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- Energy Materials (8)
This study presents electrospun niobium carbide/carbon (NbC/C) hybrid nanofibers, with an average diameter of 69 +/- 30 nm, as a facile precursor to derive either highly nanoporous niobium carbide-derived carbon (NbC-CDC) fibers for supercapacitor applications or niobium pentoxide/carbon (Nb2O5/C) hybrid fibers for battery-like energy storage. In all cases, the electrodes consist of binder-free and free-standing nanofiber mats that can be used without further conductive additives. Chlorine gas treatment conformally transforms NbC nanofiber mats into NbC-CDC fibers with a specific surface area of 1508 m2 g-1. These nanofibers show a maximum specific energy of 19.5 W h kg-1 at low power and 7.6 W h kg-1 at a high specific power of 30 kW kg-1 in an organic electrolyte. CO2 treatment transforms NbC into T-Nb2O5/C hybrid nanofiber mats that provide a maximum capacity of 156 mA h g-1. The presence of graphitic carbon in the hybrid nanofibers enabled high power handling, maintaining 50% of the initial energy storage capacity at a high rate of 10 A g-1 (64 C-rate). When benchmarked for an asymmetric full-cell, a maximum specific energy of 86 W h kg-1 was obtained. The high specific power for both systems, NbC-CDC and T-Nb2O5/C, resulted from the excellent charge propagation in the continuous nanofiber network and the high graphitization of the carbon structure.
Manganese oxide presents very promising electrochemical properties as an electrode material in supercapacitors, but there remain important open questions to guide further development of the complex manganese oxide/carbon/electrolyte system. Our work addresses specifically the influence of carbon ordering and the difference between outer and inner porosity of carbon particles for the application in aqueous 1 M Na2SO4 and 1 M LiClO4 in acetonitrile. Birnessite-type manganese oxide was hydrothermally hybridized on two kinds of carbon onions with only outer surface area and different electrical conductivity, and conventional activated carbon with a high inner porosity. Carbon onions with a high degree of carbon ordering, high conductivity, and high outer surface area were identified as the most promising material, yielding 179 F g-1. Pore blocking in activated carbon yields unfavorable electrochemical performances. The highest specific energy of 16.4 W h kg-1 was measured for a symmetric full-cell arrangement of manganese oxide coated high temperature carbon onions in the organic electrolyte. High stability during 10 000 cycles was achieved for asymmetric full-cells, which proved as a facile way to enhance the electrochemical performance stability.
MXene as a novel intercalation-type pseudocapacitive cathode and anode for capacitive deionization
(2016)
In this proof-of-concept study, we introduce and demonstrate MXene as a novel type of intercalation electrode for desalination via capacitive deionization (CDI). Traditional CDI cells employ nanoporous carbon electrodes with significant pore volume to achieve a large desalination capacity via ion electrosorption. By contrast, MXene stores charge by ion intercalation between the sheets of its two-dimensional nanolamellar structure. By this virtue, it behaves as an ideal pseudocapacitor, that is, showing capacitive electric response while intercalating both anions and cations. We synthesized Ti3C2-MXene by the conventional process of etching ternary titanium aluminum carbide i.e., the MAX phase (Ti3AlC2) with hydrofluoric acid. The MXene material was cast directly onto the porous separator of the CDI cell without added binder, and exhibited very stable performance over 30 CDI cycles with an average salt adsorption capacity of 13 +/- 2 mg g-1.
Performance stability in capacitive deionization (CDI) is particularly challenging in systems with a high amount of dissolved oxygen due to rapid oxidation of the carbon anode and peroxide formation. For example, carbon electrodes show a fast performance decay, leading to just 15% of the initial performance after 50 CDI cycles in oxygenated saline solution (5 mM NaCl). We present a novel strategy to overcome this severe limitation by employing nanocarbon particles hybridized with sol-gel-derived titania. In our proof-of-concept study, we demonstrate very stable performance in low molar saline electrolyte (5 mM NaCl) with saturated oxygen for the carbon/metal oxide hybrid (90% of the initial salt adsorption capacity after 100 cycles). The electrochemical analysis using a rotating disk electrode (RDE) confirms the oxygen reduction reaction (ORR) catalytic effect of FW200/TiO2, preventing local peroxide formation by locally modifying the oxygen reduction reaction.
