Strategies for Mitigation of Intermediate CO₂ Poisoning to Promote Electrocatalytic Efficiency during Urea Oxidation Reaction

Abstract

Urea electrolysis plays a key role in various urea waste valorization processes. In this regard, the complex six-electron transfer process, urea oxidation reaction, is the key to achieving maximum efficiency. In this study, we developed multilayered α-Ni­(OH)₂ sheets through a sonochemical approach. We used Fe³⁺ to dope the α-Ni­(OH)₂ sheets in three different compositions. The synthesized catalysts were analyzed by using XRD, SEM, EDX, FTIR, and XPS. Using EDX and XPS, successful doping of iron was confirmed. The electrocatalytic activities were characterized by CV, LSV, EIS, and CA techniques. Through CV analysis, it was identified that Fe³⁺ did not directly participate in the electrochemical reaction; rather, due to high Lewis acidity, it was modulating the electronic environment of Ni²⁺ ions. Iron doping induced a reduction in the onset potential and synergistically increased the catalytic current by promoting a higher number of active sites for urea adsorption. Tafel analysis concluded that the improved reaction kinetics was due to iron incorporation. Using Nyquist and Bode plots, it was identified that iron doping promoted CO₂ production as RDS. Therefore, catalyst poisoning due to prolonged adsorption of CO₂ was diminished, and subsequently, catalyst stability was increased. Overall, while Fe³⁺ doping has proved to be a significant method to enhance catalytic activity, careful optimization of concentration is required to build a state-of-the-art catalytic architecture for urea waste valorization.

1. Introduction

The urea oxidation reaction (UOR) has garnered significant interest due to its potential application in organic waste valorization. Urea is a key organic component of effluents generated from industrial and agricultural activities. Direct discharging of the effluents can have negative effects such as eutrophication. Apart from that, urea is also discharged from mammalian protein metabolism. An average human excretes 22–23 g/L of urea in a day. This harmful nitrogen-rich compound might seep into soil and groundwater. Urea might hydrolyze under the influence of groundwater to generate ammonia. High concentrations of ammonia are harmful to the animals, and the ammonia further decomposes into nitrates and nitrites. Therefore, treatment of urea-rich wastewater is deemed necessary for a sustainable future. Common urea removal techniques, such as adsorption, enzymatic hydrolysis, and thermal hydrolysis, require complex and energy-intensive processes. Apart from that, these processes can generate toxic products, such as ammonia and nitrates. Electrochemical oxidation of urea can be beneficial as it promises to couple the UOR with the hydrogen evolution reaction (HER) to produce H₂ from effluents. In UOR, urea is electrochemically oxidized at the anode into nontoxic N₂, CO₂, and H₂O in alkaline conditions. The overall oxidation process requires 0.37 V (vs SHE). It is theoretically ∼70% lower compared to the energy requirements of the oxygen evolution reaction (OER) (1.23 V vs SHE). Therefore, the UOR could replace the OER at the anodic side of an electrolyzer to simultaneously generate green hydrogen as well as the removal of pollutants. Apart from electrolysis, urea-rich wastewater could be utilized as a feedstock for fuel cells that can generate electricity. In direct urea fuel cells (DUFCs), urea is oxidized on the anode side, while oxygen is reduced to OH^– on the cathode side. The generated OH^– works as a charge carrier and helps to complete the circuit. A urea/O₂ fuel cell has a theoretical open-circuit voltage (OCV) of 1.146 V at room temperature, which is slightly lower than H₂/O₂ fuel cells (1.23 V). Therefore, it was suggested that UOR is an important fundamental process that has important energy applications. It promises to be an environmentally benign process with no toxic byproducts and can be used under ambient conditions. Overall, urea electro-oxidation aims to be one of the fundamental approaches of the wastewater-energy nexus.

The drawback of using UOR as a valorizing process is the requirement of highly efficient electrocatalysts. UOR is a complex six-electron transfer process with multiple adsorption and desorption steps involved. Due to the six-electron transfer process, optimizing the reaction kinetics is a fundamental demand to steer the electron transfer processes. Without efficient electrocatalysts, the UOR could follow various pathways and produce undesired outcomes. Therefore, deciphering the function of the electrocatalyst is the key to achieving maximum efficiency. While it is environmentally more attractive to perform the UOR in neutral media, as the UOR is pH sensitive. In higher pH, the energy demand is decreased. Therefore, by performing UOR in an alkaline electrolyte, the need for noble metal electrocatalysts, such as Pt/C, IrO₂/C, and RuO₂/C, is diminished. These materials are rare on Earth and expensive, making them uneconomic for large-scale systems. Therefore, transition metal-based catalysts are being widely researched. Transition metal-based catalysts are conductive, cheaper, and abundant in nature. Among these wide arrays of materials, researchers have identified Ni as a promising candidate to develop various catalysts. Ni-based catalysts are easy to synthesize, widely available, and offer significant performance. Using different synthetic approaches, catalysts could be fine-tuned according to the need. However, the Ni-based catalysts are hindered by a lack of active sites and long-term stability issues. Also, the catalyst poisoning from the reaction intermediates plays a crucial role in the overall process.

Among the different Ni-based catalysts, Nickel hydroxide (Ni­(OH)₂) is an exciting contender. Ni­(OH)₂ is easy to synthesize, is electrochemically active, and boosts a layered structure. It is found in two different phases, α and β forms. The β form is more stable than the α form, but the α form is theoretically more active due to a higher rate of electron transfer per Ni atom. Therefore, it is important to stabilize the α form, but it readily transforms into a β polymorph under alkaline conditions. Different strategies, such as heteroatom doping, defect introduction, and interface engineering, have been used to stabilize the α form during the synthesis process. α polymorphs transform into γ-NiOOH under an anodic potential. This γ-NiOOH facilitates the electrochemical reaction occurring at the electrode–electrolyte interface. It was hypothesized that urea is oxidized by γ-NiOOH because the UOR potential overlaps with the γ-NiOOH formation potential, and therefore, UOR follows an indirect mechanism under the Ni­(OH)₂ catalyst. Therefore, Ni­(OH)₂ behaves as a precatalyst. Nevertheless, the significant energy barrier encountered during the practical conversion processes restricts the overall efficiency. To address the issue, many groups have utilized different synthetic procedures to design various structures for catalytic applications. Boggs et al. presented the potential of utilizing urea-rich wastewater as a substitute energy source. According to their findings, urea electrolysis can generate 36% cheaper hydrogen compared to water electrolysis. Developing on that, Wu et al. developed Ni­(OH)₂ nanocups, which demonstrated 6 mA cm^–2 current density under 0.41 V (vs SCE) in 1 M KOH + 0.33 M urea. Tatarchuk et al. investigated cobalt-induced changes in Ni­(OH)₂ and identified that Ni₂₀Co₈₀(OH)₂ has the highest amount of current density per active sites. Yang et al. used manganese (Mn) to adjust the electrocatalytic performance of Ni­(OH)₂, and they discovered that Mn doping facilitated a decrease in onset potential and improved the catalytic activity. In another study, Cao et al. used vanadium­(V) to modify the β-Ni­(OH)₂ to V-doped α form, which exhibited a Tafel slope of 32.15 mV dec^–1, significantly improving from the undoped form. Maruthapandian et al. used a Ni MOF (BTC) to synthesize Ni­(OH)₂/NiOOH, which demonstrated an onset potential of 1.34 V vs RHE in the presence of 1 M KOH and 1 M urea. Lin et al. reported a methanol-assisted synthetic method to design single-layer α-Ni­(OH)₂ on carbon cloth. Synthesized α-Ni­(OH)₂-modified carbon cloth exhibited an exceptional UOR activity of 436.4 mA cm^–2 at 0.5 V vs Ag/AgCl. Very few reports have discussed catalyst poisoning due to intermediate adsorption. In one such report, Song et al. used selenium to dope the Ni­(OH)₂ to weaken the CO₂ intermediate adsorption and increased the catalytic activity 17.92 times more than the undoped one. However, the ultrasonic-mediated modification of Ni­(OH)₂ tested on a glassy carbon (GC) electrode for UOR applications has barely been reported, and the structural changes due to iron doping and how it affects the UOR activity are also worth exploring. The reason to choose a GC electrode is that it is functionally inert toward UOR and does not participate or self-catalyze the reaction.

Here, in this study, an ultrasonic-probe-mediated synthetic procedure was proposed to synthesize α-Ni­(OH)₂ for UOR applications. Then, iron as a heteroatomic dopant was introduced in catalysts to modify the electrocatalytic activity. The synthesized catalysts were characterized using XRD, FESEM, EDX, FTIR, and XPS. Then, the catalysts were analyzed for their OER and UOR properties under alkaline conditions using CV and LSV. Various concentrations of urea were taken to identify which concentration is an ideal concentration for UOR. EIS was employed to gain insights into the processes occurring at the electrode–electrolyte interface. Chronoamperometric measurements were performed to evaluate the long-term stability of the electrocatalysts. This overall study investigates an alternate pathway to design bimetallic architecture and its effect on electrocatalytic applications.

2. Materials and Methods

2.1. Materials

Nickel nitrate hexahydrate (Ni­(NO₃)₂·6H₂O, EMPLURA), ferric nitrate nonahydrate (Fe­(NO₃)₂·9H₂O, EMPLURA), urea (CO­(NH₂)₂), ascorbic acid (C₆H₈O₆), starch, and potassium hydroxide (KOH, EMPLURA) were purchased from Merck. Ethanol (C₂H₆O, LR) was purchased from Labnol. Isopropanol (C₃H₈O) was obtained from RANKEM. The chemicals were not modified and used as delivered. The deionized water (18.2 MΩ cm) was collected from a Milli-Q IQ 7000 Ultrapure Water system.

