Accessible Ni-Fe-Oxalate Framework for Electrochemical Urea Oxidation with Radically Enhanced Kinetics

Abstract

Urea oxidation reaction (UOR) has been utilized to substitute the oxygen evolution reaction (OER), to escalate the energy conversion efficiency in electrochemical hydrogen generation processes with denitrification of widespread urea in wastewater. This study reports breakthroughs in Ni-based UOR electrocatalysts, particularly with NiFe oxalate (O-NFF), derived from Ni3Fe alloy foam with prismatic nanostructures and elevated surface area. The O-NFF achieves cutting-edge performances, representing 500 mA cm-2 of current density at 1.47 V RHE and exceptionally low Tafel slope of 12.1 mV dec-1 (in 1 m KOH with 0.33 m urea). X-ray photoelectron/absorption spectroscopy (XPS/XAS) coupled with density functional theory calculations unveil that oxalate ligands induce charge deficient Ni center, promoting stable urea-O adsorption. Furthermore, Fe dopants enhance oxalate-O charge density and H-bond strength, facilitating C-N cleavage for N2 and NO2- formation. The extraordinary UOR kinetics by the tandem effects of oxalate and Fe prevent Ni over-oxidation, corroborated by operando XAS, minimizing OER interference. It agrees with an adaptive reconstruction to Fe-doped beta-NiOOH on top surface in extended urea electrolysis with marginal loss in UOR kinetics. This findings shed light to bimetal-organic-framework as (pre)catalysts to improve industrial electrolytic H2 production. Self-supported NiFe-oxalate (O-NFF) pillars show outstanding kinetics (500 mA cm-2 at 1.47 V RHE and Tafel slope of 12.1 mV dec-1) in urea oxidation reaction. In O-NFF, Operando X-ray absorption near edge structure confirms that the reaction with urea is dominant compared to Ni3+ being converted to Ni(3+delta)+. Density functional theory calculations highlight favorable bonding of oxalate ligand and Fe with urea-O.image