Energy Materials

A fundamental understanding of ion charge storage in nanoporous electrodes is essential to improve the performance of supercapacitors or devices for capacitive desalination. Here, we employ in situ X-ray transmission measurements on activated carbon supercapacitors to study ion concentration changes during electrochemical operation. Whereas counter-ion adsorption was found to dominate at small electrolyte salt concentrations and slow cycling speed, ion replacement prevails for high molar concentrations and/or fast cycling. Chronoamperometry measurements reveal two distinct time regimes of ion concentration changes. In the first regime the supercapacitor is charged, and counter- and co-ion concentration changes align with ion replacement and partially co-ion expulsion. In the second regime, the electrode charge remains constant, but the total ion concentration increases. We conclude that the initial fast charge neutralization in nanoporous supercapacitor electrodes leads to a non-equilibrium ion configuration. The subsequent, charge-neutral equilibration slowly increases the total ion concentration towards counter-ion adsorption.

Mechanically reversible compressible resorcinol–formaldehyde (RF) aerogels can be converted into mechanically reversible compressible carbon aerogels (CA) by carbonization in an inert atmosphere. By incorporation of polystyrene spheres into the RF gels as a sacrificial template, it is possible to create macropores with controlled size within the carbon framework during carbonization. The resulting templated carbon aerogel shows enhanced mechanical flexibility during compression compared to pristine samples. In addition, the presence of hierarchical porosity provides a porous architecture attractive for energy storage applications, such as supercapacitors.

Ion intercalation materials are emerging as a highly attractive class of electrodes for efficient energy water desalination. Most materials and concepts so far have focused on the removal of cations (especially sodium). Anion intercalation, however, remains poorly investigated in water desalination. We present a study on the capability of Mo1.33C-MXene for removing cations and anions and demonstrate the desalination performance in brackish water and seawater concentrations. Mo1.33C-MXene was prepared via acid treatment of the transition metal carbide MAX phase (Mo2/3Sc1/3)2AlC. Binder-free electrodes were obtained by entangling MXene with carbon nanotubes and tested without the use of any ion exchange membrane at low (5 mM NaCl) and high (600 mM NaCl) salt concentrations. Such electrodes showed a promising desalination performance of 15 mg/g in 600 mM NaCl with high charge efficiency up to 95%. By employing chemical online monitoring of the effluent stream, we separated the cation and anion intercalation capacity of the electrode material.

Aqueous electrolytes can be used for electrical double-layer capacitors, pseudocapacitors, and intercalation-type batteries. These technologies may employ different electrode materials, most importantly high-surface-area nanoporous carbon, two-dimensional materials, and metal oxides. All of these materials also find more and more applications in electrochemical desalination devices. During the electrochemical operation of such electrode materials, charge storage and ion immobilization are accomplished by non-Faradaic ion electrosorption, Faradaic ion intercalation at specific crystallographic sites, or ion insertion between layers of two-dimensional materials. These processes may or may not be associated with a (partial) loss of the aqueous solvation shell around the ions. Our work showcases the electrochemical quartz crystal microbalance as an excellent tool for quantifying the change in effective solvation. We chose sodium as an important cation for energy storage materials (sodium-based aqueous electrolytes) and electrochemical desalination (saline media). Our data show that a major amount of water uptake occurs during ion electrosorption in nanoporous carbon, while battery-like ion insertion between layers of titanium disulfide is associated with an 80% loss of the initially present solvation molecules. Sodiation of MXene is accomplished by a loss of 90% of the number of solvent molecules, but nanoconfined water in-between the MXene layers may compensate for this large degree of desolvation. In the case of sodium manganese oxide, we were able to demonstrate the full loss of the solvation shell.

Abstract The selective removal of ions by an electrochemical process is a promising approach to enable various water-treatment applications such as water softening or heavy-metal removal. Ion intercalation materials have been investigated for their intrinsic ability to prefer one specific ion over others, showing a preference for (small) monovalent ions over multivalent species. In this work, we present a fundamentally different approach: tunable ion selectivity not by modifying the electrode material, but by changing the operational voltage. We used titanium disulfide, which shows distinctly different potentials for the intercalation of different cations and formed binder-free composite electrodes with carbon nanotubes. Capitalizing on this potential difference, we demonstrated controllable cation selectivity by online monitoring the effluent stream during electrochemical operation by inductively coupled plasma optical emission spectrometry of aqueous 50 mm CsCl and MgCl2. We obtained a molar selectivity of Mg2+ over Cs+ of 31 (strong Mg preference) in the potential range between −396 mV and −220 mV versus Ag/AgCl. By adjusting the operational potential window from −219 mV to +26 mV versus Ag/AgCl, Cs+ was preferred over Mg2+ by 1.7 times (Cs preference).

