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Researchers at the Worcester Polytechnic Institute in the US have developed a material to remove urea from water and potentially convert it into hydrogen gas.
By building this material of nickel and cobalt atoms with carefully tailored electronic structures, the group has unlocked the potential to enable these transition metal oxides and hydroxides to selectively oxidize urea in an electrochemical reaction.
In a paper published in the Journal of Physical Chemistry Letters, the scientists explain that urea is a low-cost nitrogen agriculture fertilizer and a natural product from human metabolism. In 2021, around 180 million metric tons of urea were produced worldwide. But 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 downside of urea electrolysis has always been the lack of low-cost and highly efficient electrocatalysts that selectively oxidize urea instead of water. Yet, by making electrocatalysts consisting of synergistically interacted nickel and cobalt atoms with unique electronic structures for selective urea electro-oxidation, the WPI team found a solution to this issue.
In detail, they focused on homogeneous nickel and cobalt oxides and hydroxides and found that the key to enhancing the electrochemical activity and selectivity to urea oxidation lies 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,” head researcher Xiaowei Teng said in a media statement.
To better understand this effect, Teng’s group collaborated with Aaron Deskins, who 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.
For Teng, Deskins and their co-authors, these findings could help use urea in waste streams to efficiently produce hydrogen fuel through the electrolysis process and could be used to sequester urea from water, maintaining the long-term sustainability of ecological systems, and revolutionizing the water-energy nexus.
