Air Cathode Catalyst Research Articles

ZnS-assisted evolution of N,S-doped hierarchical porous carbon nanofiber membrane with highly exposed Fe-N4/Cx sites for rechargeable Zn-air battery

Binder-free bifunctional electrocatalysts are attractive for rechargeable Zn-air batteries (ZABs) in grid-scale energy storage and flexible electronics, but suffering from the sluggish mass transport and inadequate catalytic capability. Herein, we propose a scalable approach of in-situ engineering highly exposed Fe-N4/Cx sites on the N,S-doped porous carbon nanofiber membrane as a binder-free air electrode catalyst for ZABs. ZnS nanospheres are firstly used as integrated structure-directing agents to facilitate the electronic modulation of Fe-N4/Cx sites by S doping and construct the hierarchical macro/meso/micropores at high temperature. Neither additional step for removal of ZnS nanospheres nor doping process is required, significantly simplifying the pore formation process and improving the S doping efficiency. Benefiting from the enhanced intrinsic activity of high-density Fe-N4/Cx sites (23.53 μmol g−1) and the optimized mass transport of carbon nanofibers, as-synthesized electrocatalyst shows a positive half-wave potential of 0.89 V for oxygen reduction reaction and a small overpotential of 0.47 V at 10 mA cm−2 for oxygen evolution reaction. When used as the air cathode catalyst for ZABs, it delivers a high specific capacity of 699 mAh g−1 at 5 mA cm−2, a large peak power density of 228 mW cm−2 and a prolonged cycling over 1000 h. At 10 mA cm−2, a robust structure with atomically dispersed Fe is also remained after cycling for 420 h. Due to the flexible properties of the electrocatalyst, as-assembled quasi-solid-state ZAB shows stable cycling over 90 h at alternately flat/bent states, demonstrating great prospects in flexible electronic device applications.

Tunable dual cationic redox couples boost bifunctional oxygen electrocatalysis for long-term rechargeable Zn-air batteries

Efficient nonprecious bifunctional electrocatalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for improving the electrochemical performance of Zinc–air (Zn–air) batteries. Herein, we report a cobalt-doped Mn2(OH)3VO3 catalyst prepared by facile hydrothermal method, and the ratios of cationic redox couples of catalysts were tuned with different Co doping amounts. The as-prepared Mn1.8Co0.2(OH)3VO3 (MnCoVO-1) catalyst achieves the highest ratio of (Mn3+Mn4+)/Mn2+ and Co3+/Co2+ redox couples which serve as ORR and OER active sites respectively, and exhibits the enhanced electrocatalytic performance. Furthermore, when employed as air–cathode catalyst for rechargeable Zn–air batteries, the MnCoVO-1 catalyst reveals a high power density (278 mW cm−2), enhanced rate performance and outstanding long-term stability of over 270 h. This work demonstrates the Co-doped Mn2(OH)3VO3 with optimized electronic structure by rational doping engineering can serve as a promising bifunctional catalyst for oxygen electrocatalysis and rechargeable Zn–air batteries.

KOH-promoted in-situ construction of zeolitic imidazolate framework-derived CoO/Co-N-C hybrids jointly boosting oxygen reduction reaction

Rational design of advanced cobalt oxides/carbon nanocomposites toward electrocatalytic oxygen reduction has received increasing research attention largely due to the potential synergism between cobalt oxides and carbon support. An efficient CoO/Co-N-C hybrid electrocatalyst has been prepared by high-temperature pyrolysis of ZIF-8 (ZIF = Zeolitic Imidazolate Framework) involving cage-encapsulation of CoCl2 followed by KOH treatment. In the alkaline solution (0.1 M KOH), the optimal CoO/Co-N-C hybrid catalyst (namely C-Co(OH)2@ZIF-8–10%−1000) delivers a half-wave potential (E1/2) of 0.84 V (vs. RHE), and a high four-electron reduction selectivity. The control experiments reveal that the superb ORR activity with C-Co(OH)2@ZIF-8–10%−1000 is jointly contributed by strongly coupled CoO NPs and Co-Nx moieties. Compared to the Pt/C benchmark, C-Co(OH)2@ZIF-8–10%−1000 exhibits comparable ORR performance but better methanol tolerance and electrochemical stability. Besides, the home-made zinc-air battery with C-Co(OH)2@ZIF-8–10%−1000 as the air-cathode catalyst displays an open-circuit voltage of 1.43 V and a maximal power density of 96 mW cm−2.

