Current Density Drop Research Articles

Electrocatalytic Behavior of Hydrogenated Pd-Metallic Glass Nanofilms: Butler-Volmer, Tafel, and Impedance Analyses

Electrocatalytic activity and sorption behavior of hydrogen in nanosized Pd–Si–(Cu) metallic glass thin film and Pd thin film electrodes sputtered on a Si/SiO2 substrate were investigated by linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy. The electrode MG4 (Pd69Si18Cu13) exhibits the best performance with the highest electrocatalytic activity in the hydrogen evolution region with less than half of the Tafel slope of Pd thin film of the same thickness and lowest overpotential at 10 mA cm−2. A new approach has been adopted by a nonlinear fitting of the entire region of the polarization curve (far- and near-equilibrium cathodic and anodic regions) to the Butler-Volmer model. α parameter is lowest for the MG2 electrode (Pd79Si16Cu5), marking that nonequilibrium conditions change the reaction kinetics. Together with MG2, MG4 shows the lowest Bode magnitude values for hydrogen sorption and evolution regions, indicating that the bonding and release of hydrogen atoms to the electrode is easier. MG4 electrode shows a dramatic decrease of the overpotential after 100 cycles, yielding an increase in hydrogen activity. Besides, MG4 exhibits the sharpest current density drop in the HER region in cyclic voltammetry compared with other MG and Pd electrodes, indicating higher electrocatalytic activity towards hydrogen evolution. The findings highlight the influence of the selected metallic glasses for the design and development of metal catalysts with higher sorption kinetics and/or electrocatalytic turnover.

Preparation of Ti–Zr-Based Conversion Coating on 5052 Aluminum Alloy, and Its Corrosion Resistance and Antifouling Performance

A chemical conversion coating on 5052 aluminum alloy was prepared by using K2ZrF6 and K2TiF6 as the main salts, KMnO4 as the oxidant and NaF as the accelerant. The surface morphology, structure and composition were analyzed by SEM, EDS, FT–IR and XPS. The corrosion resistance of the conversion coating was studied by salt water immersion and polarization curve analysis. The influence of fluorosilane (FAS-17) surface modification on its antifouling property was also discussed. The results showed that the prepared conversion coating mainly consisted of AlF3·3H2O, Al2O3, MnO2 and TiO2, and exhibited good corrosion resistance. Its corrosion potential in 3.5 wt % NaCl solution was positively shifted about 590 mV and the corrosion current density was dropped from 1.10 to 0.48 μA cm−2. By sealing treatment in NiF2 solution, its corrosion resistance was further improved yielding a corrosion current density drop of 0.04 μA cm−2. By fluorosilane (FAS-17) surface modification, the conversion coating became hydrophobic due to low-surface-energy groups such as CF2 and CF3, and the contact angle reached 136.8°. Moreover, by FAS-17 modification, the corrosion resistance was enhanced significantly and its corrosion rate decreased by about 25 times.

Direct Electrolytic Splitting of Seawater: Activity, Selectivity, Degradation, and Recovery Studied from the Molecular Catalyst Structure to the Electrolyzer Cell Level

AbstractSeawater electrolysis faces fundamental chemical challenges, such as the suppression of highly detrimental halogen chemistries, which has to be ensured by selective catalyst and suitable operating conditions. In the present study, nanostructured NiFe‐layered double hydroxide and Pt nanoparticles are selected as catalysts for the anode and cathode, respectively. The seawater electrolyzer is tested successfully for 100 h at maximum current densities of 200 mA cm−2 at 1.6 V employing surrogate sea water and compared to fresh water feeds. Different membrane studies are carried out to reveal the cause of the current density drop. During long‐term dynamic tests, under simulated day‐night cycles, an unusual cell power performance recovery effect is uncovered, which is subsequently harnessed in a long‐term diurnal day‐night cycle test. The natural day‐night cycles of the electrolyzer input power can be conceived as a reversible catalyst materials recovery treatment of the device when using photovoltaic electricity sources. To understand the origin of this reversible recovery on a molecular materials level, in situ extended X‐ray absorption fine structure and X‐ray near‐edge region spectra are applied.

