Rechargeable Metal-air Batteries Research Articles

Robust InNCo3–xMnx Nitride-Supported Pt Nanoparticles as High-Performance Bifunctional Electrocatalysts for Zn–Air Batteries

Electrocatalysts at the air cathode play a vital role in determing the cycle life of rechargeable metal-air batteries. Herein, we demonstratred a highly durable and efficient nitride-based bifuncti...

(Invited) Nanocarbon Catalysts for Reversible Energy Storage and Conversion Via Oxygen Electrocatalysis

Energy conversion and storage via direct electrochemical oxidation and reduction processes, such as the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), can provide a clean, efficient, and economically viable means of meeting this sustainability challenge, by allowing stored renewable energy to be reliably delivered where and when it is needed. During the development of the highly demanded nonprecious metal catalysts to replace precious metals for these applications, carbon catalysts have many advantages over metal oxides, including excellent electrical conductivity, high surface area, and easy functionalization. However, most carbon-based catalysts suffer from poor activity and stability for the more challenging OER under a highly oxidative conditions. This represents one of grand challenges in energy conversion research societies.For the first time, this work demonstrated that a new class of large-diameter tubular nanocarbon (>500 nm in diameter) catalyst is capable of catalyzing both the ORR and the OER with activity comparable to Pt and exceeding Ir, respectively, for these two reactions. Most importantly, the tube catalyst exhibited good stability upon cycling across a wide potential window (0 to 1.9 V vs RHE) in 0.1 M NaOH electrolyte. The high OER activity was verified using well-designed electrochemical tests, demonstrating that the current generated during the OER arose solely from oxygen evolution, rather than carbon oxidation. This is the first example of such excellent OER activity and durability of a nanostructured carbon-based electrocatalyst across the complete ORR-OER potential window. The newly developed carbon tubes were synthesized via a one-step template-free graphitization process using inexpensive dicyandiamide (DCDA) precursor as carbon/nitrogen source and a ternary FeCoNi alloy as a catalyst. Generation of a mixed metal catalyst, i.e., FeCoNi during the graphitization process is the key factor to yield the largest tube size for maximum electrochemically accessible surface areas, dominant graphitic nitrogen doping, and thicker tube walls, which are associated with the significantly improved activity and durability. The cost effective, easily scalable, single step and environmentally benign synthesis procedure further favors this nanocarbon’s application as a practical bifunctional electrocatalyst for the ORR and OER, with potential to advance the development and commercialization of regenerative fuel cell systems and rechargeable metal-air batteries. The use of nanostructured carbon as bifunctional ORR/OER catalysts will open up new research direction in this field and further advance the role of carbon nanomaterials in oxygen electrocatalysis for energy conversion and storage.

