Pulling Hydrogen from Water Polluted by Urea

WPI researchers have developed a material to efficiently remove urea from polluted water and potentially convert it into hydrogen gas. By building these materials of nickel and cobalt atoms with carefully tailored electronic structures, the group believes it has unlocked the secret to upcycling the common organic compound. The study, led by Xiaowei Teng, the James H. Manning Professor of Chemical Engineering, was recently published in the Journal of Physical Chemistry Letters.

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 nontoxic, 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.  

The WPI team found that the key to enhancing its 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,” Teng says.

A major nitrogen fertilizer and feed additive, urea was commercially produced as early as the 1920s; around 180 million metric tons were produced in 2021. Urea can be derived from natural sources; an adult human produces 1.5 liters of urine daily, equivalent to 11 kg of urea and 0.77 kg of hydrogen gas yearly.  

The team’s 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.