By Arnav, Winter 2022.
Just a few years ago, LG revealed their rollable TV and Samsung unveiled their Z Fold phone with a foldable display [1]. It wasn’t long ago, however, that TVs had small screens and massive bodies, making the viewing experience bulky [2]. So what actually made the development of rollable, flexible, ultra-thin screens possible? The answer is organic electronics, and specifically the development of OLED (Organic LED) screens. OLEDs have no backlighting, leading to incredibly rich colors and super-black blacks. Additionally, OLEDs are made from organic molecules (built from a primarily carbon and hydrogen backbone), which, unlike inorganic materials, are not arranged in an ordered crystalline structure. This lack of rigid structure allows organic materials to be flexible and, in the case of the LG TV, rollable! Organic materials are showing promise in not just displays, but electronics components as well, and have the potential to compose electronic devices that are both functionally superior and better for the environment than traditional inorganic silicon based electronics.
Organic electronics have the potential to do things that inorganic electronics cannot [3]. We already saw this with the flexibility of displays, but there are several other benefits. Organic materials are chemically compatible with biological systems and can detect biological signals, which are essentially just differences in voltage (i.e., they act as sensors). Organic electronics are also more “eco-friendly” than their inorganic counterparts, helping mitigate the effects of climate change, as they can be engineered to be biodegradable and more material efficient. By contrast, electronic devices manufactured with inorganic materials contain materials harmful to the environment, such as lead, mercury, etc [4]. In India, for example, almost 2.7 million tons of e-waste are generated annually [4]. Electronic waste is often not disposed of correctly, exposing the environment to an excess of toxic chemicals. Disposal of organic electronics would be much easier, as organic materials would not harm the environment. Finally, the production of organic electronics is more resource-friendly. Rather than “top down” (removal of material) approaches used in the production of silicon devices, organic electronics are produced using “bottom up” (addition of material) techniques like printing, thus wasting less resources [3]. Additionally, to protect inorganic electronics from short-circuiting, the potent greenhouse gas sulfur hexafluoride (SF6) is used. Even small leaks of this gas into the atmosphere can cause incredibly dangerous effects, as SF6 is 23,500 times more potent than carbon dioxide [5].
Organic electronics have the potential to not only be more environmentally friendly than previous inorganic electronics, but also create new breakthroughs and push the boundaries of what electronics can do. One emerging application of organic electronics is in energy storage devices [6]. While societal reliance on nonrenewable energy sources is contributing to climate change, many well-intentioned solutions such as solar and wind energy harvesting devices are not reliable for continuous use. For these technologies to be viable, an efficient and sustainable storage mechanism is necessary. Currently, inorganic redox-flow batteries (RFBs), utilizing toxic heavy metal ions as the “energy carriers”, are used for large scale energy storage [7]. A diagram of a typical RFB is shown in Figure 1 below:
Figure 1: Redox Flow Battery Diagram [8]
The anolyte and catholyte are chemicals that act as “energy carriers”. The anolyte releases electrons, which then flow through the circuit connected to the battery, and are finally gained by the catholyte. The nature of both the catholyte and anolyte are extremely important, as we want them to be able to release and gain electrons with ease, which is the appeal of the current heavy metal ions. However, these metals are rare and expensive, while organic materials are cheap and readily available, so research is currently underway to engineer high energy density organic electrolytes. One important requirement for organic electrolytes is the reversibility of their electron flow. For example, anolytic organic materials must easily release electrons. However, once the material has released all the electrons, we want to reuse the material by applying some energy, reversing the reaction, forcing the anolyte to regain electrons, and “charging” the battery. We want the recharging process to demand as little energy as possible, but not so much so that the forward reaction (the discharging process) becomes unfavorable. Therefore, we want the reaction to be as close to reversible as possible, so neither the charging or discharging process is unfavorable [9]. Organic polymers with disulfide (S-S) bonds seem to be perfect for this requirement. The bond can accept electrons to form a negatively charged thiolate (S- anion) and lose electrons to reform the neutral disulfide, and thus the battery can be charged or discharged with relative ease.
