Active Electrode Area Research Articles

Characterization of Electrochemical Surface Area and Porosity of Zirconia Sensors

The platinum electrode/yttria-stabilized zirconia metal/solid electrolyte interface is used in a wide variety of sensing applications including lambda oxygen sensors, wide range oxygen sensors, HC and NOx sensors.1–3 In many cases the electrode is protected by a porous ceramic layer to shield the device from exhaust particle impingement,1 or in the case of some types of mixed potential sensors, the electrolyte itself is porous.4The electrochemically active surface area and porosity are important factors that may control the performance of all types of zirconia-based sensors. While measuring AC impedance behavior in the gas phase is possible, the high diffusivity of gases through the porous layer makes extracting diffusion parameters difficult. Mixed potential electrochemical sensors being electro-kinetic devices, are particularly sensitive to these parameters, but few techniques have been developed to measure active surface areas and characterize transport through porous layers. We have developed sensors consisting of a yttria-stabilized zirconia (YSZ) substrate coated with an insulating ceramic onto which dense electrodes of Pt, La0.8Sr0.2CrO3, or Au0.5Pd0.5alloy are screen printed. Finally, a porous YSZ electrolyte is deposited on top of the electrodes. Measuring the effective diffusivity through the porous layer and the electrochemically active surface area of the underlying electrodes is useful in optimizing the performance of the devices. The electrochemically active surface area of the Pt electrodes beneath the porous electrolyte layer was determined by immersing the sensor in aqueous H2SO4 solution and measuring the double layer capacitance or adsorption of a monolayer of H. The oxidation and reduction of [Fe(CN)6]-4/-3 observed by cyclic voltammetry (CV) acts as a probe for the effective diffusion constant through the porous layer as the kinetics of this reaction are fast and the reaction is easily reversible. The effective diffusion coefficient is obtained by fitting to the Randles-Secvik equation for the peak CV current as a function of scan rate. Figure 1 shows the CVs obtained on (a) a Pt disk and (b) the Pt sensor electrode beneath a porous YSZ electrolyte. The effective diffusion coefficient is on the order of 10-6 cm2/s for the Pt disk and is consistent with values reported in the literature.4 The diffusion coefficient is 10-10 cm2/s for the transport through the porous layer on sensor indicating significant hindrance by the YSZ. Measuring these characteristics for both sensors produced by screen printing (ESL ElectroScience) and sensors produced by additive manufacturing in our lab will guide the optimization of sensor performance characteristics. The measurements are correlated to the device sensing and AC impedance response.

Electrochemical Assay for a Total Cellulase Activity with Improved Sensitivity.

Electrochemical methods allow fast and inexpensive analysis of enzymatic activities. Here, we report a simple and yet efficient electrochemical assay for the total activity of cellulase, a hydrolytic enzyme widely used in food and textiles industries, and for production of bioethanol. The assay exploits the difference in electrochemical signals from a soluble redox indicator, ferricyanide, on nitrocellulose films treated by cellulases. Ferricyanide electrochemistry is totally inhibited on graphite electrodes modified with an insulating nitrocellulose film and is evoked after the cellulase treatment. Ferricyanide voltammetric responses correlate with the increased permeability of the films and electrochemically active surface area of electrodes becoming accessible to the ferricyanide reaction after nitrocellulose digestion by cellulase. Trichoderma and Aspergillus niger cellulases activities were determined in a 5 min assay with a sensitivity of 10-8 U per assay, being 103-104-fold more sensitive than the standard commercially available optical assays. That makes the developed electrochemical approach the most prospective cost-effective alternative both for research and automated industrial applications.

Novel method for determining the width of the electrochemically active electrode area along the triple phase boundary

The width δ of the active electrode area along the triple phase boundary, TPB, is determined using a novel method based on ac impedance measurements between two interdigitated electrodes. The theoretical basis is presented and used to derive the width δ for gold electrodes applied onto the solid electrolyte (SE) Y2O3 stabilized ZrO2 (YSZ). Gold electrodes are used in the demonstration of the method due to their ion blocking nature and poor cathodic properties which requires that all or at least most of the electrochemical reaction steps take place on the free surface of the SE near the TPB. The measurements are done at relatively low temperatures of 250–300°C. The width δ is determined to be 0.4, 0.3 and 0.2μm at 250, 275 and 300°C, respectively, decreasing with increasing temperature. In addition to the width δ, the ionic conductivity of YSZ at those low temperatures is determined as 2.2×10−7, 7.2×10−7 and 2.3×10−6S/cm at 250, 275 and 300°C, respectively, with an activation energy of 1.27eV.