2.2. Catalyst Preparation

• (a)Preparation of Ni­(OH)₂: To synthesize the catalyst, a general sonochemical approach was employed. Initially, 0.12 g of starch was dispersed in 50 mL of deionized water and allowed to stir. Then, 0.1 M of Ni­(NO₃)₂·6H₂O was dissolved, and subsequently, 30 mL of 0.2 M solution of ascorbic acid was added to the mixture. The pH of the total solution was determined to be 1.96. Then, the mixture was stirred for 1 h. Following this, 1 M NaOH was added dropwise to the solution while stirring. Then, the solution was subjected to ultrasound irradiation with a 20 kHz frequency probe sonicator (Q-700, Q-Sonica) for 90 min at 40% amplitude with a cycle of 20 s ON and 10 s OFF. After the sonication, 30 mL of ethanol was added to the sonicated solution. The catalyst particles were then centrifuged at 12,000 rpm. Collected catalyst particles were washed and rinsed using isopropanol and water to eliminate any unreacted precursors. The final product was dried in a hot-air oven at 80 °C overnight. The dried product was ground and collected for further use. The catalyst was designated as N100F0_OH.
• (b)Preparation of Fe-doped Ni­(OH)₂: The procedure for the Fe-doped Ni­(OH)₂ was similar to the synthesis of N100F0_OH, except for the addition of Fe­(NO₃)₂·9H₂O. 1, 3, and 5 mol % of Fe­(NO₃)₂·9H₂O were added after the addition of Ni­(NO₃)₂·6H₂O to modify the catalyst with iron. The catalysts were designated as N99F1_OH, N97F3_OH, and N95F5_OH, respectively.

2.3. Characterization

Powder X-ray diffraction (pXRD) analysis was performed using a PAN Analytical Bragg–Brentano diffractometer with Cu Kα radiation of 1.54 Å, in the range of 5–90°. Fourier transform infrared spectroscopy (FTIR) analysis was done by a PerkinElmer, Spectrum One FTIR Spectrometer in the range of 400–4000 cm^–1. The morphology and elemental analysis were carried out using a field emission scanning electron microscope (FESEM) (Ultra Plus, Zeiss, Germany), which was operated at 10 kV. The surface elemental composition and valence state determination were achieved by XPS characterization using AXIS SUPRA (S. No. C332905/01).

All of the electrochemical analyses were carried out using a Metrohm Autolab Potentiostat in a three-electrode system. Glassy carbon electrode (GCE), Ag/AgCl (Saturated KCl) electrode, and a Graphite rod were utilized as the working electrode (WE), reference electrode (RE), and counter electrode (CE), respectively. All of the reported potential values are reported against the Ag/AgCl electrode only. The catalyst ink was prepared by mixing 5 mg of catalyst powder with 100 μL of 5 wt % Nafion solution. Then, 2.5 mL of Ethanol and 2.5 mL of DI water were added and sonicated for another 1 h to achieve a well-dispersed catalyst ink. After the sonication, 10 μL of catalyst ink was carefully drop-cast on the working electrode (diameter 3 mm). The drop-cast catalyst ink was dried and used for all of the electrochemical studies. Impedance studies were conducted in the frequency range 10^–2 to 10⁵ Hz. All of the analysis was performed under ambient conditions.

3. Results and Discussion

3.1. Characterization of the Synthesized Catalyst

The structural characteristics of the catalysts were analyzed by pXRD. The XRD patterns depicted in Figure were consistent with the previously reported result of the rhombohedral structure of α-Ni­(OH)₂ (JCPDS no. 38-0175). The α-Ni­(OH)₂ has a hydrocalcite-like structure with randomly stacked layers of Ni­(OH)₂. The diffraction peak at 33.9° is due to the turbostratic disorders in the layers. The broadness of the spectrum could be due to the random distribution of nitrate and water molecules and suggests an amorphous nature. The characteristic peaks of the α phase of Ni­(OH)₂ at 2θ = 9.9°, 17.8°, 33.9°, and 60.4° can be assigned to (003), (006), (101), and (110) planes, respectively. The (003) peak was found to be negatively shifted, possibly due to the increased d-spacing due to water intercalation. It was observed that the XRD pattern did not change with iron doping. Therefore, the lattice structure was similar in all of the catalysts, and iron doping did not introduce any new crystal planes. However, due to ionic radius mismatch of Fe³⁺ and Ni²⁺, lattice strains could be introduced, which could be beneficial to expose more coordinatively unsaturated surface atoms. According to the ATR–IR analysis, the broad peak at 3289 cm^–1 indicates the stretching mode of the hydroxyl groups. Additionally, adsorbed water in interstitial sites contributes to peak broadening. The peak at 1589–1596 cm^–1 suggests the presence of a bending mode of the layered hydroxyl group. A peak at around 1312 cm^–1 was attributed to interstitial nitrate ions. Peaks at 1023 cm^–1 were attributed to atmospheric CO₂. Peaks at 780–610 cm^–1 were indicative of the vibrational modes of Ni–O–H and Ni–O–Ni bonds of α-Ni­(OH)₂. The peaks in the range of 754–450 cm^–1 could be attributed to M–O (M = Ni, Fe) stretching vibrations, which indicate strong coordination between metal and oxygen atoms. Iron doping could result in band overlap and band broadening. However, retention of characteristic bands suggests the preservation of the α-phase.

