Superhydrophobic Porous Coordination Polymer-Confined Co3O4 Nanoparticles: A Sustainable Strategy for Electrochemical Water Splitting

Abstract In this work, we report three robust, cost-effective, and efficient post-synthetic nanoparticle-embedded metal–organic framework (MOF)-based electrocatalysts, Co3O4@Zn2M2(μ2–OH)2(BTMB)2 (M = Zn (1), Co (2), and Ni (3)), for overall water splitting. The pristine frameworks, Zn2M2(μ2–OH)2(BTMB)2, were transformed from relatively low-conducting materials into conductive analogues through post-synthetic encapsulation of approximately 2.5–3.1 wt % Co3O4 nanoparticles via a sonochemical approach. Successful nanoparticle incorporation and preservation of framework integrity were confirmed by PXRD, SEM, TEM, XPS, EDX, ICP-OES, and TGA analyses. Notably, the Co3O4@MOF composites exhibited enhanced superhydrophobicity, displaying water contact angles of 162.41° (1), 166.42° (2), and 164.47° (3), compared to those of the corresponding pristine MOFs (155.5°, 159.3°, and 160.8°, respectively), which is one of the key steps for enhancing the electrochemical activity of this type of composite. The Co3O4@MOF composites achieved low overpotentials for the hydrogen evolution reaction (HER) of 196, 147, and 169 mV, and for the oxygen evolution reaction (OER) of 499, 425, and 486 mV, respectively, for 1–3 in 1 M KOH electrolyte. These catalysts also exhibited low Tafel slopes of 101, 94, and 96 mV dec–1 for HER and 123, 118, and 122 mV dec–1 for OER, indicating significantly improved electrocatalytic activity and reaction kinetics. Further, we also carried out an overall water-splitting study to understand their bifunctional behavior under similar electrochemical conditions, delivering a cell potential of 1.78 V at 10 mA cm–2. Compared with the parent MOFs, the nanoparticle-embedded composites demonstrated nearly fourfold enhancement in HER catalytic performance. Among the series, 2 exhibited the best overall electrocatalytic performance, combining the lowest overpotentials, superior kinetics, and remarkable operational stability, maintaining consistent activity for more than 50 h under continuous electrolysis. The integration of Co3O4 nanoparticles into a MOF matrix improves superhydrophobicity and durability, creating affordable, efficient electrocatalysts for overall water splitting and green hydrogen production.