Low Cathodic Overpotentials Research Articles

Improving Scalability of copper recovery in saline microbial fuel cells with microtubular polypyrrole-based cathodic electrocatalysts

Microbial fuel cells (MFC) are emerging energy-efficient systems for copper (Cu) electrowinning from waste streams by coupling it with anodic oxidation of organics in wastewater. However, there is a lack of research examining scalable electrocatalysts for Cu electrowinning at low cathodic overpotentials in highly saline catholytes often found in e-waste leachates. The challenge of developing resilient anodic biofilms that withstand the antagonistic effects of ions migrating from catholytes in saline MFC also needs to be addressed. In this study, polypyrrole (PPy) cathodic electrocatalysts were developed and coupled with a robust halophilic anodic biofilm in MFC to improve the kinetics of Cu electrowinning from acidic chloride-based catholytes. Electrochemical characterisation of these cathodes revealed shuttling of electrons by redox-active PPy via the formation of intermediate Cu+-complexes as an energy-efficient pathway for producing metallic Cu. High power densities ranging from 0.63 ± 0.17 to 0.73 ± 0.05 W m−2 were achieved with undoped-PPy and phytic acid doped-PPy cathodes with simultaneous recovery of ∼97% Cu. These electrocatalysts also exhibited low charge transfer resistance (3–8 mΩ m2) that met the requisites for scalable cathodes in MFC. However, a decrease in the efficiency of PPy cathodes was observed over 5 d due to competing reactions at their interfaces, including re-oxidation of deposited Cu and cathodic corrosion, with further studies suggested to enhance their corrosion resistance. Nonetheless, integrating PPy electrocatalysts for Cu electrowinning in saline MFC has advanced its outlooks as an energy-efficient downstream process for urban mining of Cu from e-waste.

Synergistic interaction of S anions with 3d electrons in the bismuth sulfide/iron sulfide interface for simultaneous overall water splitting, urea oxidation, and methanol oxidation

Lotus-flower-like heterostructures of Bi2S3/FeS with diverse morphologies and stoichiometries were successfully synthesized using a facile hydrothermal technique. The flower-like architecture and FeS content were controlled by adjusting the amount of l-cysteine during precursor synthesis. Electrochemical properties were explored through a combination of Raman and impedance spectroscopy, revealing the relationship between catalytic performance and the stoichiometry of Bi2S3/FeS. Due to the synergistic interaction of S anions with 3d electrons at the Bi2S3/FeS interface the heterostructures exhibited highly enriched overall water-splitting ability, achieving a remarkably low cathodic and anodic overpotential of 36 and 230 mV for a current density of 10 mA/cm2. Furthermore, the addition of methanol and urea in the electrolyte further enhanced the catalytic efficiency of Bi2S3/FeS, achieving a very high current density of 100 mA/cm2 at exceptionally low potentials of 1.5 V and 1.43 V for methanol and urea oxidation reaction. The excellent electrocatalytic properties of Bi2S3/FeS were attributed to the interactions and interfacial effects between Bi2S3 and FeS, leading to enhanced surface roughness, inhomogeneity, and the creation of additional electrochemically active sites. Notably, Bi2S3/FeS demonstrated long-term stability and excellent mass transportation capabilities, highlighting its potential for various practical applications. The remarkable element gradient in the heterostructures further contributes to their superior performance, making them highly promising candidates for diverse applications.

Modeling of solid oxide fuel cell (SOFC) electrodes from fabrication to operation: Microstructure optimization via artificial neural networks and multi-objective genetic algorithms

In this study, a modeling framework is proposed for the optimization of the solid oxide fuel cell (SOFC) electrode microstructures. This involves sequential simulations of the SOFCs from initial powder to final electrochemical performance with artificial intelligence-assisted multi-objective optimization. The effects of starting powder parameters such as particle size, particle size distribution (PSD) and pore former content on cathodic overpotential and degradation rate of SOFCs are studied. It is shown that fine particle size and/or low pore former content lead to low cathodic overpotential but high degradation rate in the investigated range of the parameters. Predictive models for the cathode overpotential and degradation rate are established by an artificial neural network using the simulation data. The Sobol global sensitivity study suggests that particle size and pore former content play important roles in determination of the cathode overpotential and degradation rate while the PSD effect is insignificant. A multi-objective genetic algorithm (MOGA) is used to minimize both the overpotential and degradation rate of the cathode. The Pareto front is obtained for the optimal design of cathode microstructures. Compared to the grid search method, the MOGA proves to be more robust and efficient for SOFC electrode microstructure optimization.

