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The crystalline graphitic carbon nitride, poly-triazine imide (PTI) is highly unusual among layered materials since it is spontaneously soluble in aprotic, polar solvents including dimethylformamide (DMF). The PTI material consists of layers of carbon nitride intercalated with LiBr. When dissolved, the resulting solutions consist of dissolved, luminescent single to multilayer nanosheets of around 60–125 nm in diameter and Li+ and Br− ions originating from the intercalating salt. To understand this unique solubility, the structure of these solutions has been investigated by high-energy X-ray and neutron diffraction. Although the diffraction patterns are dominated by inter-solvent correlations there are clear differences between the X-ray diffraction data of the PTI solution and the solvent in the 4–6 Å−1 range, with real space differences persisting to at least 10 Å. Structural modelling using both neutron and X-ray datasets as a constraint reveal the formation of distinct, dense solvation shells surrounding the nanoparticles with a layer of Br−close to the PTI-solvent interface. This solvent ordering provides a configuration that is energetically favourable underpinning thermodynamically driven PTI dissolution. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'. [ABSTRACT FROM AUTHOR]

Crystalline two-dimensional carbon nitrides with polytriazine imide (PTI) structure are shown to act amphoterically, buffering both HCl and NaOH aqueous solutions, resulting in charged PTI layers that dissolve spontaneously in their aqueous media, particularly for the alkaline solutions. This provides a low energy, green route to their scalable solution processing. Protonation in acid is shown to occur at pyridinic nitrogens, stabilized by adjacent triazines, whereas deprotonation in base occurs primarily at basal plane NH bridges, although NH2 edge deprotonation is competitive. We conclude that mildly acidic or basic pHs are necessary to provide sufficient net charge on the nanosheets to promote dissolution, while avoiding high ion concentrations which screen the repulsion of like-charged PTI sheets in solution. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'. [ABSTRACT FROM AUTHOR]

Although proton exchange membranes (PEMs) are widely deployed in an array of commercial applications, limitations linked to their proton conductivity, water retention, and gas permeability still limit ultimate device performance. While ex situ studies have shown additives can enhance membrane stability and mass transport, to date few have demonstrated that these performance enhancements are maintained when tested in commercially relevant electrochemical technologies, such as fuel cells or electrolyzers. Herein, a new multifunctional additive, 2D poly(triazine imide) (PTI), is demonstrated for composite PEMs, which is shown to boost proton conductivity by 37% under optimal high relative humidity (RH) conditions and 67% at low RHs. PTI also enables major improvements (over 55%) in both current and power densities in industrially relevant PEM fuel cells (PEMFCs). Most importantly, in situ and ex situ characterization suggests that the enhanced performance is due to polymer aggregate‐PTI clusters that form with increasing 2D character and improved long‐range connectivity, while acid‐base interactions with pyridinic nitrogen facilitate the critical proton hopping mechanism at all RHs. Hence, this work offers both a new membrane concept with proven benefits for important electrochemical technologies, as well as design principles for future optimization of proton transport and water management within PEMs. [ABSTRACT FROM AUTHOR]

Owing to a reputation for long lifetimes and excellent cycle stability, degradation in supercapacitors has largely been overlooked. In this work, we demonstrate that significant degradation in some commercial supercapacitors can in fact occur early in their life, leading to a rapid loss in capacitance, especially when utilized in full voltage range, high charge‐discharge frequency applications. By using a commercial 300 F lithium‐ion pseudocapacitor rated for 100,000 charge/discharge cycles as an example system, it is shown that a ∼96 % loss in capacitance over the first ∼2000 cycles is caused by significant structural and chemical change in the cathode active material (LiMn2O4, LMO). Multi‐scale in‐situ and ex‐situ characterization, using a combination of X‐ray computed tomography, Raman spectroscopy and X‐ray photoelectron spectroscopy, shows that while minimal material loss (∼5.5 %), attributed to the dissolution of Mn2+, is observed, the primary mode of degradation is due to manganese charge disproportionation (Mn3+→Mn4++Mn2+) and its physical consequences (i. e. microstrain formation, particle fragmentation, loss of conductivity etc.). In contrast to prior understanding of LMO material degradation in battery systems, negligible contributions from cubic‐to‐tetragonal phase transitions are observed. Hence, as supercapacitors are becoming more widely utilized in real‐world applications, this work demonstrates that it is vital to understand the mechanisms by which this family of devices change during their lifetimes, not just for lithium‐ion pseudocapacitors, but for a wide range of commercial chemistries. [ABSTRACT FROM AUTHOR]

