To drive renewable energy technologies for practical application on a large scale, highly efficient energy conversion and high-density power storage technologies, together with cost-effective production process, are urgently required [1]. Recently, the beneficial effect of nanotechnology for various energy harvesting, conversion and storage materials has shown great promise [2] and [3], which can be summarized briefly as follows. First, nanotechnology results in increased surface and interface area of the materials; second, short length scales within a material lead to efficient ion transportation; and, third, numerous energy-conversion processes, such as the intercalation and deintercalation in batteries and supercapacitors, lead to volume expansion which can be accommodated by using nanostructured materials. However, nanostructured materials tend to agglomerate because of their high surface energy, which usually inhibits their cycle-life stability. In addition, undesired aging effects can significantly reduce the performance gain of the energy storage devices leading to a compromise between good performance and long lifetime [4]. Thus, a strategy for maintaining the structure and functionality of nanomaterials during their life cycles is essential for energy conversion and storage. One promising approach to address this issue is the encapsulation of pre-synthesized nanostructured materials by another coating layer, which ideally should protect the inner-core nanomaterials to avoid agglomeration and to be functional. And another method is to form the heterostructured nanocomposite in a confining matrix using one-step process. These encapsulation strategies have been demonstrated to be effective in a variety of energy-conversion devices [5].
In the field of supercapacitors, the encapsulation strategy is also employed to fabricate composite electrodes with core–shell heterostructures [5]. It is well-known that the energy density of supercapacitors is inferior to that of batteries, but they are capable of storing and releasing the energy much faster and have a much longer cyclic lifespan [4]. Generally, supercapacitors can be divided into two categories: electrochemical double layer capacitors (EDLCs) and pseudocapacitors. EDLCs store charge electrostatically at the interface of high-surface-area carbon electrodes and an electrolyte, whereas pseudocapacitors, or redox capacitors, store charges through fast surface and near-surface redox reactions or through the intercalation of ions [6]. As a pseudocapacitive material, transition metal oxides enjoy a significant advantage. Due to their inherent chemical charge storage mechanism, pseudocapacitors exhibit high energy density and storage capacity, but usually suffer from slow charging and a limited lifetime. Increasing the energy density of supercapacitor electrodes without negatively impacting their power density and rate capability is an important challenge that can be addressed by electrode design and by producing a core–shell composite of metal oxide nanomaterials and conductive polymers.
The image on this issue's cover of Materials Today shows a composite consisting of core–shell Co3O4@PEDOT nanomaterials with beautiful sphere-like and dendritic microstructures synthesized on three-dimensional nickel foam skeleton. The nanocomposite with core–shell structures is designed to decrease the diffusion limitation for electrolyte ions moving through the electrode and to increase the electrode conductivity, which directly determines the performance of a supercapacitor device. To date, the high performance of Co3O4 species has been demonstrated only for thin film electrodes [7]. Three dimensional electrodes with high mass loading for practical supercapacitor application have yet to be adequately investigated due to higher areal capacitance. We address this issue by the fabrication of core–shell Co3O4@PEDOT nanocomposites. This hybrid composite was synthesized by a controllable hydrothermal process and subsequent electrodeposition methods in which the formations of core-like Co3O4 nanostructure and the deposition of shell-like conduct polymer were performed step-by-step. Moreover, it is notable that the conductive polymer shell could be further decorated by transition metal oxide nanomaterials to form sandwiched structures. We have recently reported that the core–shell–shell Co3O4@PPy@MnO2 nanocomposites for supercapacitor electrodes, which combine the benefits of hierarchical nanostructure architectures and encapsulation strategies, are capable of efficient charge and ion transport. The effective use of active nanomaterials is a key part of this fabrication strategy [8].
Further reading
1. C.W. Cheng, H.J. Fan, Nano Today, 252 (2012), p. 17
2. Y. Gogotsi, ACS Nano, 8 (2014), p. 5369
3. J. Jiang, et al., Adv. Mater., 24 (2012), p. 5166
4. G.H. Yu, et al., Nano Lett., 11 (2011), p. 4438
5. F. Schüth, Chem. Mater., 26 (2014), p. 423
6. P. Simon, Y. Gogotsi, Nat. Mater., 7 (2008), p. 845
7. K.K. Lee, W.S. Chin, C.H. Sow, J. Mater. Chem. A, 2 (2014), p. 17212
8. L.J. Han, P.Y. Tang, L. Zhang, Nano Energy, 7 (2014), p. 42