Interconnected Electrodes Research Articles

Interconnected porous composites electrode materials of Carbon@Vanadium nitride by directly absorbing VO3-

Porous carbon materials are widely used as adsorbent and electrode materials due to its high specific surface area and low cost, while vanadium nitride, with excellent conductivity, good environmental stability and high reaction selectivity has been widely used as electrode material. Herein, the interconnected porous carbon derived from the precursor polystyrene is used as adsorbent to adsorb vanadate ions, and the carbon/vanadium complex is further heat-treated under ammonia atmosphere to convert it to vanadium nitride. The final obtained hybrid composite of interconnected porous carbon@vanadium nitride consists of vanadium nitride nanoparticles on the surface of porous carbon. The electrode material prepared with the hybrid composite exhibits a high specific capacitance of 260 F g−1 at 0.5 A g−1 and a good rate capability with capacitance retention of 74.6%. Furthermore, the hybrid supercapacitor assembled with Ni (OH)2 exhibits a high specific capacitance of 129 F g−1 at the current density of 1 A g−1. Remarkably, it delivers an energy density of 40.5 Wh kg−1 and a high power density of 3760.7 W kg−1.

Flexible, strain gated logic transducer arrays enabled by initializing surface instability on elastic bilayers

Developing flexible sensors with a high strain sensing range could enable widespread downstream applications, by allowing intimate, mechanically conformable integration with soft biological tissues. By characterizing interconnected metal electrode arrays on super-flexible substrates, we have established a surface deformation control strategy of an array of strain transducers. The strain gated switches are capable of measuring various compressive strains (up to 60%) by bringing metal electrodes into self-contact via creasing elastic instability beyond a threshold substrate strain. The designed devices have been developed to explore the geometry design effect on the electrode-elastomer “stiff film on soft elastomer” surface deformation. The enabled transducer array yielded a stepwise strain-electrical resistance switching mechanism which opens up the potential of future interconnected sensor array type of super-compressible devices.

Interconnected polypyrrole nanostructure for high-performance all-solid-state flexible supercapacitor

Rational design of conductive polymer-based supercapacitor (CPSC) by integrating three-dimensional (3D) interconnected electrode materials with porosified electrolyte is of great potential to simultaneously increase the specific capacitance and rate capability of CPSC device. Herein, we describe for the first time a prototype of all-solid-state CPSC using 3D interconnected fibrous polypyrrole (PPy) and 3D porosified electrolyte. The as-made PPy was characterized to possess excellent electron conductivity (∼7.8 S cm−1), 3D interconnected structure, providing fast electron/ion transportation pathway to accelerate the kinetics of interfacial pseudocapacitive reactions. The symmetric PPy-based CPSC device initially delivered a high specific capacity up to 505 mF cm−2 (corresponding to 168 F g−1) at current stream of 2.0 mA cm−2 and was capable of retaining 50% capacitance (∼250 mF cm−2) at high rate (20.0 mA cm−2). Meanwhile, an improved energy density of 44.9 μWh·cm−2 at power of 8 mW cm−2 was achieved. In addition, PPy-CPSC exhibited excellent flexible performance, no obvious degradation of capacitive performance was observed for PPy-CPSC device under bent or twisted.

2D GEM based imaging detector readout capabilities from perspective of intense soft x-ray plasma radiation.

A detecting system based on the Gas Electron Multiplier (GEM) technology is considered for tokamak plasma radiation monitoring. In order to estimate its capabilities in processing and recording intense photon flux (up to ∼0.1 MHz/mm2), the imaging effectiveness of GEM detectors was tested with different patterned anode planes (i.e., different signal readouts): a simple hexagonal readout structure and three structures with interconnected electrodes (XY square, XY rectangular, and UXV). It was found that under intense photon flux, all the readouts fail to account for a considerable amount of the incoming signals due to mostly photon position determination ambiguity and overlapped signals. Analysis of the signals that can be used to determine photon position and energy unambiguously showed that the UXV readout structure is more effective among the readouts with interconnected electrodes. Along with similar spatial resolution and accuracy, the UXV based layout could be considered as a quite promising base of the interconnected anode electrodes configuration, keeping in mind that the photon rate capability has to be improved for the final application.

