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.     

Electrode reaction at platinum / ceria surface modified YSZ interface

The electrode reaction was examined on ceria coated YSZ by a platinum point electrode in H₂-H₂O atmosphere at 973 K- 1173 K. The thickness of the ceria coating layer was altered from 0 to 2.5 μm, fabricated by a laser ablation and by a vacuum vapor deposition method on YSZ single crystals. The electrode / electrolyte interface conductivity increased with 1/4 powers ofp(H₂) andp(H₂O) on both ceria coated and non-coated YSZ. The interface conductivity was significantly improved on a thicker ceria coating surface than 1 μm. The effective electrode reaction radius also increased in a thick ceria coating. The¹⁸O/¹⁶O exchange experiment at low oxygen partial pressure revealed that the oxygen surface exchange rate of ceria is not high compared with that of YSZ. It can be concluded that the bulk ionic conduction of ceria makes a more effective contribution to the electrode reaction than the surface catalytic activity in H₂-H₂O atmosphere. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

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.     

Antioxidant behaviour of 2′‐hydroxy‐chalcones: a study of their electrochemical properties

The electrochemical behaviour of 13 chalcone analogues was systematically studied by means of cyclic voltammetry and chronoamperometry at a glassy carbon (GC), gold and platinum working electrodes using two different supporting electrolyte/solvent combinations. It was found that chalcone analogues can be easily oxidized at both GC and gold working electrodes, but not at a platinum electrode. Principal component analysis was further employed to reveal similarities/dissimilarities between oxidation potentials, chronoamperometric signals and ability of the compounds to scavenge the reactive oxygen species H₂O₂. The study reveals the inverse proportional relationship between the scavenging ability of H₂O₂, expressed as IC₅₀, and chronoamperometric signal at 800 mV using gold as working electrode. Copyright © 2012 John Wiley & Sons, Ltd.     

Studies on modified anode polymer hydrogen sensor

An improved polymer electrolyte membrane (PEM) fuel cell based amperometric hydrogen sensor that operates at room temperature has been developed. The electrolyte used in the sensor is PVA/H₃PO₄ blend, which is a proton conducting solid polymer electrolyte. A blend of palladium and platinum coated on the membrane is used as anode and platinum as cathode. The sensor functions as a fuel cell, H₂/Pd-Pt//PVA-H₃PO₄//Pt/O₂, and the short circuit current is found to be linearly related to the hydrogen concentration. The present study aims at investigating the dependence of sensor behaviour on the anode composition. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Low-temperature zirconia oxygen gauges based on RuO2 electrode

A low temperature oxygen gauge based on zirconia electrolyte has been developed. It makes use of RuO₂ as electrode material in place of platinum in conventional gauges. The low interfacial impedance of the RuO₂ electrode makes it possible to keep the cell resistance below 10⁶ ω even at low temperatures. Nernst's law tests indicate that this cell can give theoretical outputs down to 498 K campared to 923 K for gauges with platinum electrodes. Faraday's law tests confirm its good performance over a wide range of oxygen concentrations. High electronic conductivity, single oxide phase, slight non-stoichiometry and good adherence are responsible for the good performance of RuO₂ as electrode. An activation energy of 90.95 kJ/mole observed for the interface shows that the vacancy movement in the electrolyte is the rate controlling step. The evaporation of RuO₂ as RuO₄ gives rise to flow dependent output. This can be overcame by operating the cells at low temperatures though at the cost of speedy response. The low operating tempratures lead to a compact gauge with good stability.     

