Taehan Yeo

Effects of chemical fuel composition on energy generation from thermopower waves

Thermopower waves, which occur during combustion within hybrid structures formed from nanomaterials and chemical fuels, result in a self-propagating thermal reaction and concomitantly generate electrical energy from the acceleration of charge carriers along the nanostructures. The hybrid structures for thermopower waves are composed of two primary components: the core thermoelectric material and the combustible fuel. So far, most studies have focused on investigating various nanomaterials for improving energy generation. Herein, we report that the composition of the chemical fuel used has a significant effect on the power generated by thermopower waves. Hybrid nanostructures consisting of mixtures of picric acid and picramide with sodium azide were synthesized and used to generate thermopower waves. A maximum voltage of ∼2 V and an average peak specific power as high as 15 kW kg(-1) were obtained using the picric acid/sodium azide/multiwalled carbon nanotubes (MWCNTs) array composite. The average reaction velocity and the output voltage in the case of the picric acid/sodium azide were 25 cm s(-1) and 157 mV, while they were 2 cm s(-1) and 3 mV, in the case of the picramide/sodium azide. These marked differences are attributable to the chemical and structural differences of the mixtures. Mixing picric acid and sodium azide in deionized water resulted in the formation of 2,4,6-trinitro sodium phenoxide and hydrogen azide (H-N3), owing to the exchange of H(+) and Na(+) ions, as well as the formation of fiber-like structures, because of benzene π stacking. The negative enthalpy of formation of the new compounds and the fiber-like structures accelerate the reaction and increase the output voltage. Elucidating the effects of the composition of the chemical fuel used in the hybrid nanostructures will allow for the control of the combustion process and help optimize the energy generated from thermopower waves, furthering the development of thermopower waves as an energy source.

Investigation of the effect of the structure of large-area carbon nanotube/fuel composites on energy generation from thermopower waves

Thermopower waves are a recently developed energy conversion concept utilizing dynamic temperature and chemical potential gradients to harvest electrical energy while the combustion wave propagates along the hybrid layers of nanomaterials and chemical fuels. The intrinsic properties of the core nanomaterials and chemical fuels in the hybrid composites can broadly affect the energy generation, as well as the combustion process, of thermopower waves. So far, most research has focused on the application of new core nanomaterials to enhance energy generation. In this study, we demonstrate that the alignment of core nanomaterials can significantly influence a number of aspects of the thermopower waves, while the nanomaterials involved are identical carbon nanotubes (CNTs). Diversely structured, large-area CNT/fuel composites of one-dimensional aligned CNT arrays (1D CNT arrays), randomly oriented CNT films (2D CNT films), and randomly aggregated bulk CNT clusters (3D CNT clusters) were fabricated to evaluate the energy generation, as well as the propagation of the thermal wave, from thermopower waves. The more the core nanostructures were aligned, the less inversion of temperature gradients and the less cross-propagation of multiple thermopower waves occurred. These characteristics of the aligned structures prevented the cancellation of charge carrier movements among the core nanomaterials and produced the relative enhancement of the energy generation and the specific power with a single-polarity voltage signal. Understanding this effect of structure on energy generation from thermopower waves can help in the design of optimized hybrid composites of nanomaterials and fuels, especially designs based on the internal alignment of the materials. More generally, we believe that this work provides clues to the process of chemical to thermal to electrical energy conversion inside/outside hybrid nanostructured materials.

Facile One-pot Transformation of Iron Oxides from Fe2O3 Nanoparticles to Nanostructured Fe3O4@C Core-Shell Composites via Combustion Waves.

The development of a low-cost, fast, and large-scale process for the synthesis and manipulation of nanostructured metal oxides is essential for incorporating materials with diverse practical applications. Herein, we present a facile one-pot synthesis method using combustion waves that simultaneously achieves fast reduction and direct formation of carbon coating layers on metal oxide nanostructures. Hybrid composites of Fe2O3 nanoparticles and nitrocellulose on the cm scale were fabricated by a wet impregnation process. We demonstrated that self-propagating combustion waves along interfacial boundaries between the surface of the metal oxide and the chemical fuels enabled the release of oxygen from Fe2O3. This accelerated reaction directly transformed Fe2O3 into Fe3O4 nanostructures. The distinctive color change from reddish-brown Fe2O3 to dark-gray Fe3O4 confirmed the transition of oxidation states and the change in the fundamental properties of the material. Furthermore, it simultaneously formed carbon layers of 5–20 nm thickness coating the surfaces of the resulting Fe3O4 nanoparticles, which may aid in maintaining the nanostructures and improving the conductivity of the composites. This newly developed use of combustion waves in hybridized nanostructures may permit the precise manipulation of the chemical compositions of other metal oxide nanostructures, as well as the formation of organic/inorganic hybrid nanostructures.