Morphological evolution of mossy structures during the electrodeposition of zinc from an alkaline zincate solution

The morphological evolution of zinc during electrodeposition from an alkaline zincate solution, i.e., corresponding to charging operation in secondary zinc batteries, was investigated. The initiation and propagation of mossy structures were particularly focused; mossy structures represent one of the undesired, irregular morphologies at low cathodic overpotential. Scanning electron microscopy observations showed that mossy structures formed transiently from layer-like morphology followed by the formation of protrusions with a size of submicrometers. These protrusions still exhibited layer-like features in the initial stage. In addition to this transition behavior, the propagation of mossy structures was monitored in situ by optical microscopy. At −5.0mAcm−2, mossy structures formed successively and propagated, while there was no clear change in layer-like morphology during deposition outside these characteristic structures. At −20mAcm−2, the layer-like growth proceeded for longer duration and the population of the mossy structures apparently decreased. However, they continuously increased in size followed by successive formation of mossy structures once initiated. From these results, growth mode of zinc at low cathodic overpotential transiently changed from layer-like growth to mossy structure evolution accompanied with dynamic variations in local current density.

Investigation of adsorption and corrosion inhibition of mild steel in hydrochloric acid solution by 5-(4-Dimethylaminobenzylidene)rhodanine

The adsorption and corrosion inhibition of mild steel in 0.5M HCl solution by 5-(4-Dimethylaminobenzylidene)rhodanine (DABRh) were investigated by electrochemical and scanning electron microscopy (SEM) techniques. It was found that DABRh has high inhibitory efficiency against the corrosion of mild steel in HCl solution. This compound is classified as the mixed type corrosion inhibitor with predominant control of cathodic reaction. The high inhibitory efficiency of DABRh was related with the adsorption of DABRh molecules at the meal/solution interface and a protective film formation. The surface inhibitor film was found to be very stable at low anodic and cathodic overpotentials.

Mechanistic aspects of electrodeposition of Ni–Co–SiC composite nano-coating on carbon steel

The nucleation and early-stage growth mechanism and kinetics of electrodeposited Ni–Co–SiC composite coating on carbon steel was investigated by cyclic voltammetry, current–time transient measurements and atomic force microscopy characterization. The conventional Guglielmi's model for metal-inert particle co-deposition was modified to consider the effect of the bath electrolyte hydrodynamics on amount of nano-particles deposited in the coating. It is determined that the nucleation and early-stage growth of the coating depends on depositing overpotential. At low cathodic overpotentials, it is between an instantaneous and progressive mechanism; while at high overpotentials, it follows the instantaneous mechanism. Addition of SiC nano-particles in the bath electrolyte reduces the electrodepositing efficiency by inhibiting nucleation and growth of metallic coating. An empirical model is developed to estimate the amount of nano-particles contained in Ni–Co–SiC composite coating during pulse electrodeposition.

Three-dimensional assembly and electrical properties of Cu2O/ZnO heterojunction via an electrochemical superfilling method

Three-dimensional (3D) heterojunctions consisting of Cu2O and ZnO nanorods were assembled via the electrochemical superfilling approach. This procedure included two electrochemical steps: the initial low-temperature nucleation of Cu2O on the surface of ZnO nanorods at high cathodic overpotential and the subsequent high-temperature growth at low cathodic overpotential. The structural morphologies, electrical properties and the interface features of the 3D heterojunction were characterized with SEM observation, I–V and Mott–Schottky testing and the impedance measurement. The bottom-up growth of Cu2O in ZnO nanorod arrays was realized. The filling depth was over 4.0μm. The dense structure had good electrical contact and large forward current, but the enlarged junction area and grain boundaries produced great interface state density, which contributed to the relative lower conversion efficiency of 0.08%.

Synthesis and Characterization of Well Aligned Ru Nanowires and Nanotubes

Arrays of well aligned Ru nanowires and nanotubes were prepared by electrodeposition through porous anodic aluminum oxide membranes. It is shown that Ru nanowires are formed at low cathodic deposition overpotential, while nanotubes are prepared at higher cathodic overpotential. The inner diameter of the nanotube can be controlled by the deposition potential, and it varies from 0 to 70% of the outer diameter as the deposition potential is changed from -0.30 to -0.45 V vs SCE.