To promote the commercial application of sodium ion batteries (SIBs), new high‐stability and high‐capacity anode materials are needed. Metal oxides show great promise, but significant issues relating to their low electrical conductivity and unstable structure during charge storage must be resolved. Herein, a new composite anode consisting of pseudohexagonal Nb2O5 (TT‐Nb2O5) nanoparticles strongly anchored to carbon nanotubes (CNT) through a glucose‐derived carbon framework (Nb2O5/g‐CNT) was synthesised and assessed for use as the anode in SIBs. This represents the first application of TT‐Nb2O5 in SIBs. The composite is shown to offer high specific capacity (203 mAh g−1 at 0.2 A g−1), good rate capability (53 mAh g−1 at high current density of 5 A g−1), and a high‐capacity retention rate (135 mAh g−1 at 0.2 A g−1 between 25 and 300 cycles). This is attributed to high electrical conductivity and flexibility offered by the designed carbon framework, the superior capacity of the TT‐Nb2O5 and the linkage between the two provided by the bonding glucose derived carbon. This work therefore demonstrates a scalable route for the application of metal oxides in future high‐performance SIB systems. [ABSTRACT FROM AUTHOR]

Poly(ethylene oxide) (PEO) is a promising solid electrolyte material for solid‐state lithium–sulfur (Li–S) batteries, but low intrinsic ionic conductivity, poor mechanical properties, and failure to hinder the polysulfide shuttle effect limits its application. Herein, a polymer of intrinsic microporosity (PIM) is synthesized and applied as an organic framework to comprehensively enhance the performance of PEO by forming a composite electrolyte (PEO‐PIM). The unique structure of PIM‐1 not only enhances the mechanical strength and hardness over the PEO electrolyte by an order of magnitude, increasing stability toward the metallic lithium anode but also increases its ionic conductivity by lowering the degree of crystallinity. Furthermore, the PIM‐1 is shown to effectively trap lithium polysulfide species to mitigate against the detrimental polysulfide shuttle effect, as electrophilic 1,4‐dicyanooxanthrene functional groups possess higher binding energy to polysulfides. Benefiting from these properties, the use of PEO‐PIM composite electrolyte has achieved greatly improved rate performance, long‐cycling stability, and excellent safety features for solid‐state Li‐S batteries. This methodology offers a new direction for the optimization of solid polymer electrolytes. [ABSTRACT FROM AUTHOR]

Atomically dispersed transition metal‐nitrogen‐carbon catalysts are emerging as low‐cost electrocatalysts for the oxygen reduction reaction in fuel cells. However, a cost‐effective and scalable synthesis strategy for these catalysts is still required, as well as a greater understanding of their mechanisms. Herein, iron, nitrogen co‐doped carbon spheres (Fe@NCS) have been prepared via hydrothermal carbonization and high‐temperature post carbonization. It is determined that FeN4 is the main form of iron existing in the obtained Fe@NCS. Two different precursors containing Fe2+ and Fe3+ are compared. Both chemical and structural differences have been observed in catalysts starting from Fe2+ and Fe3+ precursors. Fe2+@NCS‐A (starting with Fe2+ precursor) shows better catalytic activity for the oxygen reduction reaction. This catalyst is studied in an anion exchange membrane fuel cell. The high open‐circuit voltage demonstrates the potential approach for developing high‐performance, low‐cost fuel cell catalysts. [ABSTRACT FROM AUTHOR]

Although lithium, and other alkali ion, batteries are widely utilized and studied, many of the chemical and mechanical processes that underpin the materials within, and drive their degradation/failure, are not fully understood. Hence, to enhance the understanding of these processes various ex situ, in situ and operando characterization methods are being explored. Recently, electrochemical atomic force microscopy (EC‐AFM), and related techniques, have emerged as crucial platforms for the versatile characterization of battery material surfaces. They have revealed insights into the morphological, mechanical, chemical, and physical properties of battery materials when they evolve under electrochemical control. This critical review will appraise the progress made in the understanding batteries using EC‐AFM, covering both traditional and new electrode–electrolyte material junctions. This progress will be juxtaposed against the ability, or inability, of the system adopted to embody a truly representative battery environment. By contrasting key EC‐AFM literature with conclusions drawn from alternative characterization tools, the unique power of EC‐AFM to elucidate processes at battery interfaces is highlighted. Simultaneously opportunities for complementing EC‐AFM data with a range of spectroscopic, microscopic, and diffraction techniques to overcome its limitations are described, thus facilitating improved battery performance. [ABSTRACT FROM AUTHOR]

