High Peak Power Density Research Articles

Designing Thiophene‐Enriched Fully Conjugated 3D Covalent Organic Framework as Metal‐Free Oxygen Reduction Catalyst for Hydrogen Fuel Cells

AbstractThe application of three‐dimensional (3D) covalent organic frameworks (COFs) in renewable energy fields is greatly limited due to their non‐conjugated skeletons. Here, we design and successfully synthesize a thiophene‐enriched fully conjugated 3D COF (BUCT‐COF‐11) through an all‐thiophene‐linked saddle‐shaped building block (COThTh‐CHO). The BUCT‐COF‐11 exhibits excellent semiconducting property with intrinsic metal‐free oxygen reduction reaction (ORR) activity. Using the COF as cathode catalyst, the assembled anion‐exchange membrane fuel cells (AEMFCs) exhibited a high peak power density up to 493 mW cm−2. DFT calculations reveal that thiophene introduction in the COF not only improves the conductivity but also optimizes the electronic structure of the sample, which therefore boosts the ORR performance. This is the first report on the application of COFs as metal‐free catalysts in fuel cells, demonstrating the great potential of fully conjugated 3D COFs as promising semiconductors in energy fields.

Designing Thiophene-Enriched Fully Conjugated 3D Covalent Organic Framework as Metal-Free Oxygen Reduction Catalyst for Hydrogen Fuel Cells.

The application of three-dimensional (3D) covalent organic frameworks (COFs) in renewable energy fields is greatly limited due to their non-conjugated skeletons. Here, we design and successfully synthesize a thiophene-enriched fully conjugated 3D COF (BUCT-COF-11) through an all-thiophene-linked saddle-shaped building block (COThTh-CHO). The BUCT-COF-11 exhibits excellent semiconducting property with intrinsic metal-free oxygen reduction reaction (ORR) activity. Using the COF as cathode catalyst, the assembled anion-exchange membrane fuel cells (AEMFCs) exhibited a high peak power density up to 493 mW cm-2 . DFT calculations reveal that thiophene introduction in the COF not only improves the conductivity but also optimizes the electronic structure of the sample, which therefore boosts the ORR performance. This is the first report on the application of COFs as metal-free catalysts in fuel cells, demonstrating the great potential of fully conjugated 3D COFs as promising semiconductors in energy fields.

Bulk preparation of free‐standing single‐iron‐atom catalysts directly as the air electrodes for high‐performance zinc‐air batteries

AbstractThe keen interest in fuel cells and metal‐air batteries stimulates a great deal of research on the development of a cost‐efficient and high‐performance catalyst as an alternative to traditional Pt to boost the sluggish oxygen reduction reaction (ORR) at the cathode. Herein, we report a facile and scalable strategy for the large‐scale preparation of a free‐standing and flexible porous atomically dispersed Fe–N‐doped carbon microtube (FeSAC/PCMT) sponge. Benefiting from its unique structure that greatly facilitates the catalytic kinetics, mass transport, and electron transfer, our FeSAC/PCMT electrode exhibits excellent performance with an ORR potential of 0.942 V at −3 mA cm−2. When the FeSAC/PCMT sponge was directly used as an oxygen electrode for liquid‐state and flexible solid‐state zinc‐air batteries, high peak power densities of 183.1 and 58.0 mW cm−2 were respectively achieved, better than its powdery counterpart and commercial Pt/C catalyst. Experimental and theoretical investigation results demonstrate that such ultrahigh ORR performance can be attributed to atomically dispersed Fe–N5 species in FeSAC/PCMT. This study presents a cost‐effective and scalable strategy for the fabrication of highly efficient and flexible oxygen electrodes, provides a significant new insight into the catalytic mechanisms, and helps to realize significant advances in energy devices.

Low Pt loading for high-performance fuel cell electrodes enabled by hydrogen-bonding microporous polymer binders.

A key challenge for fuel cells based on phosphoric acid doped polybenzimidazole membranes is the high Pt loading, which is required due to the low electrode performance owing to the poor mass transport and severe Pt poisoning via acid absorption on the Pt surface. Herein, these issues are well addressed by design and synthesis of effective catalyst binders based on polymers of intrinsic microporosity (PIMs) with strong hydrogen-bonding functionalities which improve phosphoric acid binding energy, and thus preferably uphold phosphoric acid in the vicinity of Pt catalyst particles to mitigate the adsorption of phosphoric acid on the Pt surface. With combination of the highly mass transport microporosity, strong hydrogen-bonds and high phosphoric acid binding energy, the tetrazole functionalized PIM binder enables an H2-O2 cell to reach a high Pt-mass specific peak power density of 3.8 W mgPt-1 at 160 °C with a low Pt loading of only 0.15 mgPt cm-2.

