An efficient metal-free bifunctional oxygen electrocatalyst of carbon co-doped with fluorine and nitrogen atoms for rechargeable Zn-air battery

It is highly critical to explore efficient bifunctional oxygen electrocatalysts for regenerative fuel cells and metal-air batteries. Herein, N, F co-doped carbon material (NF@CB) was synthesized as metal-free efficient bifunctional electrocatalysts by directly pyrolyzing a mixture of carbon black, polytetrafluoroethylene and melamine. Benefiting from the synergistic effects between N and F atoms, NF@CB exhibits a positive half-wave potential (E_1/2) of 0.814 V (vs. RHE) for oxygen reduction reaction, and an operating potential (E₁₀) of 1.609 V at 10 mA cm⁻² for oxygen evolution reaction in alkaline electrolyte. The bifunctional oxygen electrocatalytic activity index (ΔE = E₁₀−E_1/2) is 0.795 V, which is notably better than that of the single N-doped carbon (1.238 V), and similar to that of the commercial Pt/C and RuO₂ mixture catalyst (0.793 V). Impressively, the assembled Zn-air battery (ZAB) with NF@CB as an air-electrode catalyst displays a small charge/discharge voltage gap of 0.852 V at 20 mA cm⁻². Moreover, the NF@CB catalyzed ZAB exhibits good rechargeability and long-lasting cycling stability with over 49 h. This investigation introduces a cheap and simple way to develop highly efficient bifunctional N, F co-doped electrocatalysts.     

Transitional metal alloyed nanoparticles entrapped into the highly porous N-doped 3D honeycombed carbon: A high-efficiency bifunctional oxygen electrocatalyst for boosting rechargeable Zn-air batteries

Rational development of low-cost, durable and high-performance bifunctional oxygen catalysts is highly crucial for metal-air batteries. Herein, transition metal alloyed FeCo nanoparticles (NPs) embedded into N-doped honeycombed carbon (FeCo@N-HC) was efficiently prepared by a one-step carbonization method in the existence of NH₄Cl and citric acid. Benefiting from the honeycomb-like architectures and the synergistic effects of the FeCo alloy with the doped-carbon matrix, the as-synthesized FeCo@N-HC exhibited outstanding oxygen reduction reaction (ORR) with the more positive onset potential (E_onset = 0.98 V vs. RHE) and half-wave potential (E_1/2 = 0.85 V vs. RHE), coupled with outstanding oxygen evolution reaction (OER) with the lower overpotential (318 mV) at 10 mA cm^?2. Besides, the home-made Zn-air battery has the larger power density of 144 mW cm^?2 than Pt/C + RuO₂ (80 mW cm^?2). This research offers some valuable guidelines for constructing robust oxygen catalysts in clean energy storage and conversion technologies.     

Facile synthesis of BSCF perovskite oxide as an efficient bifunctional oxygen electrocatalyst

We present a facile way to synthesize BSCF by using glycine-nitrate auto-combustion followed by annealing at different conditions, which work as high-performance bifunctional electrocatalyst for oxygen evolution (OER) as well as oxygen reduction (ORR) reactions in alkaline solution with comparatively better efficiency for OER. Annealing condition plays an important role towards catalytic performance due to morphological control and surface composition. Although, there is no significant change in onset potentials but these catalysts afford a current density >10 mA cm^?2 at the potential of 1.65 V for oxygen evolution reaction and a current density >2.5 mA cm^?2 at the potential of 0.009 V for oxygen reduction reaction with respect to RHE in 0.1 M KOH. The underlying mechanism for ORR and OER as well as catalytic activity differences were understood with the help of different analytical characterization techniques.     

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.     

A dual ligand coordination strategy for synthesizing drum-like Co,N co-doped porous carbon electrocatalyst towards superior oxygen reduction and zinc-air batteries

We herein propose a dual ligand coordination strategy for deriving puissant non-noble metal electrocatalysts to substitute valuable platinum (Pt)-based materials toward oxygen reduction reaction (ORR), a key reaction in metal-air batteries and fuel cells. In brief, cobalt ions are firstly double-coordinated with proportionate 2-methylimidazole (2-MeIm) and benzimidazole (BIm) to obtain drum-like zeolitic imidazolate frameworks (D-ZIFs), which are then carbonized to output the final Co, N co-doped porous carbon (Co–N–PC_D) catalyst inheriting the drum-like morphology of D-ZIFs. The Co–N–PC_D is featured by mesopores and exhibits superb electrocatalytic behavior for ORR. Impressively, the half-wave potential of Co–N–PC_D catalysts is 0.886 V with finer methanol-tolerance and stability than those of commercial Pt/C. Additionally, a zinc-air battery assembled from the Co–N–PC_D displays an open-circuit voltage of 1.413 V, comparable to that of commercial Pt/C (1.455 V), suggesting the potentials of Co–N–PC_D in practical energy conversion devices.     

