Isolated Binary Fe-Ni Metal-Nitrogen Sites Anchored on Porous Carbon Nanosheets for Efficient Oxygen Electrocatalysis through High-Temperature Gas-Migration Strategy

Atomically dispersed dual-site catalysts can regulate multiple reaction processes and provide synergistic functions based on diverse molecules and their interfaces. However, how to synthesize and stabilize dual-site single-atom catalysts (DACs) is confronted with challenges. Herein, we report a facile high-temperature gas-migration strategy to synthesize Fe-Ni DACs on nitrogen-doped carbon nanosheets (FeNiSAs/NC). FeNiSAs/NC exhibits a high half-wave potential (0.88 V) for the oxygen reduction reaction (ORR) and a low overpotential of 410 mV at 10 mA cm-2 for the oxygen evolution reaction (OER). As an air electrode for Zn-air batteries (ZABs), it shows better performances in aqueous ZABs and excellent stability and flexibility in solid-state ZABs. The high specific surface area (1687.32 m2/g) of FeNiSAs/NC is conducive to electron transport. Density functional theory (DFT) reveals that the Fe sites are the active center, and Ni sites can significantly optimize the free energy of the oxygen-containing intermediate state on Fe sites, contributing to the improvement of ORR and the corresponding OER activities. This work can provide guidance for the rational design of DACs and understand the structure-activity relationship of SACs with multiple active sites for electrocatalytic energy conversion.

Exploring Cu-Doped Co3O4 Bifunctional Oxygen Electrocatalysts for Aqueous Zn-Air Batteries

The efficiency of oxygen electrocatalysis is a key factor in diverse energy domain applications, including the performance of metal-air batteries, such as aqueous Zinc (Zn)-air batteries. We demonstrate here that the doping of cobalt oxide with optimal amounts of copper (abbreviated as Cu-doped Co3O4) results in a stable and efficient bifunctional electrocatalyst for oxygen reduction (ORR) and evolution (OER) reactions in aqueous Zn-air batteries. At high Cu-doping concentrations (≥5%), phase segregation occurs with the simultaneous presence of Co3O4 and copper oxide (CuO). At Cu-doping concentrations ≤5%, the Cu ion resides in the octahedral (Oh) site of Co3O4, as revealed by X-ray diffraction (XRD)/Raman spectroscopy investigations and molecular dynamics (MD) calculations. The residence of Cu@Oh sites leads to an increased concentration of surface Co3+-ions (at catalytically active planes) and oxygen vacancies, which is beneficial for the OER. Temperature-dependent magnetization measurements reveal favorable d-orbital configuration (high eg occupancy ≈ 1) and a low → high spin-state transition of the Co3+-ions, which are beneficial for the ORR in the alkaline medium. The influence of Cu-doping on the ORR activity of Co3O4 is additionally accounted in DFT calculations via interactions between solvent water molecules and oxygen vacancies. The application of the bifunctional Cu-doped (≤5%) Co3O4 electrocatalyst resulted in an aqueous Zn-air battery with promising power density (=84 mW/cm2), stable cyclability (over 210 cycles), and low charge/discharge overpotential (=0.92 V).

Mn Single-Atom Tuning Fe-N-C Catalyst Enables Highly Efficient and Durable Oxygen Electrocatalysis and Zinc-Air Batteries

Fe-N-C catalyst is one of most promising candidates for oxygen electrocatalysis reaction in zinc-air batteries (ZABs), but achieving sustained high activity is still a challenging issue. Herein, we demonstrate that introducing Mn single atoms into Fe-N-C (Mn1@Fe-N-C/CNTs) enables the realization of highly efficient and durable oxygen electrocatalysis performance and application in ZABs. Multiple characterizations confirm that Mn1@Fe-N-C/CNTs is equipped with Mn-N2O2 and Fe-N4 sites and Fe nanoparticles. The Mn-N2O2 sites not only tune the electron structure of Fe-Nx sites to enhance intrinsic activity, but also scavenge the attack of radicals from Fe-Nx sites for improvement in ORR durability. As a result, Mn1@Fe-N-C/CNTs exhibits enhanced ORR performance to traditional Fe-N-C catalysts with high E1/2 of 0.89 V vs reversible hydrogen electrode (RHE) and maintains ORR activity after 15 000 CV. Impressively, Mn1@Fe-N-C/CNTs also presents excellent OER activity and the difference (ΔE) between E1/2 of ORR and OER potential at 10 mA cm-2 (Ej10) is only 0.59 V, outperforming most reported catalysts. In addition, the maintainable bifunctional activity of Mn1@Fe-N-C/CNTs is demonstrated in ZABs with almost unchanged cycle voltage efficiency up to 200 h. This work highlights the critical role of Mn single atoms in enhancing ORR activity and stability, promoting the development of advanced catalysts.

