中文版本

Opportunity

The rapid development of flexible and wearable electronics, including electronic skin, human health monitoring, and physiological activity sensors, has created a pressing demand for self-powered technologies that can harvest mechanical energy from human gestures. Traditional piezoelectric materials such as polyvinylidene fluoride (PVDF) and its copolymers offer promising energy conversion capabilities, but they suffer from significant drawbacks. The reliance on toxic constituents, complicated synthesis procedures involving mechanical stretching, annealing, or strong electric fields, and poor sustainability raise serious concerns. Moreover, the random distribution and lack of long-range alignment of dipole moments within polymer chains severely restrict domain evolution and the achievement of permanent polarization during electric poling. Metal-organic frameworks (MOFs) have been explored, but they often exhibit limited piezoelectric response due to low asymmetry. Consequently, there is an urgent need for piezoelectric materials that combine low toxicity, excellent stability, ordered structure, and high performance to enable efficient and sustainable mechanical energy harvesting for next-generation self-powering systems.

Technology

This invention introduces a class of two-dimensional (2D) covalent organic frameworks (COFs) designed specifically for piezoelectric energy harvesting. The core innovation lies in the structural design: piezoelectric monomers within the COFs are interconnected via covalent bonds, and each monomer features fluorinated alkyl chains tethered onto the peripheral positions of an aromatic moiety. These fluorinated chains—such as the 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl side chain—introduce asymmetry and strong polarization owing to the high electron negativity of fluorine atoms, creating robust C–F dipoles. The synthesis involves anchoring fluorinated side chains onto aromatic precursors (e.g., 2,5-dihydroxyterephthalaldehyde) via substitution reactions under alkaline conditions, followed by condensation with amino monomers (e.g., 1,3,5-tris(4-aminophenyl)benzene or tris(4-aminophenyl)triazine) to form imine-linked 2D COFs with high crystallinity and ordered AA-stacking structures. The resulting frameworks, exemplified by CityU-13 and CityU-14, exhibit non-centrosymmetry and permanent porosity, enabling uniform dipole alignment and net spontaneous polarization. When incorporated into a piezoelectric nanogenerator (PENG) with a sandwich structure (top electrode/active layer/bottom electrode), the aligned dipoles generate an electric pulse upon mechanical deformation, efficiently converting stress into electricity.

Advantages

• Highest output open-circuit voltage among all reported organic piezoelectric materials (60 V for CityU-13, 50 V for CityU-14)
• High piezoelectric coefficient (d33 = 20.9 pC/N for CityU-13, 18.9 pC/N for CityU-14)
• Excellent short-circuit current (2.4 μA for CityU-13, 1.5 μA for CityU-14) and instantaneous power (3.3 μW and 2.8 μW at 5 MΩ load)
• Superior long-term stability: negligible performance degradation over 600 seconds and up to 14 days
• Environmentally friendly synthesis without toxic constituents or harsh poling conditions
• Superhydrophobicity (water contact angle >154°) and high thermal stability (stable up to ~350°C)
• Ordered crystalline structure ensures uniform dipole alignment and enhanced piezoelectric response