Design and Engineering of Hierarchically Porous Transition Metal-based Electrocatalysts for Rechargeable Zn-air Batteries
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Electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the critical cathodic and anodic reactions, respectively, in electrically rechargeable Zn-air battery. With a variety of advantages including relatively high energy density (1218 Wh kg-1), the abundance of zinc in the earth, and secure handling and safe operation, electrically rechargeable (secondary) Zn-air battery technology has been regarded as highly promising energy applications in consumer electronics, electric vehicles, and smart grid storage. Zn-air batteries consist of not only zinc anode, polymer separator, and an alkaline electrolyte that are typical battery components, but also air-breathing cathode that makes Zn-“air” battery unique technology. Unlike other general battery systems such as lithium-ion batteries, there is no active material stored in cathode, but gaseous oxygen molecules in the air are used as the fuel for energy-generating reaction in the air cathode of Zn-air technology. The reactions occurring during battery discharge and charge are ORR and OER, respectively, which are mostly dominating the overall energy efficiency of the Zn-air battery system due to their intrinsically sluggish kinetics. The high energy barrier attributed to conversions between oxygen molecules diffused from the air and hydroxide ions in the electrolyte at the thin layer of the electrode leads to low charge/discharge energy efficiency and insufficient cycle stability hindering the commercialization of rechargeable Zn-air batteries to the market. Therefore, it is necessarily required to facilitate the slow kinetics of oxygen electrocatalytic reactions by using bifunctionally active and durable oxygen electrocatalyst materials to progress the reactions at practically viable and stable rates. With the use of bifunctional oxygen catalysts, kinetics of ORR and OER can be improved, leading to enhancement of Zn-air battery performances such as higher operating voltage and longer battery cycling life. The current best-known catalysts for ORR and OER are noble metals, including platinum (Pt) and iridium (Ir), respectively. However, high cost and scarcity of the precious metal-based catalysts hinder their employment in large scale energy applications. Furthermore, the electrochemical stability of these materials is well known to be very insufficient for long term usage even under typical device operating conditions. Therefore, the development of non-precious transition metal-based electrocatalysts has significantly been a momentous research field. Along with this movement, the facile synthesis and inexpensive preparation of highly active and durable electrocatalysts will take the top priority for the fulfillment of practically available rechargeable Zn-air battery technology in a variety of energy applications from portable electronics to electric vehicles and smart grid storage systems. In this work, novel design strategies of bifunctionally active and durable electrocatalysts possessing robust three-dimensional framework with hierarchical porosity are presented. A porous structure with a large surface area is essential to improve the oxygen electrocatalysis since the oxygen reactions take place at the surface of materials, where active sites reside, and thereby the large surface indicating plenty of catalytically active sites enhances kinetics of the reactions. Additionally, the porous architecture facilitates diffusion of oxygen gas molecules during the oxygen electrocatalysis, leading to enhanced mass transport of reactants and reduced overpotentials for ORR and OER polarizations, eventually resulting in improved activities. In addition to the improvement of activities, electrochemical stability is an essential fundamental property for the rational design of electrocatalysts. Thus, the 3D porous structure must have robust framework which can endure the highly oxidative environment in OER potential range. Therefore, the work presented in this thesis is aiming for the design and engineering of hierarchically porous transition metal-based electrocatalysts involving high porosity as well as electrocatalytically robust frameworks to improve the oxygen electrocatalytic activities and durability and thereby put the rechargeable Zn-air battery technology at a commercially viable level. In the first study, a facile polymer template-derived method has been used to synthesize three-dimensionally ordered meso/macro-porous (3DOM) spinel cobalt oxide as a bifunctional oxygen electrocatalyst. Physicochemical characterizations have revealed the morphology of the designed electrocatalyst to be a hierarchically meso/macro-porous metal oxide framework. As investigated by electrochemical characterizations, 3DOM Co3O4 shows far enhanced ORR and OER activities with improved kinetics compared to the bulk material. The enhancement is majorly attributed to the five times higher specific surface area and significantly greater pore volume, leading to the increased number of catalytic active sites and facilitated diffusion of oxygen molecules into and out of the structure, respectively. Moreover, the robust frameworks of 3DOM Co3O4 helps to withstand harsh cycling environments by exhibiting significantly small performance reduction and retaining the original morphology. The improved oxygen electrocatalytic activity and durability have been well demonstrated in the rechargeable Zn-air battery system. 