Hetero-Anionic Structure Activated CoS Bonds Promote Oxygen Electrocatalytic Activity for High-Efficiency Zinc-Air Batteries

The electronic structure of transition metal complexes can be modulated by replacing partial ion of complexes to obtain tuned intrinsic oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) electrocatalytic activity. However, the anion-modulated transition metal complexes ORR activity of is still unsatisfactory, and the construction of hetero-anionic structure remains challenging. Herein, an atomic doping strategy is presented to prepare the CuCo2 O4-x Sx /NC-2 (CCSO/NC-2) as electrocatalysts, the structrual characterization results favorably demonstrate the partial substitution of S atoms for O in CCSO/NC-2, which shows excellent catalytic performance and durability for OER and ORR in 0.1 m KOH. In addition, the catalyst assembled Zinc-air battery with an open circuit potential of 1.43 V maintains performance after 300 h of cyclic stability. Theoretical calculations and differential charges illustrate that S doping optimizes the reaction kinetics and promotes electron redistribution. The superior performance of CCSO/NC-2 catalysis is mainly due to its unique S modulation of the electronic structure of the main body. The introduction of S promotes CoO covalency and constructs a fast electron transport channel, thus optimizing the adsorption degree of active site Co to the reaction intermediates.

Tailoring Spin State of Perovskite Oxides by Fluorine Atom Doping for Efficient Oxygen Electrocatalysis

Promoting the initially deficient but economical catalysts to high-performing competitors is important for developing superior catalysts. Unlike traditional nano-morphology construction methods, this work focuses on intrinsic catalytic activity enhancement via heteroatom doping strategies to induce lattice distortion and optimize spin-dependent orbital interaction to alter charge transfer between catalysts and reactants. Experimentally, a series of different concentrations of fluorine-doped lanthanum cobaltate (F_x -LaCoO₃ ) exhibiting excellent electrocatalytic activity is synthesized, including a low overpotential of 390 mV at j = 10 mA cm^-2 for OER and a large half-wave potential of 0.68 V for ORR. Meanwhile, the assembled rechargeable Zn-air batteries deliver an excellent performance with a large specific capacity of 811 mAh/g_Zn under 10 mA cm^-2 and stability of charge/recharge (120 h). Theoretically, taking advantage of density functional theory calculations, it is found that the prominent OER/ORR performance arises from the spin state transition of Co³⁺ (Low spin state (LS, t_2g ⁶ e_g ⁰ ) → Intermediate spin state (IS, t_2g ⁵ e_g ¹ ) and the mediated d-band center upshift by F atom incorporation. This work establishes a novel avenue for designing superior electrocatalysts in perovskite-based oxides by regulating spin states.

Materials Screening by the Descriptor G max(η): The Free-Energy Span Model in Electrocatalysis

To move from fossil-based energy resources to a society based on renewables, electrode materials free of precious noble metals are required to efficiently catalyze electrochemical processes in fuel cells, batteries, or electrolyzers. Materials screening operating at minimal computational cost is a powerful method to assess the performance of potential electrode compositions based on heuristic concepts. While the thermodynamic overpotential in combination with the volcano concept refers to the most popular descriptor-based analysis in the literature, this notion cannot reproduce experimental trends reasonably well. About two years ago, the concept of G _max(η), based on the idea of the free-energy span model, has been proposed as a universal approach for the screening of electrocatalysts. In contrast to other available descriptor-based methods, G _max(η) factors overpotential and kinetic effects by a dedicated evacuation scheme of adsorption free energies into an analysis of trends. In the present perspective, we discuss the application of G _max(η) to different electrocatalytic processes, including the oxygen evolution and reduction reactions, the nitrogen reduction reaction, and the selectivity problem of the competing oxygen evolution and peroxide formation reactions, and we outline the advantages of this screening approach over previous investigations.

