The influence of the type of N dopping on the performance of bifunctional N-doped ordered mesoporous carbon electrocatalysts in oxygen reduction and evolution reaction

To develop more ideal bifunctional heteroatom-doped carbon electrocatalysts toward the oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) for regenerative fuel cells and rechargeable metal–air batteries, herein, tobacco-derived N-containing ordered mesoporous carbon(N-OMC) electrocatalysts with different N species distributions are designed. Results indicate that the as-prepared N-OMC with more pyrrolic and pyridinic Ns exhibits much higher activities for the ORR and OER than N-OMC with more graphitic N in both acidic and alkaline media, suggesting that the increase of pyrrolic and pyridinic Ns favors the improvement of ORR and OER activities of the N-containing carbon catalysts, and showing a great potential for the designing of more effective, lower-cost ORR and OER bifunctional electrocatalysts for future regenerative fuel cells and rechargeable metal–air batteries.     

Ultrafine Fe/Fe3C decorated on Fe-Nx-C as bifunctional oxygen electrocatalysts for efficient Zn-air batteries

Efficient bifunctional oxygen electrocatalysts for ORR and OER are fundamental to the development of high performance metal-air batteries.Herein,a facile cost-efficient two-step pyrolysis strategy for the fabrication of a bifunctional oxygen electrocatalyst has been proposed.The efficient non-preciousmetal-based electrocatalyst,Fe/Fe₃C@Fe-N_x-C consists of highly curved onion-like carbon shells that encapsulate Fe/Fe₃C nanoparticles,distributed on an extensively porous graphitic carbon aerogel.The obtained Fe/Fe₃C@Fe-N_x-C aerogel exhibited superb electrochemical activity,excellent durability,and high methanol tolerance.The experimental results indicated that the assembly of onion-like carbon shells with encapsulated Fe/Fe₃C yielded highly curved carbon surfaces with abundant Fe-Nxactive sites,a porous structure,and enhanced electrocatalytic activity towards ORR and OER,hence displaying promising potential for application as an air cathode in rechargeable Zn-air batteries.The constructed Zn-air battery possessed an exceptional peak power density of~147 mW cm^-2,outstanding cycling stability(200 cycles,1 h per cycle),and a small voltage gap of 0.87 V.This study offers valuable insights regarding the construction of low-cost and highly active bifunctional oxygen electrocatalysts for efficient air batteries.     

FeCo alloy/N,S dual-doped carbon composite as a high-performance bifunctional catalyst in an advanced rechargeable zinc-air battery

The rational design and development of cost-effective,high-performance,and stable bifunctional oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)electrocatalysts are essential for rechargeable zinc-air batteries.Herein,a novel FeCo composite composed of alloy nanoparticles embedded in an N,S dual-doped carbon matrix(FeCo/NSC)was prepared via one-step carbonization of amphiphilic dodecanethiol-metal salts wrapped in carbon nitride(C₃N₄).The compact combination of dual metalalloys and dual-doped carbon endowed the composite with the active sites for the ORR and OER,achieving efficient electrical transmission and highly efficient bifunctional catalytic performance.The obtained FeCo-1/NSC catalyst exhibited excellent electrocatalytic activity with a half-wave potential of 0.82 V(vs.RHE)for the ORR and a low overpotential of 0.325 V at 10 mA cm^-2 for the OER.The liquid Zn-air battery with FeCo-1/NSC as an air electrode displayed excellent charge-discharge performance,high power density,and robust charge-discharge stability for 150 h compared to the 20%Pt/C+RuO₂ counterpart.Furthermore,the FeCo-1/NSC-based flexible solid-state Zn-air battery exhibited a higher power density and good charge-discharge stability over 10 h of operation.Thus,a promising strategy for bifunctional electrocatalyst development as part of rechargeable and wearable Zn-air batteries was provided.     

