Effects of Nafion® Dehydration in PEM Fuel Cells

The current dependence of the ohmic resistance of Nafion membranes was examined with different types of humidification: cathodic (ChAd), anodic (CdAh), anodic and cathodic (ChAh) and no humidification at all (CdAd). Data show that for stacks with humidified cathodes (ChAd and ChAh), the resistance is small and relatively insensitive to the presence of the anodic humidification. On the contrary, for stacks with non-humidified cathodes (CdAh and CdAd), the membrane resistance is high and strongly dependent on current and anodic humidification. The kinetics of membrane dehydration was examined by recording the galvanostatic transients of the stack voltage and resistance, after removing the humidification. It was found that the changes in the ohmic resistance ΔR _Ω(t), although significant, cannot explain entirely the observed decay of the stack voltage. To account for the difference, an additional resistive term is introduced ΔR _p(t). Explicit equations were found for the time and current dependence of the two resistive terms ΔR _Ω(t) and ΔR _p(t) after humidification removal. A tentative explanation for the new resistive term was provided using electrochemical impedance spectroscopy (EIS). EIS data obtained at low overpotential show that dehydration of the Nafion present in the cathode catalytic layer results in an increase of the polarization resistance; the apparent deactivation of the cathode electrocatalyst appears to be due to a decrease of the electrochemically active surface area.     

Testing of PEM fuel cell performance by electrochemical impedance spectroscopy: Optimum condition for low relative humidification cathode

Electrochemical impedance spectroscopy (EIS) was used to investigate the influence of several parameters on the performance of PEMFC. The applied frequency was in the range of 50 mHz–10 kHz. The experiment was designed by using a 2 ^k factorial design to identify the effects of various parameters including cell voltage, flow rates of gaseous fuels and cell temperature at the saturated humidification in anode and 60% relative humidity cathode. The results indicated that the cell temperature, cell voltage and interactions of cell voltage, flow rate of H₂ and O₂ had a significant effect on the cell performance. In addition, the flow rate of O₂ had a strong effect on the ohmic resistance and the charge transfer resistance in the system. Models describing the relationship between previous parameters and ohmic resistance, charge transfer resistance and capacitance were also developed.     

Kinetic behavior of LiFePO4/C cathode material for lithium-ion batteries

The composite cathode materials of LiFePO₄/C were synthesized by spray-drying and post-annealing method. The crystalline structure and morphology of products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The charge-discharge kinetics of LiFePO₄/C electrode was investigated using electrochemical impedance spectroscopy (EIS). The results show that the increment of the resistance has a close relation to the appearance of the FePO₄ phase during charge-discharge course, and that the ohmic resistance, charge transfer resistance and lithium-ion diffusion coefficients of the LiFePO₄/C electrode do not change much by extended cycling tests, implying a relatively superior cyclability of the battery. The effect of cell temperature on the electrochemical reaction behaviors of LiFePO₄/C electrode was also investigated using the EIS. It is confirmed that the effect of the cell temperature on the impedance results mainly from the enhancement of the lithium-ion diffusion at elevated temperatures.     

Using electrochemical impedance spectroscopy to predict the corrosion resistance of unexposed coated metal panels

The goal of the current work was to determine if electrochemical impedance spectroscopy (EIS) testing of a series of coated but unexposed metal panels could predict the corrosion results of other sections of the same coated panels that were subjected to both continuous and cyclic corrosion testing. Variables included metal, pretreatment, primer, and topcoat. EIS results were shown to be strongly dependent upon the time-of-residence in the electrochemical cell prior to commencement of testing, and to the choice of electrolyte used in the cell. Good correlations between EIS and corrosion testing were seen for topcoat effects, but not for pretreatment effects. EIS results appear to relate mostly to barrier properties rather than electrochemical properties of coatings. It is suggested that the variation seen in EIS solution resistance values (R_s) can be utilized to quantify total system error. Total error was estimated by three techniques: total R_s variation, panel replicate variation, and EIS reading replication. The three approaches yielded similar results: total error for equivalent circuit components expressed in log₁₀ form was on the order of 50%, expressed as percent standard deviation.     

