Bifunctional Oxygen Electrocatalytic Activity Research Articles

Novel design of hollow carbon nanocage modified with nanotubes as a bifunctional electrocatalyst for high performance Zn–air batteries

Microcavities play a crucial role as microreactors in the transport of molecular/ionic guests and the exposure of active sites, thus significantly influencing the electrocatalytic performance. This study prepares Co/N-codoped hollow carbon (HT-CoN/C) with surface-distributed carbon nanotubes by pyrolysis of PDA-coated Zn/Co bimetallic ZIF (BM-ZIF@PDA). Benefiting from the hierarchical porous structure, high specific surface area (307.17 m2 g−1) and abundant Co clusters, the HT-CoN/C exhibits remarkable bifunctional oxygen electrocatalytic activity with an overpotential of the ORR/OER processes (ΔE = 0.703 V). The density functional theory (DFT) calculations also verify that the configuration of C-coated N-coordinated Co clusters (Co4-Nx) affect the electrocatalytic activity of ORR and OER, illustrating the source of the excellent oxygen electrocatalytic activity of HT-CoN/C. The aqueous rechargeable zinc-air battery (RZAB) using HT-CoN/C as the air electrode is characterized by a superior peak power density (175 mW cm−2), a prolonged cycle life (1230 cycles/410 h at 5.0 mA cm−2) and a high open-circuit voltage (1.47 V). Meanwhile, the flexible solid-state RZAB assembled by the HT-CoN/C also exhibits a higher peak power density (117 mW cm−2) and an excellent bending performance. This work is extremely valuable for the design and synthesis of Co/N co-doped carbon electrocatalysts.

Surface-Engineered Ni2P: An Efficient Oxygen Electrocatalyst for Zinc-Air Battery.

The surface engineering of electrocatalysts is one of the promising strategies to increase the intrinsic activity of electrocatalysts. It generates anion/cation vacancy defects and increases the electrochemically active surface area. We describethe surface engineering of Ni2P to favorably tune the bifunctional oxygen electrocatalytic activity and the development of a rechargeable zinc-air battery (ZAB). Ni2P encapsulated with N and P-dual doped carbon (Ni2P@NPC) is synthesized using a single-source precursor complex tris-(2,2'-bipyridine)nickel(II) bis(hexafluorophosphate). The surface engineering of the as-synthesized Ni2P@NPC is achieved by the controlled acid treatment at room temperature. The surface engineering removes carbon debris and opens the pores, exfoliates the encapsulating carbon layer, increases the P-vacancy in the crystal lattice, and boosts the electrochemically active surface area. Thesurface-engineered catalyst exhibits enhanced bifunctional activity towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalytically active sites of engineered catalysts are highly accessible for facilitated electron transfer kinetics. P-vacancy favors the facile formation of defect-rich OER active metal oxyhydroxide species. The rechargeable ZAB based on the engineered catalyst delivers a specific capacity of 770.25 mA h gZn-1, energy density of 692 Wh kgZn-1, and excellent charge-discharge cycling performance with negligible voltaic efficiency loss (0.6 %) after 100 h.

General and Facile Synthesis of Co/CoO Nanoparticals Supported by Nitrogen-Doped Graphenic Networks as Efficient Oxygen Electrocatalyst for Zn-Air Batteries.

Reasonable design of low-cost, high-efficiency and stable bifunctional oxygen electrocatalysts is of great significance to improve the reaction efficiency of Zn-air batteries, which is still a huge challenge. Here, we report a highly efficient bifunctional oxygen electrocatalyst with three-dimensional (3D) N-doped graphene network-supported cobalt and cobalt oxide nanoparticles (Co/CoO-NG), which can be in situ synthesized by inducing metal ions on metal plates via graphene oxide as an inducer. This 3D network structure and open active center show excellent bifunctional oxygen electrocatalytic activity under alkaline conditions, and can be used as an air electrode in rechargeable Zn-air batteries, with significantly better power density (244.28 mW cm-2) and stability (over 340 h) than commercial Pt/C+RuO2 mixtures. This work is conducive to advancing the practical application of graphene-based materials as air electrodes for rechargeable zinc-air batteries.

