Article pubs.acs.org/JPCC
Experimental Investigation of Potential Oscillations during the Electrocatalytic Oxidation of Urea on Ni Catalyst in Alkaline Medium Vedasri Vedharathinam and Gerardine G. Botte* Center for Electrochemical Engineering Research Chemical and Biomolecular Engineering Department, Ohio University, 182 Mill Street, Athens, Ohio 45701, United States ABSTRACT: For the first time, potential oscillations are observed during the electrochemical oxidation of urea on Ni catalyst in alkaline medium. Electrochemical measurements, in combination with in situ time-resolved surface enhanced Raman (SER) spectroscopy studies, revealed that the potential oscillations might be caused by the periodic reduction and oxidation of the electrochemically active NiOOH catalyst to its inactive Ni(OH)2 state and vice versa. Cyclic voltammetry and rotating disc electrode voltammetry suggests that mass transfer of urea to the double layer region due to local convection caused by oxygen evolution reaction does not play a role in the potential oscillation mechanism.
1. INTRODUCTION Numerous electrochemical systems exhibit complex nonlinear behavior such as periodic oscillations of potential or current, when retained far from thermodynamic equilibrium.1−3 Electrochemical oscillation is a remarkable phenomenon where the potential or current fluctuates periodically when a constant electric field is applied.4 Various researchers have reported the occurrence of electrochemical oscillations during the oxidation of formic acid,5,6 formaldehyde,2,7 methanol,8,9 and sulfide10,11 on Pt and Pt-group catalysts in pH varying from acidic to alkaline medium. The observed oscillatory behavior in the oxidation of small organic molecules into CO2 on Pt and Pt-group catalysts is due to the formation and removal of COads (poisonous intermediate) via an indirect pathway.2 This theory has been elucidated by a variety of in situ techniques such as time-resolved surface enhanced infrared absorption spectroscopy (SEIRAS)5,12 and electrochemical quartz crystal microbalance (EQCM).2 This category of oscillators has been classified as the charge transfer electrode process coupled with surface adsorption and desorption. Another type has been described in the literature, where the diffusion-limited depletion and the convection-enhanced replenishment of the electroactive species due to gas evolution constitutes the oscillation mechanism.13 Li et al. demonstrated the oscillatory mechanism involving mass transfer during the electrocatalytic reduction of iodate on Au in alkaline medium.1 They have also observed a similar mechanism during the electrodissolution of Au in acidic medium.14 In recent years, inexpensive non-Pt group metal catalysts have attained much attention due to their application in fuel cells15,16 and electrolysis.17,18 Nickel has been successfully demonstrated to possess excellent electrocatalytic activity toward oxidation of small organic molecules.19,20 Literature related to the electrochemical oscillation of organic molecules © 2014 American Chemical Society
on a non-Pt group catalyst, like nickel, is limited. Huang et al. reported the electrochemical oscillation mechanism of methanol and amino compounds on Ni(OH)2 film electrodes in alkaline medium.21−23 They reported the coupled charge transfer with diffusion and convective mass transfer accounts for the oscillation mechanism. Such behavior has been reported for the oxidation (e.g., sulfide)11 and reduction (e.g., IO3−)1 of several inorganic molecules in alkaline medium. Lately, the electrocatalytic oxidation of urea on Ni catalyst in alkaline medium has gained considerable attention due to its application in wastewater remediation,24,25 fuel cells,26 hydrogen production,27−31 and sensors.32,33 In alkaline medium, urea can be electrochemically oxidized to CO2 and N2 at the anode, while H2, a valuable