Exchange Current Density Research Articles

Electrocatalytic activity of electrodeposited CoOx thin film on low-carbon unalloyed steel substrate toward electrochemical oxygen evolution reaction (OER).

In this study, we report elaboration of a thin film of CoOx on a low carbon unalloyed steel substrate by electrochemical route and the study of its electrocatalytic performances with respect to the evolution reaction of oxygen (OER) in NaOH medium. The elaborated deposits were well-characterized using X-ray diffraction. Kinetic and thermodynamic parameters such as exchange current density, Tafel slope, reaction order with respect to OH- ions and apparent activation energy were studied. The CoOx displays satisfactory OER performance in an alkaline medium, with a low overvoltage of 362mV at 10mA/cm2 and a Tafel slope of 81mV/dec at 293K. The apparent kinetic activation energy (= 29.79kJ/mol) was similar to those obtained for the reported catalytic electrode materials. The O2 gas obtained on the cobalt oxide electrode was 2.865mmol/s.cm2, which is 28 times higher than that obtained for the platinum electrode (0.102mmol/s.cm2). Chronoamperometry demonstrates a better electrochemical stability under a polarization potential of 2V in 1M NaOH for nearly 25h. The low cost, the high OER performance, as well as the good stability of the CoOx electrode make it a promising candidate for the industrial-scale water electrolysis.

Effect of pulse-current-based protocols on the lithium dendrite formation and evolution in all-solid-state batteries

Understanding the cause of lithium dendrites formation and propagation is essential for developing practical all-solid-state batteries. Li dendrites are associated with mechanical stress accumulation and can cause cell failure at current densities below the threshold suggested by industry research (i.e., >5 mA/cm2). Here, we apply a MHz-pulse-current protocol to circumvent low-current cell failure for developing all-solid-state Li metal cells operating up to a current density of 6.5 mA/cm2. Additionally, we propose a mechanistic analysis of the experimental results to prove that lithium activity near solid-state electrolyte defect tips is critical for reliable cell cycling. It is demonstrated that when lithium is geometrically constrained and local current plating rates exceed the exchange current density, the electrolyte region close to the defect releases the accumulated elastic energy favouring fracturing. As the build-up of this critical activity requires a certain period, applying current pulses of shorter duration can thus improve the cycling performance of all-solid-solid-state lithium batteries.

Effects of F− ions on the electrochemical and interface behavior of cathodes in zinc electrowinning

The increasing use of Zn metal in the energy and industrial sectors necessitates a higher level of research into the zinc electrowinning process. In this work, the effects of fluoride ions (F−) in zinc electrolyte on the cathodic kinetics and zinc product quality during zinc electrodeposition was systematically investigated via a series analysis. The results revealed that the addition of F− in the electrolyte showed a polarization effect, with an increase in the apparent cathodic exchange current density increased by 104 orders of magnitude, and a decrease in the zinc deposition overpotential by 46 mV at a concentration of 400 mg/L. However, F− also reduces the hydrogen evolution overvoltage, when the electrolyte contains 400 mg/L F−, the hydrogen evolution overvoltage on aluminum cathode and zinc cathode decreases by 121 mV and 97 mV, respectively. The promoted hydrogen evolution kinetics increases energy consumption and surface porosity. Additionally, F− ions corrode the Al2O3 oxide film on the cathode surface, accelerating the corrosion of the aluminum cathode plate. This results in increased adhesion of zinc deposition to the damaged aluminum substrate, making it difficult to peel off.

RuO2-PdO nanowire networks with rich interfaces and defects supported on carbon toward the efficient alkaline hydrogen oxidation reaction

