Efficient Bifunctional Oxygen Catalysts Research Articles

Coaxial MWCNTs/Co-N@N-doped carbon hierarchical porous nanowires as durable air electrode for aqueous Zn-air batteries

Aqueous zinc-air batteries can replace lithium-ion batteries to be used in the future large-scale application. However, efficient bifunctional oxygen catalyst used in air electrode for aqueous zinc-air batteries generally still meet serious challenge. The catalytic activity and stability of air electrode still far from satisfactory. In this work, we designed a series of coaxial MWCNTs/Co-N@N-doped carbon hierarchical porous nanowires (MWCNTs/Co-N@NC) as bifunctional oxygen electrocatalyst with different Co contents. The Co content can facilitate to form the Co-Nx and pyridinic-N to boost the bifunctional oxygen electrocatalysis. The optimal MWCNTs/Co-N@NC-1 delivers superior electrocatalysis for oxygen reduction reaction (ORR) with E1/2 of 0.86 V (vs. RHE) and rational performance for oxygen evolution reaction (OER) with Ej=10 of 1.59 V (vs. RHE) in 0.1 mol/L KOH. In addition, the MWCNTs/Co-N@NC-1-based Zn-air battery exhibits the discharge capacity of 898.84 mAh gZn−1 and a optimal power density of 158.1 mW cm−2 with cycling stability for 300 h. This study provides a strategy to construct robust durability air electrode for aqueous zinc-air batteries.

Single-phase ruthenium-based oxide with dual-atoms induced bifunctional catalytic centers enables highly efficient rechargeable Zn-air batteries

Composite strategies for constructing dual-atom sites at the hetero-interface provide considerable prospects for designing efficient bifunctional oxygen catalysts. Given the insufficient interface site and the instability of the phase interface, we need to develop more efficient strategies for efficiently utilizing the dual-atom site. Here, we report a doping strategy to construct abundant dual-atom sites in single-phase oxide catalysts. Ru/Mn dual-atom bond formation enables electronic interaction between Ru and Mn, which reduces the oxidation state of Ru sites and meanwhile constructs electron-rich states of Mn sites. DFT calculation was further applied to explore the reaction mechanism. We found that Mn and Ru atoms serve as oxygen reduction and oxygen evolution reaction catalytic sites respectively to facilitate oxygen adsorption and OH* desorption. More importantly, co-adsorption of OOH* on Mn/Ru dual sites can greatly enhance oxygen reduction activity. The resulting Mn-RuO2 catalyst exhibits an ultra-low ORR/OER overpotential of just 0.65 V, substantially lower than RuO2 and MnOx. Remarkably, Mn-RuO2 also demonstrates excellent stability, with minimal ORR decay after repeated OER cycling. Rechargeable zinc-air batteries using Mn-doped RuO2 achieve super-durability for 2000 cycles with a final energy efficiency retention of 87.5 %.

Bifunctional ligand Co metal-organic framework derived heterostructured Co-based nanocomposites as oxygen electrocatalysts toward rechargeable zinc-air batteries

Rational construction of efficient and robust bifunctional oxygen electrocatalysts is key but challenging for the widespread application of rechargeable zinc-air batteries (ZABs). Herein, bifunctional ligand Co metal–organic frameworks were first explored to fabricate a hybrid of heterostructured CoOx/Co nanoparticles anchored on a carbon substrate rich in CoNx sites (CoOx/Co@CoNC) via a one-step pyrolysis method. Such a unique heterostructure provides abundant CoNx and CoOx/Co active sites to drive oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively. Besides, their positive synergies facilitate electron transfer and optimize charge/mass transportation. Consequently, the obtained CoOx/Co@CoNC exhibits a superior ORR activity with a higher half-wave potential of 0.88 V than Pt/C (0.83 V vs. RHE), and a comparable OER performance with an overpotential of 346 mV at 10 mA cm−2 to the commercial RuO2. The assembled ZAB using CoOx/Co@CoNC as a cathode catalyst displays a maximum power density of 168.4 mW cm−2, and excellent charge–discharge cyclability over 250 h at 5 mA cm−2. This work highlights the great potential of heterostructures in oxygen electrocatalysis and provides a new pathway for designing efficient bifunctional oxygen catalysts toward rechargeable ZABs.

