Engineering Sustainability in Ceramic Membranes for CO₂ Conversion and H₂ Utilisation

This work is based on synthesis and recycling of oxygen transport membranes (OTMs) which have potential applications in plasma assisted CO₂ conversion and H₂ utilisation. These OTMs consist of mixed ionic electronic conducting (MIEC) perovskite-type oxides, utilising their high selectivity to oxygen. For functional compatibility in these applications, OTMs are needed to control the oxygen partial pressure in the reaction zone and therefore must have high oxygen permeability, mechanical (stability) and structural stability under reactive and reducing atmospheres (i.e., CO₂, CO and H₂). Among these, La0.6Ca0.4Co1-xFexO3–d (LCCF) membranes have already demonstrated excellent CO₂ resistance. Its specific low cobalt variant La0.6Ca0.4Co0.2Fe0.8O3–d (LCCF_6428) was investigated in this study and its hydrogen tolerance was established for up to 25 h at 600 °C without significant decline in the oxygen permeation. To further enhance the hydrogen tolerance of LCCF_6428, Mn⁴⁺ was introduced into the material structure via 10% substitution of Fe with Mn at the B site which resulted in La0.6Ca0.4Co0.2Fe0.7Mn0.1O3–d (LCCFM_64271). The introduction of strong Mn⁴⁺-O bonds resulted in a two-fold increase in hydrogen tolerance albeit with a 5% decline in the oxygen permeation. The synthesis was carried with ultrasonic spray synthesis (USS) which is comparatively faster, resource efficient and generates lower direct process emissions than conventional routes such as co-precipitation, sol-gel, Pechini-based processes, etc. Given the use of critical raw materials such as cobalt (Co) and rare earth elements like lanthanum (La). It is essential to also focus on incorporating materials circularity and materials substitution along with life cycle assessment (LCA) into materials development. This study emphasised the need for developing robust (chemical) recycling techniques and identifying viable material substitutes to extend the lifespan of materials. Furthermore, LCA was used to quantify the environmental impact of materials development and recycling with assessment of potential risks to human health and ecosystems. A recycling process was developed that involved microwave assisted dissolution of the plasma exposed LCCF_6428 membrane followed by USS. The recycled LCCF_6428 membrane showcased structural equivalence at 600 °C with an oxygen permeation loss of 17% compared to primary LCCF_6428. Although, in the CO₂/H₂ plasma exposure test, traces of Co ex-solution emerged after 2 h at 1130 °C which may affect the long-term stability of the recycled membrane. LCA revealed the environmental hotspots in the primary as well as recycling process. For the recycled membrane, reduced resource depletion was observed but with a higher overall environmental impact (EI) when compared with the primary membrane due to increased electricity consumption. This result highlights the need for more energy-efficient processes and integration of renewable energy sources, while monitoring potential side effects via standardised scientific methods such as LCA. Recycling in itself is not impact free since it requires additional resources and often process chemicals which can lead to EI. This work contributes to data transparency concerning the EI, since currently the EI data of most materials and materials processing is lacking.