Engineered Two-Dimensional Transition Metal Dichalcogenides for Energy Conversion and Storage.

Designing efficient and cost-effective materials is pivotal to solving the key scientific and technological challenges at the interface of energy, environment, and sustainability for achieving NetZero. Two-dimensional transition metal dichalcogenides (2D TMDs) represent a unique class of materials that have catered to a myriad of energy conversion and storage (ECS) applications. Their uniqueness arises from their ultra-thin nature, high fractions of atoms residing on surfaces, rich chemical compositions featuring diverse metals and chalcogens, and remarkable tunability across multiple length scales. Specifically, the rich electronic/electrical, optical, and thermal properties of 2D TMDs have been widely exploited for electrochemical energy conversion (e.g., electrocatalytic water splitting), and storage (e.g., anodes in alkali ion batteries and supercapacitors), photocatalysis, photovoltaic devices, and thermoelectric applications. Furthermore, their properties and performances can be greatly boosted by judicious structural and chemical tuning through phase, size, composition, defect, dopant, topological, and heterostructure engineering. The challenge, however, is to design and control such engineering levers, optimally and specifically, to maximize performance outcomes for targeted applications. In this review we discuss, highlight, and provide insights on the significant advancements and ongoing research directions in the design and engineering approaches of 2D TMDs for improving their performance and potential in ECS applications.

Unlocking the Potential: Atomically Thin 2D Fluoritene from Exfoliated Fluorite Ore and Its Electrochemical Activity.

Fluorite mineral holds significant importance because of its optoelectronic properties and wide range of applications. Here, we report the successful exfoliation of bulk fluorite ore (calcium fluoride, CaF2) crystals into atomically thin two-dimensional fluoritene (2D CaF2) using a highly scalable liquid-phase exfoliation method. The microscopic and spectroscopy characterizations show the formation of (111) plane-oriented 2D CaF2 sheets with exfoliation-induced material strain due to bond breaking, leading to the changes in lattice parameter. Its potential role in electrocatalysis is further explored for deeper insight, and a probable mechanism is also discussed. The 2D CaF2 with long-term stability shows overpotential values of 670 and 770 mV vs RHE for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, at 10 mA cm-2. Computational simulations demonstrate the unique "direct-indirect" band gap switching with odd and even numbers of layers. Current work offers new avenues for exploring the structural and electrochemical properties of 2D CaF2 and its potential applicability.

Emerging Processes for Sustainable Li-Ion Battery Cathode Recycling.

The colossal growth in the use of Li-ion batteries (LiBs) has raised serious concerns over the supply chain of strategic minerals, e.g., Co, Ni, and Li, that make up the cathode active materials (CAM). Recycling spent LiBs is an important step toward sustainability that can establish a circular economy by effectively tackling large amounts of e-waste while ensuring an unhindered supply of critical minerals. Among the various methods of LiB recycling available, pyro- and hydrometallurgy have been utilized in the industry owing to their ease of operation and high efficiency, although they are associated with significant environmental concerns. Direct recycling, a more recent concept that aims to relithiate spent LiBs without disrupting the lattice structure of the CAMs, has been realized only in the laboratory scale so far and further optimization is required before it can be extended to the bulk scale. Additionally, significant progress has been made in the areas of hydrometallurgy in terms of using ecofriendly green lixiviants and alternate sources of energy, e.g., microwave and electrochemical, that makes the recycling processes more efficient and sustainable. In this review, the latest developments in LiB recycling are discussed that have focused on environmental and economic viability, as well as process intensification. These include deep eutectic solvent based recycling, electrochemical and microwave-assisted recycling, and various types of direct recycling.

Multifunctional Applications Enabled by Fluorination of Hexagonal Boron Nitride.

2D materials exhibit exceptional properties as compared to their macroscopic counterparts, with promising applications in nearly every area of science and technology. To unlock further functionality, the chemical functionalization of 2D structures is a powerful technique that enables tunability and new properties within these materials. Here, the successful effort to chemically functionalize hexagonal boron nitride (hBN), a chemically inert 2D ceramic with weak interlayer forces, using a gas-phase fluorination process is exploited. The fluorine functionalization guides interlayer expansion and increased polar surface charges on the hBN sheets resulting in a number of vastly improved applications. Specifically, the F-hBN exhibits enhanced dispersibility and thermal conductivity at higher temperatures by more than 75% offering exceptional performance as a thermofluid additive. Dispersion of low volumes of F-hBN in lubricating oils also offers marked improvements in lubrication and wear resistance for steel tribological contacts decreasing friction by 31% and wear by 71%. Additionally, incorporating numerous negatively charged fluorine atoms on hBN induces a permanent dipole moment, demonstrating its applicability in microelectronic device applications. The findings suggest that anchoring chemical functionalities to hBN moieties improves a variety of properties for h-BN, making it suitable for numerous other applications such as fillers or reinforcement agents and developing high-performance composite structures.

Tunable 2D Conjugated Porous Organic Polymer Films for Precise Molecular Nanofiltration and Optoelectronics.

