The depletion of fossil fuels has created the necessity for alternative power sources, renewable energy sources are intermittent by nature. Therefore, the utilization of renewable sources requires energy storage devices for the usage of the electricity generated. Metal ion batteries (Na, Li, and K) and supercapacitors have raised as the most promising energy storage technologies. However, some aspect of these devices needs to improve like service lifetime, energy density, and safety.
In recent years, novel materials based on Metal-Organic Frameworks (MOFs) have attracted attention for its application on energy storage devices with promising results for the next generation of energy storage technology with high energy density.
Li-ion batteries are the most used technology nowadays and most of the research is conducted for the improvement of these batteries. MOFs materials are developped to be used in different ways for batteries:
- Electrode materials
- Stability enhancer
We’ll have a look at these in the next paragraphs.
As electrode materials (anode and cathode), MOFs can increase the energy density and ion transport due to their high porosity and high surface area. Besides, its chemical tunability can be used for increased stability during charge/discharge cycles. An example of the implementation of MOFs was reported by the utilization of an Mn-based MOF (Mn-BTC MOF) as anode material, resulting in high specific capacity of 697 mAh/g at 0.1 A/g with 83% retention after 100 cycles.
“697 mAh/g specific capacity – 83% retention after 100 cylces”
Electrolytes are an important component and represent a way of improving energy/power density, cycle life, and safety. In this area, MOFs have attracted interest for its implementation as solid electrolytes, which eliminate the risk of explosion and fire of common liquid electrolytes. In a study, researchers created a highly conductive solid electrolyte by combining a Lithium containing ionic liquid (Li-IL) with a MOF material. The obtained electrolyte has an ionic conductivity of 3.0 × 10−4 S/cm at room temperature and is stable in a wide range of temperature from -20 to 150 °C which is rarely reported for solid electrolytes.
“ionic conductivity of 3.0 x 10-4 S/cm at room temperature”
Dendrite growth is a major issue related to the use of Lithium metal anodes leading to low coulombic efficiencies and reduced reversibility. Li anodes are one of the most promising electrode materials with theoretical capacities up to 3860 mAh/g. The implementation of MOF material has been reported as part of a separator for batteries containing Li metal anodes. This novel material allows dendrite-free and dense Li deposition (this application is known as anode protection).
“Drendrite free opperations with dense Lithium deposition”
Another application of MOFs as a protection agent is for the coating of Silicon anodes which are dimensionally unstable due to the swelling process during battery operation.
Other materials based on MOFs are available for metal ion batteries like carbon-based and metal-oxide materials. Pristine and MOF-based materials have been widely applied to variations of Lithium batteries including lithium-sulfur and Li-O2 batteries as well as sodium metal batteries.
Supercapacitors (also known as electrochemical capacitors or ultracapacitors) are energy storage devices with rapid charge/discharge rates, high power densities, and long cycle life spans. In this technology, the capacity of the device is directly related to the porosity and surface area of the electrode material. The high porosity and surface area of MOFs make them ideal for such applications.
“Record-high area capacitance of 18 µF/cm2“
Pristine MOFs can be used as sole electrode material for supercapacitors, an example of this is the application Nickel-based MOFs. In a recent study, the Ni3(HITP)2 MOF was used for the fabrication of a symmetrical electrochemical double-layer capacitor (EDLC). This MOF-based device produced an area capacitance of 18 µF/cm2 which is higher than values reported for carbon-based materials commonly used for these capacitors. Also, this MOF-based capacitor was able to retain more than 90 % of its capacity after 10,000 cycles, making evident the high stability conferred by this electrode material.
In a similar fashion as for metal-ion batteries, MOF materials can be combined with carbon-based materials and metal oxides to create composites for electrode materials with high performance.
Advantages of MOFs for energy storage devices
- High porosity
- High surface area
- Tunable pore size and shape
- High chemical and thermal stability
- High ionic conductivity
MOF-based materials represent a promising alternative as electrode, electrolyte, and structure stabilizers for different energy storage devices due to their physical and chemical properties. Such properties can be tuned by a conscious design of the Metal-Organic Framework including metal center and organic ligands. MOF-based energy storage technologies aim for applications in portable devices with high capacity and rapid response requirements such as electric cars.
Besides the opportunities around energy storage, there are many other applications for metal-organic frameworks in the electronics industry.