Effects of Counter-Anions for the Engineering of Metallo-Hydrogels for Electrocatalytic Hydrogen and Oxygen Evolution Reactions

Supramolecular metallo-hydrogels, which involve various noncovalent interactions, are considered one of the most promising methods for fabricating functional nanomaterials in energy conversion. In this work, we prepare the HO-FF-AdiA-FF–OH ((FF-AdiA-FF) AdiA = Adipic acid, F = l-phenylalanine)-based metallo-hydrogels using the sol–gel method with different cobalt salts: CoCl2·6H2O, Co(OAc)2·4H2O, CoSO4·6H2O, and Co(NO3)2·6H2O. FF-AdiA-FF forms a stable native hydrogel and metallo-hydrogel with CoCl2·6H2O (FF-AdiA-FF-1) and Co(OAc)2·4H2O (FF-AdiA-FF-2). However, gelation has not been observed with CoSO4·6H2O (FF-AdiA-FF-3) and Co(NO3)2·6H2O (FF-AdiA-FF-4). The metallo-hydrogels are prepared using a constant concentration of FF-AdiA-FF 18.37 mg (25 mM) with 3 mg of metal salt in a final volume of 1 mL. The outcomes of the hydrogen evolution reaction reveal that the gel-based FF-AdiA-FF-1 and FF-AdiA-FF-2 exhibit ultralow overpotentials of 110 and 118 mV to gain −10 mA cm–2 benchmarking current density in 0.5 M H2SO4 compared to nongel electrocatalysts. Moreover, oxygen evolution reaction measurements exhibit that the FF-AdiA-FF-1 and FF-AdiA-FF-2 show very low overpotential, specifically 151 and 159 mV, respectively, to attain 10 mA cm–2 current density compared to nongel electrocatalysts in 1 M KOH. Furthermore, the chronopotentiometry results demonstrate that the gel-based electrocatalysts remain stable for up to 50 h. The higher-order self-assembled structures of the metallo-hydrogels offer a large surface area and more electrochemically active sites, which facilitate the smooth transport and intercalation/deintercalation of electrolyte ions. Ultimately, this work demonstrates a simple method for the fabrication of efficient bifunctional electrocatalysts based on supramolecular metallo-hydrogels these can be easily processed and functionalized using nonprecious transition metals.