Ni-Based Composite Electrocatalysts for Urea Oxidation Reaction and Molecular Hydrogen Generation

Urea oxidation reaction (UOR) has been interrogated as an alternative oxidation to decrease the cell voltage in electrochemical water splitting for industrial-scale hydrogen production. The UOR not only enhances the energy conversion efficiency from renewable energy into molecular hydrogen, but also reduces carbon and nitrogen pollution loadings of urea-rich wastewater. Nickel-based catalysts are up-to-date the most widely investigated materials as the UOR electro-catalyst. This presentation introduces our recent findings on Ni-based UOR electro-catalysts that were prepared by i) galvanostatic electro-deposition of mixed metal (oxy)hydroxide (NiFeO_x or NiCoO_x) with variable mixing ratios, ii) potentiostatic dealloying of commercial NiFe alloys under variable pH, and iii) one-pot thermochemical treatment of NiFe alloy foam (NFF), both as substrate and catalyst source to create a self-supporting UOR electro-catalyst. For the anode materials under interrogation, Ni³⁺ in the form of NiOOH served as the active UOR site. The secondary elements (Fe or Co) influenced the Ni^2+/3+ redox peak potential and current as thoroughly investigated for oxygen evolution reaction (OER) electrocatalysts. This study further revealed that current inflections during potential sweep in the presence of urea could be associated with Ni³⁺/⁴⁺ redox peaks. The peak location and intensity continued to change during repetitive scans, due to dynamic dissolution and precipitation of Fe to/from the electrolyte. After the onset of OER, the presence of urea lowered the current responses which should be important consideration for industrial H₂ evolution. On the other hand, oxalic acid treatment of NFF generated prismatic NiFe-oxalate (O-NFF) which marked surprisingly low overpotential and Tafel slope for UOR. The oxalate ligand not only altered the binding site of urea, but also shifted the surface electronic structure of Ni (together with urea), which in-turn affected the rate and potential determining steps. The findings of this study could be further utilized within wastewater electrolysis cell for water-energy nexus.