(Invited) Modified Oxide Electrocatalysts for Renewable Fuels and Energy

In this talk I will discuss research to develop and understand modified oxide electrocatalysts for renewable fuels and energy. In the first part, I will focus on catalysts for the oxygen evolution reaction (OER). An important challenge for realizing cheap, stable, and efficient electrocatalysts for producing renewable fuels and energy is to enable transition metal oxides (TMOs) to replace precious metal-based electrocatalysts (e.g. IrOx and RuOx) for OER. Low energy efficiencies of OER resulting from sluggish reaction kinetics and large overpotentials is a major cause of energy losses in photocatalytic systems for solar fuels and in a number of other emergent technologies such as water splitting devices, rechargeable metal-air batteries, and unitized regenerative fuel cells. Insights from two examples of our studies will be discussed, involving a range of spectroscopic techniques for characterization of Ce-modified copper oxide (CuOx) and Ni-modified cobalt (oxy)hydroxides to reveal the importance of chemical state and local structure considerations for the rational design of improved oxide-based OER catalysts. In the case of Ce-modified CuOx, a strong promoting effect of Ce4+ for OER was observed for Ce incorporation (7 at%) into CuOx up to a concentration corresponding to CeO2 phase segregation, as probed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. In the case of Ni-modified CoOxHy, operando Raman spectroscopy and electrochemical techniques were used during OER to identify the composition and local structure of electrodeposited CoOxHy and Ni-modified CoOxHy catalyst films. During OER, Ni-modified CoOxHy films with lower initial crystallinity underwent substantial structural evolution that began with an irreversible transformation of a spinel local structure to an amorphous CoO structure at low anodic potentials. Increasing anodic polarization with elevated oxygen evolution rates caused additional structural conversion of the amorphous CoO structure to a complex phase that can be described as an amalgamation of NiOOH and layered CoO2 motifs (NiOOH-h-CoO2). Formation of this structure, independent of the initial cobalt oxide structure, suggests that this could be the universally active structure for NiCoOxHy catalysts. I will also discuss recent work on phosphorus-doped Hf-Ru oxide (denoted here as HfxRuyPO) electrocatalysts for OER in which we demonstrated a strategy for efficiently promoting the durability of RuO2. This research was motivated by the need to address the rapid corrosion and precipitation of highly efficient precious metal (Ir, Ru, and Rh) nanoparticle catalysts in the harsh environment of OER in proton exchange membrane (PEM) electrolyzers for hydrogen fuel generation. We found that when the HfxRuyPO catalysts possessed a Hf:Ru atomic ratio of 1:2, they exhibited a highly efficient and stable OER activity over 40 hours. By XPS and Raman analyses, the HfPO component was found to play a significant role in improving the durability of RuO2 through the formation of a HfOOH phase, which also possessed a higher OER activity and lower impedance.In the second part of the talk, I will briefly describe our report of an efficient acid-stable N2-plasma treated hafnium oxyhydroxide electrocatalyst for hydrogen evolution and oxidation reactions (HER and HOR). We found that processing Hf oxide with an atmospheric N2 plasma forms an acid-insoluble hafnium oxynitride material, and we propose that under electrochemical environments this material is transformed into an active oxynitride hydroxide that demonstrates unprecedented high catalytic activity and stability for both HER and HOR in strong acidic media for earth-abundant materials. The zero onset potentials and high current densities demonstrate that this material is a promising alternative to Pt group metal (PGM) catalysts, and these results have broad implications for using nitrogen incorporation to activate other non-conductive compounds and films to form new active electrocatalysts. I will also discuss our recent observations on the reaction-driven restructuring of defective PtSe2 into an ultrastable electrocatalyst for the oxygen reduction reaction (ORR), which is of interest for accelerating the commercialization of PEM fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). We have found that the restructured defective PtSe2 electrocatalyst had 1.3 times the specific activity and 2.6 times the mass activity of a commercial Pt/C catalyst, and maintained this superior ORR activity even after 126,000 cycles in accelerated durability tests.

Rapid microwave-assisted preparation of high-performance bifunctional Ni3Fe/Co-N-C for rechargeable Zn-air battery

Precious metal-free bifunctional catalysts offer scalable possibilities for achieving oxygen cathodes in rechargeable metal-air batteries. Literature thus far suggests that the oxygen reduction and evolution reactions (i.e ORR and OER) take place at different active sites. However, the design and preparation of oxygen electrocatalysts with high-performance is still a great challenge. This work develops a general approach to introduce a nanoparticle that offers oxygen evolution reaction sites onto an oxygen reduction reaction catalyst via rapid microwave treatment. The metal–organic frameworks (Co, ZIF-8) derived Co-N-C with high ORR performance is employed as a support to homogeneously disperse OER active Ni3Fe nanoparticles. Potentials of 1.54 V and 0.79 V have been achieved at current densities of 10 mA cm−2 for OER and 3 mA cm−2 toward ORR, respectively. The developed rechargeable Zn-air battery assembled with Ni3Fe/Co-N-C as an oxygen-electrode exhibits superior efficiency and robust durability, exceeding that of commercial Pt/C + IrO2. Furthermore, the satisfactory activity of our fabricated wristband shows the successfully practical application of flexible Zn-air batteries for wearable devices.

Self-Assembled Ruddlesden-Popper/Perovskite Hybrid with Lattice-Oxygen Activation as a Superior Oxygen Evolution Electrocatalyst.

