Hollow cobalt oxide nanoparticles embedded in nitrogen-doped carbon nanosheets as an efficient bifunctional catalyst for Zn–air battery

Rational design of low-cost, highly electrocatalytic activity, and stable bifunctional electrocatalysts for oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) has been a great significant for metal–air batteries. Herein, an efficient bifunctional electrocatalyst based on hollow cobalt oxide nanoparticles embedded in nitrogen-doped carbon nanosheets(Co/N-Pg) is fabricated for Zn–air batteries. A lowcost biomass peach gum, consisting of carbon, oxygen, and hydrogen without other heteroatoms, was used as carbon source to form carbon matrix hosting hollow cobalt oxide nanoparticles. Meanwhile, the melamine was applied as nitrogen source and template precursor, which can convert to carbon-based template graphitic carbon nitride by polycondensation process. Owing to the unique structure and synergistic effect between hollow cobalt oxide nanoparticles and Co-N-C species, the proposal Co/N-Pg catalyst displays not only prominent bifunctional electrocatalytic activities for ORR and OER, but also excellent durability. Remarkably, the assembled Zn–air battery with Co/N-Pg air electrode exhibited a low discharge-charge voltage gap(0.81 V at 50 mA cm~(-2)) and high peak power density(119 mW cm~(-2)) with long-term cycling stability. This work presents an effective approach for engineering transition metal oxides and nitrogen modified carbon nanosheets to boost the performance of bifunctional electrocatalysts for Zn–air battery.     

Cu/Cu_2O nanoparticles co-regulated carbon catalyst for alkaline Al-air batteries

Developing high-efficiency,inexpensive,and steady non-precious metal oxygen reduction reaction(ORR) catalysts to displace Pt-based catalysts is significant for commercial applications of Al-air battery.Here,we have prepared the Cu/Cu_2 O-NC catalyst with excellent ORR performance and high stability,due to the synergistic effect of Cu and Cu_2 O nanoparticles.The half-wave potential(0.8 V) and the limiting-current density(5.20 mA/cm~2) of the Cu/Cu_2 O-NC are very close to those of the 20% Pt/C catalyst(0.82 V,5.10 mA/cm~2).Besides,it exhibits excellent performance with a maximal power density of 250 mW/cm~2 and a stable continuous discharge for more than 90 h in the Al-air battery test The promoting effects of Cu_2 O towards Cu-based ORR catalysts are illustrated as follows:(ⅰ) Cu_2 O is the major ORR active site by the redox of Cu(Ⅱ)/Cu(Ⅰ),which provides excellent ORR activities;(ⅱ) Cu can stabilize the location of Cu_2 O by assisting the electron transfer to Cu(Ⅱ)/Cu(Ⅰ) redox,which is conducive to the high stability of the catalyst.This work provides a useful strategy for enhancing the ORR performance of Cu-based catalysts.     

Boost oxygen reduction reaction performance by tuning the active sites in Fe-N-P-C catalysts

Cost-effective atomically dispersed Fe-N-P-C complex catalysts are promising to catalyze the oxygen reduction reaction(ORR)and replace Pt catalysts in fuel cells and metal-air batteries.However,it remains a challenge to increase the number of atomically dispersed active sites on these catalysts.Here we report a highly efficient impregnation-pyrolysis method to prepare effective ORR electrocatalysts with large amount of atomically dispersed Fe active sites from biomass.Two types of active catalyst centers were identified,namely atomically dispersed Fe sites and Fe_xP particles.The ORR rate of the atomically dispersed Fe sites is three orders of magnitude higher than it of Fe_xP particles.A linear correlation between the amount of the atomically dispersed Fe and the ORR activity was obtained,revealing the major contribution of the atomically dispersed Fe to the ORR activity.The number of atomically dispersed Fe increases as the Fe loading increased and reaching the maximum at 1.86 wt%Fe,resulting in the maximum ORR rate.Optimized Fe-N-P-C complex catalyst was used as the cathode catalyst in a homemade Zn-air battery and good performance of an energy density of 771 Wh kgZn^-1,a power density of 92.9 m W cm^-2 at 137 m A cm^-2 and an excellent durability were exhibited.     

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.     

