Elastically Bendable Metal–Organic Framework Crystals for Efficient Flexocatalytic Hydrogen Evolution from Water

As a new type of porous material, metal-organic frameworks (MOFs) have emerged as promising platforms for mechanocatalytic hydrogen evolution from water splitting via piezocatalysis. However, because MOFs usually possess symmetric structures, only a few can be utilized as piezocatalysts, which generally exhibit a low catalytic efficiency. Therefore, it is desirable to explore other mechanocatalysis mechanisms in MOFs for efficient hydrogen evolution. Herein, we first report a highly efficient flexocatalysis of centrosymmetric PCN-222 MOF in hydrogen production. Our tests show that the hydrogen evolution rate of PCN-222 crystals via flexocatalysis can reach up to 259 μmol·g^-1·h^-1, six times higher than those of previous Zr(IV)-based MOF piezocatalysts. The superior flexocatalytic hydrogen evolution performance of PCN-222 crystals is attributed to the synergy among their significant flexoelectric response, large aspect ratio, and high bendability, as PCN-222 is found to possess an ultrahigh figure of merit of 0.33 for repeatable elastic bending and an ultralow Young's modulus of 0.6 GPa based on in situ bending tests. Such a high bendability of PCN-222, according to atomistic simulations, originates from the small load-bearing area and the unique self-rotation deformation mechanism of aryl and pyrrole rings. The intrinsically elastic bendability incorporated with the slender configuration of PCN-222 crystals can facilitate the generation of flexoelectric potential through buckling under ultrasonic irradiation. This study not only provides a new strategy to improve the mechanocatalytic hydrogen evolution activity of MOFs through flexocatalysis but also unveils the superior bendability of PCN-222, shedding light on its potential applications in novel flexible nanodevices.