Advanced 3D Network of N‐Doped Graphitic Carbon with FeNi Alloy Embedding for High‐Performance Rechargeable Zn‐Air Batteries

AbstractDespite the significant progress in Zn−air batteries (ZABs), their widespread use in the rechargeable sector is hindered due to the scarcity of efficient bifunctional oxygen catalysts that can catalyze both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). To address this, an ORR/OER bifunctional electrocatalyst is designed with ultrafine alloyed FeNi nanoparticles encapsulated in a 3D interconnected N‐ doped carbon network structure, featuring a carbon nitride backbone enclosed in graphitic carbon. The FeNi electrocatalyst (3DFeNiPDC) showed good bifunctional activity toward both ORR and OER in the basic medium with a low overpotential value of 30 mV for ORR and 6 mV for OER compared to its state‐of‐the‐art counterparts Pt/C, and RuO2, respectively. Utilizing 3DFeNiPDC in a rechargeable Zn‐air battery (RZAB) yields an open circuit voltage (OCV) of 1.35 V, a maximum power density of 232 mW cm−2, and an energy density of 707 W h kg−1. Additionally, a flexible RZAB employing 3DFeNiPDC demonstrates an OCV of 1.4 V with various bending angles. These finding suggest 3DFeNiPDC as a viable alternative to noble metal‐based RZABs, offering superior bifunctional electrocatalytic activity and stability, particularly with its enhanced air‐breathing properties facilitating improved operability under practical conditions.

Ultrafine Ir nanoparticles anchored on carbon nanotubes as efficient bifunctional oxygen catalysts for Zn-air batteries.

Ultrafine iridium particles anchored on nitrogen-doped CNTs were obtained from Ir(ppy)3 and CNTs using a simple annealing method and acted as highly efficient bifunctional oxygen catalysts for Zn-air batteries. A synergistic effect, efficient *OH adsorption and rapid *OOH deprotonation were demonstrated from in situ FTIR spectroscopy, EIS and activation energy measurements.

Co9S8-FeCoS2 Two-Phase Nanoparticles Anchored in N, S Co-Doped Honeycomb Carbon Spheres as Highly Efficient Bifunctional Oxygen Catalyst

Transition metal sulfides (TMS) are considered as candidate oxygen redox catalysts (OER/ORR) due to their high catalytic activity and reversible redox performance. The OER/ORR performance for TMS can be further improved through the multi-doped electrochemical active sites provided by the dual-metallic (Co,Fe) and dual-nonmetallic species (N,S). Herein, a Co9S8-FeCoS2 two-phase nanoparticle anchored in N, S co-doped honeycomb carbon sphere ((Co,Fe)S@N,S-HCS) is synthesized by a multistep process template method. It has been found that the (Co,Fe)S@N,S-HCS possesses a mesoporous structure with a highly graphitized carbon along with the key OER active site of Co9S8 and the major ORR active center of FeCoS2. Also, the synergistic effect of Co9S8-FeCoS2 two-phase nanoparticles can further enhance the activity of OER and ORR. The as-prepared (Co,Fe)S@N,S-HCS exhibits excellent bifunctional activity in alkaline media, with an OER overpotential as low as 310 mV at 10 mA cm−2 and good stability, which exceeds (Fe)S@N,S-HCS, (Co)S@N,S-HCS and the noble metal RuO2. Besides, the ORR activity exhibits a half-wave potential of 0.84 V, which exceed the other two single component sulfides and close to the commercial Pt/C. Therefore, this work provides a significant approach to prepare highly efficient and stable dual-TMS heteroatom modified carbon-based OER/ORR bifunctional oxygen catalyst.