Highly efficient iron–nitrogen electrocatalyst derived from covalent organic polymer for oxygen reduction

Developing non-precious metal catalyst with high activity, good stability and low cost for electrocatalytic oxygen reduction reaction(ORR) is critical for the wide application of energy conversion system. Here, we developed a cost–effective synthetic strategy via silica assistance to obtain a novel Fe_3C/Fe–N_x–C(named as COPBP-PB-Fe-900-SiO_2) catalyst with effective active sites of Fe–N_xand Fe_3C from the rational design two-dimensional covalent organic polymer(COPBP-PB). The nitrogen-rich COP effectively promotes the formation of active Fe–N_x sites. Additionally, the silica not only can effectively suppress the formation of large Fe-based particles in the catalysts, but also increases the degree of carbonization of the catalyst.The as-prepared COPBP-PB-Fe-900-SiO_2 catalyst exhibits high electrocatalytic activity for ORR with a halfwave potential of 0.85 V vs. reversible hydrogen electrode(RHE), showing comparable activity as compared with the commercial Pt/C catalysts in alkaline media. Moreover, this catalyst also shows a high stability with a nearly constant onset potential and half-wave potential after 10,000 cycles. The present work is highly meaningful for developing ORR electrocatalysts toward wide applications.     

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.     

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.     

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.     

Characterization of Fe/N-doped graphene as air-cathode catalyst in microbial fuel cells

This work proposed a simple and efficient approach for synthesis of durable and efficient non-precious metal oxygen reduction reaction(ORR) electro-catalysts in MFCs. The rod-like carbon nanotubes(CNTs)were formed on the Fe–N/SLG sheets after a carbonization process. The maximum power density of1210 ± 23 m W·m~(-2) obtained with Fe–N/SLG catalyst in an MFC was 10.7% higher than that of Pt/C catalyst(1080 ± 20 mW ·m~(-2)) under the same condition. The results of RDE test show that the ORR electron transfer number of Fe–N/SLG was 3.91 ± 0.02, which suggested that ORR catalysis proceeds through a four-electron pathway. The whole time of the synthesis of electro-catalysts is about 10 h, making the research take a solid step in the MFC expansion due to its low-cost, high efficiency and favorable electrochemical performance. Besides, we compared the electrochemical properties of catalysts using SLG, high conductivity graphene(HCG, a kind of multilayer graphene) and high activity graphene(HAG, a kind of GO) under the same conditions, providing a solution for optimal selection of cathode catalyst in MFCs.The morphology, crystalline structure, elemental composition and ORR activity of these three kinds of Fe–N/C catalysts were characterized. Their ORR activities were compared with commercial Pt/C catalyst.It demonstrates that this kind of Fe–N/SLG can be a type of promising highly efficient catalyst and could enhance ORR performance of MFCs.     

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.     

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.     

Red-blood-cell like nitrogen-doped carbons with highly catalytic activity towards oxygen reduction reaction

A highly active nitrogen-doped catalyst with a unique red-blood-cell(RBC) like structure is reported for oxygen reduction reaction(ORR).The catalyst Fe,N-C@carbon-900 was prepared by pyrolysis of the polyaniline(PANl) and polystyrene(PS) composites with adsorption of ferric ion on the shell of sphere structure at 900℃.Fe,N-C@carbon-900 with a unique RBC-like structure provides plenty of catalytic sites combining the electrical conductivity of the carbon sphere with the catalytic activity of the nitrogen-doped layer.The four-electron reduction pathway is selected for the catalyst Fe,N-C@carbon-900.The catalyst exhibit the ORR E_(onset) at 0.87 V(potentials is versus to reversible hydrogen electrode(RHE)),E_(1/2) at 0.78 V and high diffusion-limiting current density(5.20mA/cm~2).Furthermore,this work indicates that both N and Fe accounted for high activity of the catalyst Fe,N-C@carbon-900 toward the oxygen reduction process.It is concluded that Fe and N exhibit synergistically promotion in the ORR activity for the catalyst Fe,N-C@carbon-900.We also provide a rational design of electrocatalysts with high ORR activity to further clarify the essential ORR sites of heteroatom doped carbon materials for fuel cells and metal-air battery applications.     

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.     

3D star-like atypical hybrid MOF derived single-atom catalyst boosts oxygen reduction catalysis

Developing high-efficiency,stable and non-precious electrocatalysts for oxygen reduction reaction(ORR)is highly important for energy conversion and storage.Single atom catalysts(SACs)show good potential in enhancing ORR,however,the specifical control over the coordination surroundings around single metal center to intrinsically modify the electron structure is still a great challenge.Herein,we demonstrate that a 3 D hybrid MOF composed of cobalt doped ZIF-L and ZIF-8,featuring star morphology with six equal branches,can be used as an advanced precursor for making the Co SACs for greatly boosted ORR.The as-synthesized Co_SA-N-C exhibits excellent ORR activity with E_1/2 of 0.891 V in alkaline medium,outperforming the commercial Pt/C by 39 m V.Moreover,the E_1/2 of Co_SA-N-C(0.790 V)is merely 15 m V,less than that of Pt/C(0.805 V)in acid medium,which is among the best in the reported state-of-the-art SACs.DFT calculations demonstrate that the enhanced ORR performance is assigned to the formation of atomically isolated cobalt atom coordinated three N atoms and one C atom,which is easier to decrease the free energy of rate determining step and accelerate the ORR process than that of traditional cobalt atom coordinated four N atoms.In addition,a primary Zn-air battery with Co_SA-N-C cathode reveals a maximum power density of 92.2 m W cm^-2 at 120.0 m A cm^-2,far higher than that of commercial catalysts(74.2 m W cm^-2 at 110.0 m A cm^-2).     

