Overpotential Gap Research Articles

Rechargeable LiCO2 Batteries with Graphdiyne as Efficient Metal‐Free Cathode Catalysts

AbstractA rechargeable LiCO2 battery is one of the promising power sources for utilizing the greenhouse gas CO2 in a sustainable approach. However, highly efficient catalysts for reversible formation/decomposition of insulating discharge product, Li2CO3, are the main challenge, which can boost the cycle stability. Herein, 2D single‐atom‐thick graphdiyne (GDY) with abundant acetylenic bond sites is prepared by a bottom‐up cross‐coupling reaction strategy and used as metal‐free catalysts for reversible LiCO2 batteries. The prepared GDY has a rich diacetylenic unit and atomic‐level in‐plane pores in the network, which can chemically adsorb the CO2 molecules and easily promote the Li+ diffusion and thereby resulting in uniform nucleation and reversible formation/decomposition of the discharge product. The GDY hybrid cathodes show a small overpotential gap of 1.4 V at a current density of 50 mA · g−1, a high full discharge capacity of 18 416 mAh · g−1 at 100 mA · g−1, and outstanding long‐term stability of 158 cycles at 400 mA · g−1 with a curtailing capacity of 1000 mAh · g−1. Furthermore, a flexible belt‐shaped LiCO2 battery is fabricated as a proof of concept with a high gravimetric energy density of 165.5 Wh · kg−1 (based on the mass of the whole device) as well as excellent mechanical flexibility.

“Water-in-salt” and NASICON Electrolyte-Based Na–CO2 Battery

Super concentrated electrolytes, referred to as “water-in-salt (WiS) electrolytes”, are being increasingly employed because of their wide electrochemical stability window, cost-effectiveness, and non-flammability. However, the free water molecules present in WiS electrolytes prevent the use of highly abundant, low-cost Na metal as the anode for various Na–gas batteries. In this study, we develop a WiS-based hybrid Na–CO2 battery that utilizes CO2 and serves as an energy storage cell, where a Na super-ionic conductor enables us to directly use Na metal as the anode component and a WiS electrolyte for the cathode electrolyte. In particular, linear sweep voltammetry with corresponding differential electrochemical mass spectrometry ensures an expanded electrochemical stability window, which guarantees Na–CO2 operation without electrolyte degradation during the charge process. Furthermore, we introduce a nano-sized Ru catalyst to the current collector using the Joule-heating method for lowering the discharge–charge gap. Consequently, the Na–CO2 batteries with these Ru@carbon current collectors reduce the overpotential gap and exhibit a cycling endurance of over 75 cycles (50 days) without significant alteration. These promising results demonstrate the potential of cost-effective, WiS-based Na–CO2 batteries that utilize CO2 and can be employed as energy storage cells.

Tuning NaO2 formation and decomposition routes with nitrogen-doped nanofibers for low overpotential Na-O2 batteries

Abstract Na-O2 batteries have drawn increasing attention in recent years, owing to their high energy density and the abundance of sodium resources. Their applications still suffer, however, from lack of effective air cathodes to achieve a stable and long cycle life. Herein, we report a nitrogen-doped carbon nanofiber (NCF) material derived from polypyrrole as air cathode for the non-aqueous Na-O2 electrochemical system. Notably, Na-O2 batteries with NCF as air cathode could achieve a low overpotential gap of 500 mV, a high specific capacity of 8554.7 mA h g−1 at 100 mA g−1, and excellent cyclic stability over 90 cycles with NaO2 as the discharge product. These excellent performances can be attributed to the combination of their highly conductive three-dimensional network structure, large surface area, and superior catalytic activity, obtained by incorporating nitrogen atoms into the carbon matrix, which can facilitate electron transportation, oxygen and electrolyte diffusion, and discharge product deposition and decomposition. Besides, density functional theory (DFT) calculations indicated that pyrrolic and pyridinic-N doping can effectively optimize the surface adsorption energy of the reactants and intermediate, which facilitate to achieve excellent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity. The results reported here can point the way to the rational design of electrocatalytic air cathodes for rechargeable Na-O2 batteries.

