Oxide-Encapsulated Electrocatalysts for Solar Fuels Production
Creators: Labrador, Natalie Yumiko
Publisher: name: Columbia University
Year: 2018
Subjects: subject: Chemical engineering
subject: FOS: Chemical engineering
subject: Catalysts
subject: Electrocatalysis
subject: Solar energy--Storage
subject: Electrochemistry

description: As the cost of solar energy continues to drop, the major hurdle limiting the widespread use of intermittent renewable solar energy is the lack of efficient and cost-effective energy storage. Electrochemical technologies, such as electrolyzers, photoelectrochemical cells, and fuel cells, have the potential to compensate for solar energy intermittency on a large scale, by converting excess solar energy into storable solar fuels, such as hydrogen (H2), which can be converted back to electrical energy at a later time. However, improvements in the efficiency and lifetime of these technologies, in particular the electrocatalysts, are necessary for their commercialization. During operation, efficiency losses result from energetic penalties (overpotentials) associated with several processes occurring at or near the electrocatalyst/electrolyte (ohmic resistance, kinetic barriers, and mass transport limitations). These losses can be further exacerbated due to electrocatalyst durability issues such as dissolution, agglomeration, detachment, and poisoning. A major challenge in electrocatalysis field is developing methods to mitigate these losses without adversely affecting the electrocatalytic stability, selectivity, and/or activity. 
      One promising solution is an oxide-encapsulated electrocatalyst architecture, which has been shown to improve electrocatalyst durability and provide mechanisms for controlling reaction pathways. Previous studies on oxide-encapsulated electrocatalysts, in which metal catalysts are fully or partially covered by ultrathin layers of permeable oxide films, have mostly focused on supported nanoparticles because of their high electrochemically active surface area per catalyst loading. However, these nanoparticle-based architectures tend to have poorly defined and/or non-uniform structures which make it difficult to understand and elucidate structure-property-relationships. This dissertation investigates well-defined oxide-coated electrocatalysts, which serve as model platforms for gaining a fundamental understanding of kinetic and transport phenomena that underlie their operation. This dissertation presents three studies which highlight the versatile functionalities of oxide-encapsulated electrocatalysts to improve the electrocatalyst stability, selectivity, and activity in different electrochemical systems. This dissertation demonstrates the ability of room temperature synthesized silicon oxide (SiOx)-encapsulated Pt electrocatalysts to: i) stabilize nanoparticles and improve electron transfer, ii) mitigate catalyst poisoning and control reaction pathways through selective transport, and iii) alter reaction energetics associated with catalysis at the buried interface.
      First, this dissertation establishes the ability of room temperature synthesized SiOx coatings to stabilize nanoparticle electrocatalysts by mitigating electrocatalyst migration, coalescence, and detachment on metal-insulator-semiconductor (MIS) photoelectrodes for solar-driven water splitting. Metallic Pt nanoparticles are inherently unstable on the insulating support due to poor physical adhesion and electronic coupling between Pt and SiO2. To overcome this issue, a room temperature UV ozone synthesis process was used to deposit 2-10 nm thick SiOx overlayers on top of electrodeposited Pt nanoparticles to stabilize Pt on the electrode surface. The photoelectrodes containing oxide-encapsulated electrocatalysts exhibit superior durability and electron transfer (ohmic) properties compared to the photoelectrode that lacked the SiOx encapsulation. While this study demonstrates that the oxide-encapsulated electrocatalyst architecture improves the stability of electrocatalytic nanoparticles deposited on insulating materials, it does not elucidate how reactants and products transport through the SiOx barrier to reach the Pt surface.
      In order to gain a better understanding of kinetic and transport phenomena that govern performance of oxide-encapsulated electrocatalysts, the following studies investigate model electrodes consisting of continuous SiOx overlayers of uniform thickness deposited onto smooth Pt thin films. This planar electrode geometry allows for simple and unambiguous characterization of structure-property relationships. The next study systematically evaluates the influence of SiOx thickness on the HER performance to understand species transport through SiOx. Through detailed characterization and electroanalytical tests, it is shown that proton and H2 transport occur primarily through the SiOx coating such that the HER occurs at the buried Pt|SiOx interface. Importantly, the SiOx nanomembranes were found to exhibit high selectivity for proton and H2 transport compared to Cu2+, a model HER poison. Leveraging this property, it is shown that SiOx–encapsulation can enable poison-resistant operation of Pt HER electrocatalysts. This oxide-encapsulated architecture offers a promising approach to enhancing electrocatalyst stability while incorporating advanced catalytic functionalities such as poison resistance or tunable reaction selectivity. 
      The final study demonstrates ability of SiOx overlayers to alter reaction energetics associated with catalysis at the buried interface. Carbon monoxide (CO), methanol, and ethanol oxidation reactions are studied for their relevance in direct alcohol fuel cell applications. Oxide-supported catalysts have been shown to enhance alcohol oxidation by promoting CO oxidation at metal/oxide interfacial regions through the so-called bifunctional mechanism, in which hydroxyls on the oxide facilitate the removal of adsorbed CO−intermediates from active sites. A key advantage of the oxide-encapsulated electrocatalyst design compared to oxide–supported nanoparticles is that the former maximizes the density of metal/oxide interfacial sites. This study shows that the SiOx overlayer provides proximal hydroxyls, in the form of silanol groups, which can enhance CO and alcohol oxidation through unique interactions at the buried Pt|SiOx interface. Overall, this dissertation highlights the potential of using oxide-encapsulated electrocatalysts for stable, selective, and efficient electrochemical production and use of solar fuels.