Pomegranate-like structured FeNi-nanodots@FeNi LDH composite as a high performance bifunctional catalyst for oxygen electrocatalytic reactions in zinc-air batteries

An efficient bifunctional oxygen catalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for the widespread application of rechargeable zinc-air batteries (ZABs) with high energy density and low-cost electrically. Herein, we report an effective pomegranate-like structured FeNi–N–C@FeNi LDH catalyst using Zeolitic Imidazolate Framework (ZIF-8) as precursor via pyrolysis and hydrothermal reactions. The as-prepared FeNi–N–C@FeNi LDH composite, consisting of FeNi-nanodots embedded in carbon matrix and metal layered double hydroxides (LDHs) as outer layer, exhibits preferable performance toward ORR (E1/2 = 0.890V) and OER (Ej = 10 = 1.535V) compared to precious metal-based catalysts. Moreover, when being applied in ZABs, the FeNi–N–C@FeNi LDH composite displays a high open-circuit voltage of 1.497 V and a durability of up to 100 h, demonstrating its great possibility in practical application.

Conjugated polymerized bimetallic phthalocyanine based electrocatalyst with Fe−N4/Co−N4 dual-sites synergistic effect for zinc-air battery

The bifunctional oxygen catalyst is essential for zinc-air batteries (ZABs). Here, an efficient bifunctional oxygen catalyst, PPcFeCo/3D-G, is obtained through π-π interaction between the conjugated polymerized iron-cobalt phthalocyanine (PPcFeCo) with excellent thermal stability and three-dimensional graphene (3D-G). The bimetallic synergistic effect of PPcFeCo, verified by DFT (Density functional theory) calculation, and π-π interactions enhances the catalytic activity and durability of the PPcFeCo/3D-G. Regarding electrochemical performance, the PPcFeCo/3D-G with a high electron transfer number (3.98, @0.768 V vs. RHE) has excellent half-wave potential (E1/2 = 0.890 V vs. RHE) and exhibits outstanding reversibility (ΔE = 0.700 V, ΔE = Ej=10 − E1/2). The liquid ZAB (LZAB) employed PPcFeCo/3D-G displays a high power density (222 mW cm−2), a specific capacity (792 mA h g−1), and excellent durability (120 h). This work has guiding significance for the preparation of high-efficiency bifunctional catalysts.

Edge atomic Fe sites decorated porous graphitic carbon as an efficient bifunctional oxygen catalyst for Zinc-air batteries

The development of advanced bifunctional oxygen electrocatalysts for oxygen reduction and evolution reactions (ORR and OER) is critical to the practical application of zinc-air batteries (ZABs). Herein, a silica-assisted method is reported to integrate numerous accessible edge Fe-Nx sites into porous graphitic carbon (named Fe-N-G) for achieving highly active and robust oxygen electrocatalysis. Silica facilitates the formation of edge Fe-Nx sites and dense graphitic domains in carbon by inhibiting iron aggregation. The purification process creates a well-developed mass transfer channel for Fe-N-G. Consequently, Fe-N-G delivers a half-wave potential of 0.859 V in ORR and an overpotential of 344 mV at 10 mA cm−2 in OER. During long-term operation, the graphitic layers protect edge Fe-Nx sites from demetallation in ORR and synergize with FeOOH species endowing Fe-N-G with enhanced OER activity. Density functional theory calculations reveal that the edge Fe-Nx site is superior to the in-plane Fe-Nx site in terms of OH* dissociation in ORR and OOH* formation in OER. The constructed ZAB based on Fe-N-G cathode shows a higher peak power density of 133 mW cm−2 and more stable cycling performance than Pt/C + RuO2 counterparts. This work provides a novel strategy to obtain high-efficiency bifunctional oxygen electrocatalysts through space mediation.