Tuning the d-Band Center in Prussian Blue Analogues via Modulated Carbon and Nitrogen Coordinated Metals for Boosted Bi-Functional Oxygen Electrocatalytic Activity.

The d-band center is a promising descriptor to understand trends in electrocatalysis extensively studied in transition metal oxides but largely unexplored in metal organic frameworks (MOFs). Herein, we present our systematic study aimed at developing a mini-volcano plot demonstrating dependence of bifunctional oxygen electrocatalytic activity of Prussian blue analogues (PBAs) on their d-band center. Our results from ex-situ core level and valence band XPS, Raman, and FTIR spectroscopy suggest that the tuning of the d-band center via modulated N and C-coordinated metal centers dictates their electrocatalytic OER and ORR activities. Among PBAs, the CoCo PBA exhibits highest activity due to its optimal position of d-band center and abundant Co3+ active sites. Moreover, M-Co PBAs showcased superior performance compared to M-Fe PBAs, attributed to facile formation of surface-active sites, i.e., metal oxy(hydroxide). The study advances the understanding of the influence of metal center tuning in PBAs on the intermediate energies and catalytic activity.

Synergistic Fe and Co binary single atoms based air cathodes for high performance and ultra-stable Zn-air batteries

Cost-effective highly efficient and stable air cathodes are critical for practical applications of rechargeable Zn-air batteries (ZABs). In this work, air cathodes constructed by loading carbon nanotube supported N-doped carbon anchored Fe and Co binary single atoms, f-Fe1Co1/CNT, in carbon paper composited nickel foams, NF/CP, were developed for ZABs, exhibiting outstanding bifunctional oxygen electrocatalytic activities (ΔE = 0.671 V) with a half-wave potential (E1/2) of 0.880 V (vs. RHE) for oxygen reduction reaction (ORR) and an overpotential of 321 mV at 10 mA cm−2 (η10) for oxygen evolution reaction (OER) in 0.1 M KOH, which outperformed the benchmark (Pt/C+IrO2)-based air cathode (ΔE = 0.773 V, E1/2 = 0.850 V, η10 = 393 mV). With the unique air cathode design of compositing catalyst-loading layer (nickel foam) with gas diffusion layer (carbon paper), the f-Fe1Co1/CNT@NF/CP//Zn ZAB delivered a remarkable discharge peak power density of 282.1 mW cm−2 at an ultra-high current density of 427.7 mA cm−2 and maintained an ultra-small potential gap of 0.75 V after operations at 10 mA cm−2 for 3300 cycles (1100 h), largely outperforming the (Pt/C+IrO2)@NF/CP//Zn ZAB (162.4 mW cm−2 at 247.3 mA cm−2, 0.86 V, 590 cycles). Density functional theory calculations revealed the positive long-range synergy between Fe-N4 and Co-N4 single atoms, significantly reducing the free energies of the rate-determining steps, oxygen protonation for ORR and formation of oxyhydroxides for OER at the main active site Fe-N4, to realize the much enhanced oxygen electrocatalytic activities.

Constructing Asymmetrical Coordination Microenvironment with Phosphorus‐Incorporated Nitrogen‐Doped Carbon to Boost Bifunctional Oxygen Electrocatalytic Activity

AbstractCarbon‐based metal‐free electrocatalysts have been recognized as inexpensive alternatives to afford excellent activity in oxygen reduction/evolution reactions (ORR/OER). Nevertheless, precisely identifying the local active sites and tailoring the corresponding electronic properties to enhance the reaction kinetics remain challenging. Herein, a facile strategy to create a metal‐free electrocatalyst comprised of a mesoporous nitrogen‐doped carbon matrix with phosphorus incorporation (NPC) is described. The as‐prepared NPC‐950 electrocatalyst demonstrates superior ORR activity under alkaline and acidic conditions with half‐wave potentials of 0.88 and 0.72 V, respectively, comparable to commercial Pt/C (0.85 and 0.76 V) and overwhelmingly superior to other N‐doped carbon catalyst materials. In addition, a remarkable promotion of OER activity under alkaline conditions is observed. Notably, a zinc–air battery equipped with this NCP‐950 electrocatalyst exhibits exceptional performance in peak power density, specific capacity, and long‐term operation durability. Theoretical calculations uncover that the incorporation of phosphorus in NC material results in effective charge density redistribution, thus modulating the electronic properties of active sites to achieve optimum adsorption and desorption of ORR intermediates. The work provides a deep understanding of active sites in heteroatom‐doped carbon materials and highlights the importance of the electronic properties modulation in oxygen bifunctional electrocatalytic activity.

