First principles-based atomistic modeling of the interfacial microstructure and capacitance of graphene

textGraphene has been extensively studied for possible future technical applications due to its unique electronic, transport, and mechanical properties. For practical applications, graphene often needs to be placed in a medium or on a substrate. The interfacial interaction between graphene and other materials can greatly affect the performance of graphene-based devices, but has not been well explored. My thesis research focused on developing a better understanding of the interface of pristine and chemically/mechanically modified graphene sheets with ionic liquids (ILs) as well as amorphous silica (a-SiO₂) surfaces using first principles-based atomistic modeling which combines density functional theory, classical molecular dynamics, and Metropolis Monte Carlo. The major focus of my thesis research was on investigating the interfacial structure and capacitance between graphene and ILs; graphene-based materials and ILs have been regarded as viable candidates for supercapacitor electrodes and electrolytes, respectively. Particular emphasis was placed on elucidating the relative contributions of the electric double layer (EDL) capacitance at the graphene/IL interface and the quantum capacitance of graphene-like electrodes. More specifically, we first determined the microstructure (such as orientation, packing density, cation-anion segregation) of chosen ILs near planar graphene electrodes with various surface charge densities. Based on the calculated IL microstructure for each system, the EDL capacitance was then evaluated with particular attention to the effect of cation-anion size difference. We also examined the influence of the chemical and mechanical modifications of graphene-like electrodes on the supercapacitor performance. Especially, mechanisms underlying chemical doping-induced enhancement of the total interfacial capacitance were addressed through analysis of electrode quantum capacitance changes resulting from electronic structure modifications. A part of my effort was also devoted to examining the binding interaction of graphene with a-SiO₂ (which is not yet clearly understood despite its scientific and technological importance). In particular, we attempted to evaluate quantitatively the adsorption strength of graphene on the a-SiO₂ surface, which has been under debate mainly due to the difficulty of direct measurement.Chemical Engineerin

Batch Fabrication of High-Performance Planar Patch-Clamp Devices in Quartz

The success of the patch-clamp technique has driven an effort to create wafer-based patch-clamp platforms. We develop a lithographic/electrochemical processing scheme that generates ultrasmooth, high aspect ratio pores in quartz. These devices achieve gigaohm seals in nearly 80% of trials, with the majority exhibiting seal resistances from 20-80 GΩ, competing with pipette-based patch-clamp measurements

Understanding CO_2 capture mechanisms in aqueous hydrazine via combined NMR and first-principles studies

Aqueous amines are currently the most promising solution for large-scale CO_2 capture from industrial sources. However, molecular design and optimization of amine-based solvents have proceeded slowly due to a lack of understanding of the underlying reaction mechanisms. Unique and unexpected reaction mechanisms involved in CO_2 absorption into aqueous hydrazine are identified using ^1H, ^(13)C, and ^(15)N NMR spectroscopy combined with first-principles quantum-mechanical simulations. We find production of both hydrazine mono-carbamate (NH_2-NH-COO^−) and hydrazine di-carbamate (^−OOC-NH-NH-COO^−), with the latter becoming more populated with increasing CO_2 loading. Exchange NMR spectroscopy also demonstrates that the reaction products are in dynamic equilibrium under ambient conditions due to CO_2 exchange between mono-carbamate and di-carbamate as well as fast proton transfer between un-protonated free hydrazine and mono-carbamate. The exchange rate rises steeply at high CO_2 loadings, enhancing CO_2 release, which appears to be a unique property of hydrazine in aqueous solution. The underlying mechanisms of these processes are further evaluated using quantum mechanical calculations. We also analyze and discuss reversible precipitation of carbamate and conversion of bicarbonate to carbamates. The comprehensive mechanistic study provides useful guidance for optimal design of amine-based solvents and processes to reduce the cost of carbon capture. Moreover, this work demonstrates the value of a combined experimental and computational approach for exploring the complex reaction dynamics of CO_2 in aqueous amines

Understanding CO_2 capture mechanisms in aqueous hydrazine via combined NMR and first-principles studies

Aqueous amines are currently the most promising solution for large-scale CO_2 capture from industrial sources. However, molecular design and optimization of amine-based solvents have proceeded slowly due to a lack of understanding of the underlying reaction mechanisms. Unique and unexpected reaction mechanisms involved in CO_2 absorption into aqueous hydrazine are identified using ^1H, ^(13)C, and ^(15)N NMR spectroscopy combined with first-principles quantum-mechanical simulations. We find production of both hydrazine mono-carbamate (NH_2-NH-COO^−) and hydrazine di-carbamate (^−OOC-NH-NH-COO^−), with the latter becoming more populated with increasing CO_2 loading. Exchange NMR spectroscopy also demonstrates that the reaction products are in dynamic equilibrium under ambient conditions due to CO_2 exchange between mono-carbamate and di-carbamate as well as fast proton transfer between un-protonated free hydrazine and mono-carbamate. The exchange rate rises steeply at high CO_2 loadings, enhancing CO_2 release, which appears to be a unique property of hydrazine in aqueous solution. The underlying mechanisms of these processes are further evaluated using quantum mechanical calculations. We also analyze and discuss reversible precipitation of carbamate and conversion of bicarbonate to carbamates. The comprehensive mechanistic study provides useful guidance for optimal design of amine-based solvents and processes to reduce the cost of carbon capture. Moreover, this work demonstrates the value of a combined experimental and computational approach for exploring the complex reaction dynamics of CO_2 in aqueous amines