Tungsten carbide electrocatalysts prepared from metallic tungsten nanoparticles for efficient hydrogen evolution

Pyrolysis of hexacarbonyl tungsten, W(CO)6, in 1-octadecene has been used to prepare colloidal tungsten, W, nanoparticles (NPs). The obtained W NPs has been spin-coated on graphite (C) electrodes. Heat treatment of the W/C electrodes at elevated temperatures (≥900 °C) allows the preparation of metallic W and tungsten carbide (W2C@WC) thin films. The obtained W2C@WC electrodes were used for hydrogen evolution studies (HER) in 0.5 M H2SO4. Cyclic voltammetry tests for 1000 cycles showed that W2C@WC exhibit long term stability without significant drop in current density. The overpotential defined at 10 mA/cm2 is 310 mV vs. RHE giving an excellent catalytic activity for HER. Materials characterization has been achieved using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). Here, EIS studies were used to access the charge-transfer resistance of tungsten carbide electrodes.

Nickel-Incorporated, Nitrogen-Doped Graphene Nanoribbons as Efficient Electrocatalysts for Oxygen Evolution Reaction

We report a scalable method for the preparation of nickel incorporated nitrogen-doped graphene nanoribbon (Ni/NGNRs) through a facile solvothermal process. Significantly, we show that the incorporation of nitrogen functionalities on graphene nanoribbon with tunable nickel content not only catalyzes efficiently the water oxidation reaction but enables to tweak the catalytic reactivity. Thus, Ni/NGNRs composite with higher nickel content exhibits an overpotential of 380 mV with a Tafel slope of 60 mV dec−1 to sustain 10 mA cm−2 under alkaline conditions. Furthermore, only a negligible current density drop is witnessed during the chronoamperometric studies suggesting the robust nature of the electrocatalyst. XPS analysis of the composite before and after polarization studies confirms the formation of nickel oxide on the exposed nickel nanoparticles during the electrochemical reaction but without any major adverse effect on the performance. We attribute the formation of nickel oxide on the exposed nickel nanoparticles as the major reason for the observed enhancement of electrocatalytic performance.

Noble-metal-free tungsten oxide/carbon (WOx/C) hybrid manowires for highly efficient hydrogen evolution

Developing active, stable, and low-cost electrocatalysts to generate hydrogen is a great challenge in the fields of chemistry and energy. Nonprecious metal catalysts comprised of inexpensive and earth-abundant transition metals are regarded as a promising substitute for noble metal catalysts used in hydrogen evolution reaction (HER), but are still practically unfeasible mainly due to unsatisfactory activity and durability. Here we report a facile two-step preparation method for WOx nanowires with high concentration of oxygen vacancies (OVs) via calcination of W-polydopamine compound precursors. The resulting hybrid material possesses a uniform and ultralong 1D nanowires structure and a rough and raised surface, which can effectively improve the specific surface area. The products exhibit excellent performance for H2 generation: the required overpotentials for 1 and 10 mA cm−2 are 18 and 108 mV, the Tafel slope is 46 mV/decade, and the electrochemically active surface area is estimated to be ∼77.0 m2 g−1. After 1000 cycles, the catalyst works well without significant current density drop. Our experimental results verified metallic transition metal oxides as superior non-Pt electrocatalysts for practical hydrogen evolution reactions.

