In recent decades, materials science has provided us with true prodigies with new, improved and even combined properties to respond to the challenges we face in research, which also facilitate industrial processes or our lifestyle.

In this knowledge area, we have been talking about advanced materials, those with improved thermal, chemical and/or mechanical properties (among others), and even those designed specifically for a particular application. Among these properties, there is one that often goes unnoticed by the general public, but which is crucial for countless applications, especially for those related to energy generation and storage: porosity.

When we talk about porous materials, we might think of a sponge or a pumice stone, both porous, natural and with applications that are directly conditioned by their porous nature. The sponge can hold large volumes of liquids in its cavities, and the roughness of the pumice stone, generated by its pores, makes it an excellent abrasive for our skin. However, the pore size of these materials is in the millimeter, even centimeter range.

On this occasion, we are interested in pores that are on the nanometer scale, which is about six orders of magnitude less than the previous examples. As in other areas of knowledge, going down to the nanometer can enhance the properties we wish to exploit.

Aspects like pore volume, pore size and surface area define the texture of a solid. It is in this empty space within the material and on its inner surface that a wide variety of physical and chemical processes can take place. That is why it is important to control these properties, to get the most out of them in our applications.

A simple analogy will help us to understand this. If I need to store a substance on the surface of a material (what we know as the adsorption phenomenon), I could use an empty vessel, in which the accessible surface for the substance to be stored would be the vessel´s own inner wall. Let us assume, in this case, a few tens of cm² in a bottle of 1 L volume. Now, if I fill that same bottle with a porous material with an internal surface area of about 300 m² for each gram of material, I would be substantially increasing the surface area accessible to the substance I want to store on the surface: the tens of cm² of the container wall plus hundreds of m² for each gram of material I introduce in that container. The storage efficiency in the latter case is evident.

This happens thanks to the nanopores present in the porous solid, which increase the accessible area where the host molecules that we want to store or transport are located, where the desired chemical reactions occur or where charge transfers take place, among other possible phenomena.

Amorphous or crystalline porous materials

Historically, the most commonly used porous materials have been activated carbons or zeolites. The main difference between them is that the former are amorphous (their atoms are randomly disposed of in their structure) and, therefore, the porosity does not have an ordered layout. The latter are crystalline (their atoms and molecules are oriented in their structure based on a certain symmetry), which gives them an ordered porosity and a specific topology.

These materials stand out for being cheap, with a porosity range varying from a few m²/g to several thousand m²/g. However, in recent decades new nanoporous materials - which we could call "a la carte" - have emerged that allow us to design their structure from the atom, their chemical composition, their topology and, therefore, properties such as porosity. An excellent example is metal-organic frameworks, or MOFs, which have generated great enthusiasm in the scientific community for, among other things, having reported record surface area values.

Some examples of porous materials and their respective surface area values.

To have an idea of this, we can make an analogy with the MOF NU-110, with an S_BET value of 7140 m²/g, which has the surface area of a football pitch in one gram of material.

These properties have been applied for years in many fields: for the adsorption of toxic substances in biomedicine, for use as catalysts for chemical reactions in the pharmaceutical industry, for the adsorption and separation of gases of industrial interest or, more specifically, in the energy storage field, such as activated carbons in electrodes for supercapacitors which, due to their low cost and large surface area, allow a high specific capacitance.

Porous materials for energy conversion

Recently these materials are also gaining prominence in energy conversion. At CIC energiGUNE, research is being carried out on the mechanical and thermal energy conversion into electrical energy through water intrusion processes in the pores of a hydrophobic porous material. The Electro-intrusion project led by CIC energiGUNE, in collaboration with four other European academic institutions and a company, aims to deepen into the phenomenon of nanotriboelectrification. This is responsible for the fact that, when intruding water under pressure (generated by a mechanical (e.g. vibrations)) in the pores of a material, through friction with the hydrophobic surface inside its pores, an electric current is generated, like the one caused by rubbing a balloon against our hair. In addition, this intrusion process of water under pressure is endothermic, absorbing heat from the environment and introducing extra energy to the process that is also converted into electricity. This way, we can take advantage of the mechanical energy wasted in the vibrations of any device that generates them and, in particular, in the suspension of an electric vehicle to convert that energy into electricity that could be used to recharge the car during the journey itself.

Once again, the role of the porous materials used in the final device applied to the vehicle (since the project´s ultimate goal is a prototype shock absorber for an electric car) is of paramount importance. Not only because controlling the topology and size of the pores will ease the dissipation of mechanical energy from the vibrations of the car, but also because by controlling their surface chemistry and area, we will be able to maximize the charge transfer surface and increase the electric current in the process.

Thus, apparently, aesthetic or trivial aspects like the texture of a material can have a more than relevant impact on the final application, as seen in one of the latest studies carried out within the Electro-intrusion project: "Turning Molecular Springs into Nano-Shock Absorbers: The Effect of Macroscopic Morphology and Crystal Size on the Dynamic Hysteresis of Water Intrusion-Extrusion into-from Hydrophobic Nanopores", published in the journal ACS Applied Materials & Interfaces this 2022 by P. Zajdel and collaborators.

This research analyses the impact of the morphology of a known large surface area microporous MOF, ZIF-8. By studying ZIF-8 in three different morphologies (powder, nanometric crystals and monolithic form), it is found that its behavior in the water intrusion process is very different in each of its forms. As its crystalline particles are organized differently, its intergranular porosity is also affected, making mechanical energy dissipate in different ways based on its morphology, making it possible to control this process.

Image of the three studied forms of the microporous MOF ZIF-8 (top - left to right: monolithic ZIF-8, nanometric powder and micrometric powder) and graph of energy dissipation as a function of their morphology. Image extracted from ACS Appl. Mater. Interfaces 2022, 14, 23, 26699-26713.

All of the above is yet another example of how science sheds light on aspects of the matter at the nanoscale that may remain invisible to our intuition of the macroscopic world. The increasing complexity of tomorrow´s technological processes will make them happen in the tiny interior of materials. So, to paraphrase nanoscience figurehead Richard Feynman: "There is plenty of room at the bottom".