Size effects and nanostructured materials for energy applications

Size reduction in nanocrystals leads to a variety of exciting phenomena due to enhanced surface-to-volume ratio and reduced transport length for the mobile charges. We consider some of these anomalous phenomena restricting our discussions to the effects of nano-size on energetics and transport behaviour, with a few examples to illustrate materials challenges for efficient energy conversion and storage. We show that nano-size affects the thermodynamical aspects due to excess surface contributions giving rise to enhanced cell voltage in lithium batteries. An anomalous electrical conduction occurs at nano-size, its relevance is briefly highlighted in the context of energy conversion using fuel cells and excitonic solar cells. The benefit of narrowly spaced interfaces also results in rapid energy storage due to the reduction of the effective diffusion path. Thus, in the context of storage behaviour, nanocrystalline electrodes exhibit high capacity as well as Coulombic efficiency and in some cases high rate performance. An interfacial lithium storage mechanism which is a consequence of nanocrystallinity has been discussed to explain the extra storage when metal is brought in contact with Li2O/LiF at nanosize. Superior storage performance by supercapacitors and efficient waste heat conversion by thermoelectric devices using nanostructured materials are also briefly highlighted. This review thus emphasises fundamental understanding and novel concepts at nano-size for the development of excellent materials which achieve efficient energy conversion and storage, both of which are vital in facing the challenges posed by global warming and the predicted future limited energy resources.