Some environmentally friendly formulations as inhibitors for mild steel corrosion in sulfuric acid solution

The corrosion resistance of mild steel in 1 M H₂SO₄ solution was evaluated after addition of Sn²⁺ and Zn²⁺, N-acetylcystein (ACC) and S-benzylcystein (BzC) as a function of concentration (5–1000 μM) and solution temperature (35–50°C). Eight blends were also investigated. Both polarization resistance (R _p) and electrochemical impedance spectroscopy (EIS) were employed. For single additives, Zn²⁺ ions acted as accelerator for mild steel corrosion while the other additives showed good performance. The most effective additive was Sn²⁺. Adsorption of Sn²⁺, ACC and BzC obeyed the Temkin adsorption isotherm and had a very high negative value of free energy of adsorption (−ΔG°_ads). All blends provided good inhibition which increased with rise in temperature. Corrosion kinetic parameters such as activation energy (E _a) and the pre-exponential factor (λ) were calculated and discussed. EIS revealed that the interface of the uninhibited and inhibited systems can be represented by the simple equivalent circuit R _e(R _ct Q _dl).     

Role of some thiadiazole derivatives as inhibitors for the corrosion of C-steel in 1 M H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

Inhibition of C-steel corrosion by some thiadiazole derivatives (I–VI) in 1 M H₂SO₄ was investigated by weight loss, potentiodynamic polarization, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) techniques. The presence of these compounds in the solution decreases the double layer capacitance, increases the charge transfer resistance and increase of linear polarization. Polarization studies were carried out at room temperature, and showed that all the compounds studied are mixed type inhibitors with a slight predominance of cathodic character. The effect of temperature on corrosion inhibition has been studied and the thermodynamic activation and adsorption parameters were calculated and discussed. Electrochemical impedance was used to investigate the mechanism of corrosion inhibition. The adsorption of the compounds on C-steel was found to obey Langmuir’s adsorption isotherm. The synergistic effect brought about by combination of the inhibitors and KSCN, KI and KBr was examined and explained. The mechanism of inhibition process was discussed in the light of the chemical structure and quantum-chemical calculations of the investigated inhibitors.     

The effect of electrolyte flow rate and temperature on corrosion and protection of Al–2.5 Mg alloy by (+)-catechin

The effects of flow rate and temperature on the corrosion behaviour of the Al–2.5 Mg alloy in a 3% NaCl solution and the inhibiting efficiency of (+)-catechin on the corrosion of the same alloy have been examined. Measurements were carried out in a flow-through cell, at different flow rates (v ₁ = 0.0029 m s⁻¹, v ₂ = 0.0059 m s⁻¹ and v ₃ = 0.0118 m s⁻¹) and temperatures (20, 30, 40 °C). Electrochemical parameters for the Al–2.5 Mg alloy were determined by polarisation techniques and electrochemical impedance spectroscopy (EIS). Increased flow rate and temperature cause a stronger corrosion attack on the alloy. The addition of (+)-catechin inhibited corrosion at all temperatures and flow rates. The inhibitor efficiency decreased with increase in flow rate and temperature.     

Electrochemical properties of surface-confined films of single-walled carbon nanotubes functionalised with cobalt(II)tetra-aminophthalocyanine: Electrocatalysis of sulfhydryl degradation products of V-type nerve agents

This paper describes detailed comparative electrochemical and electrocatalytic behaviours of basal plane pyrolytic graphite electrodes (BPPGEs) modified with single-wall carbon nanotube (BPPGE-SWCNT) and SWCNTs functionalised with cobalt(II) tetra-aminophthalocyanine by physical (BPPGE-SWCNT-CoTAPc_(mix)), chemical (BPPGE-SWCNT-CoTAPc_(cov)) and electrochemical adsorption (BPPGE-SWCNT-CoTAPc_(ads)) processes. SWCNT improves both solution and surface electrochemistry of CoTAPc. Electrochemical kinetics of the SWCNT-CoTAPc modified BPPGE yielded different k_s values, indicative of different rate-determining steps for the cathodic and anodic reactions. Electrochemical impedance spectroscopy (EIS) analyses in the presence of [Fe(CN)₆]^3−/4− as a redox probe revealed that the SWCNT and SWCNT-CoTAPc_(mix) films have comparable data in terms of solution resistance (R_s), electron transfer resistance (R_et), Warburg impedance (Z_w) and electron-transfer rate constant (k_app). Also, these surface-confined films showed comparable electrocatalytic responses towards the detection of V-type nerve agent sulfhydryl hydrolysis products, dimethylaminoethanethiol (DMAET) and diethylaminoethanethiol (DEAET). Using the BPPGE-SWCNT-CoTAPc_(mix), the estimated catalytic rate constants and diffusion coefficients were higher for DEAET than for the DMAET. Also, the detection limits of approximately 8.0 and 3.0 μM for DMAET and DEAET were obtained with sensitivities of 5.0 and 6.0 × 10⁻² A M⁻¹ for DMAET and DEAET, respectively. BPPGE-SWCNT-CoTAPc showed better potential discrimination for detection of these sulfhydryl analytes than the BPPGE-SWCNT, the latter exhibited enhanced catalytic response for the sulfhydryls than the former.     