Electrocrystallisation of zinc from acidic sulphate baths; A nucleation and crystal growth process

The electrochemical nucleation and growth of zinc on low-carbon steel from acidic (pH 2.0–4.5) baths containing ZnSO 4, NaCl, and H 3BO 3, was studied by means of chronoamperometry at various cathodic potentials under a charge-transfer controlled regime. It is shown that at overpotentials in the range 0.30–0.55 V (negative to the Zn 2+/Zn redox value) the electrodeposition proceeds by instantaneous three-dimensional nucleation, which turns to progressive at higher overpotentials and/or very acidic baths. At low cathodic overpotentials (<0.30 V), a two-dimensional contribution limited by the incorporation of Zn ad-atoms in the developing lattice becomes significant at the early stages of deposition, and is more progressive in type the more acidic is the bath pH. Nucleation rate constants were calculated and correlated analytically with the respective potentials, using the classical theory of heterogeneous nucleation, which though fails to lead to reasonable values for the critical nucleus size.

Effects of surface overpotential at the La 1− xSr xCo 1− yFe yO 3-yttria stabilized zirconia interface in a model solid oxide fuel cell cathode

The cathode–electrolyte interface microstructure and surface overpotential are important factors to decide the oxygen reduction rate. Thus, the effects of the surface overpotential and temperature at the strontium- and iron-doped lanthanum cobaltite (LSCF)/gadolinia-doped ceria (GDC) electrode interface on yitria-stabilized zirconia (YSZ) have been analyzed by AC impedance. At low anodic overpotential near the equilibrium, the impedance diagram is characterized by a resistance with approximately a constant value of 0.078 Ωcm 2 at 1073 K. The resistance activation energy is 1.68 eV at zero applied overpotential. However, at high anodic overpotential such as 300 mV, the polarization resistance is significantly decreased with relatively low resistance activation energy of 1.33 eV. Under cathodic overpotential, the observed behavior is similar to that under the anodic overpotential. At low cathodic overpotential, a high resistance value of 0.215 Ωcm 2 at 1073 K is obtained; whereas a high activation energy of 1.98 eV at −300 mV overpotential is estimated. In addition, both the resistance–overpotential curves at high anodic and cathodic overpotentials obey an exponential law.

Copper electrodeposition from an acidic plating bath containing accelerating and inhibiting organic additives

Copper electrodeposition on copper from still plating solutions of different compositions was investigated utilising electrochemical impedance spectroscopy (EIS), cyclic voltammetry, and scanning electron microscopy (SEM). An acid copper sulphate plating base solution was employed either with or without sodium chloride in the presence of a single additive, either polyethylene glycol (PEG) or 3-mercapto-2-propanesulphonic acid (MPSA), and their mixture. Thallium underpotential deposition/anodic stripping was employed to determine the adsorption capability of additives on copper. In the absence of chloride ions, MPSA shows a moderate adsorption on copper, whereas PEG is slightly adsorbed. At low cathodic overpotentials, the simultaneous presence of MPSA and chloride ions accelerates copper electrodeposition through the formation of an MPSA-chloride ion complex in the solution, particularly for about 220 μM sodium chloride. The reverse effect occurs in PEG-sodium chloride plating solutions. In this case, from EIS data the formation of a film that interferes with copper electrodeposition can be inferred. At higher cathodic overpotentials, when copper electrodeposition is under mass transport control, the cathode coverage by a PEG-copper chloride-mediated film becomes either partially or completely detached as the concentration of chloride ions at the negatively charged copper surface diminishes. The copper cathode grain topography at the μm scale depends on the cathodic overpotential, plating solution composition and average current density. Available data about the solution constituents and their adsorption on copper make it possible to propose a likely complex mechanism to understand copper electrodeposition from these media, including the accelerating effect of MPSA and the dynamics of PEG-copper chloride complex adsorbate interfering with the surface mobility of depositing copper ad-ions/ad-atoms.

Measurement and Modeling of Hydrogen Environment–Assisted Cracking of Ultra-High-Strength Steel

Modern precipitation-hardened ultra-high-strength AERMET 100 steel (Fe-Co-Ni-Cr-Mo-C) is susceptible to severe transgranular hydrogen environment–assisted cracking (HEAC) in neutral 3.5 pct NaCl solution. The threshold stress intensity for HEAC, K TH , is reduced to as low as 10 pct of K IC , and the stage II subcritical crack growth rate, da/dt II, is up to 0.5 μm/s. Low K TH and high da/dt II are produced at potentials substantially cathodic, as well as mildly anodic, to free corrosion. However, a range exists at slightly cathodic potentials (–0.625 to –0.700 VSCE), where the crack growth rate is greatly reduced, consistent with reduced crack-tip acidification and low cathodic overpotential for limited H uptake. Short crack size (250 to 1000 μm) does not promote unexpectedly severe HEAC. High-purity AERMET 100 is susceptible to HEAC because martensite boundary trapping and high crack-tip stresses strongly enhance H segregation to sites that form a transgranular crack path. The stage II da/dt is H diffusion rate limited for all potentials examined. A semiquantitative model predicts the applied potential dependence of da/dt II using reasonable input parameters, particularly crack-tip H uptake reverse calculated from measured K TH and a realistic critical distance. Modeling challenges remain.

