Electrochemical performance of Ni/TiO2 hollow sphere in proton exchange membrane water electrolyzers system

This work presents the electrocatalytic evaluation of Ni/TiO₂ hollow sphere materials in PEM water electrolysis cell. All the electrocatalysts have shown remarkably enhanced electrocatalytic properties in comparison with their performance in aqueous electrolysis cell. According to cyclic voltammetric results, 0.36 A cm^?2 peak current density has been exhibited in hydrogen evolution reaction (HER) from 30 wt% Ni/TiO₂ electrocatalyst. 15 wt% Ni-doped titania sample has shown the best result in oxygen evolution reaction (OER) with the anodic peak current density of 0.3 A cm^?2. In the anodic polarization curves, the performance of 15 wt% Ni/TiO₂ hollow sphere electrocatalyst was evaluated up to 140 mA cm^?2 at comparatively lower over-potential value. 20 wt% Ni/TiO₂ hollow sphere electrocatalyst has also shown electrochemical stability in PEM water electrolyzer for 48 h long analysis. The comparative electrocatalytic behavior of hollow spherical materials with non-sphericals is also presented, which clearly shows the influence of hollow spherical structure in greater electrocatalytic activity of the materials. The physical characterization of all the hollow spherical materials is presented in this work, which has confirmed their better electrochemical behavior in PEM water electrolyzer.     

MnO2/MCMB electrocatalyst for all solid-state alkaline zinc-air cells

Nanostructured MnO₂/mesocarbon microbeads (MCMB) composite has been prepared successfully for use in zinc-air cell as electrocatalyst for oxygen reaction. The scanning electron microscope (SEM) images showed that the MnO₂ nanorods were formed and covered on the surface of MCMB in bird’s nest morphology. X-ray diffraction (XRD) pattern indicated that the MnO₂ has the hollandite structure with a composition approximating KMn₈O₁₆. By the cathodic polarization curve tests, the nanostructured material demonstrated excellent electrocatalytic activity as a kind of oxygen electrode electrocatalyst compared with electrolytic MnO₂. An all solid-state zinc-air cell has been fabricated with this material as electrocatalyst for oxygen electrode and potassium salt of cross-linked poly(acrylic acid) as an alkaline polymer gel electrolyte. The cell has good discharge characteristics at room temperature.     

Effect of MEA fabrication techniques on the cell performance of Pt–Pd/C electrocatalyst for oxygen reduction in PEM fuel cell

The effect of three different membrane electrode assembly (MEA) fabrication techniques, catalyst-coated substrate by direct spray (CCS) and catalyst-coated membrane by direct spray (CCM-DS) or decal transfer (CCM-DT), on the performance of oxygen reduction in a proton exchange membrane (PEM) fuel cell was carried out under identical conditions of Pt–Pd/C electrocatalyst loading. The results indicated that the fabrication technique had only a very slight effect on the ohmic resistance of the PEM fuel cell but it significantly affected the charge transfer resistance and open circuit voltage (OCV). The cells prepared by the CCM method, and particularly by decal transfer, exhibited a significantly higher OCV but a lower ohmic and charge transfer resistance compared with the other investigated fabrication techniques. By using cyclic voltammetry with H₂ adsorption, it was found that the electrochemical active area of the electrocatalyst prepared by CCM-DT was higher than those prepared by CCS and CCM-DS by around 1.76- and 1.05-fold, respectively. Under a H₂/O₂ system at 0.6 V, the cells with MEA made by CCM-DT provided the highest cell performance of around 350 mA/cm², significantly greater than those prepared by the CCS and CCM-DS (149 and 42 mA/cm², respectively).     

Studies to improve the Li/SO2Cl2 cell

The performance of the Li/SO₂Cl₂ primary cell was studied as a function of the type of the carbon cathode, cathode catalysts and electrolyte additives. Solutions of LiAICl₄ in S0₂ are more conductive than those in SO₂Cl₂ and, consequently, the electrolyte resistance in the Li/SO₂Cl₂ cell decreases during discharge as one mole of S0₂ is formed per mole of SO₂Cl₂. The replacement of the low surface area Chevron acetylene black carbon with the high surface area Ketjenblack carbon cathode increases cell capacity and reduces cathode polarization. The high surface area carbon, however, causes a deterioration in the storability of the cell, manifested as higher self-discharge rate. The use of carefully purified SO₂Cl₂, and of Ca²⁺ as an additive to the electrolyte, has been found to reduce the voltage delay of the cells stored at 70°C. The low temperature discharge capacity of the Li/SO₂Cl₂ cell can be increased with the addition of S0₂. Furthermore, the added S0₂ appeared to improve the storability of the cells. Cathodes doped with a catalyst consisting of a mixture of polyacrylonitrile and Co, Ni or Fe-salts, after heat treatment at 800° C, demonstrated 350–500 mV gain in the load voltage at 25 mA cm^–2. Catalysed cells stored at 70°C retained their higher cell voltage but, like the uncatalysed cells, showed a loss in capacity.     