Preparation and Characterization of a Porous Structure PbO2-ZrO2 Nanocomposite Electrode and Its Application in Electrocatalytic Degradation of Crystal Violet

A novel porous PbO2-ZrO2 nanocomposite electrode was successfully prepared by bubbles template composite electrodeposition methods for the first time. Scanning electronic microscopy (SEM) and X-ray diffraction (XRD) results show that the novel porous PbO2-ZrO2 nanocomposite electrodes have the three-dimensional porous structure and smaller crystal sizes than planar PbO2-ZrO2 nanocomposite electrodes. ZrO2 nanoparticles can be observed on the electrodes. The cyclic voltammetry measurement show that the electrochemical active surface area of porous PbO2-ZrO2 nanocomposite electrodes is 3 times than that of planar PbO2-ZrO2 electrodes. In the electrocatalytic oxidization process of crystal violet, the color and COD removal efficiency on the porous composite electrodes is 99.8% and 69.2% after 90 min. The degradation process follows pseudo-first-order kinetics. The rate constant for porous composite electrodes can reach 0.04916 min−1, which is 4.5 times than that for planar composite electrodes (0.01188 min−1). Moreover, the three-dimensional porous structure can enhance the generation capacity of hydroxyl radicals. The experiment results illustrate that the porous PbO2-ZrO2 nanocomposite electrodes have a great application potential in the pollutant degradation fields.

Galvanostatic Intermittent Titration and Performance Based Analysis of LiNi0.5Co0.2Mn0.3O2 Cathode

Galvanostatic intermittent titration technique (GITT) – a popular method for characterizing kinetic and transport properties of battery electrodes – is predicated on the proper evaluation of electrode active area. LiNi0.5044Co0.1986Mn0.2970O2 (NCM523) material exhibits a complex morphology in which sub-micron primary particles aggregate to form secondary particle agglomerates. This work proposes a new active area formulation for primary/secondary particle agglomerate materials to better mimic the morphology of NCM532 electrodes. This formulation is then coupled with macro-homogeneous models to simulate GITT and half-cell performance of NCM523 electrodes. Subsequently, the model results are compared against the experimental results to refine the area formulation. A single parameter, the surface roughness factor, is proposed to mimic the change in interfacial area, diffusivity and exchange current density simultaneously and detailed modeling results are presented to provide valuable insights into the efficacy of the formulation.

Electrochemical reduction of nalidixic acid at glassy carbon electrode modified with multi-walled carbon nanotubes

The aqueous phase electrochemical degradation of nalidixic acid (NAL) is studied in this work, using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) as instrumental techniques. The promotional effect of multi-walled carbon nanotubes (MWCNT) on the performance of glassy carbon electrodes is demonstrated, being observed that these materials catalyze the NAL reduction. The effect of surface functional groups on MWCNT −MWCNT-COOH and MWCNT-NH2–was also studied. The modification of glassy carbon electrode (GCE) with MWCNT leads to an improved performance for NAL reduction following the order of MWCNT>MWCNT-NH2>MWCNT-COOH. The best behavior at MWCNT-GCE is mainly due to both the increased electrode active area and the enhanced MWCNT adsorption properties.The NAL degradation was carried out under optimal conditions (pH=5.0, deposition time=20s and volume of MWCNT=10μL) using MWCNT-GCE obtaining an irreversible reduction of NAL to less toxic products. Paramaters as the number of DPV cycles and the volume/area (V/A) ratio were optimized for maximize pollutant degradation. It was observed that after 15 DPV scans and V/A=8, a complete reduction was obtained, obtaining two sub-products identified by liquid chromatography-mass spectrometry (LC–MS).