A Two-Dimensional Model and Numerical Treatment for Mixed Conducting Thin Films: The Effect of Sheet Resistance

A two-dimensional simulation was developed for thin-film, mixed ionic-electronic conductor (MIEC) electrodes. The equations for the steady-state (nonlinear) dc case are derived and solved through a finite volume discretization that enables a linear solver while preserving the nonlinear characteristics of the problem. Using parametric data for (LSM) collected from the literature, we found that the simulation is limited by electrochemically activated ionic transport at low cathodic overpotentials and co-limited by ionic and electronic resistance at high overpotentials. The current-voltage curves generated are similar in shape to most reported in the literature for thin LSM films. However, the simulation is much more resistive. While the cause for the discrepancy has yet to be validated, it seems clear that the sheet resistance of a thin-film electrode can critically influence its electrocatalytic behavior. This effect becomes more pronounced with the amplitude of polarization and the decrease in dimension, implying that it can be significant in patterned or nanostructured electrodes.

Electrochemical deposition of Zn on TiN microelectrode arrays for microanodes

Zn was electrochemically deposited onto square TiN electrodes with edge dimensions of 490 μm and 40 μm. These were fabricated by standard microfabrication techniques, which provide an extremely reproducible electrode for experimentation. Reliable constant-potential electrodeposition of Zn on the TiN was performed at −1.2 V, just below the Zn/Zn 2+ redox potential. At more negative potentials, the hydrogen evolution reaction on TiN interfered with bulk metal electrodeposition, resulting in poor quality Zn films. A two-step plating procedure was shown to be most efficient for electrochemical deposition of Zn, with Zn nucleation on the TiN substrate at high cathodic overpotential during the first step and a second step of bulk metal growth on the nucleated layer at low cathodic overpotential. These results were most consistent with 3D progressive nucleation of Zn on the TiN surface. Using this procedure, deposits of Zn on 490 μm TiN electrodes were uniform. In contrast, Zn deposits on 40 μm electrodes formed high-surface area and volume surface structures resulting from preferential growth at the electrode corners due to enhanced hemispherical diffusion at these sites. This should enable the formation of high surface area, high current density Zn anodes on biocompatible TiN microelectrodes, which could find application as improved microanodes for implantable miniature power supplies, e.g., implantable glucose sensors and internal cardioverter defibrillators.

CO and CH4 Electrochemical Oxidation on Ni Patterned Anodes

Single cell SOFCs with Ni patterned anodes sputtered onto single crystal YSZ electrolyte discs were used to examine electrochemical oxidation of H 2 , CO, and CH 4 under dry and wet (∼4% H 2 O) conditions. High-surface-area porous YSZ/LSM cathodes provided low cathodic overpotentials for isolation of anodic processes. Polarization curves of H 2 and CO from 750-850°C indicated that for patterns with < 100 μm line widths, power densities and impedance spectra more closely scale with Ni surface area than with three-phase-boundary lengths. CH 4 electrochemical oxidation also suggested a very weak influence of three-phase boundary lengths for temperature ≥ 800°C. Results indicate detrimental effects of H 2 O on CO oxidation power densities due to drops in open-circuit voltages, although polarization resistances did not drop significantly. Similar drops in OCV with H 2 O addition were observed for CH4, but steam-reforming rates become significant above 800°C such that polarization resistances for CH 4 increase to approach those of H 2 . Impedance spectra suggest that a Jow frequency process controls CO oxidation for temperatures ≤ 800°C but becomes less dominant at higher temperatures. Impedance spectra for CH 4 oxidation show the dramatic drop in polarization resistance as steam-reforming rates become appreciable, indicating the importance of temperature in sustaining reliable Ni anode performance.

