Nanostructured Earth-abundant Materials as Catalysts for the Hydrogen Evolution Reaction

Open Access
Author:
Popczun, Eric John
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 25, 2015
Committee Members:
  • Raymond Edward Schaak, Dissertation Advisor
  • Benjamin James Lear, Committee Member
  • Christine Dolan Keating, Committee Member
  • James Hansell Adair, Committee Member
Keywords:
  • Nanoscience
  • Metal phosphide
  • Hydrogen evolution
  • Nanoparticles
  • Electrochemistry
  • Electrocatalysis
  • Earth-abundant
  • Catalysis
Abstract:
The need for alternate fuel sources, which are not fossil fuels or hydrocarbons, has rapidly grown over the past few decades due to the development of environmental, monetary, and political issues associated with fossil fuels. Hydrogen is a possible ideal fuel, because the only byproduct upon energy release from H2 is clean water, and its energy density is much higher than conventional hydrocarbon-based fuels. Industrially, hydrogen is produced via methane reformation and the water-gas-shift reaction, which are processes that release CO2 into the atmosphere, thereby eliminating some of the environmental benefits of using hydrogen as a fuel. One clean alternative is water electrolysis, which involves the decomposition of water into hydrogen and oxygen. The hydrogen evolution reaction (HER) is the half-reaction that produces hydrogen in water electrolysis [2 H+ + 2 e- → H2 (g)]. To date, the most active catalyst for HER is platinum, a rare and expensive material that is needed for a variety of applications. There is a growing need for new Earth-abundant catalysts that can serve as alternatives to platinum. This dissertation describes the synthesis and electrochemical characterization of a new class of Earth-abundant, acid-stable HER catalysts that were inspired by the catalysts for mechanistically related chemical reactions, like hydrodesulfurization. The discussion will begin with our studies of the catalytic capabilities of hollow Ni2P nanoparticles for the hydrogen evolution reaction. The Ni2P(001) surface was previously predicted to outperform platinum as a catalyst for the hydrogen evolution reaction through density functional theory calculations by Liu and Rodriguez. The Ni2P nanoparticles were synthesized colloidally through the decomposition of tri-n-octylphosphine in the presence of in situ Ni nanoparticles. The Ni2P nanoparticles had among the highest HER activity of any non-noble metal electrocatalyst reported at the time of publication, producing H2(g) with nearly quantitative Faradaic yield, while also affording stability in aqueous acidic media The story continues with our second catalytic system, CoP nanoparticles. Cobalt phosphides had not been previously studied for HER catalysis, unlike Ni2P. While synthesized similarly to the Ni2P nanoparticles, the CoP nanoparticles vastly outperformed the Ni2P nanoparticles, becoming the first non-noble metal acid-stable HER catalyst to achieve a current density of -10 mA/cm2 at an overpotential less negative than -100 mV. Additionally, the catalyst shows excellent electrochemical stability and Faradaic efficiency. Next, morphological considerations as they pertain to HER catalysis are discussed. Highly branched CoP nanostructures that have a similar surface area to the CoP nanoparticles were synthesized. These nanostructures express a high number of (111) crystal facets, allowing us to investigate the catalytic activity of the (111) surface on this anisotropic sample. Our findings suggest the morphological control does not play a major role in the catalytic activity of CoP, while also verifying the high intrinsic activity of the material. Before concluding, we briefly discuss a few additional catalytic systems that were targeted because of their known activities for hydrodesulfurization catalysis. The catalysts discussed include several transition metal phosphides, carbides, and chalcogenides, with varying degrees of success. Ternary metal phosphide solid-solutions were electrochemically characterized, including Co1-x¬FexP, that attempts to combine the high activity of FeP with the stability of CoP, as well as Ni2-xCoxP systems that are known to show higher HDS activity than either end member. In addition, we synthesized the ternary phosphides CoMoP, CoMoP2, and NiMoP2 through phosphate reduction and investigated their catalytic activities. Nickel carbides as well as cobalt chalcogenides are also discussed briefly. Overall, these materials are active HER catalysts but their overpotentials are significantly higher than those of the phosphides presented earlier in the dissertation.