In this research supported by the Analytical and Surface Chemistry Program, catalytic reaction mechanisms on electrode surfaces will be predicted and analyzed using a new theory developed in the Anderson lab for studying electrochemical interfaces. In the two-dimensional version of the new band theory program, the surface potential is adjusted by adding charge to the electrode and counter charge in the double layer. The double layer charge distribution is modeled by a modified Poisson-Boltzmann theory within a dielectric continuum model. The whole interfacial system is varied to charge self-consistency, yielding predictions of the Gibbs free energies as functions of structure and electrode potential. The theory, already found useful for predicting reversible potentials for forming reaction intermediates adsorbed on electrode surfaces as functions of potential, will be extended to characterizing transition state structures and activation energies during electron transfer reactions on electrode surfaces. Efficient energy conversion and energy storage will in the future depend more and more on making the best use of the Gibbs energy of chemical reactions in the form of electrical work and less and less on the enthalpy of chemical reactions in the form of heat and pressure-volume work. Catalysis is required to generate electrical work directly from fuels, whether by enzymes in biological systems or catalytic electrodes in fuel cells. It is critical to develop an understanding of electrocatalysis at the level of molecular structures and reaction mechanisms. Work toward this goal using the theory developed in the Anderson lab has begun and the research supported by the program will yield significant advancements. Two major thrusts are (i) gaining fundamental understanding of the factors that so far limit the efficiency and stability of platinum electrocatalysts in fuel cells, (ii) applying the insight gained to proposing new electrocatalysts, possibly not including platinum, that are as yet undiscovered. The results will be helpful to a broad scientific community and should speed the development of efficient portable electric power sources for transportation and mobile electronic device applications.
Normal 0 false false false EN-US X-NONE X-NONE Electricity can cause chemical reactions and chemical reactions can generate electricity. Sometimes these processes are inefficient and wasteful of material, which renders them, though promising for important applications, impractical in practice. For several decades the generation of electricity in fuel cells has been heavily researched but their broad applications such as powering transportation vehicles have remained just around the corner. There are several reasons for this and the two most significant ones are inefficient reaction rates at the anode compartment, where hydrogen or an alcohol are stripped of electrons to produce protons, water, and carbon dioxide and at the cathode compartment, which takes the electrons, which have gone through a wire outside the fuel cell to run motors and light bulbs, and combines them at the cathode with oxygen and the protons, having passed through the interior of the fuel cell, to produce water. Platinum alloyed with other metals is used for the electrodes in the anode and cathode in fuel cells because these metal alloys are the best known catalysts for these chemical reactions. However, at the cathode the working potential is about five tenths of a volt less than it would be if platinum were a perfect catalyst for oxygen electroreduction. The result is power loss, which means the fuel cell doesn’t convert all of the chemical reaction energy available in the fuel into electricity. The lost power is wasted heating up the fuel cell and can cause the need to expend power to run fans or refrigeration to cool the fuel cell. When hydrogen is used as the fuel, the anode reaction is much more efficient than the cathode reaction but methanol and ethanol fuels generate very little electricity, in part because these fuels are only partially oxidized and form acids and aldehydes which remain stable within the anode compartment. When all the chemical energy is extracted, carbon dioxide is the only reaction product at the anode, along with the electrons and protons which go to the cathode. Our research was to develop means to understand the inefficiencies of the platinum alloy catalysts and predict properties of possible electrode materials that would increase the power outputs of fuel cells. We did accomplish these goals. We now have a good useful theory for predicting the voltages at which fuel cells will operate and to predict the voltages that must be applied to cause chemical reactions of the type that consume electricity. The predictions depend on the strengths of chemical bonds between reaction intermediates and the catalyst. The bond strengths can be calculated using special quantum mechanical theory written as computer programs. We call the procedure the linear Gibbs energy relationship or LGER theory. The first step in oxygen reduction on a fuel cell cathode when the solution in the fuel cell is acidic, as is usually used, is an electron and a proton combine with an oxygen molecule, O2, to form the intermediate molecule OOH, which is held to the catalyst by chemical bonds. The next step is the dissociation of OOH to the O atom, O, and the hydroxyl radical, OH, which also are held to the surface by chemical bonds. We found that on platinum alloys this dissociation reaction releases heat and the amount of heat generated accounts for most of the voltage and power loss in hydrogen fuel cells when platinum alloy catalysts are used. The problem with the platinum alloy catalysts is that the chemical bonds holding the OOH intermediate molecule are too weak and the chemical bonds holding the O and OH intermediates are too strong. We predicted values for the bond strengths for each intermediate to an ideal catalyst, one for which no heat would be generated and also for which the reaction steps would be fast enough for generating high levels of power. These chemical bond strengths have been introduced to other scientists in the scientific literature. We are hoping our findings will spark catalyst developers to try wholly new materials that they think might provide the needed bond strengths. From our study of thirteen platinum alloy catalysts and from the many studies of fuel cells using these alloys, it is evident that catalyst development needs new approaches since the performance differences among the alloys are very small. We have applied LGER theory to the methanol and ethanol electrooxidation reactions on the anode and we have found the chemical bond strengths between the intermediate molecules and platinum and have developed fuller understanding of why platinum is not a very successful catalyst for these reactions. We have predicted the ideal chemical bond strengths that would be possessed by the ideal catalyst. This work is being prepared for publication. We plan to use LGER theory in the future to understand electrochemical synthesis reactions.
