Nafion based amperometric hydrogen sensor

A Nafion based amperometric hydrogen sensor that operates at room temperature has been developed. The electrolyte used in the sensor is Nafion 117, which is a proton conducting solid polymer electrolyte. Palladium catalyst was used on the sensing side and platinum supported on carbon on the air side. The sensor functions as fuel cell, H₂/Pd//Nafion//Pt/O₂ and the short circuit current is measured. The short circuit current is found to be linear with respect to concentration of hydrogen on the sensing side. The sensor is able to detect the concentration of hydrogen in argon down to ppb level. Details of assembly of the sensor, response behavior and applications are discussed. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Three-electrode solid-state fuel cell for evaluating working electrodes

Development of a commercial solid-state fuel cell depends on identification of suitable catalytic electrodes to replace platinum. A three-electrode test cell for electrode evaluation is reported. The solid protonic electrolyte used was dodecamolybdophosphoric acid, H₃Mo₁₂PO₄₀·29H₂O, and a thin platinum wire inserted into the electrolyte served as the third electrode. Reproducibility and insensitivity to third-electrode position were demonstrated. The third electrode measures separately the anode and cathode interfacial resistances, thus providing a direct measure of the relative catalytic activity of a given test electrode. Application of the technique is illustrated.     

Proton conducting interpenetrating polymer networks

In order to improve stability and performance of the polymer electrolyte-based hydrogen sensor developed for on-line analysis, modifications to the PVA/H₃PO₄ proton conducting polymer blend were made. Water insoluble, increased bulk modulus, and higher conductivity polymer membranes have been fabricated by development of interpenetrating polymer networks that incorporate the PVA/H₃PO₄ (host) blend. The guest polymer is a three dimensional polymer network composed of methacrylic acid and methylenebisacrylamide. High protonic conductivity results from the phosphoric acid and water, the poly (methaacrylic acid-methylenebisacrylamide) contributes to the increased modulus water insolubility. Hydrogen sensors have been demonstrated using these membranes.     

Characterization of high ionic conducting PVAc–PMMA blend-based polymer electrolyte for electrochemical applications

A solid polymer blend electrolyte is prepared using poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) polymers with different molecular weight percentage (wt%) of ammonium thiocyanate (NH₄SCN) by solution casting technique with tetrahydrofuran (THF) as a solvent. The structural, morphological, vibrational, thermal and electrical properties of the prepared polymer blend electrolytes have been studied. The incorporation of NH₄SCN into the polymeric matrix causes decrease in the degree of crystallinity of the samples. The complex formation between the polymer and salt has been confirmed by FTIR technique. The increase in T _g with increase in salt concentration has been investigated. The maximum conductivity of 3.684?×?10^?3 S cm^?1 has been observed for the composition of 70PVAc/30PMMA/30 wt% of NH₄SCN at 303 K. This value of ionic conductivity is five orders of magnitude greater than that of 70PVAc/30PMMA polymer membrane. Dielectric and transport studies have been done. The highest conducting polymer electrolyte is used to fabricate proton battery with the configuration Zn/ZnSO₄·7H₂O (anode) ||polymer electrolyte||PbO₂/V₂O₅ (cathode). The open circuit voltage of the fabricated battery is 1.83 V, and its performance has been studied.     

Hydrogenase-based nanomaterials as anode electrode catalyst in polymer electrolyte fuel cells

We consider hydrogenase-based nanomaterials for possible use as anode electrode catalysts in polymer electrolyte fuel cells (PEFCs). We choose Fe-only hydrogenase component of Desulfovibrio desulfuricans (DdHase) as a hydrogenase complex, and investigate its catalytic activity for H₂ dissociation using ab initio calculations based on density functional theory (DFT). We found two possible H-H bond cleavage pathways, which are heterolytic and possess low activation barriers. Moreover, the H₂ dissociation can be promoted by inducing spin polarization of the H₂ adduct. We report that hydrogenase or hydrogenase-based nanomaterials can manipulate to exhibit the catalytic activity equivalent to the well-known platinum catalyst.     