Three-dimensional entangled and twisted structures of nitrogen doped poly-(1,4-diethynylbenzene) chain combined with cobalt single atom as a highly efficient bifunctional electrocatalyst

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have been explored extensively for various energy conversion and storage system. Development of metal single atom doped well designed carbon substrates are considered as a new effective strategy to enhance catalytic activity and stability, and focused on as promising air cathode materials for metal-air battery. In this work, we report a novel method to fabricate cobalt (Co) single-atom species supported on porous carbon substrate using poly-(1,4-diethynylbenzene) (PDEB) chain as a new carbon substrate. Nitrogen doped PDEB chain combined with cobalt single-atom (Co-N-PDEB) shows excellent ORR and OER bifunctional catalytic performance with 1.08 V of oxygen electrode activity due to the synergistic effect between unique meso and macroporous carbon matrix with doped Co-N4 moiety. Finally, Co-N-PDEB air cathode adopted hybrid sodium (Na) air battery exhibits outstanding charge-discharge characteristics with low overpotential gap (0.296 V) and high round trip efficiency (91.02 %).

Pt‐Ir‐Pd Trimetallic Nanocages as a Dual Catalyst for Efficient Oxygen Reduction and Evolution Reactions in Acidic Media

AbstractThe development of dual catalysts with high efficiency toward oxygen reduction and evolution reactions (ORR and OER) in acidic media is a significant challenge. Here an active and durable dual catalyst based upon cubic Pt39Ir10Pd11 nanocages with an average edge length of 12.3 nm, porous walls as thin as 1.0 nm, and well‐defined {100} facets is reported. The trimetallic nanocages perform better than all the reported dual catalysts in acidic media, with a low ORR‐OER overpotential gap of only 704 mV at a Pt‐Ir‐Pd loading of 16.8 µgPt+Ir+Pd cm−2geo. For ORR at 0.9 V, when benchmarked against the commercial Pt/C and Pt‐Pd nanocages, the trimetallic nanocages exhibit an enhanced mass activity of 0.52 A mg−1Pt+Ir+Pd (about four and two times as high as those of the Pt/C and Pt‐Pd nanocages) and much improved durability. For OER, the trimetallic nanocages show a remarkable mass activity of 0.20 A mg−1Pt+Ir at 1.53 V, which is 16.7 and 4.3 fold relative to those of the Pt/C and Pt‐Pd nanocages, respectively. These improvements can be ascribed to the highly open structure of the nanocages, and the possible electronic coupling between Ir and Pt atoms in the lattice.

Synergistic effect of quinary molten salts and ruthenium catalyst for high-power-density lithium-carbon dioxide cell

With a recent increase in interest in metal-gas batteries, the lithium-carbon dioxide cell has attracted considerable attention because of its extraordinary carbon dioxide-capture ability during the discharge process and its potential application as a power source for Mars exploration. However, owing to the stable lithium carbonate discharge product, the cell enables operation only at low current densities, which significantly limits the application of lithium-carbon dioxide batteries and effective carbon dioxide-capture cells. Here, we investigate a high-performance lithium-carbon dioxide cell using a quinary molten salt electrolyte and ruthenium nanoparticles on the carbon cathode. The nitrate-based molten salt electrolyte allows us to observe the enhanced carbon dioxide-capture rate and the reduced discharge-charge over-potential gap with that of conventional lithium-carbon dioxide cells. Furthermore, owing to the ruthernium catalyst, the cell sustains its performance over more than 300 cycles at a current density of 10.0 A g−1 and exhibits a peak power density of 33.4 mW cm−2.

Highly dispersed Co nanoparticles decorated on a N-doped defective carbon nano-framework for a hybrid Na-air battery.

