Ni/Co Bimetallic Flower-Like Metal-Organic Frameworks with Enhanced Performance for High-PowerEnergy Storage Applications

Ilias Ntoukas, Alexander Roberts, Rong Lan, Chiara Milanese, Evangelos Gkanas

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Abstract

Metal-Organic Frameworks (MOFs) are compounds consisting of metal ions coordinated toorganic ligands to form crystalline porous structures by self-assembly. They have emerged as a class ofcrystalline materials with high surface area and porosity, tuneable pore size and functionalized surface. Due tothe metal nodes in the framework, they provide redox centres facilitating faradaic reactions, and due to theircrystalline porous structure they provide easier access for electrolyte diffusion. Thus, they own greatelectrochemical properties, making them ideal for electrode materials for supercapacitors.
However, most reported MOFs own an insulating nature which is a major drawback for their electrochemicalapplications. A common method to solve this restriction is to incorporate another metallic element to enhancethe properties of the electrode material.
Nickel materials demonstrate high specific capacitance and exhibit promising electrochemical properties aselectrode materials for supercapacitors, but they own low rate capability and cycle life, which is attributed totheir poor structural stability during the fast charge-discharge process. With the addition of another metal ionsuch as Co
into the structure, more active sites and improved conductivity can be obtained, while stabilizingthe Ni species. The partial substitution of the second metal ions in the inorganic nodes will provide synergisticeffects for the bimetallic framework. The ratio between the metals can also be adjusted to tune thephysiochemical properties of MOFs.
For all these reasons, bimetallic Ni/Co MOFs are promising materials for electrodes in energy storageapplications. Currently, Activated Carbon (AC) is used as electrode material for supercapacitors due to its highconductivity, large number of micropores and high surface area. However, the micropore structure of activatedcarbon limits pore accessibility and ion diffusion, thus reducing the capacitance and energy density. Transitionmetal oxides are also used as electrode materials for supercapacitors due to their metal centres, which providefaradaic redox reactions, and their low cost. Though, their poor conductivity leads to a low specific capacitanceand causes structural destruction of the materials, thus decreasing the stability of the system.
In this study, using terephthalic and trimesic acid as the linkers, binary Ni/Co MOFs (Ni:Co = 4:1, 3:1, 2:1, 1:1)have been synthesized utilizing a solvothermal method. Various parameters that affect the structure andproperties of MOFs such as time, temperature, ligands, Ni/Co ratio, and additives have been investigated. Themain focus is on the effect of Co ions that substitute the Ni ones in the framework, how they affect thestructure’s stability and the electrochemical properties of the bimetallic Ni/Co MOFs.
These materials have been structurally characterised by SEM, EDX, XRD, XPS, FTIR, TGA, BET and TEM.The morphology consists of 2D interlayered nanosheets with smooth surface that assemble to form 3Dmicroflower-like crystalline structures. The addition of Co
leads to a more dense, hierarchical and sphericalmorphology. The phase of these materials is also identified by Rietveld analysis and is: Ni(OH)(CHO(CCDC no. 985792).
The addition of Co does not alter the framework but only provides stability.
Moreover, these materials have been electrochemically tested by CV, GCD, EIS and cycling measurements. Alarge specific capacitance of 1503 F/g at 1A/g has been achieved with 70% retention after 3000 cycles. Finally,asymmetric coin cells using these materials and Activated Carbon (AC) have been developed and tested fortheir capacitance and cycle life.
In conclusion, Ni/Co MOFs have been synthesized and the effect of Co
in the framework has beeninvestigated. The incorporation of Co
provides stability and advanced electrochemical properties, making these materials promising candidates as electrode materials for supercapacitors.
Original languageEnglish
Publication statusSubmitted - 28 Oct 2022
EventMRS Spring 2023 - USA, San Fransisco, United States
Duration: 10 Apr 202314 Apr 2023
https://www.mrs.org/meetings-events/spring-meetings-exhibits/2023-mrs-spring-meeting

Conference

ConferenceMRS Spring 2023
Abbreviated titleMRS
Country/TerritoryUnited States
CitySan Fransisco
Period10/04/2314/04/23
Internet address

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