Manganese-Based Tunnel-Type Cathode Materials for Secondary Li-Ion and K-Ion Batteries.

The rational design of novel cathode materials remains a key pursuit in the development of (post) Li-ion batteries. Considering the relative ionic and Stokes radii and open frameworks with large tunnels, Na-based compounds can act as versatile cathodes for monovalent Li-ion and post-Li-ion batteries. Here, tunnel-type sodium insertion material Na0.44MnO2 is demonstrated as an intercalation host for Li-ion and K-ion batteries. The rod-shaped Na0.44MnO2 was synthesized by a solution combustion method assuming an orthorhombic structure (space group Pbam), which led to Na0.11K0.27MnO2 (NKMO) and Na0.18Li0.51MnO2 (NLMO) cathodes for K-ion batteries and Li-ion batteries, respectively, via facile electrochemical ion exchange from Na0.44MnO2. These new compositions, NKMO and NLMO, exhibited capacities of ∼74 and 141 mAh g-1, respectively (at a rate of C/20), with excellent cycling stability. The underlying mechanistic aspects (structural changes and charge storage mechanism) in these cathode compositions were probed by combining ex situ structural, spectroscopy, and electrochemical tools. Tunnel-type Na0.44MnO2 forms a versatile cathode material for non-aqueous alkali-ion batteries.

Iron-Based Mixed Phosphate Na4Fe3(PO4)2P2O7 Thin Films for Sodium-Ion Microbatteries.

Iron-based polyanionic materials can be exploited to realize low cost, durable, and safe cathodes for both bulk and thin film sodium-ion batteries. Herein, we report pulsed laser deposited mixed phosphate Na4Fe3(PO4)2P2O7 as a positive electrode for thin film sodium-ion microbatteries. The bulk material and thin films of Na4Fe3(PO4)2P2O7 are employed by solution combustion synthesis (SCS) and the pulsed laser deposition (PLD) technique, respectively. Phase purity and the nature of the crystallinity of the thin films were confirmed by grazing incidence X-ray diffraction and transmission electron microscopy. Identification of surface roughness and morphology was obtained from atomic force microscopy and scanning electron microscopy, respectively. Emerging electrochemical properties were observed from charge-discharge profiles of the thin films, which are well comparable to bulk material properties. The Na4Fe3(PO4)2P2O7 thin film electrodes delivered a highly reversible Na+ storage capacity of ∼120 mAh g-1 with an excellent stability of over 500 cycles. Electrochemical analysis results revealed that the thickness of the film affects the storage capacity.

Cobalt and Nickel Phosphates as Multifunctional Air-Cathodes for Rechargeable Hybrid Sodium-Air Battery Applications.

Noble-metal-free bifunctional electrocatalysts are indispensable to realize low-cost and energy-efficient rechargeable metal-air batteries. In addition, power density, energy density, and cycle life of these metal-air batteries can be improved further by utilizing the fast faradaic reactions of metal ions in the catalyst layer together with the oxygen evolution/reduction reactions (OER/ORR) for charge storage. In this work, we propose mixed metal phosphates of nickel and cobalt, NixCo3-x(PO4)2 (x = 0,1, 1.5, 2, and 3), as multifunctional air-cathodes exhibiting bifunctional electrocatalytic activity and reversible metal redox reaction (M3+/2+, M = Ni and Co). Submicron-sized NixCo3-x(PO4)2 particles were synthesized by a solution combustion synthesis technique with urea acting as the fuel. Electrocatalytic activity toward the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in 0.1 M NaOH was systematically tuned by varying the Ni-to-Co ratio. The synthesized NixCo3-x(PO4)2 with x = 1.5 (NCP11) showed superior bifunctional catalytic activity to other samples. Moreover, the catalyst material delivered a specific capacity of ∼110 mAh g-1 by the redox reactions of its metal sites. The hybrid Na-air battery fabricated using the NCP11 catalyst-loaded air-cathode exhibited low overpotential, stable cycling performance, and round-trip energy efficiency exceeding 78% in a 0.1 M NaOH aqueous electrolyte.

Highly Electroactive Ni Pyrophosphate/Pt Catalyst toward Hydrogen Evolution Reaction.

