Energy Materials

Asymmetric hybrid supercapacitors (AHSCs) combine high specific energy and power by merging two electrodes with capacitive and Faradaic charge storage mechanisms. In this study, we introduce AHSC cells that use lithium titanate and activated carbon electrodes in an alkali-ion containing ionic liquid electrolyte. With this cell concept, it is possible to operate the activated carbon electrode in a higher potential window. Consequently, higher cell voltages and a reduced carbon electrode mass can be used, resulting in significantly increased energy compared to aqueous or organic electrolytes. We demonstrate the feasibility of this cell concept for both lithium- and sodium-ion intercalation, underlining the general validity of our approach. Our prototype cells already reach high specific energies of 100 W h/kg, while maintaining a specific power of up to 2 kW/kg and cycling stability of over 1500 cycles. Owing to the IL electrolyte, stable cycling of an AHSC at 80 °C is demonstrated for the first time.

In recent years, a wealth of new desalination technologies based on reversible electrochemical redox reactions has emerged. Among them, the use of redox-active electrolytes is highly attractive due to the high production rate and energy efficiency. Yet, these technologies suffer from the imperfect permselectivity of polymer membranes. Our present work demonstrates the promising desalination performance of a sodium superionic conductor (NASICON) for selective removal of sodium against iodide in a half-cell configuration consisting of an activated carbon electrode in aqueous 600 mM NaI solution. For feedwater with aqueous 600 mM NaCl, the desalination cell exhibited a stable performance over a month with more than 400 operation cycles with the aid of high sodium permselectivity of the NASICON membrane against iodide (99.9–100%). The cell exhibited a maximum sodium removal capacity of 69 ± 4 mg/g (equivalent to the NaCl salt uptake capacity of 87 ± 4 mg/g) with a charge efficiency of 81 ± 3%.

Over recent decades, a new type of electric energy storage system has emerged with the principle that the electric charge can be stored not only at the interface between the electrode and the electrolyte but also in the bulk electrolyte by redox activities of the electrolyte itself. Those redox electrolytes are promising for non-flow hybrid energy storage systems, or redox electrolyte-aided hybrid energy storage (REHES) systems; particularly, when they are combined with highly porous carbon electrodes. In this review paper, critical design considerations for the REHES systems are discussed as well as the effective electrochemical characterization techniques. Appropriate evaluation of the electrochemical performance is discussed thoroughly, including advanced analytical techniques for the determination of the electrochemical stability of the redox electrolytes and self-discharge rate. Additionally, critical summary tables for the recent progress on REHES systems are provided. Furthermore, the unique synergistic combination of porous carbon materials and redox electrolytes is introduced in terms of the diffusion, adsorption, and electrochemical kinetics modulating energy storage in REHES systems.

In recent years, electrochemical water desalination with battery electrode materials has emerged as a promising solution for energy-efficient salt-water desalination. Here, we report the promising desalination performance of a hydrothermally synthesized vanadyl phosphate material (mixed phases of sodium vanadyl phosphate dehydrate and vanadyl hydrogen phosphate hemihydrate) as a new electrode material. We observed robust stability of the synthesized electrode material over 280 cycles during desalination operation for 100 mM NaCl feedwater which was continuously flowing along the electrode material. During the first 100 cycles, the charge storage capacity was enhanced by 47%. This enhancement seems to be caused by a continuous conversion to vanadyl phosphate monohydrate from initial phases according to the post-mortem analysis by X-ray diffraction and infrared spectroscopy. The maximum sodium uptake capacity of the vanadyl phosphate electrode was 24.3 mg g−1 with charge efficiency of around 85%. We found no detectable level of contamination by phosphor nor vanadium from the treated water stream indicating that our synthesized electrode is also environmentally safe for water desalination applications.

The performance of nanoporous carbons, used for hydrogen storage, ionic charge storage, or selective gas separation, is strongly determined by their pore shape and size distribution. Two frequently used experimental techniques to characterize the nanopore structure of carbons are gas adsorption combined with quenched-solid density functional theory and small angle X-ray scattering. However, neither of the two techniques can unambiguously derive a valid pore model for disordered pore structures without making assumptions. Here, we quantitatively compare pore size distributions from X-ray scattering and gas adsorption data. We generate three-dimensional pore models of activated carbons using small angle scattering and the concept of Gaussian Random Fields. These pore models are used to generate pore size distributions inherently containing a slit-pore assumption, making them comparable to pore size distributions obtained from gas adsorption analysis. This is realized by probing the effective adsorption potential via sampling of the three-dimensional pore structure with a probing adsorbate and calculating a “Degree of Confinement” parameter accounting for local pore geometry effects. We also generate pore size distributions with an alternative definition of pore size and discuss intricacies of gas adsorption results, such as the general tendency to underestimate the pore size dispersity in disordered microporous carbons.

