You probably know that simply tossing dead batteries in the trash is no bueno – they release toxic heavy metals as they break down over time in landfills, contaminating the soil and nearby water supplies. Thankfully, we now have e-waste recycling facilities around the world that can prevent these from polluting the environment – but surely we can also make safer batteries that decompose naturally, no?
That's what propelled researchers at Canada's McGill University to develop an eco-friendly alternative. Inspired by children's science projects that used a lemon and copper wire to power a lightbulb, the small team explored how citric acid could enhance a gelatin-based electrolyte to increase its conductivity.
Right, so the battery uses gelatin as the electrolyte, and magnesium and molybdenum as electrodes – both of which are relatively benign elements and can safely degrade in soil. That wouldn't work great on its own, as "Magnesium can generate a layer that stops the reaction between electrolyte and electrode," explained PhD student Junzhi Liu, who handled battery testing for the study that appeared in Advanced Energy and Sustainability Research this August.
Liu followed research supervisor Sharmistha Bhadra's advice, who said "Many people make a lemon battery as kids. The lemon has enough ions to conduct electricity. I suggested Junzhi look at citric acid."
The engineers found that mixing citric acid, and even lactic acid, with the gelatin electrolyte, broke down the layer that accumulated on the magnesium electrode. This increased the battery's lifetime and voltage.
That's neat on its own. What's even cooler is that once the team suspended both acids in the gelatin electrolyte, the researchers cut the battery in a pattern inspired by kirigami, the Japanese art of folding and cutting paper into three-dimensional designs. This allowed for the battery to stretch by up to 80% beyond its original length, while maintaining stable voltage.
To test this, the team developed a simple pressure sensor that could be worn on a finger, and powered it with the battery. The 0.4 x 0.4-inch (1 x 1-cm ) battery successfully powered the wearable device, producing only slightly less power than a standard AA-sized battery.
The scientists also found that when this stretchy battery was depleted and immersed in a phosphate-buffered saline solution, its electrolyte and magnesium electrode fully degraded over the course of just under two months. The molybdenum electrode has a slower degradation rate, so it needs more time to fully decompose.
With that, the team demonstrated that it's possible to make a more environmentally friendly battery that can help reduce e-waste, and find applications in wearables, medical implants, and in future Internet-of-Things devices.
Source: McGill University