Robust electrocatalysts toward the resourceful and sustainable generation of hydrogen by splitting of water via electrocatalytic hydrogen evolution reaction (HER) are a prerequisite to realize high-efficiency energy research. Highly electroactive catalysts for hydrogen production with ultralow loading of platinum (Pt) have been under exhaustive exploration to make them cutting-edge and cost-effectively reasonable for water splitting. Herein, we report the synthesis of hierarchically structured nickel pyrophosphate (ß-Ni2P2O7) by a precipitation method and nickel phosphate (Ni3(PO4)2) by two different synthetic routes, namely, simple cost-effective precipitation and solution combustion processes. Thereafter, Pt-decorated nickel pyrophosphate and nickel phosphate (ß-Ni2P2O7/Pt and Ni3(PO4)2/Pt) were prepared by using potassium hexachloroplatinate and ascorbic acid. The fabricated novel nickel pyrophosphate and nickel phosphate/Pt materials were utilized as potential and affordable electrocatalysts for HER by water splitting. The detailed electrochemical studies revealed that the ß-Ni2P2O7/Pt (1 µg·cm-2 Pt) electrocatalyst showed excellent electrocatalytic performances for HER in acidic solution with an overpotential of 28 mV at -10 mA·cm-2, a Tafel slope of 32 mV·dec, and an exchange current density ( j0) of -1.31 mA·cm-2, which were close to the values obtained using the Vulcan/Pt (8.0 µg·cm-2 Pt), commercial benchmarking electrocatalyst with eight times higher Pt amount. Furthermore, the ß-Ni2P2O7/Pt electrocatalyst maintains an excellent stability for over -0.1 V versus RHE for 12 days, keeping j0 equal after the stability test (-1.28 mA cm-2). Very well-distributed Pt NPs inside the "cages" on the ß-Ni2P2O7 structure with a crystalline pattern of 0.67 nm distance to the Ni2P2O7/Pt electrocatalyst, helping the Volmer-Tafel mechanism with the Tafel reaction as a major rate-limiting step, help to liberate very fast the Pt sites after HER. The high electrocatalytic performance and remarkable durability showed the ß-Ni2P2O7/Pt material to be a promising cost-effective electrocatalyst for hydrogen production.

Electrochemical potassium-ion intercalation in NaxCoO2: a novel cathode material for potassium-ion batteries.

Reversible electrochemical potassium-ion intercalation in P2-type NaxCoO2 was examined for the first time. Hexagonal Na0.84CoO2 platelets prepared by a solution combustion synthesis technique were found to work as an efficient host for K+ intercalation. They deliver a high reversible capacity of 82 mA h g-1, good rate capability and excellent cycling performance up to 50 cycles.

A Metal-Organic Framework Derived Porous Cobalt Manganese Oxide Bifunctional Electrocatalyst for Hybrid Na-Air/Seawater Batteries.

Spinel-structured transition metal oxides are promising non-precious-metal electrocatalysts for oxygen electrocatalysis in rechargeable metal-air batteries. We applied porous cobalt manganese oxide (CMO) nanocubes as the cathode electrocatalyst in rechargeable seawater batteries, which are a hybrid-type Na-air battery with an open-structured cathode and a seawater catholyte. The porous CMO nanocubes were synthesized by the pyrolysis of a Prussian blue analogue, Mn3[Co(CN)6]2·nH2O, during air-annealing, which generated numerous pores between the final spinel-type CMO nanoparticles. The porous CMO electrocatalyst improved the redox reactions, such as the oxygen evolution/reduction reactions, at the cathode in the seawater batteries. The battery that used CMO displayed a voltage gap of ∼0.53 V, relatively small compared to that of the batteries employing commercial Pt/C (∼0.64 V) and Ir/C (∼0.73 V) nanoparticles and without any catalyst (∼1.05 V) at the initial cycle. This improved performance was due to the large surface area (catalytically active sites) and the high oxidation states of the randomly distributed Co and Mn cations in the CMO. Using a hard carbon anode, the Na-metal-free seawater battery exhibited a good cycle performance with an average discharge voltage of ∼2.7 V and a discharge capacity of ∼190 mAh g-1hard carbon during 100 cycles (energy efficiencies of 74-79%).

Hydrothermal synthesis and electrochemical performances of 1.7 V NiMoO4⋠xH2O||FeMoO4 aqueous hybrid supercapacitor.

One-dimensional (1D) NiMoO4⋅xH2O nanorods and ß-FeMoO4 microrods are successfully synthesized by simple hydrothermal method without using any organic solvents. X-ray diffraction (XRD) patterns reveal the single phase formation of nickel molybdate (NiMoO4⋅xH2O) and pure monoclinic phase of ß-FeMoO4. The growth of one dimensional morphology of both the molybdates are identified from scanning and transmission electron microscopes (SEM and TEM). The cyclic voltammogram envisage the pseudocapacitance behavior of NiMoO4⋅xH2O and ß-FeMoO4 through the reversible redox reactions of Ni(3+)/Ni(2+) and Fe(3+)/Fe(2+) ions. An asymmetric supercapacitor is fabricated using NiMoO4⋅xH2O nanorods and ß-FeMoO4 as a positive and negative electrode, respectively. The ß-FeMoO4||NiMoO4⋅xH2O asymmetric supercapacitor delivers a capacitance of 81 F g(-1) at a current density of 1 mA cm(-2). The cell exhibits a high energy density of 29 W h kg(-1) and good cycling stability even after 1000 cycles.