MnO2 cathode in an aqueous Li2SO4 solution for battery applications

A manganese dioxide (MnO₂) cathode with zinc (Zn) as the anode has been investigated using lithium sulphate (Li₂SO₄) as an electrolyte. Previously we demonstrated that cells comprising MnO₂ and lithium hydroxide (LiOH) as an electrolyte can be made rechargeable to over one-electron capacity with a discharge capacity of 150 mAh g⁻¹. Here we have extended our work to assess Li₂SO₄ as an electrolyte and have found that the battery is not rechargeable. Based on the electrochemical (discharge/charge) performance and the products formed following discharge and charge, the mechanism proposed for the sulphate-based media is one of proton insertion into the MnO₂ cathode, rather than the lithium ion insertion observed for the LiOH electrolyte. The addition of bismuth species to the Li₂SO₄-based cell results in a transition to rechargeable behaviour. This is believed to be due to the influence of Bi ions on the formation of soluble Mn³⁺ soluble intermediates. However, the coulombic efficiency of the cell diminishes rapidly with repeated charge/discharge cycles. This confirms that the nature of the Li-containing electrolyte has a marked influence on the electrochemistry of the cell.     

Ionic Conductivity and Discharge Characteristic Studies of PVA-Mg(CH3COO)2 Solid Polymer Electrolytes

Ion conducting solid polymer electrolytes based on a polymer polyvinyl alcohol (PVA) complexed with magnesium acetate (Mg(CH₃COO)₂) were prepared by solution cast technique. Various experimental techniques, such as XRD, DSC, composition-dependent conductivity, temperature-dependent conductivity, and transport number measurements are used to characterize these polymer electrolyte films. The transference number data indicated the dominance of ion-type charge transport in these polymer electrolyte systems. An electrochemical cell with the configuration Mg/(80PVA + 20Mg(CH₃COO)₂)/(I₂ + C + electrolyte) has been fabricated and its discharge characteristics were studied. The Open Circuit Voltage (OCV) is 1.84 V.     

An investigation of the solid-state cell Cu/Rb4Cu16I7Cl13/CuxTiS2

The solid electrolyte cell Cu,Z/Z/Cu_xTiS₂,Z, graphite (Z=Rb₄Cu₁₆I₇Cl₁₃) was investigated in a search for a cell with high and stable open circuit voltage (OCV). The cells Cu/Cu_x Tis₂ (x=0 and 0.3–0.5) showed only a small reduction in OCV values over 200 days. The cause of the slight decrease of OCV was discussed from the viewpoint of degradation of the cell-component materials. Polarization curves and constant load discharge curves were obtained to indicate that the cathode polarization was predominant and that Cu_xTiS₂ of 0.3x0.5 performed well, as did TiS₂.     

Evaluation of concepts for hydrogen – chlorine fuel cells

Three different concepts for H₂–Cl₂ fuel cells have been evaluated. An ordinary PEM fuel cell based on a Nafion membrane, a fuel cell based on a combination of circulating hydrochloric acid and a Nafion membrane and a system based on a phosphoric acid doped Polybenzimidazole (PBI) membrane. None of the investigated systems were able to demonstrate stable operation under the conditions used in this study, due to electrocatalyst corrosion, membrane dehydration and/or electrode flooding. All systems studied achieved open circuit voltages close to the reversible thermodynamic value for production of aqueous hydrochloric acid, suggesting formation of dissolved HCl in the electrolyte and fast electrode kinetics.     

X-ray absorption and diffraction studies of La0.6Ca0.4CoO3 perovskite, a catalyst for bifunctional oxygen electrodes

The chemical stability of La_0.6Ca_0.4CoO₃ perovskite as an electrocatalyst in bifunctional oxygen electrodes has been studied by ex-situ and in-situ X-ray absorption spectroscopy and X-ray diffraction measurements. The catalyst was investigated ex-situ as a powder mixed with BN, and in-situ on carbon supports in bilayer air electrodes at different potentials in an electrochemical cell with an alkaline electrolyte. It was found to be stable for at least 1300 h in electrodes operated under the conditions of rechargeable Zn/air cells.     