Bimetallic ZIFs-derived electrospun carbon nanofiber membrane as bifunctional oxygen electrocatalyst for rechargeable zinc-air battery

The recharged zinc-air battery (ZAB) has drawn significant attention owing to increasing requirement for energy conversion and storage devices. Fabricating the efficient bifunctional oxygen catalyst using a convenient strategy is vitally important for the rechargeable ZAB. In this study, the bimetallic ZIFs-containing electrospun (ES) carbon nanofibers membrane with hierarchically porous structure was prepared by coaxial electrospinning and carbonization process, which was expected to be a bifunctional electrocatalyst for ZABs. Owing to the formed dual single-atomic sites of Co–N4 and Zn–N4, the obtained ES-Co/Zn-CNZIF exhibited the preferable performance toward oxygen reduction reaction (ORR) with E1/2 of 0.857 V and JL of 5.52 mA cm−2, which were more than Pt/C. Meanwhile, it exhibited a marked oxygen evolution reaction (OER) property with overpotential of 462 mV due to the agglomerated metallic Co nanoparticles. Furthermore, the ZAB based on the ES-Co/Zn-CNZIF carbon nanofibers membranes delivered peak power density of 215 mW cm−2, specific capacity of 802.6 mA h g−1, and exceptional cycling stability, far larger than Pt/C+RuO2-based ZABs. A solid-state ZAB based on ES-Co/Zn-CNZIF showed better flexibility and stability with different bending angles.

In situ growth of N-doped carbon nanotubes in Fe-Nx/Fe2O3/Fe3O4-encapsulated carbon sheets for efficient bifunctional oxygen catalysis

Development of efficient bifunctional oxygen catalysts is of interest for zinc-air batteries (ZABs). However, achieving both highly active and durable oxygen reduction and evolution reactions (ORR/OER) in one catalyst remains challenging. Herein, a composite catalyst (m-Fe/N-C@CNT) integrating N-doped carbon nanotubes (NCNTs) and Fe-Nx/Fe2O3/Fe3O4-encapsulated carbon nanosheets is obtained by in situ growth and interface induction strategy. SiO2 is modified with amine groups to anchor Fe3+ and inhibit Fe3+ reduction, and the resulting Fe2O3/Fe3O4 nanoparticles convert g-C3N4 to NCNTs. m-Fe/N-C@CNT shows superior activity and stability to commercial Pt/C and RuO2 catalysts in ORR and OER, respectively. The high activity of m-Fe/N-C@CNT is revealed to mainly derive from Fe-Nx moieties, and the contributions of N-C and Fe2O3/Fe3O4 species are reflected in ORR and OER, respectively. Moreover, the constructed ZABs exhibit a peak power density of 160 mW cm−2 and recyclability. This work provides a new idea for the preparation of bifunctional oxygen catalysts.

Co, N co-doped carbon nanosheets coupled with NiCo2O4 as an efficient bifunctional oxygen catalyst for Zn-air batteries

Developing of inexpensive and efficient bifunctional oxygen catalysts is important for the zinc-air batteries (ZABs). Here, a composite of Co, N co-doped carbon nanosheets coupled with NiCo2O4 (NiCo2O4/CoNC-NS) is developed as oxygen catalyst, which has good bifunctional oxygen catalytic activity and durability. Specifically, the half-wave potential of oxygen reduction reactions (ORR) is 0.849 V, and the overpotential of oxygen evolution reactions (OER) is 1.582 V at a current density of 10 mA cm−2. And the assembled liquid ZABs based on NiCo2O4/CoNC-NS exhibit high open circuit potential (OCP, 1.482 V), high peak power density (148.3 mW cm−2) and large specific capacity (699.9 mAh g−1) with long-term stability. Moreover, the further assembled solid ZABs can also provide high OCP (1.401 V), good power density (58.1 mW cm−2) and superior stability. This work would provide a good reference for the development of other advanced oxygen catalyst in future.

