Recent Progress in Extending the Cycle-Life of Secondary Zn-Air Batteries

Secondary Zn-air batteries with stable voltage and long cycle-life are of immediate interest to meet global energy storage needs at various scales. Although primary Zn-air batteries have been widely used since the early 1930s, large-scale development of electrically rechargeable variants has not been fully realized due to their short cycle-life. In this work, we review some of the most recent and effective strategies to extend the cycle-life of Zn-air batteries. Firstly, diverse degradation routes in Zn-air batteries will be discussed, linking commonly observed failure modes with the possible mechanisms and root causes. Next, we evaluate the most recent and effective strategies aimed at tackling individual or multiple of these degradation routes. Both aspects of cell architecture design and materials engineering of the electrodes and the electrolytes will be thoroughly covered. Finally, we offer our perspective on how the cycle-life of Zn-air batteries can be extended with concerted and tailored research directions to pave the way for their use as the most promising secondary battery system of the future

Heterostructured redox-based nanomaterials for supercapacitor application

Nanomaterials for energy storage application play an important role in versatile and reliable use of energy, including the pressing requirement on clean energy resources. Huge proliferation of portable electronic devices and intermittent nature of renewable energy generations demand for an efficient and sustainable energy storage device. At the same time, energy storage devices are expected to store extraordinary amount of energy within small area of footprint while delivering the energy rapidly. Hence, revolutionary advances in structural design and material selection for high performance energy storage device are necessary in order to meet this challenging requirement. Supercapacitor is an energy storage device that is able to balance the need of high power density of capacitor and large energy density of battery. Pseudocapacitor which is based on redox materials is of interest due to its fast and reversible faradaic charge storage process, leading to the high capacitance of supercapacitor. Nanostructuring has been explored to increase the capacitance of electrode materials. However, as some of the intrinsic properties of the materials cannot be tailored easily by nanostructuring method, single phase nanostructured material may not always yield an optimum capacitance. In this thesis, integrating two or more materials to form heterostructured nanomaterials was adopted to overcome the limitation of single phase nanomaterials on meeting the criteria of large capacitance, good cycling stability and high rate capability. Three strategies were proposed in designing high performance heterostructured nanomaterial. First, shell material of coaxial nanowire structure has provided additional redox reaction and prevented the dissolution of core material into electrolyte, resulting in the high capacitance and outstanding cycling stability of supercapacitor electrode. Second, nanostructured current collector has enhanced both ions and electrons transport on wide accessible surface area of electrode material and provided mechanical support for electrode material, leading to the excellent rate capability, high capacitance and good cycling stability of supercapacitor electrode. Third, free-standing electrode has reduced the contact resistance and poor adhesion at electrode material/current collector interface by eliminating the need of current collector, yielding the high capacitance and good cycling stability of supercapacitor electrode. In addition, its flexible paper-like nature and high mechanical property was promising for its application in flexible supercapacitor device. Furthermore, localized and in-situ electrochemical characterization at micron/nano level was of interest to further advance the performance of supercapacitor electrode. In this thesis, Scanning Electrochemical Microscopy (SECM) was introduced as a promising technique to probe the kinetics of localized interfacial processes with high spatial resolution and accuracy. SECM study was focused on SECM feedback mode, in particular approach curve measurement to extract heterogeneous charge transfer rate constant and illustration of SECM tip and substrate voltammetry. SECM feedback mode measurements on porous nanostructure and thin film redox-based electrodes have shown favourable behaviour as facile charge transfer medium with low kinetics barrier property. This study has provided an insight to the relationship among the capacitive behaviour, physical and kinetics properties of electrode material. The work presented has provided significant contribution on synthesis, design approach and kinetics study of redox-based supercapacitor electrode. Furthermore, it has given enlightenment on fundamental understanding and advancement to optimize capacitive performance of supercapacitor electrode, for the benefit of an excellent supercapacitor device.Doctor of Philosophy (MSE

Interfacial study of electroless Ni-Sn-P plating and Sn-3.5Ag solder after multiple reflows

