Context

Urea is a widely abundant chemical produced naturally within the nitrogen cycle and industrially by ammonia synthesis. Approximately 87% of industrially synthesized ammonia is used to make fertilizer, about half of which is further converted to urea fertilizer. Urea is the predominant type of fertilizer, favored because of its high nitrogen content and safety in handling and transport. It is readily converted in the soil by enzymatic catalysis with urease to form ammonia and carbon dioxide. Ammonia is the source of nitrogen that cascades through a variety of plant based proteins, amino acids, and other compounds necessary to sustain life. Ultimately, that nitrogen is eliminated as urea, the most abundant compound in urine.

Urea is non-toxic, odorless, solid in its pure state, and highly soluble and colorless in aqueous solution. In addition to fertilizer, urea is widely used in pharmaceuticals, cosmetics and dermatology, pesticides and herbicides, as a de-icer, as a corrosion inhibitor, and for control of NOx emissions from diesel engines. Urea’s intrinsic fuel value makes it suitable for a direct urea fuel cell (DUFC). It has higher energy content than does ammonia per volume, though slightly less than that of methanol. Urea can also serve as a hydrogen carrier, with H₂ being produced by thermal catalytic and electrocatalytic means. In comparison with a wide variety of possible fuels and hydrogen carriers (including hydrogen itself), urea alone has the significant advantage of being safe to handle and transport.

Because urea is so stable, its production and use create a need to remove it. Some examples include:

• Mitigating fertilizer run-off from agricultural land into rivers, lakes, and estuaries that cause the formation of algae blooms harmful to aquatic life;
• Wastewater treatment at large scale or point source (residential) applications; and
• Removal of dialysate urea in the artificial kidney.

Technical Aspects

An especially promising means of removing urea, or converting it to energy in a DUFC, is by electrocatalysis. The figure below illustrates a DUFC. Urea enters the cell at upper left and reacts on the anode electrocatalyst (black) to form N₂, CO₂ and H₂O, which exit at lower right. Oxygen and water enter at top and react to form hydroxide ions (OH^–) on the cathode electrocatalyst (behind, not shown), while excess water exits the cathode at upper right. The OH^– ions migrate through the electrolyte (green) from cathode to anode. This arrangement removes urea with the benefit of producing electricity.

A closer look at the processes within the DUFC is shown in the diagram below, and the specific reactions are listed in the table below. In the diagram red is oxygen, black is carbon, blue is nitrogen, yellow is hydrogen, and light green is the electron traveling through the circuit from anode to cathode.

Reactions for Urea Oxidation in a Fuel Cell

Anode	(NH2)2CO + 6 OH–	—>	N2 + CO2 + 5 H2O + 6 e–
Cathode	3/2 O2 + 3 H2O + 6 e–	—>	6 OH–
Overall	(NH2)2CO + 3/2 O2	—>	N2 + CO2 + 2 H2O

 

If the objective is just to remove urea or produce H₂ (or both), water, rather than oxygen, is used at the cathode. In this case, the reactions are:

Reactions for Urea Electrolysis with H₂ Production

Anode	(NH2)2CO + 6 OH–	—>	N2 + CO2 + 5 H2O + 6 e–
Cathode	6 H2O + 6 e–	—>	6 OH– + 3 H2
Overall	(NH2)2CO + H2O	—>	N2 + CO2 + 3 H2

Here, the polarities of the anode and cathode are reversed due to the need to supply electricity for the reaction.

Whether the application is electricity production, H₂ production, or urea removal, the anode reaction is the same. The anode reaction is known as the urea oxidation reaction (UOR) or as urea electrooxidation. Our group studies the UOR at the fundamental level to determine reaction mechanism and kinetics as a function of anode potential, temperature, and urea concentration. The main challenges to the UOR are its slow reaction kinetics and limited selection of electrocatalysts.

While the UOR can occur on electrodes of Pt or Rh, the highest kinetics occur on Ni and Ni-based electrodes such as mixtures or alloys of NiMo, NiFe, or NiCr. This requires the use of an alkaline electrolyte to stabilize a passive layer on Ni. The alkaline electrolyte is usually in the  form of an anion exchange membrane (AEM – dark green in the diagrams above), which has an effective pH of 14.7. A number of applications, such as wastewater treatment and kidney dialysis, involve neutral solutions (pH ≈ 7) and high concentrations of chloride, which leads to corrosion of the Ni-based electrode. Our research also addresses the stability of Ni electrodes in neutral solutions.

Approach

We study Ni and Ni-based electrocatalysts in three-electrode cells and in two-electrode devices. The three-electrode cell is used for precise kinetic measurements. The two-electrode device, such as a fuel cell or electrolyzer, is used to test electrodes in actual use. Measurement techniques include cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy.