Summary
Electrochemical energy storage and conversion technologies are at the forefront of modern science and engineering research efforts due to decreasing global fossil fuel supplies and increasing environmental concerns. In order to provide sustainable energy infrastructures and resources for future generations, significant improvements to the current state of these technologies is imperative. Our group focuses on the development of unique, high performance nanostructured materials for use in clean, sustainable energy technologies. Specifically, our interests lie in nanostructured material developments for polymer electrolyte membrane fuel cells (PEMFC), lithium ion (Li-ion) batteries and metal-air batteries. These include nitrogen-doped carbon nanotubes and graphenes, unique nanostructured metal and metal oxides, along with innovatively combined nanocomposite materials.
PEMFCs produce only water and heat by products at the point of emission, operating off of a fuel (i.e. hydrogen or methanol) and oxygen. With the potential to operate off clean, renewable source feeds, PEMFCs are expected to find mass appeal for numerous applications including transportation, portable and stationary backup power supply. Sustainable commercialization of these devices is still hindered by their high cost and low operational stability, factors limited by the expensive platinum (Pt) based electrode catalysts. Our research focuses on the development of efficient, high performance and inexpensive catalyst materials in order to perpetuate the looming wide-spread commercialization of this technology. Our approach involves investigation of: i) non-precious catalysts, traditionally based on a combination of transition metal, carbon and nitrogen species; ii) unique catalyst support materials to provide favourable Pt-support interactions; and iii) Pt and Pt-alloy materials with controllable nanostructures such as nanowires or core-shell structures.
To realize the application potential of the unique catalysts developed in our laboratory, it is important to apply engineering strategies to design electrode structures that can be integrated into PEMFC systems for device demonstration. As different catalyst properties will affect important parameters such as electrode porosity, mass transport, ionomer interactions and electronic conductivity, various electrode design strategies must be investigated to determine the optimal configuration. This challenge is even more pronounced in the case of non-precious metal catalyst electrodes, that, owing to their minimal cost in comparison to Pt-based materials, employ relatively thick (i.e., 100 µm) catalyst layers. At this thickness, issues with ionomer dispersivity and mass transport become even more of an issue and must be addressed by careful engineering strategies to achieve high fuel cell performance and durability.
One of the great challenges in the twenty-first century is undoubtedly energy utilization. In response to the increasing needs of modern society and ecological concerns, it is essential to develop next-generation rechargeable batteries with environmental benignity and low cost. The performance of these systems strongly depends on the properties of materials; therefore, new materials hold great promise in fundamental advances in new generation of energy storage devices. Our research mainly focuses on design and development of novel electrode materials and electrode architectures for next-generation rechargeable energy storage systems.
1. Silicon-Sulfur Battery
Current lithium-ion batteries can only provide an energy density of ~150 Wh kg-1 due to the low specific capacities of the electrode materials. By comparison, Si-S battery can boost the energy density by an order of magnitude to ~1550 Wh kg-1, and significantly reduce the size and cost of the power system. However, current technology for fabricating Si-S batteries is limited by the rapid capacity fading of both the anode and cathode materials, mainly resulting from the large volume change of Si causing severe cracking and pulverization of the anode, and the dissolution of intermediate lithium polysulfide products in the electrolyte and the insulating nature of Li2S. Our group is developing new technologies to stabilize both Si anode and S cathode for high-performance Si-S battery.
2. Supercapacitors
The ever-growing needs for portable electronics and electric vehicles require energy storage devices/power sources with both high energy and power densities. Compared to lithium batteries, supercapacitors possess much higher power but lower energies. To overcome this barrier, lithium-based hybrid supercapacitors with lithium-battery anode and supercapacitor cathode have been attracting tremendous research interest. Our group is mainly focusing on developing transitional metal oxides-carbon composites for high-performance asymmetric supercapacitors. Particularly, we design and build robust electrode architectures with both ion and electron pathways in order to improve the electrode kinetics.
