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durability coincidence of oxygen evolution reaction in the presence of carbon corrosion: the case study of MnCo2O4 spinel with carbon black carbon bla...
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Activity-durability coincidence of oxygen evolution reaction in the presence of carbon corrosion: the case study of MnCoO spinel with carbon black 2

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Juchan Yang, Seungyoung Park, Kyoung young Choi, HanSaem Park, Yoon-Gyo Cho, Hyunhyub Ko, and Hyun-Kon Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01879 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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ActivityActivity-durability coincidence of oxygen evolution reaction in the presence of carbon corrosion: the case study of MnCo2O4 spinel with carbon black Juchan Yang, Seungyoung Park, Kyoung young Choi, Han-Saem Park, Yoon-Gyo Cho, Hyunhyub Ko and Hyun-Kon Song* School of Energy and Chemical Engineering, Ulsan National Institute of Science Technology, UNIST-gil 50, Ulsan, 44919, Korea * E-mail: [email protected] ABSTRACT: Highly oxygen-evolution-reaction-(OER)-active electrocatalysts often exhibit improved OER durability in the presence of carbon corrosion or oxidation (COR) in literature. The activity-durability coincidence of OER electrocatalysts was theoretically understood by preferential depolarization in galvanostatic situations. At constant-current conditions for a system involving multiple reactions that are independent and competitive, the overpotential is determined most dominantly by the most facile reaction so that the most facile reaction is responsible for a dominant portion of the overall current. Therefore, higher OER activity improves durability by mitigating the current responsible for COR. The activity-durability coincidence was then proved experimentally by comparing between two catalysts of the same chemical identity (MnCo2O4) in different dimensions (5 nm and 100 nm in size). Carbon corrosion responsible for inferior durability was suppressed in the smaller-dimension catalyst (MnCo2O4 in 5 nm) having more number of active sites per a fixed mass.

KEYWORDS: carbon corrosion, electrocatalyst, oxygen evolution reaction, metal air battery, triple phase boundary

trocatalysts for ORR and OER, which proceeds especially in highly oxidative situations.10 The loss of carbon mass working as electric pathways results in the loss of active electrocatalysts participating in reactions. The oxidative situations causing carbon corrosion are met on cathodes of fuel cells during normal steady-state operation of ORR and moreover, startup/shutdown processes11; and on air electrodes of metal-air batteries during charging where OER proceeds.12 Therefore, ORR and, more likely, OER proceeds with carbon oxidation reaction (COR or carbon corrosion).

INTRODUCTION Oxygen electrochemistry including oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER) is the working principles of fuel cells (ORR),1 water electrolysis (OER)2 and metal air batteries (ORR and OER).3, 4 To accelerate the kinetics of ORR and OER for decreasing overpotentials, electrocatalysts have been used, including novel metals,5 transition metal oxides6 and nitrogen-doped carbons.7 Carbonaceous materials (e.g., carbon black) have been used for supporting electrocatalysts in order to keep catalysts dispersed as a form of nanoparticles or nanoislands on the surface of supports without agglomeration8 and to provide electric pathways to each catalyst particle.9 Pt nanoparticles supported by carbon show high ORR activities due to their dispersion on high surface area. The electroactivities of a highly conductive perovskite oxide catalysts were not fully realized in the absence of carbon black9. Bulk conductivity of the oxide catalysts did not guarantee electric pathways from base electrodes to active sites due to the high contact-to-contact resistance between electrocatalyst particles. The stability in electroactivity or the durability of electrocatalysts for long-term operation is the most important issue from a practical viewpoint. Carbon corrosion is one of the main reasons responsible for the instability of elec-

