Electron Delocalized Ni Active Sites in Spinel Catalysts Enable Efficient Urea Oxidation

Area of Science:

• Electrochemistry
• Catalysis
• Materials Science

Background:

• Urea oxidation reaction (UOR) is a promising alternative to oxygen evolution reaction (OER) due to lower overpotential.
• Nickel-based catalysts show promise for UOR, but selectivity and mechanism remain unclear.
• Controlling product selectivity and understanding urea dissociation are key challenges.

Purpose of the Study:

• Investigate how electron delocalization in nickel sites affects urea dissociation and product selectivity in UOR.
• Utilize spinel NiCo2O4 as a model catalyst to tune electronic properties.
• Clarify the fundamental mechanisms governing UOR efficiency and selectivity.

Main Methods:

• Employing phosphorus substitution in NiCo2O4 spinel to tune the charge delocalization of nickel active sites.
• Conducting electrochemical experiments to measure UOR performance, including current density and Faraday efficiency.
• Utilizing theoretical calculations to understand urea dissociation pathways and hydroxide binding sites.

Main Results:

• Complete phosphorus substitution in NiCo2O4 significantly enhanced nitrogen (N2) selectivity to 26.8% with a peak current density of 300 mA·cm−2.
• Partial phosphorus substitution achieved a high Faraday efficiency of 78.1% for liquid products (NOx−).
• Strong charge delocalization promoted remote-site attack of hydroxide on urea, favoring N2 selectivity and suppressing OER.
• Weak delocalization favored nearby-site attack, enhancing UOR efficiency.

Conclusions:

• Tuning electron delocalization at Ni sites is crucial for controlling OH− binding and urea dissociation pathways.
• Phosphorus substitution in NiCo2O4 offers a viable strategy for selective UOR catalysis.
• Understanding these mechanisms can guide the design of highly efficient electrocatalysts for urea oxidation.