Physicochemically Engineered Perovskite Oxides as Oxygen Electrocatalysts for Rechargeable Zinc-air Battery

Majee, Rahul (2020) Physicochemically Engineered Perovskite Oxides as Oxygen Electrocatalysts for Rechargeable Zinc-air Battery. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Rahul Majee (14RS027)) 14RS027.pdf - Submitted Version Restricted to Repository staff only Download (22MB)

Abstract

Perovskite oxides are one of the most abundant minerals on the Earth’s surface. They are a family of composite oxides with basic unit cell formula of ABO3, where A is rare earth or alkaline earth metal ion and B is the is transition metal ion. This class of materials offers enormous opportunities for structural variations such as adjusting different A- and B-site ions, doping at lattice sites, oxygen nonstoichiometry, tunable lattice site occupancy and a wide range of crystal structures that help to engineer their physicochemical characteristics. The modulations of chemical structure endow alteration of the electronic environment at the perovskite oxide surface during heterogeneous catalysis. In particular, the oxygen nonstoichiometry and semi-metallic character of perovskite oxides anticipates efficient electrochemical oxygen activation. Therefore structural optimization of these complex oxide systems can bring significant improvement to the electrocatalytic activities, for example oxygen electrolysis, water splitting, carbon dioxide and nitrogen reduction reactions. Consequently, the alteration of first row transition metal ions at the B-site containing reaction specific active centers enable facile charge transfer resulting in an enhanced catalytic rate. Therefore first row transition metal based perovskites accentuate promising electrocatalysts to be used in devices where the thermodynamically constricted oxygen electrolysis is found to be the key reaction. In this context, the rechargeable zinc air batteries (ZAB) are yet to be widely commercialized because of the dearth of efficient and durable cathode materials that can drive the bifunctional oxygen electrocatalysis (OER/ORR). To circumvent the search, utilization of perovskite oxides as bifunctional electrode shows promising avenue for commercializing ZAB, beyond the lithium ion batteries. Chapter 1 presents the introduction of the thesis work. At the onset, the energy crises, recent outlook of energy resources and energy storage systems are concisely discussed. In the context of electrochemical energy storage systems, major drawbacks of metal ion batteries and thus importance of metal air batteries are described. A comparative analysis is presented to show the usefulness of ZAB over other metal air batteries. The setback of rechargeable ZAB is analyzed in the following section and a general overview on the progress of developing bifunctional oxygen electrocatalysts is summarized. The feasibility of perovskite oxide electrocatalysts as bifunctional cathode for ZAB is summarized. Moreover strategies citing all sort of possibilities and the optimistic routes to design the efficient perovskite oxide electrocatalysts are described. In chapter 2 brief descriptions of synthesis and characterization methods used in this thesis are given. All the principal instruments such as powder X-ray diffractometer (PXRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscope (AFM), Brunauer, Emmett and Teller (BET) surface area analyzer, X-ray photoelectron spectrometer (XPS), Raman spectrometer, Fourier transform infrared spectrometer (FTIR), inductively coupled plasma-mass spectrometer (ICP-MS) and potentiostat with electrochemical impedance analyzer and their working principles are presented in detail with appropriate schematics. In addition, methods used for this thesis such as Rietveld refinement of the PXRD patterns and methods of ZAB performance evaluation are elaborated. Chapter 3 describes the atomistic details of an electrochemically reversible lattice of double perovskite oxide nanosheet (NS) where the oxygen coordination of A-site is found to influence the redox oxygen activation. A unique synthesis approach of Ba₀.₅Pr₀.₅Mn₂₋xCoxO₅₊δ (BPMC-x, x = 0.25-1.75) NSs is shown, where the impact of Co concentration is discussed. A significant portion of this chapter deals with surface structural reversibility of the NSs on application of reversible electrochemical bias, which is a rare exploration in the literature. The BPMC-0.25 NS sample having 4.1 nm (~5 unit cell) thickness illustrates reversible appearance of PrO₁.₈ secondary phase on applying reversible electrochemical bias, which indicates an unprecedented involvement of the oxygen coordination of A(Pr³⁺)-site. The structural changes are demonstrated through a combined analysis using Rietveld refinement of PXRD patterns, high-resolution TEM (HRTEM), XPS, FTIR and cyclic voltammetry (CV). This structural reversibility is also observed during practical OER/ORR cycling. Having apparently perfect NS morphology, BPMC-0.25 NSs shows superior oxygen electrocatalysis among all BPMC samples. When BPMC-0.25 NSs is used as cathode for ZAB, it demonstrates decent performance which is however limited by the low electrical conductivity of the NSs owing to high fraction of oxygen vacancies. To circumvent the low electronic conductivity of BPMC-0.25 NSs, spatially connected nitrogen doped multiwalled carbon nanotube (NCNT) composites (BPMC/NCNT) are engineered