Dissertation
Rapid and energetic solid-state metathesis reactions for metal boride formation and their investigation as bifunctional water splitting electrocatalysts
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2023
DOI: 10.25820/etd.006998
Abstract
Scientists and chemists are pursuing new and sustainable approaches to solutions for the energy and environmental issues that the world is currently experiencing and will encounter in the near future. Finding a long-term, environmentally friendly fuel source as an alternative to rapidly depleting fossil fuels and meeting future energy demand is one of the greatest global issues. Many natural and artificial alternative energy sources have been discovered, but most of them have several limitations that make them unsuitable as future energy sources. Hydrogen (H2) has grown in popularity as an energy source due to mainly its high gravimetric energy density and ability to produce pollution-free clean energy. However, a few key issues in H2 must be addressed before it can be used as the world's future fuel, specifically the current high cost and environmental impact of hydrogen production. Water-electrolysis to produce H2 and O2 using abundant water has gained enormous attraction. Electrocatalysts are commonly associated with the improvement of H2 and O2 production in the cathode and anode of water electrolyzers. Therefore, the development of cost- and energy-effective electrocatalysts for water electrolysis is a vast area of research worldwide. Transition metal compounds (for example, metal phosphides, sulfides, carbides, oxides, and borides) have demonstrated their feasibility as low-cost, high-active, and long-lasting electrocatalyst alternatives to currently commercially used noble and expensive electrocatalysts. Even though, some transition metal compounds have been investigated as electrocatalysts for water electrolysis thus far, many more remain to be studied.
Metal borides have long been used in some applications due to their desirable chemical and physical properties, such as high melting points, hardness, electrical conductivity, and chemical stability. The properties of metal borides have the potential to produce active and long-lasting electrocatalysts. The synthesis method/reaction conditions determine the metal boride morphology, size, number of active sites on the surface, and crystallinity of the final product (for example, temperature, pressure, and time). Typical metal boride preparations use high-energy and/or slow thermal heating processes, resulting in aggregated particles with fewer surface-active sites and a high production cost. To screen and evaluate metal boride overall electrocatalytic activity, it is critical to develop a method for quickly synthesizing metal borides under mild reaction conditions/using a cost-effective method and examining their electrocatalytic activity for water electrolysis. Solid-state reactions have many advantages over liquid and gas phase reactions such as easy sample preparation and purification, easy reaction handling, less apparatus and solvent requirement, etc., but typically they are slow and use harsh reaction conditions (high temperature and/or high pressure) to material synthesis. Several chemical and physical methods have been successfully applied in recent years to speed up solid-state reactions and lower the required temperatures. A chemical reaction pathway called solid-state metathesis uses a small amount of external energy to produce solid materials rapidly. Using the intrinsic energy of the reaction, the reaction self-propagates as the temperature increases, yielding crystalline nano- and micrometer-sized particles in a matter of seconds.
This dissertation details rational, solvent-free single-step synthesis of several crystalline metal monoborides containing earth-abundant late transition metals (Chapter 3) and rigid early transition metals (Chapter 6) using solid-state metathesis reactions. In addition to typical MClx/MgB2 solid-state metathesis reaction pathway, we newly recognized MClx/Mg/B reaction pathway were examined for monoborides formation. Late transition metal borides can be synthesized from traditional MClx/MgB2 SSM reactions, but only TiB2 can be formed from early transition metal borides, nevertheless, all late and early transition metal borides were successfully obtained from new MClx/Mg/B reactions. SSM reactions yield crystalline monoborides in seconds without the need for continuous external heating and with high isolated product yields (~80%). These SSM reactions are sufficiently exothermic to theoretically raise reaction temperatures to the boiling point of the MgCl2 byproduct (1412 °C). The chemically robust monoborides were examined for their ability to perform electrocatalytic water oxidation and reduction. Crystalline CoB and NiB embedded on carbon wax electrodes demonstrate moderate and stable bifunctional electrocatalytic water splitting activity, whereas FeB only exhibits detectable hydrogen evolution activity. Early transition metal borides exhibit only HER activity and no oxygen evolution. Analysis of catalyst particles after extended electrocatalytic experiments shows that the bulk crystalline metal borides remain intact during electrochemical water-splitting reactions though surface oxygen species may impact electrocatalytic activity.
