Improved electrochemical impedance response induced by morphological and structural evolution in nanocrystalline MnO2 electrodes

The electrochemical cycling behavior of nanocrystalline MnO₂ coatings with a defective antifluorite structure, prepared by anodic electrodeposition, was investigated in terms of morphological and structural evolution. MnO₂ electrodes cycled at 100 mV s⁻¹ exhibited improved electrochemical frequency response, compared with as-prepared electrodes. The improved frequency response is attributed to morphological and structural evolution from equiaxed oxide nanocrystals to petal-shaped, single-crystal nanosheets, which follows a dissolution-redeposition mechanism.     

Indirect identification of Hads relaxation on different metals by electrochemical impedance spectroscopy

For the hydrogen evolution reaction, the coverage rate of the electrode surface by adsorbed hydrogen is generally difficult to evidence especially in presence of bubbles. In the present paper, a parallel competing reaction was incorporated to the interface by the addition of ferricyanide to a NaOH solution. This reaction was supposed to take place on the free electrode surface and allowed the relaxation of adsorbed hydrogen to be identified. Models taking into account hydrogen evolution, hydrogen absorption, and ferricyanide reduction were proposed. Three metals were tested: platinum, iron, and palladium, which absorb hydrogen in very different quantities. In these conditions, low frequency loops related to the H_ads surface coverage appeared in the electrochemical impedance diagrams, whose properties depend on the tested metal. Very good agreement was found between the experimental data and model predictions.     

Fabrication of porous Cu supported Ni-P/CeO2 composite coatings for enhanced hydrogen evolution reaction in alkaline solution

Cost-effective electrodes with high activity for hydrogen evolution reaction (HER) and durability are required to develop clean and renewable hydrogen energy. In this work, a porous Cu supported Ni-P/CeO₂ composite coating was fabricated by a facile electrodeposition technique. Owing to the contribution from the 3D porous Cu support and the incorporating agglomeration-free and uniformly distributed CeO₂ particles into the Ni-P matrix, the optimal composite coating (porous Cu supported Ni-P/CeO₂ (20 g L^-1)) exhibits outstanding electrocatalytic performance with small overpotentials (η) of 118 and 320 mV at a cathodic current density of 10 and 100 mA cm². Moreover, the composite electrode also presents excellent electrochemical stability in the alkaline solution. This work provides a feasible option to fabricate composite electrodes that may have desirable electrochemical properties for HER.     

Electrocatalytic activity of electrodeposited composite films of polypyrrole and CoFe2O4 nanoparticles towards oxygen reduction reaction

Electrochemical behaviour of sandwich-type composite electrodes of polypyrrole (PPy) and CoFe₂O₄ nanoparticles (Ox) were investigated in an aqueous solution of 0.5 M K₂SO₄ and 5mM KOH at 25 °C using electrochemical impedance (EI), cyclic voltammetry (CV) and Tafel polarization techniques. EI and CV studies indicated that the incorporation of oxide nanoparticles influenced the charge transfer and transport behaviours of the polymer matrix greatly. The bulk electrical resistances of pure polymer (4.5 ± 1.7 Ω) as well as composite (2.7 ± 0.8 Ω) electrodes were practically constant in the potential region, +0.1 to −0.7 V. The latter electrode showed a good electrocatalytic activity towards the oxygen reduction reaction (ORR).     

Characterization by electrochemical impedance spectroscopy of magnetite nanoparticles supported on carbon paste electrode

Magnetite nanoparticles were supported on carbon paste electrode and characterized by low scan rate voltammetry and electrochemical impedance spectroscopy (EIS) to obtain mechanistic information related to its oxidation and reduction in acid media.The voltammograms showed only one reduction and one oxidation peak for the supported magnetite, which were attributed to formation of ferrous ion and ferric oxide, respectively. Both peaks are fairly wide, indicating complex mechanisms.Using EIS, a mechanism showing up to three time constants, capacitive all of them, was evidenced, both in anodic and cathodic domain. These were attributed to charge transfer at the highest frequencies, adsorption of generated species at intermediate frequencies, and proton adsorption at low frequencies. Discussion about the nature of the adsorbed species and the concerned mechanism for each domain is developed.     