Cuboid-like phosphorus-doped metal–organic framework-derived CoSe2 on carbon cloth as an advanced bifunctional oxygen electrocatalyst for rechargeable zinc-air batteries

Zinc-air batteries (ZABs) are regarded as attractive devices for electrochemical energy storage and conversion due to their outstanding electrochemical performance, low price, and high safety. However, it remains a challenge to design a stable and efficient bifunctional oxygen catalyst that can accelerate the reaction kinetics and improve the performance of ZABs. Herein, a phosphorus-doped transition metal selenide/carbon composite catalyst derived from metal-organic frameworks (P-CoSe2/C@CC) is constructed by a self-supporting carbon cloth structure through a simple solvothermal process with subsequent selenization and phosphatization. The P-CoSe2/C@CC exhibits a low overpotential of 303.1 mV at 10 mA cm−2 toward the oxygen evolution reaction and an obvious reduction peak for the oxygen reduction reaction. The abovementioned electrochemical performances for the P-CoSe2/C@CC are attributed to the specific architecture, the super-hydrophilic surface, and the P-doping effect. Remarkably, the homemade zinc-air battery based on our P-CoSe2/C@CC catalyst shows an expected peak power density of 124.4 mW cm−2 along with excellent cycling stability, confirming its great potential application in ZABs for advanced bifunctional electrocatalysis.

Positive regulation of active sites for oxygen evolution reactions by encapsulating NiFe2O4 nanoparticles in N-doped carbon nanotubes in situ to construct efficient bifunctional oxygen catalysts for rechargeable Zn–air batteries

Benefitting from the structure of NiFe2O4 nanoparticles encapsulated in N-doped carbon nanotubes, the Fe–Me–Ni exhibits excellent catalytic activities for ORR and OER, and superior long term discharge–recharge durability.

Aerosol‐assisted synthesis of bimetallic nanoparticle‐loaded bamboo‐like N‐doped carbon nanotubes as an efficient bifunctional oxygen catalyst for Zn‐air batteries

Considering fabrication of highly efficient and low-cost bifunctional oxygen electrocatalysts, which can substitute noble metal-based catalysts, is imperative for advanced rechargeable Zn-air batteries (ZABs). Among the various candidates, N-doped carbon and transition metal composite catalyst is considered as promising bifunctional oxygen electrocatalysts. In this study, one-dimensional structured bamboo-like N-doped carbon nanotubes (bNCNTs) were synthesized by spray pyrolysis followed by a post-treatment. CoFe nanoalloys embedded within SiO2 microspheres were employed as a nanocatalyst to grow homogeneous bNCNTs, which have known as excellent oxygen reduction reaction (ORR) activity. To compensate for the low oxygen evolution reaction (OER) activity of bNCNTs, NiFe nanoparticles were loaded on bNCNTs (NiFe/bNCNT) by a “drop and dry” process and post-treatment. Synergistic effect arising from bNCNTs showing high surface area and controlled infiltration method, NiFe nanoparticles exhibited high dispersity on bNCNTs. Owing to the synergistic effect of NiFe metal nanoparticles and bNCNTs, NiFe/bNCNT exhibited superior ORR properties with a high limiting current density (5.1 mA cm−2) and a low Tafel slope (94 mV dec−1), as well as outstanding OER activity with a low overpotential (350 mV) in an alkaline media compared to those of single metal-loaded bNCNTs and Pt/C-RuO2. Furthermore, when applied as ZAB cathode catalyst, NiFe/bNCNT demonstrated a high power density (224 mW cm−2) and 330 hours long cycling stability.

