Paper Session III-A - Electrolytic Oxygen Enrichment Using Supernoxide Ion in a Solid Polymer Membrane Electrolyte

Electrochemical cells are among the technologies under consideration for gaseous oxygen concentration or enrichment in both aerospace and civilian applications. Current electrochemical technology involves the electro-reduction of molecular oxygen, O2, to water at one electrode, and the electro-oxidation of water to oxygen at the other. In terms of the overall chemical mechanism, this is a 4-electron, 4-proton process. From an economic point of view, one would like to use as little energy as possible to effect oxygen transport. The simplest possible mechanistic scenario would be if the O-, reduction product is the superoxide ion, O2~, involving only a single electron exchange: O2 + e = O 2 Superoxide anion can be produced electrochemically via reduction of O 2 in an organic aprotic solvent, such as dimethyl formamide or acetonitrile. Moreover, production of superoxide via electrolysis is electrochemically reversible (i.e., the forward and reverse reaction is so rapid that it proceeds under diffusion control near the thermodynamic potential). Considerable energy savings may be realized if electrochemical O, transport could be performed using superoxide ion

Sulfur Recovery From Oil And Gas Refinery Waste Streams Using Semiconductor Particulates

A new tail-gas sulfur-recovery system is described in which the H2S, having been absorbed into an alkaline scrubber unit, is decomposed to its constituent elements in a solar photoelectrochemical scheme based on semiconductor particulates. The semiconductor particles would be immobilized in or flowed through a flat-bed photoreactor, along with the alkaline-sulfide stream. The exit stream, containing mostly sulfur as polysulfide anions, would be treated to yield free sulfur and regenerated alkali to be returned to the scrubber. In an evaluation of semiconductor compounds that could possibly bring about H2S decomposition, only CdS showed the ability to decompose H2S. Consequently, the author projects that a photoreactor based on CdS occupying 6.5 acres of land would be required to recover 1.0 ton of sulfur per day

Solar Photocatalytic Hydrogen Production from Water Using a Dual Bed Photosystem - Phase I Final Report and Phase II Proposal

In this work we are attempting to perform the highly efficient storage of solar energy in the form of H{sub 2} via photocatalytic decomposition of water. While it has been demonstrated that H{sub 2} and O{sub 2} can be evolved from a single vessel containing a single suspended photocatalyst (Sayama 1994; 1997), we are attempting to perform net water-splitting by using two photocatalysts immobilized in separate containers, or beds. A schematic showing how the device would work is shown

Development Of High Performance, Low Cost Pem Electrolytes

One of the major cost components in a PEM fuel cell is the solid polymer electrolyte separating anode and cathode. The high cost of the membrane is mainly due to the need to perfluorinate the organic polymer. This is thought to be necessary to protect the membrane from oxidative attack by hydroxyl radicals that are generated as by-products of the electrochemical process. Aryl ether chain polymers such as polyetheretherketone (PEEK) are hydrocarbon-based compounds that are known for their oxidative resistance. Sulfonated PEEK (SPEEK) polymers were synthesized and exposed to 3.5 % peroxide solution at 25-80 °C. Optical microscopy, water permeability, and conductivity of aqueous extracts were used to monitor polymer degradation. It was shown that 20% SPEEK was more susceptible to oxidation than perfluorinated ionomers such as Nafion?, but considerably less than polystyrene-based ion-exchange membranes. Copyright © (2006) by AFHYPAC

Solar hydrogen via a photosynthetic Z-scheme analogue based on semiconductor powders

Mother Nature uses solar energy to oxidize water to O2 and reduce protons onto NADP+ via dual photosystems connected by a string of redox agents. By splitting the energetically challenging task of decomposing water between two separate photochemical reactions, more abundant and lower energy solar photons can be employed. This same approach can be utilized with semiconductor powders. Based upon their electronic band structure, semiconductors can be chosen that selectively oxidize or reduce water. One can then select O2-evolving and H2-evolving photocatalysts, disperse or immobilize them in separate containers, and use an appropriate reversible redox agent as an electron shuttle between them. We have had some success with this photosynthetic Z-scheme analogue using an alkaline iodate redox electrolyte and a variety of semiconductor compounds, such as TiO2 and InP. Proof of concept has been demonstrated, but various materials problems underscore the need to identify other photocatalysts

Uv Photochemical Oxidation Of Aqueous Sodium Sulfide To Produce Hydrogen And Sulfur

Sodium sulfide solutions were illuminated with ultraviolet light (λ=253.7nm) to produce hydrogen and disulfide ion in equimolar amounts. The quantum efficiency for hydrogen production was as high as 27% for a 0.5M Na2S solution in a batch reactor. While light intensity and sulfide concentration had a pronounced effect on reaction rate, the photochemical process was found to be pH independent within the range 8.5-13.3. A mechanism involving adsorption of bisulfide ion on the inner sleeve of the photoreactor is postulated. Elemental sulfur could be recovered from the disulfide solution via purging with H2S and then filtering. The resulting filtrate was also photoactive, suggesting the possibility of a continuous closed cycle photochemical operation for H2S decomposition into its constituent elements. © 2004 Elsevier B.V. All rights reserved

X-Ray Photoelectron Investigation Of Phosphotungst1C Acid As A Proton-Conducting Medium In Solid Polymer Electrolytes

An X-ray photoelectron spectroscopic study was performed on phosphotungstic acid, H3PW12O40, a cluster compound based on interlinked WO3 units. It was observed that the W4∫ transition could span a range of 2.4 eV, depending on the type of cation substitution and the number of waters of hydration (0-13). The W4∫7/2 transition for alkali metal-substituted PTA averaged 35.7 eV, while PTA with low or no waters of hydration was 36.2 eV. Fully hydrated PTA (13 waters and higher) gave binding energies averaging 37.4 eV. Other W6+ compounds were studied as well, revealing another binding energy trend in going from octahedral to tetrahedral geometry. The chemical shift behavior in PTA was attributed to waters of hydration forming hydrogen bonds with the terminal oxygens of the Keggin ion, pulling valence electron density away from the W nucleus and increasing its net positive charge, thus increasing W4∫binding energy

Conductivity And Water Uptake Of Aromatic-Based Proton Exchange Membrane Electrolytes

Water uptake and proton conductivity as a function of temperature were determined for three aromatic-based, sulfonic acid-bearing polymers, plus the perfluoroalkyl sulfonic acid Nafion 117. Water uptake of submerged, equilibrated samples ranged from less than five water molecules per acid group for a high equivalent weight, sulfonated polyethersulfone to almost fifty waters per acid for a low equivalent weight, sulfonated polyetheretherketone. The most conductive aromatic-based polymer, sulfonated polyphenylquinoxaline (S-PPQ), had a room temperature conductivity of 9.8 × 10-3 S/cm, about an order of magnitude less than that of a perfluoroalkyl sulfonic acid under identical conditions. The slope of the S-PPQ Arrhenius conductivity plot was sufficiently steep that at 180°C, the proton conductivity, 1.3 × 10-1 S/cm, was only a factor of two lower than that of Nafion under similar conditions. The lower conductivity of the aromatic-based sulfonic acid polymers can be attributed to chain rigidity, lack of ion channels, and lower acidity