Single-use printed paper stack is water-activated and easy to dispose of

What you will learn

  • The need for single-use, low-capacity batteries.
  • How paper impregnated with a simple solution and “printed” contacts can form a basic battery.
  • The results obtained with this design and implementation of the battery.

Battery research is primarily focused on performance, constantly working towards higher energy and power densities, faster charge rates, and improved operating stability. As a result, research on biodegradable primary batteries as a complementary and versatile energy source is limited.

However, as noted by a team from the highly respected Swiss Federal Laboratories for Materials Science and Technology, known as EMPA (the German acronym for Eidgenössische Materialprüfungs und Forschungsanstalt), a large number of applications n ‘need only a modest amount of power and become one-shot demands. These include single-use electronics for point-of-care diagnostic devices, smart packaging and environmental sensing.

Although small, the cost and environmental impact of using conventional battery structures can still be significant. Faced with this situation, EMPA researchers have designed a disposable paper battery that reduces the environmental impact of batteries in such single-use applications. Their basic battery, about 1 cm2uses zinc as the biodegradable metal anode, graphite as the non-toxic cathode material, and paper as the biodegradable substrate.

The battery remains inactive and thus retains its full energy capacity until water is added and absorbed by the paper substrate, taking advantage of its natural wicking behavior. When activated, a single cell provides an open circuit potential (OCP) of 1.2 V and a peak power density of 150 µW/cm2 at 0.5mA.

Materials and inks

The materials are both simple at first glance and sophisticated (Fig.1). The anode and cathode materials they have developed are compatible with additive manufacturing techniques and can be stencil printed in a wide range of shapes and sizes. The paper, which acts as a separator between the anode and the cathode, is infused with the dry electrolyte which requires only a few drops of water to activate.

1. (a) Illustration of the water-activated paper battery. Its electrochemical (EC) cell consists of a paper membrane sandwiched between a zinc-based cathode and a graphite-based air cathode. Carbon-based current collectors are used to pull charges from the EC cell and contact external circuitry. The device remains inactive until water, which serves as the electrolyte, is supplied to the system and permeates the paper membrane. (b) Photo of a single-cell battery made by stencil printing on filter paper. The device is activated by immersing the wick in water or any aqueous solution. At the battery terminals, the filter paper is impregnated with wax to prevent electrochemical reactions of the conductive wires and to ensure mechanical stability. c) Photograph of a battery of paper stencilled with a design spelling out the name of the research institute (Empa). The battery can power low-power electronics such as the LCD alarm clock shown in this photo. The device is composed of two electrochemical cells separated by a water barrier, as shown in the photograph (d), and connected in series, as shown in the schematic cross-section (e) of the battery with its superimposed equivalent circuit ( for ideal voltage sources).

The battery consists of three inks printed on a rectangular strip of paper. Standard salt (sodium chloride) is scattered over the entire strip of paper, and one of its shorter ends is dipped in wax. An ink containing flakes of graphite and functioning as the positive end of the battery (the cathode) is printed on one of the flat sides of the paper, while an ink containing zinc powder is printed on the reverse paper as the negative end of the battery (the anode).

Another ink that contains flakes of graphite and carbon black is printed on both sides of the paper, above the other two inks. This ink forms the current collectors connecting the positive and negative ends of the battery to two wires, which are located at the end of the wax-dipped paper. The role of the current collector is to connect the cathode and the anode to the external circuits.

All inks have been specially developed and tested to ensure they exhibit anti-shear gel properties compatible with additive manufacturing techniques such as stencil printing and extrusion 3D printing.

When a small amount of water is added, the salts in the paper dissolve and charged ions are released, making the electrolyte ionically conductive. These ions activate the battery by dispersing through the paper, causing the zinc in the ink at the anode to oxidize, releasing electrons.

By closing an external circuit, these electrons are transferred from the anode containing zinc – via the ink containing graphite and carbon black, the wires and the device – to the graphite cathode where they are transferred and therefore reduced oxygen. ambient air. These redox reactions (reduction and oxidation) thus generate an electric current that can be used to power an external electrical device.

Tests and results

Performance analysis of a single-cell battery showed that when just two drops of water were added, the battery activated in 20 seconds (Fig.2). After one hour, the performance of the single-cell battery decreased significantly due to the drying of the paper.

2. (a) Graph of single-cell battery open circuit potential (OCP) versus time upon activation. Time zero is when water has been dispensed onto the activation wick. The battery has a stable OCP of 1.2 V and an activation time of 20 seconds. (b) Nyquist plot of battery before activation, showing internal resistance Rentire of 85 kΩ. (c) Nyquist curves of the battery immediately after activation (gray squares) and after one hour discharge at 100 μA (black dots), showing the internal resistances Rentire approximately 70 Ω and 90 Ω, respectively. (d) Chronopotentiogram of the battery (black solid line) and the corresponding current ramp (grey dotted line) as a function of time. (e) Graph of battery generated power versus current, showing a maximum of 150 μW at 0.5 mA. (f) Chronopotentiogram of the 100 μA constant current battery, showing the device drying and reactivation process. The discontinuity in the data points is due to additional analysis that was performed on the sample at its maximum operating voltage.

However, after the researchers added two more drops of water, the battery maintained a stable operating voltage of 0.5 V for over an additional hour. As a demonstration, the team combined two cells into a single battery to boost the operating voltage and used it to power an LCD alarm clock.

“What’s special about our new battery is that, unlike many metal-air batteries that use a metal foil which gradually wears out as the battery wears out, our design only requires adding the amount of zinc to the ink that is actually needed for the specific application,” said project manager Gustav Nyström.

The work is detailed in their readable paper “Water Activated Disposable Paper Battery” published in Nature. There is also a two-page supplemental information publication with graphical top, bottom and side views of the single cell battery manufacturing process at each stage.

About Debra D. Johnson

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