Efficient and low-cost bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are of vital importance in energy conversion. Herein, an excellent highly dispersed Co nanoparticle decorated N-doped defective carbon nano-framework (Co-N-C) derived from a ZnCo bimetal organic framework (bi-MOF) is reported. A high specific surface area originating from zinc evaporation facilitates the adsorption and desorption of oxygen, which promotes the accessibility of catalytic sites. The abundant Co-N-C species act as strong bridging bonds between Co nanoparticles and carbon materials which facilitate interfacial electron transfer. Co-N-C-0.5 (0.5 represents the molar ratio of Zn in the initial ZIF-67) exhibits a low overpotential gap of 0.94 V due to the number of active sites (e.g. N-doped defective carbon and the CoNx/Co composite) and fast interfacial electron transfer. In addition, a hybrid Na-air battery with the Co-N-C-0.5 material displays a low voltage gap of 0.31 V and a high round-trip efficiency of 90.0% at a current density of 0.1 mA cm-2. More importantly, the hybrid Na-air battery shows fantastic cyclability for charging and discharging due to its stable structure. Our results confirm Co-N-C materials derived from a bi-MOF as alternatives to high-cost Pt/C catalysts for ORR and OER activities in metal-air batteries.

Bifunctional Electrocatalyst of Low-Symmetry Mesoporous Titanium Dioxide Modified with Cobalt Oxide for Oxygen Evolution and Reduction Reactions

Hybrids of low-symmetry (disordered) mesoporous titanium dioxide modified with different weight ratios of cobalt oxide nanoparticles (Co3O4(x)/lsm-TiO2) are prepared using a one-pot self-assembly surfactant template. The physicochemical characterization of Co3O4(x)/lsm-TiO2 hybrids by scanning and transmission electron microscopy, X-ray diffraction, N2 adsorption–desorption isotherms, and X-ray photoelectron spectroscopy confirm the successful incorporation of cobalt oxide nanoparticles (2–3 nm in diameter) with preservation of the highly mesoporous structure of titanium dioxide substrate. Among these mesoporous hybrids, the ~3.0 wt.% Co3O4/lsm-TiO2 exhibits the best performance toward both the oxygen evolution (OER) and reduction (ORR) reactions in alkaline solution. For the OER, the hybrid shows oxidation overpotential of 348 mV at 10 mA cm−2, a turnover frequency (TOF) of 0.034 s−1, a Tafel slope of 54 mV dec−1, and mass activity of 42.0 A g−1 at 370 mV. While for ORR, an onset potential of 0.84 V vs. RHE and OER/ORR overpotential gap (ΔE) of 0.92 V are achieved which is significantly lower than that of commercial Pt/C, hexagonal mesoporous, and bulk titanium dioxide analogous. The Co3O4/lsm-TiO2 hybrid demonstrates significantly higher long-term durability than IrO2. Apparently, such catalytic activity performance originates from the synergetic effect between Co3O4 and TiO2 substrate, in addition to higher charge carrier density and the presence of disordered mesopores which provide short ions diffusion path during the electrocatalytic process.

Phosphate ions-functionalized and wettability-tuned nickel ferrite for boosted oxygen evolution performance

Nickel ferrite (NiFe2O4) has been explored as a promising oxygen evolution reaction (OER) electrocatalyst for water splitting owning to its earth-abundant and considerable water oxidation catalytic activity. Nevertheless, its practical electrocatalytic performance towards OER is still undesirable due to the sluggish OER kinetics and high overpotential gap on the water oxidation anode side. In this work, in order to enhance the electrochemical water oxidation performance of NiFe2O4, the surface of NiFe2O4 is functionalized with phosphate ions (Pi) by using a facile incipient impregnation and following calcination process. Results demonstrate that the OER properties of NiFe2O4 under alkaline conditions can be dramatically boosted by the surface Pi functionalization. In 1.0 M KOH solution, the resulting NiFe2O4-Pi on glassy carbon (GC) electrode demonstrates quite lower overpotential of 332 mV (10 mA/cm2) and Tafel slope of 57 mV/dec compared to that of pristine NiFe2O4 (443 mV@10 mA/cm2 and 96 mV/dec), which is also better than that of commercial RuO2 electrocatalysts (348 mV@10 mA/cm2 and 80 mV/dec). Moreover, such electrocatalyst on nickel foam electrode also realizes superior OER durability to afford a current density of 70 mA/cm2 at overpotential of only 300 mV for at least 28 h. The excellent electrocatalytic water oxidation activities of NiFe2O4-Pi can be attributed to the tuning electronic property and surface wettability by Pi ions functionalization. This work provides us a novel and effective approach to modify the photo-/electrocatalytic activity for transition metal oxides.