1.

(A) X-ray diffraction patterns and (B) ATR–IR data of catalysts.

From the SEM images depicted in Figure , an aggregated multilayered sheet-type morphology was observed. The presence of irregular and uneven surfaces suggests the presence of an amorphous character, which is evident in the XRD pattern. It could be beneficial to have an uneven surface as it could provide more adsorption sites for urea and hydroxide molecules to bind. From EDX analysis, the bulk composition was obtained. It was observed from EDX results depicted in Figure S1 (Supporting Information) that from N99F1_OH to N95F5_OH, the atomic percentage of iron increased; therefore, successful doping of Ni­(OH)₂ sheets was confirmed. The calculated bulk composition is given in Table S1.

2.

SEM images of synthesized catalysts: (A) N100F0_OH, (B) N99F1_OH, (C) N97F3_OH, (D) N95F5_OH.

X-ray Photoelectron Spectroscopy (XPS) was performed to determine the elements and their electronic states on the surface. XPS is known to be surface sensitive, as it primarily analyzes the top few nanometers. Electrons are generated by bombarding the surface with X-rays, and through inelastic collision, photoelectrons are generated from the surface. As per the XPS analysis, iron was detected on the surface. In fact, it was observed that the Ni/Fe ratios for N99F1_OH, N97F3_OH, and N95F5_OH were 1:0.69, 1:0.78, and 1:0.58, respectively. The surface of N97F3_OH was the most iron-rich among all three. Calculated surface composition results are listed in Table S2. All of the spectra in Figure were deconvoluted using the Gaussian method. The XPS full survey scan and the adventitious C1S spectra are provided in the Supporting Information (Figures S2 and S3).

3.

XPS spectra of synthesized catalysts (A,B) N100F0_OH, (C–E) N99F1_OH, (F–H) N97F3_OH, (I–K) N95F5_OH.

As shown in Figure A, the Ni 2p spectrum was fitted into spin–orbit couplets and a couple of satellite peaks. The peaks at 854.1 eV (2p _3/2) and 871.9 eV (2p _1/2) were attributed to Ni²⁺ ions. Iron was introduced in the high valent state of Fe³⁺ in N99F1_OH, N97F3_OH, and N95F5_OH. In the case of N99F1_OH, the fitted curve of Ni 2p spectra showed two spin–orbit coupling curves for 2p _3/2 and 2p _1/2 with binding energies of 854 and 871.5 eV, respectively, which correspond to Ni²⁺. Additionally, two more spin–orbit couplets at 855 and 872.9 eV were observed due to the formation of Ni³⁺. In Fe 2p spectra, 2p _3/2 peak and 2p _1/2 were observed at 712.5 and 723.9 eV, respectively, which were attributed to the presence of Fe³⁺ cation according to Xie et al. A satellite peak at 734.2 eV was observed due to shakeup processes during photoemission. Fe³⁺ acts as a strong Lewis acid due to its high positive charge and small ionic radius. Therefore, it was predicted that due to interaction with Fe³⁺, Ni²⁺ tends to donate an electron and form Ni³⁺. Therefore, doping with iron helps to form more higher-charged active sites. According to Figure F, the peaks at 855.3 and 868.1 eV correspond to Ni^2+, and the peaks at 861.5 and 873 eV correspond to the presence of Ni³⁺ species. From the Fe 2p spectra (Figure H), 2p _3/2 and 2p _1/2 maxima were identified at 707.2 and 729 eV, respectively. Along with that, a satellite peak at 737.1 eV was also observed. Clearly, there had been a peak shift due to a change in the neighboring environment. For N95F5_OH, the Ni 2p spectra showed two peaks at 854.2 eV and 871.5, corresponding to the Ni²⁺, and peaks at 855.3 eV and 872.8 were attributed to Ni³⁺. The ratio of surface Ni³⁺/Ni²⁺ sites was higher for N97F3_OH (1.93) compared to that of N99F1_OH (1.13) and N95F5_OH (1.30), which is crucial for UOR activity. All the O 1s spectra show the presence of adsorbed OH^– and lattice O^{2 –}, which suggests the presence of M–O–M interactions (M = Ni, Fe). The Lewis acidity of Fe³⁺ affected the binding energy of OH^–. Therefore, it was confirmed that Fe³⁺ affects the local environment of Ni²⁺ and increases the Ni³⁺ concentration. However, it also affects the overall concentration of surface Ni sites, which is crucial for the UOR.

3.2. Electrochemical Results

All of the catalysts were analyzed using CV to understand the redox behavior under alkaline conditions. CV was performed at a scan rate of 50 mV/s, and 1 M KOH (pH 14) was used as an electrolyte. To minimize the surface area effects on the catalytic process, electrochemical results were normalized with respect to Electrochemically Active Surface Area (ECSA). ECSA was calculated using the double-layer capacitance (C _dl) value, obtained from varying scan rates (10–50 mV/s) at the nonfaradic region (Figure S4). The C _dl value of N99F1_OH (20.3 μF) is larger than those of N100F0_OH (8.22 μF), N97F3_OH (16.1 μF), and N95F5_OH (17.9 μF) (Figure S5A). ECSA can be determined by dividing the C _dl value by the specific capacitance (C _s). C _s = 40 μF cm^–2 was used according to previous reports. It was hypothesized that doping with iron helped in introducing defects on the catalyst surfaces and activating the surface sites. As depicted in Figure S5B, it resulted in 2.5 times increase in ECSA compared to undoped α-Ni­(OH)₂.