Nanocarbons and their hybrids as catalysts for non-aqueous lithium–oxygen batteries

Rechargeable lithium-oxygen(Li–O_2) batteries have been considered as the most promising candidates for energy storage and conversion devices because of their ultra high energy density. Until now, the critical scientific challenges facing Li–O_2batteries are the absence of advanced electrode architectures and highly efficient electrocatalysts for both oxygen reduction reaction(ORR) and oxygen evolution reaction(OER), which seriously hinder the commercialization of this technology. In the last few years, a number of strategies have been devoted to exploring new catalysts with novel structures to enhance the battery performance. Among various of oxygen electrode catalysts, carbon-based materials have triggered tremendous attention as suitable cathode catalysts for Li–O_2batteries due to the reasonable structures and the balance of catalytic activity, durability and cost. In this review, we summarize the recent advances and basic understandings related to the carbon-based oxygen electrode catalytic materials, including nanostructured carbon materials(one-dimensional(1D) carbon nanotubes and carbon nanofibers, 2D graphene nanosheets, 3D hierarchical architectures and their doped structures), and metal/metal oxide-nanocarbon hybrid materials(nanocarbon supporting metal/metal oxide and nanocarbon encapsulating metal/metal oxide). Finally, several key points and research directions of the future design for highly efficient catalysts for practical Li–O_2batteries are proposed based on the fundamental understandings and achievements of this battery field.     

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 ...     

TMN4 complex embedded graphene as bifunctional electrocatalysts for high efficiency OER/ORR

Developing highly active bifunctional electrocatalysts for oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)is of great significance in energy conversion and storage technologies.In this study,we systematically investigated the OER/ORR electrocatalytic activity of TMN₄@G system by using density functional theory(DFT)calculations.Globally,IrN₄@G is a very promising bifunctional catalyst for both OER and ORR with the extremely low overpotentials of 0.30 and 0.26 V,respectively.Such outstanding electrocatalytic performance is mainly attributed to the synergistic effect of Ir and N.More importantly,by constructing 2D activity volcano plots,we obtained the limiting overpotentials of TMN₄@G system with the values of 0.26 V for OER and 0.24 V for ORR.These findings open up new opportunities for further exploring graphene-based materials for highly efficient OER/ORR electrocatalysts.     

Nitrogen-doped carbon nanotube encapsulating cobalt nanoparticles towards efficient oxygen reduction for zinc–air battery

Nitrogen-doped carbon materials encapsulating 3 d transition metals are promising alternatives to replace noble metal Pt catalysts for efficiently catalyzing the oxygen reduction reaction(ORR). Herein, we use cobalt substituted perfluorosulfonic acid/polytetrafluoroethylene copolymer and dicyandiamide as the pyrolysis precursor to synthesize nitrogen-doped carbon nanotube(N–CNT) encapsulating cobalt nanoparticles hybrid material. The carbon layers and specific surface area of N–CNT have a critical role to the ORR performance due to the exposed active sites, determined by the mass ratio of the two precursors. The optimum hybrid material exhibits high ORR activity and stability, as well as excellent performance and durability in zinc–air battery.     

Novel fusiform core-shell-MOF derived in tact metal@carb on composite:An efficient cathode catalyst for aqueous and solid-state Zn-air batteries

Owing to the varied mechanisms of ORR/OER,exploiting cost-effective bifunctional catalysts with robust ORR/OER activities and excellent performances in Zn-air batteries is still a challenge.In this work,the Co/CoO@NSC bifunctional catalyst is obtained by using Zn-MOF@Co-MOF as self-template.The Co/CoO@NSC composite has interconnected porous architecture with in tact metal@carb on structure,exhibiting superior electrocatalytic activities toward ORR and OER that can be comparable with the Pt/C and RuO₂ catalysts,respectively.The Co/CoO@NSC-based aqueous Zn-air battery achieves a high specific capacity(759.7 mAh/g)and energy density(990.5 Wh/kg),and ultra-long rechargeable property(more than 400 h/1200 cycles).The Co/CoO@NSC-based solid-state Zn-air battery also delivers an excellent performance with a long cycle life(more than 143 h/858 cycles).Most importantly,the newly synthesized and recharged Co/CoO@NSC-based solid-state Zn-air battery can be used to light up a 2 V LED lamp for more than 28 h,demonstrating the superior practicability as rechargeable power source.     