The dye-sensitized solar cells based on the interconnected ternary cobalt diindium sulfide nanosheet array counter electrode

The ternary cobalt diindium sulfide (CoIn2S4) counter electrode (CE) with interconnected nanosheet array structure is fabricated on the fluorine-doped tin oxide (FTO) substrate by in situ ethanol solvothermal synthesis. The ethanol solvent and In2.77S4 intermediate play important roles on the formation of the interconnected nanosheet array structure. The dye-sensitized solar cell (DSSC) with the optimized ternary CoIn2S4 CE gives an energy conversion efficiency of 8.83%, which is much higher than that of DSSCs based on the binary CoS2 (6.47%), and In2.77S4 (0.46%) CEs and can be comparable with that of Pt CE (8.19%) under full sunlight illumination (100 mW cm−2, AM 1.5 G). Except for the intrinsically electrocatalytic activity from the CoIn2S4, the higher conversion efficiency of CoIn2S4 based DSSC can be attributed to that the interconnected nanosheet array structure provides plentiful catalytic active sites, abundant electron transport channels, and fast electrolyte diffusion.

Substrate Engineered Interconnected Graphene Electrodes with Ultrahigh Energy and Power Densities for Energy Storage Applications.

Supercapacitors combine the advantages of electrochemical storage technologies such as high energy density batteries and high power density capacitors. At 5-10 W h kg-1, the energy densities of current supercapacitors are still significantly lower than the energy densities of lead acid (20-35 W h kg-1), Ni-metal hydride (40-100 W h kg-1), and Li-ion (120-170 W h kg-1) batteries. Recently, graphene-based supercapacitors have shown an energy density of 40-80 W h kg-1. However, their performance is mainly limited because of the reversible agglomeration and restacking of individual graphene layers caused by π-π interactions. The restacking of graphene layers leads to significant decrease of ion-accessible surface area and the low capacitance of graphene-based supercapacitors. Here, we introduce a microstructure substrate-based method to produce a fully delaminated and stable interconnected graphene structure using flash reduction of graphene oxide in a few seconds. With this structure, we achieve the highest amount of volumetric capacitance obtained so far by any type of a pure carbon-based material. The affordable and scalable production method is capable of producing electrodes with an energy density of 0.37 W h cm-3 and a power density of 416.6 W cm-3. This electrode maintained more than 91% of its initial capacitance after 5000 cycles. Moreover, combining with ionic liquid, this solvent-free graphene electrode material is highly promising for on-chip electronics, micro-supercapacitors, as well as high-power applications.

(Invited) Nanopore Batteries: Fast and Slow Ion Transport in 1D and 3D Networked Porous Nanostructure Electrodes

While interest in nanostructured materials for batteries has been growing enormously in recent years, limited systematic studies have been carried out on controlled architectures to explore the impact of structure on ion transport and resulting charge-discharge rate performance in these electrodes. Here we utilize a combination of well-defined porous 1D and 3D networked nanostructures and atomic layer deposition to fabricate a variety of systematically variable electrode architectures and describe a systematic study from 1D array electrode to 3D electrode. The structural control and electrode design are described in detail. Then, analysis of the rate performance, with a focus on distinguishing between diffusion and charge transfer limited reaction mechanisms, is carried out for two distinct electrode systems, focusing on different issues which face advanced electrode architectures. First, we study a quantitative understanding of 1D nanopore battery electrode’s superior power performance – fast electrochemistry through surface-dominant Li ion insertion. The kinetics of the charge storage reaction strongly influences high power characteristic of the electrode materials. An asymmetric nanopore battery was also fabricated to give higher energy storage. Second, we analyze the impact of transitioning from 1D to 3D structured electrodes. The traditional straight porous AAO films are fabricated for 1D nanotube array and interconnected pores 3D interconnected nanotube array electrode structures. Nanostructured V2O5 electrodes in the nanopores were examined for Li ion batteries as it is one of the well characterized cathode material. V2O5 can reversibly accommodate up to two Li ions. When V2O5 was cycled within the one Li insertion window, the 3D interconnected nanotubular structure showed superior power capabilities over the 1D nanotubular array. With the two Li ion insertion window, an opposite trend was observed, with interconnected electrode performing worse. As the structure between the one and two Li insertion windows were kept constant, possible hypothesis and explanation will be discussed through a dramatic change of V2O5 electrical and electrochemical properties. Up to one lithiation, the conductivity of the material slightly increases whereas with the second lithiation the conductivity dramatically drops by two orders of magnitude due to the disruption in the electron conduction mechanism. In the interconnected electrode, with one Li insertion, the conductivity of the electrode near the current collector increases with the degree of lithiation allowing better usage of the material farther away from the current collector due to lower ohmic loss of the overpotential needed. However, with two Li insertion the conductivity drops near the current collector which prevents the utilization of the material farther away resulting in lower power performance. This study opens up opportunities for rationally designed advanced electrode architectures to optimize the performance of electrochemical energy storage devices in the future.