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On the contrary, COR is catalyst-independent in potentiostatic situations in which each electrochemical reaction proceeds along its own way according to the applied potential. When OER proceeds in the presence of COR at a fixed potential Eapp (Figure 1c), the overall current (ioverall) is the simple sum of the two reactions: ioverall(Eapp) = iOER(Eapp) + iCOR(Eapp). There is no difference of iCOR between the catalysts 1 and 2 even if more OER-active 1 provides higher overall current than less OER-active 2. The choice of OER catalysts does not affect carbon corrosion so that the activity-durability coincidence is invalid in the potentiostatic cases. The above discussion is based on no COR-OER interaction. However, the OER activity possibly decrease as the COR proceeds because the COR triggers carbon mass loss and then catalyst loss (COR-OER interaction). It should be notified that the activity-durability coincidence at galvanostatic conditions and the catalyst-independency of COR at potentiostatic conditions are exactly valid only when OER and COR are independent of each other. Large difference in OER activities of two different catalysts possibly supports the galvanostatic activity-durability coincidence even in the presence of the OER-COR interaction. It is rarely possible for the effects of COR on OER to overcome the overpotential difference of COR triggered by the difference in OER. However, it is difficult to obtain the catalyst-independent COR at the potentiostatic conditions in the presence of the COR-OER interaction. In this work, we demonstrate the activity-durability coincidence at the galvanostatic condition by using a bifunctional electrocatalyst for ORR and OER in two different dimensions. Manganese cobalt oxide spinel catalysts (MnCo2O4; MCO) were synthesized, the sizes of which are represented by 5 and 100 nm respectively (5nm-MCO and 100nm-MCO). It is possible to assume that both catalysts have the same electroactivity per an active site because of their chemical identity. However, the specific activity (electroactivities per mass) was controlled by the size of catalyst particles loaded on carbon supports. Smaller catalysts (5nm-MCO) provided more number of active sites (higher activities) than larger catalysts (100nm-MCO) when a fixed mass of catalysts were used. Then, the durability was compared between the different-size catalysts of the same chemical identity to confirm the activity-durability coincidence at galvanostatic conditions.

Figure 1. (a) The activity-durability coincidence of OER in the presence of COR. The numbers in the parentheses indicates the data sources, the details of which are found in supplementary information. (b and c) Polarization curves of OER coupled with COR.

In the situations of OER coupled with COR, there appears to be an empirical correlation between electroactivity and durability: electrocatalysts of higher activities are more durable (activity-durability coincidence in Figure 1a). The activity-durability coincidence in galvanostatic situations, which is the practical conditions for operating batteries, is easily understood by OER polarization curves (Figure 1b). For simplicity, carbon corrosion was assumed not to be catalyzed by OER catalysts (COR). Two catalysts supported by a carbon material, 1 and 2, have their own polarization curves of OER coupled with COR (OER/COR 1 and OER/COR 2): 1 is more OER-active than 2. When a fixed current at iapp is applied, the potential E2 reached for the less OER-active catalyst 2 is higher than the potential E1 (< E2) for the more OER-active catalyst 1. The larger overpotential of poorly OER-active 2 results in a higher corrosion rate: iCOR, 1 < iCOR, 2. Therefore, at galvanostatic situations, COR is catalyst-dependent; and higher OER activity improves durability by mitigating COR. In general, at constant-current conditions for a system involving multiple reactions that are independent and competitive, the overpotential is determined most dominantly by the most facile reaction so that the most facile reaction is responsible for a dominant portion of the overall current.

RESULTS AND DISCUSSION DISCUSSION MCO is considered a promising bifunctional ORR/OER catalyst with advantages of low cost and environmentally friendly constituents.13, 14 Electroactivities of catalysts strongly depend on the effective catalyst surface area determined by their morphology and particle size since electrochemical reactions, ORR and OER, proceed at the triple phase boundary (TPB; catalyst-electrolyte-oxygen in solidliquid-gas phase; Figure S1).15 The MCO catalysts were obtained by calcining manganese and cobalt precursors. 5 nm or smaller nanoparticles of MCO (5nm-MCO) were prepared by using a mesoporous silica (SBA-15) as a hard template (Figure 2a). The size of 5nm-MCO was limited by