Nitrogen doped CuCo2O4 nanoparticles anchored on beaded-like carbon nanofibers as an efficient bifunctional oxygen catalyst toward zinc-air battery

The elaborative design and construction of first-rank bifunctional oxygen electrocatalysts featuring low price, high activity and strong stability is critical for the large-scale applications of rechargeable Zn-air batteries. Here, a resultful strategy is proposed for fabricating nitrogen-doped 1D beaded-like structure carbon nanofibers uniformly decorated with nitrogen-doped CuCo2O4 nanoparticles (N-CuCo2O4@CNFs) toward boosting oxygen evolution reaction/oxygen reduction reaction (OER/ORR) catalysis. Taking advantage of the synergistic effect between interconnected 1D hierarchical porous carbon nanofiber structure and high catalytic activity of N-doped CuCo2O4 nanoparticles derived from bimetallic MOFs, the N-CuCo2O4@CNFs catalysts possess enhanced reaction kinetics and preferable charge transfer ability. Impressively, the obtained catalysts exhibit prominent electrocatalytic ability and superior stability for OER/ORR, even surpass the commercial RuO2 and Pt/C. More significantly, the Zn-air batteries employing the N-CuCo2O4@CNFs-800 as cathode display a higher power density of 175.6 mW cm−2, a lower charge-discharge voltage gap of 0.82 V at 10 mA cm−2, as well as a better cycling stability with respect to those of Pt/C + RuO2 mixture, demonstrating the great potential of N-CuCo2O4@CNF as a high-efficiency catalyst for clean energy devices.

Engineering core–shell Co9S8/Co nanoparticles on reduced graphene oxide: Efficient bifunctional Mott–Schottky electrocatalysts in neutral rechargeable Zn–Air batteries

It is significant for the rational construction of the high–efficient bifunctional electrocatalysts for in–depth understandings of how to improve the electron transfer and ion/oxygen transport in catalyzing oxygen reduction reaction and oxygen evolution reaction (ORR and OER), but still full of vital challenges. Herein, we synthesize the novel “three–in–one” catalyst that engineers core–shell Mott–Schottky Co9S8/Co heterostructure on the defective reduced graphene oxide (Co9S8/Co–rGO). The Co9S8/Co–rGO catalyst exhibits abundant Mott–Schottky heterogeneous–interfaces, the well–defined core–shell nanostructure as well as the defective carbon architecture, which provide the multiple guarantees for enhancing the electron transfer and ion/oxygen transport, thus boosting the catalytic ORR and OER activities in neutral electrolyte. As expected, the integrated core–shell Mott–Schottky Co9S8/Co–rGO catalyst delivers the most robust and efficient rechargeable ZABs performance in neutral solution electrolytes accompanied with a power density of 59.5 mW cm−2 and superior cycling stability at 5 mA cm−2 over 200 h. This work not only emphasizes the rational designing of the high–efficient bifunctional oxygen catalysts from the fundamental understanding of accelerating the electron transfer and ion/oxygen transport, but also sheds light on the practical application prospects in more friendly environmentally neutral rechargeable ZABs.

Metal-Organic Framework-Derived Trimetallic Nanocomposites as Efficient Bifunctional Oxygen Catalysts for Zinc-Air Batteries.

Transition-metal-based multifunctional catalysts have attracted increasing attention owing to high possibilities of substituting the expensive noble-metal-based catalysts in various scenarios. Multivariate metal-organic frameworks (MTV-MOFs) are ideal precursors to prepare multimetallic nanocomposites with high catalytic activity since the uniform distribution and precise regulation of mixed metal centers, as well as the consequent strong synergistic effect, could be readily achieved. Herein, a Mn/Co/Ni trimetallic catalyst (MnCoNi-C-D) with a hollow rhombic dodecahedron shape was synthesized via pyrolysis of the corresponding trimetallic-based MTV-MOF. The catalyst shows outstanding electrochemical activity toward the oxygen reduction reaction including a half-wave potential of 0.82 V and superior tolerance against methanol as well as high stability in an alkaline medium, and its oxygen evolution reaction activity also surpasses a RuO2 catalyst. Moreover, primary and rechargeable zinc-air batteries based on MnCoNi-C-D delivered preferable performances compared with commercial Pt/C-RuO2, including higher peak power density (116.4 mW cm-2), higher specific capacity (841.3 mAh g-1), higher open-circuit potential (OCV) (1.46 V), and better stability for more than 180 h. A comprehensive comparison was also conducted to prove the necessity of employing the MTV-MOF as the precursor and investigate the intrinsic superiority of the catalyst.

