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Explorations of Novel Energy Conversion and Storage Systems

Abstract

At present, the majority of the world's energy demand is met by the consumption of exhaustible fuel supplies. Consequently, it is urgent to research and develop viable alternatives. In this dissertation, I present research that addresses fundamental questions concerning how water interacts with surfaces and solutes, with the goal of identifying novel systems for energy production and storage.

Electrokinetic currents are created when moving fluid entrains charge from the diffuse portion of an electric double layer and carries that charge downstream. The potential difference that develops on either end of the channel is known as the streaming potential. Chapter 2 of this dissertation focuses on electrokinetic energy production and conversion efficiency of liquid microjets. Section 1 of Chapter 2 presents proof-of-principle research demonstrating that molecular hydrogen is generated from electrokinetic currents in liquid water microjets. Hydrogen is generated when hydrated protons are preferentially carried downstream and recombine with electrons at a grounded target electrode. Both the current and hydrogen production scale nearly quadratically with flow rate, as predicted by equations derived from simple double layer theory and fluid mechanics. The efficiency is currently very low (ca 10-6) and is limited by the low electrokinetic current (~nA). Designs to improve this efficiency are considered.

Rather than chemical conversion efficiency, Section 2 of Chapter 2 investigates the electrical conversion efficiency of liquid water microjets. Typical electrokinetic energy conversion schemes measure current or voltage via electrodes in the fluid reservoirs on either side of a channel. With this design, the streaming potential drives a current against the flow of the fluid and, consequently, limits the conversion efficiency. In contrast, liquid microjets break up into droplets before reaching the downstream electrode and this eliminates the possibility for back conduction. As a result, liquid microjets yield conversion efficiencies exceeding 10%, much larger than channel-dependent measurements (~3%).

It is the large potentials obtainable with electrokinetic currents (tens of kilovolts) that drive up the electrical conversion efficiency. Unfortunately, low currents with high voltages are inconvenient for application. Section 3 of Chapter 2 describes efforts to utilize the high voltage of electrokinetic currents by coupling light into the process. More specifically, the streaming potential is used to modify the space charge layer in a semiconductor and, consequently, the light harvesting characteristics of that semiconductor. To this end, microchannel jets fabricated out of glass and silicon were built to allow light to impinge on the current generating surface. Although plagued with inconsistent results, streaming currents were found to increase upon illumination and some channels even gave measurable responses to ambient room lights.

Chapter 3 of this dissertation addresses the details of hydration of boron-oxides and sodium borohydride as studied by near edge x-ray absorption fine structure spectroscopy (NEXAFS) and associated theory. Boron-oxides and molecular hydrogen are products of borohydride hydrolysis which has been intensely studied for hydrogen storage purposes. In spite of their hydroxide moieties, boron-oxides turn out to not be strongly hydrated by water. The experimental spectra, as well as attending calculations, show no evidence for electronic coupling that would indicate strong hydrogen bonding between the boron-oxides and water. On the other hand, the NEXAFS spectrum of sodium borohydride is significantly altered by water. The experiment and calculations show strong evidence for short dihydrogen bonds between water hydrogens and borohydride hydrogens. Molecular dynamics simulations indicate that borohydride is hydrated at the tetrahedral corners and edge.

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