Graphene is impermeable to all gases and liquids 1-3 , and even such a small atom as hydrogen is not expected to penetrate through graphene's dense electronic cloud within billions of years 3-6 . Here we show that monolayers of graphene and hexagonal boron nitride (hBN) are unexpectedly permeable to thermal protons, hydrogen ions under ambient conditions. As a reference, no proton transport could be detected for a monolayer of molybdenum disulfide, bilayer graphene or multilayer hBN. At room temperature, monolayer hBN exhibits the highest proton conductivity with a low activation energy of 0.3 eV but graphene becomes a better conductor at elevated temperatures such that its resistivity to proton flow is estimated to fall below 10 -3 Ohm per cm 2 above 250°C. The proton barriers can be further reduced by decorating monolayers with catalytic nanoparticles. These atomically thin proton conductors could be of interest for many hydrogen-based technologies.Graphene has recently attracted renewed attention as an ultimately thin membrane that can be used for development of novel separation technologies (for review, see refs. 7,8). If perforated with atomic or nanometer accuracy, graphene may provide ultrafast and highly selective sieving of gases, liquids, ions, etc. 2,9-19 However, in its pristine state, graphene is absolutely impermeable for all atoms and molecules moving at thermal energies [1][2][3][4][5][6][7] . Theoretical estimates for the kinetic energy E required for an atom to penetrate through monolayer graphene vary significantly, depending on the employed model, but even the smallest literature value of 2.4 eV for atomic hydrogen 3-6 is 100 times larger than typical k B T which ensures essentially an impenetrable barrier (k B is the Boltzmann constant and T the temperature). Therefore, only accelerated atoms are capable of penetrating through the one atom thick crystal 20,21 . The same is likely to be valid for other two dimensional (2D) crystals 22,23 , although only graphene has so far been considered in this context. Protons can be considered as an intermediate case between electrons that tunnel relatively easily through atomically thin barriers 24 and small atoms. It has been calculated that E decreases by a factor of up to 2 if hydrogen is stripped of its electron 4,5 . Unfortunately,

formulations that can maximize surface area accessibility and ion transport within electrodes while minimizing space and environmental impact. Consequently, Additive Manufacturing (AM) technologies, which are capable of printing 3D objects and complex structures, offer unique possibilities to bring novel electrode materials into highperformance EES devices. Among the AM technologies, continuous extrusion-based 3D printing (also called direct ink writing or robocasting) is a versatile and costeffective processing route where the formulation and properties of colloidal inks directly control the printability and architecture of printed parts. It further offers the ability to integrate functional materials of different surface chemistry and dimensionality into EES devices [1] such as Li-ion batteries, [2][3][4] micro-supercapacitors (MSCs), [5,6] and wearable electronics. [7,8] Recently, 2D transition metal carbides, called MXenes (M n+1 X n T x , with M representing an early transition metal, X representing C and/or N and, and T x representing the terminal functional groups), [9,10] have shown huge potential as electrode materials for supercapacitors. [11,12] Their combination of metallic conductivity, high density (3.8 ± 0.3 g cm −3 ), and redox active, negatively charged surfaces can lead to superior charge storage and transfer capabilities when compared to other 2D materials. Their surface functional groups (O, OH, and F) further render them hydrophilic allowing them to be easily dispersed into aqueous suspensions and inks for processing electrodes using different approaches such as vacuum filtration, [10,13] spin coating, [14,15] screen printing, [16,17] stamping, [18] and spraying. [19][20][21] While these approaches show the potential of MXene for water-based processing of EES devices, limitations remain with respect to architectural control, scalability, or cost-effectiveness that could be addressed by employing 3D printing technologies. Although MXene aqueous inks have been recently employed in commercial pens for direct writing functional films, [22] the development of 3D printable MXene inks and their integration into customized 3D device architectures is still unexplored. In order to realize this challenge, these materials need to be integrated into inks with very specific rheological properties that allow smooth flow through narrow nozzles while still enabling the extruded filaments to retain their shape even after multiple layers are Additive manufacturing (AM) technologies appear as a paradigm for scalable manufacture of electrochemical energy storage (EES) devices, where complex 3D architectures are typically required but are hard to achieve using conventional techniques. The combination of these technologies and innovative material formulations that maximize surface area accessibility and ion transport within electrodes while minimizing space are of growing interest. Herein, aqueous inks composed of atomically thin (1-3 nm) 2D Ti 3 C 2 T x with large lateral size of about 8 µm possessing ideal vis...