fuel, is produced at the cathode.29 Vedharathinam and Botte have demonstrated in previous publications34,35 that the electrochemical oxidation of urea follows an indirect oxidation mechanism, where the process is mediated through the Ni(OH)2/NiOOH redox pair. To the best of our knowledge, there is no literature on the electrochemical oscillation mechanism of urea. For the first time, we report a new oscillatory mechanism for the electrochemical oxidation urea on Ni catalyst in alkaline medium by means of electrochemical measurements and in situ time-resolved Raman spectroscopy. Henceforth, the objectives of the investigation are (1) to determine the oscillatory instability and (2) to investigate the mechanism of electrochemical oscillation in the anodic oxidation of urea on Ni catalyst. Received: May 28, 2014 Revised: August 10, 2014 Published: August 28, 2014 21806
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2. EXPERIMENTAL METHODS 2.1. Chemicals and Reagents. The chemicals and supplies used were of high purity (>99.90%) and analytical grade supplied from Fisher Scientific. Ultrapure water (Alfa Aesar, HPLC grade) was used throughout the investigation. 2.2. Anode Preparation. The electrochemical measurements were performed on Ni foil as working electrode. The surface of the Ni foil was roughened using a sandblaster (Crystal Mark sandblaster, 27.5 μm aluminum oxide powder) under dry conditions at 60 psi, followed by sonication (Zenith Ultrasonic bath at 40 kHz) in a solution containing a 1:1 ratio of water and acetone for 10 min. The Ni electrode thus prepared was rinsed with ultrapure water and used for the electrochemical analysis. To obtain surface enhanced Raman (SER) spectra, the Ni(OH)2 catalyst was electrodeposited on electrochemically roughened Au foil (0.50 × 0.50 cm2, Alfa Aesar, 0.025 mm thick, 99.98%). The Au foil was roughened electrochemically by a procedure that has been described elsewhere.36 The surface roughened Au foil was then removed and washed with plenty of deionized water. The Ni(OH)2 catalyst layer was electrodeposited on the roughened Au substrate under cathodic galvanostatic conditions at 50 μA for 1200 s in 0.01 M Ni(NO3)2 (Fisher Scientific, >99.90%) using a Pt foil counter electrode (Sigma-Aldrich, 0.05 mm thick, 99.99%). A Solartron 1281 multiplexer potentiostat was used during the electrodeposition of the catalyst layer. The Ni(OH)2 electrode, thus prepared, was washed with deionized water and used as working electrode for in situ time-resolved SER spectroscopy. 2.3. Electrochemical Testing. The electrochemical tests were carried out in a conventional three-electrode cell using a Solartron 1281 multiplexer potentiostat. The cell setup consists of a Ni foil (1 × 1 cm2, Alfa Aesar, 0.025 mm thick, 99.98%) working electrode, Pt foil (2 × 2 cm2 Sigma-Aldrich, 0.05 mm thick 99.99%) counter electrode, and Hg/HgO reference electrode supported in a luggin capillary filled with 5 M KOH (electrolyte) solution. The 10th pseudosteady state voltammogram, sustained periodic state, is reported in this paper. The electrochemical impedance spectroscopy (EIS) experiments were conducted using a Solartron 1281 multiplexer potentiostat in conjunction with a Solartron model SI 1287 frequency response analyzer. The AC amplitude was maintained at 10 mV for all frequency ranges. The measurements were made between 100 kHz and 0.005 Hz. The in situ time-resolved surface spectroscopy was carried out using a Solartron 1281 multiplexer potentiostat in a threeelectrode cell setup with Ni(OH)2 deposited on Au foil as working electrode, Pt foil as counter electrode, and Hg/HgO as reference electrode. The time-resolved SER spectra were recorded at room temperature using a Bruker Senterra Raman spectrometer. The spectrometer was equipped with a microscope (10× objective lens), 50 mW power, and 785 nm laser excitation wavelength.
