Electro-reduction of hydrogen peroxide on iron tetramethoxy phenyl porphyrin and lead sulfate electrodes with application in direct borohydride fuel cells

Electroreduction of hydrogen peroxide in acidic medium is reported onto carbon-supported iron tetramethoxy phenyl porphyrin (FeTMPP/C) as well as carbon-supported lead sulphate (PbSO₄/C) electrodes. Both the catalytic electrodes can sustain electroreduction of hydrogen peroxide in direct borohydride fuel cells using hydrogen peroxide as oxidant but PbSO₄/C electrode shows catalytic activity.     

A sealed,starved-electrolyte nickel–iron battery

A sealed, starved-electrolyte, negative-limited 6V/1 Ah laboratory prototype of a nickel–iron (Ni–Fe) battery comprising five cells stacked in series with ceria-supported platinum as hydrogen–oxygen recombinant catalyst was assembled. The battery was tested under various operational conditions. While a continuous increase in gaseous pressure in the cells was observed without the recombinant catalyst, the cells with the recombinant catalyst registered a decline in gaseous pressure subsequent to the onset of hydrogen–oxygen recombination. The battery showed little decay in its capacity during its life-cycle tests conducted at C/5 rate at 25°C. The battery performance is superior to its conventional vented-counterpart.     

High surface area,supported precious metal cathodes utilizing metal microfibrous collectors for application in chlor-alkali cells

Activated cathodes were prepared from papermaking techniques for use in a membrane type chlor-alkali cell. These cathodes consisted of a nickel fiber matrix, which entrapped a platinum electrocatalyst supported on activated carbon fiber. Following optimizations of the void volume, thickness, catalyst loading, carbon support, and substrate–support ratio, these cathodes performed at an overpotential of only 58 mV @ 3 kA m^–2 in a cell containing 30 wt% NaOH at 80 °C. In addition to I/V performance, cathodes were characterized for catalyst dispersion and support surface area. When testing was discontinued, activated cathodes had demonstrated stability for greater than 60 days in a custom cell designed for continuous, steady-state operation.     

The effect of humidity and oxygen partial pressure on degradation of Pt/C catalyst in PEM fuel cell

Durability of Pt/C oxygen reduction reaction (ORR) catalyst remains one of the primary limitations for practical application of proton exchange membrane (PEM) fuel cells. In this work, the effects of relative humidity and oxygen partial pressure on platinum catalyst degradation were explored under potential cycling. At 60 °C, the loss rates of Pt mass and catalyst active surface area were reduced by about three and two times respectively when the relative humidity was lowered from 100% to 50%. The effects of oxygen partial pressure on cathode degradation were found to be insignificant. Cyclic voltammetry studies showed a slight increase in Pt electrochemical oxidation by water when the humidity increased from 50% RH to 100% RH. The rates of Pt dissolution were only slightly affected by change in humidity, and the accelerated catalyst degradation was ascribed to the increased Pt ion transport in the more abundant and larger water channel networks within the polymer electrolyte. Based on the parametric study results from our previous cathode degradation model, it was estimated that the diffusivity of Pt ions at fully humidified conditions was three times that of the value at 50% RH and 60 °C.     

Electroreduction of oxygen over iron macrocyclic catalysts for DMFC applications

The performance of macrocyclic catalysts in oxygen reduction was investigated for a direct methanol fuel cell. The dependence of catalytic activity on different factors was determined for two classes of precursors; namely, iron porphyrin (Fe-PC) and iron phthalocyanine (Fe-TPP). It was found that there was an optimal heat-treating temperature for each precursor. Heat-treated Fe-TPP shows maximum activity at 750 °C, while the highest performance in the case of Fe-PC is observed at 500 °C. It was shown that oxygen reduction activity is affected by the number of nitrogen bonds formed with iron, particle size, and formation of carbon layers.     

Enhancing thermal-stability of metal electrodes with a sputtered gadolinia-doped ceria over-layer for low-temperature solid oxide fuel cells

Cathodic activation loss is the dominant loss mechanism in the operation of low-temperature solid oxide fuel cells (LT-SOFCs). The thermal degradation of metallic cathodes decreases the performance of LT-SOFCs, causing practical issues in long-term operation. In this paper, we investigate the effect of the sputtered gadolinia-doped ceria (GDC) over-layer on the thermal stability of platinum (Pt) cathodes. The thermal stability of Pt cathodes with 23 nm-thick GDC over-layers significantly increased compared to that of the Pt-only cathodes after 2hrs’ operation at 450 °C. (<4% vs. 17% performance degradation, respectively).     