In Figure A, all of the catalysts show an oxidation peak and a reduction peak. The oxidation peak corresponds to the oxidation of divalent Nickel ions to trivalent Nickel ions. The formation process is described in eq ,

$$\mathrm{N}\mathrm{i}{(\mathrm{O}\mathrm{H})}_2+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{N}\mathrm{i}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$ 1

4.

(A) CV curves and (B) the associated Tafel plots for N100F0_OH, N99F1_OH, N97F3_OH, and N95F5_OH.

The observed cyclic voltammograms exhibited no separate redox peaks for iron, indicating that the intercalated iron does not directly participate in the electrochemical reaction. Its function appears to be a structural and electronic one, subtly influencing the catalytic activity of the Nickel centers. With the increase in iron incorporation, the oxidation peak was found to be ∼20 mV shifted toward the negative potential side. The results suggest that lower potential is required to oxidize the Ni²⁺ species under the influence of iron doping. The onset potential is a significant measurement criterion, as it determines from which potential the catalytic reaction initiates. The onset potential was determined from Figure A, and the lowest onset potential of 0.52 V was observed for N97F3_OH, which is ∼20 mV less than that of N100F0_OH. The highest amount of current density for OER was observed for N97F3_OH (5.9 mA cm^–2 under 0.65 V vs Ag/AgCl) compared to all the other catalysts and the bare GC electrode. The current density observed was 2.25 times greater than that of undoped α-Ni­(OH)₂ and 14.9 times greater than that of the bare glassy carbon (GC) electrode. Also, it was observed from the reduction peaks that N97F3_OH has the highest amount of reduction current, suggesting the presence of the highest number of Ni³⁺ ions to be reduced to Ni²⁺ among all of the catalysts. Tafel analysis was conducted to assess the kinetic activity of the synthesized electrocatalysts. The measurements were carried out at a scan rate of 1 mV/s. The Tafel slope was determined by plotting the logarithm of the current density versus the potential and calculating the slope of the resulting linear region over a ∼50 mV range.

According to Figure B, N97F3_OH demonstrated the lowest Tafel slope of 51.66 mV/dec among all four catalysts. A lower Tafel slope signifies more facile reaction kinetics, which can be supported by a high current density at low potential from the CV curves. The smaller Tafel slope could result from the Lewis acidic activity of Fe³⁺. According to Li et al, Fe³⁺ is one of the most acidic transition metals with a pK _a = 2.2. Being an electron-deficient species, it can pull the electron cloud from the 3d orbital of Ni²⁺ to the 3d orbital of Fe³⁺. This could result in the easier generation of highly charged species. Therefore, a decrease in reduction potential for Ni³⁺/Ni²⁺ couples is highly probable, and it could facilitate a higher number of Ni³⁺ species. According to XPS results, at 3% iron doping, the concentration of catalytically active Ni³⁺ is highest among all the catalysts; therefore, it is possible that 3% Fe³⁺ doping is crucial to achieve a balance between available active sites and higher stability of Nickel in the +3 oxidation state. Iron is also known for its superior conductivity; therefore, the presence of iron accelerates the charge transfer dynamics on the catalyst surface. All the results indicate that while Fe³⁺ doping significantly improves kinetic performance, precise control of the doping concentration is essential.

The catalyst structure and the surrounding electrolyte environment are crucial in influencing the reaction pathways and activation energy needed for the rate-determining step. For the electro-oxidation of urea, NiOOH is instrumental in accelerating the reaction kinetics. It is anticipated that urea will undergo oxidation via the indirect pathway in the presence of NiOOH. The indirect mechanism follows,

$$(\mathrm{N}\mathrm{i}{(\mathrm{O}\mathrm{H})}_2+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{N}\mathrm{i}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-})\times 6$$ 2

$$6\mathrm{N}\mathrm{i}\mathrm{O}\mathrm{O}\mathrm{H}+\mathrm{C}\mathrm{O}{({\mathrm{N}\mathrm{H}}_2)}_2\to 6\mathrm{N}\mathrm{i}{(\mathrm{O}\mathrm{H})}_2+{\mathrm{N}}_2+{\mathrm{C}\mathrm{O}}_2$$ 3

$$\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{l}\mathrm{l}:\mathrm{C}\mathrm{O}{({\mathrm{N}\mathrm{H}}_2)}_2+6{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{N}}_2+{\mathrm{C}\mathrm{O}}_2+5{\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}$$ 4

Under anodic potential, Ni­(OH)₂ transforms into NiOOH (eq ), which acts as the main catalytic center for urea adsorption and subsequent oxidation steps through dehydrogenation. However, the complicated 6e^– transfer depicted in eq requires efficient catalysts. Therefore, generating NiOOH at a lower potential is key to reducing the energy demand.

