Metal-Free Phthalocyanine-Based Polymer for Bifunctional Electrochemical Water Splitting

Electrochemical water splitting is an alternative approach for sustainable energy conversion with the potential to serve as an environmentally conscious technology when paired with renewable energy sources. Developing efficient, low-cost, and stable electrocatalysts is imperative for the sustainable generation of hydrogen and oxygen gas. We report on a metal-free phthalocyanine polymer synthesized through the condensation of tetraamino-phthalocyanine and terephthaloyl chloride. This material exhibits a layered morphology resulting from π–π interactions within the phthalocyanine structure, as confirmed by scanning electron microscopy images (SEM). Further characterization was performed with X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), UV–vis spectroscopy, and FT-IR spectroscopy. The polymer exhibits bifunctional performance toward the electrochemical hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with onset overpotentials (η 1 ) of 169 mV and 280 mV, overpotentials (η 10 ) of 573 mV and 380 mV (vs. RHE), and maximum current densities at the end of the potential window of 21.9 and 75.5 mA cm –2, respectively. Electrochemical characterization was performed using electrochemical impedance spectroscopy and chronoamperometry, which demonstrate improved conductivity and stability of the metal-free polymer compared to its monomeric subunit. When the molecular core is occupied by a redox-inactive metal, such as zinc, the electrochemical performance decreases, suggesting that the active site for the HER and the OER is the nitrogen-rich core. Utilizing an alternative triazole linker results in decreased electrochemical activity for the OER, implying that the aramid linker plays a role in the OER mechanism. Additionally, the overall water splitting activity was evaluated for both the HER and the OER. This work demonstrates the ability of metal-free phthalocyanines as an effective bifunctional electrocatalyst for water splitting.