Energy Storage Technologies
The steering group discussed technical challenges concerning batteries, supercapacitors, fuel cells and other chemical energy storage technologies. Below are summaries of these challenges and possible future directions for research and development identified by the group.
Conventional batteries, including nickel-cadmium (Ni-Cd), nickel-metal hydride (Ni-MH) and lead acid (Pb acid), are a commercially proven technology and are likely to continue to be used for energy storage, despite their intrinsic limitations of low energy density and high self discharge, since they are cheap to produce. However, the toxicity of Pb and Cd make them less favourable for environmentally friendly applications. More research is needed on conventional batteries since these technologies still have an important role to play.
Sodium-Sulphur (Na-S) and Zebra (Na-NiCl2) systems have potential applications in large scale static electricity storage. Zebra batteries are far safer than Na-S batteries, because the solid cathode is separated from the molten Na by both solid and liquid electrolytes. Zebra transport is already an application. However, further R & D is still required to resolve problems relating to lifetime, reliability and cost.
Lithium-ion (Li-ion) batteries have a high energy density compared to other systems because they can operate at a significantly higher voltage (3 – 4 V per cell). They are already the dominant energy storage technology for portable electronic devices and also have considerable potential to become the dominant energy storage means for media devices, power tools, electric/hybrid vehicles and even static small scale energy storage from renewable energy generation. They compliment supercapacitors which have better power density and cycle life. Depending on the choice of material for the cathode, anode and electrolyte the voltage, capacity, life, and safety of a Li-ion battery can be dramatically altered. R & D is required for the production of Li-ion batteries, at greatly reduced material and manufacturing costs, with increased cycle and calendar lifetimes.
Future Research and Development into batteries is extremely important if the Government’s energy storage goals are to be achieved. Methods of recycling raw materials or replacing strategic metals will have to be found. By developing new materials, such as an oxygen or fluoride-based cathode, or replacing the graphite anode by metal alloys, higher energy densities, important for many applications, may be achieved. Improved safety could be realised by using titanate anodes and polyoxy-anion based cathodes as well as by employing ionic liquids, polymer or glassy electrolytes. Manufacturing the electrodes out of nanomaterials should provide higher porosity and hence enhanced power density. In the long-run, the group envisages the application of enzymes for the synthesis of such materials and the development of lithium-air batteries. And the question of whether variable power loads reduce the lifetime of Li-ion batteries remains to be answered.
Supercapacitors are predominantly used in consumer electronics. Their power density is higher than that of batteries because there are no chemical reactions during charging and discharging. Therefore they can be charged/discharged quickly and hence have huge potential for use in hybrid vehicles; they could assist the start-up of engines, assist regenerative breaking systems and extend the life-time of fuel cells. Carbon only supercapacitors have high power density as well as long calendar and cycle lifetimes, but they are expensive to produce and are not particularly apt for energy storage applications since they have low energy densities. Carbon-hybrid supercapacitors have higher energy storage capacity than carbon only supercapacitors but a shorter lifetime.
Future Research and Development into supercapacitors is extremely important to address the balance of energy density with power density and lifetime. Even though hybrid supercapacitors, have improved energy density compared to carbon only supercapacitors, challenges remain in improving cycle life and increase the voltage of the capacitors. Surface chemistry and materials will be key to the development of new supercapacitors. Nanoporous electrode materials could significantly improve the interactions between electrolyte and electrodes and the introduction of new electrodes as well as electrolytes such as ionic liquids could further enhance the characteristics of supercapacitors. Furthermore, to successfully introduce supercapacitors to the hybrid vehicle market, production costs needs to be reduced.
Fuel cells are already used in niche markets. However, there is clear potential for their use in vehicles, portable electronics and static applications, all of which would help the EU to deliver its promise of reducing CO2 emissions from transport and electricity generation. However, fundamental problems remain in the development of commercially viable fuel cells.
The challenges in the development of solid oxide fuel cells (SOFCs) include the improvement of their lifetime and reliability and the reduction of production costs. Heat development is a major engineering problem and solving it could help to improve these factors.
The high cost of polymer-electrolyte membrane (PEM) fuel cells is due to their use of an expensive membrane and catalyst. Future Research and Development into fuel cells is vital in order to produce viable fuel cells. In order to improve their efficiency, water-free membranes that promote proton transfer need to be developed, thus avoiding water balance and cross-over problems. Other challenges lie in increasing the porosity of electrode materials and improving the efficiency, lifetime and durability of catalysts and finding alternatives to strategic materials. Finally, the hydrogen infrastructure, including hydrogen storage has to be developed further in order for fuel cells to make an impact as well as improvements in the purity of the fuel.