To understand the optimum electrolyte environment, various concentrations of urea along with 1 M KOH were used as electrolytes for the UOR experiments for all four catalysts. CV was performed at 50 mV/s. According to Figure S6, it was observed that 0.3 M urea produced the highest amount of current for all four catalysts. It was hypothesized that with increasing concentration from 0.3 M urea, the catalyst surface was getting saturated, and it did not improve the kinetics. The adsorption of urea on high-valent Ni ions resulted in the blocking of further adsorption, such that catalytic sites were presumed to be blocked and not renewed. The overall surface coverage was enhanced due to an increase in the urea concentration. When the concentration reached 0.3 M, the UOR current plateaued due to full occupation of the catalyst surface and it was not available for the additional reactants to adsorb from the electrolyte. Since at 0.3 M urea, the highest amount of current was achieved, at 0.3 M urea, further experiments were conducted. All four catalysts were compared at 0.3 M urea, and the results are depicted in Figure A. Doping with iron decreases the onset potential for UOR as well. While for N100F0_OH, the onset potential was at 0.43 V; for N99F1_OH, it shifted to 0.39 V, and the onset potential of N97F3_OH and N95F5_OH was 0.37 V. No Nickel oxidation peak was observed during UOR, indicating that UOR and the generation of NiOOH occur almost simultaneously. The redox peak integration method was utilized to identify the number of active sites present. The integral of the reduction peak was used to calculate the total charge and was divided by the charge of an electron (1.602 × 10^–19 C) to calculate the total number of electrons transferred. Since Ni³⁺/Ni²⁺ reduction is a one-electron transfer process, the total number of electron transfers depicts the total active Ni sites present for UOR. N97F3_OH had the highest number of active Nickel sites (8.9 × 10¹⁴), which is 6.21 times greater than that of N100F0_OH (1.43 × 10¹⁴). Tafel slopes were determined at 0.3 M urea at 1 mV/s and are presented in Figure B. According to Figure B, N100F0_OH exhibited a high Tafel slope of 146.27 mV/dec, which was attributed to the low number of catalytically active Ni³⁺ and unavailability of active sites due to the strong reaction intermediate adsorption. N99F1_OH, N97F3_OH, and N95F5_OH demonstrated Tafel slopes of 58.83 mV/dec, 44.41 mV/dec, and 54.79 mV/dec, respectively. Therefore, iron doping significantly improved the overall reaction kinetics. As identified by the redox analysis and the UOR studies, N97F3_OH was identified to have the optimum composition to deliver the superior kinetic activity to treat the urea-based wastewater effluents.

5.

(A) UOR curves and (B) the associated Tafel plots of N100F0_OH, N99F1_OH, N97F3_OH, and N95F5_OH.

EIS studies were carried out to learn about the change in kinetic processes at different potentials. Since electrochemical reactions are potential dependent, it therefore helps to decode the reaction environment under the influence of potential. EIS measurements were performed at applied potential from 0.35 to 0.65 V in increments of 0.05 V. Since 0.3 M was identified to be the optimized electrolyte concentration, it was chosen as the electrolyte for the EIS studies. The frequency range 10⁵ to 10^–2 was chosen as it is sufficient to probe different electrochemical processes. The high frequency range is crucial to identify the bulk electrolyte phenomena such as ion migration and charging/discharging of the electrical double layer, and the lower frequency range is useful to probe the processes taking part in the electrode–electrolyte interface such as faradic reactions and diffusion of redox-active species. The input sinusoidal potential amplitude was fixed at 5 mV to maintain a linear relationship between the input and output signal.

In the case of N100F0_OH, the Nyquist plot (Figure C) of 0.35 V exhibited one semicircle which corresponded to the direct urea electro-oxidation reaction. As the potential was increased, the semicircle shrank and for 0.55 V–0.60 V, a reverse flip with negative real impedance was identified. This phenomenon was ascribed to the adsorption of intermediates like –CO₂ on the catalyst surface, which obstructed the active Nickel sites. Zhang et al. reported that the proposed reaction pathway for UOR follows *CO­(NH₂)₂ → *CO­(NH·NH₂) → *CO­(NH·NH) → *CO­(NH·N) → *CO­(N₂) → *CO­(OH) → *CO­(OH·OH) → *COO. It was calculated that the binding energy of two CO₂ molecules on Nickel active sites (−0.90 × 2 = −1.80 eV) is almost identical to the two-urea molecule adsorption energy on the NiOOH surface (−1.79 eV). Therefore, it was postulated that the desorption of CO₂ could be the rate-determining step (RDS). When intermediate CO₂ desorption becomes the RDS, a reverse loop to the second quadrant was observed. However, the positive real impedance was observed again at 0.65 V, suggesting that CO₂ production became the RDS. According to the Bode plot (Figure A) of N100F0_OH, the characteristic frequency of urea electro-oxidation at 0.35 V was identified at 10^1.59 Hz. The relaxation time could be calculated using (τ = 1/2πf; f is the characteristic frequency). The estimated relaxation time for 0.35 V was 4.09 ms. As the potential increased to 0.45 V, the characteristic frequency shifted toward a higher frequency of 101.9 Hz and a lower relaxation time of 2 ms, but at 0.5 V, the characteristic frequency decreased to 10^1.41 Hz. At higher potentials of 0.55–0.60 V, an almost 180° phase angle was noticed in the Bode plot, which aligned with the second quadrant flip from the Nyquist plots. At 0.65 V, the characteristic frequency was the same as 0.5 V, indicating weak charge transfer dynamics. The reason for the decrease in characteristic frequency could be attributed to the active site blocking and CO₂ poisoning due to intermediate adsorption. Therefore, at higher potentials, a decrease in faradic reaction kinetics is probable. However, from the Bode plot (Figure B), it was observed that the total resistance at lower frequency was the least for 0.60 V, suggesting competitive intermediate oxidation processes were dominating over the UOR process. Overall, it is plausible that competition between CO₂ production and desorption decreases the catalytic efficiency.