Figure 1. Cyclic voltammogram of Ni electrode obtained in (a) 5 M KOH and (b) 0.33 M urea + 5 M KOH solution at a scan rate of 10 mV s−1. The inset shows the voltammogram with different lower potential limits of 0.58 V (gray line) and 0.7 V (black line). The noticeable crossed loop during urea oxidation is characteristic of systems that can exhibit electrochemical oscillations.
urea mediated through NiOOH/Ni(OH)2 redox couple (above 0.4 V) followed by oxygen evolution reaction at higher potentials (above 0.6 V). The descending branch noticed between the two ascending branches can be attributed to the diffusion-limited depletion of urea. However, it has been shown in our previous publication that the descending branch still persists even after rotating the electrode at 2000 rpm (revolutions per minute), suggesting that the branch is most likely controlled by kinetics rather than diffusion.34 Furthermore, an oxidation peak appears in the reverse scan of the voltammogram. The occurrence of an oxidation peak in the reverse scan has been noticed in several electrochemical reactions. The peak has been attributed to the replenishment of depleted electroactive species by the local convection caused by oxygen evolution in the forward scan.1,13,21−23 We have previously established that the electrochemical oxidation of urea on Ni catalyst in alkaline medium is a mixed control process, that is, controlled by both diffusion and electrode reaction kinetics.34 Hence, the probable effect of mass transfer through the oxygen evolution reaction has to be considered. To investigate the hypothesis, the cyclic voltammogram was stopped at 0.58 V, the potential before the oxygen evolution reaction, thus avoiding the enhanced mass transfer due to oxygen evolution. The inset shows the voltammogram obtained with (black line) and without (gray line) oxygen evolution during the electrochemical oxidation of urea. The voltammogram obtained in the absence of oxygen evolution reaction exhibits a crossed loop and a urea oxidation peak in the reverse scan. Henceforth, it can be concluded that local convection due to oxygen evolution does not play a major role in the urea oxidation peak observed in the reverse scan. 3.2. Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy (EIS) is a powerful technique to investigate the nonlinear electrochemical oscillations.2,37,38 Figure 2 shows the EIS spectra obtained at different potentials through the electrochemical oxidation of urea on Ni electrode in 0.33 M urea + 5 M KOH solution. The applied DC potentials for EIS experiments were chosen from the cyclic voltammogram for urea oxidation. The EIS spectra show different impedance behaviors at various applied potentials. A semicircle is noticed at 0.48 V, which is due to the urea oxidation charge transfer reaction. When the potential
3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetry. Figure 1 shows the cyclic voltammogram of Ni catalyst obtained in the absence and presence of 0.33 M urea in 5 M KOH solution. A detailed explanation of the voltammogram has been discussed in our previous publications.34,35 The voltammogram shows two ascending branches during the forward scan, which signifies two different reactions, that is, electrochemical oxidation of 21807
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Figure 3. Galvanodynamic polarization plot of Ni catalyst obtained in 0.33 M urea + 5 M KOH solution at a scan rate of 1 μA s−1. The polarization curve demonstrates that the potential oscillation starts before the limiting current plateau, followed by oxygen evolution reaction. This suggests that mass transfer may not play a predominant role in the oscillation mechanism.
Figure 2. (a) EIS spectra obtained as a function of applied potential during the electrochemical oxidation of urea on Ni catalyst in 0.33 M urea + 5 M KOH solution: (a) 0.48 V, (b) 0.57 V, and (c) 0.58 V. The EIS spectrum obtained at 0.57 V shows negative impedance for nonzero finite frequencies, suggesting the possibility of galvanostatic potential oscillations during the urea oxidation process.
