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The pH is one of the most important factors for electrochemical reactions. The solution pH is often measured prior to the electrochemical measurements, but it is not sufficient to consider only bulk pH, especially for the measurements in neutral pH solutions. When electrochemical reactions occur that produce/consume protons or hydroxide ions, the local pH in the vicinity of an electrode becomes considerably different from the bulk pH. This local pH is important in considering electrodeposition and corrosion. Furthermore, it also has an critical effect on the operating voltage of aqueous batteries and capacitors, as local pH affects the electrochemical stability of water.1 However it is difficult to measure the local pH, because the measurements have to be made in a small region adjacent to the electrode. Herein, rotating ring-disk electrode (RRDE) is a promising tool for the local pH measurements, though it has not been applicable to neutral pH solutions.2 In this study, the theory of local pH measurements using RRDE was extended to apply to solutions over a wide pH range.3 This extended theory was corroborated by measurements of the local pH change during hydrogen oxidation/evolution reactions on platinum disk electrode in solutions with near-neutral pH. This reversible reaction makes it possible to control the local pH on the disk electrode by the applied potential. The platinum ring electrode successfully detected the local pH changes by potentiometry, and the measured local pH was in good agreement with the theoretical predictions (Fig. 1a). In addition, using the RRDE modified with iridium oxide (IrOx) ring, we were able to evaluate the dynamic changes in the local pH of the working electrodes during oxygen evolution reaction in Ar-saturated neutral-pH solution (Fig. 1b).4 Thus, RRDE with IrOx ring has the potential to be a widely applicable tool for measuring the local pH of the electrode during any electrochemical reactions. Reference Y. Yokoyama, T. Fukutsuka, K. Miyazaki, and T. Abe, J. Electrochem. Soc., 165, A3299–A3303 (2018). W. J. Albery and A. R. Mount, J. Chem. Soc. Faraday Trans. 1, 85, 1181–1188 (1989). Y. Yokoyama, K. Miyazaki, Y. Miyahara, T. Fukutsuka, and T. Abe, ChemElectroChem, 6, 4750–4756 (2019). Y. Yokoyama, K. Miyazaki, Y. Kondo, Y. Miyahara, T. Fukutsuka, and T. Abe, Chem. Lett., 49, 195–198 (2020). Figure 1

Introduction Sodium-ion batteries (SIBs) are widely focused on as post lithium-ion batteries from the view of element strategy[1]. Hence, SIBs are attractive for large scale energy storage like electric vehicles. For the large scale energy storage, batteries with high rate capability are desired. Carbon nanosphere (CNS) covered with basal planes is expected as the candidate for carbon negative electrode material with high rate capability because CNS electrodes showed the high rate capability in LIBs owing to small particle sizes and short diffusion distances[2]. To enhance the rate capability of electrode materials, the internal resistances of electrodes should be investigated. In LIBs, the interfacial lithium-ion transfer at the interface between electrodes and electrolytes is one of the rate determining step[3]. However, the kinetic properties of interfacial sodium-ion transfer at the interface between carbon negative electrodes and electrolytes is unclear. In this study, the activation energies of interfacial sodium-ion transfer reaction at the interface between CNS electrodes and organic electrolyte solutions were evaluated. Experimental Electrochemical measurements were carried out using a three-electrode cell. Working electrode was CNS composite electrode. CNS treated at 1100°C and 2900°C were used and denoted as CNS-1100 and CNS-2900, respectively. Reference electrode was Ag/Ag+ electrode and counter electrode was natural graphite composite electrode. The electrolyte solutions were 0.9, 4 mol kg- 1 sodium bis(trifluoromethanesulfonyl)amide (NaTFSA)/ethylene carbonate (EC) + dimethyl carbonate (DMC) (1:1 by vol.) and 0.7 mol kg- 1 NaTFSA/fluoroethylene carbonate (FEC). Hereafter, all potentials are referred to as Fc/Fc+. Cyclic voltammetry (CV) was conducted between open circuit potential (OCV) and various potentials, and the scan rate was set at 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was carried out with ac amplitude of 10 mV in a frequency range of 100 kHz − 10 mHz at temperatures ranging from 30°C to 10°C. Results Figure 1 shows the cyclic voltammograms of CNS-1100 composite electrodes in 0.9 mol kg- 1 NaTFSA/EC+DMC(1:1 by vol.). The irreversible currents were observed below −2 V and more than half of the irreversible currents disappeared from the 2nd cycle, indicating the formation of solid electrolyte interphase on CNS electrodes at the same manner of lithium ion-system. The redox peaks also appeared around −3 V, and were