Principal Role of Contact-Force Distribution in Determining the Thermal Conductivity of Supported Graphene

The thermal conductivity (kappa) of graphene dramatically decreases once supported on a substrate, hindering its use for thermal management. To clarify the underlying mechanisms, we investigate the. of graphene on amorphous SiO2 by using molecular dynamics with particular attention to the graphene-substrate topography. Our analysis reveals that the suppression in. increases with the nonuniformity of the forces acting on graphene, which tends to increase as the substrate-surface roughness and graphene conformity increase. Our findings highlight the importance of the interfacial morphology on. and can provide guidance on the design of substrates to improve thermal transport through graphene.National Science Foundation CBET-0933557Robert A. Welch Foundation F-1535NSF-NASCENT Engineering Research Center EEC-1160494Donald D. Harrington Fellows ProgramChemical Engineerin

Large Capacitance Enhancement Induced by Metal-Doping in Graphene-Based Supercapacitors: A First-Principles-Based Assessment

Chemically doped graphene-based materials have recently been explored as a means to improve the performance of supercapacitors. In this work, we investigate the effects of 3d transition metals bound to vacancy sites in graphene with [BMIM]­[PF₆] ionic liquid on the interfacial capacitance; these results are compared to the pristine graphene case with particular attention to the relative contributions of the quantum and electric double layer capacitances. Our study highlights that the presence of metal-vacancy complexes significantly increases the availability of electronic states near the charge neutrality point, thereby enhancing the quantum capacitance drastically. In addition, the use of metal-doped graphene electrodes is found to only marginally influence the microstructure and capacitance of the electric double layer. Our findings indicate that metal-doping of graphene-like electrodes can be a promising route toward increasing the interfacial capacitance of electrochemical double layer capacitors, primarily by enhancing the quantum capacitance

Curvature Effects on the Interfacial Capacitance of Carbon Nanotubes in an Ionic Liquid

Carbon nanotube (CNT) electrodes in supercapacitors have recently demonstrated enhanced performance compared to conventional carbon-based electrodes; however, the underlying relationships between electrode curvature and capacitance remain unclear. Using computer simulations, we evaluate the capacitive performance of metallic (6,6), (10,10), and (16,16) CNTs in [BMIM]­[PF₆] ionic liquid (IL), with particular attention to the relative contributions of the electric double layer (EDL) capacitance (<i>C</i>_D) at the CNT/IL interface and the electrode quantum capacitance (<i>C</i>_Q). Our classical molecular dynamics simulations reveal that <i>C</i>_D improves with increasing electrode curvature, which we discuss in terms of how the curvature affects both the electric field strength and EDL microstructure. In addition, the <i>C</i>_Q of the CNTs is constant near the Fermi level and increases with curvature, as also demonstrated by density functional theory calculations. Our study shows that the electrode curvature effect on the total interfacial capacitance can be a strong function of applied voltage, which we attribute to the shifting contributions of <i>C</i>_Q and <i>C</i>_D

On the Origin of the Enhanced Supercapacitor Performance of Nitrogen-Doped Graphene

Graphene-based electrodes have been widely tested and used in electrochemical double layer capacitors due to their high surface area and electrical conductivity. Nitrogen doping of graphene has recently been demonstrated to significantly enhance capacitance, but the underlying mechanisms remain ambiguous. We present the doping effect on the interfacial capacitance between graphene and [BMIM]­[PF₆] ionic liquid, particularly the relative changes in the double layer and electrode (quantum) capacitances. The electrode capacitance change was evaluated based on density functional theory calculations of doping-induced electronic structure modifications in graphene, while the microstructure and capacitance of the double layers forming near undoped/doped graphene electrodes were calculated using classical molecular dynamics. Our computational study clearly demonstrates that nitrogen doping can lead to significant enhancement in the electrode capacitance as a result of electronic structure modifications while there is virtually no change in the double layer capacitance. This finding sheds some insight into the impact of the chemical and/or mechanical modifications of graphene-like electrodes on supercapacitor performance

The Bright Future for Electrode Materials of Energy Devices: Highly Conductive Porous Na-Embedded Carbon

High electrical conductivity and large accessible surface area, which are required for ideal electrode materials of energy conversion and storage devices, are opposed to each other in current materials. It is a long-term goal to solve this issue. Herein, we report highly conductive porous Na-embedded carbon (Na@C) nanowalls with large surface areas, which have been synthesized by an invented reaction of CO with liquid Na. Their electrical conductivities are 2 orders of magnitude larger than highly conductive 3D graphene. Furthermore, almost all their surface areas are accessible for electrolyte ions. These unique properties make them ideal electrode materials for energy devices, which significantly surpass expensive Pt. Consequently, the dye-sensitized solar cells (DSSCs) with the Na@C counter electrode has reached a high power conversion efficiency of 11.03%. The Na@C also exhibited excellent performance for supercapacitors, leading to high capacitance of 145 F g-1 at current density of 1 A g-1