Novel D-Glucose Derived Hard Carbon Anode for Sodium-Ion Batteries

The synthesis and characterization of a hard carbon anode material for room-temperature sodium-ion battery is being reported. The carbon material was prepared via two-step process including hydrothermal carbonization (HTC) and pyrolysis. Similar carbon materials were recently synthesized and investigated by our group as supercapacitor electrode materials [1,2]. Sodium-ion batteries have emerged as a promising candidate for large-scale energy storage due to sodium’s abundance and low cost of raw materials. However, significant work is needed on the development of Na-ion intercalation anodes [3,4]. The materials were investigated using SEM, HR-TEM, gas sorption techniques and Raman spectroscopy. Electrochemical characterization has been carried out using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge method (GCD). The glucose derived carbon particles have a round shape and consist of both ordered and disordered graphitic domains, as can be seen from HR-TEM images (Figure 1a). The electrode slurry was prepared by mixing the carbon active material, conductive additive (Super P) and polyvinylidene difluoride (PVdF) binder in a 75:15:10 mass ratio. The mixed slurry was cast onto copper foil using doctor-blade technique. The cast electrodes were dried under vacuum for 24h. The half-cells (EL-Cell GmbH) were assembled in an Argon-filled glovebox (O2 < 0.1 ppm, H2O < 0.1 ppm). Sodium metal was used as counter and reference electrode and 1M NaClO4 in propylene carbonate (PC) as the electrolyte. Cyclic voltammograms measured at 0.5 mV s-1 potential scan rate are shown in Figure 1b. The identical shape of the CVs indicates good reversibility. Very well developed peaks at E = 10 mV and E = 100 mV suggest that reversible Na-ion intercalation is taking place in the material. However, the electrode seems to exhibit some capacitive properties in the potential region from 225 mV to 1 V, after which the current density drops suggesting that the electrode is charged positively and therefore Na-ions are replaced by bigger and more solvated ClO4 - anions. Galvanostatic charge-discharge curves in Figure 1c show discharge capacity values of 250 mAh g-1 at 50 mA g-1 current density. The half-cells were cycled between 0.005 and 1.5 V vs Na/Na+. The effect of co-intercalating solvents [5–8] will be analysed using electrochemical techniques together with ex situ/in situ Raman spectroscopy data. Acknowledgements The present study was supported by the Estonian Centre of Excellence in Science project 3.2.0101.11-0030, Estonian Energy Technology Program project 3.2.0501.10-0015, Material Technology Program project 3.2.1101.12-0019, Project of European Structure Funds 3.2.0601.11-0001, Estonian target research project IUT20–13, NAMUR project 3.2.0304.12-0397 and projects 3.2.0302.10-0169 and 3.2.0302.10-0165.

Spin inverter and polarizer curved nanowire driven by Rashba and Dresselhaus spin–orbit interactions

We propose in theory a curved nanowire structure that can both serve as a spin inverter and a spin polarizer driven by a periodic Rashba spin–orbit coupling (SOC) and a uniform Dresselhaus SOC. The curved section of the U-shaped quasi-one dimensional nanowire with an arc of radius R and circumferential length πR is divided into segments of equal length initially having only its inherent homogeneous Dresselhaus SOC. Then a Rashba-type SOC is applied at every alternating segment. By tuning the Rashba SOC strength and the incident electron energy, this device can flip the spin at the output of an incoming spin-polarized electron. On the other hand, this same device acts as a spin filter for an unpolarized input for which an outgoing electron with a non-zero polarization can be achieved without the application of an external magnetic field. Moreover, the potential modulation caused by the periodic Rashba SOC enables this device to function as an attenuator for a certain range of incident electron energies that can make the probability current density drop to 10−4 of its otherwise magnitude in other regimes.