Two-dimensional materials, such as graphene and molybdenum disulfide (MoS2), can greatly increase the performance of electrochemical energy storage devices because of the combination of high surface area and electrical conductivity. Here, we have investigated the performance of solution exfoliated MoS2 thin flexible membranes as supercapacitor electrodes in a symmetrical coin cell arrangement using an aqueous electrolyte (Na2SO4). By adding highly conductive graphene to form nanocomposite membranes, it was possible to increase the specific capacitance by reducing the resistivity of the electrode and altering the morphology of the membrane. With continued charge/discharge cycles the performance of the membranes was found to increase significantly (up to 800%), because of partial re-exfoliation of the layered material with continued ion intercalation, as well as increasing the specific capacitance through intercalation pseudocapacitance. These results demonstrate a simple and scalable application of layered 2D materials toward electrochemical energy storage.

Carbon-based metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium have been extensively investigated with the aim of replacing the commercially available, but precious platinum-based catalysts. For the proper design of carbon-based metal-free electrocatalysts for the ORR, it would be interesting to identify the active sites of the electrocatalyst. The ORR was now studied with an air-saturated electrolyte solution droplet (diameter ca. 15 μm), which was deposited at a specified position either on the edge or on the basal plane of highly oriented pyrolytic graphite. Electrochemical measurements suggest that the edge carbon atoms are more active than the basal-plane ones for the ORR. This provides a direct way to identify the active sites of carbon materials for the ORR. Ball-milled graphite and carbon nanotubes with more exposed edges were also prepared and showed significantly enhanced ORR activity. DFT calculations elucidated the mechanism by which the charged edge carbon atoms result in the higher ORR activity.

Since the beginning of the 'graphene era' post-2004, there has been significant interest in developing a high purity, high yield, and scalable fabrication route toward graphene materials for both primary research purposes and industrial production. One suitable approach to graphene production lies in the realm of electrochemical exfoliation, in which a potential difference is applied between a graphite anode/cathode in the presence of an electrolyte-containing medium. Herein we review various works on the electrochemical fabrication of graphene materials specifically through the use of electrochemical intercalation and exfoliation of a graphite source electrode, focusing on the quality and purity of products formed. We categorise the most significant works in terms of anodic and cathodic control, highlighting the merits of the respective approaches, as well as indicating the challenges associated with both procedures.

Laminar membranes of two-dimensional materials are excellent candidates for applications in water filtration due to the formation of nanocapillaries between individual crystals that can exhibit a molecular and ionic sieving effect, while allowing high water flux. This approach has been exemplified previously with graphene oxide, however these membranes suffer from swelling when exposed to liquid water, leading to low salt rejection and reducing their applicability for desalination applications. Here, we demonstrate that by producing thin (∼5 μm) laminar membranes of exfoliated molybdenum disulfide (MoS) in a straightforward and scalable process, followed by a simple chemical functionalization step, we can efficiently reject ∼99% of the ions commonly found in seawater, while maintaining water fluxes significantly higher (∼5 times) than those reported for graphene oxide membranes. These functionalized MoS membranes exhibit excellent long-term stability with no swelling and consequent decrease in ion rejection, when immersed in water for periods exceeding 6 months. Similar stability is observed when exposed to organic solvents, indicating that they are ideal for a variety of technologically important filtration applications.

Conventional chemical oxidation routes for the production of graphene oxide (GO), such as the Hummers' method, suffer from environmental and safety issues due to their use of hazardous and explosive chemicals. These issues are addressed by electrochemical oxidation methods, but such approaches typically have a low yield due to inhomogeneous oxidation. Herein we report a two-step electrochemical intercalation and oxidation approach to produce GO on the large laboratory scale (tens of grams) comprising (1) forming a stage 1 graphite intercalation compound (GIC) in concentrated sulfuric acid and (2) oxidizing and exfoliating the stage 1 GIC in an aqueous solution of 0.1 M ammonium sulfate. This two-step approach leads to GO with a high yield (>70 wt %), good quality (>90%, monolayer), and reasonable oxygen content (17.7 at. %). Moreover, the as-produced GO can be subsequently deeply reduced (3.2 at. % oxygen; C/O ratio 30.2) to yield highly conductive (54 600 S m) reduced GO. Electrochemical capacitors based on the reduced GO showed an ultrahigh rate capability of up to 10 V s due to this high conductivity.

No abstract