Post-test analysis of electrode-supported solid oxide electrolyser cells

Three solid oxide cells have been investigated after long-term high temperature electrolysis to explain the phenomena of accelerated degradation. These cells contain a Ni-YSZ cermet (Ni-yttria-stabilised-zirconia) as hydrogen electrode (cathode), yttria-stabilised-zirconia (YSZ) as electrolyte, Ce_0.8Gd_0.2O_1.9 (CGO) as diffusion barrier layer and La_0.58Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) as oxygen electrode (anode). Cell 1, cell 2 and cell 3 were tested continuously at about 770 °C, with a current density of ?1 A cm^?2 and 80 % H₂O of absolute humidity for 9000, 1770 and 1460 h, respectively. It was found that in cell 1, the degradation rate was about 2.2 % per 1000 h, in cell 2 the degradation rate increased to 3.4 % per 1000 h and in cell 3 the degradation rate was 2.6 % per 1000 h. The mode of cell degradation was also investigated as a function of the cell fabrication in the four layers system (anode/diffusion barrier layer/electrolyte/cathode). An intergranular fractured surface along the grain boundaries of the electrolyte, and the formation of porous structures throughout the thickness of the electrolyte were observed in cell 1. LSCF, as the oxygen electrode, showed compositional fluctuations with a changed perovskite composition and formation of cobalt oxide. This phenomenon reduces the electrical conductivity and, probably, also the catalytic properties. The hydrogen electrode did not show major changes in all the three cells tested. Cells 2 and 3 showed similar features as observed for cell 1, except the fact that they retained the electrolyte structure without intergranular fracture and formation of porosity after continuous testing for long duration.     

Investigation of liquid plasma catalysis on Ti electrodes for the decolorization of a brilliant red B solution

This investigation is intended to determine the catalytic effect of liquid plasma on TiO₂, generated in situ on Ti anodes submerged in Na₂SO₄ electrolyte solution by observing the efficiency of the reaction in decolorizing a brilliant red B solution under voltage-stabilized DC power. The orthogonal test was performed in order to obtain the optimal reaction conditions for the test device. When placed under a constant voltage of 550 V, and with an electrode depth of 2 mm, Na₂SO₄ concentration of 5 g/L, pH of 2, the maximum decolorization ratio of 100 mL brilliant red B solution with the concentration of 20 mg/L was 97.8% after 40 min. The reaction rate constant was about 0.102 min^?1, conforming to the first-order reaction kinetic model. Comparative tests were conducted with: Al electrode under 450 V; Mo electrode under 550 V; and a mixture of the electrolyte and TiO₂ powder. The results showed that liquid plasma – TiO₂ on the electrode of the catalytic system naturally integrated on the discharge electrode, with an increase in reaction rate by 26.8% while utilizing the same energy consumption.     

Morphology-controlled PANI nanowire electrode by using deposition scan rate in H<Subscript>2</Subscript>SO<Subscript>4</Subscript>/PVA polymer electrolyte for electrochemical capacitor

Polyaniline (PANI) nanowire electrode was successfully prepared using electrodeposition method. The morphology, thickness, and electrochemical performance of PANI electrode can be controlled by varying the deposition scan rates. Lower deposition scan rate results in compact and aggregates of PANI nanowire morphology. The uniform nanowire of PANI was obtained at the applied scan rate of 100 mV s^?1, and it was used as symmetric electrode coupled with H₂SO₄/polyvinyl alcohol (PVA) gel electrolyte. The different concentrations of H₂SO₄ acid in polymer electrolyte have influenced the electrochemical performance as well. The optimum specific capacitance and energy density of P100 PANI electrode in 3 M H₂SO₄/PVA gel polymer electrolyte was 377 F g^?1 and 95.4 Wh kg^?1 at the scan rate of 1 mV s^?1. The good stability of the electrode in this system is applicable to many wearable electronics applications.     

Influence of electrolyte composition on deactivation mechanism of a Ti/Ru0.25Ir0.25Ti0.5O2 electrode

Deactivation of a RuO₂-IrO₂-TiO₂/Ti electrode was investigated during an accelerated life test in H₂SO₄ and NaCl solutions using cyclic voltammetry, electrochemical impedance spectroscopy, and SEM/energy dispersive spectroscopy. It is found that the deactivation mechanism depends on electrolyte composition. Intensive Ru/Ir dissolution from the oxide coating and the growth of an insulating TiO₂ interlayer are the main deactivation mechanisms of anode in H₂SO₄ and NaCl solutions, respectively. The results indicate that the determining factor in deactivation mechanism is the morphology factor of oxide anode in different solutions which restrains the diffusion process of electrolyte into coating inner layers.     