assigned to the peaks of reversible sodium-ion insertion into/from CNS-1100. Figure 2 shows the Nyquist plots of CNS-1100 composite electrode in 0.9 mol kg−1 NaTFSA/EC+DMC(1:1 by vol.). In the Nyquist plots, based on the potential dependence of the semi-circle, the semi-circle at the middle frequency region was assigned to the charge-transfer resistance. The same kinds of electrochemical behaviors were observed in the cases of other electrolytes and CNS-2900 electrodes. In the meeting, the activation energies of interfacial sodium-ion transfer will be reported. Acknowledgement This work was partially supported by ESICB, Kyoto University. References [1] Komaba et al., Adv. Funct. Mater., 21 (2011) 3859. [2] Yoshizawa et al., Carbon, 44 (2006) 2558. [3] Ogihara et al., J. Electrochem. Soc., 159 (2012) A1034. Figure 1

Enhancement of oxygen reduction and evolution reaction (ORR and OER) performance has been considered to play a key role for the development of next-generation energy storage devices such as metal-air batteries and water electrolyzers. Recently, mixed-anion compounds have been found to be highly efficient bifunctional electrocatalysts for ORR and OER[1,2]. Our group has been also clarified that layered perovskite oxychloride Sr2Co1 − x Fe x O3Cl serves as a good bifunctional catalyst, whose OER activity outperformed that of Ba0.5Sr0.5Co0.8Fe0.2O3 −δ [3]. While detailed mechanism of its enhanced OER activity has been discussed previously[4], that of ORR has not been evaluated in detail, even though ORR pathway is generally considered to be much more complicated. Moreover, further exploration of highly active bifunctional mixed-anion perovskites has to be continued for better understanding of oxygen electrocatalysis on mixed-anion perovskite compounds. In the present research, therefore, we focused on the evaluation of ORR pathway for Sr2Co1 − x Fe x O3Cl, and oxygen electrocatalysis on other mixed-anion perovskite compounds in alkaline media. Solid-state method was applied to the synthesis of Sr2Co1 − x Fe x O3Cl and other mixed-anion perovskites. Electrochemical measurements were conducted by a three-electrode cell with a rotating disk electrode (RDE). Glassy carbon RDEs coated by a catalyst layer consisting of catalysts, Vulcan XC-72 (Cabot), and AS-4 (Tokuyama) were used for the working electrode. Pt wire and reversible hydrogen electrode were used for the counter and reference electrodes, respectively. Oxygen-saturated 1 mol dm− 3 KOH solution was used for ORR activity tests, and Argon-saturated 1 mol dm− 3 KOH solution containing 5 mmol dm− 3 H2O2 was used for the activity tests of peroxide reduction reaction (PRR). Peroxide decomposition rates of Sr2Co1 − x Fe x O3Cl were performed by measuring evolved oxygen gas in the same manner as previously described[5]. While the highest OER activity was observed at x = 0.2, ORR activity of Sr2Co1 − x Fe x O3Cl was monotonously decreased with increase of Fe doping. Figure 1 shows decomposition rates of Sr2Co1 − x Fe x O3Cl in 1 mol dm− 3 KOH. Surprisingly, their HO2 − decomposition rate was higher than those of cobalt-based perovskite oxides such as LaCoO3, whose ORR activity was comparable to those of oxychlorides. Further discussion about ORR pathway of Sr2Co1 − x Fe x O3Cl, and oxygen electrocatalysis of other mixed-anion perovskites will be provided at the conference. Figure 1. H2O2 decomposition rates of Sr2Co1 − x Fe x O3Cl powders. References [1] A. Miura, C. Rosero-Navarro, Y. Masubuchi, M. Higuchi, S. Kikkawa, and K. Tadanaga, Angew. Chem. Int. Ed., 55 (2016) 7963. [2] M. A. Ghanem, P. Arunachalam, A. Almayouf, and M. T. Weller, J. Electrochem. Soc., 163 (2016) H450. [3] Y. Miyahara, K. Miyazaki, T. Fukutsuka, and T. Abe, Chem. Commun., 53 (2017) 2713. [4] Y. Miyahara, K. Miyazaki, T. Fukutsuka, and T. Abe, 232nd ECS Meeting, (2017) A1-30. [5] H. M. Zhang, Y. Shimizu, Y. Teraoka, N. Miura, and N. Yamazoe, J. Catal., 121 (1990) 432. Figure 1

For the fast charge of lithium-ion batteries, internal resistances must be decreased. Among the internal resistances, charge (lithium-ion) transfer resistances and ion transport resistances inside the composite electrode are the major ones. Charge transfer resistance of R ct can be expressed by 1/R ct = A/T exp(-E a/RT). Here, A, T, E a, and R are pre-exponential factor, absolute temperature, activation energy, and gas constant, respectively. We have extensively studied the activation energies for the charge transfer reactions, and concluded that de-solvation process is responsible for the activation energies. Then, we have tried to understand the pre-exponential factor of A. This factor corresponds to the reaction sites of insertion active materials. Here we evaluated the reaction sites of high oriented pyrolytic graphite by several methods, and the charge transfer resistances of graphite electrode were correlated with the reaction sites.

