Constructing pseudocapacitive electrodes for supercapacitors based on rationally designed nanoarchitectured current collectors

Supercapacitors are of high importance as electrochemical energy storage devices attributing to their outstanding power performance, excellent reversibility and long cycle life. However, compared with batteries, supercapacitors suffer from low energy density, which hinders their wide application. Pseudocapacitive materials with a high theoretical capacitance hold a great promise in boosting the energy storage capability for supercapacitors. Research on nanoarchitectured current collectors aims to reach their full potential in the field of charge storage by addressing challenging problems such as the inherently low electrical conductivity and the sluggish charge and discharge behavior of most pseudo-capacitive materials. In this regard, three kinds of nanoarchitectured current collectors, i.e., Ni nanorod arrays (NN), etched porous alumina membrane (EPAM) coated with SnO2 layer (EPAM@SnO2), and Ni nanowires confined into EPAM (NiNWs-EPAM), were designed to construct pseudocapacitive electrodes and they were investigated in different aspects: Firstly, the role of NN nanoarchitectured current collectors in supercapacitor electrodes with the pseudocapacitive materials in the case of high mass loading and thick layer is firstly evaluated. Through electrochemical performance and impedance analysis of the electrodes with and without the NN nanoarchitectured current collectors, the validation of thick-layer electrodes design based on nanoarchitectured current collectors is demonstrated. Secondly, EPAM@SnO2 scaffolds are designed and employed as nanoarchitectured current collectors for nanoelectrodes in order to improve the device energy density of the micro-supercapacitor (MSC). Owing to the oriented and robust nanochannels in EPAM@SnO2, the resultant nanoelectrodes can synergize both effective ion migration and abundant electroactive surface area within the limited footprint. A MSC is finally constructed and exhibits record high performance, suggesting the feasibility of the current design for energy storage devices. Thirdly, the NiNWs-EPAM nanoarchitectured current collector is fabricated to construct non-aggregative and robust one-dimensional (1D) nanoelectrode arrays. The EPAM prevents 1D nanoelectrode arrays from self-aggregating and meanwhile endows them with high structural integrity and electrochemical stability during device assembly and operation process. MSCs assembled with these non-aggregative and robust 1D nanoelectrodes attain remarkable energy storage performance. The achieved results within this work on nanoarchitectured current collectors for supercapacitors shed light on the design of future energy storage and conversion devices.Superkondensatoren sind als elektrochemische Energiespeicher von großer Bedeutung, was auf ihre hervorragende elektrische Leistung, ihre ausgezeichnete Reversibilität und ihre lange Lebensdauer zurückzuführen ist. Im Vergleich zu Batterien haben Superkondensatoren jedoch eine geringe Energiedichte, was ihre breite Anwendung einschränkt. Pseudokapazitive Materialien mit einer hohen theoretischen Kapazität sind sehr vielversprechend, um die Energiespeicherfähigkeit von Superkondensatoren zu erhöhen. Die Forschung an nanoarchitektonischen Stromspeichern zielt darauf ab, ihr volles Potenzial im Bereich der Ladungsspeicherung auszuschöpfen, indem man sich mit den herausfordernden Aspekten wie der inhärent niedrigen elektrischen Leitfähigkeit und dem trägen Lade- und Entladeverhalten der meisten pseudokapazitiven Materialien befasst. In diesem Zusammenhang wurden drei Arten von nanoarchitektonischen Stromkollektoren entworfen, um pseudokapazitive Elektroden zu konstruieren, die unter verschiedenen Aspekten untersucht werden sollten. Es handelt sich dabei um Nickel-Nanorod-Arrays (NN), geätzte poröse Aluminiumoxidmembranen (EPAM), die mit einer SnO2-Schicht beschichtet sind (EPAM@SnO2), und in EPAM eingeschlossene Nickel-Nanodrähte (NiNWs-EPAM): Zunächst wird die Rolle von NN-Nanoarchitekten-Stromkollektoren in Superkondensator-Elektroden mit den pseudokapazitiven Materialien bei hoher Massenbelastung und dicker Schicht bewertet. Durch die elektrochemische Leistungs- und Impedanzanalyse der Elektroden mit und ohne die NN-nanoarchitektierten Stromkollektoren wird die Validierung des Designs von Dickschichtelektroden auf der Basis von nanoarchitektierten Stromkollektoren demonstriert. Zweitens werden EPAM@SnO2-Gerüste als nanoarchitektonische Stromkollektoren für Nanoelektroden entworfen und eingesetzt, um die Energiedichte des Mikro-Superkondensators (MSC) zu verbessern. Dank der orientierten und robusten Nanokanäle in EPAM@SnO2 können die daraus resultierenden Nanoelektroden sowohl die effektive Ionenmigration als auch die sehr große elektroaktive Oberfläche innerhalb des begrenzten Footprints synergetisch nutzen. Ein MSC wird schließlich konstruiert und weist eine rekordverdächtig hohe Leistung auf, was auf die Umsetzbarkeit des derzeitigen Designs für Energiespeichervorrichtungen hindeutet. Drittens wird der NiNWs-EPAM nanoarchitektierte Stromkollektor hergestellt, um nicht aggregierende und robuste eindimensionale (1D) Nanoelektroden-Arrays zu konstruieren. Das EPAM verhindert die Selbstaggregation von 1D-Nanoelektroden-Arrays und verleiht ihnen gleichzeitig eine hohe strukturelle Integrität und elektrochemische Stabilität während der Montage und des Betriebsprozesses der Geräte. MSCs, die mit diesen nicht aggregierenden und robusten 1D-Nanoelektroden bestückt sind, erreichen eine bemerkenswerte Energiespeicherleistung. Die im Rahmen dieser Arbeit zu nanoarchitektonischen Stromkollektoren für Superkondensatoren erzielten Ergebnisse geben einen Ausblick auf den Entwurf zukünftiger Energiespeicher und -wandler

