The results came as something of a surprise to Wanlin Guo of China’s Nanjing University of Aeronautics and Astronautics. Guo knew that a liquid flowing under pressure through a narrow channel can generate an electronic "streaming" potential, but he emphasizes that "without a pressure gradient we get no potential".

As Guo and his colleagues expected, initial studies of fully immersed graphene moving in seawater showed no induced streaming potential, since no pressure gradient was present. "But when we checked the original record there was a very large signal at the beginning when the graphene was inserted," adds Guo. And once the graphene was fully immersed, the signals vanished.

Moving boundaries

To understand the underlying physics, Guo and his colleagues carried out first-principle calculations describing the interaction of graphene with the sodium and chloride ions in the solution. They found that graphene prefers to adsorb positive sodium ions onto its surface, with the adsorbed layer of sodium ions then attracting a diffuse layer of negative chlorine ions. This creates a so-called electron double layer, a model that’s often used to describe electrokinetic effects.

The researchers realized that this electron double layer has a boundary at the air-liquid interface when the graphene is being inserted into the seawater. "The moving boundary of the double layer is the generator," explains Guo. Fewer chlorine ions accumulate around the layer of sodium ions while the graphene is moving, which increases the concentration of holes in the graphene and generates the waving potential. The reverse effects generate an opposite potential as the graphene is removed and the charges disperse.

Graphene – a simple and beautiful platform

Researchers began to investigate how a streaming potential might be induced by the flow of liquid through carbon nanotubes in around 2001, looking also at how the response of the nanotube was affected by the flow of ionic solutions around the tube. "But the results differed from millivolt to microvolt and the conditions required were conflicting," says Guo.

Guo and his colleagues turned to graphene, which Guo describes as "a very neat and very beautiful platform." Experiments with graphene reproducibly demonstrate the generation of a waving potential, and only when the sheet is moving into or out of the liquid.

Using bilayer and trilayer graphene, the effect was greatly reduced. "To maintain the potential we need a single-layer material," explains Guo. "In bulk material the potential would be relieved by material beneath the surface."

Next steps

"Here we concentrated on the new principal of the waving potential and how to scale it up," says Guo. "Next we are making efforts to amplify the power density." To this end the team have already demonstrated the potential to scale up the electricity induced through connections in series and in parallel. The high-quality graphene required to observe a response also poses challenges for future work.

Full details of the research are reported in Nature Communications.