Degradation and lifetime prediction of high-temperature pressure seals for usage in solar thermochemical reactors

Green hydrogen production via solar thermochemical water splitting presents a promising pathway towards the decarbonisation of energy systems. Among the emerging technologies, the R2Mx solar thermochemical reactor, introduces a novel configuration with separated reduction and oxidation zones and movable redox material assemblies (RMAs). This separation enables greater process flexibility and scalability potential but also imposes demanding requirements on sealing components operating under high-temperature and vacuum conditions. Ensuring the functional integrity of these seals is critical, yet the degradation behaviour and service lifetime of polymerbased sealing materials in such environments remain poorly characterised. This work investigates the high-temperature degradation mechanisms, failure modes, and performance of fluoroelastomer (FKM) and perfluoroelastomer (FFKM) O-rings, as well as polytetrafluoroethylene (PTFE) rod seals, at temperatures between 200 °C and 300 °C. Despite their commercial availability, limited experimental data exists on these materials under prolonged use at high temperatures. This research addresses this gap through a multidisciplinary approach combining several analyses. First, finite element thermal simulations of the R2Mx prototype were used to estimate realistic operational conditions for the seals. Subsequently, both sealing materials (O-rings and rod seals) were subjected to long-term thermal ageing in air and tested in custom-built test rigs that replicate the R2Mx’s operational conditions. For O-rings, degradation was assessed through multiple chemical and mechanical degradation indicators. The thermo-oxidative degradation of FKM arises from thermally induced dehydrofluorination while FFKM’s degradation is driven by chain scission. A 75% compression set was identified as the end-of-life criterion, correlating well with leakage beyond a threshold value, enabling service lifetime predictions via time-temperature superposition. Failure of FKM and FFKM O-rings was found to occur abruptly and is predicted to arise after 220 and 400 operational days, respectively, at 200 °C under an 8-h daily operation scenario. On the other hand, PTFE lip seals were tested using different rods’ surface finishes and concluded that wear is the primary degradation factor across all conditions. Furthermore, the wear mechanism transitioned from abrasive to adhesive and finally thermally driven failure between 250 °C and 300 °C. A 1% wear was defined as end-of-life criterion, with corresponding lifetime estimates of 220 and 140 operational days at 250 °C and 275 °C, respectively. Unlike O-rings, rod seals exhibited progressive failure, marked by a steady increase in leakage over time. Finally, a numerical model was developed to assess the impact of leakage on the reactor’s solar-to-fuel efficiency. While efficiency losses remained minor (< 4%) under conservative leakage scenarios, high leakage rates could result in up to 17% efficiency reduction. This work establishes a scientific and practical foundation for understanding the degradation mechanisms of the studied polymers and predicting service lives of high-temperature sealing systems, supporting reliable and efficient operation of advanced solar thermochemical reactors.