A comparison study on single metal atoms (Fe, Co, Ni) within nitrogen-doped graphene for oxygen electrocatalysis and rechargeable Zn-air batteries

Single atom catalysts (SACs) with atomically dispersed transition metals on nitrogen-doped carbon supports have recently emerged as highly active non-noble metal electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), showing great application potential in Zn-air batteries. However, because of the complex structure-performance relationships of carbon-based SACs in the oxygen electrocatalytic reactions, the contribution of different metal atoms to the catalytic activity of SACs in Zn-air batteries still remains ambiguous. In this study, SACs with atomically dispersed transition metals on nitrogen-doped graphene sheets (M-N@Gs, M = Co, Fe and Ni), featured with similar physicochemical properties and M-N@C configurations, are obtained. By comparing the on-set potentials and the maximum current, we observed that the ORR activity is in the order of Co-N@G > Fe-N@G > Ni-N@G, while the OER activity is in the order of Co-N@G > Ni-N@G > Fe-N@G. The Zn-air batteries with Co-N@G as the air cathode catalysts outperform those with the Fe-N@G and Ni-N@G. This is due to the accelerated charge transfer between Co-N@C active sites and the oxygen-containing reactants. This study could improve our understanding of the design of more efficient bifunctional electrocatalysts for Zn-air batteries at the atomic level.

Effectively engineering in situ coupled cobalt-cobalt phosphide nanoparticles with nitrogen-doped carbon materials for advanced bifunctional oxygen electrocatalysts in rechargeable Zn-air batteries

Constructing efficient and inexpensive bifunctional catalysts with oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) is urgently requisite for advanced Zn-air batteries. Herein, a composite material consisting of cobalt-cobalt phosphide nanoparticles in situ coupled with N-doped carbon (Co-Co2P @ NC) is conveniently engineered via a facile all-in-one carbonization/phosphorization process. Specifically, Co-Co2P @ NC implements unprecedented bifunctional catalytic activities with a positive ORR half-wave potential of 0.858 V and a low OER potential of 1.598 V at 10 mA cm−2, outperforming the benchmark catalysts and is on a par with recently reported non-precious metal-based catalysts. These impressive electrocatalytic performances are attributable to the synergistic coupling effects between metallic Co, Co2P and NC, extraordinary specific surface area, rich active species and conductive carbon frameworks. Prominently, a rechargeable aqueous ZAB with Co-Co2P @ NC as an air cathode catalyst delivers an ultra-long term cycling stability of more than 2000 h (over continuous 6000 cycles) with a small change of charge-discharge voltage gap from 0.841 V to 0.890 V at 2 mA cm−2. Beyond that, both button-like and foldable quasi-solid-state ZABs also exhibit admirable performance even under the limited electrolyte condition, evidently indicating the wide adaptability and feasibility in practical energy conversion and storage.

Single atomic cobalt electrocatalyst for efficient oxygen reduction reaction

Robust oxygen reduction reaction (ORR) catalysts are essential for energy storage and conversion devices, but their development remains challenging. Herein, we design a single-atom catalyst featuring isolated Co anchored on nitrogen-doped carbon (Co-SAC/NC) via a highly efficient “plasma-bombing” strategy. With a high loading (up to 2.5 ​wt%), the well-dispersed single Co atoms in Co-SAC/NC give it robust ORR performance in an alkaline medium. It also demonstrates excellent battery performance when implemented as the air-cathode catalyst in a zinc-air battery (ZAB). Theoretical calculations reveal that the Co–N4 moiety experiences an “extraction/recovery” structural evolution during the ORR process, and the reaction's rate-determining step is the formation of OOH∗ (reaction intermediate). This work provides a new strategy for designing robust ORR catalysts for high-performance ZABs and other energy-conversion devices.