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ACS Sustainable Chemistry & Engineering the void dimension of SBA-15. Microparticles of MCO (roughly 100 nm in Figure S2; 100nm-MCO) were synthesized as a larger version of 5nm-MCO without the hard template. Their constituent elements were identified as Mn, Co and O by energy-dispersive spectroscopy (Figure S3). The absence of Si was confirmed in 5nm-MCO. The rings of electron diffraction pattern from inside to outside were assigned to (220), (311) and (400) planes of MnCo2O4 spinel, showing the polycrystalline nature of 5nm-MCO (small area electron diffraction of Figure 2a). The interplanar distance of clear lattice fringes of 100nm-MCO was 0.23 nm, which corresponded to that of (211) planes of MnCo2O4 spinel (Figure S2d). All sharp peaks of X-ray diffraction patterns of 5nm-MCO and 100nm-MCO were well matched with those of MnCo2O4 spinel having lattice constants at a = 8.269 Å and c = 8.296 Å (JCPDS card No. 231237) (Figure 2b). The absorption peaks of infrared spectra (Figure 2c) confirmed the spinel structure of 5nmMCO and 100nm-MCO: the peak at ~640 cm-1 for the stretching vibration mode of M-O where M is Mn2+ that is tetrahedrally coordinated to oxygen; the peak at ~100 cm-1 for octahedrally coordinated M-O where M = Co3+ .16 X-ray photoelectron spectra of 5nm-MCO were exactly the same as those of previously reported MnCo2O4 spinel (Figure S4).14, 16

rameters (e.g., exchange current io but not standard rate constant ko) because more number of electroactive sites on higher surface area (44 m2 g-1 for 5nm-MCO versus 9.0 m2 g-1 for 100nm-MCO by nitrogen adsorption. BrunauerEmmet-Teller equation was used for calculating the surface area.) were exposed to reactants (OH-).

Figure 3. OER in N2-saturated 0.1 M KOH. Potentials were not IR-corrected. (a) OER polarization curves at 1,600 rpm at 10 mV s−1. Geometric area of disk electrodes were used for normalizing currents. (b) Durability at 10 mA cm-2. (c) Mass change during OER at 1 mA cm-2. Catalyst layers were loaded on titanium-coated quartz crystal resonators. Frequency changes were converted to mass changes by Sauerbrey equation. Only the initial trace of mass change of 100nm-MCO was shown for clarity. Refer to the Figure S5 for the 2 h trace of 100nm-MCO.

Then, the durability of the catalyst systems of three different specific activities (carbon only, 100nm-MCO and 5nm-MCO in the ascending order of OER activities) was tested for proving the activity-durability coincidence. A high overpotential (2.65 VRHE), at which COR proceeded, was reached in a fast way when 10 mA cm-2 was applied to the only carbon layer without catalysts (None in Figure 3b). Less OER-active 100nm-MCO showed a potential plateau of OER for ~0.23 h (the duration to reach 2 VRHE) while more OER-active 5nm-MCO had a longer OER plateau during ~1.20 h. Catalysts are thought to be lost continuously with carbon loss during OER progress. After the OER plateaus, therefore, the potentials of all electrodes sharply increased to the COR potential at ~2.65 VRHE without catalysts any more. The mass losses during OER were confirmed by quartz crystal microbalance (Figure 3c). There were insignificant frequency changes of the quartz crystal resonator on which 5nm-MCO was loaded during 10 h (Figure S6), indicating that no mass change was observed during the experiment. On the other hand, 100nm-MCO showed a dramatic decrease in mass within 15 min due to carbon corrosion. The carbon corrosion or COR during OER at constant current (10 mA cm-2) was confirmed by electronmicroscopic images (Figure 4a). Hollow structures of car-

Figure 2. MCO (MnCo2O4). (a) TEM images of 5nm-MCO before (left) and after (right) etching of SBA-15. Insets: The small area electron diffraction (SAED) pattern. Refer to Figure S2 for 100nm-MCO. (b) X-ray diffraction (XRD) patterns. (b) Infrared spectra.

The particle size of MCO determined specific OER activity and therefore, areal OER activity at a fixed amount of catalyst. Both 5nm- and 100nm-MCOs electrocatalyzed OER in 0.1 M KOH. The electrocatalytic activities of the MCOs were estimated comparable to that of RuO2 known as one of the best OER catalyst (Figure 3a). 5nm-MCO was superior to 100nm-MCO and moreover RuO2 in terms of kinetics described by the onset potentials and currents at the same potential. As we intentioned, smaller dimension of MCO improved OER kinetics in terms of extensive pa-

