Highly Selective Active Chlorine Generation Electrocatalyzed by

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Highly Selective Active Chlorine Generation Electrocatalyzed by CoO Nanoparticles: Mechanistic Investigation through in situ Electrokinetic and Spectroscopic Analyses 3

4

Heonjin Ha, Kyoungsuk Jin, Sunghak Park, Kang-Gyu Lee, Kang Hee Cho, Hongmin Seo, Hyo-Yong Ahn, Yoon Ho Lee, and Ki Tae Nam J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00547 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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The Journal of Physical Chemistry Letters

Highly Selective Active Chlorine Generation Electrocatalyzed by Co3O4 Nanoparticles: Mechanistic Investigation through in situ Electrokinetic and Spectroscopic Analyses Heonjin Ha†, Kyoungsuk Jin†, Sunghak Park†, Kang-Gyu Lee†, Kang Hee Cho†, Hongmin Seo†, Hyo-Yong Ahn†, Yoon Ho Lee†, Ki Tae Nam†,* †

Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Korea

* [email protected] KEYWORDS Chlorine evolution reaction, Co3O4 nanoparticles, Electrocatalysis, Reaction mechanism, Electrochemical water treatment

ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT The reaction mechanism of electrochemical chloride oxidation at neutral pH is different from that at acidic pH, in which a commercial chlor-alkali process has been developed. Different proton concentrations and accelerated hydrolysis of the generated chlorine into hypochlorous acid at high pH can change the electrokinetics and stability of reaction intermediates. Here, we have investigated a unique reaction mechanism of Co3O4 nanoparticles for chloride oxidation at neutral pH. In contrast to water oxidation, the valency of cobalt was not changed during chloride oxidation. Interestingly, a new intermediate of Co-Cl was captured spectroscopically, distinct from the reaction intermediate at acidic pH. In addition, Co3O4 nanoparticles exhibited high selectivity for active chlorine generation at neutral pH, comparable to commercially available RuO2-based catalysts. We believe this study provides insight into designing efficient electrocatalysts for active chlorine generation at neutral pH, which can be practically applied to electrochemical water treatment coupled with hydrogen production.

TABLE OF CONTENTS


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Electrochemical water splitting, which converts water into hydrogen and dioxygen molecules, is a paramount pathway for the sustainable production of hydrogen energy.1 However, in spite of the remarkable advances in electrocatalysts for water electrolysis, the slow reaction kinetics of the oxygen evolution anodic reaction (OER) serve as the main bottleneck for the overall cost and efficiency of electrolytic hydrogen production.2-3 To overcome the current drawbacks of water electrolysis, it can be advantageous to replace OER with an oxidant generation reaction.4 Diverse oxidants including active chlorine (AC), hydrogen peroxide, ozone, and oxygen radicals can be electrochemically generated to compete with OER and utilized as an oxidizing species for electrochemical water treatment.5-6 Among the possible oxidants for electrochemical water treatment, AC is deemed an optimal oxidant on account of its effective capability to remove microorganisms, cost effectiveness, and long residual time.4 AC, which consists of dissolved chlorine (Cl2(aq)), hypochlorous acid (HClO), and hypochlorite ion (ClO-), is electrochemically generated via the chlorine evolution anodic reaction (CER) in chloride-containing electrolyte (Equation 1). Produced chlorine molecules are simultaneously hydrolyzed into hypochlorous acid and hypochlorite ions, and the chemical equilibrium concentrations are determined by the solution pH (Equation 2, 3).7 2 Cl– → Cl2 + 2 e– (E0 = 1.36 V vs. NHE)

(1)

Cl2 + H2O ↔ HClO + Cl– + H+

(2)

Cl2 + 2 OH– ↔ ClO– + Cl– + H2O

(3)

