Uranium Phthalocyanine-Anchored Acid-Functionalized Multiwalled Carbon Nanotubes for Water Electrolysis

Abstract

One of the major challenges in the commercial productionof hydrogenand oxygen from the electrochemical water splitting reaction is thenonavailability of potential and low-cost electrocatalysts for theenhancement of both the half-cell reactions. The bifunctional catalystcapable of demonstrating lower overpotentials for both the hydrogenevolution reaction (HER) and oxygen evolution reaction (OER) eliminatesthe usage of a membrane, simplifies the overall system design, reducesthe cost, and enhances the electrochemical water splitting activity.Here, a bifunctional hybrid composite catalyst comprising an organicN4 macrocycle and acid-functionalized multiwalled carbon nanotubes(MWCNTs) is fabricated and tested for the water splitting reactionto produce H-2 and O-2. The uranium tetra-[4-(2-{(E)-[(4-bromophenyl)-imino]-methyl}-phenoxy)]-phthalocyanine(UTBrImPc) is synthesized via a two-step process of imine and oxy-bridgelinkage formation. The synthesized ligands and phthalocyanine macrocycleare characterized using various spectroscopic and analytical techniques.The glassy carbon electrode (GCE) and Ni foam are used as the conductingsubstrate for the fabrication of the UTBrImPc/MWCNT bifunctional electrodeto generate H-2 and O-2. Surprisingly, the fabricatedbifunctional hybrid catalyst on the GCE exhibited a lesser overpotentialof 15 mV for the HER, which is close to that of the benchmark Pt/Ccatalyst. Furthermore, the Ni/UTBrImPc/MWCNT displayed a lower overpotentialof 368 (+/- 2) mV at a current density of 10 mA center dot cm(-2) for the OER, which is close to the overpotential shown by the preciousbenchmark catalyst IrO2. In addition, the fabricated electrodesshowed remarkable long-term stability for more than 20 h by a chronoamperometricmethod. The superior results for both the HER and OER at the compositeorganic hybrid catalyst may be due to the collusive effect of theacid-functionalized MWCNTs with UTBrImPc, which enhances the electronicconductivity and surface area. The developed state-of-the-art catalystcan be employed as a bifunctional catalyst in water electrolyzers.