Metal-organic framework-erythrocytic hybrid surfaces with enhanced oxygen reduction performance for enzymatic biofuel cells–An updated strategy

Enzymatic biofuel cells (EBFCs), which can use oxygen in the body as a cathode fuel to generate electricity, have broad application prospects. For air cathode of EBFCs, the most important factors affecting the electrocatalytic performance are the inherent fragile nature, poor oxygen enrichment performance and low activity of enzyme catalysts. Herein, based on the natural oxygen enrichment performance of red blood cells (RBCs) and its electrocatalytic activity for oxygen reduction reaction, we fabricate the hybrid surfaces of metal–organic framework-nanocoated RBCs (RBC@ZIF-8) using an in situ synthesis method. Versus native RBCs, the electrocatalytic activity and stability of RBC@ZIF-8 are enhanced by 50% and 35%, respectively, when explored as a biocathode catalyst for implantable biofuel cells. And compared with enzyme catalysts, RBC@ZIF-8 has better stability and handleability. Mechanistic studies show that the coating surface increases the O2-enriched capacity of RBCs and then provides more reaction raw materials for the hemoglobin with catalytic active. Moreover, direct electron transfer capability of RBC@ZIF-8 is found on carbon cloth electrode without promoters and mediators. Our research expands the types of air cathode catalyst for EBFCs and suggests a promising strategy to enhance the electrocatalytic performance of gas-based biocatalysts, such as bioelectrocatalytic reduction of CO2 and N2.

Hydrophobization Engineering of the Air–Cathode Catalyst for Improved Oxygen Diffusion towards Efficient Zinc–Air Batteries

AbstractPoor oxygen diffusion at multiphase interfaces in an air cathode suppresses the energy densities of zinc–air batteries (ZABs). Developing effective strategies to tackle the issue is of great significance for overcoming the performance bottleneck. Herein, inspired by the bionics of diving flies, a polytetrafluoroethylene layer was coated on the surfaces of Co3O4 nanosheets (NSs) grown on carbon cloth (CC) to create a hydrophobic surface to enable the formation of more three‐phase reaction interfaces and promoted oxygen diffusion, rendering the hydrophobic‐Co3O4 NSs/CC electrode a higher limiting current density (214 mA cm−2 at 0.3 V) than that (10 mA cm−2) of untreated‐Co3O4 NSs/CC electrode. Consequently, the assembled ZAB employing hydrophobic‐Co3O4 NSs/CC cathode acquired a higher power density (171 mW cm−2) than that (102 mW cm−2) utilizing untreated‐Co3O4 NSs/CC cathode, proving the enhanced interfacial reaction kinetics on air cathode benefiting from the hydrophobization engineering.

Hydrophobization Engineering of the Air-Cathode Catalyst for Improved Oxygen Diffusion towards Efficient Zinc-Air Batteries.

Poor oxygen diffusion at multiphase interfaces in an air cathode suppresses the energy densities of zinc-air batteries (ZABs). Developing effective strategies to tackle the issue is of great significance for overcoming the performance bottleneck. Herein, inspired by the bionics of diving flies, a polytetrafluoroethylene layer was coated on the surfaces of Co3 O4 nanosheets (NSs) grown on carbon cloth (CC) to create a hydrophobic surface to enable the formation of more three-phase reaction interfaces and promoted oxygen diffusion, rendering the hydrophobic-Co3 O4 NSs/CC electrode a higher limiting current density (214 mA cm-2 at 0.3 V) than that (10 mA cm-2 ) of untreated-Co3 O4 NSs/CC electrode. Consequently, the assembled ZAB employing hydrophobic-Co3 O4 NSs/CC cathode acquired a higher power density (171 mW cm-2 ) than that (102 mW cm-2 ) utilizing untreated-Co3 O4 NSs/CC cathode, proving the enhanced interfacial reaction kinetics on air cathode benefiting from the hydrophobization engineering.