Electrochemical Characterization of PEMECs Operating at Various Current Densities

This conference contribution touches upon electrochemical characterization of operating polymer electrolyte membrane electrolysis cells (PEMECs) by the application of electrochemical impedance spectroscopy (EIS). Analysis of differences in impedance spectra (ADIS) (Jensen et al., 2007) can be applied to gain insight into the relative magnitudes of resistance contributions from the two electrodes, the electrolyte and of mass transfer limitations and can help identifying the time scale of the respective processes. The gained knowledge may facilitate further development of the PEMECs. A state-of-the-art PEMEC obtained from IRD with a 2.9 cm2 active electrode area and loaded with 0.3 mg/cm2 IrO2 on the anode and 0.5 mg/cm2 Pt on the cathode was operated at current densities ranging from 0.07 to 1 A/cm2 while examined with EIS. The cell temperature at operation was 61 ˚C at all current densities, and the cell was conditioned for 20 minutes at each current step prior to EIS measurements. A Solartron 1260 was used for the data acquisition, the frequency range was 100 kHz - 0.01 Hz, the amplitude was 17.3 A/cm2 and 12 points were measured per decade. Nyquist plots of the EIS results are shown in the upper graph in the figure. One arc is evident in the spectrum at 0.07 A/cm2, whereas the spectra at 0.35 and 0.69 A/cm2 include two arcs, and two overlapping arcs are observed at 1 A/cm2. ADIS plots, with the impedance measured at 0.07 A/cm2 subtracted, are shown in the lower graph in the figure. Based on the ADIS plots it can be concluded that the impedance is independent of the current density at frequencies above 100 Hz, whereas two processes observed at approximately 0.4 and 10 Hz are current density dependent, the one increasing and the other decreasing in size with increasing current density. Acknowledgement The work is part of the research project e-STORE funded by the Innovation Fund Denmark. Reference S. H. Jensen, A. Hauch, P. V. Hendriksen, M. Mogensen, N. Bonanos, T. Jacobsen. (2007). A Method to Separate Process Contributions in Impedance Spectra by Variation of Test Conditions. Journal of The Electrochemical Society, 154 (12), B1325-B1330. Figure Nyquist plots (upper graph) and ADIS plots (lower graph) of EIS measured on a PEMEC operating at 0.07, 0.35, 0.69 and 1 A/cm2. The ADIS plots represent the difference between the differentiated real part of the impedance at each current density and the differentiated real part of the impedance collected at 0.07 A/cm2. Figure 1

Numerical Simulation of the Diffusion Processes in Nanoelectrode Arrays Using an Axial Neighbor Symmetry Approximation.

Nanoelectrode arrays have introduced a complete new battery of devices with fascinating electrocatalytic, sensitivity, and selectivity properties. To understand and predict the electrochemical response of these arrays, a theoretical framework is needed. Cyclic voltammetry is a well-fitted experimental technique to understand the undergoing diffusion and kinetics processes. Previous works describing microelectrode arrays have exploited the interelectrode distance to simulate its behavior as the summation of individual electrodes. This approach becomes limited when the size of the electrodes decreases to the nanometer scale due to their strong radial effect with the consequent overlapping of the diffusional fields. In this work, we present a computational model able to simulate the electrochemical behavior of arrays working either as the summation of individual electrodes or being affected by the overlapping of the diffusional fields without previous considerations. Our computational model relays in dividing a regular electrode array in cells. In each of them, there is a central electrode surrounded by neighbor electrodes; these neighbor electrodes are transformed in a ring maintaining the same active electrode area than the summation of the closest neighbor electrodes. Using this axial neighbor symmetry approximation, the problem acquires a cylindrical symmetry, being applicable to any diffusion pattern. The model is validated against micro- and nanoelectrode arrays showing its ability to predict their behavior and therefore to be used as a designing tool.

Voltammetric determination of melatonin using a graphene-based sensor in pharmaceutical products.

Melatonin can be sensitively detected in pharmaceuticals by cyclic voltammetry and fixed-potential amperometry using a graphene-based sensor. The sensor characterization of cyclic voltammetry constantly provides high values of electrode active area and heterogeneous rate constant. In optimal conditions, the sensor was applied for the determination of melatonin in different pharmaceutical samples. The sensitivity to melatonin was 0.0371 A M−1, and the limit of detection was 0.87×10−6 M. The data obtained by using the graphene-based sensor for the detection of melatonin in pharmaceutical products were in good agreement with the data provided by the producer. Since no interferences from the excipients were found, using a separation technique was not necessary. Additionally, the low price, ease of handling, small amount of sample, short time per analysis, and possibility of automation are the important advantages that recommend this methodology for quality control of pharmaceuticals.