occurrence of potential oscillations before the limiting current plateau and the oxygen evolution reaction (∼6 mA cm−2), thus indicating that mass transfer is not involved in the oscillatory mechanism. Also, the amplitude of the oscillations depends on the applied current density and is located in the potential range from 0.42 to 0.6 V, where urea is electrochemically oxidized (Figure 1). It should be noted that the potential oscillations occur on the positive slope of the galvanodynamic curve confirming the presence of H-NDR behavior, as discussed in section 3.2. The inset shows the expanded region of the oscillation window to clearly demonstrate the shape of the potential oscillations. The shape of the potential oscillations will be explained further in the following sections. 3.4. Galvanostatic Potential Oscillation. The effect of urea concentration and applied current on potential oscillations was investigated and the results are shown in Figures 4 and 5. Figure 4 shows the galvanostatic potential oscillations obtained in 0.1 M urea + 5 M KOH solution at 10 and 11 mA. At an applied current of 10 mA, the potential starts oscillating
was increased to 0.57 V, the EIS spectrum changed its shape dramatically, where the loop in the first quadrant reverses to the second and third quadrants. The real impedance is positive in the high frequency region, followed by negative impedance in the middle and low frequency regions. The system with negative impedance for nonzero finite frequencies can exhibit unstable behavior leading to electrochemical oscillations.37,38 Hence, it can be stated that the electrochemical oxidation of urea on Ni catalyst may exhibit oscillations. The N-shaped cyclic voltammogram in Figure 1 indicates that the electrochemical oxidation of urea belongs to either negative differential resistance (N-NDR) or hidden negative differential resistance (H-NDR), where the electrical double layer potential is an autocatalytic variable that leads to the negative differential resistance.4,10 The low frequency end of the EIS spectrum has negative real impedance for a finite range of nonzero frequencies (third quadrant), but has positive impedance (fourth quadrant) for the lowest frequencies. The negative impedance cannot be noticed in the polarization curve and hence is “hidden”, but does lead to potential oscillations at fixed applied current. Such electrochemical oscillatory behavior belongs to the H-NDR category. From the above-mentioned EIS spectra, one can infer that the electrochemical system will lead to an oscillatory instability under galvanostatic control. When the potential is increased further to 0.58 V, the EIS spectra go back to a semicircle with positive impedance and an inductive arc at low frequency region. The inductive behavior at low frequencies is characteristic of electrochemical systems capable of exhibiting galvanostatic potential oscillations.2 3.3. Galvanodynamic Polarization Measurement. Figure 3 shows the galvanodynamic curve obtained in 0.33 M urea +5 M KOH solution. The slow variation in current density acts as a bifurcation control parameter, thus directing the electrochemical system through an oscillatory window. The current density is scanned linearly from 0 to 10 mA cm−2. When the current density is scanned in the positive direction, potential oscillations are noticed around 2.3 mA cm−2 and vanish at 4.0 mA cm−2. The potential increases linearly until a limiting current plateau is reached, which is preceded by the oxygen evolution reaction. It is important to notice the
Figure 4. Chronopotentiometric curves of Ni electrode obtained in 0.1 M urea + 5 M KOH solution at different applied current: (a) 10 mA and (b) 11 mA. The galvanostatic potential oscillations obtained at low concentration of urea are loosely packed and have a shorter lifetime of 16 min. The amplitude of oscillation increases with the increment in applied current and vanishes at 11 mA when the potential reaches the oxygen evolution reaction. 21808
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Figure 5. Chronopotentiometric curves of Ni electrode obtained in 1 M urea + 5 M KOH solution at different applied current: (a) 2 mA, (b) 3 mA, (c) 4 mA, (d) 5 mA, (e) 6 mA, and (f) 7 mA. The inset shows the expanded region of the potential oscillations. The galvanostatic potential oscillations obtained at high urea concentration are densely packed and have a longer lifetime (60 min). The amplitude of oscillation increases with the increment in applied current and vanishes at 7 mA when the potential reaches the oxygen evolution reaction.