CeO2 nanoparticle-decorated reduced graphene oxide as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions

Highly efficient bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for the development of renewable energy technologies such as fuel cells and metal-air batteries. Herein, a ceria (CeO₂) – modified reduced graphene oxide (CeO₂/rGO) nanocomposite was fabricated via a facile yet cost-effective method under a mild condition. The prepared CeO₂/rGO nanocomposite showed remarkable catalytic activity, high tolerance to methanol and durability toward ORR in alkaline media. Meanwhile, the catalyst also displayed remarkable activity for the OER with more negative onset potential and higher current compared with commercial Pt/C catalyst. The high oxygen reaction activity of the catalyst could contribute to synergistic effect of the combination of the oxygen vacancies of CeO₂ and excellent electronic conductivity of rGO. The results suggested that the CeO₂/rGO nanocomposite has potential advantages as a bifunctional electrocatalyst in the practical applications.     

Tunable Fe/N co-doped 3D porous graphene with high density Fe-Nx sites as the efficient bifunctional oxygen electrocatalyst for Zn-air batteries

Nitrogen-doped transition metal materials display promising potential as bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, Fe/N co-doped three-dimensional (3D) porous graphene (FeN-3D-PG) is prepared via a template method using sodium alginate as the carbon source and low polymerization degree melamine resin as the nitrogen source. The low polymerization degree melamine resin can form complexes with Fe³⁺ in the aqueous solution and further forms high density Fe-N_x active sites during pyrolysis. Meanwhile, the formed 3D porous structure efficiently promotes the uniform distribution of Fe-N_x active sites. The FeN-3D-PG catalyst exhibits pH-independent ORR activity. For OER, the catalyst possesses a low over potential (370 mV at 10 mA cm⁻²) in alkaline electrolyte. The Zn-air batteries (ZABs) using FeN-3D-PG as cathode exhibits a power density up to 212 mW cm⁻², a high specific capacity of 651 mAh g⁻¹, and the charge-discharge stability of 80 h. This work provides new sight to transition metal materials based ZABs with excellent performance.     

Co nanoparticles and ZnS decorated N,S co-doped carbon nanotubes as an efficient oxygen reduction catalyst in zinc-air batteries

The economical, efficient and durable oxygen reduction catalysts facilitate the enhancement of electrochemical energy devices competitiveness towards widespread applications. In view of this, we provide an innovative sulfuration inducing method for the synthesis of ZnS and cobalt nanoparticles decorated N, S co-doped CNTs (ZnS/Co-NSCNTs) catalyst. S introduced into the zinc-based zeolitic imidazolate frameworks (ZIF-8) and cobalt-based zeolitic imidazolate frameworks (ZIF-67) precursors via pyrolysis, and induced the generation of ZnS/Co-NSCNTs have been confirmed by XRD, SEM, TEM, and XPS techniques. The key features including activity sites, transfer channels and adsorption energy back up the excellent electrocatalytic activity of the as-prepared ZnS/Co-NSCNTs towards oxygen reduction reactions (ORR). ZnS/Co-NSCNTs additionally exhibited a positive half-wave potential of 0.871 V (vs. RHE) with improved current density towards ORR. In alkaline medium, ZnS/Co-NSCNTs catalyst displayed a high tolerance towards methanol and an excellent long-term cycling stability. The observed onset potential for our prepared ZnS/Co-NSCNTs catalyst is analogous with the commercially available noble metal catalysts. Also, ZnS/Co-NSCNTs catalyst as a cathode in zinc-air battery displayed an enhanced electrochemical performance with a highly specific capacity of 750.1 mAh g⁻¹, outstanding cycling stability, and high rate behavior. This work provides a new approach for the construction of stable low-cost alternative air-cathode catalysts for other energy conversion and storage applications.     

