UC San Diego

Avid consumption of fossil fuels by humans has led to record high levels of the greenhouse gas carbon dioxide (CO2). An energy storage medium to replace dwindling and finite fossil fuel supplies must be found. Hydrogen gas (H2) is an attractive alternative to hydrocarbon fuels but presently natural gas is the primary feedstock for H2 production globally. Therefore, an alternative process for hydrogen production must be developed. Photoelectrochemical electrolysis of water (H2O) presents a promising method for the clean generation of H2. The oxidation of water to oxygen (O2), four protons (H+), and four electrons (e-) must precede the reduction of H+ to H2 and is the more demanding reaction both kinetically and thermodynamically. Transition metal catalysis can realize this challenging transformation. Careful choice of metal and ligand design can facilitate each stage of the oxidation of water; in particular, the management of H+ during catalysis can help improve both the speed and the durability of the catalyst. This dissertation describes the synthesis, characterization, and water oxidation activity of two new water oxidation catalysts featuring heteroatom H+ relays and the development of instrumentation to better quantify the O2 produced during H2O oxidation.

Chapter 2 describes the incorporation of phosphonate monoester and sulfonate and pendant bases into the first coordination sphere of the well-studied water oxidation platform [(2,2';6',2"- terpyridine)(2,2’-bipyridine)Ru(OH2)]2+. The complexes were characterized by combustion analysis, NMR spectroscopy, and x-ray crystallography. The catalytic performance of the complexes was evaluated for water oxidation catalysis using ceric ammonium nitrate (CAN) as a sacrificial oxidant via manometry and by square wave and cyclic voltammetry in a buffered aqueous milieu. The phosphonate monoester was found to perform poorly under chemical oxidation conditions but did show electrocatalytic behavior by cyclic voltammetry. The sulfonate system performed very well with CAN as the oxidant demonstrating a turnover frequency of 0.88 s-1 and turnover number of 7402. The sulfonate system also demonstrated electrocatalytic behavior suggesting homogenous electrocatalysis is maintained. Pourbaix analysis and a computational study suggest the intermediacy of a unprecedented ruthenium (III) oxyl, with the sulfonate acting as a pendant base late in the catalytic cycle.

Chapter 3 describes the development of two pieces of instrumentation for the detection of oxygen produced by our water oxidation catalysts. The first instrument is a dual manometry/optical oxygen sensing cell which was constructed for evaluating catalyst performance. Problems with the fluorescent oxygen sensor including drift and sensitivity to humidity led us to focus on pressure as the primary indication of oxygen production. The cell is a robust and easy to use system that provides excellent repeatability and reliability, with <2% drift at more than twice typical operating pressure over 60 h. The second piece of instrumentation was an automated system for sampling headspace gas in a bulk electrolysis (BE) cell. The design and construction of a panel that interfaces a custom BE cell, potentiostat, and gas chromatograph/mass spectrometer (GC/MS) is described.