Anion exchange membranes with high power density and energy efficiency for aqueous organic redox flow batteries

Membrane development in organic redox flow batteries (ORFBs) is of significant importance. Herein, we designed a series of anion exchange membranes made from poly(p-phenylene oxide) (PPO) with different degrees of functionalization, cationic moieties and crosslinking degrees. We tested them in a N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride/1,1′-dimethyl-4,4′-bipyridinium dichloride (TMA-TEMPO/MV) based ORFB single cell to investigate the structure-property-performance relationship. The common problem of active species crossover is solved by controlling the membrane water uptake below 70 wt.%, resulting in stable systems with coulombic efficiency of over 99.3%. The cell employing a membrane prepared from PPO and trimethylamine moiety attached via a C6-spacer achieved the best overall cell performance. It delivered high capacity retention (94% after over 100 consecutive cycles) and higher peak power density (293 mW·cm−2 at 16 mL·min−1) and energy efficiency (80%) than the commercial reference memane FAA-3-50.

Electrical Pulse Induced One-step Formation of Atomically Dispersed Pt on Oxide Clusters for Ultra-Low-Temperature Zinc-Air Battery.

Atomically dispersed sites anchored on small oxide clusters are attractive new catalytic materials. Herein, we demonstrate an electrical pulse approach to synthesize atomically dispersed Pt on various oxide clusters in one step with nitrogen-doped carbon as the support (Pt1 -MOx /CN). As a proof-of-concept application, Pt1 -FeOx /CN is shown to exhibit high activity for oxygen reduction reaction (ORR) with a half-wave potential of 0.94 V vs RHE, in contrast to the poor catalytic performance of atomically dispersed Pt on large Fe2 O3 nanoparticles. Our work has revealed that, by tuning the size of the iron oxide down to the cluster regime, an optimal OH* adsorption strength for ORR is achieved on Pt1 -FeOx /CN due to the regulation of Pt-O bonds. The unique structure and high catalytic performance of Pt1 -FeOx /CN enable the Zinc-Air batteries an excellent performance at ultralow temperature of -40 °C with a high peak power density of 45.1 mW cm-2 and remarkable cycling stability up to 120 h.

Electrical Pulse Induced One‐step Formation of Atomically Dispersed Pt on Oxide Clusters for Ultra‐Low‐Temperature Zinc‐Air Battery

AbstractAtomically dispersed sites anchored on small oxide clusters are attractive new catalytic materials. Herein, we demonstrate an electrical pulse approach to synthesize atomically dispersed Pt on various oxide clusters in one step with nitrogen‐doped carbon as the support (Pt1−MOx/CN). As a proof‐of‐concept application, Pt1−FeOx/CN is shown to exhibit high activity for oxygen reduction reaction (ORR) with a half‐wave potential of 0.94 V vs RHE, in contrast to the poor catalytic performance of atomically dispersed Pt on large Fe2O3nanoparticles. Our work has revealed that, by tuning the size of the iron oxide down to the cluster regime, an optimal OH* adsorption strength for ORR is achieved on Pt1−FeOx/CN due to the regulation of Pt−O bonds. The unique structure and high catalytic performance of Pt1−FeOx/CN enable the Zinc‐Air batteries an excellent performance at ultralow temperature of −40 °C with a high peak power density of 45.1 mW cm−2and remarkable cycling stability up to 120 h.

NCNT grafted perovskite oxide as an active bifunctional electrocatalyst for rechargeable zinc-air battery

