Energy Materials

Owing to MXenes’ tunable mechanical properties induced by their structural and chemical diversity, MXenes are believed to compete with state-of-the-art 2D nanomaterials such as graphene regarding their tribological performance. Their nanolaminate structure offers weak interlayer interactions and an easy-to-shear ability to render them excellent candidates for solid lubrication. However, the acting friction and wear mechanisms are yet to be explored. To elucidate these mechanisms, 100-nm-thick homogeneous multilayer Ti3C2Tx coatings are deposited on technologically relevant stainless steel by electrospraying. Using ball-on-disk tribometry (Si3N4 counterbody) with acting contact pressures of about 300 MPa, their long-term friction and wear performance under dry conditions are studied. MXene-coated specimens demonstrate a 6-fold friction reduction and an ultralow wear rate (4 × 10–9 mm3 N–1 m–1) over 100 000 sliding cycles, outperforming state-of-the-art 2D nanomaterials by at least 200% regarding their wear life. High-resolution characterization verified the formation of a beneficial tribolayer consisting of thermally/mechanically degraded MXenes and amorphous/nanocrystalline iron oxides. The transfer of this tribolayer to the counterbody transforms the initial steel/Si3N4 contact to tribolayer/tribolayer contact with low shear resistance. MXene pileups at the wear track’s reversal points continuously supply the tribological contact with fresh, lubricious nanosheets, thus enabling an ultra-wear-resistant and low-friction performance.

Abstract Owing to high electrical conductivity and ability to reversibly host a variety of inserted ions, 2D metallic molybdenum disulfide (1T-MoS2) has demonstrated promising energy storage performance when used as a supercapacitor electrode. However, its charge storage mechanism is still not fully understood, in particular, how the interlayer spacing of 1T-MoS2 would affect its capacitive performance. In this work, molecular dynamics simulations of 1T-MoS2 with interlayer spacing ranging from 0.615 nm to 1.615 nm have been performed to investigate the resulting charge storage capacity in ionic liquids. Simulations reveal a camel-like capacitance-potential relation, and MoS2 with an interlayer spacing of 1.115 nm has the highest volumetric and gravimetric capacitance of 118 F cm-3 and 42 F g-1, respectively. Although ions in MoS2 with an interlayer spacing of 1.115 nm diffuse much faster than with interlayer spacings of 1.365 nm and 1.615 nm, the MoS2 with larger interlayer spacing has a much faster charging process. Our analyses reveal that the ion number density and its charging speed, as well as ion motion paths, have significant impacts on the charging response. This work helps to understand how the interlayer spacing affects the interlayer ion structures and the capacitive performance of MoS2, which is important for revealing the charge storage mechanism and designing MoS2 supercapacitor.

The preparation of carbons for technical applications is typically based on a treatment of a precursor, which is transformed into the carbon phase with the desired structural properties. During such treatment the material passes through several different structural stages, for example, starting from precursor molecules via an amorphous phase into crystalline-like phases. While the structure of non-graphitic and graphitic carbon has been well studied, the transformation stages from molecular to amorphous and non-graphitic carbon are still not fully understood. Disordered carbon often contains a mixture of sp3-, sp2-and sp1-hybridized bonds, whose analysis is difficult to interpret. We systematically address this issue by studying the transformation of purely sp3-hybridized carbons, that is, nanodiamond and adamantane, into sp2-hybridized non-graphitic and graphitic carbon. The precursor materials are thermally treated at different temperatures and the transformation stages are monitored. We employ Raman spectroscopy, WAXS and TEM to characterize the structural changes. We correlate the intensities and positions of the Raman bands with the lateral crystallite size La estimated by WAXS analysis. The behavior of the D and G Raman bands characteristic for sp2-type material formed by transforming the sp3-hybridized precursors into non-graphitic and graphitic carbon agrees well with that observed using sp2-structured precursors.

Capacitive deionization with porous carbon electrodes is an energy-efficient water treatment technique limited to the remediation of only brackish water due to the severe efficiency drop at high molar strength. Combining experiment and simulation, our work demonstrates the ability of subnanometer pores for permselective ion electrosorption, which enables capacitive deionization for saline media with high concentrations. Molecular dynamics simulations reveal the origin of permselective ion electrosorption in subnanometer pores at high molar strength. Within the subnanometer range, carbon pores with smaller size become more ionophobic and then express a higher ability of permselective ion electrosorption. This can be understood by the effects of the pore size on the microstructure of in-pore water and ions and the nanoconfinement effects on the ion hydration. These findings provide a new avenue for capacitive deionization of saline water (seawater-like ionic strength) to enable the application of highly concentrated saline media by direct use of porous carbons.

Electrolyte-filled subnanometre pores exhibit exciting physics and play an increasingly important role in science and technology. In supercapacitors, for instance, ultranarrow pores provide excellent capacitive characteristics. However, ions experience difficulties in entering and leaving such pores, which slows down charging and discharging processes. In an earlier work we showed for a simple model that a slow voltage sweep charges ultranarrow pores quicker than an abrupt voltage step. A slowly applied voltage avoids ionic clogging and co-ion trapping—a problem known to occur when the applied potential is varied too quickly—causing sluggish dynamics. Herein, we verify this finding experimentally. Guided by theoretical considerations, we also develop a non-linear voltage sweep and demonstrate, with molecular dynamics simulations, that it can charge a nanopore even faster than the corresponding optimized linear sweep. For discharging we find, with simulations and in experiments, that if we reverse the applied potential and then sweep it to zero, the pores lose their charge much quicker than they do for a short-circuited discharge over their internal resistance. Our findings open up opportunities to greatly accelerate charging and discharging of subnanometre pores without compromising the capacitive characteristics, improving their importance for energy storage, capacitive deionization, and electrochemical heat harvesting.