Polymer electrolyte fuel cells (PEFCs) are a promising replacement for the fossil fuel–dependent automotive and energy sectors. They have become increasingly commercialized in the last decade; however, significant limitations on durability and performance limit their commercial uptake. Catalyst layer (CL) design is commonly reported to impact device power density and durability; although, a consensus is rarely reached due to differences in testing conditions, experimental design, and types of data reported. This is further exacerbated by aspects of CL design such as catalyst support, proton conduction, catalyst, fabrication, and morphology, being significantly interdependent; hence, a wider appreciation is required in order to optimize performance, improve durability, and reduce costs. Here, the cutting‐edge research within the field of PEFCs is reviewed, investigating the effect of different manufacturing techniques, electrolyte distribution, support materials, surface chemistries, and total porosity on power density and durability. These are critically appraised from an applied perspective to inform the most relevant and promising pathways to make and test commercially viable cells. This holistic view of the competing aspects of CL design and preparation will facilitate the development of optimized CLs, especially the incorporation of novel catalyst support materials. [ABSTRACT FROM AUTHOR]

Aqueous zinc-ion batteries (ZIB) with Zn metal anodes are promising candidates for future electrochemical energy storage devices. However, Zn dendrite growth greatly limits their practical application. Many recent studies have developed methods to hinder dendrite formation and growth, including interfacial barrier layers, alternative anode and cathode materials, new electrolyte chemistries, or complex structured separators. However, relatively little attention has been paid to the structure of the Zn foil itself. Herein, by simply polishing the Zn foil before electrochemical operation, the morphological uniformity and reversibility of the deposited Zn layer on the metal anode is significantly improved during cycling, compared to that of as-received Zn foils. By combining ex situ optical microscopy (OM) and in situ electrochemical atomic force microscopy (EC-AFM), it is demonstrated that the initial roughness of the Zn foil electrode surface defines the subsequent plating/stripping morphology. The use of flatter foil anodes notably increases ZIB cycle life. This methodology offers a simple and industrially scalable route to wider ZIB utilisation, as well as highlighting an important consideration for future battery research. [ABSTRACT FROM AUTHOR]

The spontaneous dissolution of 2D polytriazine imide (PTI) carbon nitrides in certain solvents diverges dramatically from the inherent insolubility of other 2D materials such as graphene. Here, the mechanism of dissolution and underlying factors which govern PTI solubility are probed, uncovering a complex and adaptable system. At high concentrations, multi-layered species are co-dissolved, and these solubilised stacks may be further exfoliated to few- or single-layer species with the addition of water. While the PTI sheets are fundamentally soluble, the presence of intercalated lithium salts increases yield and improves the degree of exfoliation, with lithium cations adsorbed on the solvated PTI layers. The tuneable degree of delamination modifies the solutions' photoluminescent properties, which may be trapped in a solid phase following vitrification of the solvent. [ABSTRACT FROM AUTHOR]

Abstract: Two‐dimensional (2D) layered graphitic carbon nitride (gCN) nanosheets offer intriguing electronic and chemical properties. However, the exfoliation and functionalisation of gCN for specific applications remain challenging. We report a scalable one‐pot reductive method to produce solutions of single‐ and few‐layer 2D gCN nanosheets with excellent stability in a high mass yield (35 %) from polytriazine imide. High‐resolution imaging confirmed the intact crystalline structure and identified an AB stacking for gCN layers. The charge allows deliberate organic functionalisation of dissolved gCN, providing a general route to adjust their properties. [ABSTRACT FROM AUTHOR]

The combustion of mercury(II) thiocyanate to form 'Pharaoh's serpents' is a spectacular reaction first described nearly two centuries ago. The large volume of distinctive yellow branches that grow from a tiny quantity of flaming reactants makes this an enchanting demonstration, often used to depict the magic of chemistry. In recent years several videos of this bizarre process have 'gone viral' online. Formally, the reaction should yield a carbon nitride with the ideal formula C3N4 along with HgS. However, since early characterization attempts there has been little further study of the materials produced. Herein we apply modern characterization techniques to reinvestigate the nature of the carbon nitride and its intimate relationship to the HgS produced. The HgS phase forms nanoparticles that decorate the surfaces of a C3N4 foam matrix. Both of these compounds are important wide-gap semiconductors and we propose that the 'Pharaoh's serpents reaction' could be used to produce a potentially important heterojunction nanocomposite materials that could be useful optoelectronic and photocatalytic applications. [ABSTRACT FROM AUTHOR]