Electroless Ni-P plating has been a good candidate for Under Bump Metallization (UBM) in IC packaging due to its lower cost and slower chemical reaction with solder compared to Cu based UBM. However, during its reaction with Sn-based lead free solder, it faces the problem of rapid Ni out diffusion which can lead to the voids formation and brittle fracture. To enhance the property of electroless Ni-P alloy, alloyment of Sn into Ni-P to form Ni-Sn-P ternary alloy can be one of the options. Besides having good thermal stability and solderability, the presence of Sn in electroless Ni-Sn-P could influence the diffusion process during soldering reflow process. Electroless Ni-Sn-P film was prepared from alkaline citrate plating bath with hypophosphate ion as the reducing agent. Effect of sodium stannate as Sn source and plating time were investigated to maximize the quality of the plated surface. Multiple reflow soldering was performed to investigate the interfacial layer composition and growth under liquid state reaction. Tensile test was carried out to study the effect of the interfacial layer composition and growth on the mechanical property of the solder joint. Increasing stannate concentration and plating time resulted in smoother and more compact plated surface. There were two distinctive layers identified during multiple reflow of Ni-Sn-P/Sn-Ag solder joint. They were P-rich layer and Sn-rich layer which grew thicker as the reflow cycle increase. The presence of Sn in electroless nickel plating had affected the diffusion process in the solder joint. The changes could be observed by the presence of Sn in the P-rich layer and significant amount of P and Ni content in the Sn-rich layer during the reflow processes, as well as the voids location which was observed at different location from the voids in the Ni-P/Sn-Ag solder joint. In addition, Sn-rich layer was observed to have a slower growth rate compared to P-rich layer. This observation was on the opposite trend of the interfacial growth in electroless Ni-P/Sn-Ag solder joint. Slow growth of Sn-rich layer might affect the tensile strength of the solder joint, thus small drop in tensile strength was observed up to 40 cycles of reflow. As the stable mechanical property is required for solder joint in IC packaging, hence electroless Ni-Sn-P can be used as UBM in lead free solder joint application.Bachelor of Engineering (Materials Engineering

Facile coating of manganese oxide on tin oxide nanowires with high-performance capacitive behavior

In this paper, a very simple solution-based method is employed to coat amorphous MnO2 onto crystalline SnO2 nanowires grown on stainless steel substrate, which utilizes the better electronic conductivity of SnO2 nanowires as the supporting backbone to deposit MnO2 for supercapacitor electrodes. Cyclic voltammetry (CV) and galvanostatic charge/discharge methods have been carried out to study the capacitive properties of the SnO2/MnO2 composites. A specific capacitance (based on MnO2) as high as 637 F g−1 is obtained at a scan rate of 2 mV s−1 (800 F g−1 at a current density of 1 A g−1) in 1 M Na2SO4 aqueous solution. The energy density and power density measured at 50 A g−1 are 35.4 W h kg−1 and 25 kW kg−1, respectively, demonstrating the good rate capability. In addition, the SnO2/MnO2 composite electrode shows excellent long-term cyclic stability (less than 1.2% decrease of the specific capacitance is observed after 2000 CV cycles). The temperature-dependent capacitive behavior is also discussed. Such high-performance capacitive behavior indicates that the SnO2/MnO2 composite is a very promising electrode material for fabricating supercapacitors

Nanoarchitectured current collector for high rate capability of polyaniline based supercapacitor electrode

Indium tin oxide (ITO) nanowires array was used as current collector and building block for polyaniline based supercapacitor. Thin polyaniline coating was deposited on the nanowires and resulted in the formation of polyaniline ITO coaxial nanowires. This hybrid heterostructure design improved the specific capacitance, rate capability, and cycling stability of the supercapacitor electrode. Good conductivity harnessed by these directly grown ITO nanowires is useful to improve the charge transport during the charge discharge processes which were confirmed by the electrochemical impedance spectroscopy measurement. Electrochemical test in 1 M H2SO4 at 4 A g−1 delivered specific capacitance as high as 738 F g−1. In addition, sub-micron size of the intercoaxial nanowires spacing ensures the fast penetration of electrolyte ions which resulted in the superior rate capability (98% capacitance retention when applied current was varied from 4 to 25 A g−1). The capacitance retention is significantly higher as compared to other polyaniline composite electrodes and it is one of the best reported performances to date for polyaniline based supercapacitor electrodes. This work illustrates a promising platform that can be adopted for other redox nanocomposite materials while reaping the benefit as low cost and binder free electrode material for supercapacitor application

