The Electrochemical Society Interface • Spring 2011 63 T his article focuses on graphene-based electrodes for electrochemical energy conversion and storage devices. 1,2 As elaborated in the other feature articles in this issue, graphene is a 2D “fat mat” consisting of a honeycomb-like structure of carbon atoms with sp 2 bonding character for each carbon. It exhibits excellent electrical conductivity and mechanical strength, and can be synthesized in a number of ways. Most syntheses involve frst oxidizing graphite to graphene oxide (GO), a widely used method developed many years ago and referred to as Hummers Method. 3 The graphitic carbon is dissolved in sulfuric acid along with other oxidants such as sodium nitrate during sonication, and KMnO 4 is slowly added to complete the oxidation process. Washing and rinsing allows for a fully exfoliated GO suspension in water, ethanol, and other polar solvents, that can be prepared as a stable colloid for months. The GO can be subsequently reduced in a number of ways by UV irradiation or thermal treatment, 4 sonolysis, 5 or chemical treatment with a strong reducing agent such as hydrazine 6 and is referred to as reduced graphene oxide (RGO). The use of RGO in lithium ion batteries is still in its infancy and may provide a practical and inexpensive way to substantially improve the performance of these electrochemical energy storage devices, serving markets ranging from the long-sought electric vehicle as well as simple applications such as cell phones and laptop computers. GO is an extraordinarily useful material for the development of composite materials as a result of the oxygen moieties spread throughout the formerly-pristine carbon sheet. These highly electronegative species allow for the stability of the GO colloidal suspension as well as the binding of cations. Binding cationic or organic protonated anions (i.e., acetate) produces GO composites, and subsequently RGO composites that are formed from further chemical processing. The ability to selectively bind cationic materials has led to a swarm of activity surrounding the development of novel GO/RGO- nanoparticle composite materials for a variety of applications. The development of electrodes for lithium ion batteries using the following scheme has produced a number of RGO-metal oxide composites already. 7-18 Graphene in a Li Ion Battery Graphene offers many advantages over using typical Li ion battery electrode materials in a standalone fashion. 19 First, the graphene can serve as a binder material, eliminating the use of binding polymer materials such as poly(vinylidene fuoride). 20 Second, the high conductivity associated with graphene sheets lends itself to rapid transport of electrons to and from the active material intercalation sites, 21 particularly given the close physical association of the nanoparticles and the RGO sheets. Also the mechanical strength of the graphene has the potential to absorb some of the expansion and contraction of the anchored nanoparticles during the intercalation and de- intercalation processes, 22 which typically lead to mechanical failure of the electrode and performance reduction through the loss of intimate contact of the active material and the conductive carbon black mixed into the electrode material for enhanced conductivity. The electrode can ultimately be pulverized if the expansion is large enough, hence the use of active materials that exhibit small changes upon Li + intercalation. Graphene has also been utilized as a standalone material for Li ion battery anodes in place of traditional graphite and shows some marked improvement in storage and cycling. The storage capacity and cycling ability of graphene was shown to outperform graphite in a number of studies. 20,23-30 Some have attributed this improvement to the higher surface area achieved in graphene relative to graphite. 24 Lithium diffusion Graphene-based Composites for Electrochemical Energy Storage by James G. Radich, Paul J. McGinn, and Prashant V. Kamat along the edges of graphene following chemical reduction and production of “zig-zag” and “armchair” edge effects was calculated from frst principles and supports the experimental fndings of better charge/discharge rates and capacity. 31 The features of RGO (i.e., zig-zag, armchair, creases) were further shown to support enhanced Li + intercalation when compared to single-layer graphene where Li + repulsion prevented signifcant intercalation. 32 The imperfections achieved in the RGO via reduction of GO provide electronic barriers to Li + repulsion effects, which are signifcant during intercalation into mostly electrically neutral host sites. The use of carbon families in conjunction with graphene as anode materials was also shown to yield enhanced Li + storage. In fact, by controlling the interlayer spacing of the graphene sheets through the use of carbon nanostructures such as fullerenes and carbon nanotubes (CNT), signifcant improvements were realized beyond the improvement of graphene alone. 30 Kinetic barriers may exist in the Li + diffusion process into the graphene layers as shown by the over 150% increase in capacity of a graphene anode 20 when the current rate was decreased from 50 mA∙g -1 to 10 mA mA∙g -1 . The improvements realized upon incorporation of the fullerenes and CNT are likely a result of increasing the interlayer spacing and thus decreasing the kinetic barriers to the diffusion of Li + into the graphene nanostructures as illustrated in Fig. 1. The theoretical capacity for graphite anodes is 372 mAh∙g -1 . When graphene was used as anode material in the aforementioned studies, initial capacities were as high as 1264 mAh∙g -1 at a current density of 100 mA∙g -1 with some decrease following the buildup of the solid electrolyte interface (SEI). 24 Reversible capacities in this study ranged from 718 mAh∙g -1 at 500 mA∙g -1 and up to 848 mAh∙g -1 at 100 mA∙g -1 after 40 cycles, almost a 100% increase above the theoretical capacity for graphite. Even graphene nanoribbons have been synthesized for Li ion battery electrodes by unraveling multi- walled carbon nanotubes (MWCNT) in the hopes of limiting the diffusion resistance of the Li + into the packed carbon material. 23 Initial capacities up to 1400 mAh∙g -1 were recorded with reversible capacities starting near 800 mAh∙g -1 . However, long-term cycling with the nanoribbons leads to continued loss of capacity, albeit small at ~3% per cycle as shown in Fig. 2, yet suggesting additional work is required for optimization. Designing Nanocomposites Graphene-based nanocomposites with SnO 2 9,11,14,16,18 and other materials such as Sn, 22 Si, 33,34 Mn 3 O 4 , 7,10 Co 3 O 4 , 8,12 and Ti 5 O 12 , 21 have also shown promise as anode materials for Li ion batteries. Figure 1 depicts a common wet-chemical synthesis technique for the complexation of the Mn 3 O 4 with RGO, showing that excellent coverage by highly crystalline active materials is possible on RGO. All of the nanocomposites exhibit additional capacity relative to the pure graphene, which is to be expected given the intercalation capability of each of the compounds. With regard to cycling, the SnO 2 exhibits similar stability to that of graphene while the Si capacity decreases considerably with increasing cycle number. The Sn-graphene anode material on the other hand shows exquisite cycling stability, even beyond 100 cycles with an overall high reversible capacity of 508 mAh∙g -1 . Cu 2 O nanocubes were also synthesized in situ on GO sheets via complexation of Cu 2+ with GO 15 . The GO was reduced, and the composite anode material was tested and showed initial capacity of 1100 mAh∙g -1 , although poor cycling renders this material unstable in its current form. Copper compounds will present challenges in that the Cu + oxidation state rapidly undergoes disproportionation in most organic solvents. (continued on next page) . © ECS 2017 ecsdl.org/site/terms_use address. See 80.82.77.83 Downloaded on 2017-06-17 to IP