Pristine Metal-Organic Frameworks for Efficient Oxygen Evolution Electrocatalysis

Abstract

Green energy technologies propose the use of hydrogen as fuel instead of fossil fuels that have drawbacks of limited supplies and greenhouse gas emission during their combustion. Though hydrogen is the most abundant element on the Earth, hydrogen gas is commonly not found in pure form, but bound in compounds such as water or hydrocarbons. Thus, hydrogen gas is a synthetic fuel that needs to be produced. For the energy to be completely green, hydrogen should be produced using renewable energy. Currently, most of the hydrogen is produced using fossil fuels and only ca. 5% is produced using electrolytic water splitting. It is the high cost of the water-splitting process that limits its use on the wider scale. To reduce the associated high energy input to break a water molecule, resulting in high cost of hydrogen produced by water splitting, expensive electrocatalysts are typically used. The aim of this work is to design novel and low cost metal-organic frameworks (MOF) or coordination polymers (CP) utilizing different ligands (for instance, amide functionalized ligands) towards their application in energy conversion reactions such as oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). MOFs are rarely used in their pristine form; literature reports typically focus on MOF-derived carbon materials. Moreover, herein special attention is given to OER as the efficiency-limiting reaction within the water electrolysis process. The first part of the work focuses on the synthesis and characterization of ligands and MOFs/CPs based on transition metals, nickel and cobalt. The morphology and structure of these composites is examined using electron microscopy and X-ray diffraction analysis. The second part of the work focuses on the pristine materials’ electrocatalytic activity involving standard electrochemical techniques, i.e., voltammetry, impedance spectroscopy, and chronoamperometry to identify the material with the optimum performance and stability. Main reactions parameters including onset potential, overpotential at defined current density as well as current density at defined potential and Tafel slope, are determined. This work presents step towards green hydrogen production within search for solutions for the current energy crisis. Acknowledgments This work was supported by the Science Fund of the Republic of Serbia, grant number 7750219, Advanced Conducting Polymer-Based Materials for Electrochemical Energy Conversion and Storage, Sensors and Environmental Protection-AdConPolyMat (IDEAS programme).