An electric power supply driven by water evaporating from a carbon nanomaterial has been unveiled by researchers in China. Their device is about 2.5 cm long and can create a voltage of about 1.5 V – on par with a standard AA battery. While the power supply only delivers a few hundred nanoamps, the team connected several devices together to run a liquid-crystal display. With further improvements, the researchers say, the device could be used to run sterilization equipment and to purify or desalinate water in warm regions of the world.

The power supply has been built by a team led by Wanlin Guo at Nanjing University and Jun Zhou at Huazhong University. It involved depositing multi-walled carbon nanotubes (MWCNTs) onto a quartz substrate to create two electrodes. The substrate is about 25 mm long and the 2 mm electrodes are positioned at each end. Carbon black – tiny particles of carbon about 20 nm in diameter – was then deposited, covering the substrate to a thickness of about 70 µm. Copper wires were then attached to each electrode and a circuit was completed via a voltmeter.

Dunked in water

One end of the device is placed in a beaker of deionized water so that the bottom few millimetres of the device are immersed. Capillary action draws water up the previously dry portion of the device, reaching a maximum distance of about 20 mm from the wet end in about 1 h. As the water rises through the device, the voltage across the electrodes increases, reaching a maximum value of about 1 V in 1 h.

When the device and beaker were placed in an enclosed environment from which water vapour cannot escape, the voltage dropped to zero in about 15 min – and recovered quickly when ventilation was provided. Air flow, which is known to boost evaporation, increased the voltage on the device up to 1.5 V. An increase in ambient humidity, on the other hand, lowered the voltage by inhibiting evaporation. Taken together, say the researchers, these observations confirm that evaporation is driving the operation of the device.

Streaming potential

An infrared spectroscopy study of the device suggests that electrical energy is created via a streaming potential. This is an electrochemical process that occurs when an electrolyte is driven by a pressure gradient through a channel or pore.

The team then connected four of their devices in series to create a power source that can deliver about 380 nA at 4.8 V – which was enough to drive a liquid-crystal display. The team says that the performance of the device could be enhanced by optimizing the streaming and evaporation processes.

The research is reported in Nature Nanotechnology.