Precious-metal-free electrocatalysts with high activity to bifunctionally activate both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are key enablers for the scalable adoption of zinc-air batteries (ZABs). In this work, a unique hybrid catalyst comprising nitrogen-doped carbon nanotube (NCNT) in-situ grown directly on La0·8Sr0·2Ti0·65Fe0·35O3-δ perovskite oxide (denoted as LSTFO/NCNT) has been developed as a bifunctional ORR and OER electrocatalyst for ZAB applications. The compositionally optimized LSTFO/NCNT hybrid delivers remarkable bifunctional oxygen electrocatalysis performances with a characteristic potential gap (ΔE) of 0.76 V. On a device level, the primary and rechargeable homemade ZABs demonstration using such an LSTFO/NCNT hybrid as an air electrode catalyst exhibit reliable electrochemical performances of high peak power density and cycling stability with a final round-trip efficiency of 62.8% at 10 mA cm−2 during 123 cycles. The active oxygen electrocatalytic activity is believed to be related to the considerable level of pyridinic-N and graphitic-N on NCNT, abundant oxygen vacancies, improved electrical conductivity and the strong electronic interaction between LSTFO and NCNT. The DFT calculation results demonstrated that the electron transfer from NCNT to LSTFO not only reduces the oxygen adsorption and activation energy, but also activates the surface lattice oxygen and favors the formation of O–O bond through reinforcing the Fe/Ti–oxygen covalency, leading to exceptional bi-functionality. This work demonstrates a robust method to obtain exceptional bi-functional oxygen electrocatalysis based on transitional metal oxides for high-performance metal-air batteries.

Free-standing Pd/SnO2/CP cathode for high-efficiency magnesium-hydrogen peroxide semi-fuel cell

The free-standing Pd/SnO2/CP electrodes are prepared by a simple hydrothermal process combined with electrodeposition. The morphology of SnO2 nanorods is readily optimized by tuning the holding time of the tin precursor. With a holding time of 20 min, the quadrangular prism of SnO2 nanorods grows into bundles vertically on the surface of carbon paper. Pd nanoparticles are modified on the top of SnO2 nanorods, showing excellent mass normalized activity(8885 A gPd-1) owing to the corresponding low Pd loadings. The Mg-H2O2 semi-fuel cell with Pd/SnO2/CP as cathode displays a high peak power density(192 mW cm−2 at 60 °C) and excellent stability during the discharge process of 50 h. The microscopic 3D structure constructed by SnO2 interlayer growing on carbon paper accelerates the electrolyte delivery. The synergistic effect between SnO2 and Pd ensures the efficient utilization of Pd and provides more Pd active sites for enhancing the electrode performance further.

Greatly enhancing the bifunctional oxygen catalytic activities of cobalt-based perovskites by anchoring metal sulfides

Co-based perovskites have been considered as the promising bifunctional oxygen catalysts for zinc-air batteries (ZABs) though their catalytic activities toward oxygen reduction reaction (ORR) are still need to be improved. In this work, we adopt a facile method to anchor MnS, CoS2 with high ORR activity on SrNb0.1Co0.7Fe0.2O3-δ (SNCF) surface. The optimized perovskite hybrid (SNCF-Mn-S4) exhibits the highest bifunctional oxygen catalytic activity among most of the perovskites, and its half-wave potential (E1/2) positively shifts as high as 180 mV compared with that of SNCF. The aqueous ZAB with SNCF-Mn-S4 has the high peak power density of 129.4 mW cm−2 and excellent cycling stability during 4500 charge-discharge cycles. Moreover, the energy efficiency of ZAB with SNCF-Mn-S4 can reach as high as 82%. This can be ascribed to the high oxygen catalytic activity of SNCF-Mn-S4 and hybrid effects of oxygen catalytic reactions and ion conversion reactions caused by CoS2, CoO and Co-based perovskite. Our works provides a feasible strategy for developing the Co-based perovskites with superior bifunctional catalytic properties for the high-performance ZABs.

Cu-N-C assisted MnO nanorods as bifunctional electrocatalysts for superior long-cycle Zn-air batteries

The main factor constraining the performance of zinc-air batteries (ZABs) is the slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), for which high performance electrocatalysts are greatly needed. However, the state-of-the-art oxygen electrocatalysts still suffer from the low activity and limited stability. Herein, we report the synthesis of a novel MnO@Cu-N-C core-shell nanostructure comprising MnO nanorods coated by a Cu-N-C shell that is derived from zeolitic-imidazolate frameworks. The as-prepared MnO@Cu-N-C exhibits fairly high bifunctional electrocatalytic activity toward both ORR and OER, showing a combined overpotential as low as 0.80 V. Both experiments and theoretical calculations indicate that the superior performance is benefited from the synergy between MnO and Cu-N-C, which facilitates the interfacial electron transfer and thus leads to the lower overpotentials. When being employed as the air electrode in the rechargeable ZABs, MnO@Cu-N-C displays a high peak power density of 196.8 mW cm−2 and an impressively prolonged cycle life over 650 h at 10 mA cm−2, outperforming most of recently reported Mn-based counterparts. Moreover, its excellent electrochemical performance can also be transferred to solid-state ZABs, as showcased by a power density of 66.6 mW cm−2 and a cycle life of 16 h at 2 mA cm−2.