New polymer electrolyte based on (PVA–PAN) blend for Li-ion battery applications

A polymer blend electrolyte based on polyvinyl alcohol (PVA) and polyacrylonitrile (PAN) was prepared by a simple solvent casting technique in different compositions. The ionic conductivity of polymer blend electrolytes was investigated by varying the PAN content in the PVA matrix. The ionic conductivity of polymer blend electrolyte increased with the increase of PAN content. The effect of lithium salt concentrations was also studied for the polymer blend electrolyte of high ionic conductivity system. A maximum ionic conductivity of 3.76×10⁻³ S/cm was obtained in 3 M LiClO₄ electrolyte solution. The effect of ionic conductivity of polymer blend electrolyte was measured by varying the temperature ranging from 298 to 353 K. Linear sweep voltammetry and DC polarization studies were carried out to find out the stability and lithium transference number of the polymer blend electrolyte. Finally, a prototype cell was assembled with graphite as anode, LiMn₂O₄ as cathode, and polymer blend electrolyte as the electrolyte as well as separator, which showed good compatibility and electrochemical stability up to 4.7 V.     

Perborate as novel fuel for enhanced performance of membraneless fuel cells

This paper presents the development of membraneless sodium perborate fuel cell using acid/alkaline electrolyte. In the acid/alkaline electrolyte, perborate works both as an oxidant as well as reductant. Sodium perborate affords hydrogen peroxide in aqueous medium. The cell converts the energy released by H₂O₂ decomposition with H⁺ and OH^? ions into electricity and produces water and oxygen. Such a novel design eliminates the need of a membrane, in which acid and alkaline electrolytes contact with each other. At room temperature, the laminar flow-based microfluidic membraneless fuel cell can reach a maximum power density of 34 mW/cm² with the molar ratio of [Perborate]/[NaOH]?=?1 as fuel and [Perborate]/[H₂SO₄]?=?2 as oxidant. The paper reports for the first time the use of sodium perborate as the oxidant and reductant. The developed fuel cell emits no CO₂, features no proton exchange membrane, inexpensive catalysts, and simple planar structure, which enables high design flexibility and easy integration of the microscale fuel cell into actual microfluidic systems and portable power applications.     

Hydrogen and oxygen generation with polymer electrolyte membrane (PEM)-based electrolytic technology

With dwindling liquid fuel resources, hydrogen offers a credible alternative. The use of hydrogen in a fuel cell offers the highest fuel conversion efficiency compared with all other technologies and it also has the potential to substantially reduce greenhouse gas and particulate emissions at least at the end-user sites. One of the major barriers to the introduction of the hydrogen economy and its wider acceptance is the lack of the rather costly hydrogen generation, transportation and distribution infrastructure to meet the local transport fuel demands. On-site or distributed hydrogen generation would remove the need for this up-front infrastructure requirements and assist with the early large-scale trials of the fuel cell technology for both transport and stationary applications and also introduction of the hydrogen economy. In this paper, the development of polymer electrolyte membrane electrolysis technology for on-site, on-demand hydrogen generation has been discussed. The major emphasis is given on reducing catalyst cost; interface design and modifications; interconnect materials, design and fabrication; and investigation of the sources of degradation. Stacks to 2 kW_{H 2} capacity have been constructed and tested and show initial efficiencies of >87% at 1 A cm⁻².     

Electrochemical properties of sputtered iridium oxide films

Iridium oxide films previously used in association with WO₃ and a proton conducting electrolyte in electrochromic devices, have been directly sputtered on both sides of an anhydrous membrane of a new proton conducting polymer electrolyte (nylon 6–10, 2H₃PO₄). Charge-discharge experiments show that this symmetric cell works reversibly in the “rocking chair” mode owing to the presence of roughly equal amounts of Ir³⁺ and Ir⁴⁺ in the as-deposited films. Impedance spectroscopy has been applied to the bare and sputtered polymer in order to characterize the main elements of the equivalent circuits for the two systems. Paper presented at the 2nd Euroconference in Funchal, Madeira, Portugal. Sept. 10–16, 1995     