On the Distribution of Lightning Current among Interconnected Grounding Systems in Medium Voltage Grids

This paper presents the results of a first investigation on the effects of lightning stroke on medium voltage installations’ grounding systems, interconnected with the metal shields of the Medium Voltage (MV) distribution grid cables or with bare buried copper ropes. The study enables us to evaluate the distribution of the lightning current among interconnected ground electrodes in order to estimate if the interconnection, usually created to reduce ground potential rise during a single-line-to-ground fault, can give place to dangerous situations far from the installation hit by the lightning stroke. Four different case studies of direct lightning stroke are presented and discussed: (1) two secondary substations interconnected by the cables’ shields; (2) two secondary substations interconnected by a bare buried conductor; (3) a high voltage/medium voltage station connected with a secondary substation by the medium voltage cables’ shields; (4) a high voltage/medium voltage station connected with a secondary substation by a bare buried conductor. The results of the simulations show that a higher peak-lowering action on the lighting-stroke current occurs due to the use of bare conductors as interconnection elements in comparison to the cables’ shields.

Electrochemical capacitive energy storage in PolyHIPE derived nitrogen enriched hierarchical porous carbon nanosheets

Porous and interconnected electrodes based on carbon nanoarchitectures offer comprehensive advantages of large specific surface area and high porosity consequently increasing the specific capacitance of ultracapacitor energy storage systems. Emulsion-templated polymers, PolyHIPEs (Polymerized High Internal Phase Emulsions) are highly porous polymers with a structure of cages interconnected by windows thus provide suitable framework to create such porous carbon nanostructures. Herein, nitrogen enriched porous carbon nanosheets are synthesized by pyrolysis of polymer-silica hybrid PolyHIPE and subsequent silica removal. This nitrogen enriched porous carbon nanosheets when tested as an electrode for ultracapacitor, showed specific capacitance as high as 209 F/g at a current density of 1 A/g in 1 M H2SO4with excellent capacity retention over long cycling.

Improved Optoelectronic Performance of High-Voltage Ultraviolet Light-Emitting Diodes Through Electrode Designs

High-efficiency high-voltage ultraviolet light-emitting diodes (HV-UVLEDs) consisting of a ${4} \times {4}$ microcells array with an area of ${61} ~\text {mil} \times {33} ~\text {mil}$ were designed and fabricated via the electrode pattern and interconnect technique. Polymer material was used to fill trench with planarization, and interconnection technology was used for metal layer connection to address wiring defect issue. In comparison with the conventional lateral UV-LED (C-UVLED), which had 33.6% and 34.7% enhancement in the light output power and wall-plug efficiency (at 1.5 W) of the HV-UVLEDs. A 56% improvement in the external quantum efficiency droop behavior was achieved for the HV-UVLEDs when compared with that of C-UVLED. This improvement can be attributed to superior current spreading in the HV-UVLED due to its smaller microcells, which increases light-emission efficiency overall.

Investigations on effects of the hole size to fix electrodes and interconnection lines in polydimethylsiloxane

Translational research in bioelectronics medicine and neural implants often relies on established material assemblies made of silicone rubber (polydimethylsiloxane-PDMS) and precious metals. Longevity of the compound is of utmost importance for implantable devices in therapeutic and rehabilitation applications. Therefore, secure mechanical fixation can be used in addition to chemical bonding mechanisms to interlock PDMS substrate and insulation layers with metal sheets for interconnection lines and electrodes. One of the best ways to fix metal lines and electrodes in PDMS is to design holes in electrode rims to allow for direct interconnection between top to bottom layer silicone. Hence, the best layouts and sizes of holes (up to 6) which provide sufficient stability against lateral and vertical forces have been investigated with a variety of numbers of hole in line electrodes, which are simulated and fabricated with different layouts, sizes and materials. Best stability was obtained with radii of 100, 72 and 62 µm, respectively, and a single central hole in aluminum, platinum and MP35N foil line electrodes of 400 × 500 µm2 size and of thickness 20 µm. The study showed that the best hole size which provides line electrode immobility (of thickness less than 30 µm) within a central hole is proportional to reverse value of Young’s Modulus of the material used. Thus, an array of line electrodes was designed and fabricated to study this effect. Experimental results were compared with simulation data. Subsequently, an approximation curve was generated as design rule to propose the best radius to fix line electrodes according to the material thickness between 10 and 200 µm using PDMS as substrate material.

