Plugging devices into the air around us is set to be a reality, says S.Ananthanarayanan.

The current imperative is that sources of power be carbon neutral. This limits the truly green sources to wind-power, solar power and hydropower. These three sources, unfortunately, share the features of being intermittent or limited to geographical locations.

Xiaomeng Liu, Hongyan Gao, Joy E. Ward, Xiaorong Liu, Bing Yin, Tianda Fu, Jianhan Chen, Derek R. Lovley and Jun Yao, an interdisciplinary team from the University of Massachusetts, Amherst, write in the journal, Nature, of their work in using the levels of moisture in the air around us to generate electricity to power simple devices. This would be revolutionary, as moisture and air are around us at all times and everywhere!

The authors refer to other moisture-based attempts that have been made to harvest energy. The basis of the methods are that thin films in contact with water develop a gradient, from lesser to greater content of moisture. The moisture content itself ionizes or induces atoms within the film material to ionize, or split into positively and negatively charge components. And, as there is a gradient in the quantity of moisture, a gradient develops in the charges induced, which can be used to drive an electric current.

In one report that the paper cites, a power generator and a respiratory monitor are energized by human breath. In other reports, harvesting energy from ambient moisture is suggested as a power source for ‘wearable electronics’. The paper cites other methods, where films of carbon materials are engineered to have different compositions on the two sides, or devices based on polymer materials. The methods, however, the paper says, either depend on a liquid water source or are limited to small dimensions and low output. In many of these instances, in fact, the application suggested is that of a humidity sensor.

Apart from not being scalable, the paper says, devices that work on ambient moisture do not have sustained performance. All that has been possible are short, less than 50 second bursts of current, followed by a recharging phase of more than 100 seconds, the paper says.

In contrast, authors present an arrangement, based on a film of electrically conducting biological material, in the form of a nanowire mesh, which outperforms predecessors both in the current output as well as in the length of time it keeps working. At least 20 hours at a time and output as high as four milliwatts., against the earlier 30 microwatts, for one cubic cm of material, the paper says. At four milliwatts a cubic cm, a one inch cube device, which is about 15 cubic cm, could charge a cell phone, just from the moisture in the air!

The device consists of a thin film, about 7 microns thick, or the tenth of the width of a human hair, of protein nanowires, which is deposited on a gold foil, some 5mm x 5mm, on a glass base. The protein is extracted from a rod-like bacterium, geobacter sulfurreducens, which belongs to a group of bacteria that is electrically conducting. G. sulfurreducens, in fact, is known to create electric currents, has been studied, and is being used in microbial fuel cells.

The nanowires, or protein material from the electrically active species, are sandwiched between the gold foil base and smaller, just 1mm x 1mm, electrode placed above. The distribution of moisture, through the 7-micron film, creates a voltage difference between the two faces of the conducting film. The voltage, when no current is drawn, is about 0.5 Volts and when the gold plates are connected, the current is about 250 nano Amperes.

The behavior did not change with the illumination of the device, which showed that light had nothing to do with the effect. The voltage and current lasted for more than 12 hours, which showed that the effect was not caused by transient charging. And, as both electrodes were gold, it was not some metal-metal effect that was in play. Even with carbon as the electrodes, the paper says, the voltages were of the same order. Chemical tests eliminated any decomposition of the nanowires as being the source of electricity and even removing nitrogen or oxygen from the surrounding air had no effect.

It was found that current could flow, through a load, continuously for as long as 20 hours. In the course of 20 hours, the voltage fell from 0.5V to 0.35V, but could regenerate and reach 0.5V in just 5 hours. Over a longer time span, the voltage remained at 0.4V - 06V for more than two months of cycles of regeneration. The fluctuation of the voltage was associated with changes in the relative humidity of the ambient air. The best voltage output was at 40-50% relative humidity, which may be typical on a fine day in Mumbai or Kolkata. But substantial voltages were still generated at relative humidity as low as 20% (desert) or 100% (soaking, wet day), the paper says.

The authors clearly identified the moisture as the source of electricity by sealing the top face of the film – which led to disappearance of the voltage – which reappeared when the seal was removed. And then, of course, the dependence of the voltage on the relative humidity. It was seen that the moisture content at the surface of the thin film was some 27%, against about 3% at the bottom face for an 8-micron film. And the voltage generated increased with thickness of the film, till a plateau of about 0.55V at about 10 microns.

The device can be made as small as desired, the paper says, and devices can be connected in series or in parallel, to provide higher voltage or to deliver more power. It is practical to think of power density, when films are stacked, of over 1kilowatt per cubic metre, the paper says. In one trial, the authors achieved output at 10V by connecting 17 devices in series.

------------------------------------------------------------------------------------------ Do respond to : response@simplescience.in -------------------------------------------