Electrospinning of ultrafine metal oxide/carbon and metal carbide/carbon nanocomposite fibers
(2015)
Electrospinning has emerged as a facile technology for the synthesis of ultrafine fibers and even nanofibers of various materials. While carbon nanofibers have been extensively investigated, there have also been studies reported on metal oxide and metal carbide fibers. Yet, comparative studies, especially following the same general synthesis approach, are lacking. In our comprehensive study, we use a sol gel process by which a carrier polymer (cellulose acetate or polyvinylpyrrolidone) is mixed with titanium butoxide, zirconium(iv) acetylacetonate, or niobium n-butoxide to yield nanotextured titania/carbon, zirconia/carbon, or niobia/carbon nonwoven textiles. Carbothermal reduction between 1300 °C and 1700 °C effectively transforms the metal oxide/carbon fibers to metal carbide/carbon nanocomposite while preserving the fiber integrity. As a beneficial effect, the fiber diameter decreases compared to the as-spun state and we obtained ultrafine fibers: 294 +/- 108 nm for ZrC/C, 122 +/- 28 nm for TiC/C, and 65 +/- 36 nm for NbC/C. The highly disordered and porous nature of the carbon matrix engulfing the metal carbide nanocrystals enables a high specific surface area of up to 450 m 2 g -1 (TiC/C) after carbothermal reduction.
Atomic layer deposition has proven to be a particularly attractive approach for decorating mesoporous carbon substrates with redox active metal oxides for electrochemical energy storage. This study, for the first time, capitalizes on the cyclic character of atomic layer deposition to obtain a highly conformal and atomically controlled decoration of carbon onions with alternating stacks of vanadia and titania. The addition of 25 mass% TiO2 leads to an expansion of the VO2 unit cell, thus greatly enhancing lithium intercalation capacity and kinetics. Electrochemical characterization revealed ultrahigh discharge capacity of up to 382 mAh[middle dot]g-1 of the composite electrode (554 mAh[middle dot]g-1 per metal oxide) with an impressive capacity retention of 82 mAh[middle dot]g-1 (120 mAh[middle dot]g-1 per metal oxide) at a high discharge rate of 20 A[middle dot]g-1 or 52 C. Rigorous stability benchmarking showed superior stability over 3,000 cycles when discharging to a reduced potential of -1.8 V vs. carbon. These capacity values are among the highest reported for any metal oxide system, while in addition, supercapacitor-like power performance and longevity are achieved. On a device level, high specific energy and power of up to 110 Wh[middle dot]kg-1 and 6 kW[middle dot]kg-1, respectively, were achieved when employing the hybrid material as anode versus activated carbon cathode.
We introduce a high performance hybrid electrochemical energy storage system based on an aqueous electrolyte containing tin sulfate (SnSO4) and vanadyl sulfate (VOSO4) with nanoporous activated carbon. The energy storage mechanism of this system benefits from the unique synergy of concurrent electric double-layer formation, reversible tin redox reactions, and three-step redox reactions of vanadium. The hybrid system showed excellent electrochemical properties such as a promising energy capacity (ca. 75 W h kg-1, 30 W h L-1) and a maximum power of up to 1.5 kW kg-1 (600 W L-1, 250 W m-2), exhibiting capacitor-like galvanostatic cycling stability and a low level of self-discharging rate.
Electrospun carbon hybrid fibers as binder-free electrodes for electrochemical energy storage
(2018)
There is a great need for the development and improvement of electrochemical energy storage devices for applications ranging from energy and power management to portable electronic devices. My work explores electrode materials for devices with higher energy storage capacity and rate handling, namely electrical double-layer capacitors, lithium-ion batteries, and sodium-ion batteries. To this end, I report the synthesis and properties of electrospun fiber mats composed of nanoporous carbon, transition metal oxide/carbon hybrid material, or silicon oxycarbide. Based on a comprehensive array of structural and chemical analysis and electrochemical benchmarking, this work evaluates the potential and drawbacks of electrospun materials as electrodes. Key findings demonstrate that electrospinning of molecular precursor is an attractive approach for the synthesis of carbon and hybrid fiber mats as free-standing electrodes. By following a one-pot synthesis approach, material properties such as phase composition, crystal structure, and phase distribution are well tuned to achieve the desired electrochemical properties. Compared to polymer-bound free-standing electrodes, the continuous fiber network yields a superior gravimetric electrochemical performance, related to the absence of additives and the continuous path for electron transport. However, the large interfiber space and low electrode density limit the usefulness of adopting electrospun fiber mats to size-sensitive applications
