Interfaces

Let’s face it, we all have to interact. Our entire universe revolves around interactions. From the sensation of touch to the catalysis of chemical reactions to the waterproof coat you might wear on a rainy day, all rely on a specific behaviour at an interface.

These interfaces are of enormous interest to researchers in pretty much all fields, notably catalysis, material separation, and energy storage and conversion. Moreover, it is at the nano-scale that things get truly exciting: the surface area of nanomaterials is immense in comparison to macroscale counterparts, maximising interfacial interactions, and, upon intrusion/extrusion of liquids, they can exhibit unique properties that possess the potential to facilitate the otherwise impossible.

The energy of interfacial phenomena can be optimised and harnessed for chromatography, smart nanomachines, and energy storage applications. There are myriad potential applications to obtain, recycle, and redistribute energy and control systems with smart materials. Due to the current limitations of electric vehicles, for example, the possibility of saving energy from waste heat is particularly appealing, as is the conversion of other forms of energy.

Here at CIC energiGUNE, the research group Interfacial Phenomena and Porous Media study these phenomena to adapt and optimise them for a plethora of applications, such as the flagship project triboelectrification of porous materials as part of the Electro-Intrusion project (FET-PROACTIVE of the Horizon 2020 Programme) and the development of aqueous HPLC technology with the NoDRY ERC proof of concept project, in tandem with computational investigations conducted at Sapienza Università di Roma.

Intrusion/Extrusion

Let’s get into it! Porous nanomaterials are being investigated for a wide range of applications, such as shock-absorbers and bumpers, molecular springs, thermally-regulated artificial muscles (actuators), smart valves, chromatography, smart fluids, and even porous water!

They possess large volumes and enormous surface areas for their size, making them promising storage media for gases, medicine, and even lithium-ions in potential battery technologies. Some can compress and expand under specific conditions, allowing them to behave like smart valves, switches, and artificial muscles. On the other hand, there are others that are hydrophobic and possess fascinating properties when forced to interact with water that can be exploited for a wide variety of energy applications, such as shock-absorbers, molecular springs, and triboelectric generators.

Intrusion is the entry of a material into a cavity or channel under pressure, and extrusion is the reverse process. One of the main focuses of research is storage and separation. Porous materials are also commonly used in column chromatography, where materials are separated for purification in the lab or in industrial settings, and there is even work being done on drug delivery, where porous materials can retain a specific drug/chemical until a specific trigger in vivo (e.g., temperature, pH) causes it to be released into the body. In addition, some porous materials can even be synthesised around drug molecules, avoiding the step of intruding the material.

In  Figure 1, the intrusion/extrusion of water into the material is illustrated schematically, with water entering the porous material at sufficiently high pressure (1c). Depending on the flexibility of the material (amongst other factors) there can be different degrees of energy loss during cycling of intrusion/extrusion. Whilst this may be energy lost if the system is considered as a battery, it is advantageous for a shock absorber, in which energy needs to be dissipated effectively.

^{Figure 1 - Scheme of intrusion/extrusion inside a sealed chamber with piston. (a) The piston starts to move, when sufficient pressure is reached in the chamber (b), the liquid enters the pores of the material producing intrusion (c). Conversely, when the piston releases pressure (d), the liquid exits the porous material resulting in extrusion (e).}

In application

The difference in intrusion pressure and extrusion pressure is dependent on the material, and with greater differences between these two (hysteresis), the systems can be tailored for different technologies. Materials with a greater hysteresis are effective dampers and shock absorbers, dissipating more energy than materials with a smaller hysteresis. On the other hand, smaller hysteresis is more advantageous for energy storage systems, as a similar amount of energy is returned compared to what is put in.

We are also looking at properties that can tune the threshold pressure of intrusion, thereby allowing materials that require pressures outside those of conventional operation to become viable. If these can be tuned without the need to synthesise new materials, they immediately become more appealing for a wide range of applications.

HPLC: a dry topic?

The NoDRY project is focused on determining a system that can allow for the use of water in high-performance liquid chromatography (HPLC), a common technique for material separation in a variety of commercial industries. Traditionally, these technologies use organic solvents that are produced by the petrochemical industry. However, challenges with the wetting/drying of the solid phase has hitherto prohibited the effective use of water.

The project aims to overcome this issue, yielding more efficient and more eco-friendly technology. They work with materials that have a larger hysteresis, making the intruded state stable enough for operation whilst also allowing the material to be dried (for extrusion to occur) when required for cleaning.

Shocking Discoveries

The intrusion/extrusion of water into hydrophobic, porous materials can yield intriguing properties, such as electricity generation, heat generation, and even the swelling of the material under pressure (so called negative compressibility), a very rare trait. Here at CIC energiGUNE, the research group Interfacial Phenomena and Porous Media investigates the generation of electricity by the intrusion/extrusion of water into these porous materials as part of the Electro-Intrusion project, exploiting unique properties unforeseen by classical theories to improve energy harnessing, recovery, and dissipation.

The law of conservation of energy states that energy cannot be created or destroyed, only transformed. Therefore, the dissipated energy must be converted and released via another route. Although some loss will be heat, in the same way that conventional batteries get hot when charging/discharging, another possible route is electrification (Figure 2).

^{Figure 2 - Charge exchange in a pore produced by friction (triboelectrification) of the liquid during the intrusion-extrusion process. (a) Pre-intrusion equilibrium of the liquid in the pore, (b) intrusion and charge exchange and (c) charged material when extrusion occurs. Friction-induced charge (triboelectrification) due to charge exchange during contact between the nanomaterial pores and the surface.}

^{Figure 3 - PVT (pressure, volume and temperature) device cell for triboelectrification measurement where electrical leads are connected to a conductive electrode in contact with a nanoporous hydrophobic material.}

Friction-induced charging/discharging is a much more familiar phenomenon than you think: maybe you have received a shock after walking on a carpet or whilst wearing a woollen jumper, or you have rubbed a balloon on your head and lifted your hair with it. The principle here is more or less the same, but with the added benefit of the enormous internal surface area of nanoparticles: some can have a surface area in excess of 1800m²/g – nearly seven tennis courts for just 1g of material!

At CIC energiGUNE, we have equipment that can measure the contact electrification during cycles of applied pressure (Figure 3). The PVT equipment can reach pressures of 800 bar with efficient temperature regulation, and bespoke caps for the cell allow electrical wires to pass through, with which we can track voltage and current during cycles of high pressure. We also have a mercury porosimeter, which can efficiently measure the intrusion/extrusion pressure and intruded volume of smaller samples, allowing for the more effective screening of candidate materials. Many materials here have been assessed for HPLC applications and nano-triboelectrification, and results are promising. Next steps will include optimisation and development of the systems for commercial applications.

In conclusion, the study of porous nanomaterials and their interfaces is a captivating journey into the world of limitless possibilities for energy conversion and material applications. As we continue to unlock the secrets of these nanostructures, their potential to revolutionise industries such as energy storage, chromatography, and smart materials becomes increasingly evident. With ongoing research endeavours focused on optimising their performance and sustainability, the future holds exciting prospects for harnessing the power of porous nanomaterials to address pressing global challenges and drive innovation in the field of materials science.