Impact of Interconnections, Dynamic Conductivity, Pore Size on the Performance of V2O5 Cathode for Lithium Ion Batteries

It is well known that a nanostructured electrodes have superior performance over their bulk counter parts. It has been pursued by the battery community to explore different nanostructures to discover critical parameters for designing next generation batteries. Here, we describe a systematic study from 1D array electrode to 3D electrode enabled by nanostructured scaffolds and atomic layer deposition. Nanostructured scaffolds have been used as templates for the fabrication of nanostructured materials for wide range of applications. One of the commonly used scaffolding material is porous anodized aluminum oxide films (AAO). AAO are used for their hexagonally ordered nanoporous structures and the ease of controlling the structural parameters such as the pore diameter and the pore length by altering the anodization conditions. Here we use voltage control to fabricate not only the traditional straight porous AAO templates, for 1D nanotube array, but also interconnected porous AAO templates, 3D interconnected nanotube array. From previous work, by deviating from a constant voltage, which results in straight pores, to lower voltage and back to the initial voltage cause the pores to deviate from their straight structure to interconnecting layer and back to their straight structure allowing controllable insertion of the interconnecting layers throughout the film. Using atomic layer deposition, V2O5 was conformally coated along the walls of these high aspect ratio AAO with sputtering of gold current collector at the top. Using AAO as a template which can be tuned systematically, the resulting nanostructured V2O5 electrodes were examined for Li ion batteries as it is one of the well characterized cathode material. V2O5 can reversibly accommodate up to two Li ions. From the previous study, when V2O5 was cycled within the one Li insertion window, the 3D interconnected nanotubular structure showed superior power capabilities over the 1D nanotubular array. This was due to the interconnections allowed more of the materials closer to the top current collector. As faradaic reactions are much more sensitive to the ohmic loss, this close proximity to the current collector allowed more efficient utilization of the overall electrode. With the two Li ion insertion window, an opposite trend was observed, with interconnected electrode performing worse. As the structure between the one and two Li insertion windows were kept constant, we hypothesized two possibilities. One, because we have more material within the interconnected electrodes, it will require more Li ions than the straight electrodes. However the pore opening is the same which could lead to ion starvation in the interconnected electrodes because the diffusion of the Li ions into these interconnected electrodes could not keep up with the demand. Two, V2O5 with the second lithiation causes a dramatic change the electrochemical properties more severely in the interconnected electrode. To test these hypothesis, different pore diameter electrodes were used. With the larger pores, the electrodes overall performed better but the difference between the interconnected and the straight electrodes could not be explained. Literatures show the electronic conductivity of V2O5 varies dramatically depending on the degree of lithiation. Up to one lithiation, the conductivity of the material slightly increases whereas with the second lithiation the conductivity dramatically drops by two orders of magnitude due to the disruption in the electron conduction mechanism. This could explain the performance difference between the straight and the interconnected electrodes. When the current density is low, the electrons have enough time to travel through the electrode regardless of the structure. When the current density is high, now the conductivity matters. In the interconnected electrode, with one Li insertion, the conductivity of the electrode near the current collector increases with the degree of lithiation allowing better usage of the material farther away from the current collector due to lower ohmic loss of the overpotential needed. However, with two Li insertion the conductivity drops near the current collector which prevents the utilization of the material farther away resulting in lower power performance. Comsol simulations are in progress to better understand the overpotentials and ion depletion in these straight and interconnected electrodes. This work is able to show the need to match the properties of the electroactive material with the properties of the nanostructure for high performance electrodes.