Bilirubin oxidase based enzymatic air-breathing cathode: Operation under pristine and contaminated conditions

The performance of bilirubin oxidase (BOx) based air breathing cathode was constantly monitored over 45days. The effect of electrolyte composition on the cathode oxygen reduction reaction (ORR) output was investigated. Particularly, deactivation of the electrocatalytic activity of the enzyme in phosphate buffer saline (PBS) solution and in activated sludge (AS) was evaluated. The greatest drop in current density was observed during the first 3days of constant operation with a decrease of ~60μAcm−2day−1. The rate of decrease slowed to ~10μAcm−2day−1 (day 3 to 9) and then to ~1.5μAcm−2day−1 thereafter (day 9 to 45). Despite the constant decrease in output, the BOx cathode generated residual current after 45days operations with an open circuit potential (OCP) of 475mV vs. Ag/AgCl. Enzyme deactivation was also studied in AS to simulate an environment close to the real waste operation with pollutants, solid particles and bacteria. The presence of low-molecular weight soluble contaminants was identified as the main reason for an immediate enzymatic deactivation within few hours of cathode operation. The presence of solid particles and bacteria does not affect the natural degradation of the enzyme.

A novel minimally invasive method for monitoring oxygen in microbial fuel cells

Oxygen availability is a potential rate-limiting step in the bioelectrochemical process catalyzed by microbes in microbial fuel cells (MFC). Determination of oxygen availability using a minimally invasive oxygen sensor is advantageous in terms of ease of usage, maintenance and cost-effectiveness as compared to using conventional probe-type oxygen sensors. The utility of this method is substantiated by using this sensor to demonstrate the relationship between oxygen availability and current density. 10 % drop in oxygen concentration resulted in a concomitant drop in current density by about 36 %, further establishing the criticality of monitoring oxygen levels in the MFC. The detachable sensor membrane of the minimally invasive sensor confers multiple advantages. The novel method would enable real-time monitoring of oxygen in MFCs, simplify process optimization and validation and more importantly, provide an impetus for development of more efficient MFC designs.

Time stability of dye-sensitized solar cell based on ZnO sphere aggregates

Time stability of dye-sensitized solar cells based on ZnO sphere aggregates sensitized by N719 was systematically investigated. Eight cells in two groups were involved in this study. Each group has four cells that were connected in series and were used to supply the power to a light emission diode. Group 1 was kept in a room and Group 2 was kept outside ambient condition. The duration was half a year. The cells of the two groups can maintain about 90% of their initial efficiency after half a year running. The decrease of efficiency mainly results from about 15% drop of current density in the first week. Then the current density remained unvaried in the following days no matter whether the cells were kept in the room or outside ambient condition. The open-circuit voltages of these cells increased from 0.67–0.72 to 0.71–0.76V in the first week and then kept unvaried in the following operation. These results indicate an excellent time stability of DSCs based on aggregates of ZnO spheres sensitized by N719 and suggest a great potential for practice use.

Modeling the cation transport in an operating polymer electrolyte fuel cell (PEFC)

A computational fluid dynamics model is developed to investigate the multicomponent cation transport in polymer electrolyte membranes and to predict the performance degradation of the polymer electrolyte fuel cell (PEFC) due to the cationic contamination. A Maxwell–Stefan approach is implemented by modifying the Nernst–Planck equations to model the multicomponent cationic species transport in the membrane. Langmuir isotherms are used to model the non-ideal species adsorption in the membrane. Cation transport model shows good agreement with the experimental data found in the literature. Following the validation of the cation transport model, it is incorporated in a PEFC model framework, which solves for mass, momentum, species and charge conservations. Both fuel side and air side impurities are considered in analyses, which show that for air side impurities current density drops from 0.9 to 0.7 A/cm 2 whereas for fuel side impurities current density drops to impractical values as low as 0.05 A/cm 2, at the steady-state. Effect of cationic impurity on water transport in the membrane is also investigated and found that cathode dry-out occurs due to decreased water generation in case of fuel side contamination.