The oxygen evolution reaction (OER) is pivotal in multiple gas-involved energy conversion technologies, such as water splitting, rechargeable metal-air batteries, and CO2 /N2 electrolysis. Emerging anion-redox chemistry provides exciting opportunities for boosting catalytic activity, and thus mastering lattice-oxygen activation of metal oxides and identifying the origins are crucial for the development of advanced catalysts. Here, a strategy to activate surface lattice-oxygen sites for OER catalysis via constructing a Ruddlesden-Popper/perovskite hybrid, which is prepared by a facile one-pot self-assembly method, is developed. As a proof-of-concept, the unique hybrid catalyst (RP/P-LSCF) consists of a dominated Ruddlesden-Popper phase LaSr3 Co1.5 Fe1.5 O10-δ (RP-LSCF) and second perovskite phase La0.25 Sr0.75 Co0.5 Fe0.5 O3-δ (P-LSCF), displaying exceptional OER activity. The RP/P-LSCF achieves 10 mA cm-2 at a low overpotential of only 324 mV in 0.1 m KOH, surpassing the benchmark RuO2 and various state-of-the-art metal oxides ever reported for OER, while showing significantly higher activity and stability than single RP-LSCF oxide. The high catalytic performance for RP/P-LSCF is attributed to the strong metal-oxygen covalency and high oxygen-ion diffusion rate resulting from the phase mixture, which likely triggers the surface lattice-oxygen activation to participate in OER. The success of Ruddlesden-Popper/perovskite hybrid construction creates a new direction to design advanced catalysts for various energy applications.

One-step In-situ Synthesis of Vacancy-rich CoFe2O4@Defective Graphene Hybrids as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries

Developing efficient catalysts toward both oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) is the core task for rechargeable metal-air batteries. Although integration of two active components should be an effective method to produce the bifunctional catalysts in principle, traditional techniques still can not attain fine tunable surface structure during material-hybridization process. Herein, we present a facile short-time in-situ argon(Ar) plasma strategy to fabricate a high-performance bifunctional hybrid catalyst of vacancy-rich CoFe2O4 synergized with defective graphene(r-CoFe2O4@DG). Reflected by the low voltage gap of 0.79 V in two half-reaction measurements, the striking capability to catalyze ORR/OER endows it excellent and durable performance in rechargeable Zn-air batteries, with a maximum power density of 155.2 mW/cm2 and robust stability(up to 60 h). Further experimental and theoretical studies validate its remarkable bifunctional energetics root from plasma-induced surface vacancy defects and interfacial charge polarization between DG and CoFe2O4. This work offers more opportunities for reliable clean energy systems by rational interfacial and defect engineering on catalyst design.

Atomic species derived CoOx clusters on nitrogen doped mesoporous carbon as advanced bifunctional electro-catalysts for Zn-air battery

The development of earth-abundant and cost-effective electro-catalysts at the air cathodes for both oxygen reduction and evolution reactions (ORR and OER) with sluggish kinetics are central to the overall performance of rechargeable metal-air batteries, remaining an issue that is gaining increasing interest in recent years. Herein, we report a nitrogen-doped mesoporous carbon supported clustered CoOx composite catalyst (CoOx/NMC) synthesized from the anchored atomically dispersed CoOOH species by an efficient two-step nucleation method. Due to the synergistic and size effects, CoOx/NMC exhibits an enhanced bifunctional electrocatalytic performance in alkaline media than the state of the art commercial Pt/C catalyst for ORR and the OER catalyst (Ir/C). More importantly, the advanced CoOx/NMC catalyst enables recharge Zn-air batteries (ZABs) to achieve long-term cycling performance over 400 ​h by galvanostatic charge-discharge at current density of 10 ​mA ​cm−2 with high efficiency, suggesting a very promising alternative to the conventional Pt/C and Ir/C catalysts for an air cathode.

