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Solar Fuels
"Solar Fuels" is a renewable energy technology that involves using energy from the sun to convert abundant substances to fuel (e.g. Converting water to hydrogen). This is accomplished on an industrial scale by using solar panels to electrochemically split water to hydrogen and oxygen (PV approach). My research focused on the molecular approach which asks the question: Can we get rid of the solar panel altogether? Instead, can we design single molecules that can be dissolved in water, which act as molecular batteries to drive water splitting upon absorption of sunlight? Such molecular structures are known as "photosensitizers" and have been successfully developed over the last several decades. However, state-of-the-art photosensitizers still rely on ultra scarce metals such as iridium and ruthenium. The scarcity of these metals precludes the scalability of this technology.
With this limitation in mind, I decided to figure out how to make copper behave like iridium and ruthenium. I approached this problem by identifying five criteria that make the scarce metals work so well. Once the criteria were identified, I systematically designed the copper molecules to satisfy each criteria individually. After hitting each milestone individually, my team and I put our copper molecules to the test by dissolving them in THF spiked with water, and attempting to convert the water to hydrogen by shining visible light on the sample. The results can be seen in the following video: small bubbles are rapidly generated in the beam path. Gas chromatography measurements confirmed that these bubbles were pure hydrogen. Follow this link to read more about this exciting work!
Organic Light Emitting diodes
Organic LED (OLED) displays are an emerging technology which offer thinner devices with enhanced picture quality compared to traditional LED displays. OLED displays are lined with millions of pixels that contain electroluminescent molecules. When the device is turned on, positive and negative charges deposit onto the electroluminescent molecule (see the following image). The charged molecule is in the "excited state" and it can release its energy by emitting a photon, which lights up the display. While green and red pixels are stable for >20,000 hours of operation, blue pixels notably degrade within 1,000 hours of operation, which gradually diminishes the display quality. This is because emission out of the excited state is not spontaneous; there exists some amount of time where the molecule is sitting around charged up with energy. Two molecules lingering in the excited state can combine their energy, resulting in an ultra-high energy state. This isn't such a problem for red and green emitters, but doubling the energy of blue photons is enough to destroy the emissive molecule, which is why the blue pixel fails relatively quickly.
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One of the contemporary solutions to the "blue problem" is to speed up the molecule's rate of emission, which allows the excited molecule to quickly release its energy before it can perform energy transfer and decompose. In Mark Thompson's group at USC, we discovered a molecule with the highest reported blue emission rate with >95% quantum efficiency. However, we did not understand why our new molecules were emitting so quickly. I proposed a simplified model based on quantum physics that relates the rate of emission in these compounds to the charge separation on the molecule: larger charge separation leads to a faster emission rate (see following figure). We made 18 new molecules to verify teste this theory. A clear trend was observed in the experimental data; larger charge separation results in faster emission. Not only did we set a record for fastest blue emitter, we provided a guide to the OLED industry to make even faster emitting molecules. If you want to break our record, follow this link and you'll know how to do it!
Ocean Water Desalination
Drinkable water is a necessary resource for life, but 97% of the water on the planet is ocean water. Desalinating ocean water is currently performed by a process known as "reverse osmosis", which involves pumping ocean water through membranes with very fine pores to filter out the salt (see following figure). Ocean water and fresh water that are separated by a membrane have a natural tendency to mix. The pressure associated with this mixing force is the "osmotic pressure". Thus, one must apply enough pressure to overcome the osmotic pressure (~3kWh/m^3) to successfully perform reverse osmosis. This process is extremely energy intensive; reverse osmosis alone markedly contributes to global CO2 emissions. The substantial CO2 footprint comes from the high energy demand required to pump and deionize the water.
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During my time at Oregon State, I thought about designing a chemical desalination approach that would not require any external energy input. Since the primary salt in ocean water is sodium and chloride ions (Na+ and Cl-), my initial thought was to run ocean water through two ion-exchange resins. The first resin would exchange Na+ for H+ and the second would exchange Cl- for OH-. This would produce pure water as the final product as H+ and OH- react to make water (See the following figure). However, H+ and OH- ion exchange resins are made from organic materials that decompose and leach into water at high sodium chloride concentrations; they are better for treating brackish water. Additionally, regenerating the ion exchange column for future consumes more water than they are able to purify, yielding a net negative in fresh water production. Thus, I shifted my focus to inorganic materials because they are more robust than organic resins.
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I learned about an exotic class of aqueous, inorganic molecules called "polyoxometalates" under the mentorship of Prof. May Nyman. These are metal-oxo clusters that are typically highly charged (see the following figure: M is a metal ion, n = 2 to 8). I identified this as a useful property for desalination, as each cluster can accommodate a large number of ions. For example, Ta6O19 has an 8- charge, thus it can accommodate 8 Na+ per cluster. I identified two materials that bind to Na+ and Cl- respectively. Preliminary data shows that these molecules efficiently exchange Na+ for H+ and Cl- for OH- followed by precipitation out of solution. The precipitated material is easily filtered off, and I designed a treatment that allows the precipitate to be recycled for future use. In summary, I designed a system that behaves like a molecular salt sponge: the molecules spontaniously desalinate ocean water without any external energy input, and the bound Na+ and Cl- can be easily expelled from the precipitate for re-use of the starting material.
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Fast Charging Ion Batteries
Rechargeable lead acid batteries were invented in the mid-1800s. These batteries lacked long term stability and took hours to recharge. Over 100 years later, the first lithium-ion batteries (LIBs) were developed in the 1970s, which are significantly more stable and charge faster than the lead acid battery predecessor. Even so, increasing the charging rate of batteries remains a modern commercial challenge as consumers want to cut down the charging time on their smartphones and vehicles.
Charging LIBs requires lithium ions to eject out of the cathode and intercalate into the anode (see following video). The rate limiting step of this process is the sluggish lithium-ion mobility. In all batteries to date, the only force that drives lithium mobility is the potential difference applied to the electrodes. What if one could add another force to enhance ion mobility? I have written a proposal that details the synthesis of new electrode materials that could increase the charging rate of modern batteries by up to 6 orders of magnitude. This technology could unlock instant charging (less than 0.001 seconds to reach 100% charge). If you are interested in working on this with me, please reach out. Stay tuned for more details on this technology.
Coordination Polymers
My professional scientific career began in 2013 as an undergraduate researcher in Prof. May Nyman's lab. Even though I was a freshman at OSU, I asked Prof. Nyman if I could spearhead my own project without grad student supervision. She graciously accepted and I started a project that was initially related to lithium-ion battery technology. I threw myself into the project which resulted in a serendipitous discovery of a new class of rare-earth coordination polymers. These structures feature trivalent rare earth ions that polymerize with pentavalent niobium through oxalate bridging ligands (See the following figure). This project was a great opportunity for me to gain experience in making new materials, characterizing them, and write a manuscript on their contribution to science. Follow this link to read more!