Synthesis–structure correlations of manganese–cobalt mixed metal oxide nanoparticles

Mixed metal oxides in the nanoscale are of great interest for many aspects of energy related research topics as water splitting, fuel cells and battery technology. The development of scalable, cost-efficient and robust synthetic routes toward well-defined solid state structures is a major objective in this field.While monometallic oxides have been studied in much detail, reliable synthetic recipes targeting specific crystal structures of mixed metal oxide nanoparticles are largely missing. Yet, in order to meet the requirements for a broad range of technical implementation it is necessary to tailor the properties of mixed metal oxides to the particular purpose. Here, we present a study on the impact of the nature of the gas environment on the resulting crystal structure during a post-synthesis thermal heat treatment of manganese–cobalt oxide nanoparticles. We monitor the evolution of the crystal phase structure as the gas atmosphere is altered from pure nitrogen to synthetic air and pure oxygen. The particle size and homogeneity of the resulting nanoparticles increase with oxygen content, while the crystal structure gradually changes from rocksalt-like to pure spinel. We find the composition of the particles to be independent of the gas atmosphere. The manganese–cobalt oxide nanoparticles exhibited promising electrocatalytic activity regarding oxygen evolution in alkaline electrolyte. These findings offer new synthesis pathways for the direct preparation of versatile utilizable mixed metal oxides.     

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,biomassderived heteroatom-doped porous carbons with large surface area,rich edge defects,numerous micropores and thin 2 D nanoarchitecture could be suitable as efficient bifunctional oxygen catalysts.In the present article,synthesis,N-doping,ORR/OER mechanism and electrocatalytic performance of biomassderived 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 selfdoping 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.     

Metal-organic-framework-derived formation of Co–N-doped carbon materials for efficient oxygen reduction reaction

Non-precious metal nitrogen-doped carbonaceous materials have attracted tremendous attention in the field of electrochemical energy storage and conversion. Herein, we report the designed synthesis of a novel series of Co-N-C nanocomposites and their evaluation of electrochemical properties. Novel yolkshell structured Co nanoparticles@polymer materials are fabricated from the facile coating polymer strategy on the surface of ZIF-67. After calcination in nitrogen atmosphere, the Co–N–C nanocomposites in which cobalt metal nanoparticles are embedded in the highly porous and graphitic carbon matrix are successfully achieved. The cobalt nanoparticles containing cobalt metal crystallites with an oxidized shell and/or smaller(or amorphous) cobalt-oxide deposits appear on the surface of graphitic carbons. The prepared Co–N–C nanoparticles showed favorable electrocatalytic activity for oxygen reduction reactions,which is attributed to its high graphitic degree, large surface area and the large amount existence of Co–N active sites.     

Melamine-assisted pyrolytic synthesis of bifunctional cobalt-based core–shell electrocatalysts for rechargeable zinc–air batteries

Designing a highly active-and stable non-noble metal bifunctional oxygen catalyst for rechargeable Znair battery remains a great challenge. Herein, we develop a facile and melamine-assisted-pyrolysis(MAP)strategy for the synthesis of core–shell Co-based electrocatalysts@N-doped carbon nanotubes(Co@CNTs)derived from metal–organic frameworks. The Co@CNTs exhibited excellent bifunctional electrocatalytic performance for both oxygen evolution and reduction. DFT calculations demonstrated that the Gibbs free energy of the rate-determining step was small enough to improve ORR activities. As a result, a Zn-air battery assembled with Co@CNTs proves a lager power density, low voltage gap between charge–discharge and excellent stability. Thus, this work offers a facile strategy to realize the synthesis of non-noble metal electrocatalyst for Zn-air battery materials with high electrochemical performance.     

Enhancing bifunctionality of CoN nanowires by Mn doping for long-lasting Zn-air batteries

Tailoring the nanostructure and composition of transition metal nitrides is highly important for their use as potent low-cost electrocatalysts. Cobalt nitride(CoN) exhibits strong catalytic activity for oxygen evolution reaction(OER). However, its poor catalytic efficiency for oxygen reduction reaction(ORR) hinders its application in rechargeable zinc-air batteries(ZABs) as the air cathode. In this work, we deploy the effective strategy of Mn doping to improve both OER and ORR activity of CoN nanowires as the cathode material for ZAB. Theoretical calculation predicts that moderate Mn doping in cobalt nitride results in a downshift of the d-band center and reduces the adsorption energy of reaction intermediates. With ～10 at% Mn dopants, stronger catalysis activities for both OER and ORR are achieved compared to pure CoN nanowires. Subsequently, both aqueous and flexible quasi-solid-state ZABs are constructed using the Mn-doped CoN nanowires array as additive-free air cathode. Both types of devices present large open circuit potential, high power density and long-cycle stability. This work pushes forward the progress in developing cost-effective ZABs.     