Triboelectric nanogenerators powered electrodepositing tri-functional electrocatalysts for water splitting and rechargeable zinc-air battery: A case of Pt nanoclusters on NiFe-LDH nanosheets

Abstract Designing highly efficient electrocatalysts with superior performance on electrochemical water splitting and rechargeable metal-air batteries is an urgent and challenging task due to required large overpotentials. A pulsed direct current with high voltage produced by a layered triboelectric nanogenerator (TENG) provides a novel type of power sources to electrodeposit sub-2 nm Pt nanoclusters onto NiFe-LDH nanosheets in the absence of any capping reagents to improve hydrogen evolution reaction (HER) activity and to electronically reduce part of Fe3+ cations in the lattice of NiFe-LDH to Fe2+ species to enhance oxygen evolution reaction (OER) activity. The particle size of Pt nanoclusters could be easily tuned from 0.8 nm to 1.2 nm by alternating the working frequency of the layered TENG. By loading sub-2 nm Pt nanoclusters onto NiFe-LDH (Pt–NiFe-LDH), the HER overpotential drops from 345 mV to 86 mV at a current density of 50 mA cm−2 in alkaline electrolyte. The synergistic effect between Pt nanoclusters and NiFe-LDH nanosheets on enhancing the cleavage of HO–H bond and recombination of hydrogen intermediates to form molecular hydrogen dramatically improves the HER activity. Meanwhile, benefiting from tuning the local atomic structure and crystal defects by reducing Fe3+ to Fe2+, NiFe-LDH nanosheets demonstrates enhanced OER activity. The deposited sub-2 nm Pt nanoclusters also produces elevated oxygen reduction reaction (ORR) activity with a half-wave potential of 0.84 V (vs RHE). Hence, the as-prepared Pt–NiFe-LDH catalysts exhibits tri-functional properties towards HER, ORR and OER and could be used as the electrode catalysts for overall electrocatalytic water splitting and rechargeable zinc-air batteries. With the optimal catalyst (Pt–NiFe-LDH-0.5-12) as the electrode for overall water splitting, the potential difference between OER and HER drops to 1.63 V at a current density of 50 mA cm−2, much lower than that of the mixed noble-metal catalysts (Pt/C and RuO2, 1.98 V). As an air electrode of rechargeable zinc-air batteries, Pt–NiFe-LDH-0.5-12 exhibits much better performance than that of bare NiFe-LDH in terms of higher open-circuit voltage, higher round-trip efficiency and durability.

Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future.

The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO2 emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O2 cells but include Na-O2, K-O2, and Mg-O2 cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O2 reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O2 cells.

Flower-like NiCo2O4–CN as efficient bifunctional electrocatalyst for Zn-Air battery

Bifunctional transition metal oxide based electrocatalysts play an important role in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for rechargeable metal-air batteries. Herein, we synthesize a kind of three-dimensional (3D) flower-like composite catalyst with NiCo2O4 and N doped carbon (NC) derived from g-C3N4, under hydrothermal conditions. The best NiCo2O4–CN catalyst shows a hierarchical structure composed of interconnected nanosheets, and exhibits versatile bifunctional ORR/OER electroactivities with a half-wave potential of 0.81 V for ORR and an overpotential of 383 mV for OER under a current density of 10 mA cm−2. The zinc-air battery with the NiCo2O4–CN cathode demonstrates superior peak power density (149.6 mW cm−2), excellent capacity performance up to 738.7 mAh g−1 and considerable durability, showing the feasibility of NiCo2O4–CN in electrochemical energy devices.

Enhancement of oxygen evolution activity of perovskite (La0.8Sr0.2)0.95MnO3-δ electrode by Co phase surface modification

Rechargeable metal-air battery and regenerative fuel cells are highly demanded given their high energy density and sustainable nature. Both of them require robust oxygen electrocatalysts improving electrical efficiency. In this work, we use cobalt compounds to modify (La, Sr)MnO3 (LSM) by a hydrothermal method to ameliorate the oxygen evolution reaction (OER) performance. The effects of loading amounts and phase-microstructure of the cobalt phase by adjusting the calcination temperature are systematically investigated. Much improved OER geometrical, specific and mass activities over its individual phase, an onset voltage of 1.38 V and the lowest voltage of 1.68 V at 10 mA cm−2 that are among the best values based on LSM matrix electrocatalysts in the open literature, are obtained. The increased surface oxygen vacancy concentration, water absorption capability, and the modulated surface cations’ electronic structure are ascribed to be the source of the significantly improved electrochemical properties. LSM-Co-RT-12 also showed good durability in constant voltage testing for 10 h. Therefore, integrating with the intrinsically high oxygen reduction activity, the LSM-Co hybrid catalyst possesses promise for bi-functionality oxygen electrocatalysis. This work also highlights the surface modification is an effective method to modify the intrinsic activity of OER inert perovskite materials for the multifunctional application.