Quantifying phosphoric acid in high‐temperature polymer electrolyte fuel cell components by X‐ray tomographic microscopy

Synchrotron‐based X‐ray tomographic microscopy is investigated for imaging the local distribution and concentration of phosphoric acid in high‐temperature polymer electrolyte fuel cells. Phosphoric acid fills the pores of the macro‐ and microporous fuel cell components. Its concentration in the fuel cell varies over a wide range (40–100 wt% H₃PO₄). This renders the quantification and concentration determination challenging. The problem is solved by using propagation‐based phase contrast imaging and a referencing method. Fuel cell components with known acid concentrations were used to correlate greyscale values and acid concentrations. Thus calibration curves were established for the gas diffusion layer, catalyst layer and membrane in a non‐operating fuel cell. The non‐destructive imaging methodology was verified by comparing image‐based values for acid content and concentration in the gas diffusion layer with those from chemical analysis.     

Influence of methanol impurity in hydrogen on PEMFC performance

Proton exchange membrane fuel cells [PEMFC] have become highly attractive for stationary as well as mobile energy applications due to their good efficiency compact cell design and zero emissions. PEM fuel cells mainly consist of anode and cathode containing platinum/platinum alloy electrocatalysts and Nafion membrane as the electrolyte. They operate on hydrogen fuel, which is generally produced by reforming of hydrocarbons, alcohols such as methanol and may contain large amounts of impurities such as methanol, carbon dioxide, trace amounts of carbon monoxide, etc. The studies on the effect of methanol impurity in hydrogen on fuel cell performance and methods of mitigation of poisoning are very important for the commercialization of fuel cells and are described in a limited number of papers only. In this paper, we present the studies on the influence of methanol impurity in hydrogen for the PEM fuel cells. The effect of various parameters such as methanol concentration, cell voltage, current density, exposure time, reversibility, operating temperature, etc. on the cell performances was investigated using pure hydrogen. Various methods of methanol poisoning mitigation were also investigated.     

Fabrication and characterization of solid state proton conductor (NH4)2SiP4O13–NH4PO3 for fuel cells operated at 150–250 °C

NH₄PO₃–(NH₄)₂SiP₄O₁₃ composite, a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C, was synthesized with a solid-state reaction method. Electromotive forces (emfs) as measured with hydrogen concentration cells showed that the composite was a pure proton conductor at hydrogen partial pressure from 10² to 10⁵ Pa. Its proton transference numbers were determined to be 1.0 at 150 °C, 0.99 at 200 °C, and 0.99 at 250 °C. Fuel cells that used NH₄PO₃–(NH₄)₂SiP₄O₁₃ as electrolytes were also fabricated. Maximum power density was 6.6 mW/cm² at 250 °C when dry hydrogen and dry oxygen were used as the fuel and oxidant, respectively. Improved cell performance is expected by increasing cathode activity, increasing the electrolyte density, and decreasing the electrolyte thickness.     

Performance of palladium electrode for electrochemical hydrogen pump using strontium-zirconate-based proton conductors

An electrode design with no use of three-phase boundary was investigated using palladium electrode. The hydrogen evolution rate of the palladium electrode cell using SrZr_0.9Y_0.1O_3 − α electrolyte followed Faraday’s law up to 180 mA cm⁻², and the anode and cathode overpotentials were significantly lower than those of a platinum electrode cell, suggesting that the palladium electrode is effective to improve the performance of the hydrogen-pumping cell using SrZrO₃-based electrolyte. The rate-determining step (RDS) for electrode reaction was also investigated by changing the electrode morphology and hydrogen partial pressure, and it was suggested that the RDS of the anode is a reaction at electrode/electrolyte interface.     