Combustion-derived BaNiO3 nanoparticles as a potential bifunctional electrocatalyst for overall water splitting

Electrochemical water electrolyser though an assuring solution for clean hydrogen production, the sluggish kinetics and high cost of existing precious metal electrocatalyst remains a barrier to its effective utilization. Herein, solution combustion route derived perovskite type barium nickelate (BaNiO₃) nanoparticles were developed and studied for their bifunctional electrocatalytic properties towards overall water splitting. The unannealed BaNiO₃ nanoparticles exhibited the highest OER and HER activity with overpotentials 253 mV and 427 mV respectively to attain 10 mAcm⁻² in 1.0 M KOH. Using unannealed BaNiO₃ as a bifunctional electrocatalyst in a two-electrode alkaline electrolyser, the cell was able to achieve the benchmark current density at a low cell voltage of 1.82 V. Impressively the setup's electrocatalytic performance improved 4.9% after continuous overall water splitting for 24 h at 30 mAcm⁻². Therefore, BaNiO₃ nanoparticles can be a low-cost and efficient alternative for noble metal electrocatalysts for clean H₂ production.     

A robust bifunctional electrocatalyst for rechargeable zinc-air batteries: NiFe nanoparticles encapsulated in nitrogen-doped carbon nanotubes

Developing high-efficiency, low-cost, and stable bifunctional oxygen electrocatalysts is essential for the commercialization of rechargeable metal-air batteries. Herein, three-dimensional self-assembled microspheres via in situ encapsulation of NiFe alloy nanoparticles (NPs) into N-doping carbon nanotubes (NiFe@NCNTs) have been achieved through pyrolyzing a mixture of nickel-iron alkoxide and melamine. The as-prepared electrocatalyst exhibits outstanding oxygen reduction reaction (ORR) performance with a half-wave potential of 0.79 V and oxygen evolution reaction (OER) activity with a low overpotential of 330 mV at 10 mA cm^?2. The eminent activity of NiFe@NCNTs is ascribed to high dispersion of active sites (zero-dimensional core-shell structure of NiFe@NC) and one-dimensional conductive network (NCNTs). Accordingly, the zinc-air battery assembled with NiFe@NCNTs as the air cathode exhibits a long cycling life of 200 h with a high energy efficiency of 65.6%. This work may shed new light on the design of advanced bifunctional electrocatalysts toward metal-air batteries.     

Petal-like NiCoP sheets on 3D nitrogen-doped carbon nanofiber network as a robust bifunctional electrocatalyst for water splitting

Searching high-active, stable and abundant bifunctional catalysts to replace noble metals for hydrogen and oxygen evolution reactions (HER and OER) is desired. Herein, petal-like NiCoP sheets were synthesized on carbon paper covered with a 3D nitrogen-doped carbon nanofiber network (NiCoP/CNNCP) by a simple hydrothermal process followed by phosphorization. The HER overpotential in 0.5 M H₂SO₄ and OER overpotential in 1 M KOH of the NiCoP/CNNCP electrode only required 55 mV and 260 mV to drive a current density of 10 mA cm^?2, respectively, which was comparable or even better than most nickel-and cobalt-based phosphide catalysts. The overall water-splitting electrolyzer with an asymmetric electrolyte system assembled using NiCoP/CNNCP as bifunctional electrodes required an extremely low cell voltage of 1.04 V to achieve a current density of 10 mA cm^?2, which was much lower than almost all alkaline electrolysis systems.     

N-doped graphene as a bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions in an alkaline electrolyte

In this work, a nitrogen-doped graphene (NG) catalyst was prepared using a hydrothermal method with ammonia as the nitrogen precursor, which was followed by a freeze-dry process. The catalyst was characterized using X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscope, and X-ray photoelectron spectroscopy. The bifunctional catalytic activities for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) were investigated using cyclic voltammetry in an alkaline electrolyte. The results indicate that nitrogen is successfully doped in the NG catalyst, and the catalyst has a loose structure that was produced during the freeze-dry process. The catalyst exhibits an excellent ORR activity with an onset potential of −0.08 V and a high OER activity with an obvious OER current at 0.7 V. The rotating-disk-electrode test results indicate that the ORR process catalyzed by the NG catalyst involves a mix of the two-electron and four-electron transfer pathways. This work preliminarily explores the bifunctional catalytic properties for the ORR and the OER of nitrogen-doped graphene materials in alkaline electrolyte.     