Oxygen reduction at electrochemically deposited crystallographically oriented Au(1 0 0)-like gold nanoparticles

A novel electrocatalyst for oxygen reduction has been proposed based on the electrodeposition of Au nanoparticles onto glassy carbon (nano Au/GC) electrodes in the presence of some additives, typically cysteine. Steady-state hydrodynamic voltammetric studies for the oxygen reduction reaction (ORR), in O 2-saturated 0.5 M KOH solution, at the thus-prepared nano Au/GC disk electrode have revealed an increasing disk current ( I D) (current maxima) up to −0.5 V vs. Ag/AgCl/KCl(sat) compared to the other nano Au/GC disk electrodes prepared under different conditions. This high value (maxima) of I D is associated with a low value of the corresponding Pt ring current ( I R) and in turn a higher number of electrons ( n) is exchanged at the disk electrode during the ORR in alkaline medium in this potential window. At more cathodic potentials, this disk current maxima vanished and the nano Au/GC disk current showed a minimal value associated with a significant increase in I R. This resembles the behaviour of the ORR at single crystalline Au(1 0 0) disk electrodes in the similar media. Thus, a 4-electron reduction of oxygen to water can be accomplished at a reasonably low cathodic overpotential.

Some aspects of the EIS study of the rhodium electrode in the hydrogen evolution region

General expression for the frequency dependence of the faradaic electrode admittance is derived for the case when the rate of the hydrogen evolution reaction (HER) depends on the potential, potential dependent surface fraction of adsorbed hydrogen, and potential dependent surface or subsurface concentration of the molecular or absorbed hydrogen, respectively. The simplified version of the general expression was adjusted to take into account two different specific reaction mechanisms of HER, and applied in the analysis of the experimentally measured impedance spectra of the rhodium/H 2SO 4 electrode in the region of low cathodic overpotentials. The results of the fitting procedure suggested proceedings of HER in parallel with diffusion associated with some absorption of hydrogen within the rhodium metal layer.

La-Doped BaCoO3 as a Cathode for Intermediate Temperature Solid Oxide Fuel Cells Using a LaGaO3 Base Electrolyte

Cathodic behavior of doped with La was investigated in this study for an intermediate temperature solid oxide fuel cell (ITFC). Electrical conductivity of increased monotonically with an increasing amount of La doped for the Ba site at 1073 K Cathodic overpotential at 1073 K decreased with increasing La content and attained a minimum at in Since the cathodic overpotential of kept a small value at decreased temperature, is the optimum composition for the cathode of an ITFC among -based oxides. When was used for the electrolyte, the power density of the cell using for the cathode at 1073 K attained a value of 550 mW/cm2, which is slightly higher than that using for the cathode. In addition, a low cathodic overpotential of was also maintained in air. exchange reaction was performed to estimate the surface activity for oxygen dissociation. It was found that exhibits high activity for the oxygen exchange reaction. Therefore, superior cathode property was assigned to the high surface activity of © 2002 The Electrochemical Society. All rights reserved.

Electrochemical Formation of Palladium Islands on HOPG: Kinetics, Morphology, and Growth Mechanisms

Scanning tunneling microscopy (STM) and conventional electrochemical techniques were utilized to investigate the growth kinetics and mechanism of palladium island electroformation on highly oriented pyrolitic graphite (HOPG) from aqueous acid palladium chloride solutions at 298 K. Initially, the electrodeposition reaction at low cathodic overpotentials involves an activation process in which a PdCl2 surface intermediate is formed. At intermediate overpotentials, the growth of palladium islands involves a progressive nucleation and growth model under diffusion control, whereas at high overpotentials, the bulk discharge of soluble palladium species undergoes a free convective-diffusion process. As the cathodic overpotential is shifted negatively, the aspect ratio of the islands, defined as the ratio of the maximum height of the island to the island radius, and the island size decrease, whereas the island density increases. As the cathodic overpotential becomes a few millivolts more positive than the thresho...

Electrochemical Characterization of Mixed Conducting Ba(Ce0.8−yPryGd0.2)O2.9 Cathodes

The high proton conductivities of doped barium and strontium cerates has led to their consideration as electrolytes in fuel cells operating at intermediate temperatures (600-800°C). The electrochemical properties of Pr-doped barium gadolinium cerate have been investigated as part of our assessment of new mixed-conducting electrodes in fuel cells with barium cerate electrolytes. Substitution of Pr for Ce increases the total and electronic conductivities, and the total conductivity of exceeds ∼0.75 S/cm at 800°C in dry air. Electromotive force measurements indicate that the Pr-substituted samples also retain high proton and oxygen-ion conductivities despite their decreased ionic transport numbers. Cathodes prepared using these mixed-conducting Pr-doped barium-gadolinium cerates exhibit low cathodic overpotentials in electrolyte fuel cells. For example, the cathodic overpotential resistance of a cathode is at 800°C at current densities This is the lowest cathodic overpotential resistance reported for a perovskite electrode on a proton-conducting electrolyte fuel cell at this temperature. © 2000 The Electrochemical Society. All rights reserved.