Recently, an efficient AC generation method for water treatment has garnered great interest due to the Ballast Water Management (BWM) convention, which recently entered into force to prevent invasive aquatic species in ship ballast water from causing ecological and health issues.8 AC generation by sea water electrolysis is now considered a highly efficient and adoptable methodology of BWM systems.9-12 Therefore, highly selective electrocatalysts for AC generation is of great importance for achieving the efficient electrochemical water treatment systems. Among the reported electrocatalysts for CER, a dimensionally stable anode (DSA) comprised of RuO2 and TiO2-based mixed metal oxides (MMOs) has been considered as a highly efficient catalyst.7, 13-15 Nevertheless, owing to instability of Ru under applied anodic bias and the high cost of noble metals, attempts to enhance the



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stability of the DSAs16-20 and to develop low-cost and earth-abundant catalysts21-25 for CER have intensively proceeded.26 However, chlorine-evolving catalysis so far is usually discussed for chlorine gas production systems, i.e., an industrial chlor-alkali process.7, 27 Since gaseous chlorine (Cl2(g)) is chemically stable and the parasitic OER is thermodynamically unfavorable under acidic conditions, most of the reported works for CER have been conducted at acidic pH, typically lower than pH 4 to ensure high efficiency of the chlorine gas evolution.14 In this respect, we would like to point out that the electrolysis conditions of AC generation for water treatment are totally different from those of chlorine gas production. Since typical wastewater and brine are circumneutral and the AC species are predominately formed at circumneutral pH, AC generation should be conducted under neutral conditions. Thermodynamically, the undesired OER occurs more favorably at neutral pH, thus the development of selective electrocatalysts for AC generation at neutral pH is more challenging than that for chlorine gas production at acidic pH. To surmount the thermodynamic disadvantage of AC generation, metal oxides exhibiting high overpotentials for OER, such as SnO2 or PbO2-based MMOs19, 25, and multi-layered heterojunction electrodes26, 28 have recently been suggested as selective electrocatalysts for AC generation at neutral pH. However, the mechanistic understanding has still remained unclear due to complexity of the multi-component catalysts and the lack of in situ experimental analyses, such as changes of the catalytic surface and verification of reaction intermediates during the catalysis. The mechanistic investigations for CER have been conducted mainly based on the DSA, which is summarized in Table S1. At the beginning of work on the chlorine-evolving mechanisms, the DSA and Pt were known to proceed through the Volmer-Heyrovsky29 and Volmer-Tafel30 mechanisms, respectively. In 1975, Krishtalik et al. suggested a new reaction mechanism (Krishtalik mechanism) and a reaction intermediate of (Clad)+ by measuring the first reaction order with respect to chloride concentration.31 In 1981, Erenburg found a pH effect during CER and suggested a proton-involved chlorine evolution mechanism formed a reaction intermediate of HOCl (Erenburg mechanism).32 However, the mechanistic understanding of CER is still insufficient, because the suggested mechanisms were based solely on the results of electrokinetic analyses and direct experimental


