Biodegradable batteries made out of paper

Scientists from Nanyang Technological University, Singapore (NTU Singapore) have developed paper-thin biodegradable zinc batteries that could one day become an environmentally sustainable option for powering flexible and wearable electronic systems. Their work has been published in the journal Advanced Science.

The internal workings of a battery are usually housed within a metal or plastic case. Inside this case are the cathode and anode — the electrodes where the electrochemical reactions occur. A separator added between the cathode and anode creates a barrier and prevents the electrodes from touching while allowing electrical charge to flow freely between them, avoiding short circuits. Also inside the battery is a medium known as the electrolyte, which allows the electric charge to flow between the cathode and anode.

The NTU batteries are made up of electrodes screen-printed onto both sides of a piece of cellulose paper that has been reinforced with hydrogel. The scientists adopted a ‘sandwich design’ for their batteries — the electrodes are like the bread slices and the cellulose paper that the electrodes are printed on is like the sandwich filling.

The fabrication process starts with reinforcing the cellulose paper with hydrogel to fill up the fibre gaps found naturally in cellulose. This forms a dense separator that effectively prevents the mixing of the electrodes, which are formulated as ‘electrode inks’ and screen-printed onto both sides of the hydrogel-reinforced cellulose paper.

The anode ink is mainly made up of zinc and carbon black, a conductive type of carbon. For the cathode ink, the scientists developed one type with manganese and another with nickel as a proof of concept, though the research team said that other metals could possibly be used.

After the electrodes are printed, the battery is immersed in an electrolyte. A layer of gold thin foil is then coated on the electrodes to increase the conductivity of the battery. The final product is about 0.4 mm thick — about the thickness of two strands of human hair.

In a proof-of-concept experiment, the NTU team demonstrated how a 4 x 4 cm square of printed paper battery could power a small electric fan for at least 45 minutes; bending or twisting the battery did not interrupt the power supply. In another experiment using a 4 x 4 cm battery to power an LED, the scientists showed that despite cutting away parts of the paper battery, the LED remained lit, indicating that cutting does not affect the functionality of the battery.

With hydrogel and cellulose naturally broken down by bacteria, fungi and other microorganisms, the battery can simply be buried in soil at the end of its life span. It breaks down in a matter of weeks, making it a fully biodegradable product.

“When decomposition happens, the electrode materials are released into the environment,” said Professor Fan Hongjin, co-lead author on the study. “The nickel or manganese used in the cathodes will remain in their oxide or hydroxide forms, which are close to the form of natural minerals. The zinc found in the anode will be naturally oxidised to form a non-toxic hydroxide. This points to the battery’s potential as a more sustainable alternative to current batteries.”

“We believe the paper battery we have developed could potentially help with the electronic waste problem, given that our printed paper battery is non-toxic and does not require aluminium or plastic casings to encapsulate the battery components,” added co-lead author Assistant Professor Lee Seok Woo. “Avoiding the packaging layers also enables our battery to store a higher amount of energy, and thus power, within a smaller system.”

Going forward, the NTU researchers hope to demonstrate the complete integration of the printed paper battery in printed electronics, electronic skins and energy storage systems. By developing a single large piece of battery that can be cut to desired shapes and sizes without loss of efficiency, Prof Fan said the team has created a simpler, cheaper way of manufacturing batteries that are “ideal for integration in the sorts of flexible electronics that are gradually being developed”.

Image credit: NTU Singapore.

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