3DOM Co3O4 presents remarkably enhanced rechargeability over 200 cycles while retaining quite comparable operating voltage gap in comparison with the precious benchmark catalyst. In the second study, palladium (Pd) nanoparticle is deposited on the surface of 3DOM Co3O4 via a simple chemical reduction process. The morphological advantages of the 3DOM framework, as confirmed in the previous study, are expected to facilitate diffusion of oxygen molecules into and out of the structure leading to the decreased overpotentials during ORR and OER. However, using metal oxides as electrocatalysts restricts fast electron transfer leading to limited activity for oxygen catalysis due to their intrinsically low electrical conductivity. Therefore, Pd nanoparticles are introduced into 3DOM Co3O4 by expecting synergy from the combination of the morphological advantage of 3DOM architecture and the significant thermodynamic stability as well as the excellent ORR activity of palladium metal. Electrochemical characterizations have revealed that the combination demonstrates synergistically improved bifunctional electrocatalytic activity and durability. Moreover, computational simulation via density-functional-theory (DFT) verifies Pd@Co3O4(3DOM) is superior in two ways; (i) Activity-wise: the d-band center of Pd deposited on 3DOM Co3O4 was found to decrease significantly, resulting in increased electron abundance at the Fermi level, which in turn enhanced the overall electrical conductivity; (ii) Durability-wise: synergistic hybrid of Pd and 3DOM Co3O4 resulted in a significantly improved corrosion resistance, due to the much higher carbon oxidation potential and bulk-like dissolution potential of Pd nanoparticles on 3DOM Co3O4. The remarkable electrochemical activities and stabilities of Pd@3DOM-Co3O4 obtained from the half-cell testing resulted in excellent rechargeability of a prototype Zn-air battery, demonstrating the synergistic introduction of Pd into 3DOM Co3O4. In the last study, a type of metal-organic-framework (MOF) is selected as a template to synthesize MOF-based electrocatalyst possessing robust framework with multi-level porosity. Typically, MOF materials consist of metal centers linked by functional organic ligands, which gives them unique material characteristics such as high porosity and surface area, morphological and compositional flexibility, and high crystallinity. Especially, transition metal-based Prussian blue analogue (PBA) nanocubes with a chemical formula MxII[MyIII(CN)6]z▪H2O, where MII and MIII are divalent and trivalent transition metal cations, respectively, are employed as the MOF precursors due to a several material advantages such as simple precipitation synthesis, various possible compositions, and robust structure with high porosity. Specifically, a ternary nickel-cobalt-iron (Ni-Co-Fe) PBA nanocube precursor is designed to maintain the nano-cubic structure while optimizing the composition of transition metal centers for efficient OER and hydrogen evolution reaction (HER) activities for water-splitting technique. Physicochemical characterizations have revealed several advantages of the PBA-derived electrocatalysts, including (i) enlarged active surface area with high porosity and robust architecture, (ii) active nitrogen species formed during synthesis, and (iii) favorably tuned electronic structure by hybridizing the transition metals possessing the different number of valence electrons. This NCF-MOF is highly promising because (i) the unique rigid and interconnected porous structure significantly improves material utilization and allows rapid mass transport to be maintained, (ii) the fine-tuned ternary transition metal composition optimizes the electronic structure to accelerate the intrinsically sluggish OER, and (iii) the in-situ nitrogen-doping during synthesis enhances the interaction of reaction intermediates with active sites during HER. The unique material advantages successfully render reduced overpotential and significant durability for OER and HER, resulting in excellent water-splitting reactions.
Cite this version of the work
Moon Gyu Park (2020). Design and Engineering of Hierarchically Porous Transition Metal-based Electrocatalysts for Rechargeable Zn-air Batteries. UWSpace. http://hdl.handle.net/10012/15494