First insights into the borohydride oxidation reaction mechanism on gold by electrochemical impedance spectroscopy

Direct borohydride fuel cells (DBFC) exhibit some potential regarding the powering of small portable electronic devices, thanks to their high energy density as well as the facile and safe storage of borohydride salts. However, DBFC are hindered because (i) the borohydride oxidation reaction (BOR) is complex, (ii) its mechanism imperfectly determined yet and (iii) no practical electrocatalyst exhibits both fast BOR kinetics and high faradaic efficiency. In this context, we characterized the BOR mechanism for polycrystalline bulk gold (a classical model BOR electrocatalyst) in the rotating disk electrode (RDE) setup. Modeling cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data, we propose a simplified reaction pathway, the theoretical behavior of which agrees with the experimental findings. This pathway includes at least a first irreversible electrochemical step (E) for BH₄⁻ oxidation, which competes with the electrochemical adsorption reaction (EAR) of OH⁻ anions at high potentials.     

An electrochemical impedance study of hydrogen evolution and absorption on Sn-modified steel electrodes

In this work we report results related to hydrogen evolution and absorption, on AISI 1045 steel and AISI 1045 steel modified by Sn electrodeposition, in 1 M NaOH. The simulation of experimental impedance, steady-state j/E data and electrochemical permeation data in terms of the Volmer–Heyrovsky mechanism coupled to H absorption and diffusion, allowed the estimation of kinetic and transport parameters leading to an appropriate interpretation of diminished H absorption on Sn modified samples.     

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/H₂SO₄ 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.     

Evidence of redox interactions between polypyrrole and Fe3O4 in polypyrrole-Fe3O4 composite films

The electrical properties of conducting polymers make them useful materials in a wide number of technological applications. In the last decade, an important effect on the properties of the conducting polymer when iron oxides particles are incorporated into the conductive matrix was shown. In the present study, films of polypyrrole were synthesized in the presence of magnetite particles. The effect of the magnetite particles on the structure of the polymer matrix was determined using Raman spectroscopy. Mass variations at different concentrations of Fe₃O₄ incorporated into the conducting matrix were also measured by means of quartz crystal microbalance. Additionally, the changes in the resistance of the films were evaluated over time by electrochemical impedance spectroscopy in solid state. These results show that the magnetite incorporation decreases polymeric film resistance and Raman experiments have evidenced that the incorporation of magnetite into polymeric matrix not only stabilizes the polaronic form of the polypyrrole, but also preserves the polymer from further oxidation.     

Hydrogen adsorption and hydrogen evolution reaction on a polycrystalline Pt electrode studied by surface-enhanced infrared absorption spectroscopy

Hydrogen evolution reaction (HER) on a polycrystalline Pt electrode has been investigated in Ar-purged acids by surface-enhanced infrared absorption spectroscopy and electrochemical kinetic analysis (Tafel plot). A vibrational mode characteristic to H atom adsorbed at atop sites (terminal H) was observed at 2080-2095 cm⁻¹. This band appears at 0.1 V (RHE) and grows at more negative potentials in parallel to the increase in hydrogen evolution current. Good signal-to-noise ratio of the spectra enabled us to establish the quantitative relation between the band intensity (equivalently, coverage) of terminal H and the kinetics of HER, from which we conclude that terminal H atom is the reaction intermediate in HER and the recombination of two terminal H atoms is the rate-determining step. The quantitative analysis of the infrared data also revealed that the adsorption of terminal H follows the Frumkin isotherm with repulsive interaction.     