This dissertation also describes the SSM reaction's further development to the rapid, single-step, solvent-free synthesis of more advanced mixed metal borides, and metal boride/nitride composites using MClx/Mg/B reaction directions. he successful formation of these complex structures demonstrates the versatility of SSM reactions for quick, effective, and inexpensive way of complex structure formation in high isolated product yields (>80%). Mixed metal borides are moderately electrocatalytically active for both hydrogen and oxygen evolution reactions, and they exhibit systematic electrocatalytic activity behavior while distributing activity between parent structures' electrocatalytic activities. SSM-derived crystalline MBs exhibit excellent crystallinity retention in post-electrocatalyst materials, demonstrating that crystalline metal borides are stable, long-lasting electrocatalysts for water-splitting reactions.
Details
- Title: Subtitle
- Rapid and energetic solid-state metathesis reactions for metal boride formation and their investigation as bifunctional water splitting electrocatalysts
- Creators
- Janaka Prasad Abeysinghe
- Contributors
- Edward G Gillan (Advisor)Johna Leddy (Committee Member)Mark A Arnold (Committee Member)Alexei V Tivanski (Committee Member)Scott R Daly (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Chemistry
- Date degree season
- Spring 2023
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.006998
- Number of pages
- xxx, 366 pages
- Copyright
- Copyright 2023 Janaka Prasad Abeysinghe
- Language
- English
- Date submitted
- 04/28/2023
- Date approved
- 06/30/2023
- Description illustrations
- illustrations, tables, graphs
- Description bibliographic
- Includes bibliographical references (pages 263-283).
- Public Abstract (ETD)
- The fast-growing world is driving through its energy reserves. Energy is used for every aspect of our day-to-day life including cooking, transportation, electricity and thermal generation. The world energy requirements keep increasing with increasing the population as well as increasing the human needs. Fossil fuels continue to be our primary energy source, accounting for more than 80% of total energy, but they are rapidly depleting. Fossil fuels are formed over millions of years from the remains of long-dead plants and animals, and as such, they are a non-renewable source, which means that once consumed, they disappear. It is urgent requirement to find and utilize efficient and long-time supply energy sources as alternative to fossil fuels. Hydrogen (H2) has been identified as one of best energy source because of its high energy density and low environmental impacts. However, the large-scale use of hydrogen as a future fuel has been severely hampered by the lack of an environmentally friendly, low-cost, and efficient hydrogen production methods. Currently, the major hydrogen production methods, natural gas reforming and gasification, use fossil fuel sources and contribute to the greenhouse effect (CO2 release during H2 formation). Water electrolysis or water splitting to H2 and O2 gas molecules are environmentally friendly methods of producing pure hydrogen, but their popularity has remained low due to process and reaction limitations. A major limitation of water electrolysis is the lack of an effective, low-cost, and abundant electrocatalyst for conducting hydrogen and oxygen evolution reactions at low overpotentials. The noble metals currently used as electrocatalysts are expensive and scarce. Transition metal compounds have demonstrated good to moderate electrocatalytic activities and high stability as electrocatalysts for water splitting reactions with being abundant and inexpensive, but more research is needed to identify the best transition metal electrocatalysts. Metal borides have been used for a long time in some applications due to their beneficial chemical and physical properties, such as high melting points, hardness, electrical conductivity, and chemical stability. Metal borides' characteristics have the potential to create electrocatalysts that are both active and durable. It is essential to develop a method for quickly synthesizing metal borides under mild reaction conditions using a cost-effective method and assessing their electrocatalytic activity for water electrolysis. Solid-state reactions have many advantages over liquid and gas phase reactions, including simple sample preparation and purification, simple reaction handling, a reduction in the need for apparatus and solvents, etc. However, they are typically slow and use harsh reaction conditions (high temperature and/or high pressure) to synthesize materials. More progress in fast reaction rates is required in order to benefit from solid- state reactions for the synthesis of metal borides. This dissertation details the development of rapid solid-state reactions that release energy during their reaction to quickly (in seconds) form crystalline metal borides and stable MgCl2 salt. The characterized metal borides using a wide variety of transition-metals were studied as possible water splitting electrocatalysts. Several metal borides were identified as effective H2 and O2 evolution electrocatalysts, particularly those with earth abundant iron, cobalt, and nickel metals. These metal boride electrocatalysts may provide a foundation for effective and less expensive water splitting electrocatalysis.
- Academic Unit
- Chemistry
- Record Identifier
- 9984425312402771
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