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Nanostructured Core-shell Materials based on Manganese Cobalt Oxide and Nickel Molybdenum Oxide for Asymmetric Supercapacitor Applications

Jaafar Mehrez

Electrochemical energy storage devices with emphasis on rational design at the material and device level are paving the way towards finding a worldwide solution to the global energy challenge as they provide an effective implementation of the electricity generated from renewable energy sources. Among the different energy storage devices, supercapacitors are well noted for their ultrafast charge and discharge ability, long operating life, and high power density (1−10 KW kg $^{−1}$). However, supercapacitors still face the problem of low energy density. In order to increase the energy density, tremendous research efforts have recently been made to enhance the specific capacitance and working voltage window. One of the approaches towards achieving higher specific capacitance is the implementation of pseudocapacitive electrode materials, which are mainly composed of transition metal oxides and conducting polymers that store charges faradaically at the surface/near surface, resulting in high capacitance and energy density. On the other hand, a wider potential window will be realized when implying an asymmetric cell configuration, combining two different materials with different capacitive mechanisms.

Owing to the high capacitance of pseudocapacitive electrodes, transition metal oxides have been well studied and exploited to enhance the energy densities of supercapacitors. Nevertheless, the practical implementation of metal oxide based materials as supercapacitor electrodes still suffer from moderate electronic conductivity and relatively low specific capacitance. The unique synergistic effect of hierarchical core-shell structures demonstrates great potential for designing the next-generation electrode materials for supercapacitors. In this thesis, manganese cobalt oxide and nickel molybdate oxide were studied as electrode materials for supercapacitors. The MnCo2O4 nanowire arrays (NWAs) were firstly grown directly over a nickel foam substrate without any binders, and then characterized as a cathode material for supercapacitors. Subsequently, NiMoO4 nanosheets were grown and coated over the previous MnCo2O4 nanowire arrays, resulting in a unique hybrid Core-Shell structure. The as-prepared electrodes were studied and characterized individually before fabricating a full asymmetric supercapacitor device. The main results are the following:

• Simple and facile hydrothermal method combined with post-annealing was introduced to controllably design MnCo2O4 nanowire arrays over a substrate of nickel foam without using any binders. The sample was characterized by TG, XRD, XPS, SEM and TEM and then carefully tested as cathode for supercapacitors.

• The electrochemical performance of the manganese cobalt nanowire hybrid arrays was tested and it displays a specific capacitance of 457.58 F g $^{−1}$ and a 74% rate capability between 1−10 A g $^{−1}$ capacitance range and a high capacitance retention of 100% after 10000 cycles at 5 A g $^{−1}$.

• Using a second hydrothermal process with post-annealing, the previous manganese cobalt nanowires were coated with a layer of nickel molybdate nanosheets. The sample was characterized by XRD, XPS, SEM, TEM and BET to confirm the chemical composition and crystal structure. After that, the MnCo2O4@NiMoO4 core-shell were tested as a cathode for supercapacitors.

• The electrochemical performance of the MnCo2O4@NiMoO4 core-shell NWAs was tested displaying great improvements, such as a high specific capacitance of 1244 F g $^{−1}$ and a 91% rate capability between 1–10 A g $^{−1}$ capacitance range and a capacitance retention of 85% after 2500 cycles upon 5 A g $^{−1}$.

• For further comparison, NiMoO4 nanosheets were grown directly over nickel foam substrate. The electrochemical performance of the pure-NiMoO4 was 762.5 F g $^{−1}$ and a 64% of rate capability between 1−10 A g $^{−1}$ capacitance range and a capacitance retention of 73% after 2500 cycles upon 5 A g $^{−1}$.

• Using the MnCo2O4@NiMoO4 core-shell NWAs as a cathode and commercially available active carbon (AC) as an anode, we assembled and tested an asymmetric supercapacitor (ASC). The device displayed high energy density of 42 W h kg $^{−1}$ at a power density of 852.3 W kg $^{−1}$ and a good cyclability with 93% of the initial capacitance retained at the end of 8000 successive charge/discharge cycles.

For this consideration and building upon the study within this thesis, the MnCo2O4@NiMoO4 core-shell NWAs electrode material holds a great promise for manifesting reliable energy storage devices.

Publication:

[1] J. A.-A. Mehrez, K. A. Owusu, Q. Chen, L. Li, K. Hamwi, W. Luo, and L. Mai, Hierarchical mnco2o4@nimoo4 as free-standing core-shell nanowire arrays with synergistic effect for enhanced supercapacitor performance, Inorg. Chem. Front. 6, 857 (2019). (https://doi.org/10.1039/C8QI01420E)