Probing Expired Metformin as a Host to Modulate Active Sites for Carbon-Neutral Hydrogen Production: A Multi-Analytical Approach to Trigger Total Water Splitting Activity

Abstract

A nickel-based electrocatalyst for carbon-neutral hydrogen production could be developed via a meticulous synthetic strategy of tuning nickel oxidation states using the same scaffold host. We used expired metformin (MET) as a host for strategic tuning of Ni oxidation states in Ni-derived electrocatalysts (Ni-MET), namely, Ni²⁺-doped metformin (Ni2MET) and Ni⁰-doped metformin (Ni0MET), which improved the catalytic activity due to synergistic Ni–N interactions. The Ni2MET and Ni0MET catalyzed the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with overpotentials of 361 mV (61 mV dec^–1; 2.6 Ω) and 276 mV (71 mV dec^–1; 3.1 Ω) to attain 50 mA cm^–2, respectively. The optimized Ni2MET and Ni0MET exhibit higher TOFs (0.4073 and 0.3254 s^–1, respectively) with faradaic efficiencies of 98.03 and 91%, respectively. The long-term stability of Ni2MET and Ni0MET over 100 h supported their robustness. The kinetic study via operando EIS reveals less resistance with more conductivity and enhanced kinetics of Ni2MET and Ni0MET. The improved activity was sustained by the Bode study at various potentials. The low activation energy of Ni2MET (2.21 and 2.86 kJ mol^–1) signifies its potential for the OER and HER. The higher rate constant derived from Trumpet plot revealing that Ni2MET at various pH inferring rapid formation of gas bubbles. Finally, the alkaline and solar-driven electrolyzer Ni2MET//Ni0MET shows a low cell voltage of 1.48 V to attain 10 mA cm^–2 with great catalytic stability (100 h). All of the results explored that the metformin scaffold host ensured even dispersion and stabilization of Ni-active sites in a corrosive environment. The scaffold’s substantial stability is attributed to the N-rich core with extensive H-bonding and van der Waals forces, promoting electron rearrangement to lower the energy barrier, which unlocks the potential of metformin-based electrocatalysts in future energy applications.