The unique properties and great variety of two-dimensional (2D) nanomaterials make them highly attractive for energy storage applications. Here, an insight into the progress made towards the application of 2D nanomaterials for capacitive energy storage is provided. Synthesis methods, and electrochemical performance of various classes of 2D nanomaterials, particularly based on graphene, transition metal oxides, dichalcogenides, and carbides, are presented. The factors that directly influence capacitive performance are discussed throughout the text and include nanosheet composition, morphology and texture, electrode architecture, and device configuration. Recent progress in the fabrication of 2D-nanomaterials-based microsupercapacitors and flexible and free-standing supercapacitors is presented. The main electrode manufacturing techniques with emphasis on scalability and cost-effectiveness are discussed, and include laser scribing, printing, and roll-to-roll manufacture. Various issues that prevent the use of the full energy-storage potential of 2D nanomaterials and how they have been tackled are discussed, and include nanosheet aggregation and the low electrical conductivity of some 2D nanomaterials. Particularly, the design of hybrid and hierarchical 2D and 3D structures based on 2D nanomaterials is presented. Other challenges and opportunities are discussed and include: control of nanosheets size and thickness, chemical and electrochemical instability, and scale-up of electrode films.

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The revival of the Na‐ion battery concept has prompted intense research activities toward new sustainable Na‐based insertion compounds and their implementation in full Na‐ion cells. Efforts are parted between Na‐based polyanionic and layered compounds. For the latter, there has been a specific focus on Na‐deficient layered phases that show cationic and anionic redox activity similar to a Na0.67Mn0.72Mg0.28O2 phase. Herein, a new alkali‐deficient P2‐Na2/3Mn7/9Zn2/9O2 phase using a more electronegative element (Zn) than Mg is reported. Like its Mg counterpart, this phase shows anionic redox activity and no O2 release despite evidence of cationic migration. Density functional theory (DFT) calculations show that it is the presence of an oxygen nonbonding state that triggers the anionic redox activity in this material. The phase delivers a reversible capacity of 200 mAh g−1 in Na‐half cells with such a value be reduced to 140 mAh g−1 in full Na‐ion cells which additionally shows capacity decay upon cycling. These findings establish Na‐deficient layered oxides as a promising platform to further explore the underlying science behind O2 release in insertion compounds based on anionic redox activity.

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Graphene electrodes of high power density were manufactured by a surfactant-water based exfoliation method followed by a scaleable spray-deposition process. Cyclic voltammetry and galvanostatic charge-discharge experiments revealed a combination of electric double layer and pseudocapacitive behavior that, unlike the many graphene-oxide derived electrodes, was maintained to unusually high scan rates of 10,000 mV s −1 , reaching a maximum capacitance of 543 μF cm −2 and with a capacitive retention of 57% at 10,000 mV s −1 . The performance of graphene electrodes was contrasted with carboxylated single walled carbon nanotubes that showed a sharp decrease in capacitance above 200 mV s −1 . Electrochemical impedance spectroscopy analysis showed a fast capacitor response of 17.4 ms for as manufactured electrodes which was further improved to 2.3 ms for surfactant-free 40 nm thick electrodes. A maximum energy density of 75.4 nW h cm −2 gradually decreased as power density increased up to 2.6 mW cm −2 . Graphene electrodes showed 100% capacitance retention for 5000 cycles at the high power scan rate of 10,000 mV s −1 .

