Rapid and energetic solid-state metathesis reactions for metal boride formation and their investigation as bifunctional water splitting electrocatalysts
Creators: name: Abeysinghe, Janaka Prasad
Publisher: name: University of Iowa
Year: 2024
Subjects: Crystalline metal borides
Hydrogen evolution electrocatalysis
Metal boride solid-solutions
Oxygen evolution electrocatalysis
Solid-state metathesis
Thermochemistry
public abstract

description: 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.
description: 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.
description: 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.
description: 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 (&gt;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.