The internal resistance of a microbial fuel cell and its dependence on cell design and operating conditions

The internal resistance R_int of a mediator-less microbial fuel cell (MFC) has been determined as a function of cell voltage using electrochemical impedance spectroscopy (EIS) for a MFC with and without Shewanella oneidensis MR-1. The same tests were performed for a MFC containing small stainless steel (SS) balls in the anode compartment with a graphite feeder electrode as in a packed bed cell. It has been found that R_int decreased with decreasing cell voltage as the increasing current flow decreases the polarization resistance of the anode and the cathode. The ohmic components of R_int played a very minor role. In the presence of MR-1 R_int was lower by a factor of about 100 than R_int of the MFC with buffer and lactate as anolyte. R_int was also significantly lower for the anode containing SS balls with buffer and lactate as anolyte. For the MFC containing SS balls in the anode compartment no significant further decrease of R_int could be obtained when MR-1 was added to the anolyte since in this case the polarization resistance of the anode was lower than that of the cathode. Similar trends were observed in the cell voltage (V)-current (I) curves that were obtained using potentiodynamic sweeps and the power (P)-V curves that were calculated from the V-I curves.     

Electro-Catalytic Oxidation of Methanol on a Ni–Cu Alloy in Alkaline Medium

The electro-catalytic oxidation of methanol on a Ni–Cu alloy (NCA) with atomic ratio of 60/40 having previously undergone 50 potential sweep cycles in the range 0–600 mV vs. (Ag/AgCl) in 1 m NaOH was studied by cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS). The electro-oxidation was observed as large anodic peaks both in the anodic and early stages of the cathodic direction of potential sweep around 420 mV vs. (Ag/AgCl). The electro-catalytic surface was at least an order of magnitude superior to a pure nickel electrode for methanol oxidation. The diffusion coefficient and apparent rate constant of methanol oxidation were found to be 2.16 × 10⁻⁴ cm² s⁻¹ and 1979.01 cm³ mol⁻¹ s⁻¹, respectively. EIS studies were employed to unveil the charge transfer rate as well as the electrical characteristics of the catalytic surface. For the electrochemical oxidation of methanol at 5.0 m concentration, charge transfer resistance of nearly 111 Ω was obtained while the resistance of the electro-catalyst layer was ca. 329 Ω.     

Investigation of La0.8Sr0.2CoO3/Ce0.85Sm0.15O2-x cathode performance of solid oxide fuel cell by electrochemical impedance spectroscopy: Effect of firing temperature

Perovskite type complex oxide L_0.8Sr_0.2CoO_3-δ symmetrical cells were prepared on Samaria doped ceria electrolyte Ce_0.85Sm_0.15O_2-x by using the screen-printing method in a laboratory scale. The performance of the symmetrical cell was investigated by using electrochemical spectroscopy at frequency ranging from 0.1–300 kHz. Effect of firing temperature from 975–1,050 °C was investigated under the controlled oxygen pressure from 0.002–0.21 atm and controlled measuring temperature from 635–782 °C. The preliminary results indicated that, for all cells prepared at different firing temperatures, the SEM and XRD did not indicate any differences between them. By using EIS, however, two impedance arcs were obviously observed. This first arc was found at high frequency region (<1,000 Hz) and the second one was observed at low frequency region (>10 Hz). The high frequency arc corresponded to the impedance of electron-transfer and ion-transfer processes occurring at the current collector/electrode and electrode/electrolyte interfaces. The low frequency arc was the convoluted contribution of the diffusion processes (non-charge transfer processes). Changing firing temperature, measuring temperature and oxygen pressure leads to changing of symmetrical cell performances. The activation energy of these symmetrical cells was around 1.5–2.0 eV depending on the firing temperature and oxygen pressure.     