an induction period of 13 min. The potential increases slowly from 0.51 to 0.62 V and stays at the high potential region for few seconds after which it drops down steeply to a lower value. Similar oscillatory behavior was noticed at 3 mA. Between 4 and 7 mA, a plateau is reached in the high potential region and the oscillatory time of the plateau increases with applied current. Fewer oscillations are noticed at 7 mA, which fade away when the oxygen evolution potential is reached at 0.64 V. At low applied currents between 2 and 6 mA, monoperiodic (P1) potential oscillations are noticed. Further increasing the current to 7 mA results in a transition from monoperiodic (P1) to period-doubled oscillations (P2) with a large peak followed by a smaller one in one period. Since the P2 oscillations occur at high current, 7 mA, that is, close to the oxygen evolution region, it could be hypothesized that the competing oxygen
periodically after an induction period of 20 min. The potential oscillations persisted only for 16 min, after which they disappeared. The oscillations belong to the monoperiodic (P1) type with one peak in one period, where the potential jumps quickly from a high potential to a low potential value. The potential then drifts back to high potential. The amplitude of the oscillations increase with time, until the upper limit reaches the potential for oxygen evolution reaction, after which the oscillations disappear. The oscillation vanished when the applied current is increased to 11 mA. Figure 5 shows the chronopotentiometric curves obtained in 1 M urea + 5 M KOH solution at various applied potentials. When the solution was changed to higher concentration, the oscillations persisted for a longer time of 60 min. At an applied current of 2 mA, the potential starts oscillating periodically after 21809
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evolution reaction along with the periodic reduction and oxidation of NiOOH during the oxidation of urea may lead to the period doubled behavior. The results obtained in Figures 4 and 5 imply that the potential oscillations are strongly dependent on the concentration of urea and applied current. 3.5. Effect of Stirring on Potential Oscillation. It is imperative to examine the participation of convective mass transfer due to the oxygen evolution reaction in the electrochemical oscillation mechanism of urea on Ni catalyst. Figure 6 shows the effect of rotation on galvanostatic potential
Figure 7. In situ time-resolved SER spectra obtained at the Ni(OH)2 catalyst at an applied current of 10 mA: (a) 5 M KOH and (b) 0.33 M urea + 5 M KOH solution. In the presence of urea, the intensity of NiOOH doublet peak (479/559 cm−1) varies periodically with time, which indicates the transition between the electrochemically active and passive states of Ni catalyst.
Figure 6. Chronopotentiometric curves of Ni catalyst obtained at an applied current of 5 mA in 1 M urea + 5 M KOH solution at various rotation speeds: 0 and 2000 rpm. The potential oscillations continued even after rotating the electrode at 2000 rpm, inferring that the oscillations are not due to mass transfer.
3.7. Proposed Mechanism for Potential Oscillation. For the first time, potential oscillations are reported during the electrochemical oxidation of urea on Ni catalyst in alkaline medium. The experimental results confirmed that local convection due to oxygen evolution reaction is not involved in the oscillatory mechanism. Taking into account the SERS experimental observations, it is hypothesized that the oscillations could be caused by the surface reactions, that is, reduction/oxidation of NiOOH/Ni(OH)2 as explained next. We have previously reported that the electrochemical oxidation of urea on a Ni catalyst in alkaline medium follows an indirect oxidation mechanism, as per the following reactions.34,35
oscillations at a constant applied current of 5 mA. In the absence of rotation, the potential oscillations start at 0.74 V with an induction period of 180 s. After ∼600 s, the electrode was rotated at 2000 rpm. The potential drops steeply from 0.83 to 0.64 V, where the system does not show signs of oscillations. The potential increases slowly to 0.74 V, after which the periodic galvanostatic potential oscillations are noticed. The above result illustrates that the mass transfer of urea to the Ni catalyst surface due to local convection caused by oxygen evolution reaction does not play an imminent role in the potential oscillation mechanism. 3.6. Time Resolved SERS Monitoring of the Potential Oscillation. Figure 7a and b shows the in situ time-resolved SER spectra on Ni(OH)2 catalyst acquired in 5 M KOH and 0.33 M urea + 5 M KOH solution, respectively. The SER spectra were collected every 25 s at an applied current of 10 mA. The spectra obtained in 5 M KOH solution (Figure 7a) show a doublet band at 476 and 560 cm−1 that corresponds to the Ni−O bending and Ni−O stretching vibrations, characteristic of NiOOH species.35,39 No considerable change in the intensity of the doublet peak is noticed with time, which suggests that the surface concentration of NiOOH species is stable. A similar experiment was performed in 0.33 M urea + 5 M KOH solution in the presence of potential oscillations, and the corresponding SER spectra are shown in Figure 7b. Interestingly, the intensity of the doublet peak varies in phase with the potential oscillations. The periodic change in intensity indicates that the active form of nickel (NiOOH) for the oxidation of urea is fluctuating. Therefore, it is reasonable to think that this change in the nickel form could be associated with the potential oscillations as discussed in section 3.7.