Effects of Carboxylated Polythiophenes in Fe3O4 Li-Ion Battery Anodes

The high energy/power density of Li-ion batteries elicits intensive research efforts related to high capacity anode materials. The main obstacles which retard the practical employment of high-capacity electrochemically active particles including Si, Sn, metal oxide and their derivatives, stem from large volume changes associated with Li insertion/extraction and the resultant electrical contact loss, thereby leading to poor cycling performance. Efforts have been made to circumvent the breakdown of electron pathways through the introduction of electrical conducting functionalities to the surface of active materials, such as carbon coatings and conductive polymeric binders, or the fabrication of a stable battery anode with electrically inactive polymeric binders that enable the system to maintain its integrity. These approaches are reasonably effective, however, incorporation of a porous entity which helps ion transport, into a composite electrode is also essential for improved performance. In principle, electrochemical reactions that occur within the electrode, reveal the intrinsic energy capacity when a Li ion encounters an electron inside an active site. Electron and ion transport are both critical factors to determine the internal resistance of the electrodes, which in turn influences their electrochemical performance. Here we present how to improve both electronic and ionic transport in Li-ion battery electrodes using conjugated polymers. The approach includes poly[3-(potassium-4-butanoate)thiophene] (PPBT) — a water-soluble, carboxylate substituted polythiophene — as a binder component, and polyethylene glycol (PEG) as a surface coating on active material, namely, Fe3O4 nanoparticles. Additionally, carbon nanotubes (CNTs) are considered for use as the conducting networks to facilitate design of a light-weight, flexible web electrode. To enhance the electronic conduction in the electrodes, connection between active materials (PEG- Fe3O4) and conducting agents (or CNTs) through binding components is of importance. PEG coating and carboxylated polythiophenes play an important role in dispersing the Fe3O4 nanoparticles and conducting agents (or CNTs), respectively, in a water medium, which allow for well-developed and interconnected electrode structures. Furthermore, carboxylated polythiophenes (e.g. PPBT) can boost electronic conduction, based on their high conducting properties and through electrochemical doping during electrochemical testing. The presence of carboxylic moieties on the side chain of polythiophenes such as PPBT could facilitate the formation of stable electrodes via chemical interactions between PPBT moieties (COO-) and the Fe3O4 surface (–OH). The results will show that this methodical consideration of both ion and electron transport through introduction of a carboxylated PPBT component, can remarkably enhance the performance of Fe3O4 based high-capacity anodes.

Ultrahigh specific capacitance of spray deposited nanoporous interconnected ruthenium oxide electrode fabric for supercharged capacitor

In present work, nanoporous interconnected ruthenium oxide electrode fabric have been designed and prepared successfully by spray pyrolysis technique on thin stainless steel substrates via aqueous route. X-ray diffraction analysis gives amorphous nature of the prepared sample. The existence of non-covalent interactions between ruthenium and oxygen was confirmed by Fourier transform infrared spectroscopy. The elemental analysis was observed by using elemental dispersive spectrum and X-ray photoelectron spectroscopy. Morphological analysis of the ruthenium oxide fabric revealed the nanoporous granular interconnected architecture, producing high specific surface area and good electronic or ionic conducing path. The use of nanoporous interconnected ruthenium oxide electrode fabric is as free-standing electrodes for supercharged capacitor, which offers excellent ability of ultrahigh specific capacitance 1429 F g−1 at scan rate 1 mV s−1, higher energy density 172 Wh kg−1, power density 320 KW kg−1 and columbic efficiency is 94.74% in 1 M KOH. Also it exhibits very low charge transfer resistance (0.89 Ω) and relatively excellent cycle stability at scan rate 100 mV s−1 for 3000 cycles, which concluded that it has a potential application as a supercharged capacitor electrode fabric.

Self assembled sulfur induced interconnected nanostructure TiO2 electrode for visible light photoresponse and photocatalytic application

Pristine TiO2 and sulfur doped TiO2 (S-TiO2) thin films were coated over the glass substrates by varying the concentration of sulfur source (thiourea – 2, 4, 6, 8 and 10at%) using a cost-effective Jet nebulizer spray technique. The deposited thin films were in anatase phase with the tetragonal structure analyzed from the XRD pattern. The chemical state of the elements was determined from XPS analysis. Pristine TiO2 and S-TiO2 thin films depict the presence of spherical particles embedded over 3-D interconnected wire-like structure from SEM analysis. Optical studies revealed reduction in band gap of S-TiO2 films on increasing the sulfur concentration (3.2–2.8eV). The sulfur incorporation in TiO2 lattice confirmed by the fall in intensity of near band edge emission as observed from room temperature PL spectra. The charge carrier dynamics of the prepared thin films were studied by means of steady state and transient photoconduction measurements. The photocatalytic performance of pristine TiO2 and S-TiO2 thin films for the degradation of malachite green dye was investigated under visible light.