DEVELOPING NANOPORE ELECTROMECHANICAL SENSORS WITH TRANSVERSE ELECTRODES FOR THE STUDY OF NANOPARTICLES/BIOMOLECULES

This study concerns development of a technology of utilizing metallic nanowires for a sensing element in nanofluidic single molecular (nanoparticle) sensors formed in plastic substrates to detect the translocation of single molecules through the nanochannel. We aimed to develop nanofluidic single molecular sensors in plastic substrates due to their scalability towards high through and low cost manufacturing for point-of-care applications. Despite significant research efforts recently on the technologies and applications of nanowires, using individual nanowires as electric sensing element in nanofluidic bioanalytic devices has not been realized yet. This dissertation work tackles several technical challenges involved in this development, which include reduction of nanowire agglomerates in the deposition of individual nanowires on a substrate, large scale alignment/assembly of metallic nanowires, placement of single nanowires on microelectrodes, characterization of electrical conductance of single nanowire, bonding of a cover plate to a substrate with patterned microelectrodes and nanowire electrodes. Overcoming the abovementioned challenges, we finally demonstrated a nanofluidic sensor with an in-plane nanowire electrode in poly(methyl methacrylate) substrates for sensing single biomolecules. In the first part of this study, we developed the processes for separation and large-scale assembly of individual NiFeCo nanowires grown using an electrodeposition process inside a porous alumina template. A method to fabricate microelectrode patterns on plastic substrates using flexible stencil masks was developed. We studied electrical and magnetic properties of new composite core-shell nanowires by measuring the electrical transport through individual nanowires. The core-shell nanowires were composed of a mechanically stable FeNiCo core and an ultrathin shell of a highly conductive Au gold (FeNiCo-Au nanowires). In the second part of this study, we simulated the effects of the nanopore geometry on the current drop signal of the translocation through a nanopore via finite element method using COMSOL. Using the above techniques, we developed for the fabrication and alignment of the microelectrodes and nanowires, we studied the optimum conditions to integrate the transverse nanoelectrode with the nanochannel on plastic substrates. The main challenge was to find the conditions to embed the micro-/nanoelectrodes into the nanochannel substrate as well as the nanochannel cover sheet

Three-dimensional nanotube arrays for solar energy harvesting and production of solar fuels