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evidence of surface change and reaction intermediates of CER has rarely been reported. Herein, we report Co3O4 nanoparticles (NPs) as an earth-abundant 3d transition metal-based catalyst with comparable AC-generating performance to the precious DSA under neutral brine conditions. We not only measured the electrochemical performance of Co3O4 NPs for AC generation but also investigated the chlorineevolving mechanism of Co3O4 NPs at neutral pH through comprehensive electrokinetic analyses and in situ spectroscopic analyses. Co3O4 NPs were synthesized by a hot injection method modifying synthetic methods in previous reports33-34 and spin-coated on a fluorine-doped tin oxide (FTO) substrate for electrode preparation. Structural morphology and characterization of Co3O4 NPs are described in Supporting Information. We first measured electrochemical performance of Co3O4 NPs in neutral 0.6 M NaCl, which is equivalent to the molar concentration of sea water. Figure 1a illustrates linear sweep voltammetry (LSV) of Co3O4 NPs in 0.6 M NaCl compared with that in 0.6 M NaClO4. We chose NaClO4 as an appropriate electrolyte where only OER occurs because the perchlorate ions (ClO4–) are fully oxidized oxyanions of chlorine and further oxidation of the anions does not happen at anodic bias. Under anodic polarization in the chloride-containing 0.6 M NaCl, the onset potential was near the 1.28 V (vs. NHE) and the potential required to reach 10 mA/cm2 was 1.40 V (vs. NHE), which was 210 mV less than that potential in the chloride-absent 0.6 M NaClO4. The cathodic shift in the LSV under 0.6 M NaCl in comparison with that under 0.6 M NaClO4 indicates CER occurs much more favorably occurred than does OER in the chloride-containing electrolyte. We also compared the LSV of Co3O4 NPs with that of bulk CoO, bulk Co3O4, electrodeposited amorphous CoOx (Co-Pi)35, and the commercial DSA in 0.6 M NaCl (Figure 1b). The commercial DSA (Siontech Inc., Korea) exhibited comparable catalytic performance for CER to previously reported DSAs14,

19-20,

and we

therefore believed that the commercial DSA we measured could be a relevant reference catalyst in this work. Figure 1b shows the Co3O4 NPs exhibited remarkable CER activity, which was higher than the bulk cobalt oxides and comparable to the DSA in 0.6 M NaCl. We further measured the electrochemically active surface areas (ECSA) of all the catalysts as shown in Figure S3. The ECSA of Co3O4 NPs was slightly smaller than that of the DSA, and the ECSAs of the bulk-sized Co-Pi, CoO, and Co3O4 were about 100 times smaller than that of Co3O4 NPs.



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Figure 1c shows current efficiencies of AC generation on Co3O4 NPs and the DSA at different applied current densities in 0.6 M NaCl. The amount of generated AC was measured by a standard AC quantification method called DPD (N,N-diethyl-p-phenylenediamine) colorimetry.36 Figure S4 shows the generated amounts of AC at four different current densities in 0.6 M NaCl. The corresponding current efficiencies of AC generation were calculated from Equation 4 ηAC =

2VF[ACDPD] 𝑗At

(4)

where V, F, [ACDPD], j, A, and t are the volume of electrolyte, the Faraday constant, the molar concentration of the AC measured by DPD colorimetry, the applied current density, the area of electrode, and the electrolysis time. As shown in Figure 1c, Co3O4 NPs exhibited high selectivity for AC generation with an efficiency of over 80% within the range of current densities we applied, which was better than the DSA. The current efficiency of Co3O4 NPs obtained at 1 mA/cm2 was significantly higher than that of the DSA because of the lower capacitive current of Co3O4 NPs (Figure S5). In addition, it is worth mentioning that the current efficiencies on both Co3O4 NPs and the DSA slightly decreased as the applied current density increased. This decrease is inevitable because of the competition of OER at high anodic potentials. Nevertheless, the AC generating efficiency on Co3O4 NPs remained above 80%, suggesting Co3O4 NPs are more selective electrocatalysts for AC generation in neutral 0.6 M NaCl than the DSA. To confirm the accuracy of our product analysis by DPD colorimetry, we crosschecked the current efficiencies on Co3O4 NPs in two different current densities by iodometric titration which is another AC quantification method. This method also shows the similar current efficiencies compared to the DPD results (Figure S6). Furthermore, we examined the catalytic stability of Co3O4 NPs through chronopotentiometry (CP) at 10 mA/cm2 in 0.6 M NaCl (Figure 1d). As a result, the measured potential of Co3O4 NPs at the applied current density of 10 mA/cm2 increased from 1.37 V (vs. NHE) to 1.39 V (vs. NHE) during the 12-hour electrolysis. According to the Pourbaix diagram of cobalt37, Co3O4 are thermodynamically stable under the applied potential where CER can be occurred. We checked the amount of dissolved cobalt during the stability test by inductively coupled plasma mass spectrometry (ICP-MS), resulting in cobalt atoms of 0.111 µg and 0.203 µg were dissolved