Electrochemical solid electrolyte gas sensors — hydrocarbon and NOx analysis in exhaust gases

For the in situ measurement of the free oxygen concentration and the equilibrium oxygen partial pressure oxygen sensors based on zirconia solid electrolytes are widely used in order to monitor and control technical high temperature processes. Similarly combustibles (HC, CO) and NO_x can be determined in non equilibrated oxygen containing gas mixtures of exhausts by mixed potential sensors and amperometric solid electrolyte sensors. It is expected that their long-term stability is similar to that of oxygen sensors. In both cases the electrode material with the desired electrochemical and catalytic properties is the key component. Different electrode materials made of perovskites (La_1-xSr_xCr_1-yGa_yO_3-δ) and composites (Au/Metal oxide) were investigated in different combustibles including CO, C₃H_6/8, C₇H₈ and CH₄. The response behaviour of mixed potential sensors is determined by the catalytic activity of the measuring electrode, which is closely connected with the defect structure and depends on the measuring conditions. Furthermore the electrode response can be understood by electrokinetic data. Gas symmetrical mixed potential sensors with electrodes made of Au/Nb₂O₅ composites show maximum sensitivity. By using Ptreference electrodes without equilibrium behaviour the sensors are applicable in lean and rich mixtures as well. In the amperometric sensor mode the consecutive determination of oxygen and NO_x or combustibles at two working electrodes is possible. The catalytic activity of the oxygen pumping electrode should be low in order to avoid the decomposition of NO and HC respectively. Alternatively, the electrochemical reduction of NO can be performed at a single working electrode, made of materials with improved NO selectivity, without the previous reduction of oxygen. Paper presented at th 8th EuroConference on Ionics, Ixia, Rhodos, Greece, Sept. 15–21, 2002.     

Electrocatalysis of oxygen reduction reaction on Nafion/platinum/gas diffusion layer electrode for PEM fuel cell

In this work a new membrane electrode based on Pt-coated Nafion membrane was fabricated. Chemical deposition process was used to coat platinum on Nafion 117 membrane and then Pt-coated Nafion membrane was hot pressed on gas diffusion layer (GDL) to make new membrane electrode. The electrochemical and chemical studies of the Pt-coated Nafions were investigated by electrochemical techniques, X-ray diffraction and scanning electron microscopy. The electrochemical results indicated that as the concentration of H₂PtCl₆ increased, the oxygen reduction reaction rate increased until the concentration was reached where the reduction reaction was limited by the problem of mass transport. The electrochemical results for oxygen reduction reaction showed that the new electrode which prepared by plating Nafion membrane with 0.06 M H₂PtCl₆ in electroless plating solution, has a higher performance than other electrodes. The XRD results showed that the average platinum particle size of the best sample was about 3 nm. The loading of platinum for this electrode was 0.153 mg cm⁻².     

The use of the platinum electrode coated with ultrathin poly(allylamine hydrochloride)/Nafion films for selective detection of hydrogen peroxide

In this paper, a novel polyelectrolyte multilayer (PEM) film-coated platinum electrode for the selective detection of H₂O₂ was presented. The PEM film was formed by the layer-by-layer assembly technique. The quartz crystal microbalance experiments showed that the thickness of the prepared Nafion layer was about 8 nm and depended on the pH of poly(allylamine hydrochloride) solution. The combination of different polyanions and polycations layers was investigated, and it is found that ploy(allylamine hydrochloride) (PAH) and Nafion composited film functioned best as a diffusion barrier toward uric acid (UA) and ascorbic acid (AA) while allowed H₂O₂ to pass through smoothly. When the platinum electrode coated with two-bilayer film, (PAH/Nafion)₂, the amperometric responses of 0.1 mM UA and 0.1 mM AA were respectively 0.008 and 0.006 μA, which were only 0.2% or less of the response of 0.1 mM H₂O₂ (4.0 μA). The linear response range of the electrode toward H₂O₂ was from 1.0 μM to 1.0 mM, and the detection limit was 0.3 μM. The electrode also displayed high operational stability and long-term storage stability.     

