Bifunctional porphyrin-based metal-organic polymers for electrochemical water splitting

Electrochemical water splitting offers the potential for environmentally friendly hydrogen and oxygen gas generation. Here, we present the synthesis, characterization, and electrochemical analyses of four organic polymers where metalloporphyrins are the active center nodes. These materials were obtained from the polymerization reaction of poly(p-phenylene terephtalamide) (PPTA) with the respective amino-functionalized metalloporphyrins, where M = Fe, 1; Co, 2; Ni, 3; Cu, 4. Scanning and transmission electron microscopy images (SEM and TEM) show that these polymers exhibit a layer-type morphology, which is attributed to hydrogen bonding and π-π stacking between the metalloporphyrin nodes. The synthesized materials were characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), UV-Vis spectroscopy, and Fourier-transform infrared spectroscopy (FT-IR). Among the materials studied, the cobalt-based polymer, 2, demonstrates a bifunctional electrocatalytic activity for oxygen (OER) and hydrogen (HER) evolution reactions with overpotentials (η₁₀) of 337 mV and 435 mV, respectively. The Fe, 1, and Ni, 2, polymers are less active for HER with maximum current densities (j_max) of 12.6 and 19.1 mA cm^-2 and η₁₀ 678 mV, 644 mV. Polymer 2 achieves a j_max of 37.7 mA cm^-2 for HER and 133 mA cm^-2 for OER. The copper-based material, 4, on the other hand, shows selectivity towards HER with an overpotential (η) of 436 mV and a maximum current density (j) of 45.5 mA cm^-2. The bifunctional electrocatalytic performance was tested in the overall water-splitting setup, where polymer 2 requires a cell voltage of 1.64 V at 10 mA cm^-2. This work presents a novel approach to heterogenized molecular systems, providing materials with exceptional structural characteristics and enhanced electrocatalytic capabilities.