Encapsulating atomic molybdenum into hierarchical nitrogen-doped carbon nanoboxes for efficient oxygen reduction

Construction of single-atom catalysts (SACs) with maximally exposed active sites remains a challenging task mainly because of the lack of suitable host matrices. In this study, hierarchical N-doped carbon nanoboxes composed of ultrathin nanosheets with dispersed atomic Mo (denoted as hierarchical SA-Mo-C nanoboxes) were fabricated via a template-engaged multistep synthesis process. Comprehensive characterizations, including X-ray absorption fine structure analysis, reveal the formation of Mo-N4 atomic sites uniformly anchored on the hierarchical carbon nanoboxes. The prepared catalysts offer structural and morphological advantages, including ultrathin nanosheet units, unique hollow structures and abundant active Mo-N4 species, that result in excellent activity with a half-wave potential of 0.86 V vs. RHE and superb stability for the oxygen reduction reaction in 0.1 M KOH; thus, the catalysts are promising air–cathode catalysts for Zn-air batteries with a high peak power density of 157.6 mW cm−2.

Three-dimensional hierarchical graphitic carbon encapsulated CoNi alloy/N-doped CNTs/carbon nanofibers as an efficient multifunctional electrocatalyst for high-performance microbial fuel cells

A novel Co/Ni metal-organic framework (MOF)-derived three-dimensional (3D) nanoarchitecture in which graphene carbon-coated CoNi alloy nanoparticles grow on the tips of N-doped carbon nanotubes that are vertically-aligned on carbon nanofibers (Co/Ni@GC/NCNTs/CNFs) is fabricated. The Co/Ni@GC/NCNTs/CNFs electrode has a half-wave oxygen reduction reaction (ORR) potential of 0.80 V, a superior standing stability with 84.8% current retention after 175 000 s. The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) reach a lower overpotential of 0.395 V and 0.15 V, respectively. Moreover, the resultant sample is an air cathode catalyst in a microbial fuel cell and indicates an utmost power density of 2100 ± 45 mW cm −2 , which is much better than that of the Pt/C electrode (1334 ± 61 mW m −2 ). The density functional theory calculation demonstrates that the designed Co/Ni@GC/NCNTs/CNFs catalyst can remarkably promote OER, ORR, and HER performance. • Co/Ni MOF-derived graphene carbon-coated CoNi alloy/NCNTs/CNFs was fabricated. • Co/Ni@GC/NCNTs/CNFs endows excellent ORR, OER and HER properties. • The as-prepared electrode exhibits high power density of 2100 ± 45 mW cm −2 in MFCs. • Superior property is due to the synergy of abundant active sites and 3D structure.

A simultaneous phosphorization and carbonization strategy to synthesize a defective Co2P/doped-CNTs composite for bifunctional oxygen electrocatalysis

Integration of heteroatom doped carbon material and metal phosphide is an ideal strategy for bifunctional oxygen reduction and evolution catalysis. In this work, heteroatoms doped carbon nanotubes, together with cobalt phosphide (Co2P/doped-CNTs) are prepared with a simultaneous phosphorization and carbonization strategy. It is revealed that the in-situ generated Co2P weakens the catalytic graphitization effect of metallic Co during the carbonization process, thus bringing about large numbers of defects, which is conducive to generating more pyridinic-N species in the carbon nanotube. As a result, the ORR-active pyridinic-N species and the OER-active Co2P are enriched in the carbon nanotube after the introduction of phosphorus, therefore providing dual active sites. The resultant Co2P/doped-CNTs catalyst shows outstanding ORR and OER performances, which are comparable to those of the commercial counterparts. The bifunctional activity makes Co2P/doped-CNTs an effective air–cathode catalyst for the rechargeable zinc-air batteries, showing a high power density and prominent cycling stability. This work abandons the traditional post-treatment phosphating method and realizes the integration of simplicity, safety and high efficiency.

