Protons (H⁺) and hydronium ions (H₃O⁺) in free aqueous solutions seem to migrate faster than other ions due to the the Grotthuss-mechanism. Individual protons do not really migrate at all. Instead, bonds of the hydronium ions are broken and new bonds to other water molecules are formed, so that the individual proton does not migrate. Rather charges are transported directly from one water molecule to the next. This process is quicker than the diffusion of an ion through the solution.

Behavior in Confined Spaces Unexplored

So far, many studies have investigated the transport of protons in free aqueous solution. “In real life such conditions are relatively rare,” says Professor Martina Havenith, speaker of RESOLV and an author of the study. “Most protons transport processes actually occur in confined spaces or in nanopores.” Hydronium ions are involved in defining the pH value. Up to now, the effect the of confinement has not yet been completely understood.

To change that, researchers from Bochum and Berkeley combined theoretical and experimental methods. They created tiny water pools, whose size could be precisely controlled. As soon as the diameter of the droplets became smaller than two nanometers, the proton transport mechanism in the experiment and simulations changed abruptly. “Under two nanometers the proton migration is restricted by confinement effects. This effect is reduced when the water pool is enlarged,” explains Martina Havenith. “Suprisingly we found that above two nanometres, where the formation of hydronium ions is possible, there is a proton traffic jam.” The proton is stuck in an oscillatory state, where it bounces back and forth along the surface of the water pool, but makes no progress forward, resulting in the conductivity not increasing further — as originally expected.

Short-Circuit in the Hydrogen-Bonding Network

In addition to the size of the pools, the acid concentration also influences the proton migration behavior. When the research team increased the acid content, they created a type of short-circuit in the hydrogen bonding network of the droplet, so that the proton no longer migrated from its position, but rather paused in oscillatory bouncing state. “That has consequences for every system that relies on proton transport, because the size of the system or the proton concentration can lead to a traffic jam and for example disrupt the signaling process,” concludes Havenith.

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Materials provided by Ruhr-University Bochum. Note: Content may be edited for style and length.

One major challenge stretchable electronics must overcome is the stiff and inflexible nature of their energy storage components, batteries and supercapacitors. Supercapacitors that use electrodes made from transitional metal carbides, carbonitrides or nitrides, called MXenes, have desirable electrical properties for portable flexible devices, such as rapid charging and discharging. And the way that 2D MXenes can form multi-layered nanosheets provides a large surface area for energy storage when they’re used in electrodes. However, previous researchers have had to incorporate polymers and other nanomaterials to keep these types of electrodes from breaking when bent, which decreases their electrical storage capacity. So, Desheng Kong and colleagues wanted to see if deforming a pristine titanium carbide MXene film into accordion-like ridges would maintain the electrode’s electrical properties while adding flexibility and stretchability to a supercapacitor.

The researchers disintegrated titanium aluminum carbide powder into flakes with hydrofluoric acid and captured the layers of pure titanium carbide nanosheets as a roughly textured film on a filter. Then they placed the film on a piece of pre-stretched acrylic elastomer that was 800% its relaxed size. When the researchers released the polymer, it shrank to its original state, and the adhered nanosheets crumpled into accordion-like wrinkles.

In initial experiments, the team found the best electrode was made from a 3 µm-thick film that could be repetitively stretched and relaxed without being damaged and without modifying its ability to store an electrical charge. The team used this material to fabricate a supercapacitor by sandwiching a polyvinyl(alcohol)-sulfuric acid gel electrolyte between a pair of the stretchable titanium carbide electrodes. The device had a high energy capacity comparable to MXene-based supercapacitors developed by other researchers, but it also had extreme stretchability up to 800% without the nanosheets cracking. It maintained approximately 90% of its energy storage capacity after being stretched 1,000 times, or after being bent or twisted. The researchers say their supercapacitor’s excellent energy storage and electrical stability is attractive for stretchable energy storage devices and wearable electronic systems.

The authors acknowledge funding from the Key Research and Development Program of Jiangsu Provincial Department of Science and Technology of China, China Postdoctoral Science Foundation and High-Level Entrepreneurial and Innovative Talents Program of Jiangsu Province.

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Materials provided by American Chemical Society. Note: Content may be edited for style and length.

Shape memory polymers alternate between an original, undeformed state and a secondary, deformed state. The deformed state is created by stretching the polymer and is held in place by molecular changes, such as dynamic bonding networks or strain-induced crystallization, that are reversed with heat or light. The polymer then returns to its original state through the release of stored entropic energy. But it’s been challenging for scientists to make these polymers perform energy-intensive tasks. Zhenan Bao and colleagues wanted to develop a new type of shape memory polymer that stretches into a stable, highly elongated state, allowing it to release large amounts of energy when returning to its original state.

The researchers incorporated 4-,4′-methylene bisphenylurea units into a poly(propylene glycol) polymer backbone. In the polymer’s original state, polymer chains were tangled and disordered. Stretching caused the chains to align and form hydrogen bonds between urea groups, creating supermolecular structures that stabilized the highly elongated state. Heating caused the bonds to break and the polymer to contract to its initial, disordered state.

In tests, the polymer could be stretched up to five times its original length and store up to 17.9 J/g energy — almost six times more energy than previous shape memory polymers. The team demonstrated that the stretched material could use this energy to lift objects 5,000 times its own weight upon heating. They also made an artificial muscle by attaching the pre-stretched polymer to the upper and lower arm of a wooden mannequin. When heated, the material contracted, causing the mannequin to bend its arm at the elbow. In addition to its record-high energy density, the shape memory polymer is also inexpensive (raw materials cost about $11 per lb) and easy to make, the researchers say.

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Materials provided by American Chemical Society. Note: Content may be edited for style and length.