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after 6 and 12 hours, respectively. These amounts corresponded to only 0.13% and 0.28% of the total loading amount of Co3O4 NPs, respectively. Hence, we concluded Co3O4 NPs exhibited highly selective and stable catalytic activity for AC generation in neutral 0.6 M NaCl. Most of electrochemical measurements in our work were conducted in a two-compartment cell separated by Nafion membrane. The reason why the two compartment cell was used is to prevent the possible interference of OH– and H2 produced at the cathode. In the range of neutral pH, dominant cathodic reaction is hydrogen evolution reaction expressed by the following equation, 2 H2O + 2e– → H2 + 2OH–, resulting in increasing pH at the cathodic compartment. Another observation was that even in an unseparated one-compartment cell, the overall pH increased due to the imbalance of H+ and OH– generated at the anode and the cathode, respectively. As above-discussed with Equation 1, 2, and 3, the chemical equilibrium of AC including Cl2, HClO, ClO– determines the H+ concentration at the anodic compartment, thereby generating less H+ at the anode than OH– at the cathode as the electrolysis goes. In short, the pH change during the electrolysis seems inevitable both in the two- and one-compartment cell. Nevertheless, the current efficiencies under the two conditions were similarly measured, indicating the selectivity of Co3O4 NPs was not significantly affected by the pH change (Figure S8). Therefore, the two-compartment cell setup was adopted for our electrochemical measurements. We obtained the same CV curves at every potential sweep although the solution pH at the anodic compartment continuously decreased (Figure S9). At least, with our Co3O4 NPs, this analysis can be fair because Co3O4 NPs did not have the pH dependency at neutral pH as discussed later in Figure 2f. Indeed, even in a flow system which maintained the constant pH by refreshing the solution, the same CV and Tafel slope were obtained (Figure S10), leading to the same conclusion. Tafel slopes of Co3O4 NPs and DSA in 0.6 M NaCl were measured to 44 mV/dec and 32 mV/dec, respectively (Figure 2a). In order to avoid the OER effect in our Tafel analysis, we also analyzed Tafel slope of Co3O4 NPs under saturated 4 M NaCl condition (Figure S11), and the Tafel slope was measured to 40 mV/dec, which was similar to that in 0.6 M NaCl. Theoretically, the Tafel slope (b) can be extracted from the current density (j)– potential (E) relationship (Equation 5, 6), where j0, α, F, E0, R, and T are the exchange current density, the charge transfer coefficient, the Faraday constant, the thermodynamic equilibrium potential, the gas constant, and the temperature.38 In addition, in a multi-electron reaction, the charge transfer coefficient is expressed as



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Equation 7 where nb is the number of electrons transferred before the rate determining step (RDS), ν is the number of the RDS occurrences in an overall catalytic cycle, nr is the number of electrons transferred in the RDS, and β is the symmetry factor, which is typically 0.5.2 Therefore, the Tafel slope of 44 mV/dec on Co3O4 NPs corresponds to the charge transfer coefficient (α) of 1.5 and indicates the second discharging step is probably the RDS where the reversible one-electron transfer step occurs prior to the RDS. 𝛼𝐹(𝐸 ― 𝐸0)

𝑗 = 𝑗0 exp (

b=

α=

∂E ∂log 𝑗

𝑛𝑏 𝜈

=

𝑅𝑇

)

2.303𝑅𝑇

(5)

(6)

𝛼𝐹

(7)

+ 𝑛𝑟𝛽

Furthermore, chloride concentration dependence on catalytic activity of Co3O4 NPs was analyzed via chloride dependent polarization-corrected j-E curves (Figure 2b). Similar Tafel slopes were measured from 0.0375 M NaCl to 0.6 M NaCl, and the average Tafel slope was 42 mV/dec. The potentials required to reach 1 mA/cm2 under the different chloride molarities is depicted in Figure 2c, and the slope was estimated to - 45 mV/dec. Therefore, the reaction order with respect to chloride concentration can be calculated from Equation 8, which resulted in unity. The first reaction order with respect to chloride concentration means the discharge of a chloride ion occurred only once in the RDS or the preceding step, which will be discussed later.