Herein we use Nitrogen-doped reduced Graphene Oxide (N-rGO) as the active material in supercapacitor electrodes. Building on a previous work detailing the synthesis of this material, electrodes were fabricated via spray-deposition of aqueous dispersions and the electrochemical charge storage mechanism was investigated. Results indicate that the functionalised graphene displays improved performance compared to non-functionalised graphene. The simplicity of fabrication suggests ease of up-scaling of such electrodes for commercial applications.Recent research has shown that the unique physical, chemical and electronic properties of graphene may be exploited in a wide range of applications; namely sensing, 1,2 electronics 3,4 and energy. 5,6 In particular, energy conversion and storage technologies that take advantage of graphene's excellent mechanical strength, chemical stability and high surface area may be developed. Indeed, many research publications detail the use of graphene and related materials in lithium ion battery electrodes, 7 solar energy conversion, 8 supercapacitors 9 and as electrode materials in other electrochemical energy devices. 10 For many of these applications it has been shown that the presence of heteroatoms in the graphene lattice improves material performance. Chemical modification allows for tuning of graphene properties such as surface chemistry and electronic properties; 11 which renders them more suited to certain applications than pristine graphene. Nitrogen-doped (N-doped) graphene has been demonstrated to be of use as an electrocatalytic material for oxygen reduction in hydrogen fuel cells, 12 improves biocompatibility of carbon devices in biosensing 13 and enhances the performance of graphene-based supercapacitors. 14 Graphene may be synthesised via mechanical cleavage of graphite flakes, 15 Chemical Vapour Deposition (CVD) 16 or the decomposition of silicon carbide (SiC). 17 However, while the latter two have great potential in electronics fabrication, it is not feasible to produce gram-scale quantities using these methods. Liquid phase exfoliation and processing of graphene is one method which shows great potential as a means of producing large quantities of material. 18,19 Reduction of graphene oxide (GO) is one method which shows promise as a means of producing large quantities of material suited for energy applications. [20][21][22] Graphite oxide is easily exfoliated and the oxygen functional groups can be removed via thermal or chemical means. While the reduction of GO produces a graphene material with structural defects and residual heteroatoms, these issues may not be problematic for applications such as catalysis and energy storage, where crystallinity and structural integrity of the graphene material is not a priority. In fact, the presence of a few oxygen-containing groups at the surface can advantageously influence the interfacial activity between graphene electrodes and the electrolyte.Much work has been carried out towards production of N-doped graphene via several m...

Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in materials science, and developing a capability to observe the mechanisms of such processes on the atomic scale can offer new insights across a wide range of materials systems. Aberration correction in scanning transmission electron microscopy (STEM) has enabled atomic resolution imaging at significantly reduced beam energies and electron doses. It has also made possible the quantitative determination of the composition and occupancy of atomic columns using the atomic number (Z)-contrast annular dark-field (ADF) imaging available in STEM. Here we combine these benefits to record the motions and quantitative changes in the occupancy of individual atomic columns during a solid-state chemical reaction in manganese oxides. These oxides are of great interest for energy-storage applications such as for electrode materials in pseudocapacitors. We employ rapid scanning in STEM to both drive and directly observe the atomic scale dynamics behind the transformation of Mn3O4 into MnO. The results demonstrate we now have the experimental capability to understand the complex atomic mechanisms involved in phase changes and solid state chemical reactions.

E -m a i l : b e a t r i x . m e n d o z a @ g m a i l . c o m"These authors contributed equally to this work Abstract Manganese oxide nanosheets were synthesized using liquid-phase exfoliation that achieved suspensions in isopropanol with concentrations of up to 0.45 mg ml -1 . A study of solubility parameters showed that the exfoliation was optimum in N,N-Dimethylformamide followed by isopropanol and diethylene glycol. Isopropanol was the solvent of choice due to its environmentally friendly nature and ease of use for further processing. For the first time, a hybrid of graphene and manganese oxide nanosheets was synthesized using a single-step co-exfoliation process. The 2D hybrid was synthesized in isopropanol suspensions with concentrations of up to 0.5 mg ml -1 and demonstrated stability against re-aggregation for up to 6 months. The co-exfoliation was found to be a energetically favorable process in which both solutes, graphene and manganese oxide nanosheets, exfoliate with an improved yield as compared to the single-solute exfoliation procedure. This work demonstrates the remarkable versatility of liquid-phase exfoliation *To whom correspondence should be addressed 2 with respect to the synthesis of hybrids with tailored properties, and it provides proof-of-concept ground work for further future investigation and exploitation of hybrids made of two or more 2D nanomaterials that have key complementary properties for various technological applications.