Steady-state Current Density Research Articles

Electrocatalytic Performance of Palladium-Based Electrocatalysts Supported on Carbon Nanotubes for Formic Acid Oxidation

In the present work, Pd and PdFe nanoparticles supported on CNT with and without functionalization (CNT and CNTox) were used for Formic Acid Oxidation Reaction (FAOR) in acid media. Electrocatalysts were synthesized by the borohydride reduction method with 20 wt.% metal loading. The CNTs were synthesized by the methane catalytic decomposition, and subjected to an oxidation treatment with nitric acid, named as CNTox. Themorphology, composition and structural properties were studied by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX) and X-Ray Diffraction (XRD). The FAOR was evaluated in acid media in a conventional three-electrode cell by means of cyclic voltammetry and chronoamperometry. From the steady state current density, it was found that Pd and PdFe supported at CNTox allowed improving the catalytic activity in comparison with the nonoxidized support.

Performance improvement of self-humidifying PEM fuel cells using water injection at various start-up conditions

A critical issue in self-humidifying proton electrolyte membrane (PEM) fuel cells is to achieve stable performance during start-up. In this study, the start-up characteristics of self-humidifying PEM fuel cells are investigated at various temperatures and voltages. Aquivion and Nafion are used as electrolytes for the performance comparison. Moreover, to overcome start-up failure, the direct water injection strategy is proposed and tested in the self-humidifying PEM fuel cell according to the injection side and amount of injected water. Generally, Aquivion membranes exhibit better start-up performance than Nafion membranes in the self-humidifying PEM fuel cell owing to its higher water absorption capability. The water injection into the anode side is more effective than that into the cathode side in achieving high stability and steady-state current density in the self-humidifying PEM fuel cells. With the water injection during start-up, the steady-state current density of the self-humidifying PEM fuel cell with Aquivion is 32% on average higher than that with Nafion. In addition, the optimal amount of water injection is dependent on the water absorption capability of the membrane at the start-up conditions.

Electron injection characteristics of a cathodic interface for an organic light emitting diode in a dark injection space-charge-limited current experiment

The injection and transporting characteristics of an Al/metal fluoride (or Alq3: Cs2CO3)/Alq3/Al electron-only device are determined by the dark injection space-charge-limited current (DI-SCLC)method. The deviation from the ratio of peak current density JDI to the steady-state current density JSS for the ideal ohmic contact, which should be 1.21, was obtained to compare the effects of modifications by four metal fluoride films and Alq3 doped with Cs2CO3 on the injection of electrons. The value of JDI/JSS, 1.29, for the device modified by 0.5 nm LiF approaches the ideal value of 1.21, confirming the formation of quasic-ohmic contact between the organic layer and cathode, although the deviation increases significantly with the thickening of the film. The value of JDI/JSS for the device modified by NaF remains stable within 1.6 in a wide NaF film thickness range of 0–2 nm. The minimum value of JDI/JSS, for Alq3:Cs2CO3 doped film is 1.24, which means that the theoretical ohmic contact between the organic layer and cathode is formed. These characterization results are consistent with the observations in steady-state current–voltage(J–V) experiments. The great electron injection enhancements in the devices pave the way for an application of the cathodic injection in the preparation of OLEDs which are confirmed by doping Alq3 with Cs2CO3.

Investigation of the electrocatalytic activity for ethanol oxidation of Pt nanoparticles modified with small amount (≤5 wt%) of CeO2