Heteroatom-Doped Carbon-Encapsulated FeP Nanostructure: A Multifunctional Electrocatalyst for Zinc-Air Battery and Water Electrolyzer

The rational design and synthesis of efficient multifunctional electrocatalysts for renewable energy technologies is of significant interest. Herein, we demonstrate a novel approach for the synthesis of a nitrogen and phosphorus dual-doped mesoporous carbon-encapsulated iron phosphide (FeP@NPC) nanostructure and its multifunctional electrocatalytic activity toward an oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction for zinc-air battery (ZAB) and alkaline water-splitting applications. FeP@NPC is obtained by the carbothermal reduction of the precursor complex [Fe(bpy)₃](PF₆)₂ in the presence of melamine without any traditional phosphidating agent. The PF₆^- counteranion is used for the phosphidation of Fe. FeP@NPC obtained at 900 °C (FeP@NPC-900) exhibits excellent bifunctional oxygen electrocatalytic performance with a very low potential gap (ΔE = E_1/2^ORR - E_j10^OER) of 670 mV. The ZAB device delivers a peak power density of 190.15 mW cm^-2 (iR-corrected), specific capacity of 785 mA h g_Zn^-1, and energy density of 706.5 Wh kg_Zn^-1 at 50 mA cm^-2. The ZAB exhibits excellent charge-discharge cycling stability for over 35 h with negligible voltaic efficiency loss (0.9%). Three CR2032 coin-cell-based ZABs made of an FeP@NPC-900 air cathode connected in series power 81 LEDs for 15 min. FeP@NPC-900 also has promising electrocatalytic activity toward water splitting in acidic as well as in alkaline pH. The benchmark current density of 10 mA cm^-2 is achieved with a two-electrode alkaline water electrolyzer at a cell voltage of 1.65 V. ZAB-powered water electrolyzer is made by integrating two rechargeable ZABs connected in series with the two-electrode water electrolyzer. The ZAB powers the electrolyzer for 24 h without a significant loss in the open-circuit voltage. The catalyst retains its initial structural integrity even after continuous water electrolysis for 24 h.

A "Pre-Constrained Metal Twins" Strategy to Prepare Efficient Dual-Metal-Atom Catalysts for Cooperative Oxygen Electrocatalysis

Dual-metal-atom-center catalysts (DACs) are a novel frontier in oxygen electrocatalysis, boasting functional and electronic synergies between contiguous metal centers and higher catalytic activities than single-atom-center catalysts. However, the definition and catalytic mechanism of DACs configurations remain unclear. Here, a "pre-constrained metal twins" strategy is proposed to prepare contiguous FeN₄ and CoN₄ DACs with homogeneous conformations embedded in a N-doped graphitic carbon (FeCo-DACs/NC). A programmable phthalocyanines dimer is used as a structural moiety to anchor the bimetallic sites (containing Co and Fe) in a metal-organic framework (MOF) to achieve delocalized dispersion before pyrolysis. The resultant FeCo-DACs/NC exhibits excellent electrochemical performance in oxygen electrocatalysis and rechargeable Zn-air batteries. Theoretical calculations demonstrate that the synergetic interaction of adjacent metals optimizes the d-band center position of metal centers and balances the free energy of the *O intermediate, thereby improving the oxygen electrocatalytic activity. This work opens up an avenue for the rational design of DACs with tailored electronic structures and uniform geometric configurations.

Embedding Co2P nanoparticles in Cu doping carbon fibers for Zn-air batteries and supercapacitors

Developing highly efficient and non-precious materials for Zn-air batteries (ZABs) and supercapacitors (SCs) are still crucial and challenging. Herein, electronic reconfiguration and introducing conductive carbon-based materials are simultaneously conducted to enhance the ZABs and SCs performance of Co₂P. We develop a simple and efficient electrospinning technology followed by carbonization process to synthesize embedding Co₂P nanoparticles in Cu doping carbon nanofibers (Cu-Co₂P/CNFs). As a result, the 7% Cu-Co₂P/CNFs presents high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity (half-wave potential of 0.792 V for ORR, an overpotential of 360 mV for OER). The ZABs exhibit a power density of 230 mW cm^-2and excellent discharge-charge stability of 80 h. In addition, the 7% Cu-Co₂P/CNFs show the specific capacitance of 558 F g^-1at 1 A g^-1. Moreover, the 7% Cu-Co₂P/CNFs//CNFs asymmetric supercapacitor was assembled applying 7% Cu-Co₂P/CNFs electrode and pure CNFs, which exhibits a high energy density (25.9 Wh kg^-1), exceptional power density (217.5 kW kg^-1) and excellent cycle stability (96.6% retention after 10 000 cycles). This work may provide an effective way to prepared Co₂P based materials for ZABs and SCs applications.