In Situ Enhancement of Flow-through Porous Electrodes with Carbon Nanotubes via Flowing Deposition

This study describes a novel flowing deposition method which can be used in situ on a fully assembled electrochemical cell to enhance the electrochemical active surface area of flow-through porous electrodes. As a test case, vanadium redox reaction rates are enhanced by flowing deposition of carbon nanotubes on carbon paper electrodes. The deposition is characterized using a recently developed analytical flow cell technique and shown to increase the electrochemical active surface area and therefore the exchange current for the limiting V2+/V3+ reaction by over an order of magnitude. When applied to the anode of an assembled vanadium redox flow cell, the method nearly eliminates the overpotential associated with this reaction, thereby significantly improving the power output of the cell by up to 70% in the high voltage efficiency regime. As a surface area treatment, this flowing deposition method could be used to similarly enhance the energy conversion efficiency of many other electrochemical flow cells such as fuel cells, flow batteries and capacitive deionization cells.

Nanostructured heated gold electrodes for DNA hybridization detection using enzyme labels

We report about two new types of nanostructured gold electrodes that can be heated electrically. At the directly heated gold micro-wire electrode, the microelectrode behavior is diminished upon galvanic deposition of gold nanostructures. The voltammetric waves observed with ferrocyanide turned into peaks. This suggests that it is the active electrode area rather than the mere geometric dimensions, what determines microelectrode behavior. Direct heating of this nanostructured gold wire electrode brought back the wave-like voltammogramms typical for microelectrodes. We also demonstrate with a second type of heated gold electrode, an indirectly heated sensor based on low temperature co-fired ceramics (LTCC), that DNA hybridization response of covalently attached enzyme labels is greatly improved by nanostructures. The voltammetric signal increase for the nanostructured electrode was 22-fold at 5°C and 6-fold at 70°C electrode temperature. We think that a trapping effect in the gold pores greatly increases sensitivity, in particular at lower electrode temperature.

Effects of Pb on Electrocatalysis of Alcohol Oxidation Reaction on Pt/C Catalysts in Alkaline Media

Electrocatalysis of ethanol oxidation reactions (EORs) and methanol oxidation reactions (MORs) in alkaline media has gained great research interests due to the development of alkaline anion exchange membranes. The EOR and MORs kinetics are more facile in alkaline media than those in acid media. The most importantly, there are more choices of materials to be used as the electrocatalysts for the EOR or MOR since these materials are stable in the alkaline media, but not in the acid media. Although extensive research has been done worldwide related to electrocatalysis of EORs and MORs in both acid and alkaline media, the EOR or MOR mechanisms are still not well understood. It still remains as a major challenge of reducing the overpotentials of EORs and MORs in low temperature fuel cells. In this work, we aim to study effects of Pb on electrocatalysis of EORs and MORs on Pt/C catalysts in alkaline media. The commercial available Pt/C catalysts were performed electrochemical characterizations in a standard tri-electrode electrochemical cell with a graphite column serving as the counter electrode and a Hg/HgO/1.0 M OH- electrode (0.098 V vs. NHE) as the reference electrode in an argon-saturated or ethanol or methanol-contained 1 M NaOH solution without or with 1mM lead(II) acetate (Sigma-Aldrich) presented. The EOR or MOR activities of the catalysts were also characterized with a single direct alcohol fuel cell. The membrane electrode assemblies (MEA) with an active electrode area of 4.5 cm2 were comprised of a catalyst coated membrane (CCM), a Ni foam (Hohsen Corp.) as the anode backing layer and a TGP-H-090 carbon paper (Toray) as the cathode backing layer. The anode catalyst ink was prepared by mixing the catalyst with Nafion solution. The cathode catalyst ink was prepared by ultrasonicating 8mg MnO2 nanorod catalyst, 8 mg active carbon (BP2000, Cabot Corp. ) and the A4 ionomer (Tokuyama Co.). The catalyst/ionomer (Nafion or A4) weight ratio is kept at 80:20 in both anode and cathode. A fuel cell test system (Scribner Associates Model 850e) was used for controlling the cell temperature, cathode humidity, O2 flow rate. The temperature of the fuel cell was maintained with a tolerance of ± 0.2 °C. The anode solution flow rate was controlled ESI's MP2 micro peripump (elemental scientific Inc.) CV curves were obtained with the Pt/C in Ar-saturated 1.0 M NaOH or 1.0 M ethanol (or methanol) and 1.0 M NaOH solution without or with 1mM Pb2+ presented. From the CV curves obtained in the 1.0 M NaOH solutions, the Pb2+ presence in the solution was found to alter surface characteristics on Pt/C catalysts. The EOR and MOR activities were also affected accordingly. Both CV and chronoamperometry (CA) measurements indicate that the presence of Pb2+ in the solution leads to the increase of EOR and MOR kinetics on the Pt/C catalysts, agree with what have been reported. Detailed fuel cell characterizations with the Pt/C anode catalysts using fuels without and with 1mM Pb2+ presence were performed. Combining the electrochemical measurements and fuel cell test results, effects of Pb on the electrocatalysis of EOR and MOR on the Pt/C catalyst in alkaline media will be presented.Acknowledgements We acknowledge financial supports from the National Science Foundation (NSF) (Contract 1402422).