6.

Bode plots of (A,B) N100F0_OH, (D,E) N99F1_OH, (G,H) N97F3_OH, and (J,K) N95F5_OH. Nyquist plots of (C) N100F0_OH, (F) N99F1_OH, (I) N97F3_OH, and (L) N95F5_OH.

For N99F1_OH, no second quadrant flip was observed in either the Bode plot (Figure D) or the Nyquist plot (Figure F). Therefore, it was hypothesized that Fe³⁺ doping enhanced the selectivity toward urea adsorption and provided protection from catalyst poisoning. No appearance of reverse loop indicated that, instead of CO₂ desorption, CO₂ production became the RDS. However, increasing the potential resulted in an initial shifting of the frequency to a lower frequency range; later, the characteristic frequency shifted to 10^2.59, which corresponds to the Ni­(OH)₂/NiOOH transformation. Therefore, both direct and indirect kinds of UOR were identified. According to Figure E, the lowest amount of total resistance was observed in the case of 0.65 V. The reason for that could be due to Fe³⁺ intercalation; the overall conductivity of the material increased, and charge transfer resistance decreased. However, in the case of N97F3_OH, the reverse flip with negative real impedance was observed at 0.55 V (Figure G,I), due to which the very low characteristic frequency of 10^0.11 Hz and high relaxation time of 123.6 ms weres observed for 0.60 V. Despite CO₂ poisoning, at a higher potential of 0.65 V, the frequency response increased to 10^2.01 Hz, which was attributed to the redox reaction due to the Ni²⁺/Ni³⁺ reaction. Therefore, like N99F1_OH, both direct and indirect kinds of UOR were observed but catalyst poisoning affected the overall efficiency. In the case of N95F5_OH, the Nyquist plot (Figure J,K) exhibited an interesting pattern as initially, from 0.35 to 0.40 V, the characteristic frequency increased, but then at 0.40 V–0.50 V, the characteristic frequency decreased. In a higher potential of 0.55–0.65 V, the characteristic frequency shifted to a higher frequency range of Ni²⁺ oxidation. The highest characteristic frequency of 10^2.67 Hz and the lowest relaxation time of 0.3 ms were observed under the potential of 0.65 V. From Figure J–L, it was confirmed that no reverse loop was observed; therefore, the negative effect from the adsorption of the CO₂ intermediate was negligible. Based on the EIS results, it was inferred that for all of the catalysts at low potentials, direct UOR is more facile, and at higher potentials, NiOOH formation is more preferred. The reason could be that since the electrolyte contains high concentrations of urea, it is predominantly selective toward nickel sites. Therefore, the high urea concentration facilitates the formation of the CO₂ intermediate adsorption on the catalyst surface compared to the OH^– adsorption, which is required for NiOOH formation. Therefore, as the surface is free from the CO₂ intermediate at higher potential, OH^– can participate in a reaction with the catalyst surface. So, at higher potential, indirect UOR is more prevalent. Iron doping not only helps in increasing the number of active sites but also weakens the binding strength between Ni and CO₂.