Electronic Interaction between In Situ Formed RuO2 Clusters and a Nanoporous Zn3V3O8 Support and Its Use in the Oxygen Evolution Reaction

The catalytic activity and durability of RuO₂ clusters toward the oxygen evolution reaction (OER) are strongly associated with their support; however, how the electronic interaction would enhance the catalytic performance is still not quite clear. Herein, hierarchical nanoporous and single-crystal Zn₃V₃O₈ nanosheets are adopted to anchor in situ formed RuO₂ clusters. X-ray photoelectron analysis reveals significant binding energy changes of both Ru and V due to the creation of strong Ru-O-V bonding interaction, which would lead to the reconstruction of the electronic structure of the Zn₃V₃O₈ matrix and RuO₂ clusters. The ultrastrong electronic interaction also results in superior OER activity, indicated by a small overpotential at 10 mA cm^-2 (228 mV) and a shallow Tafel slope of 46 mV dec^-1. First-principles simulation further reveals the synergistic effect derived from the unique RuO₂@Zn₃V₃O₈ couple, which effectively regulates the electronic structure for the OER process. In addition, the created interfacial chemical bond and the confined microporous structure of the Zn₃V₃O₈ substrate could prevent the RuO₂ clusters from detachment and aggregation, making the nanocomposite a promising long-term stable OER electrocatalyst.

Encapsulating Cobalt Nanoparticles in Interconnected N-Doped Hollow Carbon Nanofibers with Enriched CoNC Moiety for Enhanced Oxygen Electrocatalysis in Zn-Air Batteries

Rational design of bifunctional efficient electrocatalysts for both oxygen reduction (ORR) and oxygen evolution reactions (OER) is desirable-while highly challenging-for development of rechargeable metal-air batteries. Herein, an efficient bifunctional electrocatalyst is designed and fabricated by encapsulating Co nanoparticles in interconnected N-doped hollow porous carbon nanofibers (designated as Co@N-C/PCNF) using an ultrafast high-temperature shock technology. Benefiting from the synergistic effect and intrinsic activity of the CoNC moiety, as well as porous structure of carbon nanofibers, the Co@N-C/PCNF composite shows high bifunctional electrocatalytic activities for both OER (289 mV at 10 mA cm-2 ) and ORR (half-wave potential of 0.85 V). The CoNC moiety in the composite can modulate the local environmental and electrical structure of the catalysts, thus optimizing the adsorption/desorption kinetics and decreasing the reaction barriers for promoting the reversible oxygen electrocatalysis. Co@N-C/PCNF-based aqueous Zn-air batteries (AZAB) provide high power density of 292 mW cm-2 , and the assembled flexible ZAB can power wearable devices.

Main Descriptors To Correlate Structures with the Performances of Electrocatalysts

Traditional trial and error approaches to search for hydrogen/oxygen redox catalysts with high activity and stability are typically tedious and inefficient. There is an urgent need to identify the most important parameters that determine the catalytic performance and so enable the development of design strategies for catalysts. In the past decades, several descriptors have been developed to unravel structure-performance relationships. This Minireview summarizes reactivity descriptors in electrocatalysis including adsorption energy descriptors involving reaction intermediates, electronic descriptors represented by a d-band center, structural descriptors, and universal descriptors, and discusses their merits/limitations. Understanding the trends in electrocatalytic performance and predicting promising catalytic materials using reactivity descriptors should enable the rational construction of catalysts. Artificial intelligence and machine learning have also been adopted to discover new and advanced descriptors. Finally, linear scaling relationships are analyzed and several strategies proposed to circumvent the established scaling relationships and overcome the constraints imposed on the catalytic performance.

