NANOSTRUCTURED EARTH-ABUNDANT ELECTROCATALYSTS FOR THE HYDROGEN AND OXYGEN EVOLUTION REACTIONS

Restricted (Penn State Only)
Author:
Callejas, Juan Francisco
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 15, 2016
Committee Members:
  • Raymond Edward Schaak, Dissertation Advisor
  • Raymond Edward Schaak, Committee Chair
  • Thomas E Mallouk, Committee Member
  • John V Badding, Committee Member
  • Susan E Trolier-Mckinstry, Outside Member
Keywords:
  • Nanomaterials
  • Electrocatalyst
  • Nanoparticle
  • Water-Splitting
Abstract:
Hydrogen gas obtained though the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. The widespread implementation of electrolyzers and solar-driven water-splitting systems is underpinned by the development of the individual components that comprise these systems, which must be efficient, robust and scalable. Among these components, highly active electrocatalysts are required to carry out the water-splitting half-reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Electrocatalysts based on noble metals such as Pt and Ir, are capable of carrying out the HER and OER at low overpotentials, however their high cost and scarcity has motivated the discovery and development of robust, efficient, and Earth-abundant alternatives. Transition metal phosphides have recently been identified as a promising family of Earth-abundant electrocatalysts for the HER, and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions. In this dissertation, I highlight the progress that has been made in this field and provide insights into the synthesis, characterization and electrochemical behavior of transition metal phosphides as HER electrocatalysts. I describe that iron phosphide (FeP), synthesized as nanoparticles having a uniform, hollow morphology, exhibits among the highest HER activities reported to date in both acidic and neutral-pH aqueous solutions. Additionally, under UV illumination in both acidic and neutral-pH solutions, FeP nanoparticles supported on TiO2 are able to sustain photcatalytic H2 production over several hours. I also discuss the synthesis, characterization and electrochemical performance of cobalt phosphide (Co2P) nanoparticles having a hollow, multifaceted, crystalline morphology as another highly active Earth-abundant HER catalyst material. Importantly, the Co2P nanoparticles are morphologically equivalent to previously reported CoP nanoparticle HER catalysts, allowing a direct side-by-side evaluation of their HER activities. Such comparisons of different metal phosphide HER catalysts with the same constituent elements and morphologies are important for identifying the key materials characteristics that lead to high activity. Unlike the various metal phosphide catalysts available for the HER, OER electrocatalysts that facilitate sustained oxygen production at device-relevant current densities in strongly acidic electrolytes have been limited almost exclusively to precious metal oxides. I demonstrate that nanostructured films of cobalt oxide (Co3O4) on fluorine-doped tin oxide (FTO) substrates, function as active electrocatalysts for the OER in highly acidic conditions. The Co3O4/FTO electrodes evolve oxygen with near-quantitative Faradaic yields and maintain operationally relevant current densities for over 12 h at a moderate overpotentials, making it one of the few Earth-abundant electrocatalytic systems capable of sustained oxygen evolution in acidic conditions.