The electrochemical stability of an electrolyte solution, its so-called "potential window", is simply determined by the oxidative potential and reductive potential of the solvent, if solutes dissolved in the electrolyte solution are electrochemically stable within the potential window of the solvent. Water is the most conventional solvent in the field of electrochemistry, and its potential window is as narrow as 1.23 V, thermodynamically. Practically, however, the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) need certain overpotentials, which extends the potential windows of aqueous solutions. Besides the catalytic activities of the electrode materials, the properties of aqueous solutions such as hydration structures and salt concentrations also affect the potential window.Recently, aqueous rechargeable lithium batteries (ARLBs) have attracted much attention due to their low cost and safety. However, the working voltage of ARLBs is limited to below 1.5 V due to the narrow potential window of water. Recently, a few papers have discussed the potential window of concentrated aqueous solutions with neutral pH. However, there are still fundamental unanswered questions regarding the pH-dependence of the potential window of water. In this study, the potential window of concentrated electrolyte solutions with neutral pH and the expansion of the potential window were investigated from the viewpoint of the local pH change and water concentration. Here, we shed light on the dependence of the potential window of water on local pH changes and water concentrations. Next, we focused on the water concentration, since the potential windows are determined by the water electrolysis reactions (OER and HER). The potential windows are shown in Fig. 1 as a function of the water concentrations. In this study, the onset potential of the OER/HER was defined when the current density was ± 0.1 mA cm-2 in CVs at 1 mV s-1. There are two clear tendencies in Fig. 1. First, the windows in neutral pH electrolyte solutions (closed symbols in Fig. 1) were obviously wider than those in acidic/alkaline electrolyte solutions (open symbols in Fig. 1), even at a dilute salt concentration (ca. 55 M water concentration). Second, the windows in neutral pH electrolyte solution (closed symbols in Fig. 1) were not affected by the kind of the electrolyte salt, but depended linearly on the water concentration. The windows were expanded when the water concentration decreased, i.e., the electrolyte salt concentrations increased. Potential windows of aqueous solutions were investigated systematically using various salts at different concentrations. Cyclic voltammetry measurements of Pt electrodes revealed two important points. First, the potential window in unbuffered neutral pH solution was broader than that in acidic/alkaline solutions. This expansion of potential windows can be explained by the shift in the reaction potential with local pH changes in the vicinity of the electrode. Second, the potential windows were not affected by electrolyte salts, but rather depended linearly on the water concentration. The difference for OER overpotentials was much larger than that for HER overpotentials. While HER overpotentials were derived from a local pH change, OER overpotentials were derived from both a reduced water concentration and local pH change. This study highlights the importance of these two main factors (water concentration and local pH change) in determining the potential windows of concentrated electrolyte solutions. Figure 1

Present LIBs for EV-use possess large power densities by the sacrifice of energy densities in order to shorten the charging time. In the practical LIBs for portable devices, dense and thick composite electrodes are used. In this case, the ion transport of electrolyte solution through the pores in the composite electrodes becomes a slow process. In order to enhance the energy densities with remaining high power densities, the ion transport rate should be increased for the dense and thick composite electrodes. Unfortunately, to develop dense and thick electrodes with high power densities, fundamental understanding of the ion transport in composite electrodes has not been clear. In this study, ion transportation behavior was investigated with ac impedance spectroscopy. To focus on the impedance of ion transportation of electrolyte through the pores in composite electrode, we prepared anodic aluminum membranes with homogeneous macro- and meso- pores. In addition, graphite composite electrodes were also used. As a result, we obtained semi-circles, and this semi-circle was assigned to the ion transport process through the graphite composite electrode. The detail analysis will be presented at the meeting.

