Hydroxide Ion Oxidation in Aqueous Solutions Using Boron-Doped

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Hydroxide Ion Oxidation in Aqueous Solutions Using Boron-doped Diamond Electrodes * Irkham, Takeshi Watanabe, and Yasuaki Einaga Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 10, 2017

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Analytical Chemistry

Hydroxide Ion Oxidation in Aqueous Solutions Using Boron-doped Diamond Electrodes Irkham,1 Takeshi Watanabe,1 and Yasuaki Einaga*1,2 1 2

Department of Chemistry, Keio University, 3−14−1 Hiyoshi, Yokohama 223−8522, Japan JST-ACCEL, 3−14−1 Hiyoshi, Yokohama 223−8522, Japan

E-mail: [email protected]

Abstract The electrochemical oxidation behavior of hydroxide ions at the surface of boron-doped diamond (BDD) electrodes is presented. The hydroxide ion oxidation behavior was found to be affected by the surface conditions of the BDD electrode. Over the NaOH concentration range 0.5 – 10 mM, a well-defined voltammetric wave attributed to hydroxide ion oxidation was observed at ~1.25 V versus a Ag/AgCl reference electrode when using an anodically oxidized BDD (AO-BDD) electrode, while it was observed at around ~1.15 V when a cathodically reduced BDD (CR-BDD) electrode was used. Although the hydroxide ion oxidation profiles were slightly different for the AO-BDD and CR-BDD electrodes, the peak currents was each found to have linear relationships with the NaOH concentration over the same range.

Key words Hydroxide Ion, Oxidation, Boron-doped diamond electrodes

1. Introduction Hydroxide ions are used worldwide in many industrial processes and research, including applications in the electrochemical field.1,2 Therefore it is essential to study the behavior of hydroxide ions using an electrochemical approach. Generally, hydroxide ions can be directly oxidized at the surface of an electrode with the reaction: OH− → ¼O2 + ½H2O + e

(1)

The electrochemical oxidation of hydroxide ions occurs at high positive potentials, typically in the region of 1.3 to 1.6 V (vs. Ag/AgCl).2,3 In aqueous solutions, this high overpotential makes the phenomenon quite difficult to observe due to overlapping of the oxidation wave with the oxygen evolution reaction (OER). Despite this, reports have shown that hydroxide ion oxidation can still be observed on metal electrodes such as gold, platinum, or nickel.4 Daniele et al. have already explored the electrochemical oxidation of hydroxide ions in aqueous solutions using gold microelectrodes in a steady state system.4–6 It has been demonstrated that the steady-state limiting current is proportional to the hydroxide ion concentration.4 Moreover, Mentus et al. has shown that cathodic pretreatment of gold electrodes influences the peak of the hydroxide ion oxidation.3 Later, Banks et al. reported that hydroxide ion oxidation could also be observed in the low micro-molar to milli-molar range using a nickel oxide screen-printed electrode.2 However, without special geometry or a steady state electrode system, formation of the background oxide on the metal electrode,

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which overlaps with the oxidation of the hydroxide ions, makes it difficult to observe hydroxide ion oxidation. On the other hand, the use of boron-doped diamond (BDD) as an electrode material has gained much attention due to its higher durability, smaller background current, and higher overpotential for the oxidation of water compared to conventional electrodes. In particular, BDD has been used in many applications, such as sensors,7,8 electrosynthesis,9,10 metal recovery,11 CO2 reduction,12 and so on. Despite its popularity, there have yet to be any reports concerned with the study of hydroxide ion oxidation at BDD electrodes. Since BDD has a high overpotential for the oxidation of water, it should be possible to distinguish the oxidation wave due to hydroxide ions from the OER. In addition unlike metals electrodes, no oxide layer forms on BDD electrodes. This makes the study of hydroxide ion oxidation even easier, without the need of any particular geometry or electrode system such as micro-sized electrodes or a steady state system. In this paper we report on a study of hydroxide ion oxidation at BDD electrodes in basic aqueous solutions. The study includes the effect of a surface pretreatment, that were conducted before each measurement, of the BDD electrodes on the behavior of the hydroxide ions oxidation. This is also important because pretreatment of BDD electrodes has been reported as producing different surface conditions, making differences to the electrochemical behavior.13–16

