Chemical deposition from a liquid crystal template: A highly active mesoporous nickel phosphate electrocatalyst for hydrogen green production via urea electro-oxidation in an alkaline solution

Abstract

Recently the urea and urinated wastewater electrolysis process has shown promising technology for hydrogen fuel green production in addition to denitrification of wastewater. However, to make the process economically valuable, the electrocatalysts nanoarchitecture, nanometer size, shape, facets, and composition need to be engineered to boost the urea oxidation reaction (UOR). This work demonstrates a simple and novel approach to the synthesis of mesoporous nickel phosphate nanoparticles (meso-NiPO) via chemical deposition from a surfactant liquid crystal template. Typically, the nickel ions dissolved in the aqueous domain of the hexagonal liquid crystalline phase of the Brij®78 template were chemically reacted with the sodium phosphate solution to precipitate the mesoporous nickel phosphate nanoarchitecture after washing up the surfactant. The physicochemical characterizations show the meso-NiPO exhibits an amorphous highly mesoporous structure with a higher specific surface area (43.50 m2/g) compared to the bare nickel phosphate (bare-NiPO, 4.26 m2/g) prepared in the absence of surfactant. The electrochemical performance of meso-NiPO electrocatalyst for the urea oxidation reaction in alkaline solution exhibits superior activity including a lower oxidation onset potential (0.30 V vs. Ag/AgCl), charge transfer resistance (3.35 O) and mass activity of 700.7 mA/cm2 mg at the oxidation potential of 0.6 V vs. Ag/AgCl. Moreover, the meso-NiPO reveals long-term stability, and 97.5% of the steady-state oxidation current was maintained after the 3-hour urea electrolysis test. Using an H-shape urea electrolyzer, the corresponding cathodic hydrogen production rate reached 415 µmol/h and a Faradic efficiency of 96.8 % at an applied bias of 2.0 V. The electroactivity high performance of the mesoporous nickel phosphate is ascribed to the high specific surface area and mesoporous architecture that provide efficient charge transfer, as well as mass transport of the electroactive species. The chemical deposition from a surfactant liquid crystal template has the advantages of a one-pot template, applicable to the synthesis of a wide range of nanomaterials with various compositions and nanoarchitectures at room temperature for application in electrochemical energy production and storage systems.