Biomass-derived bifunctional electrocatalysts for oxygen reduction and evolution reaction: A review

Oxygen electrode catalysts are important as inter-conversion of O2 and H2O is crucial for energy technologies. However, the sluggish kinetics of oxygen reduction and evolution reactions (ORR and OER) are a hindrance to their scalable production, whereas scarce and costly Pt and Ir/Ru-based catalysts with the highest electrocatalytic activity are commercially unviable. Since good ORR catalysts are not always efficient for OER and vice versa, so bifunctional catalysts on which OER and ORR occurs on the same electrode are very desirable. Alternative catalysts based on heteroatom-doped carbon nanomaterials, though showed good electrocatalytic activity yet their high cost and complex synthesis is not viable for scalable production. To overcome these drawbacks, biomass-derived heteroatom-doped porous carbons have recently emerged as low-cost, earth-abundant, renewable and sustainable environment-friendly materials for bifunctional oxygen catalysts. The tunable morphology, mesoporous structure and high concentration of catalytic active sites of these materials due to heteroatom (N)-doping could further enhance their ORR and OER activity, along with tolerance to methanol crossover and good durability. Thus, biomass-derived heteroatom-doped porous carbons with large surface area, rich edge defects, numerous micropores and thin 2D nanoarchitecture could be suitable as efficient bifunctional oxygen catalysts. In the present article, synthesis, N-doping, ORR/OER mechanism and electrocatalytic performance of biomass-derived bifunctional catalysts has been discussed. The selected biomass (chitin, eggs, euonymus japonicas, tobacco, lysine and plant residue) except wood, act as both C and N precursor, resulting in N self-doping of porous carbons that avoids the use of toxic chemicals, thus making the synthesis a facile and environment-friendly green process. The synthetic strategy could be further optimized to develop future biomass-based N self-doped porous carbons as metal-free high performance bifunctional oxygen catalysts for commercial energy applications. Recent advances and the importance of biomass-based bifunctional oxygen catalysts in metal-air batteries and fuel cells has been highlighted. The material design, perspectives and future directions in this field are also provided.

Strengthening absorption ability of Co–N–C as efficient bifunctional oxygen catalyst by modulating the d band center using MoC

Co–N–C is a promising oxygen electrochemical catalyst due to its high stability and good durability. However, due to the limited adsorption ability improvement for oxygen-containing intermediates, it usually exhibits inadequate catalytic activity with 2-electron pathway and high selectivity of hydrogen peroxide. Herein, the adsorption of Co–N–C to these intermediates is modulated by constructing heterostructures using transition metals and their derivatives based on d-band theory. The heterostructured nanobelts with MoC core and pomegranate-like carbon shell consisting of Co nanoparticles and N dopant (MoC/Co–N–C) are engineered to successfully modulate the d band center of active Co–N–C sites, resulting in a remarkably enhanced electrocatalysis performance. The optimally performing MoC/Co–N–C exhibits outstanding bi-catalytic activity and stability for the oxygen electrochemistry, featuring a high wave-half potential of 0.865 V for the oxygen reduction reaction (ORR) and low overpotential of 370 mV for the oxygen evolution reaction (OER) at 10 mA cm−2. The zinc air batteries with the MoC/Co–N–C catalyst demonstrate a large power density of 180 mW cm−2 and a long cycling lifespan (2000 cycles). The density functional theory calculations with Hubbard correction (DFT + U) reveal the electron transferring from Co to Mo atoms that effectively modulate the d band center of the active Co sites and achieve optimum adsorption ability with “single site double adsorption” mode.