2. Experimental Materials. All the reagents were obtained commercially (WAKO, Japan) and used without further purification. Pure water was doubly distilled with a maximum conductivity of 18 MΩ, obtained from a Simply−Lab water system (DIRECT-Q 3 UV, Millipore). Preparation of BDD. The BDD films were deposited on silicon (111) wafers using a microwave plasma-assisted chemical vapor deposition (MPCVD) system (CORNES Technologies / ASTeX−5400). Acetone and trimethoxyborane were used as the carbon and boron sources, respectively, with an atomic ratio of B/C = 1%. The surface morphology of the BDD was examined with a field emission scanning electron microscope (SEM, JEOL JCM-6000). Raman spectra were recorded with an Acton SP2500 (Princeton Instruments) with excitation at 532 nm from a green laser diode at ambient temperature. Electrochemical Measurements. The electrochemical measurements were conducted with a potentiostat (PGSTAT302N, AUTOLAB Instrument) using a single-compartment threeelectrode cell with 1% BDD as the working electrode, a Pt spiral as the counter electrode, and Ag/AgCl (Saturated KCL) as the reference electrode. The electrode area of the BDD was 0.096 cm2. Unless otherwise specifically mentioned, there were two pretreatment steps conducted before each measurement to stabilize the surface of the electrode. The first step was electrochemical cleaning by performing 10 voltammetric cycles between −3.0 and 3.0 V. The second step was either anodic oxidation by 10 voltammetric cycles between 0 and 3.0 V or cathodic reduction by 10 voltammetric cycles between 0 and −3.0 V. These BDD electrodes are hereafter referred to as anodically oxidized BDD (AO-BDD) and cathodically reduced BDD (CR-BDD) electrodes, respectively. The entire pretreatment was conducted at a scan rate of 300 mV/s in 0.1 M NaClO4 solution. All the measurements were carried out at room temperature after bubbling N2 gas through the solutions for 1 hour to remove dissolved oxygen.

3. Result and discussion 3.1 Hydroxide Ion Oxidation at AO-BDD


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The SEM image of the polycrystalline BDD used in these experiments shows triangular shaped (111) facets, which are known to be electrochemically more active than square shaped (100) facets, (Figure 1a).17–21 The Raman spectrum of highly boron-doped diamond shown in Figure 1b is typical, with the zone-center optical phonon of diamond observed as a shoulder peak at around 1300 cm−1. Moreover, the Raman spectrum shows no peak for sp2-bonded carbon around 1500 cm−1.22 Figure 2a (solid line) displays a linear sweep voltammogram (LSV) using an AOBDD electrode in 1.0 mM NaOH in 0.1 M NaClO4 solution after bubbling N2 gas through it for 1 hour. Bubbling was performed to remove the dissolved oxygen from the solution because this might influence the equilibrium of reaction 1. The peak at around 1.25 V is assumed to be due to hydroxide ion oxidation at the surface of the electrode. The oxidation peak could not be observed both on a glassy carbon electrode and a platinum electrode. Waves similar to those shown in Figure 2a (solid line) have been reported as being due to hydroxide ion oxidation at metal electrodes in steady state systems, although a limited current wave instead of a peak was formed.2–4,6,23 In the present case, hydroxide ion oxidation can be observed at a BDD electrode without any particular electrode system or cell geometry. Such peaks near the OER have often been reported as the oxidation peak for sp2-bonded carbon at BDD electrodes.24–28 However, as previously mentioned, the sp2-bonded carbon peak was not observed in the Raman spectrum of the BDD used in this experiment (Figure 1b). Additionally, there were no peaks observed in the background measurement, for which there was no NaOH (Figure 2a, dotted line). In order to confirm whether the main peak is due to hydroxide ion oxidation or surface oxidation of sp2-bonded carbon, LSV measurements with a range of scan rates were also conducted (Figure 2b). The results show that the peak current is proportional to the square root of the scan rate. Regression analysis of the experimental data yielded (ip/µA) = 1.49 × 10–6 + 4.71 × 10–5 (V/s)½ r2 = 0.992. This indicates that the main peak is due to a diffusion-controlled reaction rather than surface oxidation of the electrode. Thus, our conclusion is that the main peak is due to the oxidation of hydroxide ions. Direct observation of hydroxide ion oxidation with a BDD electrode is possible due to the high overpotential for the OER at a BDD electrode, leaving the hydroxide ion oxidation separate from the OER which occurs at a more positive potential. Moreover unlike metal electrodes, no oxide is formed at the surface of the BDD electrode, making it easier to observe the oxidation of hydroxide ions. 3.2 Effect of Surface Pretreatment on Hydroxide Ion Oxidation at BDD Electrodes It has been reported that BDD shows different electrochemical behavior with different surface pretreatments.13,14,29–31 In this experiment, two kinds of pretreatment were done before each measurement, anodic oxidation (AO-BDD) and cathodic reduction (CRBDD). Anodic oxidation has been reported as being able to convert the surface termination of BDD into oxygen termination.16,32,33 It has also been reported that anodic oxidation of asdeposited BDD increases the oxygen to carbon (O/C) ratio from 0.02 up to 0.3.16,32,33 Characterization of such an anodically-oxidized BDD surface by XPS shows that it contains many C-O functional groups.16,29,32,34–36 On the other hand, cathodic pretreatments have been reported as being able to reduce the O/C ratio of the BDD surface.13 Compared to AO-BDD, the hydroxide oxidation peak potential for CR-BDD is slightly shifted toward a more negative potential (Figure 3a). One possible reason for this result is the electrostatic interaction between the surface and the hydroxide ions. An AO-BDD surface that is rich in C-O functionalities is believed to be in an electrostatically negative state due to the relatively high electronegativity of the oxygen atoms.32 The relatively negative surface of AO-BDD gives higher repulsion to hydroxide ions, which also have negative charge, making it harder for hydroxide ions to be oxidized at the surface of the electrode. As a result, the oxidation potential of hydroxide ions was higher with AO-BDD. Additionally, there is a noticeable difference between the background currents for AO-BDD and CR-BDD. The difference perhaps is a result of the cathodic pretreatment, which reduces some electrochemically active



