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Dopant interaction in binary metal oxide system: Towards the development of an improved supercapacitor material

Sharma, Pratigya (2022) Dopant interaction in binary metal oxide system: Towards the development of an improved supercapacitor material. PhD thesis, Murdoch University.

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Technological advancement has raised the expectation on the role of energy storage systems. With the transition towards renewable technologies like solar and wind, demand for efficient energy storage systems capable of harvesting energy from various sources and delivering stable electricity output to meet customers' demand is in priority. Supercapacitors (SCs) are complementary to battery technology. However, SCs can provide high power density, longer life cycle, and superior round-trip efficiency compared to battery systems. Hence, it holds an important space in energy storage technology. This thesis presents a series of modified materials of binary metal oxides (BMOs) family and biomass-derived activated carbon detailing material synthesis and the strategies to improve its performance, optimization, and performance testing for supercapacitor applications.

A new insight into existing knowledge in BMOs having a structural formula ABOx to improve energy storage capacity by introducing dopants is presented in this thesis. Doping introduces impurities in a pristine material that alters its physical and chemical properties. Detailed experimental findings along with vital theoretical validations on ABOx crystal structure (where A and B represents a wide range of cation and anion metal ion site such as Ni, Co, Mg, Bi, Mo, etc., O represents oxygen and x = 1, 2, 3, etc. in a BMO family) for both anode and cathode material is presented. A detailed electrochemical characterization is presented by cyclic voltammetry and charge/discharge to quantify its capacitance value, energy and power densities, and cyclic stability in an aqueous alkaline medium. The investigated materials are physically characterized using various techniques. The change in morphology of BMOs resulting from the synthesis procedure or dopant additive is visualized by field-emission scanning electron microscopy (FE-SEM). The ABOx crystal structure has been validated via X-ray diffraction (XRD), which has also been used as a primary component for theoretical validation. A surface-sensitive technique, X-ray photoelectron spectroscopy (XPS), has been used to identify the elemental composition (Ni, Co, Mg, Bi, Mo, O, etc. and its chemical state. Change in the functional group is studied by infra-red spectroscopy (IR) along with synchrotron sourced radiation.

In a cathode material, nickel molybdate (NiMoO4), 0.2 wt% of zinc (Zn) dopant improved the electrochemical performance by 15% compared to the pristine material. Computational study (density functional theory, DFT) showed that the optimal Zn doped NiMoO4 (ZNM) resulted in a thermodynamically stable structure with an excess of oxygen vacancy, hence enhancing the charge transport between the electrode surface and OH− ion of the NaOH electrolyte, which agrees with the experimental finding. In the subsequent chapter, Bismuth-based BMO, bismuth molybdate (Bi2MoO6), a highly redox-active system, is chemically tuned using Gadolinium (Gd) as a dopant for anode material. Gd dopant (0.05 wt.%) stabilized the structure as it is energetically favourable while enhancing energy storage. Increase in areal capacitance value from 0.417 F cm−2 to 2.152 F cm−2 corresponding to Bi2MoO6 and Gd0.05Bi1.95MoO6 respectively was observed.

Further down towards my Ph.D. thesis, to tune the nanoparticle interfacial properties and its stability, a core@shell structure is studied. A synergistic effect of dopant along with a hierarchical core@shell structure exhibited enhanced and stable charge storage behaviour. ZNM as a core and cobalt tungstate (CoWO4, termed as CW) as a shell, i.e., ZNM@CW led to an improved faradic reaction contributing to efficient charge transfer. An alteration of the interfacial region explains this enhanced performance in core@shell nanocomposites, and the surface reconstruction is due to the core@shell combination. ZNM alone delivered an areal capacitance value of 2.40 F cm−2, whereas ZNM@CW showed superior performance with 7.12 F cm−2 energy storage capacity. When ZNM@CW is paired with commercial activated carbon (AC) as an asymmetric SC device, it retained 96% of its initial capacitance even after 1000 continuous charge-discharge cycles.

To understand the difference in electrochemical behaviour as a result of anion (B site) substitution in the ABOx system, molybdenum (Mo) has been replaced by tungsten (W) and reported in one of the chapters. Both Mo and W are electrochemically active materials and are widely used in energy storage, catalyst, and sensor technologies. Nickel molybdate (NiMoO4), compared to nickel tungstate (NiWO4), delivered enhanced electrochemical performance as a contribution from the faradic reaction between Ni2+ and Ni3+. Although a similar faradic reaction occurs in NiWO4, the formation of tungsten oxide (WO3) passivation layer occurs on the electrode surface due to the reaction of tungstate with the electrolyte, resulting in a decreased energy storage capacity. NiMoO4 vs. AC device exhibited discharge capacitance of 124 F g−1, whereas NiWO4 vs. AC device showed discharge capacitance of 77 F g−1. Regardless of the storage value, both the materials were fairly stable, capable of retaining 87.14% (NiMoO4) and 82.22% (NiWO4) even after a 1000 continuous charge-discharge cycle.

This thesis is concluded by exploring biomass-derived activated carbon as an efficient anode material. It is paired with an excellent Zn-doped NiMoO4 cathode material, both are synthesized in the laboratory. Wheat straw, an agricultural waste, is converted into a value-added product after chemical and physical treatments. Acid (H2SO4) activated wheat-straw (WS-acid) delivered a specific capacitance of 162 F g−1 in a one-electrode configuration. An asymmetric device, i.e., ZNM vs.WS-acid derived AC, delivered a specific capacitance of 118 F g−1. This device presents a similar energy storing ability to commercial activated carbon as described in the Zn-doped NiMoO4 chapter delivering specific capacitance of 122 F g−1. This opens a path for exploring a sustainable carbon source without any mining hazard. Coal, if used as a source of carbon, would require energy to dig out from the pit, which would also involve greenhouse gas emissions. Recommendations for further work are put forward in the conclusion chapter.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Engineering and Energy
Supervisor(s): Sundaram, Manickam Minakshi, Laird, Damian and Pivrikas, Almantas
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