Effects of Pb in Electrocatalysis of Ethanol Oxidation Reaction on Pd/C and Pdru/C Catalysts in Alkaline Media

Electrocatalysis of ethanol oxidation reactions (EORs) in alkaline media has gained great research interests due to the development of alkaline anion exchange membranes. The EOR kinetics are more facile in alkaline media than those in acid media. The most importantly, there are more choices of materials to be used as the electrocatalysts for the EOR since these materials are stable in the alkaline media, but not in the acid media. Although extensive research has been done worldwide related to electrocatalysis of EORs in both acid and alkaline media, the EOR mechanisms are still not well understood. It still remains as a major challenge of reducing the overpotentials of EORs in low temperature fuel cells. The oxidation of ethanol in alkaline media is very complicated and involves many types of intermediates, such as C1 species (most likely COad and CHx,ad), OHad, CH3COad and other forms of acetaldehyde in alkaline media. Single metal catalysts are often found to perform much poorer than those modified with the second metal. “Multi-functional” electrocatalyst systems have been designed and tested for EORs. In this work, we aim to study effects of Pb on electrocatalysis of EORs on Pd/C and PdRu/C catalysts in alkaline media.Pd/C and Pd2Ru/C (the atomic ratio of Pd:Ru is 2:1) catalysts with 20 wt. % Palladium loading were prepared by an impregnation method. The morphologies of the catalysts were studied by transmission electron microscope (TEM) and x-ray diffraction (XRD) characterizations. Then the catalysts were performed electrochemical characterizations in a standard tri-electrode electrochemical cell with a graphite column serving as the counter electrode and a Hg/HgO/1.0 M OH- electrode (0.098 V vs. NHE) as the reference electrode in an argon-saturated or ethanol-contained 1 M NaOH solution without or with 1mM lead(II) acetate (Sigma-Aldrich) presented.The EOR activities of the catalysts were also characterized with a single direct ethanol fuel cell. The membrane electrode assemblies (MEA) with an active electrode area of 4.5 cm2 were comprised of a catalyst coated membrane (CCM), a Ni foam (Hohsen Corp.) as the anode backing layer and a TGP-H-090 carbon paper (Toray) as the cathode backing layer. The anode catalyst ink was prepared by mixing the 16mg catalyst (Pd/C and Pd2Ru/C) with Nafion solution. The cathode catalyst ink was prepared by ultrasonicating 8mg MnO2 nanorod catalyst, 8 mg active carbon (BP2000, Cabot Corp. ) and the A4 ionomer (Tokuyama Co.). The catalyst/ionomer (Nafion or A4) weight ratio is kept at 80:20 in both anode and cathode. A fuel cell test system (Scribner Associates Model 850e) was used for controlling the cell temperature, cathode humidity, O2 flow rate. The temperature of the fuel cell was maintained with a tolerance of ± 0.2 °C. The anode solution flow rate was controlled ESI's MP2 micro peripump (elemental scientific Inc.)CV curves were obtained with the Pd/C and Pd2Ru/C in Ar-saturated 1.0 M NaOH or 1.0 M ethanol and 1.0 M NaOH solution without or with 1mM Pb2+ presented. From the CV curves obtained in the 1.0 M NaOH solutions, the Pb2+ presence in the solution was found to alter surface characteristics on both Pd/C and PdRu/C catalysts. The EOR activities were also affected accordingly. Both CV and chronoamperometry (CA) measurements indicate that the presence of Pb2+ in the solution leads to the reduction of EOR kinetics on the Pd/C or PdRu/C catalysts, which are different from what were observed with Pt-based catalysts. Detailed fuel cell characterizations of these various catalysts were performed. Combining the electrochemical measurements and fuel cell test results, effects of Pb on the electrocatalysis of EOR on Pd/C and PdRu/C catalysts in alkaline media will be discussed.