Synergistic Coupling Derived Cobalt Oxide with Nitrogenated Holey Two-Dimensional Matrix as an Efficient Bifunctional Catalyst for Metal-Air Batteries.

Developing cost-effective, efficient bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the heart of metal-air batteries as a renewable-energy technology. Herein, well-distributed nanopolyhedron (NP) Co3O4 grown on iron (Fe) encapsulated in graphitic layers on a nitrogenated, porous two-dimensional (2D) structure, namely, a C2N matrix, (NP Co3O4/Fe@C2N), presents an outstanding bifunctional catalytic activity with a comparable overpotential and Tafel slope to those of benchmark Pt/C and IrO2. The rationally designed atomic configuration of Co3O4 on the C2N matrix has a well-controlled NP morphology with a (111) plane, leading to bifunctional activities for the ORR and OER. Interestingly, the specific interaction between the NP Co3O4 nanoparticles and the C2N matrix introduces synergistic coupling and changes the electronic configuration of Co atoms and the C2N framework. Benefiting from the synergistic coupling of Co3O4 with the C2N matrix, the NP Co3O4/Fe@C2N electrocatalyst exhibits exceptionally high stability and an even lower charge-discharge overpotential gap of 0.85 V at 15 mA cm-2 than that of the Pt/C+IrO2 catalyst (1.01 V) in Zn-air batteries. This work provides insights into the rational design of a metal oxide on a C2N matrix for bifunctional, low-cost electrochemical catalysts.

A highly efficient cathode catalyst γ-MnO2@CNT composite for sodium-air batteries

The γ-MnO2@CNT catalyst was prepared by in situ solid phase synthesis and first applied into sodium-air batteries (SABs). The initial discharge specific capacity of SABs with γ-MnO2@CNT catalyst can reach 8804 mA h g−1 and the overpotential gap is only 140 mV, which is better than the batteries that is catalyzed by α-MnO2@CNT and pure CNT. Besides, the batteries also exhibit excellent cycle performance, which can keep relatively stable for 246 cycles at 500 mA g−1 and 140 cycles at 1000 mA g−1.

Dihydrogen phosphate ion functionalized nanocrystalline thallium ruthenium oxide pyrochlore as a bifunctional electrocatalyst for aqueous Na-air batteries

The sodium-air (Na–air) batteries are considered as a new promising energy storage devices due to their 1683 W h kg−1 of high theoretical specific energy density. However, these outstanding theoretical energy density performances cannot be achieved because undesirable sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) reaction kinetics on the air cathode side causes the high overpotential gap and low round-trip efficiencies during charge-discharge process. Therefore, the development of highly active bifunctional oxygen electrocatalysts is the key strategy to obtain the low overpotential gap with high energy density performance of Na-air batteries. In this work, a novel single crystalline thallium ruthenium oxide (Tl2Ru2O7) with pyrochlore structure was firstly revealed as an effective bifunctional electrocatalyst. Moreover, in order to enhance the surface chemical reactivity of Tl2Ru2O7, the surface of Tl2Ru2O7 is functionalized by using the dihydrogen phosphate ion (P-Tl2Ru2O7) for achieving the effective and rapid charge transfer behavior. The superior bifunctional catalytic activity of P-Tl2Ru2O7 can be explained by the degree of covalency of Ru-O bonds in Tl2Ru2O7 and enhanced covalent character by functionalized dihydrogen phosphate ion, which can afford favorable oxidation nature to Tl and Ru ions.

