Stability investigation of 1-octadecanol and palmitic acid binary mixture for latent heat thermal energy storage application

Standardized evaluation protocols are crucial for assessing long-term stability of phase change materials (PCM). This study investigated the thermal stability, possible degradation mechanisms, and material compatibility of an organic-organic eutectic PCM, while evaluating the relevance of different stability assessment methodologies. The representative case study focused on a biobased eutectic composed of 1-octadecanol (OD) and palmitic acid (PA) (0.548:0.452 mass ratio of OD:PA) exhibiting a melting temperature of 46.9 ± 0.1 °C and a latent heat of 194 ± 1 J/g. This PCM was subjected to a series of stability assessments including benchmark testing, accelerated thermal cycling in a heating/cooling device and differential scanning calorimetry (DSC), thermal stress exposure, and compatibility tests with metals. The results showed severe degradation characterized by a progressive reduction in latent heat and significant modification of phase transition behavior for the 5 mL samples cycled in the refrigerated heating/cooling device compared to the 20 μL samples cycled in DSC. Thermal stress tests for large samples confirmed degradation. Corrosion compatibility tests indicated moderate corrosion of copper while stainless steel, carbon steel, and aluminum exhibited low or negligible corrosion. However, corrosion effects on PCM properties are secondary compared to thermally induced degradation. A regeneration test after 600 cycles showed no recovery, indicating chemical degradation, as further confirmed by 1 H NMR spectroscopy. The findings highlight the limitations of relying solely on DSC-based cycling and emphasize the need for multi-scale stability assessments. Beyond the specific OD–PA PCM, this work provides methodological insights for the evaluation of long-term reliability in organic-organic phase change materials.