Metal organic frameworks from unconventional metal feedstock for energy storage applications

Abstract

The study focuses on exploring ways to extract valuable metals from waste materials such as spent lithium-ion batteries (LIBs) and coal fly ash (CFA) and thus converting them into Metal-organic frameworks (MOFs) suitable for energy storage applications. The rise in clean energy demand requires that there be suitable storage systems that can be utilized. LIBs have a large energy density and a drawback exhibiting low power density and short cycle stability, which supercapacitors (SCs) complement by possessing high-power density with high operational charge and discharge rates at a high number of cycles. However, the main drawback of developing energy storage materials and devices is the cost of fabrication and operation. The study aims to introduce cost-effective ways of extracting valuable metals for the use of preparing porous MOFs for use in energy storage. In the first instance, spent LIBs were physically dismantled and discharged before subjecting the cathode material to calcination to convert the metallic structure into metal oxides. The metal oxides were mixed with either hydrochloric acid (HCl) or sulphuric acid (H2SO4) to convert them into a metal salt that can react with an organic linker (under a conducive environment) to obtain manganese-based MOFs. The prepared MOFs exhibited structural, morphological and textural properties similar to manganese-based MOF obtained using commercial metal salts. The Mn-MOF(Cl2) was the MOF prepared using HCl as a salt convertor and had better structural and textural properties. It was tested as an anode material in LIBs application and its electrochemical properties were compared to manganese-based MOF obtained using commercial metal salts (Mn-MOF(Com)). Mn-MOF(Cl2) achieved coulombic efficiency (CE) of approximately 99% and discharge capacity of 1355 mAh g-1 as compared to Mn-MOF(Com) obtained using commercial metal salt, which had a discharge capacity of 772.55 mAh g-1 at 100 cycles. Mn-MOF(Cl2) had good reversibility of the redox process and remarkable electrochemical performance ascribed to its flaky sheet-like structures, allowing shorter ion diffusion path, which compliments the strain incurred through severe volume variations amid frequent intercalation/de-intercalation of lithium ions. In the second instance, coal fly ash was leached using an optimized method to obtain an aluminium sulphate leachate solution. The solution was used as a metal feedstock with fumaric acid as an organic linker in the synthesis of aluminium-based MOF (CFA-FumMOF). The CFA-FumMOF was synthesized as a mimic of aluminium fumarate MOF (Al-FumMOF). Despite the presence of other metals in the synthesis solution, the prepared coal fly ash derived MOF (CFA-FumMOF) exhibited almost similar characteristics to Al-FumMOF synthesized using commercial chemicals. Achieving a surface area of 1236 m2/g with respect to Al-FumMOF (1266 m2/g). The prepared MOFs were subjected to direct carbonization to prepare MOF derived carbons (MDCs) for use in supercapacitors. The three-electrode measurements of Al-FumMOF and novel CFA-FumMOF were successfully prepared with a specific capacity of 28.62 mAh g-1 and 9.88 mAh g-1, respectively at a specific current of 0.5 A g-1. Their respective MDCs, Al-MDC and CFA-MDC obtained a specific capacitance of 111.94 F g-1 and 306.59 F g-1, respectively at 0.5 A g-1. When assembled into an asymmetric device, Al-FumMOF//Al-MDC had a specific capacity of 5.09 mAh g-1 at current densities of 0.5 A g-1, with coulombic efficiency of 99.10%, indicating good cyclability and capacity retention of 46.80% after 5000 cycles. CFA-FumMOF//CFA-MDC achieved a specific capacity of 4.0 mAh g-1 at 0.5 A g-1, with a good coulombic efficiency of 98.90% and capacity retention that gradually decreased to 42.36% after 5000 cycles. In summary, the waste-to-MOF concept has attracted attention in the academia space and scientist have recorded the use of waste materials to obtain organic linkers and metals for MOF preparation. Herein, the authors wish to introduce a new synthesis route of using spent LIBs and CFA as metal feedstock to prepare MOFs. There are no reports of using spent LIBs to prepared Mn-based MOF, which is also used as anode for LIBs application. There are also no known reports on preparing Al-based-MOFs from CFA that are also used as sacrificial templates in preparation of MOF-derived-carbons. The electrochemical performance of these materials in LIBs and SCs application is the highest compared to the commercially prepared (pristine) materials. The reported synthesis routes exhibit innovative ways of utilization of cheap waste materials. The preparation of both Mn-MOF(Cl2) and CFA-FumMOF signifies the potential for new market creation and relatively cheap ways to mitigate climate change.