Feasibility of using hollow double walled Mn2O3 nanocubes for hybrid Na-air battery

Synthesis of the strongly anisotropic materials, such as highly porous hollow double walled cubes are considered excellent approach to maximize the diffusion of electrolytes. Herein, hollow doubled walled (HDW) Mn2O3 nanocubes (NCs) were synthesized by facile hydrothermal method followed by calcination method. The growth of nanocubes were studied by performing hydrothermal reaction at different times ranging from 3 to 9 h. Thereafter, the feasibility of prepared HDW NCs as air cathode in hybrid Na-air battery was systematically investigated. Among all, the sample prepared by 9 h hydrothermal treatment showed superior performance than 3 and 6 h samples. The fabricated hybrid Na-air cell using HDW Mn2O3 NCs displayed 330 mV overpotential gap and 90% electrical energy efficiency at 5 mA g−1 current density, maximum of 0.2 W g−1 power density and good cyclic stability up to 75 cycles which is attributed to the highly porous nature of material that allows efficient diffusion of electrolyte ions and oxygen from air. Thus, present investigation suggests that HDW Mn2O3 NCs can be a potential air cathode and can be utilized in other metal-air battery systems.

Metal-Organic Framework Supported on Food Waste-Derived Carbon As an Efficient Bifunctional Catalyst for Oxygen Electrocatalysis

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are a core pair in many energy storage and conversion systems, such as regenerative fuel cells and rechargeable metal-air batteries. Unfortunately, both reactions possess sluggish kinetics, involving multiple charge-/proton-transfers and oxygen bond breaking (in ORR)/recombining (in OER) processes at the “solid-liquid-gas” triple phase interfaces. Therefore efficient bifunctional catalysts are urgently needed. The most efficient oxygen-relative electrocatalysts are made from precious metal, but there is no a single catalyst based on precious metals that can serve the need for both ORR and OER. Metal-organic frameworks (MOFs) are porous crystalline materials self-assembled with metal nodes and organic linkers with various compositions, high specific surface areas, tunable pore structures, and easily functionalized. Therefore, MOFs have provided opportunities for advancements in oxygen electrocatalysis. MOFs-based ORR/OER electrocatalysts are generally used in two ways: (1) by self-sacrificing through pyrolysis, and (2) in pristine phases. The former is at the expense of MOFs’ properties, while the latter falls short of meeting the practical target due to the poor conductivity of pristine MOFs, requiring carbon support. Conventional carbon supports are synthesized from fossil fuel precursors with harsh synthetic conditions and high energy consumption. Sustainable carbon materials should be derived from renewable resources (especially agriculture/food waste), and the preparation process should be green, facile and easy to scale up. Herein, a Co(mIm)2 (mIm=methyl-imidazole) MOF was incorporated with N-doped pomelo-peel-derived carbon (NPC) as a synergistic ORR/OER bifunctional catalyst in an alkaline electrolyte (Fig. 1A). Through rotating disk electrode voltammetry, an optimized composite exhibits an ORR/OER overpotential gap, ΔE (E OER@10 mAcm-2-E ORR@-1 mAcm-2), as small as 0.79V (Fig. 1B), putting the catalyst among the best of the reported bifunctional electrocatalysts so far. The catalyst also demonstrated a 4-electron transfer pathway and superior catalytic stability. The Co-N4 ligation in MOF, electrochemical active surface area, and the strong interactions between MOF and support are attributed as the main contributors to the bifunctional catalytic activity. These factors act synergistically, resulting in substantially enhanced bifunctional catalytic activity and stability. Figure 1

Brush-Like Cobalt Nitride Anchored Carbon Nanofiber Membrane: Current Collector-Catalyst Integrated Cathode for Long Cycle Li-O2 Batteries.