at room temperature. The composite samples exhibit p-n junction characteristics generating facile charge transfer from p-type BPMC-0.25 NS to n-type NCNT. 10wt% NCNT containing sample (BPMC/NCNT-10) exhibits the lowest bifunctionality index (BI) of 0.794 mV for O2 activation which is attributed to the optimized Fermi energy levels. The ZAB constructed with BPMC/NCNT-10 cathode shows specific discharge capacity of 789.2 mA.h/gZn and cyclic stability over 85h at current density of 5 mA/cm². Chapter 4 deals with utilization of grain boundary defects in perovskite oxide for bifunctional O2 activation, from acting as a resistive barrier for electronic percolation, the grain boundaries are transformed to have a crystalline nature by doping noble metal, Pd. This chapter also elucidates the Δx variation of the Pd-dopant (x) between the grain and grain boundary that results in quantified grain boundary defects. When La₀.₇Sr₀.₃CoO₃₋δ (LSC) is doped by an aliovalent Pd⁴⁺ species, the grain boundary defects originate from an excess Pd doping as compared to the grain. The grain boundary defects were thoroughly analyzed by HRTEM and subsequently quantified by an empirical formula. A comprehensive analysis shows an increasing trend of defect concentration with the extent of Pd doping. Highest grain boundary defect of 1.29% was found in case of 5at% Pd doped sample (LSCP-3). At higher Pd-doping, these defects are accompanied by Pd-exsolution from the B-site as surface segregated metallic Pd and PdOx species. The grain boundary defect% shows a linear correlation to the oxygen activation activity where LSCP-3 demonstrates superior activity with the lowest BI of 0.91 V, which is duly corroborated by computational studies. The ZAB with LSCP-3 cathode shows specific capacity of 740 mA.h/gZn when discharge is performed at 10 mA/cm². Galvanostatic charge discharge cycling with 1h cycle time shows 60 h stable performance. Chapter 5 presents circumvention of the instable surface of perovskite oxide under sustained oxidative bias by sheltering the surface with two-dimensional layered double hydroxide (LDH). A polyethyleneimine attachment of 10-50wt% NiFe-LDH on 40 nm interconnected particles of Ba₀.₆Sr₀.₄Co₀.₈Fe₀.₂O₃₋δ (BSCF) is obtained through strategic surface functionalization. HRTEM analysis shows crystalline junction between BSCF and NiFe-LDH indicating an atomic interaction that can facilitate the required bifunctional OER/ORR activity. The synergistic interfacial charge percolation enables BSCF/NiFe-LDH as a promising electrocatalyst for oxygen activation. When the composite contains 25wt% NiFe-LDH (BSCF/NiFe-25) a superior activity is demonstrated in all figures of merit. Electronic permeability between BSCF and NiFe helps BSCF to retain structural stability during OER which boosts unhindered long term rechargeable Zn-air battery performance. The overall performance as well as durability of the battery increase by employing BSCF/NiFe-25 at the air-electrode. BSCF/NiFe-25 outperforms both BSCF and Pt/C-RuO₂ cathodes in all the standard figures-of-merit and has a specific capacity of 697.1 mA.h/gZn at 10mA/cm² operational discharging voltage, low discharge-charge voltage gap (0.89 V at 10 mA/cm2), perfect cyclic stability at 100 mAcm-2 over 100h and an energy density of 776.3 mW.h/gZn. In chapter 6, a general design principle to maneuver the physicochemical descriptors of a perovskite oxide is elucidated that have a profound impact on the electrocatalytic activities. La₀.₇Sr₀.₃Co₀.₇Fe₀.₃O₃₋δ (LSCF, δ = 0.05-0.11) is taken as the model system and its descriptor namely spin states of B-site cations, crystal structure, electronic conductivity and surface area are altered by varying the calcination temperature from 750-1200oC. The one calcined at 975⁰C (LSCF-975) demonstrates interconnected grain morphology suitable to enhance the electronic conductivity. LSCF-975 outperforms LSCF-750 and LSCF-1200 in driving the bifunctional oxygen activations requiring overpotentials of 440 mV at 10 mA/cm² and 641 mV at -1 mA/cm² for OER and ORR, respectively. With an electronic conductivity close to bulk sample and midway surface area, LSCF-975 has larger electrochemically active surface area, lower charge transfer resistance and optimum eg orbital occupancy of 1.26. Density functional theory calculations envisage LSCF-975 with moderate Fermi energy (EF) level and work function (Φ) that fulfill the prerequisite for facile OH- binding and faster O2 evolution. Further improvement in electrocatalytic activities is demonstrated by employing a composite consisting of 20wt% graphene oxide nanoribbon (GOR) prepared by oxidative unzipping of multiwalled CNT. LSCF-975/GOR shows 180 mV suppression of combined overpotential (BI) of OER and ORR. Besides increasing the electronic conductivity of the sample and increasing reactant binding sites by the GOR additive, Mott-Schottky analysis shows a raised Fermi energy (Ef) level, that facilitates the interfacial charge transfer. Proof of concept ZAB with LSCF-975/GOR cathode has power density of 40.2 mW/cm² and specific capacity of 815.8 mA.h/gzn. Chapter 7 provides the work summary and future prospects. The main theme of individual chapters is concisely described. The future scope is discussed based on perovskite oxide systems focusing on the electrochemical energy conversion and storage systems.

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