Preparation, electrochemical behavior and performance of gallium hexacyanoferrate as electrocatalyst of H2O2

The gallium hexacyanoferrate (GaHCF) was synthesized chemically and characterized by FTIR technique. Its electrochemical behavior was carefully investigated by fabricating a GaHCF modified carbon paste electrode in various supporting electrolyte. The experimental results showed that in KNO₃, K₂SO₄, KCl and other supporting electrolyte, GaHCF yielded one pair of ill-defined redox waves with a formal potential of 0.9 V (versus SCE). In 0.050 mol L⁻¹ phosphate buffer solution (PBS, pH 6.8), however, GaHCF yielded one pair of well-defined redox peaks with a formal potential of 0.222 V. Furthermore, this modified electrode exhibited a high electrocatalytic activity toward the reduction of H₂O₂ in pH 6.8 PBS, with over-potential dramatically lower than that of on the bare carbon paste electrode. Amperometry was used for the determination of H₂O₂, under the optimal conditions, a linear dependence of the catalytic current versus H₂O₂ concentration was obtained in the range of 4.9 × 10⁻⁶ to 4.0 × 10⁻⁴ mol L⁻¹ with a detection limit of 1 × 10⁻⁶ mol L⁻¹ when the signal-to-noise ratio was 3, and a sensitivity of 27.9 μA mM⁻¹ (correlation coefficient of 0.997). Chronoamperometry was used to conveniently determine the diffusion coefficient of H₂O₂ in the solution.     

Electrochemical behaviour of PANi/polyurethane antifouling coating in salt water studied by electrochemical impedance spectroscopy

Electrochemical behaviour of polyaniline–polyurethane (PANi–PU) antifouling coating in 3.5 wt% NaCl is studied by electrochemical impedance spectroscopy (EIS). A thick coating (∼1 mm) of 10, 15 and 20% PANi in marine grade PU, is cast over corrosion resistant aluminium alloy 2024 and its impedance characteristics are measured by EIS and compared with neat PU. On addition of 10% PANi, the impedance of the coating drastically comes down from 10⁹ to 10⁷ Ω. 20% is the maximum processable amount of PANi for the selected PU system. The coatings are exposed to 3.5 wt% NaCl and its impedance characteristics are monitored as a function of time. Changes in the impedance characteristics of the systems were found to occur as a function of the exposure time in all cases, though their evolution with time showed marked differences with PANi content. Water sorption and break down frequency are derived from the experimental results and analysed.     

Mechanistic study of the reduction of copper oxides in alkaline solutions by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy was used to study the mechanism by which copper oxides are reduced in alkaline solutions. For the reductions of CuO and Cu₂O, a capacitive loop and also an inductive loop under certain conditions were observed in the complex plane. The electrochemical impedance for CuO reduction was not greatly dependent on the solution alkalinity and the kind of alkali hydroxide. However, the electrochemical impedance for Cu₂O reduction was considerably affected by the kind and concentration of alkali hydroxide. The diameter of the capacitive loop, i.e., the charge-transfer resistance (R_ct), was increased with increase in solution alkalinity. It should also be noted that R_ct was increased in the order of KOH < NaOH < LiOH. These dependences were consistent with the good separation between the reduction potentials of CuO and Cu₂O in chronopotentiometric and voltammetric measurements with strongly alkaline electrolytes containing Li⁺. The inductive loop observed for the Cu₂O reduction at higher concentrations of KOH (>6 M) and LiOH (>0.2 M) suggested the existence of an intermediate species (probably CuOH). The specific inhibitory effect of Li⁺ ions on the reduction of Cu₂O might be explained by a possible stabilization of CuOH by Li⁺ ions.     

Corrosion inhibition of carbon steel in alkaline chloride media by Na3PO4

The decrease of the pH of the concrete due to the interaction with the CO₂ contained in the air creates an electrolyte in the pores with a pH < 8 that destroys the passive film. Also the presence of chloride ions at the rebar surface in alkaline media making susceptible to the aggression of the chlorides as pollutant in marine environment. The inhibiting efficiency of Na₃PO₄ against corrosion of carbon steel in alkaline medium simulating the interstitial solution of the concrete (pH 12.5) polluted with chloride ions were carried out. Two manners for adding the Na₃PO₄ were examined: one where the inhibitor was added into the pore solution also containing the aggressive solution, and other where the rebar was first pre-treated by immersion in the inhibitor solution. The protective effectiveness of these two procedures for the inhibitor addition was evaluated by means of several electrochemical techniques, EIS, polarisation curves and microbalance.The results allow concluding that:

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The pre-passivation procedure is more effective for the rebar protection.