Pt is still the state of the art electrocatalyst for the oxidation of ethanol but it is poisoned by the CO-intermediates resulting in a slow ethanol oxidation reaction kinetic. Here, we show that the electrocatalytic activity of unsupported Pt nanoparticles for the ethanol electro-oxidation in acid medium can be significantly enhanced by the addition of small amounts of CeO2 nanoparticles (≤5wt%). Comparative studies on bare Pt, Pt-CeO21wt% and Pt-CeO2—5wt% catalysts composed of Pt nanoparticles with different sizes (from 10 to >40nm for bare Pt, and from 6 and 20nm for the Pt-CeO2 composites) revealed an increase of the microstrain with the decrease of the Pt's average particle size. Besides, there is a strong electronic interaction between the Pt and CeO2 nanoparticles, demonstrated by a systematic shift of the Pt 4f7/2 peak to lower binding energy with the addition of CeO2. Chronoamperometric data showed that the steady state current density normalized to Pt's electrochemical surface area for ethanol electro-oxidation is mainly governed by the microstrain, and it decreases with increasing strain. This trend is explained in terms of a more facile activation of the ethanol molecule at the surface of less strained Pt nanoparticles. Thus, the best performance was obtained with Pt-CeO2 1wt% formed by Pt nanoparticles with an average size of 18nm, reaching 1.0A·m−2 after 900s at 0.6V vs SCE.

Microwave-assisted facile synthesis of cobalt[sbnd]iron oxide nanocomposites for oxygen production using alkaline anion exchange membrane water electrolysis

In this study, a rapid, scalable, and cost-effective method was developed for synthesizing cobalt–iron metal oxide catalysts for water electrolysis. Cobalt-iron metal oxide catalysts were synthesized using the microwave-assisted hydrothermal methods by varying the molar ratios of cobalt and iron. When the cobalt to iron ratio was 2:1, its electrolytic cell yielded the onset potential of only 1.56 V at 10 mA cm−2, which is close to the thermodynamically reversible potential. When its cell potential was at 1.8 V, the cell current density was approximately 130 mA cm−2. The results of the stability test showed a steady-state cell current density of 130 mA cm−2 and remained constant for more than 16 h at a continuous cell potential of 1.8 V. Compared with other catalysts, cobalt–iron metal oxide catalysts showed lower overpotential and lower Tafel slope than did conventional precious metal catalysts such as PtO2 and IrO2. Cobalt-iron metal oxide catalysts serve as an inexpensive route to large-scale commercialization through facile synthesis for enhanced electrochemical water splitting.

Enhanced kinetics of CO2 electro-reduction on a hollow gas bubbling electrode in molten ternary carbonates

Electrochemical reduction of CO2 to value-added carbon and oxygen in lithium-containing molten carbonates at 723 K is a promising approach to the efficient utilization of CO2. It was recently reported that the cathodic process in this transformation is controlled by the sluggish diffusion of the generated O2− ions. The formation of Li2O on the cathode results in partial cathodic passivation of the cathode. To accelerate the reaction kinetics and also eliminate the concentration polarization of the resulting Li2O, the effect of bubbling CO2 through a hollow electrode was examined in this work using a home-made hollow gas bubbling (HGB) electrode. The localised CO2 bubbling not only accelerates the transport of O2− ions by agitating the electrolyte nearby, but the CO2 also reacts with Li2O to form the more soluble Li2CO3. Cyclic voltammetry (CV), linear sweep voltammetry (LSV) and constant current electrolysis were conducted in the melt at 723 K to study the depolarization mechanisms involved in the CO2 bubbling reaction. Using the HGB electrode, the steady-state current density increased from 15.3 mA/cm2 to ~200 mA/cm2 at a potential of −2.4 V (vs. Ag/Ag2SO4). The HGB electrode effectively improved the cathodic kinetics, which is beneficial for CO2 capture and electrochemical conversion.

Effects of alloying elements and heat treatment on hydrogen diffusion in SCRAM steels

In this paper, the effects of alloying elements Ti and N, different heat treatments and permeation temperature on hydrogen diffusion in SCRAM steels were investigated. Devanathan-Stachurshi electrochemical hydrogen permeation technique was used to determine the hydrogen diffusivity. The results show that increasing the content of Ti and N can decrease the hydrogen diffusion coefficient of SCRAM steels. Compared with the traditional quenching and tempering heat treatment process, the process including an intermediate hold can refine the M23C6 phase, increase the volume fraction of MX phase but coarsen the martensitic lath. The twice quenching and tempering heat treatment process can not only refine the martensitic lath and M23C6 phase, but also increase the volume fraction of MX phase, resulting in a decrease of hydrogen diffusivity. The hydrogen permeation steady-state current density and the hydrogen diffusivity increase with the rise of permeation temperature. The temperature dependence of hydrogen diffusion coefficient conforms with the Arrhenius relationship. And the activation energy for hydrogen diffusion of two SCRAM steels with different heat treatments was determined.

