Bipyridine-Based Dicationic Salts as Solid–Solid Phase-Change Materials for Thermal Energy Storage

Phase-change material (PCM)-based thermal energy storage (TES) systems enable more compact thermal devices, which is especially relevant in industrial environments with limited space. A large share of industrial heat demand falls within the 80–200 °C range, yet very few PCMs operate efficiently and stably in this window. Even scarcer are solid–solid PCMs (SS-PCMs), which offer unique advantages for system integration, such as eliminating leakage and enabling direct incorporation of thermal conductivity enhancers. In this work, we present the first 4,4′-bipyridine-based dicationic salts with bromide counterions that function as SS-PCMs. By varying the peripheral alkyl chains (R = C4–C18), we systematically explore their phase transition behavior, thermal properties, and cyclability. The resulting materials exhibit solid–solid transitions around ∼125 °C. Latent heat values reach up to 110.1 J/g in this industrially relevant range. All salts show decomposition temperatures >268 °C and ≤1% enthalpy loss over 500 thermal cycles, ensuring excellent long-term reliability. Thanks to their solid-state behavior, these materials also accommodate 3 wt % expanded graphite without segregation or agglomeration over 50 cycles, doubling the thermal conductivity (up to 0.35 W m–1 K–1), a stability level not achievable with solid–liquid PCMs. This study establishes 4,4′-bipyridine-based dications as a molecular platform for the rational design of high-performance SS-PCMs tailored to midtemperature TES applications.