Electrolytes for hybrid carbon-MnO2 electrochemical capacitors

Different aqueous-based electrolytes have been tested in order to improve the electrochemical performance of hybrid (asymmetric) carbon/MnO₂ electrochemical capacitor (EC). Chloride and bromide aqueous solutions lead to the formation of Cl₂ and Br₂ respectively upon oxidation of the corresponding salt, thus limiting the useful electrochemical window of the MnO₂ electrode and producing gas evolution (in the case of chloride salts) detrimental to the cycling ability of an hybrid device. For sulfate and nitrate salts, MnO₂ electrode exhibits a 20% increase in capacitance when lithium is used as the cation compared to sodium or potassium salts, probably due to partial lithium intercalation in the tunnels of α-MnO₂ structure. The higher ionic conductivity and solubility of LiNO₃ has led to the investigation of this electrolyte in carbon/MnO₂ supercapacitor compared to standard hybrid cell using K₂SO₄. A lower resistance increase was evidenced when the temperature was decreased down to −10 °C. Long term cycling ability of carbon/MnO₂ supercapacitor was also evidenced with 5 M LiNO₃ electrolyte.     

Electrochemical studies on solid state cells with mol % 70AgI-20Ag2O-10(0.8V2O5-0.2P2O5) amorphous electrolyte and organic cathodes

The highest conducting amorphous solid electrolyte composition (mol %) 70AgI-20Ag₂O-10(0.8V₂O₅-0.2P₂O₅) in the system AgI-Ag₂O-V₂O₅-P₂O₅ was investigated as an electrolyte material for solid state cells with organic cathodes by studying the electrochemical properties. The variation of open circuit voltage (OCV) with temperature, current discharge and the load characteristics were determined for the cells with iodine, tetramethyl ammonium iodide and tetrabutyl ammonium iodide as cathode materials. The cell capacities were estimated from the load characteristic curves and, in general, the cells were found to have very good stability at low current discharges even at high temperatures. In addition, it was found that the silver on the working electrode is electrochemically active and can be oxidized to Ag⁺ ions, making the organic cathode cells rechargeable. Thus these cells find potential use in rechargeable micropower sources and uninterrupted power supplies for microelectronic circuit devices.     

A highly conductive Ce0.8Nd0.2O1.9 electrolyte with double sintering aids for intermediate-temperature solid oxide fuel cells

In this paper, the influence of Bi/Zn mass ratio on the phase composition, microstructure, sintering properties, and electrical properties of Bi/Zn co-added Nd_0.2Ce_0.8O_1.9 (NDC) used for intermediate-temperature solid oxide fuel cells (SOFCs) was investigated. At 700 °C, the total conductivity of the NDC-based electrolyte (3Bi/1Zn-NDC) with the mass ratio 3:1 for Bi₂O₃ and ZnO was as high as 5.89 × 10^?2 S cm^?1, 4.60 and 4.51 times higher than the single addition of 4 wt% Bi₂O₃ and 4 wt% ZnO, respectively. In addition, the 3Bi/1Zn-NDC electrolyte exhibited a good physical and chemical compatibility with the electrode materials. The open circuit voltage (OCV) of the cell supported by the 3Bi/1Zn-NDC electrolyte was 0.67 V, and the output power density could reach 402.25 mW cm^?2 at 700 °C. It showed stable power output and OCV in the long-term stability test within 50 h. Overall, the combination of 3 wt% Bi₂O₃ and 1 wt% ZnO was a very effective dual sintering aid for NDC electrolyte.     

Preparation of highly dispersed SiO2 and Pt particles in Nafion112 for self-humidifying electrolyte membranes in fuel cells

We have developed preparation protocol of practically large size self-humidifying polymer electrolyte membranes (PEMs) with highly dispersed nanometer-sized Pt and/or SiO₂ for fuel cells. The Pt particles were expected to catalyze the recombination of H₂ and O₂, leading to a suppression of the chemical short-circuit reaction at the electrodes, while the SiO₂ particles were expected to adsorb the water produced at the Pt particles together with that produced at the cathode reaction. Stable SiO₂ particles were formed in a commercial PEM (Nafion^®112) via in situ sol-gel reactions at 70 °C. It was found by SAXS that the hydrophilic cluster size increased by water adsorbed SiO₂, which may contribute to the increase in the proton conductivity once SiO₂ adsorbed water. Pt particles were uniformly dispersed in a Na⁺-form normal-PEM or SiO₂-PEM by an ion-exchange reaction with [Pt(NH₃)₄]Cl₂, followed by a reduction with 1-pentanol at 125 °C. The newly prepared Pt-SiO₂-PEM was found to perform a self-humidifying operation in a standard-size PEFC (25 cm² electrode area) with H₂ and O₂ humidified at 30 °C. The performance of the Pt-SiO₂-PEM cell operated with the low humidity reactant gases was as high as the normal-PEM cell fully humidified, because the ohmic resistance of the former cell was as low as the latter cell.     