Carbon supported PdM (M = Fe, Co) electrocatalysts for formic acid oxidation. Influence of the Fe and Co precursors

Pd and PdM (M = Fe or Co) nanostructured electrocatalysts were synthesized by the impregnation method and supported on carbon black Vulcan XC-72R for the formic acid oxidation reaction, FAOR, in acid medium. Nitrates or chlorides were used as Fe and Co precursors to study the counter ion role on the physicochemical features and electrochemical performance of the electrocatalysts. TEM analysis showed that PdM was deposited on the carbon material with a particle size around 2–3 nm. From XRD, peaks associated with the fcc palladium planes were observed along with evidence of PdM alloy formation, particularly when the nitrate salts were used as metal precursors. Furthermore, XPS analyses indicated that nitrates promote the metal oxide formation to a greater extent than chlorides, mainly for Pd. PdCo electrocatalyst obtained from nitrates exhibited the highest performance for FAOR with a steady state current density of 451 and 313 μA cm−2 at 200 and 400 mV respectively, which is in both cases, 3 times larger than that developed for a commercial Pd/C catalyst.

Core-Shell Nanostructured Mixed Ligand Directed ZnO Nanoparticles with Excellent Structural, Optical and Electronic Properties for Application in Light Emitting Devices

In the present study, three salicylaldehyde based surface directing agents (L1, L2, L3) were synthesized from the same amine, but different aldehydes using condensation reaction for their employment as capping layer. Surface decorated zinc oxide (ZnO) nanoparticles (N1, N2 and N3) were then synthesized by wet chemical precipitation. The optical studies of mixed ligand directed ZnO nanoparticles were carried out using ultraviolet-visible (UV-Vis) absorption spectroscopy and photoluminescence techniques. The capping with ligand resulted in the successful amendment of surface of ZnO nanoparticles as it reduced the surface defects and controlled the defect-related emissions. The photoluminescence studies of surface modified ZnO nanoparticles N1, N2 and N3 were further carried out to check their binding ability. N2 and N3 exhibited selective behavior towards Al(III) and Cu(II) respectively with change in the fluorescence emission profile; however, no such change was noticed for other metal ions. An increased fluorescence intensity together with narrow spectrum is observed in N2 with the addition of Al(III) ion. The surface modified ZnO nanoparticles demonstrated semiconducting behavior when incorporated in Light emitting diode (LED) structure with steady-state current densities of up to 1.9 A/cm2.

Recoverability of electrical conductivity of a Geobacter-enriched biofilm

The electrical conductivity of a Geobacter-enriched biofilm anode fed with acetate medium was assessed using small gold electrode systems as substrate concentration was changed. At 1.86 A/m2 of the steady-state current density, biofilm conductivity was stable at 0.48–0.51 mS/cm. This high biofilm conductivity was not changed at short-term starvation (< 7 h). In comparison, biofilm conductivity significantly decreased down to 0.16 ± 0.002 mS/cm after 4 d of long-term starvation (current density 0.03 ± 0.01 A/m2). Biofilm conductivity, however, was recovered to 0.35 ± 0.03 mS/cm after 25 mM acetate spiking in a few days. The decrease and increase of biofilm conductivity in response to substrate conditions for long-term starvation was consistently observed in three consecutive cycles. This dynamic biofilm conductivity indirectly indicates that catabolism of exoelectrogens (i.e., energy generation) is associated with biofilm conductivity.

