The crystal structure rainbow – Indigo in your batteries?

What does it look like?

The indigo carmine structure found by Yao et al.  Image take from thier paper.

The indigo carmine structure found by Yao et al. Image take from thier paper.

What is it?

This molecule is colourful, and perhaps an answer to humankind’s energy storage issues? Indigo carmine (or 5,5′-indigodisulfonic acid sodium salt) already has a number of uses because of its colour. This molecule can be used as an indicator of acidity. It also has medical uses, often being used to investigate how a urinary tract is working. It turns your pee indigo blue and can easily be broken down by your kidneys.

But some investigators have looked into the possibility of it being used as a battery material. This is because of the sodium ion in the structure (in the picture above these are purple). Much of the focus on increasing the effectiveness of batteries is investigating lithium ion batteries, but there’s an issue in that there’s only so much lithium the world has to offer. There’s a lot more sodium available, but it’s difficult to use as a battery material because the sodium atoms are quite a bit bigger than lithium ones (and you need them to be able to flow past the rest of the material in the electrode). Electrode materials (those that store up the charge in batteries) are often made out of inorganic materials, which are themselves quite big atoms. The difficulties is finding an electrode that can reproducible give away it’s sodium to generate charge, but then take it back to store up energy again. Could an electrode be made out of a smaller, organic material?       

Where did the structure come from?

These Japanese researchers investigated the potential that indigo carmine has as a battery material, and monitored how the structure changed as it was charged up and as the energy was used. As they were preparing the same they saw that the structure had changed, and this new structure had lots of ‘potential’ as an electrode material.


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