Phthalocyanine-Based Catalysts For Electrochemical Small Molecule Activation: Iron-Catalyzed Nitrate Reduction And Metal-Free Water Splitting

A growing global population and concurrent industrialization have contributed to the world’s dependence on fossil fuels to power various aspects of daily life, including electricity and heat generation and transportation. Heavy reliance on fossil fuels has led to rising carbon emissions, damaging the Earth’s atmosphere and contributing to global warming. The industries powered by fossil fuels also contribute to the life cycles of pollutants that contaminate our water supplies. Nitrate is a persistent pollutant that is commonly found in drinking water supplies. Nitrate pollution primarily originates from anthropogenic sources, such as agricultural runoff. To meet food demands, nitrogen-based fertilizers are overapplied, leading to poor soil health and excess runoff into drinking water streams. The Haber-Bosch process produces most of the world’s ammonia, which is then used as fertilizer, leading to nitrate contamination. Electrochemical nitrate reduction is a promising technique that can transform nitrate back to ammonia. Nitrate can serve as an alternative nitrogen source compared to the traditional Haber-Bosch process. We developed an iron-based phthalocyanine polymer via the Cu(I)-catalyzed “click” reaction, which we benchmarked against an amino-functionalized iron phthalocyanine. Nitrate reduction was examined under galvanostatic and potentiostatic conditions to assess both fundamental and practical approaches, and the overall nitrate conversion and ammonia selectivity were measured. The iron phthalocyanine polymer outperformed the molecular subunit, achieving up to 80% nitrate conversion and 90% ammonia selectivity. X-ray absorption spectroscopy operando studies revealed a decrease in the oxidation of the iron active site during electrolysis, which could later be regenerated following removal of the applied current. The search for alternatives to traditional fossil fuels has led to the development of renewable energy sources, including solar, wind, and hydroelectric. Additionally, secondary energy sources, such as hydrogen, can augment the use of alternative fuels. Hydrogen is considered a secondary energy source or energy carrier, and it has a higher energy density than gasoline or diesel. Traditionally, hydrogen is generated via methane steam reforming, an energy-intensive process that also produces carbon dioxide. When paired with renewable energy sources, electrochemical hydrogen generation can provide a cleaner platform for hydrogen generation. We developed a metal-free phthalocyanine-based polymer for the simultaneous generation of hydrogen (HER) and oxygen (OER) through electrochemical water splitting. Our material exhibits overpotentials of 576 mV and 380 mV for HER and OER, respectively. We corroborated the HER mechanism with our previous work on porphyrin-based polymers, and further explored the role of the chemical linker in the OER mechanism. The electrochemical performance of our material was comparable to that of other metalated phthalocyanine systems.