Hierarchical Hollow MOF-Derived Bamboo-like N-doped Carbon Nanotube-Encapsulated Co0.25Ni0.75 Alloy: An Efficient Bifunctional Oxygen Electrocatalyst for Zinc-Air Battery

The synthesis of nonprecious electrocatalysts for oxygen electrocatalysis is of considerable interest for the development of electrochemical energy devices. Herein, we demonstrate a facile approach for the synthesis of bamboo-like nitrogen-doped carbon nanotube-encapsulated Co_0.25Ni_0.75 alloy electrocatalyst (Co_0.25Ni_0.75@NCNT) and its bifunctional oxygen electrocatalytic performance toward oxygen reduction and oxygen evolution reactions. The Co_0.25Ni_0.75 alloy wrapped with NCNT is obtained by a one-step carbothermal reduction approach using dicyandiamide and NiCo-MOF precursors. Dicyandiamide acts as a nitrogen source, and the in situ generated Co_0.25Ni_0.75 alloy nanoparticles catalyze the growth of bamboo-like NCNTs. The hollow NiCo-MOF plays a sacrificial role in providing a suitable environment for the controlled growth of Co_0.25Ni_0.75 alloy and NCNT. Co_0.25Ni_0.75@NCNT efficiently catalyzes both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) at a favorable overpotential. It shows a low potential gap (ΔE) of ∼0.8 V between the two reactions, and it qualifies for the development of air cathode in metal-air batteries. The enhanced bifunctional activity and excellent durability stem from the chemical composition and the synergistic effect between Co_0.25Ni_0.75 alloy and encapsulating NCNT. The original phase and morphology of the catalyst is preserved after an extensive durability test. Aqueous rechargeable Zn-air battery (ZAB) is fabricated using a Co_0.25Ni_0.75@NCNT-based air cathode. The battery has high open-circuit voltage (1.53 V) and a maximum peak power density of 167 mW cm^-2 with only 1.6% loss in the voltaic efficiency after 36 h charge-discharge cycles. As a proof-of-concept demonstration, the as-fabricated ZAB is successfully used for the electrochemical water splitting in alkaline solution.

Three-Dimensional Graphene-Based Macrostructures for Electrocatalysis

Electrochemical energy storage and conversion is an effective strategy to relieve the increasing energy and environment crisis. The sluggish reaction kinetics in the related devices is one of the major obstacles for them to realize practical applications. More efforts should be devoted to searching for high-efficiency electrocatalysts and enhancing the electrocatalytic performance. 3D graphene macrostructures (3D GMs) are one kind of porous crystalline materials with 3D structures at both micro- and macro-scale. The unique structure can achieve large accessible surface area, expose many active sites, promote fast mass/electron transport, and provide wide room for further functional modification. All these features make them promising candidates for electrocatalysis. In this review, the authors focus on the latest progress of 3D GMs for electrocatalysis. First, the preparation methods of 3D GMs are introduced followed by the strategies for functional modifications. Then, their electrocatalytic performances are discussed in detail including monofunctional and bifunctional electrocatalysis. The electrocatalytic processes involve oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and carbon dioxide reduction reaction. Finally, the challenges and perspectives are presented to offer a guideline for the exploration of excellent 3D GM-based electrocatalysts.

Cation Modulation of Cobalt Sulfide Supported by Mesopore-Rich Hydrangea-Like Carbon Nanoflower for Oxygen Electrocatalysis

Transition-metal sulfide is pursued for replacing scare platinum-group metals for oxygen electrocatalysis and is of great importance in developing low-cost, high-performance rechargeable metal-air batteries. We report herein a facile cationic-doping strategy for preparing nickel-doped cobalt sulfide embedded into a mesopore-rich hydrangea-like carbon nanoflower. Nickel cations are introduced to induce the formation of Co³⁺-active species and more oxygen vacancies due to higher electronegativity and smaller ionic radius, thereby strengthening the intrinsic activity for oxygen electrocatalysis. Moreover, hydrangea-like superstructure composed of interconnected carbon cages provides abundant accessible active sites and hierarchical porosity. As a result, it shows excellent catalytic performance with a superior mass activity for the oxygen reduction reaction to the state-of-the-art Pt/C catalyst and a low overpotential of 314 mV at 10 mA cm^-2 for the oxygen evolution reaction. When used as an air cathode for the rechargeable Zn-air battery, it delivers a peak power density of 96.3 mW cm^-2 and stably operates over 214 h. This work highlights the importance of cationic doping in strengthening the electrocatalytic performance of 3d-transition-metal chalcogenides.