Over the past decade extensive research has been carried out on photovoltaic semiconductors to provide a solution towards a renewable energy future. Fabricating high-efficiency photovoltaic devices largely rely on nanostructuring the photoabsorber layers due to the ability of improving photoabsorption, photocurrent generation and transport in nanometer scale. Vertically aligned, highly uniform nanorods and nanowire arrays for solar energy conversion have been explored as potential candidates for solar energy conversion and solar-fuel generation owing to their enhanced photoconversion efficiencies. However, controlled fabrication of nanorod and especially nanotube arrays with uniform size and shape and a pre-determined distribution density is still a significant challenge. In this research work, we demonstrate how to address this issue by fabricating nanotube arrays by confined electrodeposition on lithographically patterned nanoelectrodes defined through electron beam as well as nanosphere photolithography. This simple technique can lay a strong foundation for the study of novel photovoltaic devices because successful fabrication of these devices will enhance the ability to control structure-property relationships. The nanotube patterns fabricated by this method could produce an equivalent amount of photocurrent density produced by a thin film like device while having less than 10% of semiconducting material coverage. This project also focused on solar fuel generation through photoelectrocatalytic water splitting for which efficient electrocatalysts were developed from non-precious elements. Lastly, a protocol was developed to disperse these electrocatalysts into a butadiene based polymeric catalytic ink and further processing to yield free-standing catalytic film applicable for water electrolysis”--Abstract, page iv

Diagnostics and Degradation Investigations of Li-Ion Battery Electrodes using Single Nanowire Electrochemical Cells

Portable energy storage devices, which drive advanced technological devices, are improving the productivity and quality of our everyday lives. In order to meet the growing needs for energy storage in transportation applications, the current lithium-ion (Li-ion) battery technology requires new electrode materials with performance improvements in multiple aspects: (1) energy and power densities, (2) safety, and (3) performance lifetime. While a number of interesting nanomaterials have been synthesized in recent years with promising performance, accurate capabilities to probe the intrinsic performance of these high-performance materials within a battery environment are lacking. Most studies on electrode nanomaterials have so far used traditional, bulk-scale techniques such as cyclic voltammetry, electrochemical impedance spectroscopy, and Raman spectroscopy. These approaches give an ensemble-average estimation of the electrochemical properties of a battery electrode and does not provide a true indication of the performance that is intrinsic to its material system. Thus, new techniques are essential to understand the changes happening at a single particle level during the operation of a battery. The results from this thesis solve this need and study the electrical, mechanical and size changes that take place in a battery electrode at a single particle level. Single nanowire lithium cells are built by depositing nanowires in carefully designed device regions of a silicon chip using Dielectrophoresis (DEP). This work has demonstrated the assembly of several NW cathode materials like LiFePO4, pristine and acid-leached α-MnO2, todorokite – MnO2, acid and nonacid-leached Na0.44MnO2. Within these materials, α-MnO2 was chosen as the model material system for electrochemical experiments. Electrochemical lithiation of pristine α-MnO2 was performed inside a glove box. The volume, elasticity and conductivity changes were measured at each state-of-charge (SOC) to understand the performance of the material system. The NW size changes due to lithiation were measured using an Atomic Force Microscope (AFM) in the tapping mode. Electronic conductivity changes as a function of lithiation was also studied in the model α-MnO2 NWs and was found to decrease substantially with lithium loading. In other measurements involving a comparison between the alpha and todorokite phases of this material system, it was observed that the rate capability of these materials is limited not by the electronic but, by the ionic conductivity. Mechanical degradation of a battery cathode represents an important failure mode, which results in an irreversible loss of capacity with cycling. To analyze and understand these degradation mechanisms, this thesis has tested the evolution of nanomechanical properties of a battery cathode. Specifically, contact-mode AFM measurements have focused on the SOC-dependent changes in the Young’s modulus and fracture strength of an α-MnO2 NW electrode, which are critical parameters that determine its mechanical stability. These changes have been studied at the end of the first discharge step, 1 full electrochemical cycle, and 20 cycles. The observations show an increase in Young’s modulus at low concentrations of lithium loading and this is attributed to the formation of new Li-O bonds within the tunnel-structured cathode. As the lithium loading increases further, the Young’s modulus was observed to reduce and this is hypothesized to occur due to the distortions of the crystal at high lithium concentrations. The experimental-to-theoretical fracture strength ratio, which points to the defect density in the crystal at a given stoichiometry, was observed to reduce with electrochemical lithium insertion / cycling. This capability has demonstrated lithiation-dependent mechanical property measurements for the first time and represents an important contribution since degradation models, which are currently in use for materials at any size scale, always assume constant values regardless of the change in stoichiometry