An Efficient Electrocatalyst for Oxygen Evolution Reaction in Alkaline Solutions Derived from a Copper Chelate Polymer via In Situ Electrochemical Transformation

Efficient oxygen evolution reaction (OER) electrocatalysts are highly desired in the field of water electrolysis and rechargeable metal-air batteries. In this study, a chelate polymer, composed of copper (II) and dithiooxamide, was used to derive an efficient catalytic system for OER. Upon potential sweep in 1 M KOH, copper (II) centers of the chelate polymer were transformed to CuO and Cu(OH)2. The carbon-dispersed CuO nanostructures formed a nanocomposite which exhibits an enhanced catalytic activity for OER in alkaline media. The nanocomposite catalyst has an overpotential of 280 mV (at 1 mA/cm2) and a Tafel slope of 81 mV/dec in 1M KOH solution. It has a seven-fold higher current than an IrO2/C electrode, per metal loading. A catalytic cycle is proposed, in which CuO undergoes electrooxidation to Cu2O3 that further decomposes to CuO with the release of oxygen. This work reveals a new method to produce an active nanocomposite catalyst for OER in alkaline media using a non-noble metal chelate polymer and a porous carbon. This method can be applied to the synthesis of transition metal oxide nanoparticles used in the preparation of composite electrodes for water electrolyzers and can be used to derive cathode materials for aqueous-type metal-air batteries.

Tuning cobalt eg occupation of Co-NCNT by manipulation of crystallinity facilitates more efficient oxygen evolution and reduction

Co encapsulated in N-doped carbon nanotubes (Co-NCNT) catalysts are of high interest as bifunctional electrocatalyst material for both efficient oxygen evolution and reduction (OER/ORR) in applications of rechargeable metal-air batteries. Up to now, the role played by the functional metal species in OER/ORR is still insufficiently understood. The main focus of our research is to shed light on the mechanistic role of the Co species that serve as active sites in the bi-functional Co-NCNT catalysts. It is found that S700 exhibits an outstanding OER/ORR activity. We thus hypothesize that CoII and CoIII clusters predominately function as active sites in the OER and ORR processes, respectively. Furthermore, OER/ORR activity for Co-NCNT catalyst primarily correlates to eg occupation. A near-unity occupancy of the eg orbital of S700 is revealed to be the cause for the maximum intrinsic OER/ORR activity, which provides guidelines for the design of highly active catalysts.

In-situ formation of N doped hollow graphene Nanospheres/CNTs architecture with encapsulated Fe3C@C nanoparticles as efficient bifunctional oxygen electrocatalysts

The exploration of cost-effective and efficient bifunctional catalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is currently a critical obstacle in the development of regenerative fuel cells and rechargeable metal-air batteries. In this reported study, we detail the synthesis of an N-doped hollow graphene nanospheres/CNTs architecture with encapsulated Fe3C@C nanoparticles (Fe3C@C-NGns-NCNTs) via a facile, in situ fabrication method. The resulting catalyst had a high specific surface area (450 m2 g−1) with a hierarchical meso-macroporous structure. In an alkaline electrolyte, the catalyst manifested remarkable bifunctional catalytic activity with a half-wave potential for ORR of ca. 23 mV that was higher than that produced by a commercial Pt/C and the former had a small overpotential of 412 mV at a current density of 10 mA cm−2 for the OER. The ΔE (oxygen electrode activity parameter) value for the Fe3C@C-NGns-NCNTs was only 0.787 V. We posit that the high electrocatalytic performance of this catalyst was due to the following factors: (1) a synergistic effect, induced by the presence of multiple types of active sites, including Fe3C@C nanoparticle, nitrogen dopant and possible Fe-Nx-C sites; (2) the unique hollow graphene nanospheres/CNTs architecture, which facilitated the adsorption/diffusion of reactants/products and rapid electron transfer; and (3) the high degree of graphitization of the catalyst that improved the electrocatalytic stability.

