Self-Supported Transition Metal-Based Nanoarrays for Efficient Energy Storage.

Rechargeable batteries and supercapacitors are currently considered as promising electrochemical energy storage (EES) systems to address the energy and environment issues. Self-supported transition metal (Ni, Co, Mn, Mo, Cu, V)-based materials are promising electrodes for EES devices, which offer highly efficient charge transfer kinetics. This review summarizes the latest development of transition metal-based materials with self-supported structures for EES systems. Special focus has been taken on the synthetic methods, the selection of substrates, architectures and chemical compositions of different self-supported nanoarrays in energy storage systems. Finally, the challenges and opportunities of these materials for future development in this field are briefly discussed. We believe that the advancement in self-supported electrode materials would pave the way towards next-generation EES.

Tuning oxygen reduction activity on chromia surface via alloying: a DFT study.

Economical electro-catalysts for the oxygen reduction reaction (ORR) are highly desirable for a range of advance energy storage technologies. Chromium compounds have been suggested as one possible source of non-precious metal based catalysts for oxygen reduction reaction (ORR), especially chromia (Cr2 O3 ) which is the most stable form of Cr oxide at room temperature. Using density functional theory+U calculations, we investigate the 4-electron ORR on the hydroxylated Cr2 O3 surfaces alloyed with 17 different transition metals. On the one hand, we find that the ORR overpotential is lower when the Cr2 O3 surface alloyed with elements towards the end of both the first and second rows of transition metals. Among these elements, Cd alloyed Cr2 O3 surface is found to promote the ORR the most, but due to its high toxicity and price it loses out to Zn as the recommended alloyant. On the other hand, we find that the ORR overpotential is generally higher and less varied on the Cr2 O3 surface alloyed with the early-to-mid row transition metal elements (e. g. Zr, Ti). As Cr2 O3 is also a major component in the passive film on stainless steels, where a low ORR rate is desirable to reduce the impact of localized corrosion. This implies that alloying with early-to-mid row transition elements could be beneficial to stainless steels. The difference in oxygen reduction activity is attributed to the tendency of forming stable ORR intermediates during the oxygen reduction process.

Three Dimensionally Free-Formable Graphene Foam with Designed Structures for Energy and Environmental Applications.

Three-dimensional assemblies of graphene have been considered as promising starting materials for many engineering, energy, and environmental applications due to its desirable mechanical properties, high specific area, and superior thermal and electrical transfer ability. However, little has been done to introduce designed shapes into scalable graphene assemblies. In this work, we show here a combination of conventional graphene growing technique-chemical vapor deposition with additive manufacturing. Such synthesis collaboration enables a hierarchically constructed porous 3D graphene foam with large surface area (994.2 m2/g), excellent conductivity (2.39 S/cm), reliable mechanical properties (E = 239.7 kPa), and tunable surface chemistry that can be used as a strain sensor, catalyst support, and solar steam generator.

Development of a Nanostructured α-MnO2/Carbon Paper Composite for Removal of Ni2+/Mn2+ Ions by Electrosorption.

Toxic metal ions, such as Ni2+ and Mn2+, in industrial waste streams are nonbiodegradable and can cause damage to the human body. Electrochemical cleaning techniques are attractive as they offer more control and produce less sludge than do chemical/biological approaches without the high pressures needed for membranes. Here, nanoneedle-structured α-MnO2/carbon fiber paper (CFP) composites were synthesized by a hydrothermal approach and used as electrodes for combined electroadsorption and capacitive deionization removal of nickel and manganese ions from pseudoindustrial waste streams. The specific performance of α-MnO2/CFP (16.4 mg Ni2+ per g of active material) not only shows a great improvement in comparison with its original CFP substrate (0.034 Ni2+ mg per g), but also is over 6 times that of activated carbon (2.5 mg Ni2+ per g). The high performance of α-MnO2/CFP composites is attributed to their high surface area, desirable mesoporosity, pore-size distribution that permits the further access of ions, and their property as a pseudocapacitor, which contributes to a more efficient electron/charge transfer in the faradic process. Unfortunately, it was also found that some Mn2+ ions are released due to the partial reduction of MnO2 when operated as a negative electrode. For the removal of Mn2+ ions, an asymmetric arrangement, consisting of a MnO2/CFP positive electrode and an activated carbon negative electrode, was employed. This arrangement reduced the Mn2+ concentration from 100 ppm to less than 2 ppm, a vast improvement over the systematical two-activated carbon electrode system that could only reach 42 ppm under the same conditions. It was also observed that as long as the MnO2/CFP composite was maintained as a positive electrode, it was completely stable. The technique was able to reduce both Ni2+ and Mn2+ ions to well below the 10 ppm requirement for discharge into public sewers in Singapore.

Rational Design of Self-Supported Ni3S2 Nanosheets Array for Advanced Asymmetric Supercapacitor with a Superior Energy Density.

We report a rationally designed two-step method to fabricate self-supported Ni3S2 nanosheet arrays. We first used 2-methylimidazole (2-MI), an organic molecule commonly served as organic linkers in metal-organic frameworks (MOFs), to synthesize an α-Ni(OH)2 nanosheet array as a precursor, followed by its hydrothermal sulfidization into Ni3S2. The resulting Ni3S2 nanosheet array demonstrated superior supercapacitance properties, with a very high capacitance of about 1,000 F g-1 being delivered at a high current density of 50 A g-1 for 20,000 charge-discharge cycles. This performance is unparalleled by other reported nickel sulfide-based supercapacitors and is also advantageous compared to other nickel-based materials such as NiO and Ni(OH)2. An asymmetric supercapacitor was then established, exhibiting a very stable capacitance of about 200 F g-1 at a high current density of 10 A g-1 for 10,000 cycles and a surprisingly high energy density of 202 W h kg-1. This value is comparable to that of the lithium-ion batteries, i.e., 180 W h kg-1. The potential of the material for practical applications was evaluated by building a quasi-solid-state asymmetric supercapacitor which showed good flexibility and power output, and two of these devices connected in series were able to power up 18 green light-emitting diodes.