One-step transformation of MnO2 into MnO2−x@carbon nanostructures for high-performance supercapacitors using structure-guided combustion waves

The manipulation of micro/nanostructured metal oxides is crucial to advancing their diverse applications, including as electrodes in supercapacitors or batteries, catalysts, and pigments. However, controlling the physicochemical properties of metal oxides requires complex procedures with bulky setups that incur high costs and long processing times. Herein, we present a facile one-step manipulation of the reduced states of manganese oxides and the synthesis of carbon coatings surrounding them, using structure-guided combustion waves (SGCWs), which is induced by incomplete combustion through the chemical fuel-wrapped materials. Controlled oxygen release from MnO2 using SGCWs in air and in an Ar atmosphere enabled direct fabrication of reduced Mn2O3/Mn3O4/MnO and MnO, respectively. Furthermore, control of the incompletely combusted carbonaceous fuel facilitated the synthesis of carbon coating layers to form Mn2O3/Mn3O4/MnO@C and MnO@C. These core–shell nanostructures of reduced manganese oxides and carbon layers were applied as supercapacitor electrodes. These electrodes showed better specific capacitance (maximum 438 F g−1 at 10 mV s−1 scan rate for Mn2O3/Mn3O4/MnO@C) and improved stability in charge–discharge performance compared with bare MnO2, due to the carbon coatings enhancing electrical conductivity in the percolation network of electrodes and facilitating the reversible redox reaction without degradation during cycling operations. SGCWs are applicable for fast, low-cost, and large-scale fabrication of reduced metal oxides and organic material coatings, which could significantly contribute to electrochemical applications.

DC-field-driven combustion waves for one-step fabrication of reduced manganese oxide/multi-walled carbon nanotube hybrid nanostructures as high-performance supercapacitor electrodes

Micro–nanostructured metal oxides can facilitate the development of electrochemical electrodes with enhanced features for supercapacitors and batteries. However, the fabrication of electrodes using precisely controlled metal oxides generally requires high-cost, multi-step procedures, which limits the scalability. Herein, we report that a direct current-field-driven combustion wave (DC-CW) enables the one-step fabrication of high-performance supercapacitor electrodes from hybrid nanostructures comprising reduced manganese oxides and multi-walled carbon nanotubes (MWCNTs). A layered film of MnO2 nanoparticles (NPs) and MWCNTs on a nitrocellulose membrane is prepared and subsequently subjected to a DC-electric field, thereby igniting and propagating CWs throughout the film surface within one second. The underlying mechanism of the DC-CW process is elucidated by comparative analysis of the electrodes generated by the laser irradiation-driven combustion wave process without the DC-field and the as-prepared MnO2/MWCNT film. The MnxOy/MWCNT hybrids via DC-CWs exhibit higher specific capacitance (757 F g−1) and capacitance retention (100%) than the other two systems over 10 000 charge–discharge cycles, due to the improved inter-conductivity and substantial contact interfaces in heterogeneously mixed, less agglomerated nanostructures. The DC-CWs may enable various manipulation methods of micro–nanostructured metal oxides and their hybrid structures via a low-cost, fast, and scalable process for high-performance electrochemical electrodes.

Voltage amplification of thermopower waves via current crowding at high resistances in self-propagating combustion waves

Combustion wave propagation in micro/nanostructured materials generates a chemical-thermal-electrical energy conversion, which enables the creation of an unusual source of electrical energy, called a thermopower wave. In this paper, we report that high electrical resistance regimes would significantly amplify the output voltage of thermopower waves, because the current crowding creates a narrow path for charge carrier transport. We show that the structurally defective regions in the hybrid composites of chemical fuels and carbon nanotube (CNT) arrays determine both the resistance levels of the hybrid composites and the corresponding output voltage of thermopower waves. A sudden acceleration of the crowded charges would be induced by the moving reaction front of the combustion wave when the supplied driving force overcomes the potential barrier to cause charge carrier transport over the defective region. This property is investigated experimentally for the locally manipulated defective areas using diverse methods. In this study, thermopower waves in CNT-based hybrid composites are able to control the peak voltages in the range of 10-1000 mV by manipulating the resistance from 10 Ω to 100 kΩ. This controllable voltage generation from thermopower waves may enable applications using the combustion waves in micro/nanostructured materials and better understanding of the underlying physics.

