A water-activated paper battery biodegrades once used

A water-activated disposable paper battery is being developed by scientists at Switzerland's Empa institute that biodegrades once discarded.

The Empa institute has developed a disposable paper battery aiming to reduce the environmental impact of single-use electronics for applications such as point of care diagnosis, smart packaging and environmental sensing. The battery uses Zinc as a biodegradable metal anode, graphite as a nontoxic cathode material and paper as a biodegradable substrate. To facilitate additive manufacturing, it has developed electrodes and current collector inks that can be stencil printed on paper to create water-activated batteries of arbitrary shape and size.

The battery remains inactive until water is provided and absorbed by the paper substrate, taking advantage of its natural wicking behaviour. Once activated, a single cell provides an open circuit potential of 1.2 V and a peak power density of 150 µW/cm2 at 0.5 mA. As a proof of concept, Empa fabricated a two cell battery and used it to power an alarm clock and its liquid crystal display.

Over the last decades, an ever-increasing use of electronic devices, leading in turn to electronic waste (e-waste) becoming the world's fastest growing waste stream. Mitigating the associated environmental risks requires advances at the material and device levels, for instance by moving towards more environmentally friendly materials and improving the resource recovery rate. There has been notable progress in this direction, for example with the development of green power supply technologies such as biodegradable photovoltaics, energy harvesters and supercapacitors. However, there is still limited research on biodegradable primary batteries, a complementary and versatile source of energy that can provide higher energy density and more stable operation.

Battery research predominantly focuses on performance, constantly progressing towards higher energy and power densities, faster charging rates and improved operation stability. This is mainly achieved by developing new materials tailored to the requirements of lithium-ion cells that currently dominate the market. However, with a rising awareness of the e-waste problem and the emergence of single-use electronics for applications like environmental sensing and food monitoring, there is a growing need for low environmental impact batteries. This shift from traditional performance-oriented figure of merits creates new opportunities for unconventional materials and designs that can provide a balance between performance and environmental impact.

Aqueous primary batteries based on inorganic materials such as magnesium (Mg), iron (Fe), tungsten (W) and molybdenum (Mo) have recently emerged as promising candidates for use in high-energy density transient batteries. Organic alternatives have also been demonstrated using, for instance, naturally occurring melanin and quinone in a biodegradable aqueous redox flow battery. Although promising advances have been made in recent years, additive manufacturing of biodegradable batteries remains an important scientific challenge.

Cellulose, in the form of paper, has an undeniable historical importance. Following its millennial use as a carrier substrate for information and knowledge transfer, it has over the last decade experienced renewed interest as an advanced material in a wide range of applications including in biomedical diagnostics, as information display and for energy storage. Batteries and supercapacitors have also been developed using cellulose as a high surface area template for redox active materials, or as a low-cost substrate by coating functional dispersions onto pre-formed paper or foam. However, several unique properties of cellulose such as its intrinsic biodegradability, hygroscopic nature and wicking behavior have so far been poorly utilised.

Empa has created a printed paper battery developed to power single-use disposable electronics and to minimise their environmental impact. The battery is based on a metal-air electrochemical cell that uses Zinc as a biodegradable metal in the anode, graphite in the cathode, paper as a separator between the electrodes, and a water-based electrolyte. In addition to paper’s inherent biodegradability, sustainability and low cost, this design takes advantage of its natural wicking behaviour and hygroscopic nature; The battery remains inactive until it contacts with water which then passively absorbs and transports across the paper membrane, thus activating the battery.

 

 

The single cell battery is composed of a paper substrate sandwiched between the air cathode and a current collector on one side, and the zinc anode and a current collector on the opposite side. Figure 1a (above) presents a schematic cross-section of the device and illustrates its water activation process. The battery is manufactured without electrolyte, effectively maintaining the anode and cathode isolated from one another. When water is provided to the system, it readily absorbs and diffuses through the paper substrate, thus dissolving NaCl dispersed in the paper and thereby activating the electrochemical cell. During discharging, the zinc anode is oxidized while an oxygen reduction reaction occurs at the cathode.

Because the cathode reaction uses oxygen from ambient air, the airtight current collector located on this side of the device is limited in size. This design maximizes oxygen flow while maintaining the contact resistance as low as possible. Figure 1b presents a picture of the fabricated battery where the cathode appears in gray, the current collector in black and the substrate in white. The paper substrate extends beyond the 1 cm2 active area to create an activation wick where water can absorb into the system. Contrarily, the substrate is made hydrophobic on the terminals end to avoid undesired electrochemical reactions with the connecting wires. This single cell battery provided a 1.2 V open circuit potential.

Multiple electrochemical cells can be printed on the same substrate and connected in series to achieve higher open circuit potentials. Figure 1c shows a two cell battery with a 2.4 V open circuit potential powering an alarm clock and its liquid crystal display. As a demonstration of the design flexibility, the battery spells the name of our research institution (Empa). A closer view is presented in Fig. 1d where the anode of the left cell appears in black and the cathode of the right cell in appears in grey. The cells are separated by a hydrophobic region as illustrated in the schematic cross-section presented in Fig. 1e, overlaid with the equivalent circuit (for ideal voltage sources).

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