Paper power from the air for disposable sensors

Researchers in the US have developed an paper-based energy harvesting technology that can generate power from moisture in the air for disposable sensors.

The bacteria-based ‘papertronic’ power cell developed at the Bioelectronics and Microsystems Lab at Binghamton University in New York state, use spores to create a gradient with more positive ions on top than on the bottom and an asymmetric electrode to trap the ions. This leads to an electric charge that can be used for sensors and even wearable displays.

Moisture absorption is enhanced by adding a Janus paper layer that is hydrophobic on one side and hydrophilic on the other, drawing in water molecules and keeping them inside the device until they are processed. All of this has been combined on a paper substrate for a low cost, flexible power source.

When the spore-biofilm was prepared at 37 °C, the device sustainably and reliably produced 0.45 V for more than 2000 min. As the formation temperature decreased to 20 and 0 °C, the voltage significantly reduced to 0.37 and 0.15 V, respectively.

The nanochannel-based spore biofilm generated 0.5 µW/cm2, five times better than other biofilms. The short-circuit current is close to 2.5 µA at 90% RH by directionally adsorbing water molecules from the atmosphere, and even under low humidity conditions (e.g., 20% RH), the generator can still deliver a short-circuit current of ≈1.6 µA. Although the power output of moisture–electric generators is inherently low, they can still provide sufficient electric power for external loads. 

The power cell can also be easily scaled with cells in series for a higher voltage with a strong linear relationship between voltage output and the number of devices.

Two devices series generates an open circuit voltage of 1.2 V at 90% RH, while six units in a series achieve 2.7 V. When 20 units were connected in a series, the system could charge an external 470nF supercapacitor and sufficient power to illuminate a liquid–crystal display (LCD), highlighting its potential for powering small electronic devices.

“Wearable electronics will use energy-harvesting techniques in the future, but right now, the techniques are very irregular in time, random in location and inefficiently converted,” said Professor Seokheun “Sean” Choi who runs the lab focused on papertronics. “The reason why I was interested in this topic is that the moisture in our air is ubiquitous, and I realized that energy harvesting from moisture is very easy.

Potential improvements and refinements include increasing power output, developing a method for energy storage and integrating with other energy-harvesting techniques. Choi also hopes to shrink the device to the same scale as micro-electromechanical systems (MEMS).

“The size of one device is too big for me. I’m a MEMS guy!” he said. “By decreasing each individual unit and connecting more cells within a small footprint, we can improve the power density significantly. Also, because we are using paper, we can try many other ideas, including origami techniques.”

“I don’t want to wear something all day for four months,” he said. “I want to use it for short time and then throw it away — so in that way, paper is the best.”