Interfacial engineering is a promising approach for enhancing electrochemical performance, but rich and efficient interfacial active sites remain a challenge in fabrication. Herein, RuO2-PdO heterostructure nanowire networks (NWs) with rich interfaces and defects supported on carbon (RuO2-PdO NWs/C) for alkaline hydrogen oxidation reaction (HOR) was formed by a seed induction-oriented attachment-thermal treatment method for the first time. As expected, the RuO2-PdO NWs/C (72.8% Ru atomic content in metal) exhibits an excellent activity in alkaline HOR with a mass specific exchange current density (j0,m) of 1061 A gRuPd−1, which is 3.1 times of commercial Pt/C and better than most of the reported non-Pt noble metal HOR electrocatalysts. Even at the high potential (∼0.5 V vs. RHE) or the presence of CO (5 vol%), the RuO2-PdO NWs/C still effectively catalyzes the alkaline HOR. Structure/electrochemical analysis and theoretical calculations reveal that the interfaces between RuO2 and PdO act as the active sites. The electronic interactions between the two species and the rich defects for the interfacial active sites weaken the adsorption of Had, also strengthen the adsorption of OHad, and accelerate the alkaline HOR process. Moreover, OHad on RuO2 can spillover to the interfaces, keeping the RuO2-PdO NWs/C with the stable current density at higher potential and high resistance to CO poisoning.

Sphere shaped Mo- doped transition metal oxide as electrocatalyst for oxygen evolution reaction in alkaline medium

Designing and development of the low cost and efficient transition metal based electrocatalyst for the electrochemical water splitting is a challenging task because in general, costly noble metals are used as electrocatalyst for water splitting reactions. Herein Mo dopped ferric oxide (MFO) was synthesized by hydrothermal method followed by the heat treatment and characterized by using different techniques such as XRD, FESEM, HRTEM, FTIR, EDS, M-S plot and BET etc. The electrocatalytic properties of ferric oxide (FO) and MFO towards water splitting in alkaline medium were studied and compared with RuO2. The prepared material (MFO) exhibited higher electrocatalytic activity towards the oxygen evolution reaction (OER) in the alkaline medium as compared to ferric oxide (FO). The overpotential for MFO at 10 mA/cm2 current density was found as 359 mV vs. RHE and Tafel slop value was found as 80 mV/ decade. The investigated MFO also offered high value of the exchange current density and large number of active sites (8.6568 *10−8 mol) as well as large BET surface area (179.41 m2/g).). Thus MFO acts as efficient low cost electrocatalyst towards the oxygen evolution reaction to produce the molecular oxygen for the renewable energy production.

Approach to Evaluation of Electrocatalytic Water Splitting Parameters, Reflecting Intrinsic Activity: Toward the Right Pathway.

The development of transition metal-based non-precious-metal electrocatalysts for energy storage and conversion systems has received a lot of interest recently. To further this subject in the proper way given the development of electrocatalysts, a fair comparison of their respective performance is necessary. This Review investigates the parameters used for the comparison of electrocatalyst activity. Significant evaluation criteria employed in electrochemical water splitting studies are the overpotential at defined current density usually at 10 mA per geometric surface area, Tafel slope, exchange current density, mass activity, specific activity and turnover frequency (TOF). This Review will discuss how to identify the specific activity and TOF by electrochemical and non-electrochemical methods to represent intrinsic activity as well as the benefits and uncertainties of each technique, ensuring that each method is applied correctly when calculating intrinsic activity metrics.

Electrocatalytic performance investigation of NiCo2O4 nanostructures prepared by hydrothermal method and thermal post-annealing treatment

Nickel-cobalt oxide (NiCo2O4), also known as nickel–cobalt ore, is a spinel-structured mixed oxide. It is more environmentally friendly, abundant in nature, has better electrical conductivity, and has higher electrochemical activity than two single-component oxides, nickel oxide and cobalt oxide. Here, NiCo2O4 is synthesized using a simple hydrothermal method and then annealed in the air for 3 h at 300, 350, 400 and 450 °C. The resulting materials were characterized using X-ray diffraction (XRD), Fourier-Transform-Infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), ultraviolet–visible (UV–vis) spectroscopy and electrochemical techniques. A single-phase face-centered cubic spinel structure with a nanorod-like morphology is obtained at 350 °C, and samples above this temperature show mixed phases consisting of nanowires and nanorods.. The sample annealed at 350 °C demonstrated excellent electrochemical performance for hydrogen evolution reaction (HER), with an overpotential of 435 mV at a current density of 10 mA/cm2, a pseudo-capacitance of 0.287 mF/cm2, a Tafel slope of 71 mV/dec, an exchange current density of 3.5 × 10-3 mA/cm2, and a minimum charge transfer resistance of 98 O. The study thus demonstrated the efficacy of the hydrothermal method and thermal post-annealing treatment in producing NiCo2O4 nanostructures with excellent electrocatalytic performance for energy conversion and storage applications.