Nb2O5 has been explored as a promising anode material for use as lithium-ion batteries (LIBs), but depending on the crystal structure, the specific capacity was always reported to be usually around or below 200 mAh/g. For the first time, we present coarse-grained Nb2O5 materials that significantly overcome this capacity limitation with the promise of enabling high power applications. Our work introduces coarse-grained carbide-derived Nb2O5 phases obtained either by a one-step or a two-step bulk conversion process. By in situ production of chlorine gas from metal chloride salt at ambient pressure, we obtain in just one step directly orthorhombic Nb2O5 alongside carbide-derived carbon (o-Nb2O5/CDC). In situ formation of chlorine gas from metal chloride salt under vacuum conditions yields CDC covering the remaining carbide core, which can be transformed into metal oxides covered by a carbon shell upon thermal treatment in CO2 gas. The two-step process yielded a mixed-phase tetragonal and monoclinic Nb2O5 with CDC (m-Nb2O5/CDC). Our combined diffraction and spectroscopic data confirm that carbide-derived Nb2O5 materials show disordering of the crystallographic planes caused by oxygen deficiency in the structural units and, in the case of m-Nb2O5/CDC, severe stacking faults. This defect engineering allows access to a very high specific capacity exceeding the two-electron transfer process of conventional Nb2O5. The charge storage capacities of the resulting m-Nb2O5/CDC and o-Nb2O5/CDC are, in both cases, around 300 mAh/g at a specific current of 10 mA/g, thereby, the values are significantly higher than that of the state-of-the-art for Nb2O5 as a LIB anode. Carbide-derived Nb2O5 materials also show robust cycling stability over 500 cycles with capacity fading only 24% for the sample m-Nb2O5/CDC and 28% for o-Nb2O5/CDC, suggesting low degree of expansion/compaction during lithiation and delithiation.

Abstract Continuous and low-energy desalination technologies are in high demand to enable sustainable water remediation. Our work introduces a continuous desalination process based on the redox reaction of a dual-zinc electrode. The system consists of two zinc foils as redox electrodes with flowing ZnCl2 electrolyte, concentrated and diluted salt streams with three anion- and cation-exchange membranes (AEM and CEM) separated configuration (AEM|CEM|AEM). If a constant current is applied, the negative zinc electrode is oxidized, and electrons are released to the external circuit, whereas the positive zinc electrode is reduced, causing salt removal in the dilution stream. The results showed that brackish water can be directly desalted to 380.6 ppm during a continuous batch-mode process. The energy consumption can be as low as 35.30 kJ mol−1 at a current density of 0.25 mA cm−2, which is comparable to reverse osmosis. In addition, the dual-zinc electrode electrochemical desalination demonstrates excellent rate performance, reversibility, and batch cyclability through electrode exchange regeneration. Our research provides a route for continuous low-energy desalination based on metal redox mediators.

There is an urgent global need for electrochemical energy storage that includes materials that can provide simultaneous high power and high energy density. One strategy to achieve this goal is with pseudocapacitive materials that take advantage of reversible surface or near-surface Faradaic reactions to store charge. This allows them to surpass the capacity limitations of electrical double-layer capacitors and the mass transfer limitations of batteries. The past decade has seen tremendous growth in the understanding of pseudocapacitance as well as materials that exhibit this phenomenon. The purpose of this Review is to examine the fundamental development of the concept of pseudocapacitance and how it came to prominence in electrochemical energy storage as well as to describe new classes of materials whose electrochemical energy storage behavior can be described as pseudocapacitive.

Many metal sulfides present a layered structure with large interlayer space and a high theoretical capacity for lithium-ion battery applications. Compared to other transition metal dichalcogenides, vanadium sulfides remain little explored. Vanadium sulfides are commonly obtained by hydrothermal synthesis, which requires further post-processing and coating with binder and carbon additives. Here, we introduce a route to obtain free-standing vanadium sulfide fiber mats with in-built carbon. The combination of electrospinning and thermal sulfidation with H2S produces homogeneous vanadium sulfide particles embedded in carbon fibers that provide electrical conductivity and mechanical resistance for the electrode. The fibers were tested as a binder-free lithium-ion battery cathode within different potential ranges to evaluate insertion and conversion mechanisms and contributions to the overall capacity. Between 1.2 V and 3.5 V vs. Li/Li+, lithium intercalation provides a specific capacity up to 138 mAh∙g−1 at 0.01 A g−1 with good rate handling. When operating in a larger potential range between 0.1 V and 3.0 V vs. Li/Li+, the contribution by conversion reactions increases the capacity to 790 mAh∙g−1, but there is a fast capacity fading.

MAX phases are etched using an ionic liquid–water mixture to produce titanium carbide MXenes. The process avoids the use of any acid. Hydrolysis of the fluorine-containing ionic liquid leads to the selective removal of Al, while the ionic liquid is intercalated in-between the transition metal carbide layers.