3D nitrogen-doped graphene foam with encapsulated germanium/nitrogen-doped graphene yolk-shell nanoarchitecture for high-performance flexible Li-ion battery

Flexible electrochemical energy storage devices have attracted extensive attention as promising power sources for the ever-growing field of flexible and wearable electronic products. However, the rational design of a novel electrode structure with a good flexibility, high capacity, fast charge–discharge rate and long cycling lifetimes remains a long-standing challenge for developing next-generation flexible energy-storage materials. Herein, we develop a facile and general approach to three-dimensional (3D) interconnected porous nitrogen-doped graphene foam with encapsulated Ge quantum dot/nitrogen-doped graphene yolk-shell nano architecture for high specific reversible capacity (1,220 mAh g−1), long cycling capability (over 96% reversible capacity retention from the second to 1,000 cycles) and ultra-high rate performance (over 800 mAh g−1 at 40 C). This work paves a way to develop the 3D interconnected graphene-based high-capacity electrode material systems, particularly those that suffer from huge volume expansion, for the future development of high-performance flexible energy storage systems.

Modeling of phase separation across interconnected electrode particles in lithium-ion batteries

Non-equilibrium lithiation and its relaxation towards equilibrium in a particle network with phase-separating materials.

Spinel ZnCo2O4 nanosheets as carbon and binder free electrode material for energy storage and electroreduction of H2O2

We present simple surfactant free hydrothermal method followed by calcination process to in situ fabricate ZnCo2O4 material on Ni foam with two distinctly different morphologies. The interconnected nanosheets (ZnCo2O4-H) and bundle-like nanosheets (ZnCo2O4-U) morphologies of ZnCo2O4 material are seeded on Ni foam using hexamethylenetetramine and urea as precipitating agents. These nanosheets on Ni foams consist of large number of mesopores with BET surface areas varying between 88 and 112 m2 g−1. The Ni foams seeded with ZnCo2O4 nanosheets are directly used as electrodes to evaluate their charge storage performance and electrocatalytic activity for hydrogen peroxide reduction in alkaline medium. The interconnected nanosheets exhibit high specific capacitance value of 1102 F g−1 compared to 653 F g−1 of bundle-like nanosheets at a current density of 1 A g−1. The bundle-like nanosheets of zinc cobalt oxide material shows excellent rate capability with 77% capacitance retention at 16 A g−1. Interestingly both showed excellent capacitance retention over 5000 galvanostatic charge-discharge cycles at very high current densities. Furthermore, both the materials are very effective for hydrogen peroxide reduction. In the presence of 0.5 M H2O2 the cathodic current increased by about 100 times for interconnected nanosheets electrode and 30 times for bundle-like nanosheets electrode compared to H2O2 free alkaline solutions. These results consistently show strong correlation between the morphology related properties and electrochemical activity of oxide materials.

Solid Dispersion Flow Battery Particle Synthesis and Electrochemical Characterization

Flow batteries have a number of advantages as an energy storage system, including the ability to decouple the power and energy of the battery. The electroactive material within traditional flow batteries consists of transition metals dissolved in aqueous acidic electrolytes, however, recently there have been reports of using solid particles as the electroactive material. The advantages of solid particles are that by starting with a solid the solubility of the electroactive species is no longer a limiting factor with regards to energy density and the energy density of the flow battery electrolyte can be increased substantially. In addition, by using organic solvents and lithium-ion battery active materials as the solid particles in the flow battery the energy density of the system can be further improved. A challenge in these so-called semisolid flow batteries is that most have carbon additives that form interconnected electrode structures. The carbon addition significantly increases the viscosity of the electrolyte, which results in additional net energy lost to pumping the electrolyte through the system as well as higher power pumps required to achieve equivalent volumetric flow rates through the channels. Our research group has been investigating flow batteries that rely on solid electroactive materials. However, within our solid dispersion flow battery system the electrochemical reactions are the consequence of collisions with the current collector. The coupling between electrochemical and rheological properties within this flow battery are highly sensitive to both the fluid flow through the channel and the electrochemically active particle morphology and loading within the electrolyte. Thus, this system requires understanding of the electrochemical properties as a function of both particle loading, particle interactions with the current collector, and the flow geometry that the electrolyte traverses through. In this talk, we will describe recent work within our group to design scalable processes to synthesize particles that form stable dispersions within the solid dispersion flow battery geometry. The synthesis route consists of sacrificial precursor components that results in loosely connected agglomerate structures that are easily separated to form small, dispersible lithium-ion battery active material particles. We will discuss recent efforts to characterize these lithium-ion battery particles within solid-dispersion flow battery systems and experiments to understand the electrochemical and rheological properties and their coupling within flow batteries comprised of dispersed solid particles. Limitations with regards to power capability and the role of the current collectors within this system will also be discussed.