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.     

Evaluation of corrosion behaviour of Ti–25Mo alloy in chloride medium

The corrosion behaviour of Ti–25Mo alloy in 0.9 wt% NaCl was evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and compared with that of commercially pure titanium (CP-Ti). The corrosion behaviour of Ti–25Mo alloy has been reported for the first time in this paper. The microstructure and structural characteristics were also examined using optical microscopy and X-ray diffraction (XRD), respectively. The study reveals that Ti–25Mo alloy possesses a β-phase microstructure. The Ti–25Mo alloy exhibits higher passivation range, lower average passive current density (i_pass) and higher charge transfer resistance (R_ct) compared to that of CP-Ti. Based on the corrosion protection ability, Ti–25Mo alloy can be a suitable alternative material for orthopaedic implant applications.     

Nafion-TiO2 hybrid electrolytes for stable operation of PEM fuel cells at high temperature

The fabrication and characterization of Nafion-TiO₂ hybrid electrolytes for proton exchange membrane fuel cell (PEMFC) operating at high temperature are reported. A low temperature sol-gel synthesis, based on the formation of a sol from Ti-peroxy complex, was used to effectively incorporate hydrophilic anatase TiO₂ nanoparticles into the Nafion matrix. Fuel cell testing at temperatures up to 130 °C revealed that the hybrid membranes exhibit an increasing ohmic drop with increasing TiO₂ content incorporated into the polymer. However, at high temperatures and low relative humidity (RH) the performance of fuel cells using the hybrid electrolytes was found to surpass the one of Nafion. Electrochemical impedance spectroscopy (EIS) measurements suggest that enhancement of the fuel cell performance at high temperature and low RH is related to a reduced polarization resistance, indicating that the hybrid electrolytes contribute for a better water management of the system. In addition, it was found that the inorganic phase confers stability to the polymer, allowing for the operation at high temperature and reduced RH.     

Influence of toluene contamination at the hydrogen Pt/C anode in a proton exchange membrane fuel cell

For fuel cells run on hydrogen reformate, traces of hydrocarbon contaminants in the hydrogen gas may be a concern for the performance and lifetime of the fuel cell. This study focuses on the influence of low concentrations of toluene on the adsorption and deactivation chemistry in a proton exchange membrane (PEM) fuel cell. For this purpose cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques were employed. Results from adsorption and desorption (by oxidation or reduction) experiments performed in a humidified nitrogen or hydrogen flow in a fuel cell test cell with a mass spectrometer system connected to the outlet are presented. The influence of adsorption potential, temperature, and humidity are discussed. The results show that toluene adsorbs on the catalyst surface in a broad potential window, up to at least 0.85 V versus RHE at 80 °C. Adsorbed toluene oxidizes to CO₂ with peak potentials above 1.0 V for temperatures below 95 °C. Some desorption of toluene (or reduced products) may take place at potentials below 0 V. In a hydrogen flow, toluene contamination in per mille concentrations leads to a continuous growth of the charge transfer resistance, while a 10-fold dilution of the toluene concentration resulted in a low and constant charge transfer resistance even for longer exposures. This indicates that a competition between toluene and hydrogen may take place on the active platinum surface at the anode.     

操作条件对质子交换膜燃料电池电化学阻抗动态行为的影响

应用交流阻抗谱法（electrochemical impedance spectroscopy,EIS）研究温度、湿度和阴、阳极过量系数4种操作条件对质子交换膜燃料电池（proton exchange membrane fuel cell,PEMFC）电化学阻抗的影响,并应用复合阻容并联等效电路对实验结果进行等效拟合。实验结果表明,PEMFC单电池的电流密度为1400 mA/cm²时,阴极过量系数对PEMFC单电池高频阻抗的影响最大,温度和湿度次之,阳极过量系数影响最小;不同操作条件的改变对高频阻抗谱中的欧姆阻抗的影响非常小,主要通过影响阴阳极法拉第阻抗来影响PEMFC单电池的输出性能;等效结果和实验结果在不同频率段的阻抗表现出一致的变化规律,各阻抗的误差值能够控制在2mΩ以内,可以有效地等效替代实验结果。     

Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy

The most common methods used to characterize the electrochemical performance of fuel cells are to record current–voltage U(i) curves. However, separation of electrochemical and ohmic contributions to the U(i) characteristics requires additional experimental techniques. The application of electrochemical impedance spectra (EIS) is an approach to determine parameters which have proved to be indispensable for the development of fuel cell electrodes and membrane electrode assemblies (MEAs). This paper proves that it is possible to split the cell impedance into electrode impedances and electrolyte resistance by varying the operating conditions of the fuel cell (current load) and by simulation of the measured EIS with an equivalent circuit. Furthermore, integration in the current density domain of the individual impedance elements enables the calculation of the individual overpotentials in the fuel cell and the determination of the voltage loss fractions.     

电流中断法在线测定直接甲醇燃料电池的欧姆阻抗

Electrochemical impedance spectroscopy (EIS) is widely used in fuel cell impedance analysis. However, for ohmic resistance (RΩ), EIS has some disadvantages such as long test period and complex data analysis with equivalent circuits. Therefore, the current interruption method is explored to measure the value of RΩ in direct methanol fuel cells (DMFC) at different temperatures and current densities. It is found that RΩ decreases as temperature increase, and decreases initially and then increases as current density increases. These results are consistent with those measured by the EIS technique. In most cases, the ohmic resistances with current interruption (RiR) are larger than those with EIS (REIS), but the difference is small, in the range from –0.848% to 5.337%. The errors of RiR at high current densities are less than those of REIS. Our results show that the RiR data are reliable and easy to obtain in the measurement of ohmic resistance in DMFC.     

Corrosion behavior of ferritic stainless steel alloyed with different amounts of niobium in hydrochloric acid solution

The corrosion behavior of stainless steel alloys containing corrosion-resistant elements was investigated. Ferritic stainless steel (FSSs) electrodes were synthesized by applying a scan rate of 1 mV s⁻¹. Stainless steels were used unalloyed and alloyed with about 0.5, 1, and 3 wt% elemental Nb. The samples were obtained from casting and forging. The samples were classified into three groups. In the first group, samples were unhomogenized and remained in production condition. In the second and third groups, samples were exposed to homogenization at 1,100 °C for 30 min or 180 min, respectively, and then quenched. The corrosion performance of the FSSs was investigated in 0.3 M HCl acid solution using electrochemical impedance spectroscopy (EIS). Corrosion resistance was calculated using the Stearn–Geary equation. SEM investigations of samples immersed in 0.3 M HCl acid solution for 60 and 360 min were performed. SEM micrographs showed generalized pitting. Consequently, it was determined that niobium has a beneficial effect on the corrosion resistance of FSS since niobium reacts with carbon to form stable carbides.     

Effect of WC on electrodeposition and corrosion behaviour of chromium coatings

To study the effect of WC particles on corrosion behaviour of chromium coating steel samples were plated in Cr(VI) baths with various concentrations of WC. XPS, EPM and XRD were used to study the chemical composition, morphology and texture of the coatings. The corrosion behaviour was studied at different exposure times in solution containing 0.01 mol L^–1 H₂SO₄ + 0.5 mol L^–1 Na₂SO₄ using cyclic voltammetry and impedance spectroscopy. Cyclic polarization measurements suggest that WC particles slow down the processes of passive film dissolution and penetration of aggressive ions to the substrate. Electrochemical impedance spectroscopy (EIS) was used to reveal the details of the corrosion process at the solution/electrode interface. The simulation of EIS data with a proposed equivalent circuit model made it possible to obtain quantitative valuation of the Y₀ (Q_c), Y₀ (Q_s) and R_pore parameters, reflecting corrosion behaviour of samples at the solution/electrode interface. Samples plated in a Cr(VI) bath with WC provided better resistance to corrosion than those plated in a bath without WC. Analysis of the data obtained suggests that WC particles enhance corrosion resistance due to the microstructural features of the coatings.