NiFe layered double hydroxide as an efficient bifunctional catalyst for electrosynthesis of hydrogen peroxide and oxygen

Two electron oxygen reduction reaction to produce hydrogen peroxide (H₂O₂) is a promising alternative technique to the multistep and high energy consumption anthraquinone process. Herein, Ni–Fe layered double hydroxide (NiFe-LDH) has been firstly demonstrated as an efficient bifunctional catalyst to prepare H₂O₂ by electrochemical oxygen reduction (2e^? ORR) and oxygen evolution reaction (OER). Significantly, the NiFe-LDH catalyst possesses a high faraday efficiency of 88.75% for H₂O₂ preparation in alkaline media. Moreover, the NiFe-LDH catalyst exhibits excellent OER electrocatalytic property with small overpotential of 210 mV at 10 mA cm^?2 and high stability in 1 M KOH solution. On this basis, a new reactor has been designed to electrolyze oxygen and generate hydrogen peroxide. Under the ultra-low cell voltage of 1 V, the H₂O₂ yield reaches to 47.62 mmol g_cat^?1 h^?1. In order to evaluate the application potential of the bifunctional NiFe-LDH catalyst for H₂O₂ preparation, a 1.5 V dry battery has been used as the power supply, and the output of H₂O₂ reaches to 83.90 mmol g_cat^?1 h^?1. The excellent electrocatalytic properties of 2e^? ORR and OER make NiFe-LDH a promising bifunctional electrocatalyst for future commercialization. Moreover, the well-designed 2e^? ORR-OER reactor provides a new strategy for portable production of H₂O₂.     

Nanoporous CoCu layered double hydroxide nanoplates as bifunctional electrocatalyst for high performance overall water splitting

The electrochemical water splitting into hydrogen and oxygen is the promising way for renewable hydrogen production as a carbon-neutral fuel, along with oxygen as a by-product. Herein, a novel nanoporous CoCu-layered double hydroxide (LDH) bifunctional electrocatalyst is fabricated by the hydrothermal method. The outstanding activity is mainly attributed to the incorporation of Cu²⁺ that promotes conductivity and enhances the electrochemical properties. As-prepared CoCu-LDH nanostructure works as efficient and stable water-splitting-electrolyzer and produces the voltage of 1.60 V at the current density of 10 mA cm^?2, which is better than catalyst based on the combination of commercial IrO₂ and Pt/C. Due to high electrocatalytic performance, together with its low cost and natural abundance of LDHs, it is expected that CoCu LDH can act as a candidate catalyst in the commercial alkaline overall water splitting.     

Rapid synthesis of porous Ni/Co/Fe-LDHs nanosheets for effective electrochemical oxygen evolution reaction and zinc-air batteries

Transition metal compounds, especially layered double hydroxide materials (LDHs), show excellent catalytic activity in oxygen evolution reaction (OER). The ethanol oxidation reaction (EOR) is an innovative alternative anodic reaction to OER for improving the efficiency of water splitting to produce hydrogen. In order to improve the reactivity and explore the similarities and differences of active sites in the two reactions, three kinds of porous LDHs (NiFe, NiCo, CoFe LDHs) were synthesized and a series of tests were carried out. Among them, the best performing OER catalyst is NiFe-LDHs with a low overpotential of 1.44 V vs. RHE at 10 mA cm^?2 and a Tafel slope of 23.85 mV dec^?1. As for the EOR reaction, NiCo-LDHs is the best, with an overpotential of only 1.38 V vs. RHE at 10 mA cm^?2 and a Tafel slope of 71.58 mV dec-1. In addition, compared with OER, the LHDs material exhibited better stability in the EOR. This work provides a new direction for studying the electrocatalytic activity of LDHs materials in OER and EOR.     

Efficient MnO and Co nanoparticles coated with N-doped carbon as a bifunctional electrocatalyst for rechargeable Zn-air batteries

Developing the low-cost, durable, and efficient bifunctional electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) plays an important role in the commercial implementation of the Zn-air batteries. Herein, we design and synthesize the MnO and Co nanoparticles coated with N-doped carbon (MC@NC) as an excellent bifunctional oxygen electrocatalyst. It is found that the optimal MC@NC-0.3 exhibits outstanding ORR performance with a positive half-wave potential of 0.82 V and excellent OER activity with a small overpotential of 360 mV at 10 mA cm⁻². When applied in the liquid Zn-air battery, MC@NC-0.3 displays a high maximum power density of 153 mW cm⁻², a large specific capacity of 776 mAh g⁻¹ and the excellent cycling stability with a negligible increase after 300 h. Furthermore, the fiber-shaped all-solid-state Zn-air battery also displays remarkable stability at high current density. This study offers a facile strategy to construct a high-efficient, low-cost, and durable transitional metal-based bifunctional electrode for renewable energy applications.     