Switching effect at a platinum electrode on stabilized zirconia during oxygen reduction in the presence of NO

A galvanic cell based upon the use of stabilized zirconia as solid oxygen ion conductor has been used to measure cathodic reduction currents at a porous platinum electrode in both nitrogen-oxygen gas mixtures with and without small amounts of NO (up to 5450 ppm). Adding small amounts of NO to the N₂/O₂ mixture induced a considerable cathodic current peak at the working electrode in the first moment after addition. After interruption of the NO exposure, the opposite effect, a high current pulse in anodic direction, was observed. The switching effect is reproducible and its magnitude depends on the concentrations of oxygen and nitrogen monoxide in the gas. As the main contribution to the current results from the reduction of the excess oxygen in the gas, it must be concluded that the presence of NO strongly affects the steady state, in particular the adsorbed oxygen at the electrode/electrolyte interface. These experimental results are interpreted in terms of a reversible change of the interface which may be due to a reconstruction of the platinum surface in the presence of NO and corresponding drastic change in the amount of adsorbed oxygen at the platinum surface. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Ethylene Hydrogenation on a Platinum Nanocoating at Various Electric Potentials

The possibility of controlling the rate of ethylene hydrogenation on a platinum nanocoating is established by applying to it electric potentials of different polarities and magnitudes from an external voltage source. At a negative potential of ?10 V, the hydrogenation rate increases by 4%, whereas at a positive potential of +10 V, the hydrogenation rate increases by 42% under the conditions of the experiment at room temperature, atmospheric pressure, and an initial mixture composition of 0.09C₂H₄ + 0.91H₂. Quantum-chemical calculations of the energy of the reaction of platinum hydride with hydrogen, Pt₂H₂ + H₂ → Pt₂H₃ + H, and the energy characteristics of similar reactions involving negatively and positively charged Pt₂H₂ are performed. It has been demonstrated that the presence of a negative or positive charge on Pt₂H₂ lowers the endothermicity of formation of H radicals by 18.4 or 22.5 kcal/mol, respectively. Based on the calculation results, a mechanism is proposed to explain the effect of the charge of a platinum coating on its catalytic activity in ethylene hydrogenation.     

Electrochemical characterisation of the Pt/YSZ interface exposed to a reactive gas phase

The Pt/yttria-stabilized cubic zirconia (YSZ) interface exposed to a reactive gas was characterised by solid electrolyte potentiometry and cyclic voltammetry. The catalytic reactions included total combustion of C₃H₈ and C₃H₆ to CO₂ and H₂O as well as NO reduction by C₃H₆ in the presence of O₂ under oxygen-rich and stoichiometric conditions. The solid electrolyte potentiometry as a function of the temperature in C₃H_x/O₂ (with x=6 or 8) reflected the catalytic properties of Pt for C₃H_x oxidation. In C₃H₆/NO/O₂, the reduction of NO was evidenced below 300 °C. The cyclic voltammetry evidenced the formation of an oxygen chemisorbed layer on the Pt surface under anodic potential. Propane had no effect on this chemisorbed layer, whereas propene weakened significantly the strength of this Pt–O bond. Addition of NO to C₃H₆/O₂ led to the disappearing of this chemisorbed layer. The use of solid electrolyte potentiometry in conjunction with cyclic voltammetry allowed us to determine the surface oxidation state of Pt during the catalytic reactions.     

