Research
Background
Figure 1. Applications for organic and polymer electrochemical systems.
A common feature in the funded proposals listed above lies in the movement of ions and charge through media which are fixed in place although locally dynamic. The well known applications for this work lie in the operation of energy generation and storage such as fuel cell membranes and lithium batteries as well as display devices. However, there are many other applications that utilize ion and charge transfer to deliver energy and reactants which are less well known. Figure 1 illustrates some samples of the wide variety of applications possible.
The left hand side of the figure shows the applications that are covered by the LBNL funded projects listed above. The right hand side shows new areas that have opportunities for funding in a wide number of areas. In particular, the use of electrochemical techniques and technology for environmental and biotechnology applications leads directly into areas of process control as new approaches to the problems of energy delivery and mass transport are necessary. Electrochemical delivery of energy and reactants is particularly useful for "green" processing as it is an inherently bio-friendly method of delivery and can lead to significant acceleration of bioprocessing and bioremediation as well as increased selectivity in chemical processing. The markets for a range of industries and their relativeenergy demands are shown in figure 2. Present applications of bioprocessing tend to be rather small volume and even the well known example of alcohol production through fermentation suffers from low space-time yields. The new opportunities provided by the biotechnology revolution, the use of biocatalysts and selective chemical catalysts can only be realized in larger volume products provided suitable methods of energy delivery are available. Electrochemistry can fill this need admirably as shown in figure 3.
Figure 2. New Markets for electrochemical processing.
The use of electrochemical processing for selective chemical synthesis has long been an area of unfulfilled promise, particularly in the U.S. Much of this has been the result of unfavorable economic comparison with catalyst technology or so called "hot rock" chemistry. Electrochemical processes that have been successful are generally ones where no practical alternative is available such as Chlor-alkali generation or Aluminum smelting. Occasionally there have been instances where a particular feedstock situation has favored electrochemical methods such as the Monsanto adiponitrile process but generally the low space-time yield of electrochemical methods and the need for complex separation schemes have kept electrochemical methods from adoption. However, the increasing demand for high selectivity in chemical processing prompts a return to electrochemical methods particularly for use with biocatalysts and biomimetic systems. The project listed above entitled "Novel & Energy Efficient Co-factor Regeneration for Enzymatic Catalysis" is an example of this. The proposed work not only centers on highly selective catalysts but also develops separations processes as an inherent component of the work.
Figure 3. Electrochemical acceleration of bio-processes.
The potential selectivity of electrochemical methods of synthesis is constantly referred to in standard organic chemistry texts but this potential has not been realized. The main reason for this is that the separation processes have not been developed that would make the method attractive. For example selective deprotection is only useful if the products may be isolated in an uncontaminated form and the protecting groups recycled. Use of electrochemical acceleration of biocatalysts by use of redox mediators as shown in figure 3 has been severely impeded by the need to completely separate the often toxic mediators from the products. There exist then great opportunities to develop new and selective electrochemical processing provided separations are provided as an integral part of the whole synthesis scheme. It is a central goal for this statement of research interest.
Figure 3 also illustrates the possibility of actually delivering energy in the form of electrons to living organisms to accelerate growth. Several examples of this are known not only for high value products but also for high volume processes such as the electro-acceleration of the growth of T. ferrooxidans for mining purposes. Bio-leaching accounts for the production of one quarter of the world's copper production and methods of acceleration of the processing has a major economic impact, particularly on account of increased space-time yields. Other important nutrients may be supplied to the organisms in a controlled fashion by use of electrochemical process control. One particular example is the enhanced supply of oxygen to bioreactors. This can be accomplished by the use of oxygen carrier materials such as modified hemoglobins, synthetic blood and fluorocarbon. In all cases separation and recovery of the oxygen carrier controls the viability. Figure 4 shows a photograph of an Aquanautics artificial gill system, capable of enhancement of oxygen flux to microorganisms of up to 100 times. The artificial blood material is regenerated by an electrochemical cell which also serves to control the redox strength of the medium. By appropriate use of current to control the redox state of the oxygen carrier the supply of oxygen to the bioreactor system can be enhanced or controlled as desired. The application of such a system requires the development of an efficient separation and recovery system.
Figure 4. Pilot-scale artificial gill system capable of enhanced supply of oxygen to bioreactors.
Figure 5. Pool process for electrokinetic remediation of soil illustrating the importance of electrolyte conditioning.
Electrochemical methods have been used to control supply of important nutrients to biosystems which are ionically charged - metal cations (e.g. zinc, copper) and anions (phosphate, nitrate, sulfate). Other additives may also be controlled in a similar manner. Electrotransport systems are a rapidly expanding field of drug delivery in human patients. However, nowhere is the control of the medium more important than in electrokinetic transport methods for soil remediation (see Figure 5). Without proper control of the electrolyte the use of electrokinetic methods to remediate contaminated soil results in worse contamination. The system shown in figure 5 emphasizes the need for complete consideration of the whole process including electrolyte circulation and conditioning and non-corroding electrodes. The electrokinetic system can be thought of as a more complex case of a reactor where the mass transport and energy flows take place at much reduced rates.