Introduction There has been increasing attention to electrochemical energy conversion devices using oxygen electrodes in alkaline media such as fuel cells, metal−air rechargeable batteries, and water-splitting devices. Because of the sluggish kinetics of the oxygen electrode reactions, many researchers have been exploring cost-effective materials which can catalyze both/either oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER). Among various kinds of catalysts, a family of perovskite-type compounds have been extensively studied because of its structural and compositional flexibility. In order to enhance the catalytic activity of perovskites, researches based on the choice of cation elements and their compositions has been extensively conducted.1 On the other hand, those based on the choice of anion elements and their compositions has been rarely conducted. These days, some research groups report that mixed-anion catalysts exhibit ORR and/or OER activities,2,3 suggesting that “anion blending” can be a great tool for enhancing catalytic activity of perovskites. Therefore, the present work focuses on the measurement of ORR and OER activities for mixed-anion perovskites. Herein we used Sr2CoO3Cl and its related materials.4 Experimental Sr2CoO3Cl and Sr3Co2O5Cl2 were synthesized by a solid-state method. Sr2Co2O5 precursor was firstly prepared by heating a stoichiometric mixture of SrCO3 and Co3O4 at 1273 K in air followed by quenching in liquid N2. Next, stoichiometric amounts of Sr2Co2O5, SrCO3, and SrCl2·6H2O were mixed and heated at 1123 K in air. For benchmarking catalysts, LaSrCoO4, which has the same crystal structure and valence state of cobalt ion, LaCoO3, which is a well-known single-perovskite catalyst with the same valence state of cobalt ion, and Ba0.5Sr0.5Co0.8Fe0.2O3 -d (BSCF), which is known as a highly-active OER electrocatalyst, were used. ORR and OER activities of the samples were carried out by using three-electrode electrochemical cell with rotating disk electrode (RDE) setup. Composite electrodes consisting of perovskites, Vulcan XC-72 (Cabot), and AS-4 (Tokuyama) were formed on a glassy carbon RDE and were used as the working electrode. Pt wire, reversible hydrogen electrode (RHE), and 1.0 mol dm-3 KOH solution were used as the counter electrode, the reference electrode, and the electrolyte, respectively. Results and discussions From scanning electron microscope-energy dispersive X-ray spectroscopy, the oxychlorides had a particle size of 1-10 μm with almost ideal atomic concentrations. X-ray diffraction patterns of the oxychlorides were almost identical to those in ICSD database. While the ORR activity of LaSrCoO4 was negligible, that of the oxychlorides was as high as that of LaCoO3, indicating that the mixing of oxide and chloride ions was effective to add the ORR activity. Fig. 1 shows OER polarization curves of perovskite catalysts. Surprisingly, the OER activity of Sr2CoO3Cl was as high as that of BSCF, proving that “anion blending” is a powerful tool to enhance the OER activity. Roles of chloride ion in improving ORR and OER activities will be discussed in the conference. Fig. 1. OER polarization curves of (solid line) Sr2CoO3Cl, (dashed line) LaSrCoO4, and (dotted line) Ba0.5Sr0.5Co0.8Fe0.2O3 -d in 1 mol dm-3 KOH solution. References 1. D. Chen, C. Chen, Z. M. Baiyee, Z. Shao, and F. Ciucci, Chem. Rev., 115, 9869 (2015). 2. A. Miura, C. Rosero-Navarro, Y. Masubuchi, M. Higuchi, S. Kikkawa, and K. Tadanaga, Angew. Chem. Int. Ed., 55, 7963 (2016). 3. M. A. Ghanem, P. Arunachalam, A. Almayouf, and M. T. Weller, J. Electrochem. Soc., 163 H450 (2016). 4. Y. Miyahara, K. Miyazaki, T. Fukutsuka, and T. Abe, Chem. Comm., 53 2713 (2017). Figure 1