Thermally Pretreated 46% Pt/Vulcan XC72: Characterisation by TGA/DSC/TEM and Cyclic Voltammetry

AbstractWe report a study of thermal stability and impact of thermal pretreatment procedures for 46% Pt/Vulcan XC72 (Tanaka) fuel cell catalyst. Stability in air and in inert gas (nitrogen, argon, helium) has been investigated by thermal gravimetric analysis (TGA), TGA‐mass spectrometry (TGA‐MS) and differential scanning calorimetry (DSC). Two distinct low temperature mass loss processes (100–200 and 285–300 °C) were observed, each exhibiting unique pretreatment temperature dependencies. TGA‐MS data in helium show fragment ions that suggest the thermal degradation processes are associated with decomposition of materials such as processing aids. Transmission electron microscopy (TEM) reveals a modest increase in average Pt nanoparticle size upon thermal pretreatment. After a pretreatment protocol based on TEM and thermal characterisation (300 °C/15 min, N2), the electrochemically active surface area did not increase. At the kinetically controlled potential region (E &gt;0.8 V) there was a small drop in current density for treated 46% Pt/C in comparison with as‐received catalyst. The slowing in ORR kinetics is significant. Apparently, the removal of organic components, which would improve mass transport, is negated by increased nanoparticle size.

Performance Deterioration of Ni-based Cermet Induced by Electrochemically-generated Steam in Anode

Nickel-yttria stabilized zirconia (Ni-YSZ) cermet is a conventional anode for use in solid oxide fuel cells, and its composition and microstructure are carefully controlled to achieve high performance and long-term stability. In this study, the performance stability of electrolyte-supported cell (Ni-YSZ | YSZ | LSM) was examined at 1000ºC by feeding humidified fuel, x% H2O-(100-x)% H2. The influence of cermet composition on deterioration was also studied. The deterioration behavior was significantly dependent on the fuel humidity and cermet composition. In the case of Ni-YSZ with a volumetric ratio of 50 to 50, peculiar phenomena were observed. When the fuel of 30% H2O-70% H2 was supplied to anode and the terminal voltage was fixed at 0.7 V, the current density decreased gradually soon after the discharge up to 70 h. Then a sudden drop in current density occurred. After the subsequent open-circuit holding, the performance was partially recovered in discharge operation. This behavior of deterioration-recovery was reversibly repeated upon discharge-open-circuit holding operation.

Sudden Deterioration in Performance During Discharge of Anode-supported Solid Oxide Fuel Cells

The stability of the anode-supported solid oxide fuel cell consisting of Ni-YSZ|YSZ|LSM was examined at 800°C by monitoring the current density at a constant terminal voltage of 0.5 V. Although stable power generation was possible within initial 60 min, gradual decrease and subsequent sudden drop in current density were observed: a series of this behavior was different from that observed in limiting current density. This behavior is suggested to be one of the degradation factors related with transport of gas species in the pores of the electrode. In response to performance deterioration, a drastic increase in ohmic loss and impedance was confirmed soon after the discharge operation. It is noted, however, that the deteriorated performance was reversibly recovered depending on the holding time under the open-circuit condition. Analogous phenomena occurred with the cell consisted of Ni-YSZ|YSZ|Ce0.8Sm0.2O1.9|La0.6Sr0.4Co0.2Fe0.8O3, indicating that the cause of the sudden deterioration was in the anode side.

J0401-5-2 ポリエチレンによる摩耗がCo-Cr合金の腐食挙動に及ぼす影響(生体材料およびその表面改質材(5))

Cobalt-Chromium (Co-Cr) alloys possess outstanding corrosion resistance and wear resistance because of passive film of a few nanometers' thickness on their surfaces. In this study, we have clarified the damage accumulation mechanism of Co-Cr alloys under the simultaneous reaction of corrosion and wear in sodium chloride solution. A corrosion wear characteristic of these alloys was evaluated by static polarization test under simultaneous wear damage. Wear was applied using a reciprocating rotational pin-on-flat abrasion machine. The pin material in all tests was polyethylene. Applied loads were 9.8 N. The disk rotation sliding rates were 10, 1.0, and 0.1 mm/s. The results show the current density increases significantly early only when the sliding rate is 10 mm/s. This is thought to occur because the wear on the passive film affects the corrosion reaction. The results also show that the current density drops (recovers) over time and approaches the Control. Therefore, if you assume the prolonged use of implants, the sliding rate in early use greatly affects the corrosion reaction.

