Bench Talk

(Source: Vera Kuttelvaserova - stock.adobe.com)

As the demands for power become ever greater in modern-day society—with higher power demands being driven by larger populations, more powerful electronics, and more advanced electronic systems—the need to use all available resources becomes more important. 2D materials have been gaining interest for a range of clean energy harvesting devices because the properties of the 2D material class are vast, tunable, and often tailorable to each application (in part due to the breadth of available materials as well as the ability to tune these materials at the atomic level).

2D material energy harvesting devices are being tested for both small-scale applications to power individual devices as well as larger-scale setups that can produce more significant amounts of power. With the dependence on fossil fuels and non-renewable energy sources expected to diminish, we need to harness all the Earth’s natural resources (including different forms of energy) if clean energy is to eventually phase out fossil fuels completely.

Water-energy harvesting is already a focus area in the clean energy space and has been showcased by the power that can be obtained from hydroelectric dams and tidal harvesting systems, as well as harvesting energy from other flowing water systems such as rivers. The Earth is composed of 70 percent water, so it is a natural resource that has the potential to provide large amounts of energy.

Even though the larger energy harvesting systems dominate the lion’s share of generated energy from water, a significant amount of energy can be harvested from the low-frequency flow of water—such as in the motion of raindrops, tide dynamics, or the osmotic effects in salinity gradients. So, while 2D materials can help build more efficient devices for larger energy harvesting systems, they also have the potential for harvesting energy from water that isn’t otherwise possible because of their small size and excellent energy-harvesting properties.

A number of different water-energy harvesting areas are targeted for 2D materials. It also includes water splitting applications, but because those are a photocatalytic process rather than pure water harvesting, that area is for another time, but it is still worth mentioning. Beyond water splitting, there are many different areas where 2D materials are gathering a lot of interest.

While we see 2D materials used in larger-scale energy harvesting and energy storage systems, the ability to harvest other types of energy from water that other systems can make for a more interesting talking point. In the area of smaller-scale harvesting, there are currently two stand-out areas being tested and developed:

• Harvesting osmotic energy from salinity gradients
• Harvesting the motion of water to generate electricity.

Osmotic Energy

The harvesting of osmotic energy involves transforming the chemical energy with salinity gradients into an electrical output. It is employed at the interface where oceans and rivers meet (as there’s a big difference in salinity between the two water bodies creating a salinity gradient). Even though it is a low-magnitude harvesting approach, it’s a process that can produce a power output up to 0.8kWh m³.

The harvesting of osmotic energy is driven by a charged ion flow through an ion-selective, partially permeable membrane. These membranes have varying thicknesses, pore diameters, and densities, and they separate the fresh and the salty water reservoirs, leading to a chemical potential difference. It is in these very thin, semi-permeable membranes where 2D materials have been gathering a lot of interest.

Harvesting Water Motion

The second approach requires less physical infrastructure to harness water energy and uses either electrokinetic or piezoelectric effects to convert the motion of water into electricity. In this process, an electric double layer forms at the interface of a charged solid surface and a fluid, where the electric double layer contains a diffusion layer rich in counter ions. When exposed to an external stimulus, this diffusion layer modifies itself, generating a motion between the fluid and solid known as the electrokinetic effect.

Water-solid interactions can induce several different electrokinetic effects, including the drawing and splashing motions of rain droplets and moisture exposure, based on a material’s ability to interact with the water molecules in the moisture, as some graphene-based sheets can. This opens the possibility of using 2D materials to harness energy from rainfall and moisture in the atmosphere.

Many different properties of 2D materials—beyond their versatility and large numbers—make them suitable for these low-magnitude water energy harvesting operations. One of the key properties is that all 2D materials have a very high surface-to-volume ratio of atoms, which provides an excellent platform for interacting with water molecules. This is something that can’t typically be achieved with conventional bulkier technologies and materials. Beyond the large surface area, there are 2D materials—especially graphene derivatives—that exhibit an enhanced triboelectric interaction with water molecules, making them highly suited to water energy harvesting applications. This property extends to other types of harvesters at both a small device and large installation level.

2D materials possess some distinct and unique advantages for harvesting osmotic energy compared to other materials. The high surface of the 2D materials enables the creation of semi-permeable membranes with a higher working surface area than when they are made of other materials. Beyond this, the ability for 2D materials to be functionalized and adapted in a number of ways allows the creation of much larger pores in the membrane, reducing the level of fouling across the membrane.

Membranes made of 2D materials have already shown performances that are several orders of magnitude higher than their bulkier counterparts. The inherent thinness of 2D material membranes also increases the ion-concentration gradient across the harvesting device, leading to higher power densities. As it stands, several osmotic energy membranes have been created using graphene and graphene oxide derivatives, transition metal dichalcogenides (such as molybdenum disulfide), and hexagonal boron nitride.

Moving on to harvesting water motions, this area is less developed. The most potential is currently in harvesting energy from rainfall. The splash from a raindrop on a tilted 2D material sheet can generate an electric voltage—and a voltage that is higher than the drawing of a water droplet because of the high spreading of the water droplet on impact across a high surface area sheet.

The other way is harvesting the moisture that can become present in some electronic devices. The ability to interact with water vapor has meant that a number of 2D material-based—predominantly graphene derivatives—moisture sensors have been created over the years, and now this moisture can be changed into an electrical output in the form of a harvesting device. There are already several graphene-based moisture harvesters being tested, some of which can harness energy across a moisture variation of up to 90 percent.

In the devices that have been created, the moisture is converted via a direct electric generation process, where the water vapor causes charged hydrogen ions to move through the 2D sheet and induce an open circuit voltage output. In recent years, the output has already improved from an initial output of 35 mV (in the early developments) to 1.5V. These membranes can also be fabricated into larger assemblies to provide greater amounts of power—with some ambitious target application areas including calculators, LEDs, LCD displays, and self-powered IoT devices.

The area of small-scale water-energy harvesting may not be as prevalent as larger-scale water-energy harvesting, such as hydroelectric dams and tidal energy, but it offers a way of harnessing energy that wouldn’t otherwise be harvestable using other materials and/or conventional energy-harvesting technologies. It’s ultimately the combination of a high surface area and excellent interaction properties that enables 2D materials to be used in these types of energy harvesting devices, with harvesting osmotic energy from salinity gradients at ocean-river interfaces and harvesting water motions, such as rainfall, being the most promising application areas.