Novel synthesis and characterization of nanocomposite Pt-WO3-TiO2/C electrocatalyst for PEMFC

In this study, an effective preparation of Pt-WO₃-TiO₂/C electrocatalysts has been developed for polymer electrolyte membrane fuel cell (PEMFC) application. The single cell performance of Vulcan XC-72R carbon-supported Pt-WO₃-TiO₂ electrocatalysts with various compositions (as weight percentage Pt-W-Ti 0:5:5, 2:4:4, 4:3:3, 6:2:2, and 8:1:1) as anode materials are investigated in PEMFC. These catalysts are compared with 10 % Pt/C on the same Vulcan XC-72R carbon support and 10 % Pt/C (commercial) electrocatalyst. The physical and morphological characterization of the optimized Pt-WO₃-TiO₂/C, 10 % Pt/C, and 10 % Pt/C (commercial) electrocatalysts are further investigated by X-ray diffraction (XRD), cyclic voltammetry, scanning electron microscopy with energy-dispersive X-ray analysis, and transmission electron microscopy (TEM) techniques. Among all the molar ratio of the catalysts, the Pt-W-Ti (4:3:3) molar ratio catalyst exhibited the larger electrochemical active surface area. The electrochemical performance of Pt-WO₃-TiO₂/C (with a weight percentage of Pt-W-Ti 4:3:3) as anode material is better than those of other compositions of Pt-WO₃-TiO₂/C catalysts. The amount of platinum was also reduced from 1.76 to 0.704 mg cm^?2 which exhibited higher performance in single cell tests. Platinum shows a smaller-sized crystalline structure in XRD and TEM analysis. High performance indicates that enhanced proton transport occurs through the use of this catalyst.     

Fuel quality and operational issues for polymer electrolyte membrane (PEM) fuel cells

Polymer electrolyte membrane (PEM) fuel cells are susceptible to degradation due to the catalyst poisoning caused by CO present in the fuel above certain limits. Although the amount of CO in the fuel may be within the permissible limit, the fuel composition (% CO₂, CH₄, CO and H₂O) and the operating conditions of the cell (level of gas humidification, cell temperature and pressure) can be such that the equilibrium CO content inside the cell may exceed the permissible limit leading to a degradation of the fuel cell performance. In this study, 50 cm² active area PEM fuel cells were operated at 55–60 °C for periods up to 250 hours to study the effect of methane, carbon dioxide and water in the hydrogen fuel mix on the cell performance (stability of voltage and power output). Furthermore, the stability of fuel cells was also studied during operation of cells in a cyclic dead end / flow through configuration, both with and without the presence of carbon dioxide in the hydrogen stream. The presence of methane up to 10% in the hydrogen stream showed a negligible degradation in the cell performance. The presence of carbon dioxide in the hydrogen stream even at 1–2% level was found to degrade the cell performance. However, this degradation was found to disappear by bleeding only about 0.2% oxygen into the fuel stream.     

Measurement of electrode overpotentials for direct hydrocarbon conversion fuel cells

Cathodic and anodic overpotentials were measured using current interruption and AC impedance spectroscopy for two separate solid oxide fuel cells (SOFCs). The fuel cells used yttria-stabilized zirconia (YSZ) as the electrolyte, strontium-doped lanthanum manganite (LSM) as the cathode, and a porous YSZ layer impregnated with copper and ceria as the anode. The Cu/CeO₂/YSZ anode is active for the direct conversion of hydrocarbon fuels. Overpotentials measured using both current interruption and impedance spectroscopy for the fuel cell operating at 700 °C on both hydrogen and n-butane fuels are reported. In addition to providing the first electrode overpotential measurements for direct conversion fuel cells with Cu-based anodes, the results demonstrate that there may be significant uncertainties in measurements of electrode overpotentials for systems where there is a large difference between the characteristic frequencies of the anode and cathode processes and/or complex electrode kinetics.     

