If you’ve never heard of covalent organic frameworks before, that could be about to change. To the eye, these delicate, super lightweight crystalline materials look like nothing more than a pile of fluffy powder, but they are beginning to emerge as a force to be reckoned with in the energy storage field. In the latest development, a research team based at Northwestern University has figured out how to get one such material to behave like a battery and a supercapacitor, all in one.
That could be an important development for the EV market. An improved battery means greater range without adding more weight, and a more efficient supercapacitor translates into faster charging and more powerful discharging.
“A new nanomaterial acts as both battery and supercapacitor: A conductive polymer (green) formed inside the small holes of a hexagonal framework (red and blue) works with the framework to store electrical energy rapidly and efficiently,” by William Dichtel, Northwestern University.
What Is A Covalent Organic Framework (COF), Anyways?
For a quick primer on COF’s you can check out Lawrence Berkeley National Laboratory, which provides this rundown:
“COFs and their cousin materials, metal organic frameworks (MOFs), are porous three-dimensional crystals with extraordinarily large internal surface areas that can absorb and store enormous quantities of targeted molecules.”
If that sounds a little vague, CleanTechnica offered up the origin story last year:
“COFs were first developed at the University of Michigan back in 2005, when researchers ‘coaxed’ rigid plastics to organize into predictable crystal structures — something that had never been done before. The research team did it by slowing down the reaction time involved in synthesizing rigid plastics.”
Like their MOF cousins, COFs benefit from a hub-and-strut structure (think Tinker Toys and you’re on the right track) that in effect loads the material with nanoscale pores, like a sponge.
The pores can be altered by adjusting the angles at which the struts connect to the hubs, to provide a COF with unique properties.
As for why COFs instead of MOFs, for one thing, they contain no metal, so they are much lighter than MOFs (COFs are typically made with hydrogen, boron, carbon, nitrogen, and oxygen).
As an organic (aka plastic) material, COFs also have the potential to come in at a far lower cost than other materials.
Solving The COF Energy Storage Riddle, With PEDOT
MOFs have previously been investigated for energy storage, but COFs are at a huge disadvantage in that field because they lack sufficient conductivity.
One key problem is the disordered nature of COFs as applied to electrodes. So far, the body of research has arrived at one solution, which is to apply them to an electrode as a two-dimensional material:
“Two-dimensional covalent organic frameworks (2D COFs) address these limitations by predictably and deliberately organizing redox-active groups into insoluble, high-surface area polymer networks with uniform micropores.”
That’s all well and good, but the problem is that the charge/discharge rate is very slow, so you can kiss any high-performance applications goodbye.
That’s where the new Northwestern University research comes in. Instead of just fiddling with the angles of their COF, they modded it out with the conducting polymer PEDOT, which is short for poly(3,4-ethylenedioxythiophene).
Northwestern provides a rundown on the process:
“… Two organic molecules self-assembled and condensed into a honeycomb-like grid, one 2-D layer stacked on top of the other. Into the grid’s holes, or pores, the researchers deposited the conducting polymer.”
The pores in the new material are 2.3 nanometers wide, and each pore translates into increased surface area:
“A small amount of the fluffy COF powder, just enough to fill a shot glass and weighing the same as a dollar bill, has the surface area of an Olympic swimming pool.”
I know right? Here’s the result in plain language:
“The modified COF showed a dramatic improvement in its ability to both store energy and to rapidly charge and discharge the device. The material can store roughly 10 times more electrical energy than the unmodified COF, and it can get the electrical charge in and out of the device 10 to 15 times faster.”
The new device is also quite durable. According to the study, it can withstand 10,000 charge/discharge cycles.
You can get all the details from the American Chemical Society journal Central Science under the title, “Superior Charge Storage and Power Density of a Conducting Polymer-Modified Covalent Organic Framework.”
Baby Steps To A Potentially Giant Energy Storage Transformation — Thanks, Army!
The EV market is slowly gathering steam, and if the Northwestern research pans out, it looks like you ain’t seen nothing yet. The researchers are confident that this is a breakthrough for EV battery technology:
“This work represents the first time that electroactive COFs or crystalline framework materials have shown volumetric energy and power densities comparable with other porous carbon-based electrodes, thereby demonstrating the promise of redox-active COFs for EES [electrical energy storage] devices.”
In other words, if the EV of tomorrow is equipped with a COF battery (or some similar future technology), it will go significantly farther, cost less, and charge faster than the current crop.
Make that the day after tomorrow. So far, the Northwestern research has been tested out on a coin-cell-sized prototype that can light up an LED for 30 seconds.
So, don’t hold your breath. But, when that affordable, sporty, 500-mile range EV of the future does come rolling into your garage, the US Army (yes, the Army) might take partial credit.
Among other funders, the Northwestern team is supported by a US Army Research Office Multidisciplinary University Research Initiatives grant.
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