Preparation and evaluation of hybrid composites of chemical fuel and multi-walled carbon nanotubes in the study of thermopower waves

When a chemical fuel at a certain position in a hybrid composite of the fuel and a micro/nanostructured material is ignited, chemical combustion occurs along the interface between the fuel and core materials. Simultaneously, dynamic changes in thermal and chemical potentials across the micro/nanostructured materials result in concomitant electrical energy generation induced by charge transfer in the form of a high-output voltage pulse. We demonstrate the entire procedure of a thermopower wave experiment, from synthesis to evaluation. Thermal chemical vapor deposition and the wet impregnation process are respectively employed for the synthesis of a multi-walled carbon nanotube array and a hybrid composite of picric acid/sodium azide/multi-walled carbon nanotubes. The prepared hybrid composites are used to fabricate a thermopower wave generator with connecting electrodes. The combustion of the hybrid composite is initiated by laser heating or Joule-heating, and the corresponding combustion propagation, direct electrical energy generation, and real-time temperature changes are measured using a high-speed microscopy system, an oscilloscope, and an optical pyrometer, respectively. Furthermore, the crucial strategies to be adopted in the synthesis of hybrid composite and initiation of their combustion that enhance the overall thermopower wave energy transfer are proposed.

Scalable Synthesis of Triple‐Core–Shell Nanostructures of TiO2@MnO2@C for High Performance Supercapacitors Using Structure‐Guided Combustion Waves

Core-shell nanostructures of metal oxides and carbon-based materials have emerged as outstanding electrode materials for supercapacitors and batteries. However, their synthesis requires complex procedures that incur high costs and long processing times. Herein, a new route is proposed for synthesizing triple-core-shell nanoparticles of TiO2 @MnO2 @C using structure-guided combustion waves (SGCWs), which originate from incomplete combustion inside chemical-fuel-wrapped nanostructures, and their application in supercapacitor electrodes. SGCWs transform TiO2 to TiO2 @C and TiO2 @MnO2 to TiO2 @MnO2 @C via the incompletely combusted carbonaceous fuels under an open-air atmosphere, in seconds. The synthesized carbon layers act as templates for MnO2 shells in TiO2 @C and organic shells of TiO2 @MnO2 @C. The TiO2 @MnO2 @C-based electrodes exhibit a greater specific capacitance (488 F g-1 at 5 mV s-1 ) and capacitance retention (97.4% after 10 000 cycles at 1.0 V s-1 ), while the absence of MnO2 and carbon shells reveals a severe degradation in the specific capacitance and capacitance retention. Because the core-TiO2 nanoparticles and carbon shell prevent the deformation of the inner and outer sides of the MnO2 shell, the nanostructures of the TiO2 @MnO2 @C are preserved despite the long-term cycling, giving the superior performance. This SGCW-driven fabrication enables the scalable synthesis of multiple-core-shell structures applicable to diverse electrochemical applications.

Tunable fabrication of core-shell Ni-MnO2 hybrid foams through structure-guided combustion waves for binder-free high-performance supercapacitor electrodes

Hybrid foam structures of metal and carbon are extensively used for electrochemical applications. However, their fabrication involves solution- or vacuum-processing, which damages the metal backbones or increases the fabrication time and cost. Herein, we report a tunable method for the scalable fabrication of core–shell metal–carbon hybrid foams using structure-guided combustion waves (SGCWs) and their application for the synthesis of core–shell Ni–MnO2 hybrid foams as binder-free supercapacitor electrodes. SGCWs passing through the hybrids of nickel foams and chemical fuels, prepared by a wet impregnation method, enabled the direct fabrication of carbon coatings on the surfaces of the inner nickel backbones and yielded core–shell Ni@C. The incompletely combusted carbonaceous fuels in a few seconds, which were formed in the narrowly confined foam structures reaching 430 °C, acted as amorphous carbon coatings, while the total amount and uniformity of the carbon content could be controlled by the number of times SGCWs were applied. The developed carbon coatings were used as templates for MnO2 shells to synthesize core–shell Ni@MnO2 hybrid foams as binder-free supercapacitor electrodes. The core–shell Ni@MnO2 foams fabricated by applying SGCWs three times exhibited a high specific capacitance of up to 660 F g−1 and stable capacitance retention (∼95.4% over more than 10 000 cycles) because of their lower serial resistance and optimal diffusion during the redox reaction. This tunable fabrication method using SGCWs in a vacuum-free, open-air environment enables the synthesis of scalable carbon coatings on metal- or ceramic-based foams for electrochemical applications.