Molten salt-assisted carbonization and unfolding of Fe, Co-codoped ZIF-8 to engineer ultrathin graphite flakes for bifunctional oxygen electrocatalysis

Zeolitic imidazolate frameworks (ZIFs) are already utilized to synthesis precursors for transition metal and nitrogen co-doped carbon catalysts. However, there are few reports on deliberately tailoring catalyst structure except for the hierarchical pores formed from Zn evaporation during the pyrolysis. Herein, NaCl salt-assisted carbonization and unfolding of Fe, Co-codoped ZIF-8 polyhedron (FeCo/ZIF-8) result in ultrathin Fe, Co, N-codoped graphite flake (FeCo/NG), while FeCo/ZIF-8 pyrolysis without NaCl yields Fe, Co, N-codoped carbon spheres (FeCo/NC). Because of the high specific surface area, porosity, and degree of defects, the active site is further exposed. The synthesized FeCo/NG demonstrates excellent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) properties with half-wave potential (E 1/2 ) at 0.88 V and overpotential at 460 mV and good cycling stability, superior to those of FeCo/NC. In addition, the assembled Zn-air batteries with FeCo/NG catalysts deliver high peak power density at 109 mW cm −2 , specific capacity at 783 mAh g −1 and an operating time for 100 h. The study will provide a feasible synthesis method to design high-performance electrocatalysts by deliberate tailoring structures with molten salt-assisted carbonization and unfolding of ZIFs. • Molten salt-assisted pyrolysis tailors ZIF-8 polyhedron into ultrathin graphite flake. • Molten NaCl assists carbonization and unfolding of FeCo/ZIF-8 to form ultrathin FeCo/NG. • Evaporation of Zn in pyrolysis and removal of NaCl result in porous FeCo/NG. • FeCo/NG demonstrates excellent bifunctional oxygen electrocatalytic performances. • Zn-air battery with FeCo/NG delivers high peak power density and specific capacity.

N, S co-doped carbon with embedment of FeNi alloy as bifunctional oxygen electrocatalysts for rechargeable Zinc-air batteries

Zinc-air batteries (ZABs) have sparked great interest, but their wide-ranging applications are limited by sluggish cathode reactions (ORR and OER). In this work, a multifunctional 3D catalyst (FeNi alloy/porous carbon) was easily fabricated by introducing Fe3+ and Ni2+ to modulate in-situ vapor phase grown carbon nanotubes. The as-prepared catalyst FNSNC73-800 displays abundant mesoporous, massive structural defects, and multiple active sites, which greatly facilitate the transport of oxygen species and charge transfer during the reaction. Due to the excellent ORR and OER performance, FNSNC73-800 shows a narrow voltage gap (ΔE) of 0.76 V, which is superior to recent reports. Notably, the secondary zinc-air battery with this catalyst displays a high-peak power density (210 mW cm−2), while maintaining an ultra-low potential gap (0.74 V) after a long-term charge-discharge cycle of 200 h. This effort presents a facile strategy for designing economical and efficient 3D catalysts for zinc-air batteries and more energy devices.

Isolating Fe Atoms in N-Doped Carbon Hollow Nanorods through a ZIF-Phase-Transition Strategy for Efficient Oxygen Reduction.

Transition metal-nitrogen-carbon (TM-N-C) catalysts have been intensely investigated to tackle the sluggish oxygen reduction reactions (ORRs), but insufficient accessibility of the active sites limits their performance. Here, by using solid ZIF-L nanorods as self-sacrifice templates, a ZIF-phase-transition strategy is developed to fabricate ZIF-8 hollow nanorods with open cavities, which can be subsequently converted to atomically dispersed Fe-N-C hollow nanorods (denoted as Fe1 -N-C HNRs) through rational carbonization and following fixation of iron atoms. The microstructure observation and X-ray absorption fine structure analysis confirm abundant Fe-N4 active sites are evenly distributed in the carbon skeleton. Thanks to the highly accessible Fe-N4 active sites provided by the highly porous and open carbon hollow architecture, the Fe1 -N-C HNRs exhibit superior ORR activity and stability in alkaline and acidic electrolytes with very positive half-wave potentials of 0.91 and 0.8V versus RHE, respectively, both of which surpass those of commercial Pt/C. Remarkably, the dynamic current density (JK ) of Fe1 -N-C HNRs at 0.85V versus RHE in alkaline media delivers a record value of 148mA cm-2 , 21 times higher than that of Pt/C. The assembled Zn-air battery using Fe1 -N-C HNRs as cathode catalyst exhibits a high peak power density of 208mW cm-2 .

