Graphene/Boron Nitride–Polyurethane Microlaminates for Exceptional Dielectric Properties and High Energy Densities

Hexagonal boron nitride (h-BN) has tremendous potential for dielectric energy storage by rationally assembling with graphene. We report the fabrication of microlaminate composites consisting of alternating reduced graphene oxide (rGO) and h-BN nanosheets embedded in a polyurethane (PU) matrix using a novel, two-step bidirectional freeze casting process. Porous, highly-aligned rGO-PU aerogels having ultrahigh dielectric constants with relatively high dielectric losses and low dielectric strengths are fabricated by initial freeze casting. The losses are suppressed, whereas the dielectric strengths are restored by assembling the porous rGO-PU skeleton with electrically insulating BN-PU tunneling barrier layers in the second freeze casting routine. The ligaments bridging the conductive rGO-PU layers are effectively removed by the BN-PU barrier layers, eliminating the current leakage in the transverse direction. The resultant rGO-PU/BN-PU microlaminate composites deliver a remarkable dielectric constant of 1084 with a low dielectric loss of 0.091 at 1 kHz. By virtue of synergy arising from both the rGO-PU layers with a high dielectric constant and the BN-PU barrier layers with a high dielectric strength, the microlaminate composites present a maximum energy density of 22.7 J/cm3, 44 folds of the neat rGO-PU composite acting alone. The promising overall dielectric performance based on a microlaminate structure offers a new insight into the development of next-generation dielectric materials.