If we can imagine that electrochemical reactions are simply electron transfer across the electrodes and electrolyte, then we can model the mechanism through an electrochemical circuit. Like a normal circuit, EIS circuits have resistors, capacitors, inductors, etc. To accommodate the chemical changes and diffusion processes, these components have been tweaked to more accurately model the circuit. R _s corresponds to the series resistance, which was dependent on the electrolyte composition, and R _p indicates the charge-transfer resistance corresponding to the electro-oxidation. Since the coated catalyst surface does not act like an ideal capacitor, therefore, to acknowledge the surface irregularities, a constant-phase element (CPE) was utilized to model the double-layer capacitance of the electrode–electrolyte interface. The fitted data for all the catalysts for a potential range of 0.35 V–0.65 V could be found in the Supporting Information (Tables S3–S6). It was identified from Figure A,D,G,J, for the highest potential at 0.65 V, only one peak was observed. Along with that, according to Figure C,F,I,L, only one semicircle was observed. Therefore, it was hypothesized that only Ni²⁺ oxidation was taking place. Keeping that in mind, a simple Randles circuit depicted in Figure A was utilized to fit the frequency response. For simplicity of comparison at higher polarization potential, the fitted data and goodness of fit (χ²) for synthesized catalysts under 0.65 V are compared in Table . According to the fitted results, we can see that with iron doping, charge-transfer resistance toward Ni²⁺/Ni³⁺ reaction had declined. In the case of N97F3_OH, a little bit of increase was noticed, which could be due to the catalyst poisoning effect. However, N95F5_OH exhibited remarkably low resistance toward NiOOH formation. The reason could be the presence of highly conductive iron­(III) on the catalyst surface, which resulted in an overall decrease of charge-transfer resistance. Due to the drop in charge transfer resistance, easy facilitation of UOR was possible.

7.

(A) Representative diagram of the fitted circuit from the EIS data and (B) chronoamperometry of synthesized catalysts with 1 M KOH + 0.3 M urea solution.

1. Electrochemical Impedance Parameters from the Fitted Impedance Data at 0.65 V.

				CPE 1
sl. no.	catalysts	R s (Ohm)	R p (Ohm)	Y 0 (Mho*s n )	n	χ2
1	N100F0_OH	15.9	34.3	0.0028732	0.558	0.778
2	N99F1_OH	17.25	30.5	0.00019315	0.733	0.06
3	N97F3_OH	18.47	33.88	0.0011124	0.609	0.091
4	N95F5_OH	18	16.16	0.00012273	0.828	0.351

The long-term catalyst stability under high polarization potential was analyzed using chronoamperometry in 0.3 M urea at 0.65 V for 6 h. The results of the long-term chronoamperometric studies are displayed in Figure B. N100F0_OH exhibited an initial current density of 1.9 mA cm^–2, and it decreased by 60.68% in 1 h. After 6 h, the current density decreased to 0.65 mA cm^–2, which is 66.88% lower than the initial current density. Similarly, N99F1_OH showed an initial current density of 1.19 mA cm^–2, which decreased by 45.23% in 1 h, and the current density decreased to 0.27 mA cm^–2 in 6 h (77.59% change). 1.31 mA cm^–2 current density was exhibited by N97F3_OH initially. After 1 h, there was a 37.32% decrease to 0.82 mA cm^–2, which later decreased up to 0.35 mA cm^–2 (73.56% change) in 6 h. N95F5_OH demonstrated 55.86% in 1 h. In 6 h, the current density decreased by 75.46% to 0.32 mA cm^–2. Overall, for the initial first hour, N97F3_OH exhibited superior catalytic stability due to a higher number of active sites and better resistance toward intermediate poisoning. But in long-term conditions, all the catalysts exhibited similar current density, which suggests deactivation of the catalysts. The observed current density after 6 h was in a similar range to that of UOR on a bare GC electrode. Therefore, it was predicted that over a prolonged reaction period, catalyst element loss through metal leaching may occur. Therefore, tuning the catalyst for better anti-poisoning activity will be the key to achieving longer stability and better activity.

4. Conclusion

In this work, the multifaceted effects of Fe³⁺ doping into aggregated α-Ni­(OH)₂ sheets were analyzed for their electrocatalytic activity for UOR. 1, 3, and 5 mol % Fe³⁺ were used to dope the catalyst sheets. The synthesized catalyst exhibited an amorphous nature. EDX and XPS analyses confirmed successful doping of iron in bulk as well as surface sites. With CV analysis, it was confirmed that the double-layer capacitance value was highest for 1% doping (20.3 μF), and for UOR, 3% iron doping promoted the formation of the most highly valent nickel species among all the catalysts. Iron­(III) acts as a Lewis acid to reduce the electron density over the low-valent Ni²⁺ ions and promotes the inception of Ni³⁺ species, which acts as a vital element for the UOR mechanism. Catalyst with 3% iron doping exhibited the lowest onset potential of 0.37 V and the highest amount of current density of 6.13 mA cm^–2 under 0.65 V among all of the synthesized catalysts. Tafel slope values of N97F3_OH were indicative of excellent kinetic activity for OER (51.66 mV/dec) and UOR (44.41 mV/dec). At low potential, direct UOR and, at high potential, indirect UOR were identified through EIS studies. Apart from that, iron doping was identified to reduce the overall susceptibility to catalyst poisoning by altering the RDS and promoting structural stability. Three mol % iron­(III) doping was identified to be the optimum doping concentration to promote the UOR mechanism through a balance between higher generation of active sites, favorable reaction kinetics, and high CO₂ poisoning tolerance. This work may provide fundamental insights into the effect on the electronic environment due to optimization of iron doping for better design of electrocatalysts for urea-based waste valorization.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/cbe.5c00161.

• EDX, CV analysis performed at various scan rates in the nonfaradic region, double-layer capacitance plot, ECSA of synthesized catalysts, and EIS fitting data (PDF)

The authors declare no competing financial interest.