Exploring 3D-printed and freeze-cast porous SiOC for mass transport: membranes for gas-liquid phase separation and anodes for bioelectrochemical systems

Macroporous ceramics are often applied in the form of monoliths, 3D-printed grids, tapes and much more, in a wide array of industries. Mass transport is of paramount importance for almost all porous ceramics applied, and inefficient mass transport can lead to several issues if it is not designed correctly. The various problems include the high pressure drop that limits the overall reaction rate in catalytic reactors [1], and limited transport of nutrients in bioelectrochemical systems [2] or in the production of bone tissue e [3], to name a few. The solution to overcome these limitations usually lies in tuning the processing techniques of porous ceramics whether through conventional methods like replica, direct foam, and sacrificial templating [4] or through newly explored techniques like additive manufacturing [5].

Every application requires a specific set of parameters from the pore structure and the functional properties. The pore structure is usually adjusted in means of pore size distribution, open porosity, pore morphology, surface roughness, and specific surface. These parameters directly affect the mechanical and mass transport properties, while other properties like hydrophilicity, biocompatibility, and electrical conductivity can be tuned from the adjustment of the chemical composition. With this in mind, the porous structures in this work will be adapted to the requirements of two applications: screen-channel liquid acquisition devices (SC-LAD) and anodes for bioelectrochemical systems (BES). The first technology targeted is SC-LAD, which are used in propellant depots and are designed to supply gas-free propellants to the tanks of spacecraft or space exploration vehicles. The task of separating liquid from gaseous phases is a challenge, due to the difficulty of locating the liquid/gas interface in microgravity condition. The most important parameters for this application are the wicking rate, the pressure drop and the bubble point. Metallic meshes are frequently designed taking these parameters into account, but research into porous ceramics shows great potential for this application. The low thermal conductivity of ceramics would reduce the evaporation of propellants and consequently, the impairment on wicking rate, and materials with a lower density than metal mesh would meet the requirement of lightweight design of aerospace components. The pore size for capillary transport in this application (wicking) ranges from a few micrometers to approx. 200μm [6] and understanding the effect of not only the pore size, but the morphology and quantity of the pore channels is crucial for designing an SC-LAD. Additionally, manufacturing porous ceramics in SC-LAD with geometries close to the state-of-the-art materials (i.e. metal meshes), and testing their bubble point as a reliability indicator represents a step further towards this technology readiness level.

The second targeted technology is anode materials for BES like microbial fuel cells. Microbial fuel cell (MFC) is an environmentally-friendly solution that harvests energy from the purification of wastewater [7]. The energy is harvested from the oxidation of a substrate (wastewater) which is carried out by bacteria on the surface of an anode. The electrons generated from this reaction travel through the anode to an external circuit, arriving on a cathode that reacts with electrons, oxygen and protons to generate water. The current power density generated by microbial fuel cells is low and limited to applications like remote sensors. Among the various components of a MFC, the anode offers tuning possibilities in terms of its surface properties, architecture and composition, which can improve the overall performance of the MFC. Designing anodes for MFC requires pore sizes on the surface of the anodes bigger than a few microns and functional properties like hydrophilicity, electrical conductivity and biocompatibility [8,9]. The anode should have hydrophilicity [10] and biocompatibility for the adhesion of microorganisms on its surface, and high electrical conductivity [11] for transferring the electrons generated from the oxidation process of microorganisms in the anodic chamber. The pores on the surface of the anode (and the resulting surface roughness) are highly important for the initial biofilm adhesion, but even more for the biofilm growth during long-term operation. If the pores are not large enough, they will clog and limit the transport of nutrients and consequently the biofilm growth [12].
To develop porous ceramics that fulfill the requirements of the targeted applications, polysiloxanes will be used as the initial precursors for creating SiOC-based materials. SiOC monoliths and screens can be produced through solution-based freeze-casting, a process that allows the morphology, size and amount of porosity to be adjusted. Pores are generated by the solidification of a solvent from a two-phase system (polymer and solvent), subsequent sublimation of the solidified solvent’s crystals and heat treatment. The wicking rate and gas-liquid phase separation are the most important parameters in a SC-LAD, and therefore, will be explored in two different parts in this work (first wicking rate, then gas-liquid phase separation). Different morphologies of freeze-cast monoliths can be created using pure solvents or a mixture of solvents as pore-templating agents. Testing and comparing the permeability and the wicking rate of different pore morphologies can give valuable insights of the capillary transport mechanism and existing analytical models to predict this phenomena. Besides, the mechanical stability of the pore morphology is crucial for the development of crack-free screens to be tested at SC-LAD. The relationship between the pore size screens and porosity, and the pressure drop and bubble point during phase-separation is of utmost importance for designing potential materials for SC-LAD.

Finally, to study the influence of the surface porosity, roughness and available area on the biofilm formation, novel architectures and surfaces of SiOC-based anodes will be developed through suspension-based freeze-casting and additive manufacturing using carbon-based fillers. The assessment of the utility of 3D-printed anodes in MFCs compared to their non-3D-printed counterparts with the same material composition is still lacking in literature and is of great importance for further progress in the design of new anodes for BES. Additionally, the impact of the surface parameters of the anode on the initial biofilm adhesion can be further expanded.