2D and 3D photonic crystal materials for photocatalysis and electrochemical energy storage and conversion

This perspective reviews recent advances in inverse opal structures, how they have been developed, studied and applied as catalysts, catalyst support materials, as electrode materials for batteries, water splitting applications, solar-to-fuel conversion and electrochromics, and finally as photonic photocatalysts and photoelectrocatalysts. Throughout, we detail some of the salient optical characteristics that underpin recent results and form the basis for light-matter interactions that span electrochemical energy conversion systems as well as photocatalytic systems. Strategies for using 2D as well as 3D structures, ordered macroporous materials such as inverse opals are summarized and recent work on plasmonic–photonic coupling in metal nanoparticle-infiltrated wide band gap inverse opals for enhanced photoelectrochemistry are provided

ELECTROCHEMICAL SYNTHESIS, TRANSFORMATION, AND CHARACTERIZATION OF MnO2 NANOWIRE ARRAYS FOR SUPERCAPACITOR ELECTRODES

The utilization of MnO2 nanowire arrays for future light weight energy storage devices is investigated here. One of the more specific questions this work looks to answer is: Can ultra high density arrays of MnO2 nanowires really be used to create future flexible micro-supercapacitors with high energy density, high power density, and long cycle lives? This research investigates the energy storage properties of dense arrays of solely MnO2 nanowires and synergistic MnO2 nanowire composites consisting of two or more materials/architectures, where the composite materials are able to offset some of the detrimental intrinsic properties of the MnO2 nanowires. Accordingly, a complete flexible supercapacitor device was prepared utilizing a coaxial MnO2/poly (3, 4-ethylenedioxythiophene) (PEDOT) core/shell nanowire array cathode with a PEDOT nanowire array anode. This material demonstrated metrics considerably better than current devices even while being flexed. In addition, a hierarchical MnO2 nanofibril/nanowire array was synthesized by transformation of a bare MnO2 nanowire array. This material was investigated for its supercapacitor properties while altering the parameters of its nanowire and nanofibril architectures. Finally, MnO2 nanowires were investigated for their charge storage mechanism using ICP-AES to detect Li ion to Mn ion ratios during the charging and discharging process. Their charge storage process was found to differ depending on whether the electrolyte solvent used was aqueous or organic. These projects all help advance energy storage devices well beyond their current status as bulky, heavy energy sources toward their prospective use as light weight, flexible, micro- power sources

Method development for scanning electrochemical microscopy and its application for material characterization