Non-3d Metal Modulation of a 2D Ni-Co Heterostructure Array as Multifunctional Electrocatalyst for Portable Overall Water Splitting.

Portable water splitting devices driven by rechargeable metal-air batteries or solar cells are promising, however, their scalable usages are still hindered by lack of suitable multifunctional electrocatalysts. Here, a highly efficient multifunctional electrocatalyst is demonstrated, i.e., 2D nanosheet array of Mo-doped NiCo2 O4 /Co5.47 N heterostructure deposited on nickel foam (Mo-NiCo2 O4 /Co5.47 N/NF). The successful doping of non-3d high-valence metal into a heterostructured nanosheet array, which is directly grown on a conductive substrate endows the resultant catalyst with balanced electronic structure, highly exposed active sites, and binder-free electrode architecture. As a result, the Mo-NiCo2 O4 /Co5.47 N/NF exhibits remarkable catalytic activity toward the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), affording high current densities of 50 mA cm-2 at low overpotentials of 310 mV for OER, and 170 mV for HER, respectively. Moreover, a low voltage of 1.56 V is achieved for the Mo-NiCo2 O4 /Co5.47 N/NF-based water splitting cell to reach 10 mA cm-2 . More importantly, a portable overall water splitting device is demonstrated through the integration of a water-splitting cell and two Zn-air batteries (open-circuit voltage of 1.43 V), which are all fabricated based on Mo-NiCo2 O4 /Co5.47 N/NF, demonstrating a low-cost way to generate fuel energy. This work offers an effective strategy to develop high-performance metal-doped heterostructured electrode.

Simultaneously Integrating Single Atomic Cobalt Sites and Co9 S8 Nanoparticles into Hollow Carbon Nanotubes as Trifunctional Electrocatalysts for Zn-Air Batteries to Drive Water Splitting.

The development of rechargeable metal-air batteries and water electrolyzers are highly constrained by electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). However, the construction of efficient trifunctional electrocatalysts for ORR/OER/HER are highly desirable yet challenging. Herein, hollow carbon nanotubes integrated single cobalt atoms with Co9 S8 nanoparticles (CoSA + Co9 S8 /HCNT) are fabricated by a straightforward in situ self-sacrificing strategy. The structure of the CoSA + Co9 S8 /HCNT are verified by X-ray absorption spectroscopy and aberration-corrected scanning transmission electron microscopy. Theoretical calculations and experimental results embrace the synergistic effects between Co9 S8 nanoparticles and single cobalt atoms through optimizing the electronic configuration of the CoN4 active sites to lower the reaction barrier and facilitating the ORR, OER, and HER simultaneously. Consequently, rechargeable liquid and all-solid-state flexible Zn-air batteries based on CoSA + Co9 S8 /HCNT exhibit remarkable stability and excellent power density of 177.33 and 51.85 mW cm-2 , respectively, better than Pt/C + RuO2 counterparts. Moreover, the as-fabricated Zn-air batteries can drive an overall water splitting device assembled with CoSA + Co9 S8 /HCNT and achieve a current density of 10 mA cm-2 at a low voltage of 1.59 V, also superior to Pt/C + RuO2 . Therefore, this work presents a promising approach to an efficient trifunctional electrocatalyst toward practical applications.