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bon were often observed in the presence of 100nm-MCO after the galvanostatic load while there were insignificant change in carbon structure in the presence of 5nm-MCO. The hollow structures of carbon obtained after the galvanostatic load are due to the preferential gasification of small crystallites and disordered amorphous carbon in the core region.17 A well-ordered portion of the particle surface is less susceptible to oxidative attack than the central region. Raman spectra of carbon of catalyst layers (20 wt. % carbon) were compared between before and after the galvanostatic load at 10 mA cm-2 during 2 h to support the existence of COR during OER (Figure 4b and Figure S7). Characteristic peaks of carbon include D band at 1350 cm-1 for disorder-activated edge phonon and G band at 1580 cm-1 for graphitic lattice phonon. No significant change in Raman spectra was observed from the 5nm-MCOcontaining catalyst layer: D/G ratio (ID/IG) = 1.2 to 1.12. On the other hand, the D/G ratio of the carbon in the 100nmMCO-containing catalyst layer decreased from 1.41 to 1.06 with D and G bands broader after the fixed current load is applied. The decrease in D/G ratio and the increase in intensity between D and G bands supported the preferential corrosion of amorphous carbon as reported in the oxidation of non-graphitic carbon in literature.18

band of 100nm-MCO after the galvanostatic load is consistent with the larger ∆Ep. In the 5nm-MCO-containing electrode, however, there was an insignificant change in ∆Ep observed between before and after the galvanostatic load. In the same cyclic voltammetry, also, a decrease in current was observed from 100nm-MCO while 5nm-MCO showed a current increase. Faradaic current responsible for Fe(CN)6-3/-4 electrochemistry as well as capacitive current is proportional to carbon surface area. Therefore, the current change could be interpreted as carbon corrosion. Carbon corrosion proceeds preferentially on defects of carbon surface. In a limited corrosion situation, carbon corrosion results in surface area increase, encouraging rough surface and micropore formation (5nm-MCO case).22 As carbon oxidation proceeds, however, significant loss of carbon mass leads to a decrease in surface area (100nmMCO case). In addition to OER activities, the ORR electroactivities of MCOs were investigated for checking the possible use of the MCOs as bifunctional catalysts for rechargeable metal air batteries (Figure S9). The same nanodimensional benefits valid in OER were observed in ORR. 5nm-MCO was superior to 100nm-MCO in terms of ORR kinetics (e.g., onset potentials and half-wave potentials). Additionally, the number of electrons transferred during ORR (n) was improved by use of the smaller catalysts, showing the 5nm-MCO is comparable to Pt/C as the best ORR catalyst in terms of n. The ORR activities of 5nm-MCO synthesized in this work were higher than those of MnxCoyO2 previously reported.7, 23, 24 The ORR stability of 5nm-MCO was also excellent, being much superior to 100nm-MCO and Pt/C. The ORR current of 5nm-MCO decreased only by 5% after 7 h operation at 0.43 VRHE while both 100nm-MCO and Pt/C showed ~20% decrease at same condition.

Figure 4. COR at 10 mA cm-2 for 2 h. Catalyst layers = catayst : carbon black at 80 : 20 in weight. Data were compared between before and after the galvanostatic load at 10 mA cm-2 for 2 h. (a) TEM images. (b) Raman spectra of carbon with D band at 1350 cm-1 for disorder-activated edge phonon and G band at 1580 cm-1 for graphitic lattice phonon. More detailed analysis is shown in Figure S7.

The changes in microstructure of carbon caused by COR were probed electrochemically by using an inner-sphere redox couple, Fe(CN)6-3/-4 (Figure S8). It was reported that edge planes of sp2 carbon strongly accelerate the kinetics of this redox couple.19,20 The charge transfer kinetics can be measured by the peak separation between the anodic and cathodic scans in cyclic voltammograms (∆Ep = Epa Epc).21 Therefore, ∆Ep is correlated with the degree of carbon corrosion.10 The ∆Ep of the 100nm-MCO-containing electrode became larger from 83 mV to 106 mV when the galvanostatic load was applied. The larger ∆Ep indicates slower kinetics of charge transfer between the electroactive species and the carbon conductor. The reduced amount of edge planes caused by carbon corrosion is responsible for the slow kinetics. The decrease in Raman D

Figure 5. Hybrid Na-air battery. MCOs were loaded on carbon papers for air electrodes. Current density = 0.04 mA cm-2. (a) Construction. (b) Voltage profiles of the initial charge (OER) and discharge (ORR). None = carbon papers without catalysts. (c) Cyclability by repeating 5h charge and discharge. (d) Round trip efficiency (ηround).

