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

Abstract

One of the major challenges in the commercial production of hydrogen and oxygen from the electrochemical water splitting reaction is the nonavailability of potential and low-cost electrocatalysts for the enhancement of both the half-cell reactions. The bifunctional catalyst capable of demonstrating lower overpotentials for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) eliminates the usage of a membrane, simplifies the overall system design, reduces the cost, and enhances the electrochemical water splitting activity. Here, a bifunctional hybrid composite catalyst comprising an organic N4 macrocycle and acid-functionalized multiwalled carbon nanotubes (MWCNTs) is fabricated and tested for the water splitting reaction to produce H2 and O2. The uranium tetra[4-(2-{(E)-[(4-bromophenyl)imino]methyl}phenoxy)]phthalocyanine (UTBrImPc) is synthesized via a two-step process of imine and oxy-bridge linkage formation. The synthesized ligands and phthalocyanine macrocycle are characterized using various spectroscopic and analytical techniques. The glassy carbon electrode (GCE) and Ni foam are used as the conducting substrate for the fabrication of the UTBrImPc/MWCNT bifunctional electrode to generate H2 and O2. Surprisingly, the fabricated bifunctional hybrid catalyst on the GCE exhibited a lesser overpotential of 15 mV for the HER, which is close to that of the benchmark Pt/C catalyst. Furthermore, the Ni/UTBrImPc/MWCNT displayed a lower overpotential of 368 (±2) mV at a current density of 10 mA·cm–2 for the OER, which is close to the overpotential shown by the precious benchmark catalyst IrO2. In addition, the fabricated electrodes showed remarkable long-term stability for more than 20 h by a chronoamperometric method. The superior results for both the HER and OER at the composite organic hybrid catalyst may be due to the collusive effect of the acid-functionalized MWCNTs with UTBrImPc, which enhances the electronic conductivity and surface area. The developed state-of-the-art catalyst can be employed as a bifunctional catalyst in water electrolyzers.