Energy Density Recovery by Enhanced Hydrogen Bonding for High‐Performance Composite Phase Change Materials

Phase change materials (PCMs) for thermal energy storage require both high latent heat and high thermal conductivity, which is almost infeasible because the energy-dense materials are usually poor heat conductors, and increasing their thermal conductivity by making composites inevitably leads to a sacrifice in latent heat. Here, we propose a strategy for recovering the unavoidable loss of energy density of a composite PCM by strengthening the molecular connections between the fillers and matrix PCM. Taking erythritol (a polyol rich in hydroxyl groups) as an example, we use hydroxyl-modified nanofillers to reconstruct the filler-to-PCM intermolecular hydrogen bonds. Compared to unmodified graphene, we observe a remarkable recovery in the latent heat of erythritol using hydroxylated graphene, and verify the extension of this strategy to acids and hydrated salts. We show an almost full recovery of the energy density loss for composite erythritol at 1 wt.% loading, reaching an ultrahigh latent heat of fusion (328.5±0.9 J g^-1). Using molecular simulations, we confirm the formation of strong hydrogen bonds between the model PCM molecules and hydroxylated graphene. Our strategy enables the development of polyol-based composite PCMs, which can be generalized to other matrix PCMs, toward more balanced performance in high energy density and power density.