Ambient humidity energy harvesters could compete with solar cells

20 February, 2020

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Leveraging the natural adsorption–desorption exchange of water molecules from ambient air at the interface of a nanoporous mesh of protein nanowires, a team of researchers from the University of Massachusetts has demonstrated a durable form of energy harvesting that could potentially compete with solar cells on volumetric power density. By Julien Happich @ eenewseurope.com

Publishing their results in the Nature journal under the title “Power generation from ambient humidity using protein nanowires”, the researchers describe a thin-film device (only 7µm-thick) consisting of a mesh of electrically conductive protein nanowires (sheared from the microorganism Geobacter sulfurreducens) laid on top of a large gold electrode over a glass substrate for the bottom side, and covered with a thin top electrode leaving the mesh exposed to ambient air.

In this configuration, the device, only a few millimetres square, was able to produce a sustained voltage of around 0.5 volts across the two electrodes, delivering a current density of around 17µA/cm², for over 20h before self-recharging. The authors estimated the device’s power density to roughly 4mW/cm³, two orders of magnitude higher than previous reported energy-harvesting technologies relying on ambient, atmospheric moisture.

TEM images of the purified nanowire network (right panel)
produced by the microorganism Geobacter sulfurreducens
(dark shape in the left panel). Scale bars, 100 nm.
A diagram of the device structure is shown at the bottom

They tested their device for more than two months, it maintained a stable direct-current voltage of between 0.4 and 0.6V with fluctuations in voltage only associated with changes in ambient relative humidity (with 40–50% relative humidity yielding the highest voltage. The device was proven to still output power at a relative humidity as low as 20% (comparable to a desert environment) as well as at 100% humidity.

When the film is exposed to ambient humidity, a self-maintained moisture gradient (a depth-dependent difference in moisture adsorption) forms within the film, the researchers observed, which itself creates an ionization gradient in the carboxylic groups or a concentration gradient in mobile protons of the nanowires’ surface functional groups. The resulting charge diffusion induces a counterbalancing electrical field or a potential analogous to the resting membrane potential in biological systems, the authors explain, noting that a wide range of synthetic protein nanowires would likely be suitable for the design of similar ambient air energy-harvesting devices.

Because water molecules in air naturally comprise ionized species, or are ionized when adsorbed on the nanowire surface, they can donate charges to the nanowire, supplying the closed-loop current flow driven by the voltage resulting from the moisture gradient, the researchers further explain.

University of Massachusetts – www.umass.edu

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