Phosphorus-doped SrCo0.5Mo0.5O3 perovskites with enhanced bifunctional oxygen catalytic activities

Perovskite oxide has attracted wild attentions as a promising bifunctional electrocatalyst family for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in fuel cells and metal-air batteries. Here, phosphorus-doped SrCo_0.5Mo_0.5O₃ perovskites has been prepared and evaluated as bifunctional oxygen electrocatalyst. X–ray diffraction and X-ray photoelectron spectroscopy results show that P-doping benefits the formation of more stable tetragonal phase, and the generation of more surface adsorbed oxygen species. Compared with SrCo_0.5Mo_0.5O₃ perovskites without doping, rotating disk electrode measurements indicates that P-doped SrCo_0.5Mo_0.5O₃ exhibits a positively shifted half-wave potentional of 50 mV for ORR, and a reduced OER overpotential (～58 mV) at 10 mA cm^?2 in 0.1 M KOH solution. The enhanced catalytic activities and stabilities contribute to optimized surface characteristic and stable phase structure. The work not only provides a new strategy to develop efficient bifunctional oxygen catalysts, but also enriches knowledge of perovskite oxides.     

A novel bifunctional catalyst of Ba0.9Co0.5Fe0.4Nb0.1O3−δ perovskite for lithium–air battery

Ba_0.9Co_0.5Fe_0.4Nb_0.1O₃ (BCFN) perovskite has been synthesized by a solid-state reaction method, and characterized by XRD, SEM, BET. This oxide has a porous structure and a specific surface area of 10.24 m² g⁻¹ after ball-milled 24 h. The catalytic activity of the oxide for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in 0.1 M KOH solution has been studied by using a rotating ring-disk electrode (RRDE) technique. RRDE results show that the ORR mainly favors a direct four electron pathway, and a maximum cathodic current density of −5.70 mA cm⁻² at 2500 rpm was obtained, which is close to that of Pt/C (20 wt.% Pt on carbon) electrocatalyst in the same testing conditions. Compared with behaviors of pure C and Pt/C electrode, a lower onset potential of BCFN for OER is observed, and a bigger anodic current at the same applied potential is obtained. Considering small surface area of the BCFN catalyst, a big overpotential is given in the discharge–charge curves. However, the outputs of 2032 coin Li–air batteries in a dry gas mixture composed of 80 vol.% pure N₂ and 20 vol.% pure O₂ demonstrated that BCFN could be a potential bifunctional catalyst for the Li–air battery.     

Effects of the cation and anion co-doping in perovskite as bifunctional electrocatalyst for water splitting

Element doping is a very important way to modify perovskite oxide (ABO₃) electrocatalyst. Herein, the O, B and A-site of BaCoO_3-δ are doped with F, Fe and Sr elements, respectively, which are used to investigate the effect of different doping sites on water splitting performance. The results show that doping F can generate more oxygen vacancies and increase the Co valence state. On the basis of anionic doping, when Co is partially substitute by Fe, the Fe⁴⁺ formed makes the O 2P orbital close to E_F, and the bandwidth is narrower, enhancing the conductivity. Subsequently, doping Sr in A-site can change the crystal structure from P63/mmc to pm-3m, and exhibit metallic properties. This work can contribute to an effective approach for the design of perovskite oxide materials for efficient electrocatalytic water splitting.     

Improving catalytic activity of layered lithium transition metal oxides for oxygen electrode in metal-air batteries

Lithium transition metal oxide has superior performance for oxygen evolution reaction (OER), while its activity for catalyzing oxygen reduction reaction (ORR) is too low to meet the demand of practical applications. Herein, the NCM-based (NCM, LiNi_1/3Co_1/3Mn_1/3O₂) composite materials are prepared through the two steps method. The NCM-2 (Mn₂O₃/LiNi_1/3Co_1/3Mn_1/3O₂) hybrid material demonstrates excellent ORR catalytic property and high OER catalytic performance, as well as the superior stability. Besides, with NCM-2 hybrid materials as catalysts of air cathode, the Al-air battery and Zn-air battery both exhibit higher power density. Therefore, based on results of Brunauer-Emmett-Teller and O₂ temperature programmed desorption analysis, the improved catalytic performance ascribed to large specific surface area, pore structure and enhanced oxygen adsorption ability. In this work, the catalytic activity of lithium transition metal oxide has been improved, and a new method was provided to synthesize bifunctional catalysts for metal-air batteries.