Characterization of Carbon Corrosion-Induced Structural Damage of PEM Fuel Cell Cathode Electrodes Caused by Local Fuel Starvation

Postmortem thickness and morphology studies on a degraded membrane electrode assembly (MEA) caused by carbon corrosion under a local starvation operation in a proton exchange membrane (PEM) fuel cell were carried out using optical, scanning, and transmission electron microscopy. Samples used for the postmortem studies were selected and indexed with the aid of a limiting current density distribution map that was premeasured from the degraded MEA using an electrochemical diagnostic technique. An explicit correlation was found between the postmortem studies and the diagnostics: where the electrode structure damage (i.e., thickness reduction and porosity collapse) is significant, the limiting current density is low. The structure damage and current density drop were found to be in the -starved region where the carbon corrosion is severe. The results presented here are of significance in understanding the fundamentals of carbon corrosion mechanism and related structural origin of fuel cell performance degradation.

Chemically gated AlGaN∕GaN heterostructure field effect transistors for polar liquid sensing

Al Ga N ∕ Ga N heterostructure field effect transistors are investigated to monitor the polarity of liquid molecules by exposing the unmetallized gate region between source and drain contacts to chemicals with dipole moments (μ) in the range of 1.6–3.96D. The results show that the channel current can be quantitatively modulated by the magnitude of μ of tested polar molecules. At room temperature, in comparison with the value in air, the channel current density drops from 13.9to7.15mA∕mm with chlorobenzene (μ=1.6D) or to 1.35mA∕mm with diemthyl sulfoxide (μ=3.96D) at a source-drain bias (VDS) of 10V. A linear fitting shows that the current slope is around −1.8mA∕(mmD). Dynamic response of the channel current indicates excellent reversibility of devices. The response time of devices is in the range of 0.1–16s for the chemicals tested in this work.

Transient behavior of a PEMFC

The transient behavior of a proton exchange membrane fuel cell (PEMFC) with porosity is investigated in this study using a two-phase, half-cell model. The thin film agglomerate approach is used to model the catalyst layer. Both vapor transport and liquid water transport in the PEMFC are examined in this study. Proton transport is much faster than the gaseous and liquid water transport. The ionic potential reaches a steady state level in ∼10 −1 s but liquid water transport takes ∼10 s. The variation of the ionic potential loss reaches a critical value, decreasing to a steady state, and is not monotonic. The gas diffusion layer (GDL) and the catalyst layer (CL) porosity, which can affect cell performance, have been carefully investigated. The current density rises rapidly within 10 −2 s, then remaining constant. After 1 s, this is affected by the cell voltage, GDL porosity, and CL porosity, and if the GDL porosity is below 0.4, the current density drops. For the gas diffusion layer porosity, the current density increases between ɛ GDL = 0.2 and ɛ GDL = 0.5, with increased GDL porosity. For the catalyst layer porosity, the optimum value appears between ɛ CL = 0.06 and ɛ CL = 0.1.

Visualization of the spatial distribution of current densities in a photographic system with a semiconductor photodetector

This paper studies the visualization of the spatial distribution pattern of current density in a semiconductor photographic system with a gallium arsenide semiconductor photodetector. The spatial distribution of the current in the filaments was determined by photometric analysis of the gas discharge light emission when a current was passed through a photographic cell. This method ensured spatial resolution of ~ 10/mm and made it possible to describe quantitatively the distributions involving a drop in current density of ⩾ 102. Transformation of the profile and amplitude of the current density of the filaments in the different regions of the current-voltage characteristic (CVC) has been studied. The filamentation (i.e. an inhomogencous distribution of the current density) was primarily due to the formation of a space charge of positive ions in the discharge gap between the photodetector and a transparent anode plate that changed the discharge from the Townsend type to the glow type.