(

∂E

) =―(

∂log [𝐶𝑙 ― ] 𝑗

∂log 𝑗

)( ) ∂𝐸

∂log [𝐶𝑙 ― ] 𝐸 ∂log 𝑗 [𝐶𝑙 ― ]

(8)

To determine whether protons are involved in the catalytic cycle of CER, the pH dependency of the partial current density for CER was measured. Because the 0.6 M NaCl solution does change pH during the oxidation, the addition of a buffer species is necessary for pH control and a phosphate (Pi) was selected as the buffer species. When the Pi was added to the 0.6 M NaCl solution, the onset potential of Co3O4 NPs shifted cathodically and two redox couples of Co(II)/Co(III) and Co(III)/Co(IV) near 0.9 V (vs. NHE) and 1.1 V (vs. NHE)35, 39-40, respectively, became distinct, which was clearly observed in the increase of scan rates (Figure 2d). It can be understood by the enhancement of proton-coupled electron transfers facilitated by the efficient proton transport in the Pi buffer species. Generally, for OER, one proton is involved in the pre-equilibrium reaction prior to the


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RDS.34-35 Thus, we noticed that the addition of the Pi can enhance the competitive OER activity and thus suppress the desired CER, making it difficult to decouple these two reaction mechanisms. Additionally, it is possible that the high concentrated Pi ions could convert the cobalt oxide into Co-Pi species and also compete with chloride ions for the surface adsorption. Therefore, we checked how the catalytic activity for OER and CER change at different Pi concentrations and found the valid concentration point where CER still dominates OER on Co3O4 NPs. We changed the Pi concentration from 2.5 mM to 100 mM in 0.6 M NaCl and monitored the current efficiencies of AC generation on Co3O4 NPs (Figure 2e). While the current efficiencies were above 80% up to a Pi concentration of 10 mM, the current efficiencies dramatically declined to lower than 40% under higher Pi concentrations than 10 mM. The inset in Figure 2e illustrates the partial current density for AC generation at the applied potential of 1.15 V (vs. Ag/AgCl) depending on the Pi concentration, where the slope is identical to the reaction order with respect to Pi concentration. Up to a Pi concentration of 10 mM, the reaction order with respect to Pi concentration was close to zero, which implies Pi does not affect AC generation of Co3O4 NPs. Accordingly, the 10 mM Pi added in 0.6 M NaCl was chosen as an optimum electrolyte concentration in which the Pi did not disturb AC generation and had maximum buffer strength for the pH dependence measurement of Co3O4 NPs for CER. Note that ionic strength of the tested electrolyte (10 mM Pi in 0.6 M NaCl) is only 1.7% higher than 0.6 M NaCl. Figure 2f shows pH dependent partial current density for AC generation under 0.6 M NaCl with 10 mM Pi under anodic bias of 1.10 V (vs. Ag/AgCl) and 1.15 V (vs. Ag/AgCl). The zeroth reaction order with respect to proton concentration was calculated from the slope of the partial current density-pH plots, indicating protons did not participate in CER on Co3O4 NPs under neutral conditions. We further investigated the chlorine-evolving mechanism of Co3O4 NPs by in situ XANES analysis (Figure 3). The Co K-edge XANES spectra of Co3O4 NPs at 1.4 V (vs. Ag/AgCl) under CER conditions (0.6 M NaCl with 10 mM Pi) rarely shifted, compared to that of resting Co3O4 NPs. Under CER conditions at the applied 1.4 V (vs. Ag/AgCl), the average Co oxidation state of Co3O4 NPs did not change appreciably, varying from 2.71 to 2.74. This behavior is evidently different from the case of OER, in which the formation of high-valent Co(IV) species were generated as reported in Co-Pi41-43, Co3O440, 44-45, Fe-Co oxides46, and Co4O4 cubanes47-48. We