Earth-abundant coal-derived carbon nanotube/carbon composites as efficient bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries

The exploration of active and robust electrocatalysts for both the oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)is the bottleneck to realize the commercialization of rechargeable metal-air batteries and regenerative fuel cells.Here we report facile synthesis of three-dimensional(3 D)carbon nanotube(CNT)/carbon composites using earth-abundant coal as the carbon source,hydrogen reductant and heteroatom dopant to grow CNTs.The prepared composite featuring 3 D structural merits and multiple active sites can efficiently catalyze both ORR and OER,affording high activity,fast kinetics,and long-term stability.With the additional incorporation of manganese,the developed catalyst afforded a potential difference of 0.80 V between ORR at the half wave potential and OER at a current density of 10 mA cm^-2.The optimized sample has presented excellent OER performance within a constructed solar-powered water splitting system with continuously generating oxygen bubbles at anode.Notably,it can be further used as a durable air-electrode catalyst in constructed Zn-air battery,delivering an initial discharge/charge voltage gap of 0.73 V,a remained voltaic efficiency of 61.2%after 160 cycles and capability to power LED light for at least 80 h.This study provides an efficient approach for converting traditional energy resource i.e.coal to value-added alternative oxygen electrocatalysts in renewable energy conversion systems.     

MOF-derived Co-MOF,O-doped carbon as trifunctional electrocatalysts to enable highly efficient Zn-air batteries and water-splitting

Development of high-efficiency non-noble electrocatalysts for oxygen reduction reaction(ORR),oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)is urgently needed for high-performance Zn-air batteries and overall water splitting.Here,a facile strategy to synthesize novel Co-MOF,O-doped carbon(Co-MOF-T)based on Zn,Co-doped glucosamine and ZIF-8 by pyrolysis at temperature T was demonstrated.The prepared Co-MOF-800 showed a superior oxygen reduction reaction(ORR)activity comparable to that of commercial Pt/C catalyst.In addition,this catalyst shows great potential in the overall water splitting due to the excellent oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)activities.Based on the trifunctional activity,the primary Zn-air batteries using a Co-MOF-800 air electrode achieved a high open-circuit voltage of 1.38 V,a specific capacity of 671.6 mAh g^-1 Zn,and a prominent peak power density of 144 mW cm^-2.Also,the rechargeable Zn-air batteries based on CoMOF-800 air electrode could be smoothly run for 510 cycles with a low voltage gap of 0.58 V.Finally,the trifunctional Co-MOF-800 catalyst was applied to boost the electrochemical water splitting,demonstrating its promising potential as a green energy material for practical applications.     

Recent advances in spinel-type electrocatalysts for bifunctional oxygen reduction and oxygen evolution reactions

The demand for efficient and environmentally-benign electrocatalysts that help availably harness the renewable energy resources is growing rapidly. In recent years, increasing insights into the design of water electrolysers, fuel cells, and metal–air batteries emerge in response to the need for developing sustainable energy carriers, in which the oxygen evolution reaction and the oxygen reduction reaction play key roles. However, both reactions suffer from sluggish kinetics that restricts the reactivity. Therefore, it is vital to probe into the structure of the catalysts to exploit high-performance bifunctional oxygen electrocatalysts. Spinel-type catalysts are a class of materials with advantages of versatility, low toxicity, low expense, high abundance, flexible ion arrangement, and multivalence structure. In this review, we afford a basic overview of spinel-type materials and then introduce the relevant theoretical principles for electrocatalytic activity, following that we shed light on the structure–property relationship strategies for spinel-type catalysts including electronic structure, microstructure, phase and composition regulation,and coupling with electrically conductive supports. We elaborate the relationship between structure and property, in order to provide some insights into the design of spinel-type bifunctional oxygen electrocatalysts.     