Uso da voltametria cíclica e da espectroscopia de impedância eletroquímica na determinação da área superficial ativa de eletrodos modificados à base de carbono

Eletrodos à base de carbono, como os eletrodos de troca iônica, entre outros, têm aplicação principalmente no tratamento de efluentes industriais e rejeitos radioativos. Carbono é também amplamente utilizado em células a combustível como substrato para os eletrocatalisadores, por possuir elevada área superficial, que supera a sua área geométrica. O conhecimento desta superfície ativa total é importante na determinação das condições de operação de uma célula eletroquímica no que diz respeito àscorrentes a serem aplicadas (densidade de corrente). No presente estudo foram utilizadas duas técnicas eletroquímicas na determinação da área superficial ativa de eletrodos de carbono vítreo e poroso e eletrodos de troca iônica: espectroscopia de impedância eletroquímica (EIE) e voltametria cíclica (VC). Os experimentos foram realizados com soluções de KNO30,1 molL-1em célula eletroquímica de três eletrodos: eletrodo de trabalho à base de carbono, eletrodo auxiliar de platina e eletrodo de referência de Ag/AgCl. Os eletrodos de carbono vítreo e de carbono poroso utilizadospossuíam uma área geométrica de 3,14 x 10-2cm2e 2,83 х 10-1cm2, respectivamente. O eletrodo de troca iônica foi preparado misturando-se grafite, carbono, resina de troca iônica e um aglutinante, sendo esta mistura aplicada emtrês camadas sobre feltro de carbono, utilizando-se nos experimentos uma área geométrica de 1,0cm2.

Batch Fabrication of Three-Dimensional Microelectrode Arrays For The In-Vitro Investigation Of Tissue Slices And 3D Cell Cultures

