode, it demonstrates a capacity of 109.54 mAh g−1 at 0.1 C and capacity retention of 94.64 % at 0.5 C. This work provides valuable methods for regulating the structure of sodium-ion battery (SIBs) anode and enhances the potential for commercialization.

Effective recovery of Ni and Cu from low-grade nickel sulfide concentrate: Process kinetics and preparation of Ni80Cu20O material

In this work, NaCl was used to synchronously extract metal from a low-grade nickel sulfide concentrate. Optimized process parameters were determined as follows: particle size 80–96 μm, temperature 400 °C, time 100 min, and NaCl dosage 100 %. The transformation mechanism of mineral phase was clarified using the means of XRD, SEM and EDS. The roasting process contained solid–solid and gas–solid reactions, and the valuable metal was effectively extracted by the combined action of sulfation of sodium sulfate and chlorination of chlorine gas. The kinetic study of the roasting process showed that the transformation of metals depends on the diffusion control between solid and solid phases. The reaction activation energies of valuable metal were calculated to be 9.15 kJ/mol for Ni, 11.14 kJ/mol for Cu, and 12.72 kJ/mol for Co. Nickel and copper were recovered from the leaching solution by purification and solvent extraction process to prepare Ni80Cu20O material. The Ni80Cu20O electrode material prepared at calcination temperature of 400 °C has excellent electrochemical performance. A new process of recovering metals from minerals to prepare Ni80Cu20O cathode material was established, which was proved to be a clean and promising technology.

Upgrading Gel Electrolytes Through Electrostatic-Induced Dual-Salt Paradigm for Superior Zn-Ion Battery Performance.

Gel electrolytes are gaining attention for rechargeable Zn-ion batteries because of their high safety, high flexibility, and excellent comprehensive electrochemical performances. However, current gel electrolytes still perform at mediocre levels due to incomplete Zn salts dissociation and side reactions. Herein, an electrostatic-induced dual-salt strategy is proposed to upgrade gel electrolytes to tackle intrinsic issues of Zn metal anodes. The competitive coordination mechanism driven by electrostatic repulsion and steric hindrance of dual anions promotes zinc salt dissociation at low lithium salt addition levels, improving ion transport and mechanical properties of gel electrolytes. Li+ ions and gel components coordinate with H2O, reducing active H2O molecules and inhibiting associated side reactions. The dual-salt gel electrolyte enables excellent reversibility of Zn anodes at both room and low temperatures. Zn||Polyaniline cells using the dual-salt gel electrolyte exhibit a high discharge capacity of 180mAhg-1 and long-term cycling stability over 180 cycles at -20°C. The dual-salt strategy offers a cost-effective approach to improving gel electrolytes for high-performance flexible Zn-ion batteries.

Pre-introducing Li4NiWO6 defect phase by tungsten modification enables highly stabilized Ni-rich cathode

Tungsten (W) has attracted considerable attention as a promising dopant capable of enhancing the structural stability of Ni-rich cathode materials. However, investigations into W modification remain highly debated due to the elusive mechanisms involved, particularly in determining whether W is doped into the lattice structure or exists as a distinct phase on the surface of the particles. In this work, we systematically observe the effects induced by the introduction of W into LiNi0.9Co0.05Mn0.05O2 (NCM). We discovered that during high-temperature solid-state reactions, W-containing substances tend to react preferentially with surface Li, creating a lithium-deficient state on the NCM surface, resulting in the generation of Li4NiWO6. Molecular dynamics simulations tracking Li, W, and Ni elemental changes at the interface confirm the initial creation of Li6WO6, succeeded by diffusion of Ni to replace Li, forming Li4NiWO6. The pre-introducing Li4NiWO6 defect phase can effectively absorb anisotropic lattice strain, preserving the mechanical integrity of the secondary particles. Furthermore, the Li4NiWO6 phase, combined with the amorphous LixWyOz coating layer, effectively acts as a guard against undesirable interfacial side reactions, ultimately imparting excellent electrochemical performance to NCM. Understanding the dynamic processes of interfacial reactions is crucial for tailoring surface properties of Ni-rich cathodes towards long-life lithium-ion batteries.

A nickel-based dendritic electrode matrix with high surface efficiency mass transfer for highly efficient overall water splitting

Electrodes are key components in water splitting. Owing to their porous structures, commercial foam metal electrode matrices can provide numerous active sites. However, the complex and disordered surface structure impedes the release of the generated gas bubbles, and sluggish mass transfer usually becomes a bottleneck in the electrochemical reaction kinetics. For this purpose, this study proposes a pulse-jet electrodeposition method for the preparation of nickel-based dendritic electrode matrices (NDEM). This research shows that the electrode matrix prepared under the conditions of V = 26 V, PW = 10 μs, and PI = 35 μs has a unique surface structure, which can quickly release bubbles, effectively attract ions, and accelerate mass transfer on the electrode surface, resulting in excellent electrochemical performance. After loading the catalyst, the NDEM requires only 1.552 V to reach a current density of 10 mA cm−2. At an electrolysis voltage of 2.5 V, the hydrogen production was 45% higher than that of the NF electrode, and it had excellent stability. This study opens a new avenue for the design of highly efficient and low-cost electrodes for water splitting.