Tuning the Spin Density of Cobalt Single-Atom Catalysts for Efficient Oxygen Evolution

Single-atom catalysts (SACs) with magnetic elements as the active center have been widely exploited for efficient electrochemical conversions. Understanding the catalytic role of spin, and thus modulating the spin density of a single-atom center, is of profound fundamental interest and technological impact. Here, we synthesized ferromagnetic single Co atom catalysts on TaS₂ monolayers (Co₁/TaS₂) as a model system to explore the spin-activity correlation for the oxygen evolution reaction (OER). A single Co atom adsorbed at the hollow site (Co_HS) with spin-polarized electronic states serves as the active site for OER, whose spin density can be regulated by its neighboring single Co site via tuning the Co loading. Both experimental and theoretical results reveal the spin density-dependent OER activity that an optimal spin density of Co_HS can be achieved with a neighboring hetero-single Co_Ta site (substitution of Ta by Co) for a superior OER performance, in contrast to a homo-single Co_HS site, which creates an excessive spin density over vicinal Co_HS. An optimized spin density of Co_HS results in an optimal binding energy of oxygen species for the OER. Establishing the spin-activity correlation in SACs may create a descriptor for designing efficient magnetic SACs for renewable energy conversions.

Modulation of Single Atomic Co and Fe Sites on Hollow Carbon Nanospheres as Oxygen Electrodes for Rechargeable Zn-Air Batteries

Efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for metal air batteries, to replace costly metals, such as Pt and Ir/Ru based compounds, which are typically used as benchmarks for ORR and OER, respectively. Isolated single atomic sites coordinated with nitrogen on carbon supports (M-N-C) have promising performance for replacement of precious metal catalysts. However, most of monometallic M-N-C catalysts demonstrate unsatisfactory bifunctional performance. Herein, a facile way of preparing bimetallic Fe and Co sites entrapped in nitrogen-doped hollow carbon nanospheres (Fe,Co-SA/CS) is explored, drawing on the unique structure and pore characteristics of Zeolitic imidazole frameworks and molecular size of Ferrocene, an Fe containing species. Fe,Co-SA/CS showed an ORR onset potential and half wave potential of 0.96 and 0.86 V, respectively. For OER, (Fe,Co)-SA/CS attained its anodic current density of 10 mA cm^-2 at an overpotential of 360 mV. Interestingly, the oxygen electrode activity (ΔE) for (Fe,Co)-SA/CS and commercial Pt/C-RuO₂ is calculated to be 0.73 V, exhibiting the bifunctional catalytic activity of (Fe,Co)-SA/CS. (Fe,Co)-SA/CS evidenced desirable specific capacity and cyclic stability than Pt/C-RuO2 mixture when utilized as an air cathode in a homemade Zinc-air battery.

Nitrogen-Doped Cobalt Pyrite Yolk-Shell Hollow Spheres for Long-Life Rechargeable Zn-Air Batteries

Limited by the sluggish four-electron transfer process, designing high-performance nonprecious electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is urgently desired for efficient rechargeable Zn-air batteries (ZABs). Herein, the successful synthesis of porous nitrogen-doped cobalt pyrite yolk-shell nanospheres (N-CoS2 YSSs) is reported. Benefiting from the abundant porosity of the porous yolk-shell structure and unique electronic properties by nitrogen doping, the as-prepared N-CoS2 YSSs possess more exposed active surface, thus giving rise to superior activity for reversible oxygen electrocatalysis and outstanding cycling stability (more than 165 h at 10 mA cm-2) in ZABs, exceeding the commercial Pt/C and RuO2 hybrid catalysts. Moreover, the assembled ZABs, delivering a specific capacity of 640 mAh gZn -1, can be used for practical devices. This work provides a novel tactic to engineer sulfides as high efficiency and promising bifunctional oxygen electrocatalysts for advanced metal-air batteries.

Spin-Related Electron Transfer and Orbital Interactions in Oxygen Electrocatalysis

Oxygen evolution and reduction reactions play a critical role in determining the efficiency of the water cycling (H₂ O ⇔ H₂ + 1 2 O₂ ), in which the hydrogen serves as the energy carrier. That calls for a comprehensive understanding of oxygen electrocatalysis for efficient catalyst design. Current opinions on oxygen electrocatalysis have been focused on the thermodynamics of the reactant/intermediate adsorption on the catalysts. Because the oxygen molecule is paramagnetic, its production from or its reduction to diamagnetic hydroxide/water involves spin-related electron transfer. Both electron transfer and orbital interactions between the catalyst and the reactant/intermediate show spin-dependent character, making the reaction kinetics and thermodynamics sensitive to the spin configurations. Herein, a brief introduction on the spintronic explanation of the catalytic phenomena on oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is given. The local spin configurations and orbital interactions in the benchmark transition-metal-based catalysts for OER and ORR are analyzed as examples. To further understand the spintronic oxygen electrocatalysis and to develop more efficient spintronic catalysts, the challenges are summarized and future opportunities proposed. Spin electrocatalysis may emerge as an important topic in the near future and help integrate a comprehensive understanding of oxygen electrocatalysis.