In the present study, nickel matrix composites reinforced with a fine distribution of nanodiamonds (6.5 vol%) as reinforcement phase are annealed in vacuum at different temperatures ranging from 750 °C to 1300 °C. This is carried out to evaluate the in-situ transformation of nanodiamonds to carbon onions within a previously densified composite. The resulting materials are thoroughly analyzed by complementary analytical methods, including Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The proposed in-situ transformation method presents two main benefits. On one hand, since the particle distribution of a nanodiamond-reinforced composite is significantly more homogenous than in case of the carbon onions, it is expected that the transformed particles will preserve the initial distribution features of nanodiamonds. On the other hand, the proposed process allows for the tuning of the sp3/sp2 carbon ratio by applying a single straightforward post-processing step.

As population demands for freshwater increase, existing natural freshwater resources face significant strains. Currently, over 2.5 billion people live in localities that are subject to severe water scarcity at least 1 month of the year.1 Scarcity affects all types of localities, such as urban, rural, coastal areas, landlocked areas, and off-grid locations. Increasingly, active water purification technologies are being used to boost and secure freshwater supplies. A widely used desalination technology is seawater reverse osmosis (SWRO), in which pumps pressurize the feedwater to well above its osmotic pressure to pump water molecules through a membrane largely impermeable to salt ions (posmotic ∼ 25 bar).2 City-scale SWRO plants are operational in several countries, delivering on the order of 106 m3 of treated water per day (<0.1% of the total global daily water consumption). However, as the need for water purification increases and the requirements for each locality becomes more diverse, SWRO plants alone cannot meet the growing demand for a technological solution. Barriers toward increased penetration of SWRO include the enormous investment required to develop such plants, poor downscalability of the technology, the geographical limitation to coastal areas and near urban environments, and high energy requirements (typically about 4 kWh/m3).2

Vanadium oxide nanostructures are constantly being researched and developed for cathodes in lithium- and sodium-ion batteries. To improve the internal resistance and the discharge capacity, this study explores the synthesis and characterization of continuous one-dimensional hybrid nanostructures. Starting from a sol–gel synthesis, followed by electrospinning and controlled thermal treatment, we obtained hybrid fibers consisting of metal oxide crystals (orthorhombic V2O5 and monoclinic VO2) engulfed in conductive carbon. For use as Li-ion battery cathode, a higher amount of carbon yields a more stable performance and an improved capacity. Monoclinic VO2/C fibers present a specific capacity of 269 mAh·gVOx–1 and maintain 66% of the initial capacity at a rate of 0.5 A·g–1. Orthorhombic V2O5/C presents a higher specific capacity of 316 mAh·gVOx–1, but a more limited lithium diffusion, leading to a less favorable rate handling. Tested as cathodes for Na-ion batteries, we confirmed the importance of a conductive carbon network and nanostructures for improved electrochemical performance. Orthorhombic V2O5/C hybrid fibers presented very low specific capacity while monoclinic VO2/C fibers presented an improved specific capacity and rate performance with a capacity of 126 mAh·gVOx–1.

Free-standing, binder-free, titanium–niobium oxide/carbon hybrid nanofibers are prepared for Li-ion battery applications. A one-pot synthesis offers a significant reduction of processing steps and avoids the use of environmentally unfriendly binder materials, making the approach highly sustainable. Tetragonal Nb2O5/C and monoclinic Ti2Nb10O29/C hybrid nanofibers synthesized at 1000 °C displayed the highest electrochemical performance, with capacity values of 243 and 267 mAh g−1, respectively, normalized to the electrode mass. At 5 A g−1, the Nb2O5/C and Ti2Nb10O29/C hybrid fibers maintained 78 % and 53 % of the initial capacity, respectively. The higher rate performance and stability of tetragonal Nb2O5 compared to that of monoclinic Ti2Nb10O29 is related to the low energy barriers for Li+ transport in its crystal structure, with no phase transformation. The improved rate performance resulted from the excellent charge propagation in the continuous nanofiber network.

Continuous fiber mats are attractive electrodes for lithium-ion batteries, because they allow operation at high charge/discharge rates in addition to being free of polymer binders and conductive additives. In this work, we synthesize and characterize continuous Sn/SiOC fibers (diameter ca. 0.95 [small mu ]m), as a Li-ion battery anode. Our synthesis employs electrospinning of a low-cost silicone resin, using tin acetate in a dual role both as a polymer crosslinker and as a tin precursor (6-22 mass%). The hybrid electrodes present very high initial reversible capacities (840-994 mA h g-1) at 35 mA g-1, and retain 280-310 mA h g-1 at 350 mA g-1. After 100 cycles at 70 mA g-1, the hybrid fibers maintained 400-509 mA h g-1. Adding low amounts of Sn is beneficial not just for the crosslinking of the polymer precursor, but also to decrease the presence of electrochemically inactive silicon carbide domains within the SiOC fibers. Also, the metallic tin clusters contribute to a higher Li+ insertion in the first cycles. However, high amounts of Sn decrease the electrochemical performance stability. In SiOC fibers synthesized at high temperatures (1200 [degree]C), the Cfree phase has a significant influence on the stability of the system, by compensating for the volume expansion from the alloying systems (Sn and SiO2), and improving the conductivity of the hybrid system. Therefore, a high amount of carbon and a high graphitization degree are crucial for a high conductivity and a stable electrochemical performance.