Battery powered — energy storage
UK experts Peter Hall, from the University of Strathclyde, Glasgow, and Steve Tennison, from MAST Carbon Technology Ltd, Guildford, report on energy storage in the energy materials series from Materials UK.
Neither fossil fuel combustion nor nuclear are classed as sustainable energy sources and both are associated with unacceptable waste products. Renewable energy, on the other hand, has the potential to provide large amounts of carbonless energy. The main problem with energy from wind, marine and solar sources is their intermittent nature. Technologies such as pumped hydro and compressed air are limited geographically and can never provide more than a partial solution.
It is estimated that a contribution of less than 15% from renewables to an electrical grid is the limit of stability. To increase this value it is necessary to develop energy storage devices and implement them on a large scale.
The two main classes of technology that are being developed are the electrolyser/hydrogen/fuel cell (hydrogen) and electrochemical routes. The hydrogen route suffers from low overall cycle efficiency, at under 40%, the difficulty of storing hydrogen, and the safety concerns of dealing with an odourless and highly explosive gas.
Furthermore, this is a non-sustainable route because there is an insufficient global resource of platinum, an essential catalyst in low temperature fuel cells and electrolysers, to supply demand. The United States Geological Survey estimates a global reserve of 71x106 kg and an exploitation rate of 224x103 kg/yr. This gives a reserve to production (R/P) ratio of 316 years. However, the consequences of increasing the exploitation rate of platinum even by one order of magnitude would imply an R/P ratio of only 30 years.
In addition, the relatively low loading of platinum on fuel cell anodes leads to problems over its efficient recycling from these devices.
More attention is now being paid to electrochemical storage methods, which in practice means conventional batteries, supercapacitors, superconducting magnetic energy storage (SMES) and flow batteries.
Among the array of battery technology, lithium-ion (Li-ion) based devices are gaining most attention because of their superior energy density characteristics (400kWhr m-3). Although electrochemical energy storage technologies will place stress upon the supply of some strategic metals like lithium, these are much less severe than the potential stress on platinum.
Although SMES and flow batteries have been the subject of demonstration projects, there are a number of engineering and economic difficulties to overcome before they can be adopted. These technologies take longer to develop – in excess of 20 years. However, batteries and supercapacitors have the potential to commence electrification of the transport system immediately.
Progressive electrification of the transport system will bring immediate carbon dioxide reductions even if all electricity were generated from fossil fuels. There is a clear pathway to their implementation and subsequent development. The application of different energy storage devices is shown.
The biggest drawback for Li-ion batteries is the heavy metal oxide they contain, usually LiCoOx, which is environmentally toxic. The next generation of batteries will replace this with an alternative such as a silicon or tin-based system. These systems offer the potential of increases in gravimetric energy densities of up to 400%. However, recent work on oxygen cathodes in which lithium is stored as Li2O2 on a high surface area carbon with the reaction 2Li+ + 2e- +O2 → Li2O2 offers the potential to increase gravimetric energy densities by an order of magnitude over current values.
The performance of all of the devices is critically dependent upon the carbon material used in the electrodes. In current Li-ion batteries, the carbon in the anode is non-porous, graphitic or glassy. Although the active phase in the next generation of battery anodes is likely to be based on tin or silicon, the best performance has been achieved with mixtures incorporating carbon powders, or even carbon nanofibres, to enhance electrical conductivity and stabilise the inorganic nanoparticles. The supercapacitors and Li-oxide batteries are critically dependent on the use of nanoporous carbon materials with a complex combination of properties, such as a precisely tailored small nano-pore structure to ensure high storage efficiency, a larger pore transport network to allow rapid diffusion of ions for high charge and discharge rates, along with high purity and electrical conductivity for efficient charge transport.
Improved methods of production for the precise control of these properties, along with the development of novel cell configurations, will have a major impact on the implementation of renewable energy sources. Materials development for lithium batteries and supercapacitors is the principal focus of the work of the Engineering and Physical Sciences Research Council-sponsored Energy Storage Supergen Consortium in the UK universities of St Andrews, Strathclyde, Queens Belfast, Bath and Surrey.
The development and implementation of energy storage devices and the new nanostructured materials associated with them are essential to the introduction of clean renewable energy.