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monitored the Co K-edge XANES spectra under OER conditions (0.5 M Pi) at the same anodic bias, and a clear shift to a positive direction with an increase in the average Co oxidation state to 3.01 was recorded. From the results of in situ XANES analysis, we found that the surface status of Co3O4 NPs under CER conditions was highly different from that under OER conditions, and the high-valent Co species did not develop under CER conditions. Generally, the electrochemically active sites for CER and OER are thought to be the same because the two anodic reactions are in competition.7 If the surface coverage of chloride ions is superior to that of water molecules or hydroxyl ions under anodic polarization, the Co(IV) species, key reaction intermediates for OER, will hardly be generated and thus the average oxidation state of cobalt will not distinctly increase under CER conditions. In addition, high-valent Co species are unnecessary for accomplishing CER because only two electrons are transferred during the catalytic cycle of CER. Therefore, the results of in situ XANES analysis elucidated that CER outrivaled OER and the surface of Co3O4 NPs was predominantly covered by the adsorbed chlorine species under the CER conditions. In addition, in situ Raman spectroscopy of Co3O4 NPs was carried out to verify the reaction intermediates formed during CER (Figure 4). We used 4 M NaCl in order to saturate the reactants (chloride ions) for CER and increase the number of the reaction intermediates on the catalytic surface. Figure 4c and 4a show the potential-dependent Raman spectrum of Co3O4 NPs in neutral 4 M NaCl. Four main peaks at the Raman shifts of 693, 622, 484, 524 cm-1 were matched with the A1g, F2g, F2g, and Eg vibration modes of spinel Co3O4, respectively.49 No distinct change of the Raman spectrum was observed up to the anodic potential of 0.8 V (vs. Ag/AgCl), which was lower than the onset potential for CER. However, a new broad peak at 502.6 cm-1 clearly appeared at the applied potential of 1.2 V (vs. Ag/AgCl) where chlorine was evolved, and the intensity of this broad peak increased at higher applied anodic potential. Notably, the new peak at 502.6 cm-1 reversibly disappeared when the applied potential decreased to lower than the onset potential for CER (Figure S12), indicating the reversible formation of the reaction intermediates during CER. In contrast, except for the intensity decrease at 1.5 V (vs. Ag/AgCl) due to the vigorous bubbling on the surface of Co3O4 NPs, the potentialdependent Raman spectrum of Co3O4 NPs in chloride-absent electrolyte, i.e., 4 M NaClO4, was not changed under the anodic polarization (Figure 4b). Therefore, we reasoned that the detected peak appeared only under CER condition as the chlorine-containing reaction intermediates of CER on Co3O4 NPs.


