Development Of Nickel Hydroxide/Oxyhydroxide Catalysts For The Electrochemical Removal Of Urea In Alkaline Solutions

Abstract

Urea is a small organic molecule that is continually produced, circulated, and consumed through natural processes. Unfortunately, urea is a common pollutant in waste streams from industrial processes, human excrement, and drug production. High concentrations have been seen to disrupt the natural process of the nitrogen cycle, causing negative environmental impacts. The high concentrations of urea can be removed by several different types of processes, but electrochemical removal poses a double benefit. Electrochemically oxidizing urea allows for urea to removed from the environment with hydrogen as a byproduct. In first body of work, NiOOH phases were developed using selective electrochemical cycling from 0.9 VRHE to switching potentials between 1.54 and 1.64 VRHE in 0.5 M KOH at 25 ◦C. Cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) were used to characterize the surface oxide formed by the cycling procedures. Cycling with a lower switching potential (1.54 VRHE) led to preferential formation of β-NiOOH, while higher switching potentials (1.64 VRHE) favored the formation of γ-NiOOH. Moreover, a high switching potential induced surface roughening through NiOOH lattice expansion and contraction, thereby increasing electrochemical surface area (ECSA) and the number of grain boundaries. Kinetics of the OER were evaluated using Tafel analysis and turnover frequency (TOF). An electrode developed with a switching potential of 1.54 VRHE had TOF values 6–17 times larger than an electrode between 1.56 and 1.66 V developed with a switching potential of 1.64 VRHE, indicating improved OER kinetics of the β-NiOOH phase. Tafel analysis revealed little difference in Tafel slope (38.2–44.2 mV dec−1) between 1.54 and 1.69 VRHE, suggesting β-NiOOH and γ-NiOOH have similar OER mechanisms. The results here show that selective electrochemical cycling can be used to control the formation of NiOOH species can be used for studying the urea oxidation reaction. In second body of work, urea oxidation is studied to investigate the influence of urea electrooxidation on nickel oxyhydroxide by analyzing nickel phases through detailed electrochemical experiments and mass spectrometry. Urea electrooxidation was studied in KOH electrolytes with and without the presence of incidental iron to understand the UOR activity of the β-NiOOH and γ-NiOOH phases of nickel. Cyclic voltammetry, chronoamperometry and mass spectrometry were used to develop and characterize the urea oxidation and oxygen evolution reaction activities of NiOOH phases. Cycling in urea-free electrolytes with low (1.58 VRHE) and high (1.68 VRHE) switching potentials led to the formation of β-NiOOH and γ-NiOOH} (respectfully), while cycling the electrodes in 0.2 M urea lead to a phase of nickel that was more β-NiOOH-like in nature. Window opening experiments in iron-free electrolytes revealed the hysteresis at higher potentials, possibly due to the formation of NiO4 or a change of UOR reaction pathway. Chronoamperometry, combined with mass spectrometry, showed a drop in current at higher potentials (1.88VRHE) along with a drop in N2 faradaic efficiency. The results here show that the surface of nickel is susceptible to change during UOR, which in turn affects the selectivity of urea oxidation and oxygen evolution reaction products. Next, high chromium and molybdenum nickel-based alloys and pure Ni were used as electrocatalysts to study the electrochemical urea oxidation reaction with varying concentrations of Ni, Cr, and Mo. Slow cyclic voltammetry (1 mV s−1) and polarization curves were performed with 10, 50, and 100 mM urea in 1 M KOH at 37 ◦C. The electrochemically active surface area (ECSA) was determined by capacitance measurements at the open circuit potential. The ECSA tracked the extent of oxide layer growth for all electrodes. Electrodes with mature oxide layers exhibited larger ECSAs compared to those of initial oxide layers. Alloys showed higher reaction rates per geometric area than nickel. However, the alloys showed showed significantly lower urea electrooxidation turnover frequencies (TOF) (0.057–0.1 s−1) when compared to a pure Ni electrode (< 0.30 s−1). Key findings emphasize: the correlation between electrochemical surface area and roughness with urea oxidation active area, the utilization of the Langmuir-Hinshelwood model for determining the urea oxidation mechanism versus potential, and the determination that urea oxidation activity remained constant on a per-site basis through TOF ratio analysis despite the growth of the oxide layer. The last body of work focuses on developing and characterizing nickel-based catalysts to enhance urea oxidation in a wearable artificial kidney. Various catalysts (Ni, NiMn, NiCr, NiMo, NiFe) are synthesized onto nickel foam to increase the electrochemically active surface area and roughness. Mass transfer limits are observed for urea oxidation at physiological concentrations (10 mM). Urea oxidation kinetics are explored at higher concentrations (200 mM), showing improved performance during polarization but lower currents per active site. A simplified dialysis model is introduced to evaluate mass transfer coefficients’ and extent of reactions’ impact on flowrates, urea concentrations, and pH levels in streams. A nickel hydroxide catalyst is evaluated with this model and shows a minimum electrode area of 1314 cm2 is needed for continuous operation. This research combines experimental data and a computational dialysis model for a simplified continuous dialysis system, highlighting the potential of these catalysts and paving the way for future improvements. The results of these studies suggest that, although the urea oxidation reaction on nickel- based catalysts has been well studied, there is room for improvement for employing nickel- based electrodes in waste water applications. Furthermore, the methodologies developed in this research can be applied to other NiOOH-based systems to investigate and enhance their electrocatalytic performance.