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species accumulated at the surface of the BDD. This electrochemically active species then slowly oxidizes in the first scan in the positive direction, contributing to the total current with CR-BDD, causing a higher background current compared to AO-BDD. It can also be seen from Figure 3a that there is a shoulder wave at around 0.90 V using CR-BDD (referred to as a pre-peak). This kind of behavior has previously been reported for a gold electrode.3,4,6 Considering that the pre-peak was observed only with CRBDD, it might be due to oxidation of hydroxide ions at “reduced sites” on the BDD, which had become active as a result of the cathodic reduction pretreatment. To reinforce this hypothesis, various numbers of cycles of the cathodic pretreatment were conducted (Figure 3b). It can be seen that the pre-peak increases along with the increasing number of cathodic pretreatment cycles. This is perhaps due to the increase in the number of activated “reduced sites” at the BDD surface with the increasing number of reduction cycles leading to an increase in the current signal in the pre-peak. Further increases in the number of cathodic pretreatment cycles did not increase the pre-peak current (data not shown). This might be due to the limited number of “reduced sites”, which would limit the current in the pre-peak. To get a better understanding of the behavior of the pre-peak with the CR-BDD electrode, consecutive measurements without any treatment between each measurement were conducted. From Figure 4a, it can be seen that the baseline current of the second scan up to ~0.75 V has decreased, similar to that with an AO-BDD electrode, which means that the electrochemically active species at the CR-BDD electrode had been eliminated from the surface during the first scan. When the first measurement was scanned to 2.0 V, the pre-peak was not observed in the second scan (Figure 4a, blue line). This suggests that the “reduced sites” on the CR-BDD electrode were perhaps oxidized during the first LSV scan, where the potential was scanned to 2.0 V. In contrast, when the potential of the first scan went up to 1.50 V only, the second scan still had a pre-peak (Figure 4a, red line) which indicates that the potential was insufficient to oxidize the “reduced sites” on the CR-BDD electrode, yet sufficient to eliminate the electrochemical species accumulated on the surface. Moreover, the pre-peak in this second scan was higher than in the first scan. These results suggest that the electrochemically active species eliminated during the first scan were inhibiting the oxidation of hydroxide ions at the “reduced sites” on the BDD. Thus, when the potential of the first scan went up to 1.50 V, the pre-peak in the second scan could increase. On the other hand, when a greater number of consecutive scans up to 2.0 V were conducted (with new solutions each time), the current in the oxidation peak declined and shifted toward a more positive potential, indicating that the surface of the BDD changes in the oxidation process (Figure 4b). Empirically, with a BDD electrode, fairly good signal stability can be obtained by employing an electrochemical pretreatment, as mentioned in the experimental section, which is difficult to apply to other types of electrode. In order to see the correlation between the hydroxide ion concentration and the current peak, LSV measurements at different NaOH concentrations in 0.1 M NaClO4 solution using an AO-BDD electrode were conducted (Figure 5a). The peak current at around 1.25 V was plotted as a function of hydroxide ion concentration in the range from 0.5 mM to 10 mM. Regression analysis of the experimental data yielded (ip/µA) = –6.23 × 10–6 + 1.21 × 10–5 (mM) and r2 = 0.998. Meanwhile, the peak current at around 1.15 V derived from the CRBDD electrode also gives good linearity versus the concentration of hydroxide ions in the solution (Figure 5b). Regression analysis of the experimental data yielded (ip/µA) = –5.64 × 10–6 + 1.71 × 10–5 (mM) and r2 = 0.998. This good linearity indicates that BDD electrodes can be used to evaluate the concentration of hydroxide ions. Note that the experiments were carried out with bare BDD without any special geometry and without optimization of the experimental conditions. Nevertheless, the results show that AO-BDD is more suitable for evaluating the concentration of hydroxide ions, since the pre-peak phenomenon does not occur and it gives more stable measurements. 3.3 Hydroxide Ion Behavior in Other Base Solutions LSV measurements in different base solutions were also carried out. Figure 6 displays LSVs of different 1.0 mM base solutions in 0.1 M NaClO4 solution. For the same


