G. Duncan Hitchens

Direct electrochemical oxidation of organics for wastewater treatment

Abstract A single cell electrochemical reactor that utilizes a proton exchange membrane (PEM) as a solid electrolyte was investigated for the treatment of waters with low or negligible electrolyte content. The electrochemical reactor concept, test system design, the role of the proton exchange membrane and the principle of organic impurity oxidation at PEM/electrocatalyst interfacial reaction zones are outlined. Test data and kinetic analysis are presented. The feasibility and application for water reclamation processes in controlled ecological environments (e.g. lunar/Mars habitats) are discussed. The approach has potential as a terrestrial water pollution control method.

High power density proton-exchange membrane fuel cells

Abstract Proton-exchange membrane (PEM) fuel cells use a perfluorosulfonic acid solid polymer film as an electrolyte which simplifies water and electrolyte management. Their thin electrolyte layers give efficient systems of low weight, and their materials of construction show extremely long laboratory lifetimes. Their high reliability and their suitability for use in a microgravity environment makes them particularly attractive as a substitute for batteries in satellites utilizing high power, high energy-density electrochemical energy storage systems. In this investigation, the Dow experimental PEM (XUS-13204.10) and unsupported high platinum loading electrodes yielded very high power densities, of the order of 2.5 W cm−2. A platinum black loading of 5 mg cm−2 was found to be optimum. On extending the three-dimensional reaction zone of fuel cell electrodes by impregnating solid-polymer electrolyte into the electrode structures, Nafion® was found to give better performance than the Dow experimental PEM. The depth of penetration of the solid polymer electrolyte into electrode structures was 50–70% of the thickness of the platinum-catalyzed active layer. However, the degree of platinum utilization was only 16.6% and the roughness factor of a typical electrode was 274.

Enzyme electrodes based on ionomer films coated on electrodes

Abstract Experiments are described showing the feasibility of electrical communication between the enzyme glucose oxidase and a ferrocene redox mediator when both are incorporated into a (Nafion®) perfluorosulfonic acid cation-exchange polymer film coated on an electrode. It was shown that Nafion® films can serve as a stable immobilization matrix that is compatible with the conditions required for an enzyme catalyst to function. An experimental strategy for the incorporation of a redox mediator and the enzyme into the ion-exchange polymer film is described. This involved casting mixed solutions of enzyme + mediator and Nafion® onto an electrode surface. The ferrocene mediator was chemically modified so that it was strongly retained inside Nafion® through hydrophobic interactions. In the presence of glucose, the mediator was able to transfer electrons from the redox center of the film-entrapped glucose oxidase to the electrode. Slow rates of charge transfer between the enzyme and the electrode, because of the high affinity of the modified mediator for the polymer, is a difficulty with this method at present. An extension of the concepts demonstrated in this paper could lead to practical enzyme electrodes for analytical purposes.

Post-Treatment of Reclaimed Waste Water Based on an Electrochemical Advanced Oxidation Process

The purification of reclaimed water is essential to water reclamation technology life-support systems in lunar/Mars habitats. An electrochemical UV reactor is being developed which generates oxidants, operates at low temperatures, and requires no chemical expendables. The reactor is the basis for an advanced oxidation process in which electrochemically generated ozone and hydrogen peroxide are used in combination with ultraviolet light irradiation to produce hydroxyl radicals. Results from this process are presented which demonstrate concept feasibility for removal of organic impurities and disinfection of water for potable and hygiene reuse. Power, size requirements, Faradaic efficiency, and process reaction kinetics are discussed. At the completion of this development effort the reactor system will be installed in JSCs regenerative water recovery test facility for evaluation to compare this technique with other candidate processes.

Electrochemical Treatments of Wastes

The present chapter is an attempt to give a fundamental basis to the early stages of a potentially valuable electrochemical waste treatment technology for the future. The disposal of domestic waste is a matter of increasing public concern. Earlier, it was regarded as permissible to reject wastes into the apparently infinite sink of the sea, but during the last 20 years, it has become clear that this is environmentally unacceptable.(1,2) On the other hand, sewage farms and drainage systems for cities and for new housing developments are cumbersome and expensive to build and operate.(3,4) New technology whereby waste is converted to acceptable chemicals and pollution-free gases at site is desirable. The problems posed by wastes are particularly demanding in space vehicles, where it is desirable to utilize treatments that will convert wastes into chemicals that can be recycled. In this situation, the combustion of waste is undesirable due to the difficulties of dissipating heat in a space environment and to the inevitable presence of oxides of nitrogen and carbon monoxide in the effluent gases.(5,6) Here, in particular, electrochemical techniques offer several advantages including the low temperatures which may be used and the absence of any NO and CO in the evolved gases.

A NEW PHOTOCATALYTIC MATERIAL BASED ON ALGAL CELLS

Metallic platinum was deposited at surfaces of intracellular photosynthetic membranes of whole cells of a cyanobacterium (blue-green alga). The deposited platinum particles acted as a catalyst for generation of hydrogen from photosynthetic decomposition of water in the absence of other exogenous electron transfer agents. This technique represents a means of placing metal catalysts in contact with intracellular structures of microorganisms.

Microelectrode-based technology for the detection of low levels of bacteria

A microelectrode-based electrochemical detection method was used for quantitation of bacteria in water samples. The redox mediator, benzoquinone, was used to accept electrons from the bacterial metabolic pathway to create a flow of electrons by reducing the mediator. Electrochemical monitoring electrodes detected the reduced mediator as it diffused out of the cells and produced a small electrical current. By using a combination of microelectrodes and monitoring instrumentation, the cumulative current generated by a particular bacterial population could be monitored. Using commercially available components, an electrochemical detection system was assembled and tested to evaluate its potential as an emerging technology for rapid detection and quantitation of bacteria in water samples.

Electrooxidation of Organics in Waste Water

Electrooxidation is a means of removing organic solutes directly from waste waters without the use of chemical expendables. Research sponsored by NASA is currently being pursued to demonstrate the feasibility of the concept for oxidation of organic impurities common to urine, shower waters and space-habitat humidity condensates. Electrooxidation of urine and waste water ersatz was experimentally demonstrated. This paper discusses the electrooxidation principle, reaction kinetics, efficiency, power, size, experimental test results and water-reclamation applications. Process operating potentials and the use of anodic oxidation potentials that are sufficiently low to avoid oxygen formation and chloride oxidation are described. The design of an electrochemical system that incorporates a membrane-based electrolyte based on parametric test data and current fuel-cell technology is presented.