Surface Tailoring-Modulated Bifunctional Oxygen Electrocatalysis with CoP for Rechargeable Zn-Air Battery and Water Splitting.

The transition metal phosphide (TMP)-based functional electrocatalysts are very promising for the development of electrochemical energy conversion and storage devices including rechargeable metal-air batteries and water electrolyzer. Tuning the electrocatalytic activity of TMPs is one of the vital steps to achieve the desired performance of these energy devices. Herein, we demonstrate the modulation of the bifunctional oxygen electrocatalytic activity of nitrogen-doped carbon-encapsulated CoP (CoP@NC) nanostructures by surface tailoring with ultralow amount (0.56 atomic %) of Ru nanoparticles (2.5 nm). The CoP at the core and the Ru nanoparticles on the shell have a facile charge transfer interaction with the encapsulating NC. The strong coupling of Ru with CoP@NC boosts the electrocatalytic performance toward oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions. The surface-tailored catalyst requires only 35 mV to deliver the benchmark current density of 10 mA·cm-2 for HER. A small potential gap of 620 mV between ORR and OER is achieved, making the catalyst highly suitable for the development of rechargeable zinc-air batteries (ZABs). The homemade ZAB delivers a specific capacity of 780 mA·hgZn-1 and peak power density of 175 mW·cm-2 with a very small voltaic efficiency loss (1.1%) after 300 cycles. The two-electrode water splitting cell (CoP@NC-Ru||CoP@NC-Ru) delivers remarkably low cell voltage of 1.47 V at the benchmark current density. Stable current density of 25 mA·cm-2 for 25 h without any significant change is achieved. Theoretical studies support the charge transfer interaction-induced enhanced electrocatalytic activity of the surface-tailored nanostructure.

Lanthanum strontium cobaltite-based perovskite as an electrocatalyst for zinc-air battery application

Developing a low-cost and highly efficient electrocatalyst for bifunctional oxygen electrocatalytic activity is crucial in the field of energy conversion and storage devices. In this study, we employed a doping strategy to finely tune the electronic structure of lanthanum strontium cobaltite-based perovskite. Furthermore, we introduce Mn, Ni, and Cu cations as dopants at the B-site of lanthanum strontium cobaltite, La0.5Sr0.5CoO3 (LSCO), to investigate oxygen electrocatalytic activity in alkaline electrolytes. Among the electrocatalysts we synthesized, La0.5Sr0.5Co0.8Ni0.2O3 (LSCNiO) exhibited the lowest OER overpotential (ηj=10) of 429 mV in 0.1 M KOH, surpassing LSCO by 71 mV and LCO by 204 mV, respectively. Additionally, LSCNiO demonstrated a positive potential of 0.68 V @ 1 mA cm−2 for ORR, which is 50 mV less for LSCO catalyst. The improved electrocatalytic activity was investigated using spectroscopic techniques, confirming the presence of oxygen vacancies and an elevated Co oxidation state. The synergistic interplay between Co and Ni cations enhanced the hybridization between Co/Ni 3d and O 2p bands, reducing energy barriers and improving oxygen reaction kinetics. Finally, LSCNiO oxide with a bifunctional index (ΔE) of 1.08 V is tested for zinc-air battery (ZAB) applications.