Enhanced confinement synthesis of atomically dispersed Fe-N-C catalyst from resin polymer for oxygen reduction

Due to larger atom utilization,unique electronic properties and unsaturated coordination,atomically dispersed non-precious metal catalysts with outstanding performances have received great attention in electrocatalysis.Considering the challenge of serious aggregation,rational synthesis of an atomic catalyst with good dispersion of atoms is paramount to the development of these catalysts.Herein,we report an enhanced confinement strategy to synthesize a catalyst comprised of atomically dispersed Fe supported on porous nitrogen-doped graphitic carbon from the novel and more cross-linkable Melamine-Glyoxal Resin.Densified isolated grid trapping,excessive melamine restricting,and nitrogen anchoring are strongly combined to ensure the final atomic-level dispersion of metal atoms.Experimental studies revealed enhanced kinetics of the obtained catalyst towards oxygen reduction reaction(ORR).This catalytic activity originates from the highly active surface with atomically dispersed iron sites as well as the multi-level three-dimensional structure with fast mass and electron transfer.The enhanced confinement strategy endows the resin-derived atomic catalyst with a great prospect to develop for commercialization in future.     

Porous carbon coupled with an interlaced MoP–MoS2 heterojunction hybrid for efficient hydrogen evolution reaction

The design and development of electrocatalysts composed of non-noble-metal catalysts with both large surface area and high electrical conductivities are crucial for the hydrogen evolution reaction(HER).Here,a xylose-based porous carbon is coupled with a MoS2-Mo P heterojunction(MoS2-Mo P/FPC)hybrid and used as a promising catalyst for HER.The hybrid is prepared by immobilizing petal-like MoS2 nanosheets on porous carbon(MoS2/FPC),followed by controlling the phosphidation in Ar/H2 to form MoS2-Mo P/FPC.Red phosphorus provides the P species that can induce the construction of the heterojunction under the reducing atmosphere,along with the generation of a Mo P phase and the splitting of the MoS2 phase.The as-prepared MoS2-Mo P/FPC catalyst offers a low overpotential of 144 mV at a current density of 10 m A cm^-2 and a small Tafel slope of 41 m V dec^-1 for the HER in acidic media,as well as remarkable stability.Apart from the active nature of the hybrid,its outstanding activity is attributed to the MoS2-Mo P heterojunction,and the good charge/mass-transfer ability of porous carbon.This strategy provides a new method to develop and design low-cost and high-performance catalysts for the HER.     

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.     

Recent progress in fluorinated electrolytes for improving the performance of Li–S batteries

Lithium–sulfur(Li–S) batteries represent a "beyond Li-ion" technology with low cost and high theoretical energy density and should fulfill the ever-growing requirements of electric vehicles and stationary energy storage systems. However, the sulfur-based conversion reaction in conventional liquid electrolytes results in issues like the so-called shuttle effect of polysulfides and lithium dendrite growth, which deteriorate the electrochemical performance and safety of Li–S batteries. Optimization of conventional organic solvents(including ether and carbonate) by fluorination to form fluorinated electrolytes is a promising strategy for the practical application of Li–S batteries. The fluorinated electrolytes, owing to the high electronegativity of fluorine, possesses attractive physicochemical properties, including low melting point,high flash point, and low solubility of lithium polysulfide, and can form a compact and stable solid electrolyte interphase(SEI) with the lithium metal anode. Herein, we review recent advancements in the development of fluorinated electrolytes for use in Li–S batteries. The effect of solvent molecular structure on the performance of Li–S batteries and the formation mechanism of SEI on the cathode and anode sides are analyzed and discussed in detail. The remaining challenges and future perspectives of fluorinated electrolytes for Li–S batteries are also presented.     

Li–air batteries:air stability of lithium metal anodes

Aprotic rechargeable lithium–air batteries(LABs) with an ultrahigh theoretical energy density(3,500 Wh kg^-1) are known as the‘holy grail’ of energy storage systems and could replace Li-ion batteries as the next-generation high-capacity batteries if a practical device could be realized. However, only a few researches focus on the battery performance and reactions in the ambient air environment, which is a major obstacle to promote the practical application of LABs. Here, we have summar...     

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.