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ACS Sustainable Chemistry & Engineering The OER and ORR are responsible for the charge and discharge processes of metal air batteries, respectively. Based on the electroactivities of ORR as well as OER, the MCOs were tested as the bifunctional catalysts for air electrodes of hybrid-type sodium air batteries. An air electrode compartment was separated from a sodium metal electrode compartment by a solid electrolyte membrane (Na3Zr2Si2PO12). An aqueous electrolyte of 0.1 M NaOH was used for the air electrode while an organic electrolyte (1 M NaCF3SO3 in tetraethylene glycol dimethyl ether, TEGDME) was used for the sodium metal electrode (Figure 5a). During discharge, sodium is stripped while oxygen is reduced by25: Cathode: O2 (g) + 2H2O (l) + 4e- ↔ 4OH- (aq) Eo = +0.40 VSHE at pH 14 (1) + o Anode: Na (s) ↔ Na + e E = -2.71 VSHE (2) o Overall: 4Na + O2 ↔ 4NaOH E = +3.11 VSHE. (3) The OER coupled with COR proceeds during charge so that the suppression of COR during OER affects the durable operation of repeated charge and discharge. The 5nm-MCO was superior to the 100nm-MCO from the viewpoints of (1) the potential difference between charge and discharge (ΔECh-dCh) (Figure 5b and c) and (2) the durability of repeated charge/discharge operation of a sodium-air battery (Figure 5c and d). Both MCO catalysts loaded on carbon papers as the base air electrodes of sodium air batteries showed highly capacitive behaviors during the early periods of charge and discharge (Figure 5b).26, 27 On the other hand, the carbon paper without catalysts reached the potential plateaus responsible for OER and ORR within a shorter time. The 5nm-MCO showed the smallest values of ΔECh-dCh at 5 h charge and discharge indicating the highest reversibility, followed by 100nm-NCO and then the base electrode (carbon paper). From the viewpoint of the charging process based on OER, the overpotentials were less developed along repeated charge and discharge cycles (5 h for each) when 5nm-MCO was used instead of 100nm-MCO (Figure 5c and d). The maximum potential on charge was 3.5 VNa for 5nm-MCO versus 3.6 VNa for 100nm-MCO at the 55th cycles during 550 h. The round trip efficiency (ηround) was estimated at 85 % for 5nm-MCO versus 80 % for 100nm-MCO at the same cycle. The activity-durability coincidence provides a basis to the understanding on the durability superiority of 5nm-MCO over 100nm-MCO. The reversibility and durability obtained by the 5nm-MCO for sodium air batteries was considered outstanding when compared with previously reported works (Table S1). Also, the higher activity of 5nmMCO supported the more durable cyclability of lithium-air battery when compared with the 100nm-MCO-containing system (Figure S10).

kinetics obtained by using more highly OER-active catalyst increases the OER contribution to the overall current and decreases the overpotential. At the same time, the COR contribution is reduced. That is to say, carbon corrosion is mitigated in the presence of high-activity OER catalysts. The OER durability was compared between two catalysts of the same chemical identity (MnCo2O4 or MCO) in different dimensions (5 nm and 100 nm in size) to support the activity-durability coincidence experimentally. The 5nmMCO having high OER activity per a fixed mass suppressed carbon corrosion during OER and finally enabled longer operation of repeated galvanostatic charge and discharge in sodium-air and lithium-air batteries.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge in the ACS Publications website at DOI: Synthesis of materials, details of electrochemical experiments. Additional experimental data in Figure S1-S8 and Table S1. The TEM and SEM image of the samples studied, X-ray photoelectron spectroscopy details, and additional electrochemical results.

AUTHOR INFORMATION Corresponding Author *Tel.: +82-52-217-2512. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by MOTIE (Regional (KIAT): R0006515) and MOE (BK21Plus: 10Z20130011057), Korea.

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CONCLUSIONS Herein, we theoretically explained and experimentally proved the activity-durability coincidence often met in OER coupled with COR. When a constant current is applied, both OER and COR proceed competitively and contribute their own portions to the overall current. The faster OER

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TOC

We theoretically explained and experimentally proved the activity-durability coincidence often met in oxygen evolution reaction coupled with carbon oxidation reaction.

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