Supercapacitors empower sustainable energy storage

In 1957 petrol became the world's most used fuel, and nuclear energy began to gain ground as a credible energy source—power has been a dirty pleasure ever since. Yet the same year also saw Howard Becker at General Electric invent the first supercapacitor, a comparatively clean and sustainable form of stored energy. The technology did not see much further attention beyond the first patent for several years. However more recently as the world gawps at the prospect of catastrophic anthropological climate change, the technology has attracted a great deal of attention from researchers for its potential to supply a practical energy solution that might help avert environmental disaster. Far from a purely academic field of research the global supercapacitors market reached $2billion in 2015. Nanotechnology collects some of the latest developments [1] in this pacey field where advances in functional nanostructures are making headway towards the potential widespread use of supercapacitors as sustainable energy storage devices.

Progress in material science has unearthed a number of options that offer great advantages for supercapacitor nanostructures, including those based on electrostatic double-layer capacitance, electrochemical pseudocapacitance and hybrids. Poly(3,4-ethylenedioxythiophene)—PEDOT—is a conducting polymer that offers supercapacitor devices the advantages of low-cost and flexibility while delivering good electrical conductivity and pseudocapacitance, so long as issues with its stability can be overcome [2]. Another hot nanotechnology material with particular relevance to supercapacitor research is graphene. Surface area tends to be directly related to an increased capacitance or energy density, and in graphene all atoms are surface atoms. In this collection researchers at the Beijing Institute of Technology in China and the Helmholtz-Zentrum Dresden-Rossendorf in Germany review some of the advantages posed by what they describe as a 'fascinating material'—graphene fibres. The benefits they list include a 'unique and tunable nanostructure, high electrical conductivity, excellent mechanical flexibility, light weight, and ease of functionalization' [3].

The environmental motivation for supercapacitor research has also encouraged an impressive resourcefulness for sustainable synthesis of supercapacitor materials. Reports along these lines in the collection demonstrate promising supercapacitor properties from freeze drying nitrogen-doped porous carbon cryogel [4], as well as the production of activated carbon—the most common material for electrochemical double layer capacitors—from bark and coffee grounds [5].

As the name suggests, supercapacitors generally have the lead on dielectric or electrolytic capacitors in terms of capacitance. However they tend to be slow operating, which has prompted research into the use of nanostructuring, with for example arrays of pores, to decrease ionic impedance. Unfortunately the advantages these large pores bring can be offset by the excess of the other materials that are then required to support large pore arrays. By exploring the behaviour of graphene with small naturally occurring pores, Matthew H Ervin from the US Army Research Laboratory in Maryland demonstrates an approach for speeding up supercapacitors, and potentially other types of devices such as batteries and sensors as well [6]. Francesco Iacopi and colleagues at Griffith University and Queensland University of Technology in Australia, and University of South Florida, Plasma-Therm LLC and Air Force Research Laboratory in the US demonstrate another approach for producing naturally porous graphene in 'the first attempt to produce graphene with high surface area from silicon carbide thin films for energy storage at the wafer-level' [7]. The researchers suggest the method may open numerous opportunities for on-chip integrated energy storage applications.

Other nanostructures besides pores and arrays have also been exploited. When Junhong Chen, Shun Mao and their colleagues looked to improve the supercapacitor properties of transition metal sulphides to compete with oxide counterparts, they used a nanowire structure to maximise the surface area [8]. Their aim was to exploit the high conductivity and stability of metallic CoS_2. By growing the nanowires directly on to their substrates the team of researchers at the University of Wisconsin-Madison in the US and Fujian Institute of Research on the Structure of Matter in China could ensure low contact resistance, which also helped achieve good cyclability—just 0%–2.5% loss of capacity after 4250 cycles. Compared with single-phased material, these hybrid materials with a hierarchical nanostructure can provide shorter ion transport pathways, offer synergetic effects from multiple active components and have easily accessible electroactive sites for the electrolyte ions

Where finding one material that meets all the required characteristics can be elusive, combining the best properties from a range of materials is a popular solution. 'The facile design of hierarchical architecture and control over the multi-composition offers a new strategy for the fabrication of electrodes for high-performance supercapacitors', suggest Shauchun Tang, XiangKang Meng and colleagues at Nanjing University in their report on MnO₂ supercapacitors enhanced with electrochemically active Fe₃O₄ in the interior and electrically conductive SnO₂ nanoparticles in the surface layer [9]. Some other examples of successful hierarchical supercapacitor structure designs include Cu₂O/CuO/Co₃O₄ core–shell nanowires by Zhang and Zhang and colleagues in China and Singapore [10] and the polyaniline–graphene–carbon nanotube structures by Yimin Sung and Hongwei Duan and collaborators at Wuhan Institute of Technology in China and Nayang Technological University in Singapore [11].

The intrinsic reversibility of superficial redox couples, as well as good electronic conductivity, ultrahigh pseudocapacitance, and extremely long cycle life, has attracted a lot of interest in nanocrystalline hydrous ruthenium dioxide. However as Chu-Chuang Hu and colleagues at National Tsing Hua University in Taiwan point out, 'such excellent performances, unfortunately, cannot be maintained for relatively thick, nanocrystalline RuO₂ · xH₂O and amorphous RuO₂ · xH₂O films, especially in high-power applications' [12]. The solution? They incorporate RuO₂ · xH₂O onto a reduced graphene oxide/carbon nanotube conducting backbone to facilitate electron transport with impressive results: a total specific capacitance of 973 F g⁻¹ at 25 mV s⁻¹; good capacitance retention of 60.5%; and a scan rate varying from 5 to 500 mV s⁻¹.

From the exploitation of new nanomaterials and nanostructure design for optimised performance to smart and sustainable synthesis, the papers in the collection demonstrate how progress in supercapacitor research may play an important role in meeting global energy demands with environmentally friendly technology. Guest edited by Liming Dai, at Case Western Reserve University and Yury Gogotsi at Drexel University in the US, and Husnu Emrah Unalan at Middle East Technical University in Turkey, the collection gives real meaning to the old adage 'knowledge is power' [13].