At anode: E 6Ni(OH)2(s) + 6OH− ⇄ 6NiOOH(s) + 6H 2O(l) + 6e− (1)
C 6NiOOH(s) + CO(NH 2)2(aq) + H 2O(l) → 6Ni(OH)2(s) + N2(g) + CO2(g)
(2)
Net anodic reaction: EC′ CO(NH 2)2(aq) + 6OH− → N2(g) + 5H 2O(l) + CO2(g) + 6e−
(3)
The electrochemical oxidation of urea is mediated through the NiOOH/Ni(OH)2 redox couple (reaction 1). Due to the chemical oxidation of electrochemically active NiOOH to inactive Ni(OH)2 form by urea (reaction 2), the resistance of the electrode surface increases and hence the urea oxidation potential drops to a lower value. It has been reported in the 21810
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1-0001). The content of the information does not reflect the position or the policy of the U.S. government.
literature that the electrical conductivity of Ni(OH)2 is lower than that of NiOOH.40 Due to the high current density region and excess availability of OH− ions, the less conductive Ni(OH)2 is oxidized back to its conductive NiOOH state, which leads to an increase in potential. This process continues until the commencement of oxygen evolution reaction, after which the potential oscillation vanishes. An interpretation of the hidden negative impedance resistance (H-NDR) noticed in Figure 2b is discussed as follows. The EIS spectra exhibit a slow process with positive impendance and a fast process with negative impedance. It has been demonstrated in our previous publication that the electrochemical regeneration of Ni(OH)2 to NiOOH (reaction 1) is faster than the chemical reaction between NiOOH and urea (reaction 2) using in situ timeresolved Raman spectroscopy.35 The regeneration of the inactive Ni(OH)2 to active NiOOH catalyst (reaction 1) is sufficiently fast to anticipate a negative impedance, whereas the positive impedance is due to the slow chemical reduction of the active NiOOH to the inactive Ni(OH)2 by urea (reaction 2). The experimental data suggests that the self-healing phenomenon of Ni(OH)2 to NiOOH could be responsible for potential oscillations during the electrochemical oxidation of urea on Ni catalyst. However, to truly understand these oscillations, it is important to develop a mathematical model to theoretically validate the experimental findings.
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4. CONCLUSION Electrochemical methods and time-resolved SER spectroscopy were successfully employed to elucidate the potential oscillation mechanism during the electrochemical oxidation of urea on Ni catalyst. The observed crossed loop cyclic voltammogram and negative impedance in EIS are characteristics of electrochemical systems that exhibit oscillatory behavior. Galvanostatic potential oscillations are observed in the potential region where the electrochemical oxidation of urea takes place on Ni catalyst. The existence of potential oscillations even after rotating the Ni electrode at 2000 rpm implies that the mass transfer of urea to the catalyst surface by local convection due to oxygen evolution reaction does not play a key role in the oscillation mechanism. Hence, the involvement of diffusion and mass transfer of urea to the double layer region can be disregarded in the oscillatory mechanism. The change in intensity of the NiOOH doublet peak obtained by in situ timeresolved SER spectra revealed that the potential oscillations could be due to dominant surface reactions, as in indirect oxidation mechanism (reactions 1−3). The periodic formation and consumption of NiOOH, the active catalyst, could be the reason for the oscillatory mechanism. A mathematical model should be developed to verify the proposed mechanism.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Tel.: 740-593-9670. Fax: 740-593-0873. E-mail: botte@ohio. edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are thankful for the financial support of the Center for Electrochemical Engineering Research at Ohio University, and the Department of Defense through the U.S. Army Construction Engineering Research Laboratory (W9132T-0921811
dx.doi.org/10.1021/jp5052529 | J. Phys. Chem. C 2014, 118, 21806−21812
The Journal of Physical Chemistry C
Article
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