Green Energy World

Much of the current media attention on batteries, as well as significant R&D efforts and commercial investments, is focused on high-capacity, power-dense rechargeable (secondary) batteries. Such views make sense, of course, as they are used in electric vehicles (EVs) and other higher power, often mobile situations.

Still, there are countless situations where a low-power, energy-limited, non-rechargeable (primary) power source is sufficient, especially disposable applications. These include point-of-care diagnostic devices, smart packaging and labeling, and environmental sensing.

Many of these needs are currently met by small button cells or button cells, often using silver oxide chemistries, as well as lithium, alkaline, zinc-air and other chemistries. These certainly get the job done, but may actually be “too much” for modest applications.

Furthermore, they pose a serious waste stream and environmental challenge as they almost always end up in nearby trash cans. Recycling them, even when they make it to such a facility, is expensive and complicated. (Yes, AA, A, C, and D batteries should be recycled too, but that’s another story.)

An obvious question is: “how many of these batteries are used and how many are thrown away?” I’ve done extensive online research and found nothing. The few coin/coin cell usage figures I’ve found are in dollars rather than units, which doesn’t make much sense to me.

As for recycling, there are plenty of estimates for lead-acid car batteries (80% to 90% is the consensus) and larger lithium battery packs for EVs (numbers are everywhere). Also, most estimates (with the exception of lead acid) focus on dollar figures for recycling operations, not the percentage of coin cells and coin cells that I want to know about recycling. For recycling of coin cells and coin cells, one unreliable source puts it at 3% – I think it might be.

Given the need for disposable and relatively low-energy/low-power disposable batteries, a team at the well-respected Swiss Federal Laboratory for Materials Science and Technology (known as EMPA (German acronym for Eidgenössische Materialprüfungs und Forschungsanstalt)) designed a A disposable paper battery that reduces the environmental impact of batteries in such disposable applications. Their basic battery is about 1 cm 2 and uses zinc as the metal anode, graphite as the nontoxic cathode material, and paper as the biodegradable substrate.

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

These materials are both simple and complex at first glance. The anode and cathode materials they developed are compatible with additive manufacturing techniques and can be stencil-printed into various shapes and sizes. The paper serves as a separator between the anode and cathode and is infused with a dry electrolyte that can be activated with just a few drops of water.

The battery’s electrochemical (EC) cell consists of a paper membrane sandwiched between a zinc-based cathode and a graphite-based air cathode. The device remains inactive until water (electrolyte) is added and the membrane is permeated. A picture of a single cell fabricated by stencil printing on filter paper. At the battery terminals, filter paper is impregnated with wax to avoid electrochemical reactions on the leads and provide mechanical stability. Photo of a stencil-printed paper battery whose design spells out the name of the research institution (Empa); also powers a small LCD clock. The device consists of two electrochemical cells separated by a waterproof layer and connected in series, a schematic cross-sectional view of the cells and their superimposed equivalent circuit (for an ideal voltage source).

The battery consists of three inks printed on rectangular strips of paper. Standard salt (sodium chloride) is spread throughout the strip, with the shorter end dipped into the wax. Ink containing graphite flakes was printed on one side of the paper as the positive electrode (cathode) of the battery, while ink containing zinc powder was printed on the opposite side of the paper as the negative electrode of the battery. battery (anode).

Another ink containing graphite flakes and carbon black is printed on both sides of the paper, on top of the other two inks. This ink forms the current collectors that connect the battery’s positive and negative terminals to the two wires that sit on the wax-soaked ends of the paper. The role of the current collector is to connect the cathode and anode to the external circuit. All inks have been specifically developed and tested to ensure they have shear-thinning gel properties that are compatible with additive manufacturing techniques such as stencil printing and extrusion-based 3D printing.

Don’t Ignore the Need for Small, Low-Capacity Batteries

When a small amount of water is added, the salt in the paper dissolves and releases charged ions, making the electrolyte ionically conductive. These ions disperse through the paper to activate the battery, causing the zinc in the anode ink to be oxidized, releasing electrons. By shutting down the external circuit, these electrons are transferred from the zinc-containing anode — through graphite and carbon black-containing inks, wires and devices — to the graphite cathode, where they are transferred to — and thus reduced to — oxygen from the ambient air. These redox reactions (reduction and oxidation) thus generate electrical currents that can be used to power external electrical devices.

Performance analysis of a single battery showed that the battery activated within 20 seconds by adding just two drops of water. After an hour, the performance of the single-cell battery drops significantly as the paper dries out. However, after the researchers added just two extra drops of water, the battery maintained a steady operating voltage of 0.5 volts for more than an hour. As a demonstration, the team combined two batteries into one to increase the operating voltage and used it to power an LCD alarm clock.

Do you see any use for these low-capacity, disposable and easy-to-dispose batteries? Or are they just a clever idea with limited real-world applications?