To achieve a high reversibility and long cycle life for lithium-oxygen (Li-O2) batteries, the irreversible formation of Li2O2, inevitable side reactions, and poor charge transport at the cathode interfaces should be overcome. Here, we report a rational design of air cathode using a cobalt nitride (Co4N) functionalized carbon nanofiber (CNF) membrane as current collector-catalyst integrated air cathode. Brush-like Co4N nanorods are uniformly anchored on conductive electrospun CNF papers via hydrothermal growth of Co(OH)F nanorods followed by nitridation step. Co4N-decorated CNF (Co4N/CNF) cathode exhibited excellent electrochemical performance with outstanding stability for over 177 cycles in Li-O2 cells. During cycling, metallic Co4N nanorods provide sufficient accessible reaction sites as well as facile electron transport pathway throughout the continuously networked CNF. Furthermore, thin oxide layer (<10 nm) formed on the surface of Co4N nanorods promote reversible formation/decomposition of film-type Li2O2, leading to significant reduction in overpotential gap (∼1.23 V at 700 mAh g-1). Moreover, pouch-type Li-air cells using Co4N/CNF cathode stably operated in real air atmosphere even under 180° bending. The results demonstrate that the favorable formation/decomposition of reaction products and mediation of side reactions are hugely governed by the suitable surface chemistry and tailored structure of cathode materials, which are essential for real Li-air battery applications.

Three-dimensional SnS2 nanopetals for hybrid sodium-air batteries

Na-air batteries are regarded as a potential alternate to Li-air batteries due to the abundant sodium source and high theoretical energy density. However, non-aqueous Na-air battery suffers from the electrode polarization owing to the formation of insoluble discharge product, which severely limits its cyclability and performance. Herein, a high performance hybrid Na-air cell is demonstrated using a dual electrolyte (mixed aqueous and non-aqueous electrolyte) system and three dimensionally (3D) grown tin sulfide (SnS2) nanopetals based air electrode. 3D SnS2 nanopetals are synthesized by a facile solvothermal method and used as an air electrode material for hybrid Na-air battery. The vertically-grown and self-assembled ultra-thin nanosheets of 3D SnS2 nanopetals provide exposed active sites for the efficient air and electrolyte diffusion to air electrodes, resulting in high performance hybrid Na-air cell. The fabricated hybrid Na-air cell displays low overpotential gap (0.52V), high round trip efficiency (83%), high power density (300mWg−1) and good rechargeability up to 40 cycles. The proposed 3D SnS2 nanopetals as air electrodes can provide a robust platform for the future development of Na-air batteries and other energy storage devices.

Design and synthesis of hierarchical, freestanding bowl-like NiCo2O4 as cathode for long-life Li-O2 batteries

In this paper, hierarchical, freestanding bowl-like NiCo2O4 growing directly on carbon fiber paper (NiCo2O4@CFPs) are designed and synthesized through a controllable and facile electro-deposition combined with hard template method, and then applied as air electrode for long-life Lithium-O2 batteries. This novel air electrode could deliver a high discharge capacity of 9624.2 mAh g−1, low discharge/charge over-potentials (over-potential gap < 1.0 V), excellent cycleability (∼100 cycles). Moreover, it exhibits extraordinarily rate capability and reversibility. Specifically, amorphous nanosheet-like Li2O2 have been observed as the main discharge products, which might be tailored by the special bowl-like catalyst. The excellent cell performances could be ascribed to the special open structure of the synthesized bowl-like catalyst, which could provide large specific surface area, enough space for accommodating the discharge products and facilitate the transport of oxygen and electrolyte in the Lithium-O2 batteries. This facile, low-cost synthesis approach further sheds light on fabrication of various hierarchical bowl-like air electrodes including metal oxides, metals or their hybrids, and possesses promising applications on metal-air batteries and fuel cells.

Hierarchical urchin-shaped α-MnO2 on graphene-coated carbon microfibers: a binder-free electrode for rechargeable aqueous Na–air battery