Ni dispersed ultrathin carbon nanosheets as bi-functional oxygen electrocatalyst induced from graphite-like porous supramolecule

Excellent porosity and accessibility are key requirements during carbon-based materials design for energy conversion applications. Herein, a Ni-based porous supramolecular framework with graphite-like morphology (Ni-SOF) was rationally designed as a carbon precursor. Ultrathin carbon nanosheets dispersed with Ni nanoparticles and Ni-Nx sites (Ni@NiNx-N-C) were obtained via in-situ exfoliation during pyrolysis. Due to the hetero-porous structure succeeding from Ni-SOF, the Ni@NiNx-N-C catalyst showed outstanding bifunctional oxygen electrocatalytic activity with a narrow gap of 0.69 V between potential to deliver 10 mA cm−2 oxygen evolution and half-wave potential of oxygen reduction reaction, which even surpassed the Pt/C + IrO2 pair. Therefore, the corresponding zinc-air battery exhibited excellent power output and stability. The multiple Ni-based active sites, the unique 2D structure with a high graphitization degree and large specific surface area synergistically contributed to the excellent bifunctional electrocatalytic activity of Ni@NiNx-N-C. This work provided a novel viewpoint for the development of carbon-based electrocatalyst.

N, S co-doped hollow carbon nanocages confined Fe, Co bimetallic sites for bifunctional oxygen electrocatalysis

Regulating pore architecture and electronic structure is of significance for improving oxygen electrocatalytic activity of M−N−C catalysts. Herein, N, S co-doped hollow carbon nanocages confined Fe, Co bimetallic sites (FeCo-NS-HNCs) are constructed through an efficient surface bridging strategy. The surface modifier of trithiocyanuric acid trisodium salt bridges Co doped ZIF-8 surface with Fe ions to promote the formation of Fe/Co-Nx sites. The synergistic coupling of metal sites and N, S co-doping manipulates the adsorption/desorption characteristics of M−Nx sites to optimize intrinsic activity. The Kirkendall effect constructs the hollow carbon nanocages, improving the accessibility of M−Nx sites. Benefiting from the ameliorated electronic structure and hollow porous architecture, FeCo-NS-HNCs displays excellent bifunctional oxygen electrocatalytic activity (ΔE = 0.655 V). The liquid and flexible Zn-air batteries of FeCo-NS-HNCs offer prominent power density and cycling stability. This work provides estimable insights into the structure design and activity regulation of M−N−C catalysts for Zn-air battery.

Tailoring the d-band electronic structure of deficient LaMn0.3Co0.7O3-δ perovskite nanofibers for boosting oxygen electrocatalysis in Zn-Air batteries

The development and design of efficient bifunctional electrocatalysts towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial for rechargeable Zinc-air batteries (ZABs). Optimizing the d-band structure of active metal center in perovskite oxides is an effective method to enhance ORR/OER activity by accelerating the rate-determining step. Herein, we report a deficient method to optimize the d-band structure of Co ions in LaMn0.3Co0.7O3-δ (LMCO-2) perovskite nanofibers, which regulates the mutual effect between B-site Co ions and reactive oxygen intermediates. It is proved by experiment and theoretical calculation that the d-band center (Md) of transition metal ions in LMCO-2 is moved up and the electron filling number of eg orbital in B site is 1.01, thus leading to the reduction of Gibbs free energy required for ORR rate-determining step (OH*→H2O*) to 0.22 eV and promoting reaction proceeds. In this manner, LMCO-2 showed good bifunctional oxygen electrocatalytic activity, with a half-wave potential of 0.71 V vs. RHE. Furthermore, the high specific capacity of 811.54 mAh g−1 and power density of 326.56 mW cm−2 were obtained by using LMCO-2 as the cathode catalyst for ZABs. This study proved the feasibility of d-band structure regulation to enhance the electrocatalytic activity of perovskite oxides.

Geometric and Electronic Engineering of Atomically Dispersed Copper-Cobalt Diatomic Sites for Synergistic Promotion of Bifunctional Oxygen Electrocatalysis in Zinc-Air Batteries.