Au / SiO2 Hybrid Bonding with 6-μm-Pitch Au Electrodes for 3D Structured Image Sensors

Pixel counts and frame rates of broadcasting TV cameras are increasing to meet the escalating demands for high quality pictures. The pixel count could be as high as 133 million [1] in the state-of-the-art CMOS video image sensor, and the frame rate could 120 fps [2], which will further keep increasing in coming years. Owing to large pixel count, the sensors are designed to share the signal processing circuits in a form such as column-parallel processing [2], and hence the conventional image sensors face difficulties in processing the signals at fast frame rate. In other words, the signal processing speed at each pixel is the bottleneck that limits the pixel count and the frame rate. To overcome this problem, we developed a 3D structured CMOS image sensor that can process signals pixel-wise in parallel as a means to increase the pixel count and the frame rate at same time, as shown in Fig. 1 [3]. The image sensor uses stacked signal processing circuits for each pixel by placing the circuits immediately below the photodiodes. Compared to conventional image sensors, the new structure enables us to maintain a higher frame rate even if the pixel count increases.The pixel-pitch interconnection is one of a key technology to realize the 3D structured CMOS image sensors. We have previously developed an 8×8 pixel prototype 3D structured CMOS image sensor with Au interconnections in each pixel [3]. The sensor’s pixel pitch was 80 μm. To further enhance the resolution, we recently reduced the Au electrode dimension to 6-μm-pitch that could be electrically interconnected using a direct bonding technique. We also have developed a daisy-chain test device to examine the large number of interconnection.The device was fabricated by the process described in Fig. 2 that consists of the following steps: (a) Al wiring layer was formed on a fully depleted silicon-on-insulator wafer. (b) Via holes were formed in the top intermediate SiO2 layer by dry etching. (c) Ti and Au seed layers were deposited by sputtering, and then the via holes were embedded with an electroplated Au layer. (d) Au/SiO2 bonding surface was formed by chemical mechanical polishing, and the wafer was diced into chips of 20-mm and 18-mm square each. Fig. 3 shows an atomic force microscopy (AFM) image of this chip surface. The height of the Au electrode above the SiO2 surface was approximately 18 nm. The average roughness (Ra) of the polished SiO2 and Au surfaces were less than 0.4 nm and approximately 1 nm, respectively. The diameter and the pitch of the Au electrode were 3 μm and 6 μm, respectively. (e) The bonding surface was activated sequentially by Ar and O2 plasmas. (f) Two chips were directly bonded at 2,000 N for 60 min at 200 °C. Fig. 4 shows the cross-sectional scanning electrode microscopy (SEM) image of the bonded electrodes. No voids were observed at the bonded interface. Fig. 5 shows the measured daisy-chain resistance. We confirmed that a series of electrical interconnections exceeded 230,000 contacts, and that the chain resistance was proportional to the number of electrodes. The Au contact resistance for one connection was approximately 23.6 mΩ, which is considerably smaller than that of the plug electrodes of LSI circuits.To summarize, a daisy-chain test chip with 6-μm-pitch Au electrode interconnects was successfully fabricated using Au/SiO2 hybrid bonding. The result was promising to realize 3D structured CMOS image sensors for pixel-parallel signal processing. [1] R. Funatsu et al., ISSCC Digest of Technical Papers, 6.2, pp. 112-113, 2015.[2] K. Kitamura et al., IEEE Trans. Electron Devices, vol. 59, No. 12, pp. 3426-3433, 2012.[3] M. Goto et al., IEEE Trans. Electron Devices, vol. 62, No. 11, pp. 3530-3535, 2015. Figure 1