Self-Healing Redox Active Hydrogel-Stabilized Nickel Porphyrin Complex Infused with Metal Oxide-Hydroxide Heterojunction for Robust Bifunctional Water Electrolysis

A redox-active self-healing hydrogel-assisted composite heterojunction electrocatalyst, NMO/NTPPS-h@β-NH, which renders high stability and efficacy for overall water splitting, is reported here. The system integrates NiMoO₄ (NMO) and β-Ni(OH)₂ (β-NH) via a chemically modified polyacrylamide hydrogel functionalized with nickel tetraphenylporphyrin tetrasulfonic acid complex (NTPPS). This hydrogel binder introduces inherent Ni²⁺/Ni³⁺ redox activity, mechanical integrity, and improved charge transport. Mo⁶⁺ in NMO promotes Ni²⁺ → Ni³⁺ oxidation, facilitating hydrogen evolution reaction (HER), while β-NH transforms into NiOOH under anodic conditions to catalyze oxygen evolution reaction (OER) and work in redox synchronization with NMO and NTPPS. The composite achieves overpotentials of 245 mV (OER) and 95 mV (HER) with Tafel slopes of 55 and 41 mV dec^-1, respectively, outperforming binder-free in situ grown NMO/β-NH (290 mV and 165 mV; 70 and 57 mV dec^-1 for OER and HER, respectively), accompanied by a marked enhancement in the turnover frequency (TOF). The NMO/NTPPS-h@β-NH composite exhibits a Faradaic efficiency of ∼96%, with a low charge transfer resistance (R_ct). The system shows remarkable 60 h stability, which utilizes a shared Ni redox center across NMO, NTPPS-h, and β-NH for synchronized redox transitions, reduced overpotential, and suppressed charge recombination. This work highlights redox-conductive hydrogels as powerful binders enabling scalable, earth-abundant electrocatalyst design for robust overall water electrolysis.