The development of rechargeable zinc-air batteries is heavily dependent on bifunctional oxygen electrocatalysts to offer exceptional oxygen reduction/evolution reaction (ORR/OER) activities. However, the design of such electrocatalysts with high activity and durability is challenging. Herein, a strategy is proposed to create an electrocatalyst comprised of copper-cobalt diatomic sites on a highly porous nitrogen-doped carbon matrix (Cu-Co/NC) with abundantly accessible metal sites and optimal geometric and electronic structures. Experimental findings and theoretical calculations demonstrate that the synergistic effect of Cu-Co dual-metal sites with metal-N4 coordination induce asymmetric charge distributions with moderate adsorption/desorption behavior with oxygen intermediates. This electrocatalyst exhibits extraordinary bifunctional oxygen electrocatalytic activities in alkaline media, with a half-wave potential of 0.92 V for ORR and a low overpotential of 335mV at 10mAcm-2 for OER. In addition, it demonstrates exceptional ORR activity in acidic (0.85V) and neutral (0.74V) media. When applied to a zinc-air battery, it achieves extraordinary operational performance and outstanding durability (510h), ranking it as one of the most efficient bifunctional electrocatalysts reported to date. This work demonstrates the importance of geometric and electronic engineering of isolated dual-metal sites for boosting bifunctional electrocatalytic activity in electrochemical energy devices.

Perovskite oxide composites for bifunctional oxygen electrocatalytic activity and zinc-air battery application- a mini-review

Zinc-air batteries (ZABs) are considered as a potential alternative to next-generation batteries. However, the slow reaction kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), involved in the charging and discharging of ZAB hinder their real-world applicability. Thus, developing an excellent bifunctional OER/ORR electrocatalyst that exhibits solid cyclic stability is crucial. Perovskite oxides are known for their composition and structural flexibility, which allows for the modulation of the electronic structure of electrocatalysts by incorporating foreign metals into oxides, resulting in improved oxygen electrocatalytic activity. Achieving bifunctional oxygen electrocatalytic activity through a single pristine perovskite oxide is challenging due to the requirement of different reaction steps. Building heterostructures by combing perovskite oxides with metal, metal alloys, metal hydroxides, metal oxides, spinel oxides, and N-doped carbon materials enhances the active sites of the catalyst for bifunctional activity and ZAB application. In this review, we discuss the mechanism involved in ZAB and bifunctional oxygen electrocatalytic activity, mainly focusing on perovskite oxide heterostructure synthesizing strategies and their performance in ZAB. This review aims to pave the way for designing inexpensive perovskite oxide-based electrocatalysts and tailoring the active sites for improved energy conversion and industrialization.

Optimizing d-Orbital Electronic Configuration via Metal-Metal Oxide Core-Shell Charge Donation for Boosting Reversible Oxygen Electrocatalysis.

Tuning the d-orbital electronic configuration of active sites to achieve well-optimized adsorption strength of oxygen-containing intermediates toward reversible oxygen electrocatalysis is desirable for efficient rechargeable Zn-Air batteries but extremely challenging. Herein, this work proposes to construct a Co@Co3 O4 core-shell structure to regulate the d-orbital electronic configuration of Co3 O4 for the enhanced bifunctional oxygen electrocatalysis. Theoretical calculations first evidence that electron donation from Co core to Co3 O4 shell could downshift the d-band center and simultaneously weak spin state of Co3 O4 , result in the well-optimized adsorption strength of oxygen-containing intermediates on Co3 O4 , thus contributing a favor way for oxygen reduction/evolutionreaction (ORR/OER) bifunctional catalysis. As a proof-of-concept, the Co@Co3 O4 embedded in Co, N co-doped porous carbon derived from thickness controlled 2D metal-organic-framework is designed to realize the structure of computational prediction and further improve the performance. The optimized 15Co@Co3 O4 /PNC catalyst exhibits the superior bifunctional oxygen electrocatalytic activity with a small potential gap of 0.69V and a peak power density of 158.5mW cm-2 in ZABs. Moreover, DFT calculations shows that the more oxygen vacancies on Co3 O4 contribute too strong adsorption of oxygen intermediates which limit the bifunctional electrocatalysis, while electron donation in the core-shell structure can alleviate the negative effect and maintain superior bifunctional overpotential.