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concentration, the solution containing Ca(OH)2 has a bigger peak current compared to NaOH. This is simply due to there being twice the amount of hydroxide ions in the Ca(OH)2 solution compared to the same concentration of NaOH. For the same reason, the peak current for the KOH solution was found to be comparable to that for the NaOH solution for the same concentration. Meanwhile, the peak current for NH3 was found to be the lowest among the compounds shown in Figure 6. This is due to the fact that in NH3, which is categorized as a weak base, the dissociation of hydroxide ions in aqueous solution is incomplete (reaction 2) whereas in a strong base compound such as NaOH, the dissociation is complete. This makes the amount of hydroxide ions in NH3 solution smaller compared to NaOH solution at the same concentration. These results further confirm the hypothesis that the peak is due to hydroxide oxidation. NH3 + H2O → NH4+ + OH−

pKa = 9.25

(2)

4. Conclusion The oxidation behavior of hydroxide ions at BDD electrodes was investigated. Oxidation of hydroxide ions was observed at BDD electrodes without the need for either special geometry or a special electrode system. This was possible due to the high overpotential of the OER, which meant that there was no competing oxide formation mechanism at the surface of the electrode. The oxidation behavior was found to depend on the surface condition of the electrode. In comparison to AO-BDD, the peak for hydroxide ion oxidation with a CR-BDD electrode was at a more negative potential and showed a pre-peak which was interpreted as being due to hydroxide ion oxidation at “reduced sites” on the BDD surface. The values of the peak currents at both the AO-BDD and CR-BDD electrodes were similar for the same NaOH concentration, and each showed good linearity versus hydroxide ion concentration. To evaluate the hydroxide ion concentration, AO-BDD was judged to be the more suitable electrode material compared to CR-BDD due to the measurement stability.



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Figure captions Figure 1. (a) SEM image and (b) Raman spectrum of 1% BDD. Figure 2. (a) LSVs recorded at 20 mV/s with an AO-BDD electrode in 0.1 M NaClO4 solution with (solid line) and without (dotted line) the presence of 1.0 mM NaOH. (b) LSVs at various scan rates in 1.0 mM NaOH with 0.1 M NaClO4 using an AO-BDD electrode. Inset: peak current as a function of the square root of the scan rate. The potential was scanned from 0 V to 2.0 V. Figure 3. (a) LSV measurements in 0.1 M NaClO4 aqueous solution with the absence (dotted line) and presence (solid line) of 1.0 mM NaOH at a scan rate of 20 mV/s on AO-BDD (black) and CR-BDD (red) electrodes. The potential was scanned from 0 V to 2.0 V. (b) LSVs of 1.0 mM NaOH in 0.1 M NaClO4 solution with different numbers of voltammetric cycles of the cathodic pretreatment. The potential was scanned from 0 V to 1.50 V. Figure 4. (a) 1st and 2nd consecutive LSVs in 1.0 mM of NaOH in NaClO4. The black line shows the 1st LSV scan, the red line is the 2nd scan after scanning to 1.50 V, and the blue line is the 2nd scan after scanning to 2.0 V. (b) Peak current observed in consecutive LSV measurements in 2.0 mM NaOH in 0.1 M NaClO4 with and without pretreatment, respectively. The potential was scanned from 0 V to 2.0 V. Procedure of “without pretreatment” is “pretreatment – 1st scan – 2nd scan – 3rd scan – 4th scan – 5th scan”, while procedure of “with pretreatment” is “pretreatment – scan – pretreatment – scan – pretreatment – scan – pretreatment –scan – pretreatment – scan”. The solutions were fresh in each measurement. Inset: consecutive LSVs in 2.0 mM NaOH in 0.1 M NaClO4 without pretreatment. The working electrode was a CR-BDD electrode and the scan rate was 20 mV/s. Figure 5. LSVs for different concentrations of NaOH in 0.1 M NaClO4 solution with a scan rate of 20 mV/s at (a) AO-BDD and (b) CR-BDD electrodes. The potential was scanned from 0 V to 2.0 V. Inset: peak current as a function of NaOH concentration. Figure 6. LSVs of different base compounds in 0.1 M NaClO4 solution with a scan rate of 20 mV/s at a AO-BDD electrode. The potential was scanned from 0 V to 2.0 V.


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Hydroxide Ion Oxidation in Aqueous Solutions Using Boron-Doped

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