Designed Hierarchical Non-Noble Metal Electrocatalyst for Multifunctional Energy Applications


Student thesis: Doctoral Thesis

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Awarding Institution
Award date6 Sep 2017


    Electrocatalytic reactions lie at the heart of many regenerative energy conversion and storage techniques, which depict promising prospects for the sustainable development of modern society. Oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution reaction (HER), for instance, play pivotal roles in a series of pertinent renewable technologies, ranging from fuel cells through metal-air batteries to water-splitting systems. Electrocatalysts are indispensable to reduce the overpotentials and thus speed up the rate of these reactions. Pt-based catalysts have long been studied as the best catalysts for ORR and HER, while IrO2 and RuO2 are the most efficient OER materials. Regretfully, even these state-of-the-art catalysts still exhibit unsatisfactory catalytic performance in each other’s category, and the scarcity and poor durability of these noble metal-based catalyzers greatly hinder their commercialization on a large scale. Therefore, the core challenge of electrocatalysis remains on the searching for versatile, efficient, durable and cost-effective catalysts, which has also been the one of the “holy grails” of chemistry for decades.
    Recent years have witnessed the blossom of various non-noble metal based multifunctional electrocatalysts. These electrocatalytic materials can be roughly divided into two categories according to their componential elements, viz., metal-free and non-noble transitional metal-based catalysts. Though great progresses have been achieved by ever-increasing academic efforts, the general activities of these nanocatalysts are still inferior to those delivered by the noble-metal counterparts. In principle, a favored electrocatalyst should be capable to deliver a sufficiently high reaction current at low overpotential to warrant an efficient power output and energy conversion efficiency. In pursuing this advance, the most effective strategy is to investigate the coherent interplay between the component-structure-performance aspects of the catalytic materials. Chapter 1 of the present thesis gives the fundamental knowledge and background of electrocatalysis and non-noble metal based electrocatalysts, with emphases on the component-structure-performance correlationships, which were also exemplified by a series of relevant and pertinent latest studies. This advocated strategy was then adopted to investigate three types of typical electrocatalysts (including both metal-free and non-noble transitional metal-based materials) and their feasibility for practical energy applications as follows.
    Atomically precise understanding of componential influences is crucial for looking into the reaction mechanism and controlled synthesis of efficient electrocatalysts. Graphitic carbon nitride (g-C3N4) based hybrid catalyst was first studied as a typical metal-free electrocatalyzer. In contrast to prior endeavors which focus exclusively on the exterior modification over the g-C3N4 species (e.g., coupling g-C3N4 with various conductive substrates, and/or tailoring g-C3N4 into different architectures), in chapter 2, the first comprehensive study on the componential influences within g-C3N4 motifs towards its electrocatalytic performances was reported. The macroporous carbon (PC) supported g-C3N4 monoliths with three types of dopant elements (B, P and S) embedded in different sites (either C or N) of the C-N skeleton (X-CN/PC) were rationally designed and synthesized. It was demonstrated that the different site-constituted B, P and S elements exhibited substantially distinct impacts over the corresponding electroactivities, among which the S-doped hybrid afforded the best reaction kinetics and intrinsic activities for boosting both ORR and HER. Theoretic calculations revealed that the dopant elements could render different charge and spin densities within the g-C3N4 motifs, which then altered the sorption free energies of different reaction intermediates and eventually lead to the enhanced or deteriorated activities. These results highlighted the underlying componential influences within g-C3N4 in electrocatalysis domain, and also demonstrates that only proper component engineering could favor as a viable way for promoting electrocatalytic reactions. That is, in short, component matters, but not every one works.
    Porosity is another crucial factor that affects the conductivity, active site concentration and mass transport process, and appropriate porous structure engineering can render large specific surface area and facilitate harness of active sites, which eventually lead to competent electroactivities. The merit of this structure factor was also partially examplified in the study on the aforementioned X-CN/PC hybrid catalysts. To tackle with the drawbackes of traditional hard-template methods (e.g., silica needs toxic HF solution or concentrated alkaline reagent at elevated temperature), in chapter 3, a novel and facile strategy was developed for the construction of heteroatoms enriched porous carbon catalysts via a one-pot pyrolysis reaction. The simultaneous in-situ etching toward the silica template by Teflon powder, the carbonization of sucrose and the decomposition of trithiocyanuric acid (TA) precursor were integrated together during the pyrolysis process, which then rendered a hierarchically micro-, meso- and macroporous carbon featured with a decent specific surface area and abundant dopant species. The thus synthesized carbonaceous catalysts with optimized structure exhibited comparable or even better ORR activities with regard to the state-of-the-art Pt/C catalyst in both alkaline and acidic electrolytes, which also outperform most of the efficient metal-free ORR catalysts reported so far. Furthermore, this doped porous carbon could behave as an OER catalyzer as well, and its overall bi-functional ORR/OER performance ranks as one of the best values by metal-free reversible oxygen catalysts reported to date. In addition, the doped porous carbon material was demonstrated to be a low-cost and efficient catalyst with superior performance than that of the Pt/C sample in a rechargeable Zn-air battery.
    Coupling non-noble transitional metal with nitrogen-doped carbon material (M@N-C) has been proven as another efficient strategy for developing highly efficient versatile electrocatalysts. However, quite few M@N-C catalyzers could offer multifunctional activities and their activities still need further improvement to rival those by the noble metal catalysts and to meet the requirements from practical applications. In chapter 4, a novel multidimensional 0D-2D-3D hybrid structured electrocatalyst based on core/shell Co@N-C, N-doped graphene (NG) and carbon foam (CF) framework was developed by a simple impregnation-pyrolysis method. This deliberately designed yet facilely fabricated hybrid composite integrates the preferential features for efficient versatile electrocatalyst including enriched active sites, excellent conductivity, suitable porosity and applicability in both acidic and alkaline electrolytes. The resultant multidimensional 0D-2D-3D catalyst exhibited outstanding tri-functional electroactivities for ORR, OER and HER in conjunction with excellent stability. Also, this versatile electrocatalyst rendered distinguished performances toward the rechargeable Zn-air battery, and its multi-functionalities could further enable a self-powered overall water-splitting device based on HER-OER driven by a Zn-air battery based on the same catalyst for ORR, showing its great potential to replace the different traditional noble-metal based electrocatalyzers.
    In summary, three types of hierarchical electrocatalyst were rationally designed and studied in-depth in the present thesis, trageting at revealling the general interplay between the component-structure-performance of the nanocatalysts. It is believed that strategies and results in the thesis can shed new light on the design and development of economic-feasible, highly efficient and robust multifunctional catalysts applied in various electrochemical energy conversion and storage devices.