Rationally Engineering Porous Carbon-Based Metal Nanocomposites for Efficient and Durable Electrocatalysis Applications
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Date
2021-08-26
Authors
Zhang, Zhen
Journal Title
Journal ISSN
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Publisher
University of Waterloo
Abstract
Electrocatalysis plays an essential role in electrochemical energy storage and conversion, enabling a number of sustainable processes for future technologies such as metal−air batteries, sulfur-based batteries, and carbon dioxide (CO2) conversion. The grand challenge is to develop advanced electrocatalysts with enhanced activity, selectivity and durability to enable widespread adoption of clean energy technologies. Carbon-based metal hybrid materials have been receiving intense interest as promising electrocatalysts for the electrochemical transformations central to the energy conversion and chemical production technologies. Constructing effective electrocatalysts requires fundamental understanding, rational design and delicate manipulation of the catalytically active sites. This thesis work presents advanced electrocatalyst design strategies by rationally engineering porous carbon-based metal nanocomposites for promising electrochemical transformation systems including oxygen, sulfur, and CO2 electrocatalysis, providing a new route to efficiently convert abundant resources such as H2O, S, and CO2 to electricity to march toward a sustainable energy future. In the first study (Chapter 3), a unique “ship in a bottle” concept in catalyst design is proposed, which is to impregnate metal nanoparticles/nanoclusters inside the nanopores of a porous carbon matrix. As a proof-of-concept, the catalyst, composed of cobalt sulfide (CoS2) nanoparticles impregnated within the S-doped defective carbon nanopores that act as interconnected nanoreactors, is engineered for bifunctional oxygen electrocatalysis including oxygen reduction (ORR) and oxygen evolution reaction (OER). The erected 3D porous conductive architecture provides a “highway” for expediting charge and mass transfer. This design not only delivers a high surface-to-volume ratio to increase numbers of exposed catalytic sites but also precludes nanoparticles from aggregation during cycling owing to the pore spatial confinement effect. Therefore, the long-term plague inherent to nanocatalyst stability can be solved. Moreover, the synergistic coupling effects between defect-rich interfaces and chemical bonding derived from heteroatom-doping boost the catalytic activity and prohibit the detachment of nanoparticles for better stability. Consequently, the developed catalyst not only presents superior bifunctional oxygen electrocatalytic activities and durability, but also enables a long-term cyclability for over 340 hours at a high current density of 25 mA cm-2 in a practical application of rechargeable Zn−air batteries. Such a universal “ship in a bottle” design offers an appealing and instructive model of nanocatalyst engineering. Based on the proposed “ship in a bottle” design concept, the second work (Chapter 4) further introduces defect engineering and crystallinity manipulation of metal nanoclusters. The engineered ultrafine amorphous tantalum oxide nanoclusters with oxygen vacancies (Ta2O5-x) implanted inside a microporous carbon matrix are, for the first time, employed as a new electrocatalyst for polysulfide catalysis and retention. Through a pore-constriction mechanism, the dimensions of tantalum oxide are controlled to be nanosized, not only shaping the incomplete unit cell in an amorphous structure for efficient crystallinity tuning, but also exposing abundant polysulfide-retaining and catalytically active sites. The introduced oxygen vacancies in tantalum oxide manipulating electron structure with increased intrinsic conductivity function as catalytic centers to accelerate sulfur redox reactions. Moreover, the polysulfide shutting effect, sulfur agglomeration and volume expansion are well suppressed in the designed pitaya-like structure. As a result, the developed sulfur electrode in a lithium-sulfur battery presents excellent cycling stability and rate capability at practically relevant sulfur loadings and electrolyte content. The last study (Chapter 5) further optimizes the monometallic design to a bimetallic design by a “two ships in a bottle” strategy to meet higher electrocatalyst requirements. The engineered bimetallic Zn-Ag-O catalysts, where ZnO and Ag phases are twinned to constitute an individual ultrafine nanoparticle impregnated inside nanopores of an ultrahigh-surface-area carbon matrix, enable selective and durable CO2 electroreduction to CO. Bimetallic electron configurations are modulated by constructing a Zn-Ag-O interface, where the electron density reconfiguration arising from electron delocalization enhances the stabilization of the *COOH intermediate favorable for CO production, while promoting CO selectivity and suppressing HCOOH generation by altering the rate-limiting step toward a high thermodynamic barrier for forming HCOO*. Moreover, the pore-constriction mechanism restricts the bimetallic particles to nanosized dimensions with abundant Zn-Ag-O heterointerfaces and exposed active sites, meanwhile prohibiting detachment and agglomeration of nanoparticles during CO2 reduction for enhanced stability. The designed catalysts realize 60.9% energy efficiency and 94.1 ± 4.0% Faradaic efficiency toward CO, together with a remarkable stability over 6 days. Beyond providing a high-performance CO2 reduction electrocatalyst, this study presents a promising catalyst-design strategy for efficient energy conversion.
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Keywords
electrocatalysis, nanomaterials, porous carbons, electrochemistry