Determination of open-circuit potentials at gas/electrode/YSZ boundary versus molten carbonate reference electrode at medium temperatures: I. Potentials of Au and Pt in O2 and H2 + H2O atmospheres

A double gas concentration cell as combination of the cell with the yttria stabilized zirconia (YSZ) electrolyte and the cell with molten Li₂CO₃ + Na₂CO₃ eutectics is proposed as an alternative cell system with a standard reference electrode for measurements of the open-circuit potential (OCP) values of electrodes in oxygen concentration cell with the yttria stabilized zirconia (YSZ) electrolyte. In this double-cell one electrode is common for the two cells and the reference electrode is the standard molten carbonate half-cell with 0.33O₂ + 0.67CO₂ atmosphere. This reference electrode should enable the monitoring of OCP and overpotential values in polarization studies in the three-electrodes configuration. If the possible reaction between the solid YSZ and liquid molten carbonates electrolyte is very slow, the measured values of the open-circuit-voltage (OCV) of this cell may be considered equal to the respective reversible electromotive forces (EMF). Very good resistance of the smooth YSZ products to the corrosion in highly dehydrated Li/Na molten carbonates has been shown in experiments lasting few 1000 h. Hence, the consistency of OCV values with the respective EMF values have been tested at various partial pressures of CO₂ and O₂ in the gas mixtures above the molten carbonate electrolyte and at various partial pressures of O₂ + Ar or H₂ + H₂O gas mixtures at the Au or Pt electrodes/YSZ interface. The results have shown the reliability of the double-cell in determination of the open-circuit potentials (OCP) of gas electrodes at the YSZ surface as measured versus the reference electrode with molten carbonate electrolyte. The consistency of OCP and EMF values has been shown satisfying and enhances to use the proposed double-cell in further investigations of OCP and overpotential values at TPB of electrode/YSZ/mixture of reacting gases. At high differences of O₂ partial pressures on both sides of the YSZ membrane some permeation of this gas through the YSZ membrane has been observed. Probably, this effect has an electrochemical character.     

Electrical and stability performance of anode-supported solid oxide fuel cells with strontium- and magnesium-doped lanthanum gallate thin electrolyte

Anode-supported solid oxide fuel cells (SOFCs) comprising NiO-samarium-doped ceria (SDC) (Sm_0.2Ce_0.8O_1.9) composite anode, thin tri-layer electrolyte, and La_0.6Sr_0.4Co_0.8Fe_0.2O₃ (LSCF)-La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) composite cathode were fabricated. The thin tri-layer consisting of an 11-μm thick LSGM electrolyte layer and a 12-μm thick La_0.4Ce_0.6O_1.8 (LDC) layer on each side of the LSGM was prepared by centrifugal casting and co-firing technique. The performance of the cells operated with humidified H₂ as fuel and ambient air as oxidant showed a maximum power density of 1.23 W cm⁻² at 800 °C. A stability test of about 100 h was carried out and some deterioration of output power was observed, while the open circuit voltage (OCV) kept unchanged. Impedance measurements showed that both the electrolyte ohmic resistance and the electrode polarization increased with time and the latter dominated the degradation.     

O2 reduction at the Pt/Nafion interface in 85% concentrated H3PO4

The kinetics of oxygen electroreduction have been studied on a smooth platinum electrode coated with Nafion® in concentrated 85% H₃PO₄. The effects of Nafion® coatings of different thickness on O₂ electroreduction at a smooth Pt rotating disk electrode with 85% phosphoric acid as the bulk electrolyte were examined. The kinetic current increases with increasing Nafion® film thickness while the diffusion limiting current decreases with increasing Nafion® film thickness. A O₂ concentration profile model for the Pt/Nafion®/bulk electrolyte has been established, and this model can be used to explain the O₂ reduction polarization results. The performance of Nafion®-modified, high surface area Pt/carbon air cathodes for use in the H₂–air concentrated phosphoric acid fuel cell was also studied.     