With the increasing demand of cost-effective and high-energy devices, sodium–air (Na–air) batteries have attracted immense interest due to the natural abundance of sodium in contrast to lithium. In particular, an aqueous Na–air battery has fundamental advantage over non-aqueous batteries due to the formation of highly water-soluble discharge product, which improve the overall performance of the system in terms of energy density, cyclic stability and round-trip efficiency. Despite these advantages, the rechargeability of aqueous Na–air batteries has not yet been demonstrated when using non-precious metal catalysts. In this work, we rationally synthesized a binder-free and robust electrode by directly growing urchin-shaped MnO2 nanowires on porous reduced graphene oxide-coated carbon microfiber (MGC) mats and fabricated an aqueous Na–air cell using the MGC as an air electrode to demonstrate the rechargeability of an aqueous Na–air battery. The fabricated aqueous Na–air cell exhibited excellent rechargeability and rate capability with a low overpotential gap (0.7 V) and high round-trip efficiency (81%). We believe that our approach opens a new avenue for synthesizing robust and binder-free electrodes that can be utilized to build not only metal–air batteries but also other energy systems such as supercapacitors, metal–ion batteries and fuel cells. Growing metal nanostructures on graphene-coated electrodes makes batteries that run on salt, water and air more practical to operate. Sodium's abundance gives batteries that use this metal a price advantage over lithium-based cells. Hyunhyub Ko and co-workers in South Korea have developed a way around aqueous sodium-air batteries' Achilles heel – a reliance on precious-metal catalysts for recharging. The team used gas bubbles, which form on a submerged carbon microfibre mat coated with graphene, as templates to transform inexpensive manganese oxide (MnO2) catalysts into ‘sea urchin’-shaped nanostructures. The ion-accessible morphology of the MnO2 urchins, coupled with the highly conductive substrate, boosted the rechargeability and efficiency of a prototype battery. Additionally, this technique's ability to generate catalytic electrodes without binder opens opportunities for devices such as fuel cells that require lightweight materials. We have rationally synthesized a binder-free and robust electrode by directly growing urchin-shaped α-MnO2 nanostructures on porous reduced graphene oxide-coated carbon microfiber (MGC) mats and fabricated an aqueous sodium–air cell using the MGC as an air electrode to demonstrate the rechargeability of an aqueous sodium–air battery. The fabricated aqueous Na–air cell exhibited excellent rechargeability, rate capability and round-trip efficiency.

Efficient Bi-Functional Electrocatalysts of Strontium Iron Oxy-Halides for Oxygen Evolution and Reduction Reactions in Alkaline Media

Strontium iron oxyhalide perovskitic oxides were investigated by electrochemical characterization as bi-functional electrocatalysts for oxygen evolution (OE) and oxygen reduction (OR) reactions in alkaline media. The electrochemical investigations showed that the presence of lower levels of oxygen and higher levels of chloride, as in Sr4Fe3O8Cl2 and Sr2FeO2Cl2 significantly enhanced the oxygen evolution and reduction over the other perovskitic oxides. Additionally, Sr4Fe3O8Cl2 and Sr2FeO2Cl2 was proven to have high bi-functional activity due to an overpotential gap (ΔE = 0.70 V) between the OER and ORR much lower than that of other similar perovskite bi-functional catalysts. In comparison with other strontium iron oxyhalide based electrocatalysts, Sr2FeO2Cl2 was shown to have higher OER activities among all investigated catalysts. This work demonstrates that the oxygen-deficient layered perovskites of the type A2BO2X2 are a new class of high-performance electrocatalysts for oxygen evolution (OE) and oxygen reduction (OR) reactions in alkaline media.

Nitrogen and Sulfur Co-doped Graphene Supported Cobalt Sulfide Nanoparticles as an Efficient Air Cathode for Zinc-air Battery

Zinc-air battery is considered as one of the promising energy storage devices due to their low cost, eco-friendly and safe. Here, we present a simple approach to the preparation of cobalt sulfide nanoparticles supported on a nitrogen and sulfur co-doped graphene oxide surface. Cobalt sulfide nanoparticles dispersed on graphene oxide hybrid was successfully prepared by solid state thermolysis approach at 400 °C, using cobalt thiourea and graphene oxide. X-ray diffraction study revealed that hybrid electrode prepared at 400 °C results in pure CoS2 phase. The hybrid CoS2(400)/N,S-GO electrode exhibits low over-potential gap about 0.78 V vs. Zn after 70 cycles with remarkable and robust charge and discharge profile. And also the CoS2(400)/N,S-GO showing deep discharge behavior with stability up to 7.5 h.