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To verify the involvement of water molecules in the formation of the reaction intermediates of CER, isotopedependent in situ Raman spectroscopy was conducted (Figure 4d). As a result, the Raman spectrum of Co3O4 NPs under chlorine-evolving conditions in 4 M NaCl dissolved in D2O and H218O were analogous to that in 4 M NaCl dissolved in H2O, and no distinct peak shifts were observed at the applied potential of 1.3 V (vs. Ag/AgCl). If the hydrogen or oxygen atoms of water molecules participate in the formation of the reaction intermediates related to the peak at 502.6 cm-1, a peak shift should be clearly observed. From previous reports on OER catalysts, the isotope-derived spectroscopic peak shifts of 30-60 cm-1 have supported the clear evidence that the formation of the metal-oxo species during OER44, 50-51, which originates from the interaction between surface metal atoms and water molecules. Hence, the spectroscopic behavior of Co3O4 NPs under CER conditions elucidated the water molecules did not participate in the formation of the reaction intermediates during CER, suggesting the direct adsorption of a chlorine atom on a surface cobalt atom (Co-Cl). The vibration mode of the appeared peak can be originated from two possibilities: i) the vibration mode of Co-Cl intermediates based on the previous Raman data on the normal vibration mode of CoCl2 molecular complex measured in an inert gas matrix (493.4 cm-1)52-56, and ii) the newly generated vibration mode of Co-O by chlorine adsorption on the cobalt site52. Although the Raman simulation is required in the future research, both possibilities indicated the direct interaction between cobalt and chlorine atom. Therefore, we observed and confirmed the Co-Cl intermediates in the chlorine-evolving catalytic cycle, which was the first direct verification of the reaction intermediates for CER. Taken together, we propose the reaction mechanism of Co3O4 NPs at neutral pH as the Krishtalik mechanism, which is described in Scheme 1. The RDS is the second discharging step where an adsorbed Cl atom on the surface of a cobalt atom is discharged to form the adsorbed Cl+ species. The Tafel slope of 40 mV/dec, the first and the zeroth reaction order with respect to chloride and proton concentration, respectively, support the reaction steps of the suggested mechanism of Co3O4 NPs. The proposed reaction mechanism of Co3O4 NPs at neutral pH were distinct from that of Ti-supported Co3O4 at acidic pH23, involving protons and forming Co-OHCl intermediates during CER. The electrokinetic behavior of Co3O4 catalysts and the generated CER intermediates can be changed due to different proton concentrations and the promoted hydrolysis of the generated chlorine molecules into the AC species at neutral pH. Considering the measured electrokinetic parameters, the overall



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electrochemical rate law of Co3O4 NPs for CER at neutral pH was governed by Equation 9. 𝑗 = 𝑘0𝑎𝐶𝑙 ― exp

(

)

1.5𝐹𝐸 𝑅𝑇

(9)

Not only electrokinetic analyses of Co3O4 NPs but also in situ spectroscopic analyses verified the suggested Krishtalik mechanism. From the in situ XANES analysis, we revealed that CER did not require the oxidation of cobalt atoms during catalysis and the catalytic surface of Co3O4 NPs were covered by the adsorbed chlorine atoms as predicted in the proposed mechanism. In addition, the results of in situ Raman spectroscopic analysis confirmed the bonding nature of the Co-Cl reaction intermediates, which should be the reactants in the RDS of the Krishtalik mechanism. These spectroscopic results accorded with the zeroth proton reaction order from our electrokinetic studies on Co3O4 NPs. Therefore, we clearly revealed the CER mechanism of Co3O4 NPs at neutral pH with apparent electrokinetic and in situ spectroscopic evidence indicating the Krishtalik mechanism. In conclusion, we explored the chlorine-evolving mechanism of Co3O4 NPs at neutral pH by electrokinetic analyses, combined with in situ XANES and Raman spectroscopy. We revealed that Co3O4 NPs exhibited high selectivity and stability for AC generation in neutral 0.6 M NaCl, which was comparable with the precious RuO2-based DSA. A comprehensive mechanistic investigation of CER on Co3O4 NPs described the chlorine evolution for AC generation proceeded through the Krishtalik mechanism where the RDS was the second electron transfer step to form the adsorbed Cl+ species following the reversible one-electron transfer step prior to the RDS. In particular, we directly detected the Co-Cl reaction intermediates for the Krishtalik mechanism by in situ Raman spectroscopy for the first time in CER. We believe our mechanistic investigation of Co3O4 NPs provides new insight into designing efficient and selective electrocatalysts for AC generation at neutral pH, which can be practically applied to electrochemical water treatment systems.