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The impregnation duration of steel in phosphate solution and the electrode rotation speed influence considerably the phosphate-based film resistance against corrosion.

     

Effect of AB5 alloy on Ti0.10Zr0.15V0.35Cr0.10Ni0.30 hydrogen storage alloy

The structure and electrochemical characteristics of melted composite Ti_0.10Zr_0.15V_0.35Cr_0.10Ni_0.30 + x% LaNi₄Al_0.4Mn_0.3Co_0.3 (x = 0, 1, 5) hydrogen storage alloys have been investigated systematically. XRD shows that though the main phase of the matrix alloy remains unchanged after LaNi₄Al_0.4Mn_0.3Co_0.3 alloy is added, a new specimen is formed. The amount of the new specimen increases with increasing x. SEM-EDS analysis indicates that the V-based solid solution phase is mainly composed of V, Cr and Ni; C14 Laves phase is mainly composed of Ni, Zr and V; the new specimen containing La is mainly composed of Zr, V and Ni. The electrochemical measurements suggest that the activation performance, the low temperature discharge ability, the high rate discharge ability and the cyclic stability of composite alloy electrodes increase greatly with the growth of x. The HRD is controlled by the charge-transfer reaction of hydrogen and the hydrogen diffusion in the bulk of the alloy under the present experimental conditions.     

Oxygen-deficient MoOx/Ni3S2 heterostructure grown on nickel foam as efficient and durable self-supported electrocatalysts for hydrogen evolution reaction

High-performance and ultra-durable electrocatalysts are vital for hydrogen evolution reaction (HER) during water splitting. Herein, by one-pot solvothermal method, MoO_x/Ni₃S₂ spheres comprising Ni₃S₂ nanoparticles inside and oxygen-deficient amorphous MoO_x outside in situ grow on Ni foam (NF), to assembly the heterostructure composites of MoO_x/Ni₃S₂/NF. By adjusting volume ratio of the solvents of ethanol to water, the optimized MoO_x/Ni₃S₂/NF-11 exhibits the best HER performance, requiring an extremely low overpotential of 76 mV to achieve the current density of 10 mA∙cm^‒2 (η₁₀ = 76 mV) and an ultra-small Tafel slope of 46 mV∙dec^‒1 in 0.5 mol∙L^‒1 H₂SO₄. More importantly, the catalyst shows prominent high catalytic stability for HER (> 100 h). The acid-resistant MoO_x wraps the inside Ni₃S₂/NF to ensure the high stability of the catalyst under acidic conditions. Density functional theory calculations confirm that the existing oxygen vacancy and MoO_x/Ni₃S₂ heterostructure are both beneficial to the reduced Gibbs free energy of hydrogen adsorption (|∆G_H*|) over Mo sites, which act as main active sites. The heterostructure effectively decreases the formation energy of O vacancy, leading to surface reconstruction of the catalyst, further improving HER performance. The MoO_x/Ni₃S₂/NF is promising to serve as a highly effective and durable electrocatalyst toward HER.     

Electrochemical impedance spectroscopy of electrodes modified with thin films of MgMnCO3 layered double hydroxides

The electrochemical impedance spectra of MgMnCO₃ LDH films oxidized at different dc potentials were recorded. The results were fitted to a Randles type cell by replacing the Warburg impedance with a mass transfer resistance in parallel with a constant phase element. The films charge transfer resistances decreased dramatically at the onset of manganese oxidation. In thin films, R_ct decreased from 10⁴ Ω for an un-oxidized film to a minimum of 40 Ω in a film oxidized at 0.32 V, before increasing back to 10⁴ Ω in a film oxidized at 0.5 V. Iodometry measurements show these changes correspond to increases in the manganese average valence in the films from 3.09+ prior to oxidation, to 3.80+ at 0.32 V and 3.95+ at 0.5 V. In thicker films, however, a much higher dc potential, 1.0 V, was required to return R_ct to 10⁴ Ω. There was also less change in the manganese average valence in the thicker films. Oxidation at 1.0 V only increased the manganese valence to 3.33+. For the partially oxidized films, the Nyquist plots consisted of depressed semicircles at high frequency, followed by linear regions at lower frequency where the impedance was controlled by mass transport. The effective diffusion coefficient estimated from the low frequency impedance was 1 × 10⁻⁹ cm² s⁻¹, consistent with proton diffusion in solid electrodes. The impedance spectrum of a partially oxidized film reduced at −0.2 V was similar to that of the un-oxidized film.     

