The family of 2D transition metal carbides, carbonitrides and nitrides (collectively referred to as MXenes) has expanded rapidly since the discovery of Ti3C2 in 2011. The materials reported so far always have surface terminations, such as hydroxyl, oxygen or fluorine, which impart hydrophilicity to their surfaces. About 20 different MXenes have been synthesized, and the structures and properties of dozens more have been theoretically predicted. The availability of solid solutions, the control of surface terminations and a recent discovery of multi-transition-metal layered MXenes offer the potential for synthesis of many new structures. The versatile chemistry of MXenes allows the tuning of properties for applications including energy storage, electromagnetic interference shielding, reinforcement for composites, water purification, gas- and biosensors, lubrication, and photo-, electro- and chemical catalysis. Attractive electronic, optical, plasmonic and thermoelectric properties have also been shown. In this Review, we present the synthesis, structure and properties of MXenes, as well as their energy storage and related applications, and an outlook for future research. More than twenty 2D carbides, nitrides and carbonitrides of transition metals (MXenes) have been synthesized and studied, and dozens more predicted to exist. Highly electrically conductive MXenes show promise in electrical energy storage, electromagnetic interference shielding, electrocatalysis, plasmonics and other applications.

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2D metal carbides and nitrides (MXenes) for energy storage

Recently a new, large family of two-dimensional (2D) early transition metal carbides and carbonitrides, called MXenes, was discovered. MXenes are produced by selective etching of the A element from the MAX phases, which are metallically conductive, layered solids connected by strong metallic, ionic, and covalent bonds, such as Ti2AlC, Ti3AlC2, and Ta4AlC3. MXenes ­combine the metallic conductivity of transition metal carbides with the hydrophilic nature of their hydroxyl or oxygen terminated surfaces. In essence, they behave as “conductive clays”. This article reviews progress—both ­experimental and theoretical—on their synthesis, structure, properties, intercalation, delamination, and potential applications. MXenes are expected to be good candidates for a host of applications. They have already shown promising performance in electrochemical energy storage systems. A detailed outlook for future research on MXenes is also presented.

25th Anniversary Article: MXenes: A New Family of Two‐Dimensional Materials

Safe and powerful energy storage devices are becoming increasingly important. Charging times of seconds to minutes, with power densities exceeding those of batteries, can in principle be provided by electrochemical capacitors--in particular, pseudocapacitors. Recent research has focused mainly on improving the gravimetric performance of the electrodes of such systems, but for portable electronics and vehicles volume is at a premium. The best volumetric capacitances of carbon-based electrodes are around 300 farads per cubic centimetre; hydrated ruthenium oxide can reach capacitances of 1,000 to 1,500 farads per cubic centimetre with great cyclability, but only in thin films. Recently, electrodes made of two-dimensional titanium carbide (Ti3C2, a member of the 'MXene' family), produced by etching aluminium from titanium aluminium carbide (Ti3AlC2, a 'MAX' phase) in concentrated hydrofluoric acid, have been shown to have volumetric capacitances of over 300 farads per cubic centimetre. Here we report a method of producing this material using a solution of lithium fluoride and hydrochloric acid. The resulting hydrophilic material swells in volume when hydrated, and can be shaped like clay and dried into a highly conductive solid or rolled into films tens of micrometres thick. Additive-free films of this titanium carbide 'clay' have volumetric capacitances of up to 900 farads per cubic centimetre, with excellent cyclability and rate performances. This capacitance is almost twice that of our previous report, and our synthetic method also offers a much faster route to film production as well as the avoidance of handling hazardous concentrated hydrofluoric acid.

Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance

Nitrogen (N)-doped carbon materials exhibit high electrocatalytic activity for the oxygen reduction reaction (ORR), which is essential for several renewable energy systems. However, the ORR active site (or sites) is unclear, which retards further developments of high-performance catalysts. Here, we characterized the ORR active site by using newly designed graphite (highly oriented pyrolitic graphite) model catalysts with well-defined π conjugation and well-controlled doping of N species. The ORR active site is created by pyridinic N. Carbon dioxide adsorption experiments indicated that pyridinic N also creates Lewis basic sites. The specific activities per pyridinic N in the HOPG model catalysts are comparable with those of N-doped graphene powder catalysts. Thus, the ORR active sites in N-doped carbon materials are carbon atoms with Lewis basicity next to pyridinic N.

Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts

The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

Fe-based electrocatalysts are elegant due to their better performance towards the oxygen reduction reaction. Nevertheless, they commonly contain different moieties, for example Fe-Nx , Fe, Fe3 C and N-doped carbon, primarily the debatable assistance of these components towards ORR electrocatalysis, specifically for intermediate peroxide reduction reactions (PRR). In this paper, to explore the role of Fe-Nx centres for PRR, a Fe-N-C electrocatalyst rooted in nitrogen-doped carbon nanotubes with mesoporous structures was synthesized from a Fe/Zn-dicyanoimidazolate framework. The use of dicyanoimidazole coordinated with iron can introduce the Fe-Nx active sites as well as directional N-doped carbon nanotubes, which is good for enhancing electronic conductance of the catalyst. The attained electrocatalyst shows tremendous enactment to ORR, being comparable to the activity of Pt/C in acidic and better in alkaline electrolytes. This study also reveals that Fe-Nx active centres are responsible for less H2 O2 production. Though the Fe-Nx moieties and Fe3 C/Fe particles encapsulated N-doped carbon, both are active centres for ORR, however, Fe-Nx sites are more active than others for peroxide reduction reaction. These perceptions suggest rational methodologies for more active and consequently further durable Fe-N-C catalysts.

Exploring Fe-Nx for Peroxide Reduction: Template-Free Synthesis of Fe-Nx Traumatized Mesoporous Carbon Nanotubes as an ORR Catalyst in Acidic and Alkaline Solutions.

https://www.rsc.org/suppdata/cc/c1/c1cc14261e/c1cc14261e.pdf

Enhanced dispersion and durability of Pt nanoparticles on a thiolated CNT support

Hollow structured materials are widely applicable in various fields. Although many routes have been explored for getting such materials, a strategy mainly based on physical effect is still deficient. Herein, a "stresses induced orientation contraction" mechanism for preparation of hollow structures is reported. The composites constructed by zeolite imidazolate framework-8 (ZIF8) cores and polymerized dopamine (PDA) shells, upon annealing, form intensive interfacial interactions, which drag the ZIF8 cores outward to restrain their shrinkage. The gradually accumulated stresses in the central position of ZIF8 dodecahedron nanoparticles, then destroy the ZIF8 crystalline cores to form the hollow structures. In this stress-based route for creating hollow interiors with core-shell composites as the starting materials, three critical factors are necessary: 1) an intensive core-shell interfacial interaction; 2) the distinctly higher shrinkage degree of the cores than the shells; and 3) the relatively loose core structures. In oxygen reduction reaction (ORR) tested with three-electrode solution system and Zn-O2 battery, the achieved hollow nitrogen doped carbon (NC) demonstrates ultrahigh catalytic activities. This work gives an absolutely novel strategy for preparation of hollow structures, which may afford the exploration of a wider range of materials system with hollow interiors.

Preparation of Hollow Nitrogen Doped Carbon via Stresses Induced Orientation Contraction.

Pt/C trapped in activated graphitic carbon layers as a highly durable electrocatalyst for the oxygen reduction reaction

Revealing the exact roles of nitrogen configurations and precise control of the nitrogen configuration in nitrogen-doped graphene (NG) are extremely important for realizing its advanced functions in clean energy technologies. Herein, for the first time, we established that the hydrogen evolution reaction (HER) activities of NG display definite trends due to its nitrogen configurations, which were selectively generated by using layer-structured montmorillonite (MMT) with different layer distances and functions modulated by Co2+, Ni2+, Na+, and H+ ions. We found that among the three types of N, i.e., pyridine, pyrrolic, and quaternary N, quaternary N is the most active one for HER in a metal-free NG catalyst, whereas with an introduction of trace atomic cobalt, the planar (pyridine and pyrrolic) N becomes the better one. In contrast, when trace atomic Ni is involved to replace the Co, the former results in heavily depressed HER activities for the NG catalyst. Density functional theory calculations further r...

Wei Ding

Papers

Exploring Fe-Nx for Peroxide Reduction: Template-Free Synthesis of Fe-Nx Traumatized Mesoporous Carbon Nanotubes as an ORR Catalyst in Acidic and Alkaline Solutions.

Enhanced dispersion and durability of Pt nanoparticles on a thiolated CNT support

Preparation of Hollow Nitrogen Doped Carbon via Stresses Induced Orientation Contraction.

Pt/C trapped in activated graphitic carbon layers as a highly durable electrocatalyst for the oxygen reduction reaction

Understanding the Roles of Nitrogen Configurations in Hydrogen Evolution: Trace Atomic Cobalt Boosts the Activity of Planar Nitrogen-Doped Graphene