Electrocatalytic oxygen reduction of COF-derived porous Fe-Nx nanoclusters/carbon catalyst and application for high performance Zn-air battery

The development of non-previous-metal electrocatalyst with low cost, high activity and stability towards oxygen reduction reaction (ORR) remains a challenge. Covalent organic frameworks (COFs) have emerged as promising candidates for the preparation of porous metal/nitrogen co-doped carbon catalysts. In the present work, porous Fe-Nx nanoclusters/carbon catalysts are prepared by pyrolyzing iron-containing covalent-triazine organic frameworks. It is found that the synthesis conditions including pyrolysis temperature and pressure have great impacts on the surface area of micropores, mesopores and macropores, which in turn influences the ORR activity of catalysts by varying the mass diffusion and the contact of active centers with electrolytes. The optimized catalyst (denoted as Fe1.2NC-0.02-800), having the highest both BET and external surface areas, shows much higher kinetic current density and catalytic stability than commercial Pt/C catalyst in ORR process. When being assembled into Zn-air battery, it also exhibits excellent performances in terms of open circuit voltage (1.45 V), power density (210 mW cm−2 at 330 mA cm−2) and specific capacity (713.1 mA h gZn−1), which outperform the corresponding values of Pt/C-based Zn-air battery. Moreover, 15 light-emitting diodes can be powered by two series-connected Zn-air batteries with Fe1.2NC-0.02-800 as the air-cathode catalyst, implying its promising application in clean energy conversion field.

Iron/Cobalt-Decorated Nitrogen-Rich 3d Layer-Stacked Porous Biochar as High-Performance Oxygen Reduction Air-Cathode Catalyst in Biofuel Cell

Iron/Cobalt-Decorated Nitrogen-Rich 3d Layer-Stacked Porous Biochar as High-Performance Oxygen Reduction Air-Cathode Catalyst in Biofuel Cell

MOF-derived CoN/CoFe/NC bifunctional electrocatalysts for zinc-air batteries

Zinc air battery (ZAB) is one of promising devices to solve the problems of fossil energy consumption and environmental pollution, but the development of low-cost non-precious metal dual-functional catalysts for ZAB still remains a great challenge. Here, we synthesize a bifunctional catalyst (Co5.47N/Co3Fe7/NC) consisting of two kinds of active sites. The Co5.47N/Co3Fe7/NC catalyst shows excellent electrocatalytic activities with a half-wave potential of 0.89 V (versus RHE) for ORR and a low potential of 1.609 V at 10 mA cm−2 for OER in alkaline condition. When the catalyst was used in ZAB, the ZAB provides a maximal power density of 264 mW cm−2 and maintains a stable voltage gap over 180 h. The results show that Co5.47N plays a very important role in both ORR and OER processes, because it effectively improves the conductivity of catalysts. Moreover, the two-phase interface produced by the two crystal structures will produce an electronic coupling effect, forming a large number of active sites. This catalyst provides a reliable alternative to air cathode catalysts for rechargeable zinc air batteries.

Analytical impedance of two-layer oxygen transport media in a PEM fuel cell

A model for impedance of oxygen transport in two porous layers A and B (GDL and MPL) positioned between the air channel and cathode catalyst layer in a PEM fuel cell is developed. Analytical solution shows that the system transport impedance cannot be split up into a sum of separate layer A and B impedances. The two-layer system works as a unified oxygen transport media. This result suggests that in general, impedance of all oxygen transport medias in a fuel cell are interdependent.

Enhanced Fe−N active site formation through interfacial energy control of precursor impregnation solution for the air cathode of membraneless direct formate fuel cells

In the development of platinum group metal-free (PGM-free) catalysts like carbon-based catalysts for oxygen reduction reaction (ORR), one of the key factors is increasing active site density to achieve highly catalytic activity. This work proposes a facile method to effectively increase the ORR active site density by controlling the interfacial energy between the carbon support and heteroatomic precursor (FePc)-containing solution during solvothermal treatment. Decreasing the interfacial energy by purposefully choosing solvents with proper surface tension promotes the delivery of FePc into intrinsic pores of the carbon support and enhances Fe and N doping, resulting in an increase of ORR active site density by triple to 6.84 × 1018 sites g−1. The high density of active site helps to achieve higher onset potential (0.937 V vs. RHE) and half-wave potential (0.895 V) than the benchmark Pt/C in a same catalyst loading in 0.1 M KOH, as well as a higher power output (16.1 ± 0.3 mW cm−2) when used for air cathode catalyst in a membraneless direct formate fuel cell. These findings provide insights into synthesis of highly active carbon-based catalysts with high active site density and broaden the development for fuel cell application.