Recommended Practices and Benchmark Activity for Hydrogen and Oxygen Electrocatalysis in Water Splitting and Fuel Cells

Electrochemical energy storage by making H₂ an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measurements, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state-of-the-art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selection, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic- and background-correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activities of some state-of-the-art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories.

"Ship in a Bottle" Design of Highly Efficient Bifunctional Electrocatalysts for Long-Lasting Rechargeable Zn-Air Batteries

The poor durability of bifunctional oxygen electrocatalysts is one main bottleneck that suppresses the widespread application of rechargeable metal-air batteries. Herein, a "ship in a bottle" design is achieved by impregnating fine transition metal dichalcogenide nanoparticles into defective carbon pores that act as interconnected nanoreactors. The erected 3D porous conductive architecture provides a "highway" for expediting charge and mass transfer. This design not only delivers a high surface-to-volume ratio to increase numbers of exposed catalytic sites but also precludes nanoparticles from aggregation during cycling owing to the pore spatial confinement effect. Therefore, the long-term plague inherent to nanocatalyst stability can be solved. Moreover, the synergistic coupling effects between defect-rich interfaces and chemical bonding derived from heteroatom-doping boost the catalytic activity and prohibit the detachment of nanoparticles for better stability. Consequently, the developed catalyst presents superior bifunctional oxygen electrocatalytic activities and durability, out-performing the best-known noble-metal benchmarks. In a practical application to rechargeable Zn-air batteries, long-term cyclability for over 340 h is realized at a high current density of 25 mA cm^-2 in ambient air while retaining an intact structure. Such a universal "ship in a bottle" design offers an appealing and instructive model of nanomaterial engineering for implementation in various fields.

Engineering the Surface Metal Active Sites of Nickel Cobalt Oxide Nanoplates toward Enhanced Oxygen Electrocatalysis for Zn-Air Battery

Clarifying and controlling the surface catalytic active sites is at the heart of developing low-cost effective bifunctional oxygen catalysts to replace precious metals for metal-air batteries. Herein, a shape-control of hexagon nickel cobalt oxide spinel nanosheets was reported to engineer the surface metal active sites for enhanced electrocatalysis of oxygen evolution and oxygen reduction reactions (OER/ORR). Specifically, through simply tuning annealing temperature, different Ni³⁺/Ni²⁺ and Co³⁺/Co²⁺ atomic configurations on the nickel cobalt oxide surface were controllably synthesized. Electrochemical results show that the oxide treated at 250 °C (NCO-250) with the highest value of Ni³⁺/Ni²⁺ sites and the lowest value of Co³⁺/Co²⁺ sites exhibits superior OER/ORR activity in alkaline electrolytes and better discharge/charge performance in Zn-air batteries among all the samples. The optimized surface active site configuration of the NCO-250 sample leads to the optimal energy of adsorption, activation, and desorption for water molecules and oxygen species, thus promoting a high electrocatalytic activity. This work provides a strategy to design cost-effective, highly active, and durable electrocatalysts through regulating active sites on transition-metal surface for Zn-air battery and other advanced energy devices.

Coordination-Assisted Polymerization of Mesoporous Cobalt Sulfide/Heteroatom (N,S)-Doped Double-Layered Carbon Tubes as an Efficient Bifunctional Oxygen Electrocatalyst

It is a critical challenge to construct efficient precious-metal-free bifunctional oxygen electrocatalysts for fuel cell and metal-air batteries via structural and component engineering. Herein, a one-dimensional mesoporous double-layered tubular structure, where Co₉S₈ nanocrystals are incorporated into nitrogen, sulfur codoped carbon, is successfully synthesized via the coordinated-assisted polymerization and sacrificial template methods. The double-layered tubular structure provides for a large electrochemically active surface area and promotes fast mass transfer. Cobalt oxides/oxyhydroxides, which are evolved from the sulfides during the catalytic processes, as the main active sites efficiently catalyze the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), in cooperation with the Co-N-C and heteroatom-induced active sites. Hence, it demonstrates excellent bifunctional electrocatalytic activity with the overvoltage between the OER potential at 10 mA cm^-2 ( E₁₀) and ORR half-wave potential ( E_1/2) of 0.707 V, which is superior to most of precious-metal-free bifunctional oxygen electrocatalysts reported recently, as well as the state-of-art Pt/C and RuO₂ catalysts.