In situ induced growth strategy of Co2P nanocrystals encapsulated into stable 3D carbon network as a bifunctional electrocatalyst for Zn-air battery

Rational design of inexpensive and robust carbon-based bifunctional catalysts is of considerable interest for practical application of rechargeable Zn-air battery (ZAB) technology. Herein, a facile in-situ induced growth strategy is developed to construct Co2P nanocrystals encapsulated into a stable 3D carbon nanotube-modified graphene network (Co2P@NPCNG). Specifically, cobalt tetranitrophthalocyanine (CoPc(NO2)4) is employed not only as the coupling agent to form and complex Co2P nanocrystals with graphene, but also as the inducer to catalyze the graphitization of melamine to grow the uniform Co2P nanocrystal-encapsulated CNTs on graphene in situ. Encouragingly, the as-synthesized Co2P@NPCNG exhibits favorable bifunctional oxygen electrocatalytic activity, fast reaction kinetics and excellent stability. Impressively, both liquid ZAB and all-solid-state ZABs used Co2P@NPCNG as air-cathode catalysts display considerable open-circuit voltage, charge-discharge property and long lifetime. Significantly, density functional theory (DFT) calculations demonstrate that the superior properties of Co2P@NPCNG originate to the synergetic contributions between the stable configuration of 3D conductive carbon network and high metallic density of Co2P. This work may provide feasible and facile avenues to strategically construct high-efficient 3D carbon-based bifunctional electrocatalysts for portable and even wearable devices.

Tuning of cationic distribution in “partially inversed” cobalt ferrite spinel nanocubes via a nitrogen-doped graphene oxide support for enhanced bifunctional oxygen electrocatalysis

A strategy to improve the bifunctional oxygen electrocatalytic activity of cobalt ferrite spinel (CoFe2O4) nanocubes by a N-doped graphene oxide (NGO) support induced redistribution of cations at octahedral versus tetrahedral sites is presented.

Efficient bifunctional 3D porous Co–N–C catalyst from spent Li–ion batteries and biomass for Zinc–Air batteries

The recovery of valuable metals from spent Li–ion batteries (LIBs) and their full utilization has become essential to support sustainable development. Herein, we designed an efficient 3D porous Co–N–C catalyst (Co@NCW–4) by combining the Co recovered from spent LIBs with biomass. Results illustrate Co@NCW–4 with the porous lamellar architecture utilizes the multiple channel structure for effective mass transfer and provides plentiful active sites, thus exhibiting great bifunctional oxygen electrocatalytic activity and stability. According to the data, Co@NCW–4 has the half–wave potential of 0.823 V vs RHE for oxygen reduction reaction, and an overpotential of 350 mV at 10 mA cm−2 for oxygen evolution reaction. The rechargeable Zn–air batteries assembled with the Co@NCW–4 exhibit excellent power density of 215 mW cm−2, and long–term stability better than RAZBs of benchmark Pt/C + RuO2. This research represents a general method for converting abundant biomass and spent LIBs into high environmentally preferable materials for energy–related applications.

Co-modified polyoxovanadoborates derived Co/BN-CNT/VN based bifunctional electrocatalysts for rechargeable zinc-air batteries

Rational design of high-performance bifunctional electrocatalysts to accelerate the sluggish oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) in rechargeable Zn–air batteries remain an enduring challenge. The construction of multicomponent catalysts is a promising solution to achieve this goal. Herein, B and N co-doped interconnecting graphite carbon and carbon nanotube with the decoration of Co and vanadium nitride (VN) nanoparticles (Co/BN-CNT/VN) are synthesized using Co-modified polyoxovanadoborates as precursors. The optimized composite achieves superior bifunctional oxygen electrocatalytic activity and stability, which is comparable to noble metal catalysts and reported bifunctional electrocatalysts. Specifically, the half-potential of ORR reaches 0.85 V, and the overpotential of OER is low to 296 mV at a current density of 10 mA cm−2. Strikingly, zinc–air batteries assembled based on Co/BN-CNT/VN demonstrate a small charge–discharge voltage gap of 0.873 V, a remarkable peak-power density of 156.3 mW cm−2, and outstanding cycling durability (∼1000 cycles at 10 mA cm−2). This work affords a new alternative strategy to create cost-effective and high-potency bifunctional oxygen electrocatalysts for advanced air batteries.