Experimental and numerical investigation of hetero-/homogeneous combustion of CO/H2/O2/N2 mixtures over platinum at pressures up to 5 bar

The hetero-/homogeneous combustion of fuel-lean CO/H₂/O₂/N₂ mixtures over platinum is investigated at pressures up to 5 bar, inlet temperatures (T_IN) up to 874 K, and a constant CO:H₂ molar ratio of 2:1. Experiments are performed in an optically accessible channel-flow catalytic reactor and involve planar laser induced fluorescence (LIF) of the OH radical for the assessment of homogeneous (gas-phase) ignition and 1-D Raman measurements of major gas-phase species concentrations over the catalyst boundary layer for the evaluation of the heterogeneous (catalytic) processes. Simulations are carried out with an elliptic 2-D model that includes detailed heterogeneous and homogeneous chemical reaction schemes. The predictions reproduce the Raman-measured catalytic CO and H₂ consumption, and it is further shown that for wall temperatures in the range 975 ? T_w ? 1165 K the heterogeneous pathways of CO and H₂ are largely decoupled. However, for wall temperatures below a limiting value of 710–720 K and for the range of pressures and mixture preheats investigated, CO(s) blockage of the surface inhibits the catalytic conversion of both fuel components. The homogeneous ignition distance is well-reproduced by the model for T_IN > 426 K, but it is modestly overpredicted at lower T_IN. Possible reasons for these modest differences can be the values of third body efficiencies in the gas-phase reaction mechanism. The sensitivity of homogeneous ignition distance on the catalytic reactions is weak, while the H₂/O₂ subset of the CO/H₂/O₂ gaseous reaction mechanism controls the onset of homogeneous ignition. Pure hydrogen hetero-/homogeneous combustion results in flames established very close to the catalytic walls. However, in the presence of CO the gaseous combustion of hydrogen extends well-inside the channel core, thus allowing homogeneous consumption of H₂ at considerably shorter reactor lengths. Finally, implications of the above findings for the design of syngas-based catalytic reactors for power generation systems are discussed.     

Electrode materials for potentiometric hydrogen sensors

Due to their relatively high sensitivity, improved long-term stability, possibilities for miniaturization and low cost products, mixed potential solid electrolyte sensors can be competitive for the in situ measurement of hydrogen trace concentrations in oxygen containing gases. Their response behavior in non-equilibrated oxygen containing gas mixtures is mainly determined by the catalytic activity of the measuring electrode and depends strongly on preparation and measuring conditions. In this work the sensitivity of electrodes made of composites (Au/MeO) has been investigated in hydrogen containing gases in the concentration range φ(H₂) = 0…800 vol.-ppm using a two-chamber setup with Pt-air reference. Electrodes made of Au/Nb₂O₅ composites show the highest sensitivities of up to 20 mV/vol.-ppm at φ(H₂) = 10 vol.-ppm and the lowest catalytic activity for hydrogen oxidation. Selected composite materials were tested additionally in self-heated solid electrolyte sensors with both electrodes exposed to the same atmosphere (gas-symmetrical sensor).     

Novel solid acid fuel cell based on a superprotonic conductor Tl3H(SO4)2

We have fabricated a fuel cell based on a superprotonic conductor, a Tl₃H(SO₄)₂ crystal, and have measured the electrical properties of this fuel cell. It is found that the open-circuit voltage in the fuel cell based on the Tl₃H(SO₄)₂ crystal increases by supplying H₂ fuel gas and typically becomes 0.83 V. Moreover, we have observed that the cell voltage decreases with increasing current density, as observed in fuel cells such as proton exchange membrane fuel cell, solid oxide fuel cell, etc. These results indicate that it is possible to use the Tl₃H(SO₄)₂ crystal as the electrolyte of a solid acid fuel cell. In addition, we suggest that the selection of the electrode and the preparation of the very thin electrolyte are extremely important to achieve high-efficiency of power generation of this fuel cell.     

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.