The fabrication of high-quality ultramicroelectrodes (UME) was a prerequisite for a variety of scanning electrochemical microscopic (SECM) experiments carried out in this thesis. UMEs with diameters ranging from 1 to 25 µm with a thin soda lime glass insulation (RG 2-20) and desired electrochemical properties were routinely fabricated to accomplish these targets. Further, for the imaging of the reactive oxygen species (ROS) generated during electrochemical oxygen evolution reaction (OER), the formation of transient diffusion layers during electrochemical reactions at large substrates was one major limitation. To overcome this limitation the well-known advantages of convective mass transport in electrochemical systems were exploited. After the integration of a high-precision stirring device into the experimental setup, the well-defined stirring led to steady-state diffusion layer characteristics near large substrate electrodes operated as generator electrodes. The imaging of the electrochemical hydrogen evolution at a 2 mm Pt disk electrode in the substrate generation/tip collection (SG/TC) mode demonstrated that SECM with forced convection increases the amount of obtained information. The added complexity of hydrodynamic methods in the theoretical description and construction of devices with known and reproducible mass transport conditions were addressed with numerical simulations. The reliability of the simulation was verified numerically and experimentally. The simulation showed that the rotation of the cylindrical stirrer resulted in a laminar convection near the substrate electrode. The flow profile within the liquid depended on the rotational speed of the stirrer. This enabled the formation of steady-state diffusion layers with a defined layer thickness. The constructed numerical model paves the way for additional numerical studies involving other cell and substrate geometries. The combination with other simulation modules (e.g.: electrochemistry) could provide interesting and valuable information for future applications. Hydrodynamic SECM further enabled the detection and imaging of the production of ROS at Pt and boron-doped diamond (BDD) macroelectrodes during OER. The combination of the tip-substrate voltammetry with forced convection resulted in a measurement principle similar to the rotating ring disk electrode and enabled the detection of ROS at BDD and Pt. Imaging in hydrodynamic SG/TC mode revealed that both, H2O2 and another reducible ROS species, are produced simultaneously at different domains depending on the local boron content of the surface. These pioneering experiments established the advantage of hydrodynamic SECM for locally resolved studies of highly reactive species produced during electrochemical gas evolution reaction. The increased amount of accessible analytical information aids toward a better understanding of electrochemical processes. The application of SECM in combination with forced convection to other heterogeneous reactions could help to expand the knowledge in other scientific fields and opens the door for new applications. In addition, the high-resolution SECM was used to image individual gold nanowires (AuNWs) immobilized on glass and gold coated glass slides in negative and positive feedback modes, respectively. Later the enzymatic peroxidase activity of immobilized horseradish peroxidase on individual AuNWs was imaged. These images revealed a higher enzymatic activity located at the ends of the AuNWs. This work can be further extended for the characterization of other novel nanomaterials and to study their redox behavior alone or in combination with other redox enzymes. In another work, SECM was used to provide complementary information in combination with the atomic/chemical force microscopy to evaluate the surface characteristics of pretreated carbon fiber reinforced plastics (CFRP). SECM images revealed the exposure of carbon fiber strands and delivered additional information about the chemical and morphological structure of the pretreated CFRP

Nanoscale electrochemical mapping

Surfaces and interfaces, of both practical and fundamental interest, have long been recognized to be complex, yet while there are many microscopy and spectroscopy methods for imaging structure, topography and surface chemical composition at high spatial resolution, there are relatively few techniques for mapping associated chemical fluxes in the near-interface region. In this regard, scanning electrochemical probe microscopy (SEPM), which utilizes a small scale electrode probe as an imaging device, has had a unique place in the scanning probe microscopy (SPM) family of techniques, in being able to map chemical fluxes and interfacial reactivity. For a long time, techniques such as scanning electrochemical microscopy (SECM) were largely stuck at the micron –or larger –scale in terms of spatial resolution, but recent years have seen spectacular progress, such that a variety of different types of SEPM technique are now available and 10sof nm spatial resolution is becoming increasingly accessible. This step-change in capability is opening many new opportunities for the characterization of flux processes and interfacial activity in a whole raft of systems, including electrode surfaces, electromaterials, soft matter, living cells and tissues