Structural design of 2D conjugated porous organic polymer films (2D CPOPs), by tuning linkage chemistries and pore sizes, provides great adaptability for various applications, including membrane separation. Here, four free-standing 2D CPOP films of imine- or hydrazone-linked polymers (ILP/HLP) in combination with benzene (B-ILP/HLP) and triphenylbenzene (TPB-ILP/HLP) aromatic cores are synthesized. The anisotropic disordered films, composed of polymeric layered structures, can be exfoliated into ultrathin 2D-nanosheets with layer-dependent electrical properties. The bulk CPOP films exhibit structure-dependent optical properties, triboelectric nanogenerator output, and robust mechanical properties, rivaling previously reported 2D polymers and porous materials. The exfoliation energies of the 2D CPOPs and their mechanical behavior at the molecular level are investigated using density function theory (DFT) and molecular dynamics (MD) simulations, respectively. Exploiting the structural tunability, the comparative organic solvent nanofiltration (OSN) performance of six membranes having different pore sizes and linkages to yield valuable trends in molecular weight selectivity is investigated. Interestingly, the OSN performances follow the predicted transport modeling values based on theoretical pore size calculations, signifying the existence of permanent porosity in these materials. The membranes exhibit excellent stability in organic solvents at high pressures devoid of any structural deformations, revealing their potential in practical OSN applications.

Imine-Linked 2D Conjugated Porous Organic Polymer Films for Tunable Acid Vapor Sensing.

Two-dimensional (2D) films of conjugated porous organic polymers (C-POPs) can translate the rich in-plane functionalities of conjugated frameworks into diverse optical and electronic applications while addressing the processability issues of their crystalline analogs for adaptable device architectures. However, the lack of facile single-step synthetic routes to obtain large-area high-quality films of 2D-C-POPs has limited their application possibilities so far. Here, we report the synthesis of four mechanically robust imine-linked 2D-C-POP free-standing films using a single-step fast condensation route that is scalable and tunable. The rigid covalently bonded 2D structures of the C-POP films offer high stability for volatile gas sensing in harsh environments while simultaneously enhancing site accessibility for gas molecules due to mesoporosity by structural design. Structurally, all films were composed of exfoliable layers of 2D polymeric nanosheets (NSs) that displayed anisotropy from disordered stacking, evinced by out-of-plane birefringent properties. The tunable in-plane conjugation, different nitrogen centers, and porous structures allow the films to act as ultraresponsive colorimetric sensors for acid sensing via reversible imine bond protonation. All the films could detect hydrogen chloride (HCl) gas down to 0.05 ppm, far exceeding the Occupational Safety and Health Administration's permissible exposure limit of 5 ppm with fast response time and good recyclability. Computational insights elucidated the effect of conjugation and tertiary nitrogen in the structures on the sensitivity and response time of the films. Furthermore, we exploited the exfoliated large 2D NSs and anisotropic optoelectronic properties of the films to adapt them into micro-optical and triboelectric devices to demonstrate their real-time sensing capabilities.

Biotene: Earth-Abundant 2D Material as Sustainable Anode for Li/Na-Ion Battery.

Natural ores are abundant, cost-effective, and environmentally friendly. Ultrathin (2D) layers of a naturally abundant van der Waals mineral, Biotite, have been prepared in bulk via exfoliation. We report here that this 2D Biotene material has shown extraordinary Li-Na-ion battery anode properties with ultralong cycling stability. Biotene shows 302 and 141 mAh g-1 first cycle-specific charge capacity for Li- and Na-ion battery applications with ∼90% initial Coulombic efficiency. The electrode exhibits significantly extended cycling stability with ∼75% capacity retention after 4000 cycles even at higher current densities (500-2000 mA g-1). Further, density functional theory studies show the possible Li intercalation mechanism between the 2D Biotene layers. Our work brings new directions toward designing the next generation of metal-ion battery anodes.

Vapor Growth of All-Inorganic 2D Ruddlesden-Popper Lead- and Tin-Based Perovskites.

The 2D Ruddlesden-Popper (RP) perovskites Cs2PbI2Cl2 (Pb-based, n = 1) and Cs2SnI2Cl2 (Sn-based, n = 1) stand out as unique and rare instances of entirely inorganic constituents within the more expansive category of organic/inorganic 2D perovskites. These materials have recently garnered significant attention for their strong UV-light responsiveness, exceptional thermal stability, and theoretically predicted ultrahigh carrier mobility. In this study, we synthesized Pb and Sn-based n = 1 2D RP perovskite films covering millimeter-scale areas for the first time, utilizing a one-step chemical vapor deposition (CVD) method under atmospheric conditions. These films feature perovskite layers oriented horizontally relative to the substrate. Multilayered Cs3Pb2I3Cl4 (Pb-based, n = 2) and Cs3Sn2I3Cl4 (Sn-based, n = 2) films were also obtained for the first time, and their crystallographic structures were refined by combining X-ray diffraction (XRD) and density functional theory (DFT) calculations. DFT calculations and experimental optical spectroscopy support band-gap energy shifts related to the perovskite layer thickness. We demonstrate bias-free photodetectors using the Sn-based, n = 1 perovskite with reproducible photocurrent and a fast 84 ms response time. The present work not only demonstrates the growth of high-quality all-inorganic multilayered 2D perovskites via the CVD method but also suggests their potential as promising candidates for future optoelectronic applications.