Another important component of an RFB is the membrane, which prevents the anolyte and catholyte from meeting directly within the battery and ensures electrons flow through the circuit. These membranes need to allow “charge balancing” ions to pass through, keeping each side of the battery neutral, but not the redox-active materials themselves [7]. Currently, expensive ion-exchange membranes are needed to allow “charge-balancing” ions like H+ to pass through, but not the metal ion electrolytes [10]. When large organic polymeric electrolytes are used, cheap porous membranes, that discriminate solely based on size, can be implemented instead, allowing only small “charge balancing” ions to pass through the membrane [11]. Overall, utilizing organic materials in RFBs would not only be more eco-friendly and lessen our reliance on nonrenewable materials, but would also result in a cheaper production process.
Just last year, scientists at Sweden’s Linkoping University leveraged the principles behind the organic technology discussed above to engineer the first all-organic RFB with quinone-based electrolytes [12]. While not specifically containing disulfide bonds, the electrolyte still gains electrons to form a stable aromatic molecule. It can also release electrons with ease, satisfying the reversibility requirement discussed previously. Although the scientists admit the design offers less energy density as compared to current inorganic RFBs, the successful development of a completely organic material based energy storage device shows great promise for a more resource-efficient, environmentally friendly, and sustainable future.
[1] Gilbert, Ben. “We’re More than Halfway through 2019, and 2 of the Year’s Most Revolutionary Tech Gadgets Are Still Missing.” Business Insider, Business Insider, 4 July 2019, https://www.businessinsider.com/samsung-galaxy-fold-lg-rollable-tv-release-date-uncertain-2019-7.
[2] Robertson, Adi. “The Last Scan.” The Verge, The Verge, 6 Feb. 2018, https://www.theverge.com/2018/2/6/16973914/tvs-crt-restoration-led-gaming-vintage.
[3] Organic Electronics for a Better Tomorrow: Innovation … https://www.rsc.org/globalassets/04-campaigning-outreach/policy/research-policy/global-challenges/organic-electronics-for-a-better-tomorrow.pdf.
[4] Author: and Sean Gallagher Grantee. “India: The Rising Tide of e-Waste.” Pulitzer Center, 16 Jan. 2014, https://pulitzercenter.org/stories/india-rising-tide-e-waste#:~:text=%22According%20to%20recent%20studies%2C%20almost,toxic%20waste%20issues%20in%20India.
[5] McGrath, Matt. “Climate Change: Electrical Industry’s ‘Dirty Secret’ Boosts Warming.” BBC News, BBC, 13 Sept. 2019, https://www.bbc.com/news/science-environment-49567197.
[6] Thomas, Jack. “Organic Material Paves the Way for Energy Storage Devices.” Innovation News Network, 26 July 2021, https://www.innovationnewsnetwork.com/organic-material-paves-the-way-for-energy-storage-devices/12012/.
[7] “Redox Flow Batteries (RFB).” Energy Storage Association, 1 Oct. 2019, https://energystorage.org/why-energy-storage/technologies/redox-flow-batteries/#:~:text=Redox%20flow%20batteries%20(RFB)%20represent,cells%20during%20charge%20and%20discharge.
[8] Qi, Zhaoxiang, and Gary M. KoenigJr. “Review Article: Flow Battery Systems with Solid Electroactive Materials.” AVS, American Vacuum Society AVS, 1 Jan. 1970, https://avs.scitation.org/doi/10.1116/1.4983210.
[9] Potash, Rebecca A., et al. “IOPscience.” Journal of The Electrochemical Society, IOP Publishing, 8 Dec. 2015, https://iopscience.iop.org/article/10.1149/2.0971602jes.
[10] Gubler, Lorenz. “Membranes and Separators for Redox Flow Batteries.” Current Opinion in Electrochemistry, Elsevier, 12 Sept. 2019, https://www.sciencedirect.com/science/article/pii/S2451910319301322.
[11] Grocke, Garrett L, et al. “Synthesis and Characterization of Redox-Responsive Disulfide Cross-Linked Polymer Particles for Energy Storage Applications.” ACS Publications, https://pubs.acs.org/doi/abs/10.1021/acsmacrolett.1c00682.
[12] IDTechEx. “Researchers Demonstrate First Organic Battery.” Advanced Batteries & Energy Storage Research, IDTechEx, 23 Oct. 2020, https://www.advancedbatteriesresearch.com/articles/21983/researchers-demonstrate-first-organic-battery.