Carrier Dynamics Engineering for High-Performance Electron-Transport-Layer-free Perovskite Photovoltaics

Summary Electron-transport-layer-free (ETL-free) device architectures are promising designs for perovskite photovoltaics because they offer simpler configurations, low cost, and convenience for versatile optoelectronics. However, the development of ETL-free photovoltaics is hindered by their low performance. Herein, we reveal that a low electron-injection rate at the ETL-free interface is responsible for the performance loss. Moreover, we demonstrate that improving carrier lifetimes in the perovskite films can remedy the poor carrier extraction at interfaces, enabling carrier collection efficiency in ETL-free photovoltaics to approach that in ETL-containing devices. Using perovskite films with microsecond carrier lifetimes, we obtained ETL-free devices with a power conversion efficiency (PCE) of 19.5%, nearly eliminated hysteresis, and good stability. Such a PCE value is comparable to that (20.7%) of the analogous ETL-containing photovoltaics. These results offer opportunities for ETL-free architecture designs in the perovskite photovoltaics family. More importantly, this research provides a general approach to improving the performance of photovoltaics with low-injection-rate interfaces.

Periodic polarization of electroactive biofilms increases current density and charge carriers concentration while modifying biofilm structure

Anodic electroactive biofilms (EABs) need to overcome low current densities for applications such as microbial fuel cells or biosensors. EABs can store charge in self-produced redox proteins when temporarily left in open circuit, and discharge them once the electrode is appropriately repolarized, thus behaving as pseudocapacitors. Here we investigated the effect of such periodic polarization on the intrinsic nature of the EABs during their entire growth (i.e. starting from inoculation and for 10 days) on glassy carbon electrodes. An optimal periodic polarization (half-period of 10 s) greatly increased the maximum steady-state current density delivered by the Geobacter-dominated EABs (up to 1.10 ± 0.02 mA cm−2, n = 3 electrodes) when compared to continuously polarized EABs (0.41 ± 0.04 mA cm−2); and increased the amount of electric charges produced per hour by 69 ± 17% even taking into account the half-periods of open circuit. This enhancement was highly correlated with a substantial increase in charge carriers concentration (10.6 ± 0.5 mMe- vs. 2.9 ± 0.6 mMe-), allowing higher charge storage capacity and higher electron mobility across the EABs. Our results suggest that appropriate periodic polarizations may upregulate the expression of heme-containing redox proteins associated with the matrix, such as c-type cytochromes. The EABs grown under periodic polarization presented mushroom-like structures on their top layers, while EABs grown under continuous polarization were flat.

Carbon-nanotube-caged microbial electrodes for bioelectrocatalysis

A method to stably immobilize microbes on electrodes was developed. Resting cells of Methylobacterium extorquens AM1(MeAM1) were caged within multiwalled carbon nanotubes (MWNTs)by adding the cells to a water dispersion of MWNTs then allowing the resulting mixture to dry on electrodes. The MeAM1-MWCNTs electrode thus obtained displayed excellent activities in the bidirectional bioelectrocatalysis due to formate dehydrogenase(s) in the resting cells; formate oxidation and carbon dioxide reduction proceeded at steady-state catalytic current densities of 0.6 ± 0.1 and −0.8 ± 0.1 mA cm−2, respectively, using methyl viologen as mediator under very mild conditions (pH 7.0, atmospheric pressure, and 37 °C). In addition, the catalytic signal was stable for more than one week under continuous operation.

Highly stable and efficient non-precious metal electrocatalysts of Mo-doped NiOOH nanosheets for oxygen evolution reaction

Active and stable electrocatalysts for oxygen evolution reaction (OER) made from earth-abundant elements are highly attractive. Herein, we report our recent efforts in developing Mo-doped NiOOH (MoNiOOH) nanosheets as highly active and stable OER catalysts. The doping of Mo is found to be conducive for both activity and stability. The MoNiOOH nanosheets need overpotential of only 390 mV to afford the current densities of 100 mA cm−2 and show no obvious degradation under steady-state current density of 100 mA cm−2 even after 24 h.

