(664c) Stabilizing SiC for Solar Thermal Water Splitting Applications

Amanda Hoskins¹, Aidan Coffey¹, Charles B. Musgrave^1,2, Alan W. Weimer¹

¹Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80303, USA.

²Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA.

Solar thermal water splitting (STWS) has been proposed as a means for hydrogen production which has the benefit that both the energy source and the fuel are renewable and readily available. However, considerable technological challenges must first be addressed in regards to hydrogen production, storage and transport in order for hydrogen to become an economically viable fuel on a large enough scale to significantly contribute to future energy demands. A major limiting factor to the viability of this technology is the lack of high temperature materials able to withstand the extreme chemical and physical environment. Specifically, materials that can be used to fabricate efficient heat exchangers for steam/steam heat recuperation will significantly impact the progress of this technology.

Silicon Carbide (SiC) is an ideal material for high-temperature heat exchange due to its stability at much higher temperatures than the current state-of-the-art material (Inconel) which cannot be used above ~1000°C. Using SiC for heat exchangers involves challenges related to the lifetime of SiC in an oxidative environment. Silicon carbide degrades in water-rich environments through the formation and subsequent degradation of a silica (SiO₂) layer at the exposed surface. In a dry environment, the silica has a low permeability to oxygen, however the presence of water, especially at high temperatures, increases the rate of oxidation, causing cracking and volatilization of the silicon from the structure. We propose that the application of nanostructured films grown with atomic layer deposition (ALD) will significantly improve the oxidation resistance of SiC in extreme environments. We have targeted a variety of high temperature coating materials chosen based on thermal properties, structural characteristics, and stability in oxidative environments. We have grown ALD layers of our desired materials on SiC particles using a fluidized bed particle ALD reactor. We have characterized all films using energy dispersive X-ray spectroscopy (EDS) mapping, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and inductively coupled plasma mass spectrometry (ICP). Using thermogravimetric analysis, it has been shown that these coatings improve the oxidation resistance of SiC.

We have applied density functional theory (DFT) to model the performance of these coatings in an oxidative environment. Computational modeling of the chosen coating material properties, as well as oxygen diffusion through these materials can identify promising coatings to further extend the lifetime of SiC. We expect to use this understanding to develop more advanced stabilizing coatings for SiC and the myriad of other applications that require high-temperature oxidation resistant materials. This presentation will focus on both the fundamental DFT calculations and the experimental results for thin ALD films being developed to protect SiC surfaces from water oxidation at temperatures above 1000°C.