Regulating the Solvation Structure of Electrolyte via Dual–Salt Combination for Stable Potassium Metal Batteries

Batteries using potassium metal (K-metal) anode are considered a new type of low-cost and high-energy storage device. However, the thermodynamic instability of the K-metal anode in organic electrolyte solutions causes uncontrolled dendritic growth and parasitic reactions, leading to rapid capacity loss and low Coulombic efficiency of K-metal batteries. Herein, an advanced electrolyte comprising 1M potassium bis(fluorosulfonyl)imide (KFSI) + 0.05M potassium hexafluorophosphate (KPF6 ) dissolved in dimethoxyethane (DME) is introduced as a simple and effective strategy of regulated solvation chemistry, showing an enhanced interfacial stability of the K-metal anode. Incorporating 0.05M KPF6 into the 1M KFSI in DME electrolyte solution decreases the number of solvent molecules surrounding the K ion and simultaneously leads to facile K+ de-solvation. During the electrodeposition process, these unique features can lower the exchange current density between the electrolyte and K-metal anode, thereby improving the uniformity of K electrodeposition, as well as potentially suppressing dendritic growth. Even under a high current density of 4mA cm-2 , the K-metal anode in 0.05M KPF6 -containing electrolyte ensures high areal capacity and an unprecedented lifespan with stable Coulombic efficiency in both symmetrical half-cells and full-cells employing a sulfurized polyacrylonitrile cathode.

Study on microstructure and physical properties of Ba0.5Sr0.5Fe1−xCuxO3−δ–Ce0.8Sm0.15Ca0.05O1.875 cathode materials used in solid oxide fuel cell

Ba0.5Sr0.5Fe1−xCuxO3−δ (BSFCu) and BSFCu–Ce0.8Sm0.15Ca0.05O1.875 (SCDC) were prepared by solid-state reaction and sol-gel method, respectively. The possibility of BSFCu−SCDC composites as cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs) was investigated. The results show that BSFCu has a cubic BaFeO3 structure and no secondary phase. BSFCu−SCDC has cubic BaFeO3 and Ce0.8Sm0.2O1.9 structures, and BaCeO3 secondary phase. The polarization surface specific resistance (ASRRP) of BSFCu increases with increasing Cu doping. For BSFCu–SCDC, 7BSFCu10–3SCDC has the smallest ASRRP (0.106 Ω cm2, 800 °C) at 800 °C–750 °C; at 600 °C − 700 °C, 6BSFCu10–4SCDC has the smallest ASRRP (1.043 Ω cm2, 600 °C). The maximum exchange current densities of 7BSFCu10–3SCDC and 6BSFCu10–4SCDC are about 0.219 and 0.083 A cm−2 at 800 °C and 700 °C, respectively. Compared with BSFCu, 6BSFCu10–4SCDC has a lower overpotential, which shows better electrocatalytic activity, so it can increase the energy efficiency when applied to IT-SOFC.

Insight into Electrochemical Promotion of Cu/Co3O4 Catalysts for the Reverse Water Gas Shift Reaction

AbstractIn this study, Copper‐modified Co3O4 (Cu/Co3O4) catalysts were investigated for the reverse water gas shift (RWGS) reaction at 200–400 °C under different CO2 : H2 ratios. It was found that the electrocatalytic performance of Co3O4 was improved by loading 4 weight percent (wt.%) Cu nanoparticles (average size: 13 nm), which could be then used as an electrode in electrochemical promotion of catalysis (EPOC) studies. Under the applied voltages of +2 V and −1 V, the catalytic rates were suppressed and increased by about 40 % and 14 %, respectively. This was due to the changes in the active oxidation states of Cu and Co caused by O2− migration under polarization, as confirmed by XPS, XRD, and cyclic voltammetry (CV). The study also revealed that the exchange current density (i0) was counter‐correlated with the open‐circuit catalytic rate (r0) and could serve as an informative tool for predicting the catalytic rate, which was demonstrated for the first time in the instance of RWGS reaction.