Rationally Regulating Closed Pore Structures by Pitch Coating to Boost Sodium Storage Performance of Hard Carbon in Low‐voltage Platforms

AbstractHard carbon (HC) materials with rich closed pore structures and nano‐scaled soft carbon coating layer have emerged as promising anode in sodium‐ion batteries (SIBs). However, it still remains a tremendous challenge to precisely regulate closed pore structures and soft carbon coating thicknesses for achieving excellent electrochemical performance in SIBs at low‐voltage platforms. Herein, PCHC‐10 with abundant and suitable‐sized closed pore size (0.45 nm) and nano‐scaled soft carbon coating layer has been accurately designed by chemical crosslink reaction between the pre‐oxidized phenolic resin and a small addition of pitch to form ester‐based bond. As anode, PCHC‐10 delivered large reversible capacity of 359.8 mAh g−1 within 0.001–2.5 V, and high capacity of 242.8 mAh g−1 in low voltage platforms (≤0.15 V). Besides, PCHC‐10 anode exhibits 91.4% capacity retention for 100 cycles, and Na3V2(PO4)3//PCHC‐10 full cell has superior rate performance and high energy density of 231.2 Wh kg−1. Furthermore, the detailed electrochemical storage behaviors and theoretical calculations revealed that the HC owning closed pore‐size of 0.45 nm has the strongest Na+ storage abilities in low‐voltage platforms. This work presents a novel insight for constructing HC with suitable‐sized closed pore structures and soft coating layer to boost Na+ storage capability in low‐voltage platforms.

Construction of Monolayer Ti3C2Tx MXene on Nickel Foam under High Electrostatic Fields for High-Performance Supercapacitors

Ti3C2Tx MXene, as a common two-dimensional material, has a wide range of applications in electrochemical energy storage. However, the surface forces of few-layer or monolayer Ti3C2Tx MXene lead to easy agglomeration, which hinders the demonstration of its performance due to the characteristics of layered materials. Herein, we report a facile method for preparing monolayer Ti3C2Tx MXene on nickel foam to achieve a self-supporting structure for supercapacitor electrodes under high electrostatic fields. Moreover, the specific capacitance varies with the deposition of different-concentration monolayer Ti3C2Tx MXene on nickel foam. As a result, Ti3C2Tx/NF has a high specific capacitance of 319 mF cm−2 at 2 mA cm−2 and an excellent long-term cycling stability of 94.4% after 7000 cycles. It was observed that the areal specific capacitance increases, whereas the mass specific capacitance decreases with the increasing loading mass. Attributable to the effect of the high electrostatic field, the self-supporting structure of the Ti3C2Tx/NF becomes denser as the concentration of the monolayer Ti3C2Tx MXene ink increases, ultimately affecting its electrochemical performance. This work provides a simple way to overcome the agglomeration problem of few-layer or monolayer MXene, then form a self-supporting electrode exhibiting excellent electrochemical performance.

Fabrication of Fe/KB composite as sulfur host for Li-S battery.

Lithium sulfur battery is a novel kind of secondary battery which has high energy density, however its application is greatly affected by the shuttle effect of polysulfides generated in the redox reaction of cathode electrode. Metal active sites are supposed as effective catalysts which can absorb and accelerate the conversion efficiency oflithium polysulfides, thus the shuttle effect will be alleviated. In this work, we conducted a simple way to prepare a metal Fe doped ketjen black to serve as the sulfur host of lithium sulfur battery. Ketjen black has a large specific surface area and rich porous structure, while Fe nanodot is an excellent catalyst for lithium polysulfides. Because of these advantages, the Fe/KB host can effectively confine a large amount of active material and accelerate its, therefore the Fe/KB-S cathode electrode show an excellent electrochemical performance.

Preparation of biomass-derived activated carbon from golden needle mushroom roots for supercapacitor electrodes

Herein, an original activated carbon (AC) is prepared using golden needle mushroom roots and potassium hydroxide. After carbonization under 500 °C, AC-5 exhibits good graphitization degree, appropriate nitrogen and oxygen elements, large specific surface area, and micro-meso porous structures. Notably, AC-5 as electrodes in supercapacitors has excellent electrochemical performances, e. g. high specific capacitance (360.4 F/g), impressive rate capability (77 %, from 10 mV/s to 200 mV/s), and outstanding cycling durability (99.56 %, after 10,000 cycles).

Guanine-derived core-shell FeCo alloy confined in graphene-like N-doped carbon as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air batteries

Maximization the synergistic effect of each component in transition metal-carbon complexes is expected to improve the bifunctional oxygen electrocatalysis for rechargeable Zn-air batteries but is still challenging. Herein, nucleobase guanine is employed as a supramolecular precursor to generate the core (FeCo alloy)-shell (carbon) structure embedded in ultrathin graphene-like nitrogen-doped carbon nanosheets (FeCo@NCNSs) via a confinement pyrolysis strategy. Thanks to the generated core-shell structure and bimetallic synergistic effect, the as-prepared FeCo@NCNSs exhibits excellent electrochemical performance in both oxygen reduction reaction and oxygen evolution reaction. As a result, when served as the bifunctional air electrode for a practical Zn-air battery, FeCo@NCNSs exhibits a higher open-circuit voltage (1.553 V) and peak power density (197.30 mW cm−2), as well as the greatly improved long-term cyclic stability compared to the noble metal benchmarks. This work provides a promising approach to integrate various active sites for bifunctional oxygen electrocatalysis and inspires the exploration of simple but efficient electrocatalysts for energy storage and conversion.