The Challenge of Removing Urea from Water
Urea is a low-cost nitrogen agriculture fertilizer and a natural product from human metabolism. Urea-rich agricultural runoff and municipal wastewater discharge cause eutrophication–harmful algal blooms and hypoxic dead zones that adversely impact the aquatic environment and human health.
At the same time, the unique characteristics of urea make it a potential hydrogen storage medium that could offer viable on-demand hydrogen production. For instance, urea is non-toxic, has high water solubility, and has high hydrogen content (6.7% by weight). Thus, urea electrolysis for hydrogen production is more energy-efficient and economical than water electrolysis.
The weakness of urea electrolysis has always been the lack of low-cost and highly efficient electrocatalysts that selectively oxidize urea instead of water, but Teng and his team have found a solution: making electrocatalysts consisting of synergistically interacted nickel and cobalt atoms with unique electronic structures for selective urea electro-oxidation.
Unlocking Enhanced Selectivity and Activity
The WPI team’s study centered on homogeneous nickel and cobalt oxides and hydroxides. Researchers found that the key to enhancing its electrochemical activity and selectivity to urea oxidation lay in tailoring the unique electronic structures with dominant Ni2+ and Co3+ species.
“This electronic configuration is a pivotal factor to improve the selectivity of urea oxidation because we observe that higher nickel valence, such as Ni3+, indeed helps produce a fast reaction with strong electric current output; however, a large portion of current was from unwanted water oxidation,” Teng said.
To better understand this effect, Teng’s group collaborated with Aaron Deskins, a professor of chemical engineering at WPI. Deskins performed the computational simulations and found that homogenous mixing of nickel and cobalt oxides and hydroxides benefited the electron redistribution from Ni2+ to Co3+ species and shifting valence electrons to higher energy so the Ni/Co catalysts were better prepared to participate in bonding with urea and water molecules.
