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As renewable energy continues to be a priority, the ability to provide more supercapacitors to meet these demands will subsequently rise. To this end, there has been an increased interest in exploiting quantum mechanical effects to improve the performance of energy storage systems. Much is still in the theoretical modeling and testing stage; however, the assessment of the quantum effects of supercapacitors at the quantum level is yielding positive results.
What is a Supercapacitor?
A supercapacitor, which is also referred to as an ultracapacitor, can typically store anywhere between 10 to even 100 times more energy per unit volume or mass than standard electrolytic capacitors. This unique storage capability makes supercapacitors highly sought after, particularly in the transport sector, where vehicles such as buses and trains require rapid charge and discharge cycles.
Application of Supercapacitors
While the transportation sector can benefit from the use of supercapacitors, several other industries are also expected to gain from this unique technology. Elevators and cranes, for example, all require rapid charge and discharge cycles to be effective. Supercapacitors can be used in regenerative braking systems in elevators. These same systems are already being considered and integrated into newer electrical vehicles, including modern trains.
Another potential application of supercapacitors can be found in the Victoria line on the London Underground in England, which already employs a regenerative system on its rolling stock, capturing electricity from train braking at a rate of 1 Megawatt hour per day.
Salvaged energy is then fed elsewhere on the network, helping to eliminate waste. While such advantages are apparent in these situations, the quantum effects of these types of supercapacitors are more complex to quantify.
Quantum mechanical (QM) studies have already shown that QM could increase the performance of existing energy storage systems, as well as usher in a new wave of supercapacitors. These theoretical capacitors could be capable of handling well over 100 times the levels of current storage systems.
The main challenge now lies in low quantum capacitance, which is a direct result of the shortage of quantum states near the Fermi level. As a critical component in the determination of the electrical and thermal properties of solids, the Fermi level changes as a solid is warmed and more relevantly, when electrons are added or taken from the solid. The challenge here lies in the inherent unpredictability in QM. The quantum effects of supercapacitors have not been predictable enough to produce a real-world quantum supercapacitor. Yet, recent experiments show potential solutions to this problem.
In February 2019, a new model of quantum supercapacitors was introduced by a group of Cornell University researchers. To understand this model, it is essential to know that one of the most fundamental properties of the interface between matter and liquid is the capacitance of the interface. Understanding how efficient the capacitor is and how much energy can be transferred and/or stored governs its ability to carry a charge. This will always determine the final assessment of how successful any supercapacitor, including a quantum supercapacitor, can be.
The Cornell researchers predicated two chains in their model; one chain deals with electrons whereas the other chain primarily deals with holes. Arrays of quantum dots hosted both chains. However, the hole chain was particularly important, as it served as the building block of experimental architectures for the realization of charge and spin qubits. But how does this affect the theoretical supercapacitor?
The researchers employed both chains near one another, embedding the same photonic cavity that would be responsible for long-range coupling between each of the quibits. Subsequently, a variational approach was used to determine the phase part of the model that displayed ferromagnetic and anti-ferromagnetic phases to allow for the appropriate amount of freedom of movement of qubits between states. Additionally, the researchers also found that these phases worked in conjunction with those of super-radiant behavior.
In conclusion, this study confirmed that their two-chain model demonstrated a significantly enhanced level of quantum capacitance when transitioning from the ferro/anti-ferromagnetic phase, to the super-radiant phase. Whether or not it can be employed in practice depends on how soon practical working quantum computing will be available.
References and Further Reading
- “Quantum supercapacitors” – Cornell University
- Xu, Q., Yang, G., Fan, X., & Zheng, W. (2019). Improving the Quantum Capacitance of Graphene-Based Supercapacitors by the Doping and Co-Doping: First-Principles Calculations. ACS Omega 4(8); 13209-13217. DOI: 10.1021/acsomega.9b01359.
- Gudavalli, G. S., & Dhakal, T. P. (2018). Chapter 8 – Simple Parallel-Plate Capacitors to High–Energy Density Future Supercapacitors: A Materials Review. Emerging Materials for Energy Conversion and Storage; 247-301. DOI: 10.1016/B978-0-12-813794-9.00008-9.
- “London Tube’s ‘Regenerative Braking Tech’ can power an entire station” – Endgadget