V2O5 loaded on SnO2 nanowires for high-rate li ion batteries

A simple gas-phase-based method is employed to coat V2O5 on SnO2 nanowires. An electrode made from the SnO2/V2O5 core/shell-nanowires delivers a high power density of about 60 kW kg−1 while the energy density remains 282 W h kg−1. In addition, the electrode exhibits very-good cycling stability. Such good performance shows very-promising potential in the application of a high-rate lithium battery

Nickel cobalt oxide nanowire-reduced graphite oxide composite material and its application for high performance supercapacitor electrode material

In this paper, we report a facile synthesis method of mesoporous nickel cobalt oxide (NiCo2O4) nanowire-reduced graphite oxide (rGO) composite material by urea induced hydrolysis reaction, followed by sintering at 300 °C. P123 was used to stabilize the GO during synthesis, which resulted in a uniform coating of NiCo2O4 nanowire on rGO sheet. The growth mechanism of the composite material is discussed in detail. The NiCo2O4–rGO composite material showed an outstanding electrochemical performance of 873 F g−1 at 0.5 A g−1 and 512 F g−1 at 40 A g−1. This method provides a promising approach towards low cost and large scale production of supercapacitor electrode material

Large areal mass, flexible and free-standing reduced graphene oxide/manganese dioxide paper for asymmetric supercapacitor device

Well-separated RGO sheets decorated with MnO2 nanoparticles facilitate easy access of the electrolyte ions to the high surface area of the paper electrode, enabling the fabrication of a thicker electrode with heavier areal mass and higher areal capacitance (up to 897 mF cm−2). The electrochemical performance of the bent asymmetric device with a total active mass of 15 mg remains similar to the one in the flat configuration, demonstrating good mechanical robustness of the device

NiMn layered double hydroxide as efficient electrocatalyst for oxygen evolution reaction and its application in rechargeable Zn- air batteries

High performance catalyst for oxygen evolution reaction (OER) is in demand to improve the re-chargeability of Zn-air battery. In this work, atomically dispersed NiMn layered double hydroxides are prepared via simple hydrothermal synthesis and tested as an OER catalyst in rechargeable Zn-air batteries. NiMn layered double hydroxides with the optimized Ni:Mn molar feeding ratio have good crystallinity, big interlayer spacing, and large surface area, which are beneficial to enhance their catalytic activity. They are highly active and stable during OER, showing overpotential of 0.35 V, Tafel slope of 40 mV dec-1, and remarkable stability during 16 h of chronopotentiometry test. Rechargeable Zn-air batteries with NiMn layered double hydroxides as OER catalyst exhibit a low charge voltage of ≈2 V which are stable for up to 200 cycles. This study illustrates the platform to enhance catalytic activity of OER catalyst via fine-tuning the composition and physical properties of the materials and their application for rechargeable metal-air batteries.NRF (Natl Research Foundation, S’pore)Accepted versio

Cryogel synthesis of hierarchical interconnected macro-/mesoporous Co3O4 with superb electrochemical energy storage

In this contribution, we report a facile synthesis of ultrafine Co3O4 nanocrystals with an in situ construction of mesoporous and macroporous network for supercapacitor electrode material. The resultant ultrafine Co3O4 nanocrystals form an interconnected macroporous network with mesoporous hierarchical structure. The unique architecture is realized through a modified sol–gel process to formulate highly porous cryogel using freeze-drying in the presence of a soft template. Small-angle X-ray scattering and transmission electron microscopy are used to investigate the organization of the Co3O4 porous structure. The unique channels in this hierarchical pores network provide intimate electrolyte contact with cobalt oxide and facilitate electrolyte diffusion. This hierarchical structure presents superior electrochemical performance with a specific capacitance of 742.3 F g–1 measured at a potential window of 0.5 V, unveiling one of the highest performance for sol–gel synthesized oxides to date, to the best of our knowledge. The capacity retention was 86.2% after 2000 cycles. The synthesis strategy highlights a versatile and facile dual template approach to independently tailor the porosity and particle sizes using a spontaneous nucleation approach. This serves as a major milestone toward high-performance porous metal oxide material for supercapacitor electrodes