Impedance study of hydrogen evolution on Ni/Zn and Ni–Co/Zn stainless steel based electrodeposits

A study on the electrocatalytic performance of Ni/Zn and Ni–Co/Zn alloys for hydrogen evolution reaction (HER) in alkaline media (30 wt.% KOH solution) has been carried out. After preparing by electrodeposition on stainless steel supports, the alloys were leached of to remove part of the zinc and generate a porous layer. For the developed electrodes, the surface roughness factor, R_f, was evaluated by electrochemical impedance spectroscopy (EIS). The HER on these electrodes was evaluated by means of steady-state polarization curves and EIS. The obtained electrodes were characterized by large R_f for HER, and very low overpotentials at the current density of 250 mA cm⁻², η₂₅₀ ∼ 0.138 V at 30 °C. The high electrode activity was mainly attributed to the high surface area of the developed electrodes.     

Electrochemical impedance spectroscopy evidence of dimethyl-silicon-oil enhancing O2 transport in a porous electrode

Faster oxygen transport is critical to guarantee reliable power output of polymer electrolyte membrane fuel cells (PEMFCs). In order to enhance oxygen transfer in a porous electrode especially in the case of water flooding, water-proof oil (dimethyl-silicon-oil (DMS)) was introduced into the conventional Pt/C electrode. Owing to the capability of electrochemical impedance spectroscopy (EIS) in discriminating individual contribution of ohmic, kinetic, and mass transport from all PEMFC processes, EIS was carried out to evaluate the effect of the DMS on the oxygen reduction reaction (ORR). The equivalent circuits corresponding to the EIS spectra were employed. The parameters in the equivalent circuits were obtained by curve fitting to the EIS spectra with the aid of the frequency response analysis software (FRA) attached in the electrochemical station Autolab PGSYAT302. The EIS analysis has shown that the introduction of DMS reduces the oxygen diffusion resistance as well as the charge transfer resistance in the flooded state. The single cell tests show that even in the case of normal operating condition the accumulated water with PEMFC operation also worsens the oxygen transfer in the conventional Pt/C gas diffusion electrode (GDE) with more and more water produced at the cathode. GDE containing DMS, which is defined as a flooding tolerant electrode (FTE), is fortunately quite good at alleviating water flooding. Success of the FTE in alleviating water flooding is ascribed to (1) its high oxygen transfer flux due to the higher solubility of oxygen in DMS than in water as long as parts of pores are occupied beforehand by DMS rather than by water, and (2) enhanced hydrophobic property of the FTE with DMS adsorption on the walls of the pores, which makes more hydrophobic pores be open to oxygen transport.     

Investigation of electrode composition of polymer fuel cells by electrochemical impedance spectroscopy

In order to optimize the electrode composition and performance of Polymer Fuel Cells and to reduce the production cost of membrane electrode assemblies (MEAs), different MEAs using different catalyst powders, carbon supported and unsupported catalysts with different proton conducting electrolyte powder (Nafion) content were produced by using a dry powder spraying technique developed at German Aerospace Research Center (DLR, Deutsches Zentrum fuer Luft- und Raumfahrt). The electrochemical characterization was performed by recording current-voltage curves and electrochemical impedance spectra (EIS) in the galvanostatic mode of operation at 500 mA cm⁻². The evaluation of the measured impedance spectra with an adequate equivalent circuit shows that the cathode of the fuel cell is very sensitive to the electrode composition whereas the contribution of the anode is very small and invariant to the electrode composition. Furthermore, it could be shown for the first time using electrolyte powder in the electrodes that the charge transfer of the cathode decreasing monotonically with increasing electrolyte content in the cathode. These findings suggest that with increasing electrolyte content in the electrodes, in particular in the cathode, the utilization degree of the catalyst increasing linearly with increasing electrolyte content in the electrode.