Introduction Much attention has been paid to oxygen electrochemical reactions in alkaline electrolytes such as alkaline fuel cell, alkaline rechargeable metal-air battery and water-splitting devices. Many researchers have been exploring highly-active and cost-effective oxygen electrode catalysts, and one example of good oxygen electrocatalysts is perovskite-type oxide with the general formula of ABO3. Our group focuses on observing intrinsic electrochemical properties of perovskite when used as an oxygen electrode catalyst, using a carbon-free and binder-free perovskite electrode.1, 2 One of our research findings was that whereas oxygen evolution reaction activity of model electrodes were as high as that of composite electrodes consisting of perovskite powder, carbon and binder, oxygen reduction reaction activity of model electrodes was much lower than that of composite electrodes. This contrasting behavior motivated us to conduct further experiments such as hydrogen peroxide reduction reaction activity and cyclic characteristics of oxygen electrochemical reactions by using the model electrode and the composite electrode in terms of surface oxygen non-stoichiometry, which is our present research topic. Experimental section Synthesis and fabrication of La0.8Sr0.2CoO3 (LSCO) crystal electrodes were described in our previous literature.2 Perovskite powders were also synthesized by hydroxide-acid aided process3 and solid state process. The resultant oxide crystals and powders were characterized by using X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, Brunauer-Emmett-Teller for oxide powders, and iodometry for oxide powders. Electrochemical measurements were conducted by using a rotating disk electrode (RDE) in a three-electrode cell. For a working electrode, RDE of LSCO crystal electrodes were assembled by mounting a LSCO crystal in a Teflon disk holder. As for composite electrodes, a catalyst layer consisting of 250 μg cm-2 perovskite powder, 50 μg cm-2 Vulcan XC-72 (Cabot), and 50 μg cm-2 anion conductive ionomer (AS-4, Tokuyama) was formed on glassy carbon (GC) RDE. 50 μg cm-2 Pt(40wt%)/Vulcan (E-TEK) and 50 μg cm-2 anion conductive ionomer on GC RDE was also used as benchmarking of oxygen electrochemical reactions. Pt wire and reversible hydrogen electrode (RHE) were used as counter and reference electrodes, respectively. Oxygen electrochemical activity measurements were conducted using a solution of oxygen-saturated 1.0 mol dm-3 KOH for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and argon-purged 1 mol dm-3 KOH containing ca. 5 mmol dm-3 H2O2 for hydrogen peroxide reduction reaction (PRR). For qualitative electronic conductivity measurement, ferricyanide reduction reaction (FRR) was observed in a solution of argon-purged 1.0 mol dm-3 KOH containing 20 mmol dm-3 K3[Fe(CN)6]. Results and discussions XRD and EDX measurements confirmed that resultant LSCO crystals and perovskite powders had perovskite structure and the atomic compositions were quite similar as the desired composition. Figure 1 shows PRR activity on LSCO crystal. In the initial cycles, it shows clear rotation dependence indicating that LSCO crystal was active toward PRR. However, as the cycle number of cyclic voltammetry increased, reduction currents were decreased and finally showed no dependence on rotation. Similar results were observed for solid-state-synthesized LSCO composite electrode suggesting that obtained results by model electrodes were universal for practical electrodes. The decrease in activities was also observed for ORR, while the decrease in OER activity on LSCO crystal was not observed. FRR after PRR and ORR showed that electronic conductivity of LSCO crystal was decreased. The reason of activity decrease in ORR and PRR, and the influence of B-site cation in ORR and PRR in terms of surface oxygen non-stoichiometry will be discussed in the conference. References 1. Y. Miyahara, K. Miyazaki, T. Fukutsuka, and T. Abe, J. Electrochem. Soc., 161(6), F694 (2014). 2. Y. Miyahara, K. Miyazaki, T. Fukutsuka, and T. Abe, ChemElectroChem, 3, 214 (2016). 3. Y. Teraoka, H. Kakebayashi, I. Moriguchi, and S. Kagawa, J. Alloys Compd., 193, 70 (1993). Figure 1

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