Microstructure and performance of novel Ni anode for hollow fibre solid oxide fuel cells

Nickel anodes were deposited on hollow fibre yttria-stabilised zirconia (YSZ) electrolyte substrates for use in solid oxide fuel cells (SOFCs). The hollow fibres are characterised by porous external and internal surfaces supported by a central gas-tight layer (300 μm total wall thickness and 1.6 mm external diameter). The YSZ hollow fibres were prepared by a phase inversion technique followed by high temperature sintering in the range 1200 to 1400 °C. Ni anodes were deposited on the internal surface by electroless plating involving an initial catalyst deposition step with PdCl₂ followed by Ni plating (with a NiSO₄, NaH₂PO₂ and sodium succinate based solution at 70 °C). Fabrication and nickel deposition parameters (nature of solvents, air gap, temperature, electroless bath composition) and heat treatments in oxidising/reducing environments were investigated in order to improve anode and electrolyte microstructure and fuel cell performance. A parallel study of the effect of YSZ sintering temperature, which influences electrolyte porosity, on electrolyte/anode microstructure was performed on mainly dense discs (2.3 mm thick and 15 mm diameter). Complete cells were tested with both disc and hollow fibre design after a La_0.2Sr_0.8Co_0.8Fe_0.2O_3?δ (LSCF) cathode was deposited by slurry coating and co-fired at 1200 °C. Anodes prepared by Ni electroless plating on YSZ electrolytes (discs and hollow fibres) sintered at lower temperature (1000–1200 °C) benefited from a greater Ni penetration compared to electrolytes sintered at 1400 °C. Further increases in anode porosity and performance were achieved by anode oxidation in air at 1200–1400 °C, followed by reduction in H₂ at 800 °C.     

Fabrication of solid-state fuel cell based on DNA film

We have fabricated a fuel cell based on the DNA film (DNAFC) and examined its properties under various humidity conditions at room temperature. The open-circuit voltage of a DNAFC is generated by supplying H₂ gas to the anode. The open-circuit voltage strongly depends on the humidity conditions, and in a DNA film, the optimum condition in which the open-circuit voltage attains a value as high as 0.55 V is achieved under the relative humidity condition of 55%. Furthermore, the cell voltage of the DNAFC decreases with an increase in current density, as observed in fuel cells such as proton exchange membrane fuel cell, solid oxide fuel cell, and several others. These results indicate that DNA film can be used as the fuel cell electrolyte under approximately 55% humidity condition.     

Reactive sputtering deposition and electrochemical characterisation of thin solid electrolyte membranes for solid oxide fuel cells

Solid oxide fuel cells directly convert the chemical energy of a fuel into electricity. To enhance the efficiency of the fuel cells, the thickness of the gastight solid electrolyte membranes should be as thin as possible. Y₂O₃-stabilised ZrO₂ (YSZ) electrolyte films were prepared by reactive sputtering deposition using Zr/Y targets in Ar/O₂ atmospheres. The films were 5 – 8 μm thin and were deposited onto anode substrates made of a NiO/YSZ composite. After deposition of a cathode with the composition La_0.65Sr_0.35MnO₃ the electrochemical properties of such a fuel cell were tested under operating conditions at temperatures between 600 °C and 850 °C. Current-voltage curves were recorded and impedance measurements were performed to calculate apparent activation energies from the fitted resistance data. The conductivity of the YSZ films varied between 4.6·10⁻⁶ S/cm and 2.2·10⁻⁵ S/cm at 400 °C and the fuel cell gave a reasonable power density of 0.4 W/cm² at 0.7 V and 790 °C using H₂ with 3 % H₂O as fuel gas. The gas compositions were varied to distinguish the electrochemical processes of the anode and cathode in the impedance spectra. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Effect of nitrogen and carbon dioxide as fuel impurities on PEM fuel cell performances

Polymer electrolyte membrane (PEM) fuel cells are considered to have the highest power density of all the fuel cells. They operate on hydrogen fuel, which is generally produced by reforming of hydrocarbons, and may contain large amounts of impurities such as carbon dioxide, nitrogen, and trace amounts of carbon monoxide. We studied the effect of dilution of hydrogen gas with carbon dioxide on PEM fuel cells by polarization studies. The polarization curves were different when hydrogen gas was diluted with same quantities of carbon dioxide and with nitrogen. It may be due to carbon monoxide formation by reverse shift reaction and poisoning of anode platinum catalyst. Use of Pt–Ru alloy catalyst was found to suppress the poisoning. The effects of hydrogen gas composition, temperature, current density, and anode catalyst on fuel cell performances were examined in this study.