ALD PROCESSES AND APPLICATIONS TO NANOSTRUCTURED ELECTROCHEMICAL ENERGY STORAGE DEVECES

Next generation Li-ion batteries (LIB) are expected to display high power densities (i.e. high rate performance, or fast energy storage) while maintaining high energy densities and stable cycling performance. The key to fast energy storage is the efficient management of electron conduction, Li diffusion, and Li-ion migration in the electrode systems, which requires tailored material and structural engineering in nanometer scale. Atomic layer deposition (ALD) is a unique technique for nanostructure fabrications due to its precise thickness control, unprecedented conformality, and wide variety of available materials. This research aims at using ALD to fabricate materials, electrodes, and devices for fast electrochemical energy storage. First, we performed a detailed study of ALD V2O5 as a high capacity cathode material, using vanadium tri-isopropoxide (VTOP) precursor with both O3 and H2O as oxidant. The new O3-based process produces polycrystalline films with generally higher storage capacity than the amorphous films resulting from the traditional H2O-based process. We identified the crucial tradeoff between higher gravimetric capacity with thinner films and higher material mass with thicker films. For the thickness regime 10-120 nm, we chose areal energy and power density as a useful metric for this tradeoff and found that it is optimized at 60 nm for the O3-VTOP ALD V2O5 films. In order to increase material loading on fixed footprint area, we explored various 3-dimentional (3D) substrates. In the first example, we used multiwall carbon nanotube (MWCNT) sponge as scaffold and current collector. The core/shell MWCNT/V2O5 sponge delivers a stable high areal capacity of 816 μAh/cm2 for 2 Li/V2O5 voltage range (4.0-2.1 V) at 1C rate (nC means charge/discharge in 1/n hour), 450 times that of a planar V2O5 thin film cathode. Due to low density of MWCNT and thin V2O5 layer, the sponge cathode also delivers high gravimetric power density in device level that shows 5X higher power density than commercial LIBs. In the other example, Li-storage paper cathodes, functionalized of conductivity from CNT and Li-storage capability from V2O5¬, presented remarkably high rate performance due to the hierarchical porosity in paper for Li+ migration. The specific capacity of V2O5 is as high as 410 mAh/g at 1C rate, and retained 116 mAh/g at high rate of 100C. We found V2O5 capacities decreased by about 30% at high rates of 5C-100C after blocking the mesopores in cellulose fiber, which serves to be the first confirmative evidence of the critical role of mesoporosity in paper fibers for high-rate electrochemical devices. Finally, we made high density well-aligned nanoporous electrodes (2 billion/cm2) using anodic alumina template (AAO). ALD materials were deposited into the nanopores sequentially - Ru or TiN for current collection, and V2O5 for Li-storage. Ru metal by ALD shows high conductivity and conformality, and serves best as the current collector for V2O5. The capacity of V2O5 reaches about 88% of its theoretic value at high rate of 50C. Such electrodes can be cycled for 1000 times with 78% capacity retention

Integrated Nanoscale Tools for Interrogating Living Cells

The development of next-generation, nanoscale technologies that interface biological systems will pave the way towards new understanding of such complex systems. Nanowires – one-dimensional nanoscale structures – have shown unique potential as an ideal physical interface to biological systems. Herein, we focus on the development of nanowire-based devices that can enable a wide variety of biological studies. First, we built upon standard nanofabrication techniques to optimize nanowire devices, resulting in perfectly ordered arrays of both opaque (Silicon) and transparent (Silicon dioxide) nanowires with user defined structural profile, densities, and overall patterns, as well as high sample consistency and large scale production. The high-precision and well-controlled fabrication method in conjunction with additional technologies laid the foundation for the generation of highly specialized platforms for imaging, electrochemical interrogation, and molecular biology. Next, we utilized nanowires as the fundamental structure in the development of integrated nanoelectronic platforms to directly interrogate the electrical activity of biological systems. Initially, we generated a scalable intracellular electrode platform based on vertical nanowires that allows for parallel electrical interfacing to multiple mammalian neurons. Our prototype device consisted of 16 individually addressable stimulation/recording sites, each containing an array of 9 electrically active silicon nanowires. We showed that these vertical nanowire electrode arrays could intracellularly record and stimulate neuronal activity in dissociated cultures of rat cortical neurons similar to patch clamp electrodes. In addition, we used our intracellular electrode platform to measure multiple individual synaptic connections, which enables the reconstruction of the functional connectivity maps of neuronal circuits. In order to expand and improve the capability of this functional prototype device we designed and fabricated a new hybrid chip that combines a front-side nanowire-based interface for neuronal recording with backside complementary metal oxide semiconductor (CMOS) circuits for on-chip multiplexing, voltage control for stimulation, signal amplification, and signal processing. Individual chips contain 1024 stimulation/recording sites enabling large-scale interfacing of neuronal networks with single cell resolution. Through electrical and electrochemical characterization of the devices, we demonstrated their enhanced functionality at a massively parallel scale. In our initial cell experiments, we achieved intracellular stimulations and recordings of changes in the membrane potential in a variety of cells including: HEK293T, cardiomyocytes, and rat cortical neurons. This demonstrated the device capability for single-cell-resolution recording/stimulation which when extended to a large number of neurons in a massively parallel fashion will enable the functional mapping of a complex neuronal network.Physic