We present a versatile strategy to tailor the nanostructure of monolithic carbon aerogels. By use of an aqueous colloidal solution of polystyrene in the sol-gel processing of resorcinol-formaldehyde gels, we can prepare, after supercritical drying and successive carbonization, freestanding monolithic carbon aerogels, solely composed of interconnected and uniformly sized hollow spheres, which we name carbon spherogels. Each sphere is enclosed by a microporous carbon wall whose thickness can be adjusted by the polystyrene concentration, which affects the pore texture as well as the mechanical properties of the aerogel monolith. In this study, we used monodisperse polystyrene spheres of approximately 250 nm diameter, which result in an inner diameter of the final hollow carbon spheres of approximately 200 ± 5 nm due to shrinkage during the carbonization process. The excellent homogeneity of the samples, as well as uniform sphere geometries, are confirmed by small- and angle X-ray scattering. The presence of macropores between the hollow spheres creates a monolithic network with the benefit of being reversibly compressible up to 10% linear strain without destruction. Electrochemical tests demonstrate the applicability of ground and CO2 activated carbon spherogels as electrode materials.

High demand for safer and more stable lithium-ion batteries brings up the challenge for finding better electrode materials. In this work, we study the functionalities of titanium niobium oxide (TNO)/carbon hybrid materials using carbon onions (OLC) and carbon nanohorns (NS), which are synthesized by well-controlled sol-gel chemistry, for anodes in lithium-ion batteries. We used two different molar ratios of titanium to niobium oxide (1:2 and 1:5), and we compared the TNO-OLC and TNO-NS hybrid materials to conventional electrodes using physically admixed carbon black. TNO-OLC-1:2 and TNO-OLC-1:5 nanohybrid materials displayed good electrochemical performance, with initial capacity values of 284 mAh/g and 290 mAh/g, respectively, normalized to the metal oxide mass. Moreover, they maintained 68% (TNO-OLC-1:2) and 69% (TNO-OLC-1:5) of the initial capacity at 1 A/g, outperforming the carbon nanohorns hybridized and composited electrode which maintained less than 50%. The long-term cycling stability of 800 cycles presents good capacity retention of 73% (TNO-OLC-1:2) and 76% (TNO-OLC-1:5), while the TNO-NS-1:2 hybrid material yields better capacity retention of 90% despite its low capacity. Our study demonstrates that the combination of TNO with appropriate carbon substrates enables good electrochemical performance but requires careful evaluation of the interplay of crystal structure, phase content, and particle morphology.

Technologies for the effective and energy efficient removal of salt from saline media for advanced water remediation are in high demand. Capacitive deionization using carbon electrodes is limited to highly diluted salt water. Our work demonstrates the high desalination performance of the silver/silver chloride conversion reaction by a chloride ion rocking-chair desalination mechanism. Silver nanoparticles are used as positive electrodes while their chlorination into AgCl particles produces the negative electrode in such a combination that enables a very low cell voltage of only Δ200 mV. We used a chloride-ion desalination cell with two flow channels separated by a polymeric cation exchange membrane. The optimized electrode paring between Ag and AgCl achieves a low energy consumption of 2.5 kT per ion when performing treatment with highly saline feed (600 mM NaCl). The cell affords a stable desalination capacity of 115 mg g−1 at a charge efficiency of 98%. This performance aligns with a charge capacity of 110 mA h g−1.

In nanoconfinement, the reversible electrochemisorption of hydrogen extends the voltage window of aqueous electrolytes. This process has been well studied for different aqueous electrolytes but not compared to the performance of heavy water. Herein, we study hydrogen and deuterium electrosorption on a porous carbon electrode under negative polarization using sodium chloride as the salt. As electrodes, we use microporous carbons with an average pore size in the sub-nanometer range and, for comparison, mesoporous carbon nanotube bucky paper. We show that the hydrogen electrochemisorption and gas evolution processes are more pronounced than for deuterium while the same potential is applied. Our data confirm lower ion mobility of D2O compared to H2O, and a shift of the reversible charging and discharging process toward more negative potentials.

Abstract Free standing, binder free, and conductive additive free mesoporous titanium dioxide/carbon hybrid electrodes were prepared from co‐assembly of a poly(isoprene)‐block‐poly(styrene)‐block‐poly(ethylene oxide) block copolymer and a titanium alkoxide. By tailoring an optimized morphology, we prepared macroscopic mechanically stable 300 μm thick monoliths that were directly employed as lithium‐ion battery electrodes. High areal mass loading of up to 26.4 mg cm−2 and a high bulk density of 0.88 g cm−3 were obtained. This resulted in a highly increased volumetric capacity of 155 mAh cm−3, compared to cast thin film electrodes. Further, the areal capacity of 4.5 mAh cm−2 represented a 9‐fold increase compared to conventionally cast electrodes. These attractive performance metrics are related to the superior electrolyte transport and shortened diffusion lengths provided by the interconnected mesoporous nature of the monolith material, assuring superior rate handling, even at high cycling rates.