A strategy to achieve vertical iontronic energy storage via osmotic effects and electrode redox reactions

In recent years, engineers have attempted to identify new technologies for sustainably generating and storing energy. One promising solution harnesses the energy produced by osmosis when two fluids with different salt concentrations meet, for example when a body of fresh water (e.g. a river) flows into a body of salt water (e.g. the sea).

The energy produced by differences in salt concentration, known as salinity gradient or osmotic energy, has proven very difficult to transform into portable electrical energy in a practical and scalable manner. Using this process to power consumer electronics, such as smartwatches and smart bracelets, seems impractical so far.

Researchers from the Chinese Academy of Sciences, Tsinghua University and the Hong Kong University of Science and Technology recently introduced a new method that could help efficiently store iontronic energy based on osmotic effects. This method, described in Natural energyallowed them to exploit the osmotic effects and redox reactions of the electrodes to create a vertical iontronic energy storage system.

“Almost 10 years ago, we observed an interesting scientific phenomenon, that ions transported quickly in water inside graphene oxide (GO) can generate decent energy,” said Di Wei, co-author of the paper, to Tech Xplore.

“This was the first attempt to provide a very safe energy source enabling new applications, including building a foldable energy source on paper and a platform for futuristic wearable electronics Later, other studies attempted to increase its power via the concept of fractal mathematical design as a printing pattern, which unveiled the mechanism using silver (Ag) electrodes.

The recent study by Wei and co-workers builds on these previous research efforts, drawing inspiration from both the efficient ion transport dynamics of 2D nanofluidic GO channels and carefully tailored interfacial redox reactions. In their paper, the team presented a new approach to store iontronic energy based on osmotic effects, thus enabling the realization of innovative, renewable, ultrathin and safe energy sources.

“Reverse electrodialysis (RED) is one of the most commonly used methods for osmotic energy conversion and much research has focused on ion-selective membranes to increase ion transport and reduce resistance internal,” Wei explained. “There is a competitive relationship between the selectivity and permeability of the ion-selective membrane, and the optimized ideal thickness is expected to be less than 1 µm, which is fragile and difficult to achieve.”

To create their solid-state iontronic energy storage device, Wei and his colleagues first sprayed various GO-based inks onto charge collectors, using an ultrasonic spray system, and then dried them on a PET substrate. As the inks dried, the 2D nanofluidic channels of GO began to form.

“The electrode spacing of the device, covered by GO, is equal to the thickness of the ion-selective membrane in a conventional osmotic energy source,” Wei said. “We designed a vertical strategy using the edge of a PET substrate and a Kapton film, which offered the possibility of reducing the ion transport distance (equivalent to decreasing the thickness of the ion-selective membrane).”

During initial tests, it was found that the vertical structure created by the researchers effectively decreased the internal resistance of the GO film, thereby increasing the performance of their device. Notably, the electrode spacing used by Wei and colleagues in their experiments could also be reduced to the micrometer or even nanometer scale, as could the vertical design of these new transistors.

“Unlike the traditional osmotic energy conversion device, we propose a different approach to prepare a solid-state (i.e., humidity-driven) iontronic energy storage device that utilizes the properties of osmotic nanoconfined ion transport and interfacial redox reactions,” Wei said. “The vertical structure effectively decreased the internal resistance of the device and showed superior practical performance due to its improved power output with a relatively large GO film area and shorter ion transport distance.”

In initial evaluations, the device created by this research team achieved a remarkable and ultra-high output power density of 15,900 W·m.^-2, and higher volumetric specific energy and power densities, simultaneously. By connecting their devices in series, Wei and his colleagues obtained the corresponding voltage, large enough to power commercial electronics.

“Unlike traditional batteries and supercapacitors, our iontronic energy storage device could also be printed directly using commercial coating systems or printers in a cost-effective manner,” Wei said.

“Overall, fundamental understanding of nanoconfined ion dynamics and rational design of iontronic energy systems could pave the way for studying the influence of other 2D nanomaterials with various nanohierarchical designs and fine-tuning reactions redox in the development of futuristic and highly efficient products and renewable energy sources.

Recent work by Wei and colleagues opens exciting avenues for the development of alternative and renewable energy sources for consumer electronics. In the future, the approach described in their paper could inspire the development of additional energy storage systems taking advantage of osmotic effects.

“As noted in our paper, the manufacturing of our device could be scaled up using commercial coating systems or printers in a cost-effective manner,” Wei added.

“Due to the volumetric specific energy (9.46 Wh cm⁻³) and power density (106.33 W cm⁻³) obtained, the device could directly power electronic devices with relatively high power consumption, such as LCD screens. In our next studies, we will devote ourselves to the applied research of this iontronic energy storage device. »

Toward the end of 2023, Wei and his colleagues began conducting preliminary research exploring potential applications of their iontronic energy storage system. They now plan to continue exploring this research direction, focusing on their device’s potential to power medical implants, wearable devices, and other small, conformable mobile technologies, such as ultra-thin human-machine interfaces.

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