Electrocatalytic Hydrogen Evolution Reaction from Acetic Acid over Gold Immobilized Glassy Carbon Surface

A hydrogen fuel cell is a highly promising alternative to fossil fuel sources owing to the emission of harmless byproducts. However, the operation of hydrogen fuel cells requires a constant supply of highly pure hydrogen gas. The scarcity of sustainable methods of producing such clean hydrogen hinders its global availability. In this work, a noble Au-atom-decorated glassy carbon electrode (Au/GCE) was prepared via a conventional electrodeposition technique and used to investigate the generation of hydrogen from acetic acid (AA) in a neutral electrolyte using 0.1 M KCl as the supporting electrolyte. Electrochemical impedance spectroscopy (EIS), open circuit potential measurement, cyclic voltammetry (CV), and rotating disk electrode voltammetry (RDE) were performed for the characterization and investigation of the catalytic properties. The constructed catalyst was able to produce hydrogen from acetic acid at a potential of approximately −0.2 V vs. RHE, which is much lower than a bare GCE surface. According to estimates, the Tafel slope and exchange current density are 178 mV dec−1 and 7.90×10−6 A cm−2, respectively. Furthermore, it was revealed that the hydrogen evolution reaction from acetic acid has a turnover frequency (TOF) of approximately 0.11 s−1.

Theory for Heterogeneous Electron Transfer Kinetics on Nanocorrugated Atomic Stepped Metal Electrodes

We develop a novel mean-field theoretical approach for the outer-sphere heterogeneous electron transfer (OS-HET) rate constant at an atomically stepped nanocorrugated metal surface. Theory accounts for the contributions from the nature of the metal, local curvature of the nanocorrugated atomic step, the density of atomic steps and kinks, dipolar solvents, and the frontier molecular orbital of electroactive molecules. Our theoretical approach develops a novel model for the free energy of activation obtained using the alignment of the Fermi energy level in the metal with one of the frontier molecular orbitals of the electroactive species. The activation free energy is obtained as a product of a modified work function (WF) and the fractional electronic charge of activation for the alignment of levels. Metal surface local curvatures at atomic steps are modeled as a hyperbolic tangent functional with a random nanocorrugated step edge. The influence of step density and edge fluctuations (or kinks) on the activation free energy is included through the surface average mean curvature and the mean-square gradient dependent modified WF. The theoretical results highlight the step density-dependent nonmonotonic variation in WF, fractional electronic charge, and activation free energy. The increase in step density shifts the Fermi energy level toward LUMO and away from HOMO energy levels. This causes the reduced (for the LUMO) and enhanced (for the HOMO) activation free energy for ET kinetics compared to the basal plane. The dichotomy in the OS-HET kinetics at the atomically stepped metal results in anomalous enhancement and suppression for the LUMO and HOMO, respectively. The strong influence of kinks along the step edge on the HET rate constant is predicted up to 6 decades (compared to the basal surface of Pt metal). Finally, theory shows agreement with the reported experimental data of WF and exchange current density for single crystal Pt electrodes.

Effect of hetero-atom doping on the electrocatalytic properties of graphene quantum dots for oxygen reduction reaction

Oxygen reduction is an important reaction involved in a diverse variety of energy storage devices and also in many chemical and biological processes. However, the high cost of suitable catalysts like platinum, rhodium, and iridium proves to be a major obstacle for its commercialization. Consequently, many new materials have emerged in recent years such as various forms of carbon, carbides, nitrides, core–shell particles, Mxenes, and transition metal complexes as alternatives to platinum and other noble metals for oxygen reduction reaction (ORR). Among these, Graphene Quantum Dots (GQDs) as metal-free alternatives have captured universal attention, since electrocatalytic properties can be tuned not only by size and functionalization but by heteroatom doping also. We discuss electrocatalytic properties of GQDs (approximate size 3–5 nm) with specific dopants such as N and S focusing on their synergistic effects of co-doping, prepared by solvothermal routes. Cyclic Voltammetry shows benefits of doping as lowering of the onset potentials while steady-state Galvanostatic Tafel polarization measurements show a clear difference in the apparent Tafel slope, along with enhanced exchange current densities, suggesting higher rate constants.

