By Julie Fornaciari
In many electronic devices, like personal computers or electronic vehicles where quick bursts of energy are required, supercapacitors are essential for operation. One emerging research goal is to make capacitors that can store more charge and discharge faster for better device performance. However, major improvements to the electrode materials in capacitors are needed to take the efficiency and capacity to the next level for next generation energy storage technologies.
A team of Foundry staff and users synthesized a two-dimensional material that can be used in supercapacitors and have shown that it is usable in practical energy applications. Their new material showed orders of magnitude higher conductivity and fast movement of charge compared to similar materials. This collaborative work, led by Organic facility staff scientist Jian Zhang, is highlighted in their paper recently published in the Journal of the American Chemical Society (JACS).
What Exactly is this New Material?
The new material is a two-dimensional covalent organic framework (COF). These COFs – which are basically crystalline sponges with nanosized pores – are designed to store a high amount of charge but generally cannot hold the charge for long due to the poor chemical stability.
The specific COF Zhang’s team focused on is made of phthalocyanine molecules, large aromatic, organic compounds that are connected together using a unique linker molecule. The arylether linkage – which consists of two oxygen atoms bonded to two adjacent benzene molecules – is often used in engineered plastics that have high mechanical and heat robustness. Incorporating the arylether linkage into the COF helps with the overall chemical stability of the framework. The linker also maintains the ordered packing of the phthalocyanine building blocks and thus increases the charge mobility up to 19.4 cm2 V-1 s-1.
This result cleared up previous uncertainty about the possible adverse effect of arylether linkages on anisotropic charge transport. Anisotropic means that a property, like conductivity (the ability to move charge), can change depending on what direction you’re measuring it in. In the case of this COF, the arylether linker not only increases the chemical stability of the framework material, but also surprisingly facilitates both in plane and out of plane interactions in the 2D framework. Theory work done alongside the experiments confirmed the anisotropic charge transport phenomenon. Zhang explains, “We demonstrate this new material by using both theory and experiments to support the idea that the linkage enhances charge transport along the phthalocyanine stacks.”
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Faster Moving Charge for Future Applications
The team next looked at how well the new material could be used in energy storage. Zhang states, “This synthetic sponge could one day be used in your laptop or electric car.” But to be used in this way, it would need to be integrated into a functional supercapacitor.
The team incorporated the material into a solid-state supercapacitor to evaluate its viability for energy storage applications. When the researchers charged and discharged the supercapacitor, they determined that 88.5% of the energy stored could be attributed to the redox process, while the remaining stemmed from double-layer capacitance. The redox process is fast, where reversible chemical reactions act to transfer charge, and contributes higher capacitance than the double-layer alone. Additionally, the new material has a high surface area normalized capacitance of 19 μF cm-2, which is higher than comparable framework materials.
Furthermore, the researchers found that the capacitance remained high even after repeated cycles of charging and discharging, retaining 86% of the original capacitance. With minimal loss in capacitance, the material is expected to be stable for long periods of time.
Zhang explains, “This material could also extend to energy conversion in addition to energy storage devices.” Converting chemicals, such as carbon dioxide to carbon monoxide for mitigating greenhouse gas emissions or formic acid as a liquid energy fuel, is another application for these materials. The group will next investigate how these materials can be altered to be used as catalysts in these electrochemical reactions.