Oxygen reduction at Fe-N-modified multi-walled carbon nanotubes in acidic electrolyte

Multi-walled carbon nanotubes (MWCNTs) modified with iron tetramethoyxphenyl-porphyrin chloride (FeTMPP-Cl) and heat treated are active towards electrocatalytic oxygen reduction in acidic media. The activity slightly depends on the heat treatment temperature (850 < 550 °C) and the amount of porphyrin deposited onto the nanotubes before the heat treatment step. In comparison with as-received MWCNTs no increase in activity has been found with iron phenanthroline or iron acetate impregnated and heat treated MWCNTs. When MWCNTs are pretreated in an oxidation step using HNO₃, there is only a slight increase in activity after FeTMPP-Cl modification and heat treatment compared to the not pretreated MWCNTs. The HNO₃ treatment itself, however, leads to an increase in activity of the unmodified MWCNTs. TEM-measurements revealed an amorphous layer surrounding the MWCNTs after HNO₃ treatment, while XPS showed an increased amount of oxygen functional groups. It is suggested that there are different kinds of active sites at the catalyst surface, the first ones consisting of oxygen functionalities or other entities introduced by the HNO₃ treatment, and the second ones containing nitrogen (and probably iron) introduced via the porphyrin. Pyridine-type nitrogen has been found by XPS after heat treatment at both temperatures, indicating that the active sites are already formed at 550 °C.     

Oxygen reduction by Fe-based catalysts in PEM fuel cell conditions: Activity and selectivity of the catalysts obtained with two Fe precursors and various carbon supports

Fe-based catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cell conditions have been prepared by adsorbing two Fe precursors on various commercial and developmental carbon supports. The resulting materials have been pyrolyzed at 900 °C in an atmosphere rich in NH₃. The Fe precursors were: iron acetate (FeAc) and iron tetramethoxy phenylporphyrin chloride (ClFeTMPP). The nominal Fe content was 2000 ppm (0.2 wt.%). The carbon supports were HS300, Printex XE-2, Norit SX-Ultra, Ketjenblack, EC-600JD, Acetylene Black, Vulcan XC-72R, Black Pearls 2000, and two developmental carbon black powders, RC1 and RC2 from Sid Richardson Carbon Corporation. The catalyst activity for ORR has been analyzed in fuel cell tests at 80 °C as well as by cyclic voltammetry in O₂ saturated H₂SO₄ at pH 1 and 25 °C, while their selectivity was determined by rotating ring-disk electrode in the same electrolyte. A large effect of the carbon support was found on the activity and on the selectivity of the catalysts made with both Fe precursors. The most important parameter in both cases is the nitrogen content of the catalyst surface. High nitrogen content improves both activity towards ORR and selectivity towards the reduction of oxygen to water (4e⁻ reaction). A possible interpretation of the activity and selectivity results is to explain them in terms of two Fe-based catalytic sites: FeN₂/C and FeN₄/C. Increasing the relative amount of FeN₂/C improves both activity and selectivity of the catalysts towards the 4e⁻ reaction, while most of the peroxide formation may be attributed to FeN₄/C. When FeAc is used as Fe precursor, iron oxide and/or hydroxide are also formed. The latter materials have low catalytic activity for ORR and reduce O₂ mainly to H₂O₂.     

Progress in non-precious metal oxide-based cathode for polymer electrolyte fuel cells

The cathode catalysts for polymer electrolyte fuel cells should have high stability as well as excellent catalytic activity for oxygen reduction reaction (ORR). Group 4 and 5 metal oxide-based compounds have been evaluated as a cathode from the viewpoint of their high catalytic activity and high stability. Although group 4 and 5 metal oxides have high stability even in acidic and oxidative atmosphere, they are almost insulator and have poor ORR activity because they have a large band-gap. It is necessary to modify the surface of the oxides to improve the ORR activity. We have tried the surface modification methods of oxides into four methods: (1) formation of complex oxide layer containing active sites, (2) substitutional doping of nitrogen, (3) introduction of surface oxygen defect and (4) partial oxidation of carbonitrides. These modifications were effective to improve the ORR activity of the oxides. The solubility of the oxide-based catalysts in 0.1 mol dm⁻³ at 30 °C under atmospheric condition was mostly smaller than that of platinum black, indicating that the oxide-based catalysts had sufficient stability compare to the platinum. The onset potential of various oxide-based cathodes for the ORR in 0.1 mol dm⁻³ at 30 °C achieved over 0.9 V vs. a reversible hydrogen electrode.     

Proton conducting membrane containing room temperature ionic liquid

A new proton conducting membrane containing room temperature ionic liquid: 2,3-dimethyl-1-octylimidazolium trifluoromethanesulfonylimide (DMOImTFSI) and polyvinylidenefluoride-co-hexafluoropropylene (PVdF-HFP) has been developed in the present work. The addition of bis(trifluoromethanesulphonyl)imide (HN(CF₃SO₂)₂) to this membrane results in an increase in conductivity by one order of magnitude at 25 °C. The membrane shows a conductivity of 2.74 × 10⁻³ S/cm at 130 °C along with good mechanical stability. The membrane was tested in a commercial fuel cell test station at 100 °C with dry hydrogen and oxygen gas reactants using Pt/C electrodes. The membrane containing the ionic liquid has been found to be electroactive for hydrogen oxidation and oxygen reduction at the platinum electrode and can be developed for use in proton exchange membrane fuel cell (PEMFC) under non-humid conditions at elevated temperatures.