A telluride-doped porous carbon as highly efficient bifunctional catalyst for rechargeable Zn-air batteries

The development of high-efficiency, low-cost, and stable bifunctional catalysts towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of great significance to the large-scale application of zinc–air batteries. Here we synthesized a telluride (Te)-doped porous carbon catalyst (ZnCo-Te@NC) through pyrolyzing ZnCo-MOF-coated Te nanotubes. The as-prepared ZnCo-Te@NC exhibited excellent bifunctional electrocatalytic performance with an impressive half-wave potential (E1/2) of 0.873 V for ORR and a low overpotential of 400 mV at 10 mA cm−2 for OER, outperforming most non-noble metal-based catalysts previously reported. When using ZnCo-Te@NC as the air cathode catalyst in a self-assembled battery, it demonstrated smaller charge-discharge voltage gap (0.932 V at 50 mA cm−2) and higher peak power density (259.7 mW cm−2) with robust long-term (100 h) cycle stability, surpassing the benchmark precious metal catalyst-based batteries. This is the first report that telluride has been doped into a highly graphitized porous carbon for use in zinc-air batteries. This work provides an effective strategy to develop high graphitized carbon materials by telluride modification to enhance the performance of bifunctional electrocatalysts for Zn–air batteries.

Fe,N-modulated carbon fibers aerogel as freestanding cathode catalyst for rechargeable Zn–Air battery

Freestanding air cathode catalyst with exceptional catalytic bifunctions for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly desirable for exploring rechargeable Zn–air batteries (ZABs) with low cost and high reliability. Here, a facile and scalable approach is demonstrated to prepare efficient and robust freestanding cathode catalyst that consists of dispersed Fe3C nanoparticles, rich Fe-Nx/N-C species and carbon fibers networks with large specific surface area, which is based on one-pot in-situ coupling of Fe and N induced active sites within bacterial cellulose-derived carbon fibers networks. Its bifunctional activity parameter (ΔE) for ORR and OER is 0.79 V, superior to those of single Fe- or N- modulated counterparts, and comparable to the state-of-the-art Pt/C + RuO2 catalyst (0.79 V). This material can be directly used as freestanding and binder-free catalyst layer, and as-assembled ZABs exhibit a larger peak power density (173 mW cm−2), higher specific capacity (717 mA h g−1, 10 mA cm−2) and better cycle stability (negligible activity decay after 165 h charge/discharge cycles) than those of Pt/C + RuO2-based devices, representing their potential applications in advanced energy conversion systems.

Graphene composites with Ru-RuO2 heterostructures: Highly efficient Mott–Schottky-type electrocatalysts for pH-universal water splitting and flexible zinc–air batteries

Development of high-efficiency electrocatalysts for pH-universal overall water splitting is a critical step towards a sustainable hydrogen economy. Herein, graphene nanocomposites with Ru-RuO2 Mott-Schottky heterojunctions (Ru-RuO2@NPC) are prepared pyrolytically and exhibit a remarkable electrocatalytic activity at pH = 0–14 towards both oxygen/hydrogen evolution reactions and overall water splitting, as compared to commercial RuO2 and Pt/C. Ru-RuO2@NPC can also be used as an effective air cathode catalyst for flexible, rechargeable zinc-air batteries. Density functional theory calculations show that the formation of Ru-RuO2 heterojunctions moderately enhances the surface charge density of metallic Ru and brings the d states closer to Fermi level, as compared to that of RuO2 alone, leading to improved intrinsic electrocatalytic activity towards these important reactions. These results demonstrate the significance of Mott-Schottky heterojunctions in the development of high-efficiency electrocatalysts for various new energy technologies.