Hierarchically porous N-doped carbon derived from biomass as oxygen reduction electrocatalyst for high-performance Al–air battery

Developing large-scale and highly efficient oxygen reduction reaction(ORR)catalysts acts a vital role in realizing wide application of metal–air batteries.Here,we propose a gas-foaming strategy to fabricate sustainable and 3D hierarchically porous N-doped carbon with high specific surface area and abundant defects sites derived from biomass.The obtained catalyst exhibits prominent ORR property with higher half-wave potential(0.861 V)and slightly lower kinetic current density(32.44 m A cm^-2),compared to Pt/C(0.856 V and 43.61 m A cm^-1).Furthermore,employing it as catalyst of air cathode,the Al–air battery delivers remarkable discharge performance with excellent power density of 401 m W cm^-2,distinguished energy density of 2453.4 Wh kg^-1 and extremely high open-circuit voltage of 1.85 V among the reported metal–air batteries in the literatures.This gas-foaming strategy for full utilization of biomass affords a chance to explore scalable advanced catalysts in metal–air battery.     

Template-assisted synthesis of hierarchically porous Co_3O_4 with enhanced oxygen evolution activity

Oxygen evolution reaction(OER) is one of the most important reactions in the energy storage devices such as metal–air batteries and unitized regenerative fuel cells(URFCs). However, the kinetically sluggishness of OER and the high prices as well as the scarcity of the most active precious metal electrocatalysts are the major bottleneck in these devices. Developing low-cost non-precious metal catalysts with high activity and stability for OER is highly desirable. A facile, in situ template method combining the dodecyl benzene sulfuric acid sodium(SDBS) assisted hydrothermal process with subsequent high-temperature treatment was developed to prepare porous Co_3O_4 with improved surface area and hierarchical porous structure as precious catalysts alternative for oxygen evolution reaction(OER). Due to the unique structure, the as-prepared catalyst shows higher electrocatalytic activity than Co_3O_4 prepared by traditional thermal-decomposition method(noted as Co_3O_4-T) and commercial IrO_2 catalyst for OER in 0.1M KOH aqueous solution. Moreover, it displays improved stability than Co_3O_4-T. The results demonstrate a highly efficient, scalable, and low cost method for developing highly active and stable OER electrocatalysts in alkaline solutions.     

Fe-doped Co_3O_4@C nanoparticles derived from layered double hydroxide used as efficient electrocatalyst for oxygen evolution reaction

Compared with noble metal catalyst, Co_3O_4-based electrocatalysts have attracted considerable interesting as low-cost alternatives for oxygen evolution reaction(OER). However, the poor electrocatalytic activity still remains a huge challenge. Herein, we demonstrate a feasible approach through oxidation of CoFe layered double hydroxide(CoFe-LDH) to synthesize Fe-doped Co_3O_4@C nanoparticles with size of about30–50 nm. As OER catalyst, the as-synthesized Fe-doped Co_3O_4@C nanoparticles exhibited superior OER performance with a small overpotential of 260 mV at the current density of 20 mA cm~(-2), a small Tafel slope of 70 mV dec~(-1) and long-term durability(there was no obviously OER current density degradation for 100 h) in alkaline solution. The present work opens a new avenue to the exploration of cost-effective and excellent electrocatalysts based on transition metal oxide materials to substitute precious metal materials for water splitting.     

Cobalt porphyrins supported on carbon nanotubes as model catalysts of metal-N_4/C sites for oxygen electrocatalysis

Transition-metal based M-N_4/C catalysts are appealing for electrocatalytic oxygen reduction reaction(ORR) and oxygen evolution reaction(OER). Employing model catalysts, which have well-defined molecular structures and coordination environments, to investigate electrocatalytic performance of M-N_4/C sites for ORR and OER is of fundamental significance. Herein, we reported the use of Co tetra(phenyl)porphyrin 1 and Co tetra(pentafluorophenyl)porphyrin 2 as models to probe the role of Co-N_4/C sites for oxygen electrocatalysis. We showed that Co porphyrin 1 is more efficient than its structural analogue 2 for oxygen electrocatalysis in alkaline aqueous solutions, indicating that the electronrich Co-N_4/C site is more favored when noncovalently adsorbed on carbon supports. This work inspires rational design of reaction-oriented catalysts for sustainable energy storage and conversion technologies.