Electropolishing valve metals with a sulfuric acid-methanol electrolyte at low temperature

This study reports the electropolishing of Ti and Nb metals using a fluoride-free electrolyte of sulfuric acid and methanol at low temperature (−70 °C) without prior treatment. A fluoride-free electrolyte provides a less hazardous and more environmentally friendly option for electropolishing procedure. Experimental studies are presented on electropolishing with sulfuric acid electrolyte, which provides high quality macro- and micro-smoothing of the metal surfaces. Optimal conditions yielded leveling and brightening of the surface of Ti and Nb metals beyond that by the currently utilized electropolishing procedures with fluoride-containing electrolytes. The root mean square roughness (Rq) from atomic force microscopy analysis was 1.64 and 0.49 nm for Ti and Nb, respectively. Lower temperature experiments led to noticeable kinetic effects, indicated by a dramatic drop in current densities and the expansion of the steady-state current density plateau in anodic polarization curves. In addition, the voltage range of the current plateau expanded with increasing acid concentration. Surface characterization of Ti and Nb metals after polishing provided evidence of salt film formation. In addition, these metals were used as substrates in the formation of nanostructured metal oxides. The overall quality of the electropolishing led to a dramatic improvement in the uniformity of the nanostructures.

Analytical solutions and dimensional analysis of pseudo 2D current density distribution model in PEM fuel cells

Abstract In this paper, a new dimensionless pseudo 2D steady state current density distribution model along the channel of a polymer electrolyte membrane (PEM) fuel cell is presented. This model includes four fundamental phenomena observed in PEM fuel cell cathode such as the air concentration depletion along the channel, the mass transport through the gas diffusion layer (GDL), the charge transport through the membrane and the electrochemical transfer in the catalyst layer (CL). A dimensional analysis is performed, and three dimensionless parameters are found to govern the current density distribution along the channel: a Peclet number at the channel/GDL interface, a Damkholer number at the GDL/CL interface, and a Wagner number at the CL/PEM interface. Four regimes of operation are defined based on the values of these dimensionless numbers, and for each of them new analytical solutions are developed. A good agreement with the experimental measurements of current density distribution reported in literature is found, concluding that the macroscopic PEM fuel cell physic is well described by our model. The specific operating regime described with relatively simple equations of current density distributions paves the way for a better control of fuel cell operation and performance, and in situ characterisation of fuel cell material properties.

Excellent Electroactivity of Ternary Pd–Ag–Sn Nanocatalysts for Ethanol Oxidation

Multi-walled carbon nanotube (CNT) supported Pd–Ag–Sn nanocatalysts including Pd/CNT, Pd7Ag3/CNT, Pd7Sn2/CNT, Pd7Ag1Sn2/CNT, Pd7Ag2Sn2/CNT and Pd7Ag3Sn2/CNT, have been prepared in a mixture solution of ethylene glycol and water by NaBH4 reduction method. The samples have been characterized by using scanning electron microscopy, transmission election microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Compared to the Pd/CNT and Pd7Ag3/CNT catalysts, metal particles of the ternary Pd7Ag2Sn2/CNT catalyst are more uniformly dispersed on the surface of CNTs and present the size of ca. 2.4 nm. Both binary Pd–Ag and ternary Pd–Ag–Sn catalysts exhibit the alloying of Pd and Ag. Electrochemical activities of the prepared catalysts for ethanol oxidation in alkaline media were investigated by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy. Results show that Pd7Ag2Sn2/CNT catalyst reveals the highest current density of ethanol oxidation among the prepared catalysts and presents a high steady-state current density for ethanol oxidation from CA measurement, which is 33 times higher than the commercial Pd/C. Also, Pd7Ag2Sn2/CNT and Pd7Ag3Sn2/CNT catalysts show a large ethanol oxidation peak current density (against Pd loading) of 2.29 and 2.30 A mg−1, indicating the high usage efficiency of Pd particles for ethanol oxidation.

Cosputtered Calcium Manganese Oxide Electrodes for Water Oxidation.

