Electrochemical hydrogen Storage Systems

Abstract

As the global need for energy increases, scientists and engineers have found a possible solution by using hydrogen to power our world. Although hydrogen can be combusted as a fuel, it is considered an energy carrier for use in fuel cells wherein it is consumed (oxidized) without the production of greenhouse gases and produces electrical energy with high efficiency. Chemical storage of hydrogen involves release of hydrogen in a controlled manner from materials in which the hydrogen is covalently bound. Sodium borohydride and aminoborane are two materials given consideration as chemical hydrogen storage materials by the US Department of Energy. A very significant barrier to adoption of these materials as hydrogen carriers is their regeneration from 'spent fuel,' i.e., the material remaining after discharge of hydrogen. The U.S. Department of Energy (DOE) formed a Center of Excellence for Chemical Hydrogen Storage, and this work stems from that project. The DOE has identified boron hydrides as being the main compounds of interest as hydrogen storage materials. The various boron hydrides are then oxidized to release their hydrogen, thereby forming a 'spent fuel' in the form of a lower boron hydride or even a boron oxide. The ultimate goal of this project is to take the oxidized boron hydrides as the spent fuel and hydrogenate them back to their original form so they can be used again as a fuel. Thus this research is essentially a boron hydride recycling project. In this report, research directed at regeneration of sodium borohydride and aminoborane is described. For sodium borohydride, electrochemical reduction of boric acid and sodium metaborate (representing spent fuel) in alkaline, aqueous solution has been investigated. Similarly to literature reports (primarily patents), a variety of cathode materials were tried in these experiments. Additionally, approaches directed at overcoming electrostatic repulsion of borate anion from the cathode, not described in the previous literature for electrochemical reduction of spent fuels, have been attempted. A quantitative analytical method for measuring the concentration of sodium borohydride in alkaline aqueous solution has been developed as part of this work and is described herein. Finally, findings from stability tests for sodium borohydride in aqueous solutions of several different compositions are reported. For aminoborane, other research institutes have developed regeneration schemes involving tributyltin hydride. In this report, electrochemical reduction experiments attempting to regenerate tributyltin hydride from tributyltin chloride (a representative by-product of the regeneration scheme) are described. These experiments were performed in the non-aqueous solvents acetonitrile and 1,2-dimethoxyethane. A non-aqueous reference electrode for electrolysis experiments in acetonitrile was developed and is described. One class of boron hydrides, called polyhedral boranes, became of interest to the DOE due to their ability to contain a sufficient amount of hydrogen to meet program goals and because of their physical and chemical safety attributes. Unfortunately, the research performed here has shown that polyhedral boranes do not react in such a way as to allow enough hydrogen to be released, nor do they appear to undergo hydrogenation from the spent fuel form back to the original hydride. After the polyhedral boranes were investigated, the project goals remained the same but the hydrogen storage material was switched by the DOE to ammonia borane. Ammonia borane was found to undergo an irreversible hydrogen release process, so a direct hydrogenation was not able to occur. To achieve the hydrogenation of the spent ammonia borane fuel, an indirect hydrogenation reaction is possible by using compounds called organotin hydrides. In this process, the organotin hydrides will hydrogenate the spent ammonia borane fuel at the cost of their own oxidation, which forms organotin halides. To enable a closed-loop cycle, our task was then to be able to hydrogenate the organotin halides back to their hydride form. In addition to this experimental work, a parallel project was carried out to develop a new model of electrochemical impedance spectroscopy (EIS) that could be used to define the mechanisms of the electrochemical hydrogenation reactions. The EIS technique is capable of probing complex chemical and electrochemical reactions, and our model was written into a computer code that allowed the input of experimental EIS data and the extraction of kinetic parameters based on a best-fit analysis of theoretical reaction schemes. Finally, electrochemical methods for hydrogenating organic and metallo-organic materials have been explored.