Event Abstract Back to Event Batch Fabrication of Three-Dimensional Microelectrode Arrays For The In-Vitro Investigation Of Tissue Slices And 3D Cell Cultures Manuel Martina1, Stefan Klaus2, Alfred Stett3, Thoralf Herrmann4, Günther Zeck5 and Claus Burkhardt6* 1 NMI Natural and Medical Sciences Institute at the University Tubingen, Microsystems / Nanotechnology, Germany 2 NMI Naturwissenschaftliches und Medizinisches Institut, Microsystems / Nanotechnology, Germany 3 NMI Natural and Medical Sciences Institute at the University Tubingen, Technical Engineering & Biophysics, Germany 4 NMI Naturwissenschaftliches und Medizinisches Institut, Neurochip Research, Germany 5 NMI at the University Tuebingen, Neurochip Research, Germany 6 NMI Natural and Medical Sciences Institute at the University Tubingen, Microsystems / Nanotechnology, Germany Motivation Three dimensional microelectrode arrays pose a valuable tool for the in-vitro investigation of acute tissue slices [1,2]. The reason for this is a dead-cell layer, which occurs due to the preparation of tissue slices and dampens the electrical signal when using planar electrodes. 3D MEAs can overcome the dead-cell layer by penetrating the tissue and moving the active electrode array in direct proximity to the living cells. Furthermore, three dimensional cell cultures become an increasingly hot topic and are subject of more and more investigations [3,4]. Here, a 3D MEA can be applied to measure signals, preferable in different defined heights from the MEA surface. A microelectrode array combining planar and 3D electrodes, with heights ranging from some ten micrometers to a hundred micrometer, can be a valuable tool to progress in this field of research. Here, we present a batch fabrication process, which allows to combine planar electrodes and 3D electrodes with tunable geometry. Material and Methods A process scheme of the fabrication is depicted in Figure 1. First, a silicon-on-glass wafer (consisting of a 50 µm thick silicon layer bonded to a thick glass wafer) is coated with a 50 nm thick silicon nitride layer. The silicon nitride layer is structured by optical lithography into square masks, where the electrode tips will be located. The mask size correlates to the later tip height. After anisotropic etching in potassium hydroxide, the majority of the silicon is removed, yielding a transparent substrate with 3D silicon structures. From here, structuring into a MEA is done by standard processes. During the final step, namely the deposition of the electrode material, height and size of the active electrode area is determined. Results and Discussion Following the developed process, an electrode array with 50 µm high tips is fabricated. Figure 2 shows SEM images of the finished 3D electrodes. The active electrode area, consisting of a fractal titanium nitride layer is located at the upper 20 µm, respectively 15 µm. The aspect ratio (tip height / base diameter) of the structures is measured to 2 and given by the silicon crystal orientation and mask shape. The apex angle is measured to 28°, with a tip radius of 1 µm. This geometry should be well suited for tissue penetration. The developed process possess several advantages over already reported ways to fabricate 3D MEAs, e.g. the isotropic wet etching of glass or the dry etching of SU-8 resist for structure fabrication. Structures fabricated using a silicon-on-glass wafer are completely chemically inert in organic solvents (which are used in lithography processes), mechanically stable during fabrication as well as in application and provide an aspect ratio which cannot be achieved by isotropic wet etching. Conclusions and Outlook We developed a stable and batch process compatible way to fabricate MEAs with 3D electrodes. Although the electrode aspect ratio is fixed, tip height, tip distance and the size of the active electrode area can be tuned to one’s needs. The fabrication of these MEAs utilizes parallel processing and allows to produce these substrate in high numbers to affordable prices. Electrophysiological measurements with these MEAs are currently conducted. In the future, tip geometry and aspect ratio can be further tuned by adapting the mask shape. With proper mask geometry and orientation, triangular tips with apex angles below 25° (known from AFM probes) can be fabricated. It might be possible to fabricate tips with heights of 100 µm and more by using a silicon-on-glass wafer with a thicker silicon layer. Figure Legend Figure 1: Process scheme for the fabrication of an electrode array, combining planar electrodes and 3D electrodes of different heights. Figure 2: SEM images of 50 µm high tips with different sized active electrode areas (red).

Kinetic Studies on State of the Art Solid Oxide Cells: A Comparison between Hydrogen/Steam and Reformate Fuels

Electrochemical reaction kinetics at the electrodes of Solid Oxide Cells (SOCs) were investigated at 700°C for two cells with different fuel electrode microstructures as well as on a third cell with a reduced active electrode area. Three fuel mixtures were investigated – hydrogen/steam and model reformate fuels–hydrogen/carbon-dioxide and hydrogen/methane/steam. It was found that the electrode kinetics at the fuel electrode were exactly the same in both reformates. The hydrogen/steam fuel displayed 5–7% faster kinetics than the reformate fuels. 19% faster kinetics were recorded for the cell with a finer microstructure. The measured gas conversion impedance was compared with models in literature for both the 16- and the 2 cm2 cells. The continuously stirred tank reactor (CSTR) AC model approximated the overpotential of the smaller cells (2 cm2) with greater accuracy in the current density range 0–0.5 A/cm2 while the plug flow reactor (PFR) model although derived for the case of zero DC bias predicted the 16 cm2 cell ASR better than the zero bias CSTR model. Furthermore, the gas conversion impedance in the hydrogen/steam fuel split into two processes with opposing temperature behavior in the reformate fuels. By using a 87.5% smaller active electrode area the gas conversion impedance was diminished in the hydrogen/steam fuel at (the same absolute) high fuel flow rates. In both reformates, the second and third lowest frequency processes merged into a single process as the gas conversion was reduced. The SOC with finer electrode microstructure displayed improved kinetics.