Calcium manganese oxide films were prepared by cosputter deposition from Mn and CaMnO3 targets and evaluated for their suitability as catalysts for the oxygen evolution reaction (OER). Scanning electron microscopy (SEM) revealed a compact morphology for the as-deposited films and the formation of nanorodlike features on the surfaces after annealing at 600 °C. X-ray-photoelectron-spectroscopy analysis showed that the surface oxidation state is close to +III (as in Mn2O3) for the as-deposited films and increases slightly to a mixture of III and IV after annealing occurs in dry air at 400-600 °C. Glancing-incidence X-ray diffraction (GIXRD) suggested that the CaMnxOy films are amorphous even when heated to 600 °C. However, transmission electron microscopy (TEM) showed that there is actually a polycrystalline component of the film, which best matches Mn3O4 (hausmannite with the average Mn oxidation state of ∼+2.7) but may have a slightly expanded unit cell because of the incorporation of Ca. Electrochemical analyses revealed that the as-deposited CaMnxOy films were OER-inactive. In contrast, annealing at 400 or 600 °C resulted in an increase of ∼15-fold in the current densities, which reached j ≅ 1.5 mA·cm-2 at OER overpotentials of η ≈ 550 mV in cyclic voltammetry (CV) sweeps. For the same η, annealed CaMnxOy electrodes also showed good electrochemical stabilities during 2 h of electrolysis, as rather constant steady-state current densities of j ≅ 0.4-0.5 mA·cm-2 were observed. The thicknesses and surface morphologies of the CaMnxOy films did not change during the electrochemical measurements, indicating that corrosion was negligible. In comparison with a previous study in which Ca-free thin layers of MnOx were evaluated, the results demonstrate that Ca2+ incorporation can enhance the OER activity of MnOx electrocatalysts prepared by sputter deposition. This work provides guidance for designing new electrodes for water oxidation on the basis of the abundant and nontoxic elements manganese and calcium.

Electrochemical dissolution of magnetite electroplated coatings on titanium

As a part of understanding of iron and steel passivity, the dissolution rate of magnetite has been studied in anaerobic borate buffer at 25 °C between −0.5 and −0.75 V/SHE. The dissolution rates have been estimated from weight loss measurements. Linear weight losses in time have been obtained corresponding to constant dissolution rates. Nevertheless the rates depended on potential according to a Butler-Volmer law. Another Butler-Volmer law has been obtained for the steady-state cathodic current density flowing through the magnetite layer and ending by the proton reduction step. For both, the quantitative Butler-Volmer parameters have been estimated from the experiments. The cathodic feature of the electrochemical dissolution rate for magnetite was confirmed but the electron balance analysis has shown that this dissolution did not involved electrons as mentioned in the literature. One possible mechanism is proposed in which the kinetics of the proton reduction step tunes the outer voltage drop at the magnetite-solution interface and this voltage drop steers the kinetics of the dissolution rate.

Activating electrochemical catalytic activity of bio-palladium by hybridizing with carbon nanotube as \u201ce\u2212 Bridge\u201d

Nano metal catalysts produced by bacteria has received increasing attention owing to its environmental friendly synthesis route. However, the formed metal nanoparticles are associated with poorly conductive cells and challenged to be electrochemically applied. In this study, Palladium (Pd) nanoparticles were synthesized by Shewanella oneidensis MR-1. We demonstrated the limitation of palladized cells (Pd-cells) serving as electro-catalysts can be relieved by hybridizing with the conductive carbon nanotubes (Pd-cells-CNTs hybrid). Compared to the Pd-cells, the electrochemical active surface area of Pd in Pd-cells-CNTs10 (the ratio of Pd/CNTs is 1/10 w/w) were dramatically increased by 68 times to 20.44 m2·g−1. A considerable enhancement of electrocatalytic activity was further confirmed for Pd-cells-CNTs10 as indicated by a 5-fold increase of steady state current density for nitrobenzene reduction at −0.55 V vs Ag/AgCl. These results indicate that the biogenetic palladium could has been an efficient electro-catalyst but just limited due to lacking an electron transport path (e− Bridge). This finding may also be helpful to guide the way to electrochemically use other biogenetic metal nano-materials.