Enhanced dual atomic Fe-Ni sites in N-doped carbon for bifunctional oxygen electrocatalysis

Atomically dispersed catalysts with high electrocatalytic performance are emerging as promising electrocatalysts for energy conversion and storage devices. Nevertheless, achieving superior bifunctional catalytic activity with single-atom catalysts toward reactions involving multi-intermediates is still facing great challenges. Herein, dual-atomic Fe-Ni pairs dispersed in hierarchical porous nitrogen-doped carbon (FeNi-HPNC) catalysts were successfully synthesized using a facile mechanochemical strategy. By virtue of the engineered electronic structure of Fe coordinated with Ni, the as-synthesized FeNi-HPNC with atomically dispersed dual-metal active sites and pore-rich structure exhibits remarkable bifunctional activities. A high half-wave potential of 0.868 V for oxygen reduction reaction and a low potential of 1.59 V at 10 mA/cm2 for oxygen evolution reaction have been obtained for FeNi-HPNC, which are superior to the single-atom catalysts of Fe-HPNC and Ni-HPNC, respectively, and are even greater than the precious metal catalysts. Combined experimental and theoretical results have revealed that the enhanced bifunctional catalytic activity of FeNi-HPNC is ascribed to the electronic interaction of Fe-Ni sites, which decreases the adsorption energy of oxygen intermediates during oxygen reduction reaction/oxygen evolution reaction. Furthermore, the practical application of FeNi-HPNC catalysts in Zn-air batteries has been demonstrated; a high peak power density and long-term durability are delivered.

Membrane Transport and Performance in the All-Aqueous Copper Thermally Regenerative Battery

Redox flow batteries are emerging as a promising method to provide grid-scale power and long-duration energy storage safely and economically. The thermally regenerative ammonia battery (TRAB) is a new redox flow battery category that can be recharged using low-grade waste heat rather than electric energy, adding further flexibility to the applicability of flow battery systems. Recently, a new TRAB with all-aqueous electroactive species (referred to as the Cuaq-TRAB), as opposed to deposition-dissolution reactions, was found to have superior energy and power densities relative to competing TRABs. The use of bromide and ammonia stabilizes for both Cu(I) and Cu(II) oxidation states, while also creating a cell potential of up to 1.0 V. The potential can be recovered by thermally separating ammonia from the electrolyte and adding it back to alternate electrolyte chambers in successive cycles. Potential improvements in overall cell performance are possible by reducing ohmic losses associated with membrane selection. However, the ideal membrane for the Cuaq-TRAB is not obvious and raises interesting transport questions relative to the dominant redox-active copper species being negatively charged in the catholyte but positively charged in the anolyte. Furthermore, membrane crossover of ammonia, a small, uncharged molecule, was previously shown to be a primary source of parasitic losses for TRABs. Therefore, we investigated how different membrane types (cation, anion, and non-selective) affected ion transport and TRAB performance. A batch symmetry cell was used to determine membrane conductivity in the Cuaq-TRAB environment and membrane diffusion coefficients of each species in the electrolytes. Flow cell experiments were also conducted to find peak power, energy density, average power during discharge, and capacity fade over successive electric charge/discharge cycles. Tradeoffs between membrane conductivity and permeability observed in the symmetry cell are manifested in flow cell results as either high peak and average power or high energy density and low capacity fade. These results expand on potential methods for controlling transport in redox flow batteries and demonstrate the potential for non-selective membranes for electrochemical energy technologies.

Carbonate-superstructured solid fuel cells with hydrocarbon fuels

A basic requirement for solid oxide fuel cells (SOFCs) is the sintering of electrolyte into a dense impermeable membrane to prevent the mixing of fuel and oxygen for a sufficiently high open-circuit voltage (OCV). However, herein, we demonstrate a different type of fuel cell, a carbonate-superstructured solid fuel cell (CSSFC), in which in situ generation of superstructured carbonate in the porous samarium-doped ceria layer creates a unique electrolyte with ultrahigh ionic conductivity of 0.17 S⋅cm-1 at 550 °C. The CSSFC achieves unprecedented high OCVs (1.051 V at 500 °C and 1.041 V at 550 °C) with methane fuel. Furthermore, the CSSFC exhibits a high peak power density of 215 mW⋅cm-2 with dry methane fuel at 550 °C, which is higher than all reported values of electrolyte-supported SOFCs. This provides a different approach for the development of efficient solid fuel cells.

