The lifetime of electrocatalysts operated in commercial fuel cells and electrolyzers depends heavily on the composition and input quality of solutions supplied to the anode and cathode. Contaminants, in particular, are a concern for their impact on lowering current densities, removal difficulty, and corrosion. Preprocessing solutions requires a large amount of resources and energy. The energy to filter contaminants may be larger than the power output of a target fuel cell, rendering the process economically unviable. This is the case for sodium chloride’s corrosive properties on nickel anodes for urea electrooxidation such as in direct urea fuel cells (DUFC), wastewater treatment, and dialysis. Nickel corrodes in chloride-containing solutions at urea oxidation potentials within the range of 0-8pH.¹ Nickel-chromium-molybdenum (NiCrMo) alloys have corrosion resistant properties in a variety of oxidizing and reducing environments, including indefinite use in seawater. A catalyst that can oxidize urea (nickel-based) and withstand chloride-containing solutions in neutral pH is needed to urea electrooxidation processes more economically viable by removing costs affiliated with solution purification.

The principal objective of this study is to investigate and evaluate the electrochemical performance of Haynes nickel alloys BC1, C276, and C2000 vs. nickel under low temperature environments for urea oxidation. Obtaining current densities of these alloys and nickel under differing potentials, urea concentrations, and oxide layer thicknesses will be the key for understanding performance.

Experiments were conducted at 37°C in pH 14 KOH solution in a three-electrode cell with a platinum counter electrode and Hg/HgO reference electrode. Working electrodes were cut, sanded, and polished to a mirror finish and sonicated in ethanol and water before measurement. A potential hold of -1.4 V vs. Hg/HgO in 1M KOH for 30 minutes was performed in the three-electrode cell to reduce the uncontrolled oxide layer that resulted from atmosphere exposure. These electrodes were immediately tested after reduction. A 0.3 V potential was held at 37 °C for three hours to facilitate oxide layer growth. These electrodes were labeled as initial (O_i­) and mature (O_m) oxide, respectfully. Polarization and cyclic voltammetry curves were collected for 0, 10, 50 and 100 mM urea concentrations between 0 to 0.65 V vs. Hg/HgO.

Performance of alloys and nickel were conducted initially with 0 mM urea in solution to determine the base performance of electrodes in 1M KOH (pH 14). The black points and curves in Figure 1 show the potential step and 1mV/s sweep of BC1, C276, C2000, and nickel, respectively. Current density reached a local maximum near 0.55 V vs. Hg/HgO due to the transition of Ni²⁺ to Ni³⁺, which renders the electrode catalytic for urea oxidation. The current increases past 0.55 V vs. Hg/HgO signifying the onset of oxygen evolution. The current reaches a plateau at 0.52 ± 0.1 V for 10 mM urea solutions because of limiting mass transfer to the electrode surface. Current densities between 50 and 100 mM were sufficiently similar among most metals and treatments to assume little to no mass transfer limitation for potentials up to 0.55 V. A benchmark potential of 0.55 V vs Hg/HgO was chosen for evaluating maximum current densities to minimize mass transfer and oxygen evolution reaction effects. The alloys in this study exhibited excellent urea oxidation behavior, with each alloy having roughly double the current density at 0.55 V compared to nickel.

The ultimate goal of this study is to identify an anode material for urea electrooxidation that can oxidize urea in harsh industrial conditions, such as brackish water, which would result in broader applications of urea electrooxidation. The Haynes BC1, C276, and C2000 alloys show excellent urea oxidation behavior in alkaline environments. Corrosion resistance of these alloys has not yet been understood in a DUFC operation and is considered the next step for proof of concept for a viable system.

Acknowledgments. We acknowledge financial support from NSF Grant CBET 2055257 and the Center for Dialysis Innovation. We acknowledge Haynes for supplying alloy material used in this study.

1. Skilbred, Ellen Synnøve. Corrosion of Nickel-Aluminium Bronze - How Does the Different Alloying Elements Effect the Corrosion Properties? NTNU, 2016.