Oxygen-vacancy-rich MoO2 supported nickel as electrocatalysts to promote alkaline hydrogen evolution and oxidation reactions

Hydrogen energy is the most suitable solution for long-term, large-scale energy storage, and the design of efficient, economical, and stable electrocatalysts for hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) is of great significance for its commercial development. Herein, Ni nanocluster supported on oxygen-vacancy-rich MoO2 (Ni/MoO2−x) is designed which contains a large number of oxygen vacancies and Ni–O–Mo bonds. According to X-ray photoelectron spectroscopy, the formation of oxygen vacancies and Ni–O–Mo bonds facilitates the transfer of electrons from Ni to O. The prepared Ni/MoO2−x@NF shows excellent hydrogen electrocatalytic activity, which is proved by a Tafel slope of only 26 mV dec−1 for HER and an exchange current density of 3.64 mA cm−2 for HOR. Theoretical calculations showed optimized free energy of hydrogen adsorption and hydroxide adsorption on Ni/MoO2−x, which can explain the origin of the high activity of the catalyst. This work provides a feasible strategy for designing non-precious metal hydrogen electrocatalytic materials in energy storage.

Synergistic effect of MoO3/MoS2 in improving the electrocatalytic performance of counter electrode for enhanced efficiency in dye-sensitized solar cells

Engineering non-Pt-based counter electrode is an essential step toward the commercialization of DSSC. Here, we have employed MoO3 rods as a counter electrode and improved its catalytic performance towards the reduction of 3I− to I3− by embedding MoS2 on it. This combination of nanostructures composed of MoO3 rods/MoS2 sheets possessed increased surface area for electrolyte interaction by roughly 3-fold and displayed an enhanced catalytic activity. MoS2's presence lowered the charge transport resistance of MoO3 by eleven folds. Both the limiting and exchange current density were increased roughly 6 –fold. A maximum power conversion efficiency of 3.9% was achieved by M/MS2 CE, which is 2-fold higher than the efficiency obtained using MoO3 CE. This work ascertains MoO3/MoS2 counter electrode as a potential candidate for a non-Pt-based CE and thereby helps reduce the cost of DSSCs to promote commercialization.

Effect of Current Density on Preparation and Properties of TF/β-PbO2 in MSA Media.

The lead-based alloy and DSA anodes have drawbacks, such as poor corrosion resistance, easy peeling of coating, low electrocatalytic activity, and environmental pollution in electrode preparation processes. In this study, titanium foam/β-PbO2 (TF/β-PbO2) was prepared by electrodeposition in methanesulfonic acid (MSA) media. The current efficiency and the deposition rate were 89.7% and 5.36 v/(μm·min-1) at the best current density of 80 mA·cm-2, respectively. The TF/β-PbO2 was characterized by electrochemical tests, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The cyclic voltammetry (CV) test shows that the anodic peak potential of the optimum TF/β-PbO2 was as low as 2.135 V and anodic voltammetry charge was up to the maximum value of 3.564 × 10-2 C. The linear sweep voltammetry (LSV) test indicates that exchange current density of the optimum TF/β-PbO2 reached the maximum value of 8.284 × 10-6 A·cm-2. CV and LSV tests indicate that the optimum TF/β-PbO2 had a high electrocatalytic activity. Electrochemical impedance spectroscopy test Tafel polarization curves show that the optimum TF/β-PbO2 had better corrosion resistance. The XRD test shows that the crystal was mainly β-PbO2 of the optimum TF/β-PbO2 surface and the current density affected the preferential growth of the crystal surface of PbO2. SEM tests show that grains of the optimum TF/β-PbO2 coating prepared were tightly bound and uniform in size.

Microelectrode voltammetric analysis of samarium ions in LiCl–KCl eutectic molten salt

A microelectrode is useful to determine electrochemical parameters such as diffusion coefficient, exchange current density, and Tafel slope without the limitations of ion diffusion. This study aims to develop a new microelectrode for use in high-temperature molten salts, which has been evaluated by determining the parameters of the Sm(III)/Sm(II) reaction in a LiCl–KCl eutectic molten salt. The electrochemical and kinetic properties of the samarium ion in the temperature range 723–823 K were calculated using the limiting current from cyclic voltammograms and the Allen–Hickling equation. The results imply that kinetic parameters (exchange current density and charge transfer coefficient) critical for designing pyroprocesses can be obtained using the developed microelectrode.

Exciton Manipulation for Enhancing Photoelectrochemical Hydrogen Evolution Reaction in Wrinkled 2D Heterostructures.

2D materials' junctions have demonstrated capabilities as metal-free alternatives for the hydrogen evolution reaction (HER). To date, the HER has been limited to heterojunctions of different compositions or band structures. Here, the potential of local strain modulation based on wrinkled 2D heterostructures is demonstrated, whichhelpsto realize photoelectrocatalytically active junctions. By forming regions of high and low tensile strain in wrinkled WS2 monolayers, local modification of their band structure and internal electric field due to piezoelectricity is realized in the lateral direction. This structure produces efficient electron-hole pair generation due to light trapping and exciton funneling toward the crest of the WS2 wrinkles and enhances exciton separation. Additionally, the formation of wrinkles induces an air gap in-between the 2D layer and substrate, which reduces the interfacial scattering effect and consequently improves the charge-carrier mobility. A detailed study of the strain-dependence of the photocatalytic HER process demonstratesa 2-fold decrease in the Tafel slope and a 30-fold enhancement in exchange current density. Finally, optimization of the light absorption through functionalization with quantum dots produces unprecedented photoelectrocatalytic performance and provides a route toward the scalable formation of strain-modulated WS2 nanojunctions for future green energy generation.

Electrochemical determination of vitamin B6 in pharmaceutical and energy drink samples

A simple and low-cost electrochemical sensor based on poly(phenylalanine) and function­nalized multi-walled carbon nanotubes (F-MWCNTs) modified glassy carbon electrode (GCE) was developed for the determination of vitamin B6 (VB6). The surface morphology of modified glassy carbon electrodes was investigated with scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The electrocatalytic activities of the bare and modified electrodes were investigated in the presence of ferri-ferrocyanide redox couple using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The exchange current density (jo = 2462 µA cm-2) and electron transfer rate constant (ko = 0.002 cm s−1) were calculated using 5 mM K3[Fe(CN)6]. The electro­chemical activity of poly(phenylalanine)/F-MWCNT/GCE towards VB6 oxidation was investigated using CV. Parameters including the number of electrons transferred (n = 2), number of protons transferred (H+ = 2), electron transfer coefficient (α = 0.51) and surface concentration of VB6 (G = 0.24 nmol cm−2) were calculated. At the optimal experimental conditions, the oxidation peak current of VB6 measured by square wave voltammetry (SWV) was found proportional to its concentration in two linear ranges of 0.5 to 20 µM and 20 to 200 µM with a low detection limit (LOD) of 0.038 µM and limit of quantification (LOQ) of 0.125 µM. Finally, the sensor was successfully used to determine VB6 in soft drink and pharmaceutical formulation samples.

Electrochemical separation of Gadolinium from variable valence europium in molten LiCl-KCl via liquid LBE alloy electrode

The highly effective separation of two adjacent rare earth is significant to the rare earth and nuclear industries. This paper investigates the highly effective separation of Gd over Eu using liquid lead–bismuth eutectic (LBE) alloy cathode in molten LiCl-KCl-EuCl3-GdCl3 by pulse potential electrolysis (PPE). Underpotential deposition of Eu and Gd is observed on the LBE alloy, with a depolarization value far greater than that of the liquid Pb electrode. Furthermore, the higher exchange current density and the lower activation energy of the electrode reaction indicate that the LBE alloy cathode has higher kinetic performance than the liquid Pb. Then, the practical electro-separation in LiCl-KCl-EuCl3-GdCl3 systems is carried out by applying different electrolysis technology. The effects of varying electrolysis methods and parameters on separation factor (SF), extraction efficiency, cathode product morphology, and crystal size are studied and compared. The results show that both the extraction efficiency and the separation factor obtained by the PPE are the largest. The calculated extraction efficiency of Gd and the actual separation factor SFGd/Eu of Gd/Eu could be up to 99.1 % and 759, respectively.