Transformation of postsynthesized F-MOF to Fe/N/F-tridoped carbon nanotubes as oxygen reduction catalysts for high power density Zn-air batteries

The oxygen reduction reaction (ORR), an important process in Zn-air batteries (ZABs), shows sluggish reaction kinetics, which significantly impairs the further improvement of battery performance. Thus, rationally designing cathodic catalysts for ZABs has drawn sufficient attention. We herein synthesize and characterize Fe/N/F-tridoped CNTs (FeNFCs) by annealing the postsynthesized trifluoroacetic anhydride-modified Fe-MIL-88B-NH 2 nanocrystals with melamine at high temperature in a N 2 atmosphere. Benefiting from the Fe/N/F element doping, high specific surface area, and CNT structure, the FeNFC800 catalyst prepared at 800°C exhibits a preferable half-wave potential of 0.829 V vs . RHE. The Zn-air battery equipped with FeNFC800 shows a high open-circuit voltage of 1.47 V, a gratifying peak power density of 196 mW/cm 2 , and extraordinary long-term stability, outperforming the benchmark 20% Pt/C. Fe/N/F-tridoped carbon nanotubes were grown from the postsynthesized F-Fe-MIL-88B-NH 2 microcrystals through a self-catalyzed process. The synergistic effect of Fe, N, F doping and the carbon nanotube structure enhanced the ORR catalytic performance. A Zn-air battery assembled with FeNFC800 shows a high peak power density of 196 mW/cm 2 , outperforming the benchmark 20% Pt/C.

Schiff-base polymer derived FeCo-N-doped porous carbon flowers as bifunctional oxygen electrocatalyst for long-life rechargeable zinc-air batteries

Rational design and exploration of low-cost and robust bifunctional oxygen electrocatalysts are vitally important for developing high-performance zinc-air batteries (ZABs). Herein, we reported a facile yet cost-efficient approach to construct a bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) electrocatalyst composed of N-doped porous carbon nanosheet flowers decorated with FeCo nanoparticles (FeCo/N-CF). Rational design of this catalyst is achieved by designing Schiff-base polymer with unique molecular structure via hydrogen bonding of cyanuramide and terephthalaldehyde polycondensate in the presence of metal cations. It exhibits excellent activity and stability for electrocatalysis of ORR/OER, enabling ZAB with a high peak power density of 172 mW cm−2 and a large specific capacity of 811 mA h g−1Zn at large current. The rechargeable ZAB demonstrates excellent durability for 1000 h with slight voltage decay, far outperforming a couple of precious Pt/Ir-based catalysts. Density functional theory (DFT) calculations reveal that high activity of bimetallic FeCo stems from enhanced O2 and OH− adsorption and accelerated O2 dissociation by OO bond activation.

Doping Effect on Mesoporous Carbon-Supported Single-Site Bifunctional Catalyst for Zinc-Air Batteries.

Rechargeable zinc-air batteries (ZABs) require bifunctional electrocatalysts presenting high activity in oxygen reduction/evolution reactions (ORR/OER), but the single-site metal-N-C catalysts suffer from their low OER activity. Herein, we designed a series of single-site Fe-N-C catalysts, which present high surface area and good conductivity by incorporating into mesoporous carbon supported on carbon nanotubes, to study the doping effect of N and P on the bifunctional activity. The additional P-doping dramatically increased the content of active pyridine-N and introduced P-N/C/O sites, which not only act as extra active sites but also regulate the electron density of Fe centers to optimize the absorption of oxygenated intermediates, thereby ultimately improving the bifunctional activity of Fe-N-C sites. The optimized catalyst displayed a half-wave potential of 0.882 V for ORR and a low overpotential of 365 mV at 10 mA cm-2 for OER, which significantly outperforms the counterpart without P, as well as noble-metal-based catalysts. The ZABs with air cathodes containing the N,P-co-doped catalysts exhibited a high peak power density of 201 mW cm-2 and a long cycling stability beyond 600 h. Doping has shown to be an effective way to optimize the performance of single-site catalysts in bifunctional oxygen electrocatalysis, which can be extended to other catalyst systems.