Composite-structure-defined nitrogen-doped carbon nanocage embedded Co/CoxP for enhanced oxygen reduction and evolution reactions

The design and synthesis of composite-structure-defined bifunctional oxygen reduction and evolution reactions electrocatalysts for rechargeable Zn-air batteries are highly important. Herein, a facile surface functionalization-carbonization-phosphidation strategy is proposed to construct a novel composition-structure-controllable nanomaterial from a zeolite imidazole ester framework (ZIF67), which has been applied in oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and Zn-air batteries. In the typical nanomaterial, Co and CoxP (a mixture of Co2P and CoP) were embedded in hollow N-doped carbon shell (Co/CoxP/HNC-5). The relationship between the activity and shell thicknesses of the carbonization products of ZIF67 coated with different mass ratios of polydopamine has been studied. In addition, the doped P-content showed a significant impact on the bifunctional oxygen activity, especially for Co/CoxP/HNC-5. Benefiting from the unique structure, multiple compositions, and synergistic effects, the superior bifunctional oxygen electrocatalytic activity (ΔE = 0.667 V) were obtained on Co/CoxP/HNC-5. Furthermore, Co/CoxP/HNC-5 served as an air-cathode catalyst and exhibited remarkable peak power density, specific capacity and durability in a practical Zn-air battery. This work provides a strategy for the construction of efficient bifunctional electrocatalysts with special structure and adjustable compositions.

Pyrochlores for Advanced Oxygen Electrocatalysis.

Fuel cells (FCs), water electrolyzers (WEs), unitized regenerative fuel cells (URFCs), and metal-air batteries (MABs) are among the emerging electrochemical technologies for energy storage, fuel (H2), oxidant (O2), and clean energy production. Their commercial applications are hindered by the low oxygen reduction reaction/oxygen evolution reaction (ORR/OER) bifunctional activity (for URFCs and MABs), OER selectivity (brine electrolysis in seawater and Martian environments), and high cost of the benchmark electrocatalysts (OER: RuO2, IrO2 and ORR: Pt/C) which affects the performance and affordability of the devices. Low-cost electrocatalysts with highly symmetric ORR/OER bifunctional activity and high OER selectivity are crucial for large-scale FC, WE, URFC, and MAB application. Recent studies have revealed that tuning the structure of pyrochlore oxides provides a pathway to enhancing OER and ORR activity over a wide range of pH. Pyrochlore oxides commonly contain a cubic A2B2O7-x structure with two types of tetrahedrally coordinated O atoms containing (1) A-O-A and (2) A-O-B types with a cationic radii mismatch of rA/rB > 1.5 and propensity toward oxygen vacancy formation. The variety of pyrochlore oxides and their tunable properties make them attractive for a wide spectrum of applications. Among all the metal oxides, Ru-based pyrochlores (e.g., Pb2Ru2O7-x) exhibit the best bifunctional oxygen electrocatalytic activity, i.e., low bifunctionality index (BI), in alkaline medium. Furthermore, pyrochlores exhibit high OER selectivity in brine electrolytes due to the presence of surface oxygen vacancies, making them suitable for space applications (brine electrolysis on Mars) and coastal hydrogen generation. Their bifunctional activity and selectivity can be further amplified by (1) substituting "A" and "B" sites of pyrochlores (AA'BB'O7-x), (2) tuning metal oxidation states of A and B by varying synthesis conditions, and (3) modulating oxygen vacancy concentration, each of which yield favorable structural and electronic variations. In recent years, research on the synthesis and understanding of pyrochlores has significantly enhanced their viability, offering a new horizon in the quest for economical and active electrocatalysts. However, an account that focuses on critical developments in this field is still lacking.In this Account, we focus on the recent development of a variety of pyrochlore electrocatalysts to understand intrinsic structure-activity-selectivity-stability relationships in these materials. Recent developments and applications of pyrochlore-based electrocatalysts are discussed under the following headings: (1) modulation of crystal and electronic structure of pyrochlores, (2) structure-activity-stability relationships of different pyrochlores for OER and ORR, (3) development of OER-selective pyrochlores for brine electrolysis, and (4) the application of pyrochlores in electrochemical devices. Finally, we highlight some unaddressed issues such as the precise identification of active sites, which can be addressed in the future through advanced in situ and ex situ characterization techniques coupled with the density functional theory-based analyses. This Account provides foundational understanding to guide the comprehensive development of highly active, selective, stable and low-cost structurally engineered pyrochlores for high performance electrochemical devices.