3. Sodium-Ion Battery/Supercapacitors
Current energy storage methods are highly dependent on lithium-ion energy storage devices, and the expanded use of these technologies is likely to affect existing lithium reserves. The abundance of sodium makes Na-ion-based devices very attractive as an alternative, sustainable energy storage system. However, electrodes based on transition-metal oxides often show slow kinetics and poor cycling stability, limiting their use as Na-ion-based energy storage devices. Our group is developing transitional metal sulfide-graphene composites with fast kinetics and long cycling stability for both Na-ion batteries and supercapacitors. The availability of electrochemical energy storage based on Na-ion systems is an attractive, cost-effective alternative to Li-ion systems.
Metal-air batteries have extremely high energy density and are lightweight as oxygen in ambient air is used as the primary source of fuel. In particular, zinc-air batteries are interesting due to its cost effectiveness, environmental benignity, and safe operation. However, only primary (non-rechargeable) zinc-air batteries have been commercialized. Our research focus is on the development of novel bi-functional catalysts capable of catalyzing both the oxygen reduction (battery discharge) and oxygen evolution (battery recharge) reactions to create practically viable rechargeable zinc-air batteries. In addition, we focus on the design and performance optimization of both air and zinc electrodes as well as solid electrolyte membrane. Finally, we aim to combine the components into various forms of rechargeable zinc-air battery such as stationary, flexible, and flow cells.
Lithium batteries find application in the majority of commercial electronics, operating off intercalation chemistry redox chemistry occurring at each electrode. Increasing the performance, durability and safety while simultaneously reducing costs requires the design and development of unique nanostructured electrode materials. Our current focus is on developing high performance anode materials, including nanostructured porous carbons and high surface area graphene effectively coupled with silicon, tin/tin oxide, or other materials that can facilitate lithium ion interactions.
Rechargeable lithium sulfur (Li-S) batteries are safe, environmentally friendly and economical alternative energy storage systems that can potentially be combined with renewable sources including wind solar and wave energy. Our group focuses on the development of carbon-based (meso-porous carbon, CNTs, graphene) sulfur composites using various synthesis methods in order to enhance the surface area and electrical conductivity of sulfur electrodes. Our research also focuses on improving the cyclability of Li-S batteries by developing protection films (or reservoir) for dissolving and diffusing lithium polysulfides into organic electrolytes in the Li-S battery system.
Flow battery has advantages such as excellent reversibility, low cost and convenient scale-up capacity, which make it a good candidate for grid energy storage application. The conventional flow batteries include Zinc/Chlorine battery, Zinc/Bromine battery, as well as all Vanadium battery. Nowadays, the lines among different battery types change blurred, flow battery has been introduced into various battery systems, such as Li-ion batteries, super capacitors, etc. Our focus is on the development of different batteries with flow system to fulfill the requirements of energy storage application by combined advantages.
Ion conductive membranes are important components utilized in fuel cell and battery technologies. Our current focus is on the development of unique polymeric or composite membranes for these energy storage and conversion technologies. Specifically, proton conducting membranes are being developed exclusively for use in PEMFCs, with a focus on increasing the working temperature range, increasing ion transport capabilities, and improving the operational stability. Our group is also focused on the development of unique hydroxide anion exchange membranes (AEM). Developing unique AEMs with suitable hydroxide ion conductivity will allow replacement of the aqueous electrolytes used in metal-air batteries and alkaline fuel cells, where electrolyte management and replacement is a pertinent concern due to carbonate contamination.
The advanced nanostructures and materials in our group are developed by a variety of different techniques including advanced chemical vapour deposition, microwave irradiation assisted growth, solvothermal and simple wet-chemistry techniques. The materials are subjected to rigorous performance evaluation and physicochemical characterization in order to evaluate their practicality towards various applications, along with providing fundamental insight that will aid in the optimization and design of improved functional materials. Common characterization techniques applied including scanning and transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transfer infrared spectroscopy and BET analysis.