Metal-Organic-Framework-Derived Hybrid Carbon Nanocages as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are cornerstone reactions for many renewable energy technologies. Developing cheap yet durable substitutes of precious-metal catalysts, especially the bifunctional electrocatalysts with high activity for both ORR and OER reactions and their streamlined coupling process, are highly desirable to reduce the processing cost and complexity of renewable energy systems. Here, a facile strategy is reported for synthesizing double-shelled hybrid nanocages with outer shells of Co-N-doped graphitic carbon (Co-NGC) and inner shells of N-doped microporous carbon (NC) by templating against core-shell metal-organic frameworks. The double-shelled NC@Co-NGC nanocages well integrate the high activity of Co-NGC shells into the robust NC hollow framework with enhanced diffusion kinetics, exhibiting superior electrocatalytic properties to Pt and RuO₂ as a bifunctional electrocatalyst for ORR and OER, and hold a promise as efficient air electrode catalysts in Zn-air batteries. First-principles calculations reveal that the high catalytic activities of Co-NGC shells are due to the synergistic electron transfer and redistribution between the Co nanoparticles, the graphitic carbon, and the doped N species. Strong yet favorable adsorption of an OOH* intermediate on the high density of uncoordinated hollow-site C atoms with respect to the Co lattice in the Co-NGC structure is a vital rate-determining step to achieve excellent bifunctional electrocatalytic activity.

Cations in Octahedral Sites: A Descriptor for Oxygen Electrocatalysis on Transition-Metal Spinels

Exploring efficient and low-cost electrocatalysts for the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER) is critical for developing renewable energy technologies such as fuel cells, metal-air batteries, and water electrolyzers. A rational design of a catalyst can be guided by identifying descriptors that determine its activity. Here, a descriptor study on the ORR/OER of spinel oxides is presented. With a series of MnCo₂ O₄ , the Mn in octahedral sites is identified as an active site. This finding is then applied to successfully explain the ORR/OER activities of other transition-metal spinels, including Mn_x Co_3-x O₄ (x = 2, 2.5, 3), Li_x Mn₂ O₄ (x = 0.7, 1), XCo₂ O₄ (X = Co, Ni, Zn), and XFe₂ O₄ (X = Mn, Co, Ni). A general principle is concluded that the e_g occupancy of the active cation in the octahedral site is the activity descriptor for the ORR/OER of spinels, consolidating the role of electron orbital filling in metal oxide catalysis.

Development of Cobalt Hydroxide as a Bifunctional Catalyst for Oxygen Electrocatalysis in Alkaline Solution

Co(OH)2 in the form of hexagonal nanoplates synthesized by a simple hydrothermal reaction has shown even greater activity than cobalt oxides (CoO and Co3O4) in oxygen reduction and oxygen evolution reactions (ORR and OER) under alkaline conditions. The bifunctionality for oxygen electrocatalysis as shown by the OER-ORR potential difference (ΔE) could be reduced to as low as 0.87 V, comparable to the state-of-the-art non-noble bifunctional catalysts, when the Co(OH)2 nanoplates were compounded with nitrogen-doped reduced graphene oxide (N-rGO). The good performance was attributed to the nanosizing of Co(OH)2 and the synergistic interaction between Co(OH)2 and N-rGO. A zinc-air cell assembled with a Co(OH)2-air electrode also showed a performance comparable to that of the state-of-the-art zinc-air cells. The combination of bifunctional activity and operational stability establishes Co(OH)2 as an effective low-cost alternative to the platinum group metal catalysts.

Orientation-Dependent Oxygen Evolution Activities of Rutile IrO2 and RuO2

The activities of the oxygen evolution reaction (OER) on IrO2 and RuO2 catalysts are among the highest known to date. However, the intrinsic OER activities of surfaces with defined crystallographic orientations are not well-established experimentally. Here we report that the (100) surface of IrO2 and RuO2 is more active in alkaline environments (pH 13) than the most thermodynamically stable (110) surface. The OER activity was correlated with the density of coordinatively undersaturated metal sites of each crystallographic facet. The surface-orientation-dependent activities can guide the design of nanoscale catalysts with increased activity for electrolyzers, metal-air batteries, and photoelectrochemical water splitting applications.