Voltage Cycling Induced Losses in Electrochemically Active Surface Area and in H2/Air-Performance of PEM Fuel Cells

The influence of voltage cycling profiles on the degradation of MEAs (membrane electrode assemblies) for PEM (proton exchange membrane) fuel cells was investigated using MEAs with carbon supported catalysts. The loss of electrochemically active surface area (ECSA) of the cathode electrodes significantly increased with the upper potential limit. The ECSA loss was correlated with the performance decrease for hydrogen/air polarization curves, revealing purely kinetic losses at low and additional mass-transport related voltage losses at high platinum surface area normalized current densities (≡specific current densities). The absolute voltage losses only depended on the specific current density, independent of the extent of ECSA loss and the geometric current density.

Modeling a CuCl(aq)/HCl(aq) Electrolyzer using Thermodynamics and Electrochemical Kinetics

Research efforts on the CuCl(aq)/HCl(aq) electrolyzer would greatly benefit from the ability to quantify the dissipative processes that undesirably increase the cell’s applied potential, Ecell, which decreases its efficiency. To date, little is known about what impact further improvements to active surface area, extent of CuCl(aq) conversion and ohmic resistance would exactly have on the electrolyzer performance. To better understand how this electrolyzer can be improved, a model was developed to quantify and separate the effects of electrochemical kinetics, membrane transport and open circuit potential, EOCP, on Ecell for a given current density. By employing data obtained from previous studies with electrochemical cells into the developed model, it was possible to calculate Ecell values that agreed with data collected from a lab scale electrolyzer using just one adjustable parameter, the Nernst diffusion layer at limiting current. The model was then used to identify the predicted Ecell contributions as a function of CuCl(aq) conversion, active electrode area and ohmic resistance. It was found that the extent of CuCl(aq) conversion can dramatically impact the electrolyzer electrode kinetics and EOCP. More importantly, as CuCl(aq) conversion increased, the Ecell values needed consistently increased to keep the same current density. Overall, Ecell could be most readily reduced by improving Rohm, whereas improvements to electrode kinetics have limited impacts.

Carbon-ionogel supercapacitors for integrated microelectronics

To exceed the performance limits of dielectric capacitors in microelectronic circuit applications, we design and demonstrate on-chip coplanar electric double-layer capacitors (EDLCs), or supercapacitors, employing carbon-coated gold electrodes with ionogel electrolyte. The formation of carbon-coated microelectrodes is accomplished by solution processing and results in a ten-fold increase in EDLC capacitance compared to bare gold electrodes without carbon. At frequencies up to 10 Hz, an areal capacitance of 2.1 pF μm−2 is achieved for coplanar carbon-ionogel EDLCs with 10 μm electrode gaps and 0.14 mm2 electrode area. Our smallest devices, comprised of 5 μm electrode gaps and 80 μm2 of active electrode area, reach areal capacitance values of ∼0.3 pF μm−2 at frequencies up to 1 kHz, even without carbon. To our knowledge, these are the highest reported values to date for on-chip EDLCs with sub-mm2 areas. A physical EDLC model is developed through the use of computer-aided simulations for design exploration and optimization of coplanar EDLCs. Through modeling and comparison with experimental data, we highlight the importance of reducing the electrode gap and electrolyte resistance to achieve maximum performance from on-chip EDLCs.

Maximizing Output Power in a Cantilevered Piezoelectric Vibration Energy Harvester by Electrode Design

A resonant vibration energy harvester typically comprises of a clamped anchor and a vibrating shuttle with a proof mass. Piezoelectric materials are embedded in locations of high strain in order to transduce mechanical deformation into electric charge. Conventional design for piezoelectric vibration energy harvesters (PVEH) usually utilizes piezoelectric material and metal electrode layers covering the entire surface area of the cantilever with no consideration provided to examining the trade-off involved with respect to maximizing output power. This paper reports on the theory and experimental verification underpinning optimization of the active electrode area of a cantilevered PVEH in order to maximize output power. The analytical formulation utilizes Euler-Bernoulli beam theory to model the mechanical response of the cantilever. The expression for output power is reduced to a fifth order polynomial expression as a function of the electrode area. The maximum output power corresponds to the case when 44% area of the cantilever is covered by electrode metal. Experimental results are also provided to verify the theory.