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Figure 1. Electrochemical performance of Co3O4 nanoparticles (NPs) for the active chlorine (AC) generation. Linear scan voltammetry (LSV) curves of Co3O4 NPs under 0.6 M NaCl condition (a) compared with NaClO4 condition where only oxygen evolution reaction occurs, and (b) compared with various bulk cobalt oxides and the commercial DSA (scan rate: 10 mV/s). (c) Current efficiencies of AC generation on Co3O4 NPs and the DSA at various applied current densities in 0.6 M NaCl. (d) Long-term stability tests of Co3O4 NPs by chronopotentiometry at 10 mA/cm2. All the electrochemical data in Figure 1 were compensated by solution resistivity and the geometric area was used for calculation of the current density.



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Figure 2. Electrokinetic analyses of Co3O4 nanoparticles (NPs) for the active chlorine (AC) generation. (a) Tafel plots of Co3O4 NPs (red circle) and the DSA (black circle) in 0.6 M NaCl. (b) Chloride concentration dependent polarization-corrected curves of Co3O4 NPs (scan rate: 10 mV/s) and the corresponding Tafel plots (inset). (c) Potentials required to reach 1 mA/cm2 under various chloride concentrations. The reaction order with respect to chloride concentration was estimated to unity. (d) Pi concentration dependent cyclic voltammetry (CV) curves of Co3O4 NPs under various Pi concentrations added in 0.6 M NaCl (scan rate: 10 mV/s). Distinct redox couples (inset) appeared when the addition of Pi in 0.6 M NaCl increased. (e) Current efficiencies of the AC generation under different Pi concentrations added in 0.6 M NaCl. The reaction order with respect to Pi concentration (inset) was measured to be zero below the range of 10 mM Pi. (f) pH dependency over neutral pH range in 0.6 M NaCl with 10 mM Pi. The reaction order with respect to proton concentration was measured to zero at two different applied potentials. All the electrochemical data in Figure 2 were compensated by solution resistivity and the geometric area was used for calculation of the current density.


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Figure 3. In situ XANES analysis of Co3O4 nanoparticles (NPs) for active chlorine generation. (a) Co Kedge XANES spectrum of Co3O4 NPs and (b) average oxidation state of cobalt under CER and OER conditions. CER and OER conditions indicate 0.6 M NaCl with 10 mM phosphate buffer species (Pi) and 0.5 M Pi, respectively.



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Figure 4. In situ Raman spectroscopic analysis of Co3O4 nanoparticles (NPs) for the active chlorine generation. (a), (c) Raman spectrum of Co3O4 NPs at different applied potentials in 4 M NaCl and (b) in 4 M NaClO4 where only OER occurred. A broad peak around 500 cm-1 only appeared under chlorine-evolving conditions. (d) Isotope dependent Raman spectrum of Co3O4 NPs at applied 1.3 V (vs. Ag/AgCl) in 4 M NaCl dissolved in H2O, D2O, and H218O. The newly appeared peak at 502.6 cm-1 (purple) was not distinctly changed when H2O was replaced with either D2O or H218O.


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Scheme 1. Proposed chlorine-evolving mechanism of Co3O4 nanoparticles (NPs) for the active chlorine generation at neutral pH. The Krishtalik mechanism where the second electron transfer step is the rate determining step was suggested as the reaction mechanism of Co3O4 NPs. Our electrokinetic and in situ spectroscopic analyses of Co3O4 NPs supported the suggested mechanism, which was distinct from the previously reported Co3O4 catalysts for CER at acidic pH.



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ASSOCIATED CONTENT Supporting Information Detailed experimental methods, Structural Characterization, Figures S1 – S12, Scheme S1, and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF-2017M3D1A1039377), the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF2017R1A2B3012003), the Global Frontier R&D Program of the Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science and ICT, Korea (2012M3A6A7054855), and Research Institute of Advanced Materials (RIAM) at Seoul National University.


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Highly Selective Active Chlorine Generation Electrocatalyzed by

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