Preparation and characterization of gel polymer electrolyte based on PVA-K2CO3

ABSTRACT

In this study, electrolyte materials were synthesized by mixing a highly conducting salt (K₂CO₃) with the poly(vinyl alcohol) (PVA) in different proportions (from 10 to 50 wt.%). The synthesized electrolyte was characterized using Fourier transform infrared (FTIR) spectroscopy, field-emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV) for their functional groups, morphology, thermal stability, glass transition temperature (T_g ), ionic conductivity, and potential window, respectively. Characterization results show that the complex formation between PVA and K₂CO₃ salt has been established by FTIR spectroscopic study, which indicates the detailed interaction between PVA and the salts in PVA-K₂CO₃ composites while the amorphous nature of the electrolyte after incorporation of the salts has been confirmed by FESEM analysis. Similarly, TGA and DSC analysis revealed that both decomposition temperature and T_g of the synthesized electrolytes decrease with the addition of K₂CO₃ due to the strong plasticizing effect of the salt. The results confirm that the electrolytes have sufficient thermal stability for supercapacitor operation, as well as an amorphous phase to effectively deliver high ionic conductivity. The highest ionic conductivity of 4.53 × 10^?3 S cm^?1 at 373 K and potential window of 2.7 V was exhibited by PK30 (30 wt.% K₂CO₃), which can be considered as high value for solid-state electrolytes which are superior to those electrolytes from PVA salts earlier reported. The results similarly show that the prepared electrolyte is temperature-dependent as conductivity increase with increase in temperature. Based on these properties, it can be imply that the PVA-K₂CO₃ gel polymer electrolyte (GPE) could be a promising electrolyte candidate for EDLC applications. The results indicate that the PVA-K₂CO₃ as a new electrolyte material has great potential in practical applications of portable energy-storage devices.     

Evaluation of electrolytes for redox flow battery applications

A number of redox systems have been investigated in this work with the aim of identifying electrolytes suitable for testing redox flow battery cell designs. The criteria for the selection of suitable systems were fast electrochemical kinetics and minimal cross-contamination of active electrolytes. Possible electrolyte systems were initially selected based on cyclic voltammetry data. Selected systems were then compared by charge/discharge experiments using a simple H-type cell. The all-vanadium electrolyte system has been developed as a commercial system and was used as the starting point in this study. The performance of the all-vanadium system was significantly better than an all-chromium system which has recently been reported. Some metal-organic and organic redox systems have been reported as possible systems for redox flow batteries, with cyclic voltammetry data suggesting that they could offer near reversible kinetics. However, Ru(acac)₃ in acetonitrile could only be charged efficiently to 9.5% of theoretical charge, after which irreversible side reactions occurred and [Fe(bpy)₃](ClO₄)₂ in acetonitrile was found to exhibit poor charge/discharge performance.     

Electrochemical reduction of CO2 to formate at high current density using gas diffusion electrodes

The electrochemical reduction of carbon dioxide into formate was studied using gas diffusion electrodes (GDE) with Sn as electrocatalyst in order to overcome mass transport limitations and to achieve high current densities. For this purpose, a dry pressing method was developed for GDE preparation and optimized with respect to mechanical stability and the performance in the reduction of CO₂. Using this approach, GDEs can be obtained with a high reproducibility in a very simple, fast, and straightforward manner. The influence of the metal loading on current density and product distribution was investigated. Furthermore, the effect of changing the electrolyte pH was evaluated. Under optimized conditions, the GDE allowed current densities up to 200 mA cm^?2 to be achieved with a Faradaic efficiency of around 90 % toward formate and a substantial suppression of hydrogen production (<3 %) at ambient pressure. At higher current densities mass transport issues come into effect and hydrogen is increasingly produced. The corresponding cathode potential was found to be 1.57 V vs. SHE.     

Semiconductor ionic based Sm0.2Ce0.8O2−δ-La0.6Sr0.4Fe0.8Cu0.2O3-δ heterostructure for low temperature ceramic fuel cells

Structural design/doping strategy is an efficient method to prepare electrolytes with high oxygen ionic conductivity, but there is still hindrance for solid oxide fuel cell (SOFC) commercialization. Recent advances in semiconductor ionic materials have developed a novel strategy in designing low-temperature electrolyte materials. Here, a heterostructure composite of LSFC (La_0.6Sr_0.4Fe_0.8Cu_0.2O_3-δ) and SDC (Sm_0.2Ce_0.8O_2?δ) is developed. The LSFC-SDC composite exhibits a high ionic conductivity, >0.1S/cm at 550 °C. With symmetrical NCAL (Ni_0.8Co_0.15Al_0.05LiO_2-δ)-coated electrode, cells with SDC-LSFC electrolyte exhibit high open-circuit voltage (OCV), and achieve a significant power improvement (>1000 mW/cm²) compared with pure SDC electrolyte at 550 °C. The short-term stability result has proven the operating ability of SDC-LSFC electrolyte under fuel cell environment (H₂/air). This work demonstrates a new developing route of low-temperature solid oxide fuel cell (LTSOFC), which is different from the conventional SOFC.