A primary method for the production of 2D nanosheets is liquid-phase delamination from their 3D layered bulk analogues. Most strategies currently achieve this objective by significant mechanical energy input or chemical modification but these processes are detrimental to the structure and properties of the resulting 2D nanomaterials. Bulk poly(triazine imide) (PTI)-based carbon nitrides are layered materials with a high degree of crystalline order. Here, we demonstrate that these semiconductors are spontaneously soluble in select polar aprotic solvents, that is, without any chemical or physical intervention. In contrast to more aggressive exfoliation strategies, this thermodynamically driven dissolution process perfectly maintains the crystallographic form of the starting material, yielding solutions of defect-free, hexagonal 2D nanosheets with a well-defined size distribution. This pristine nanosheet structure results in narrow, excitation-wavelength-independent photoluminescence emission spectra. Furthermore, by controlling the aggregation state of the nanosheets, we demonstrate that the emission wavelengths can be tuned from narrow UV to broad-band white. This has potential applicability to a range of optoelectronic devices. [ABSTRACT FROM AUTHOR]

Graphitic carbon nitride (gCN) is shown to undergo lithium insertion reactions applicable with lithium-ion battery anodes. Lithium capacity was found to be substantially lower than theoretically expected, so the properties of gCN composited with conducting graphite (CG), which was added to improve the performance, were investigated. The electrodes exhibited a systematic increase in lithium uptake with CG content, but the capacity never exceeded that of graphite. It is shown that electron transport via conducting pathways was limiting. Li+ uptake for 10 % gCN was similar to a graphite electrode, indicating that gCN does play a role in determining the storage capacity. [ABSTRACT FROM AUTHOR]

Voltammetric studies of dopamine (DA) oxidation on pristine and acid-treated single-walled carbon nanotube (SWNT) network electrodes were undertaken in order to investigate both the effect of network density and acid treatment times on the voltammetric characteristics for DA oxidation and the susceptibility of the electrodes to fouling. Through careful control of catalysed chemical vapour deposition growth parameters, multiply interconnected and randomly oriented SWNT networks of two significantly different densities were grown (high density, HD, coverage >10 μm length of SWNT per μm−2 and low density, LD, coverage = 5 (±1) μmSWNTμm−2). Acid treatment was performed to provide materials with different electrochemical properties and SWNT coverage, as determined by field emission-scanning electron microscopy, atomic force microscopy and micro-Raman spectroscopy. A high concentration of DA (100 μM) was deliberately employed to accelerate the fouling phenomenon associated with DA oxidation in order to evaluate the lifetime of the electrodes. HD pristine SWNT networks were found to promote more facile electron transfer (ET) and were less susceptible to blocking, compared to LD pristine SWNT networks. Acid treatment resulted in both a further enhancement of the ET rate and a reduction in susceptibility towards electrode fouling. However, lengthy acid treatment detrimentally affected ET, due to a decrease in network density and significant damage to the SWNT network structure. These studies highlight the subtle interplay between SWNT coverage and degree of acid functionalisation when seeking to achieve the optimal SWNT electrode for the voltammetric detection of DA. [ABSTRACT FROM AUTHOR]

The meniscus-based microcapillary electrochemical method (MCEM) allows electrochemical measurements to be made quickly and easily at a wide range of materials, simply by connecting up the sample of interest as a working electrode and bringing a capillary containing an electrolyte solution and quasi-reference/counter electrode into meniscus contact. In this work, microcapillary-based electrochemical methodology is advanced by introducing a very high aspect ratio carbon fibre ultramicroelectrode (CF-UME) allowing generation/collection and shielding (redox competition) measurements to be made. The experimental concept is demonstrated with the outer sphere (ferrocenylmethyl) trimethylammonium ion (FcTMA +/2+ ) redox mediator on a single walled carbon nanotube (SWNT) network. It is then used to investigate electrochemical reactions on a complex electrode, i.e. a SWNT forest, which is not easily investigated using traditional techniques that would require the electrode material to be physically encapsulated. The technique is most powerful when used to probe the different mechanistic pathways of the oxygen reduction reaction (ORR). This aspect is illustrated through studies on platinum, glassy carbon and SWNT forest electrodes. By platinising the CF-UME, the electrode can be used as a local sensor for the intermediate H 2 O 2 and the O 2 reactant, and it is possible to track the evolving consumption of these species near the working electrode during voltammetric measurements. [ABSTRACT FROM AUTHOR]