N-doped porous carbon hollow microspheres encapsulated with iron-based nanocomposites as advanced bifunctional catalysts for rechargeable Zn-air battery

The design and development of low-cost, efficient, and stable bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are desirable for rechargeable metal-air batteries. In this work, N-doped porous hollow carbon spheres encapsulated with ultrafine Fe/Fe 3 O 4 nanoparticles (FeO x @N-PHCS) were fabricated by impregnation and subsequent pyrolysis, using melamine-formaldehyde resin spheres as self-sacrifice templates and polydopamine as N and C sources. The sufficient adsorption of Fe 3+ on the polydopamine endowed the formation of Fe-N x species upon high-temperature carbonization. The prepared FeO x @N-PHCS has advanced features of large specific surface area, porous hollow structure, high content of N dopants, sufficient Fe-N x species and ultrafine FeO x nanoparticles. These features endow FeO x @N-PHCS with enhanced mass transfer and considerable active sites, leading to high activity and stability in catalyzing ORR and OER in alkaline electrolyte. Furthermore, the rechargeable Zn-air battery with FeO x @N-PHCS as air cathode catalyst exhibits a large peak power density, narrow charge-discharge potential gap and robust cycling stability, demonstrating the potential of the fabricated FeO x @N-PHCS as a promising electrode material for metal-air batteries. This new finding may open an avenue for rational design of bifunctional catalysts by integrating different active components within all-in-one catalyst for different electrochemical reactions. N-doped porous hollow microspheres with encapsulated Fe-based nanocomposite show impressive activity and stability for ORR and OER, capable of serving as efficient cathode catalysts for the rechargeable Zn-air batteries.

Multifunctional electrocatalytic activity of coronene-based two-dimensional metal-organic frameworks: TM-PTC

Integrating high efficiency hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in multifunctional electrocatalysts is attractive for the unitized regenerative fuel cells and overall water splitting technologies. We demonstrated from first-principles calculations the recently- synthesized two-dimensional (2D) metal-organic frameworks (MOFs), namely transitionmetal 1,2,3,4,5,6,7,8,9,10,11,12- perthiolated coronene (TM-PTC) have multifunctional electrocatalytic activity for HER, OER and ORR. In particular, Fe-PTC monolayer can serve as a trifunctional electrocatalyst for overall water splitting and rechargeable metal-air battery with low overpotentials comparable to those of the well-developed unifunctional electrocatalysts. The origins of the trifunctional electrocatalytic activity were also discussed. This work is expected to open an avenue for the design of highly-efficient multifunctional electrocatalysts.

Synthesis of Bimetal Doped Metal-Organic Framework (MOF-5): An Electrocatalyst with Low Noble Metal Content and High Electrochemical Activity

The challenge to developing a rechargeable metal-air battery is designing and developing an oxygen reduction reaction (ORR) electrocatalyst that is reliable, low cost, and high-performance. Herein, a novel approach for the synthesis of ORR electrocatalysts presented, where Pt-Ni alloys, with ultra-low Pt contents (<1 wt%), are encapsulated in MOF-5 and then carbonized at 900 °C. Benefiting from its hierarchically porous structure and strong synergetic coupling among all prepared catalysts, the Pt/Ni/NPC-900 exhibits a positive onset potential (Eo) 1.01 V and half-wave potential (E1/2) of 0.89 V, as well as superior stability than commercial Pt/C (20 wt%) in alkaline media for ORR. This work will open up a novel route for the next generation of metal-air batteries to develop advanced bifunctional electrocatalysts.

One-Step Synthesis of N, P-Codoped Carbon Nanosheets Encapsulated CoP Particles for Highly Efficient Oxygen Evolution Reaction.

Oxygen electrocatalysis, especially oxygen evolution reaction (OER), is a central process during the actual application of rechargeable metal-air battery. It is still challenging to develop ideal electrocatalysts to substitute the commercial noble metal-based materials. In this work, we have constructed a new material, CoP nanoparticles, which are encapsulated by a biomolecule-derived N, P-codoped carbon nanosheets via a simple and facile one-step strategy. The as-prepared material releases a high electrocatalytic activity and stability for OER, with an overpotential of 310 mV to achieve 10 mA/cm2 in 1 M KOH. Importantly, we found that the phosphoric acid can not only introduce phosphorus dopant into 2D N-doped carbon nanosheets and play a role of pore-forming agent, but also participate in the formation of active center (cobalt phosphide). Moreover, the coverage of N, P-doped carbon can prevent the CoP nanoparticles from corrosion under the harsh reaction medium to achieve high and stable activity. We believe that our strategy can offer a novel pathway to synthesize new transition metal-based catalysts for electrocatalysis or other heterogeneous catalysis.