Oxygen Reduction Reaction Current Research Articles

(Digital Presentation) Improvement of Oxygen Reduction Activity on Ti Oxide-Based Electrocatalyst

For widespread utilization of fuel cell system in the future, we have focused on non-precious metal electrocatalyst against high cost of system. Especially, we have studied and reported group 4 and 5 metal oxide-based electrocatalyst as non-platinum catalysts for the oxygen reduction reaction (ORR) because of low-cost, abundant reserves, and high stability in acidic electrolytes [1-2]. We have found excellent ORR activity of Zr oxide-based electrocatalysts prepared from pyrazinecarboxylic acid as a precursor [3-4]. In the case of Ti oxide-based electrocatalysts, the sample prepared under low partial pressure of oxygen with the heat treatment shows high ORR activity when it uses nitrogen-containing porphyrin cyclic organic Ti complexes (TiOTPPz) with multi-walled carbon nanotubes (MWCNTs) as a precursor. However, the heat treatment conditions have not widely investigated yet [5]. In this study, we have investigated to improve ORR activity of Ti oxide-based electrocatalyst prepared from TiOTPPz by varying the oxygen concentration during heat treatment.Oxytitanium tetrapyrazinoporphyrazine (TiOTPPz, [TiOC24H8N16]), a porphyrin cyclic nitrogen-containing organic Ti complex, was used as a starting material. 0.39 g of TiOTPPz and 0.26 g of MWCNT were mixed and treated in a dry ball mill for 1 h to obtain the precursor. The precursor was heated up to 900 oC in an Ar atmosphere and then annealed at 900 oC for 3 h in a low-oxygen atmosphere to obtain a catalyst with precipitated carbon and Ti oxide dispersed on the MWCNTs. The conditions of the hypoxic atmosphere were 2 %H2 + n %O2 / inert gas, with n = 0.05, 0.1, 0.5, 1.0, and 2.0. Catalysts are denoted by TiCNO_(O2 concentration during heat treatment). The catalyst powder was dispersed into 1-propanol with Nafion solution to prepare a catalyst ink. The ink was dropped on a glassy carbon rod, and dried for an hour to use as a working electrode in electrochemical measurement.Electrochemical measurements were performed in 0.5 mol dm-3 H2SO4 at 30 oC with a conventional 3-electrode cell. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as used as a reference and counter electrode, respectively. Slow scan voltammetry (SSV) was performed at a scan rate of 5 mV s-1 from 0.2 V to 1.2 V vs. RHE under O2 and N2. The ORR current (i ORR) was determined by calculating the difference between the current under O2 and N2.Figure 1 shows oxygen reduction activity of Ti oxide-based electrocatalyst. The vertical axis shows the |i ORR@0.8 V|. The |i ORR@0.8 V| was less than 100 mA gcat -1 at O2 concentrations of 0.05 and 0.1 %, and was greatest at 0.5 % at 413 mA gcat -1. The concentration of O2 further decreased with increasing O2 concentration, and was the smallest at 2.0%. It is suggested that the O2 concentration during heat treatment may have an optimum value around 0.5%.We have also analyzed the catalysts by the XRD before electrochemical measurements. According to XRD spectra, the presence of TiC0.3O0.7 was confirmed in all catalysts. In Ti-CNO_0.5 %, the most active catalyst, the diffraction peaks identified TiO2 rutile and TiO2 anatase were observed. In the case of Ti-CNO_1 and 2 %, the peak intensities of TiO2 anatase increased and the ORR activity decreased. This indicates that the degree of Ti oxidation can be controlled by the atmosphere during heat treatment.Acknowledgement: The authors thank for providing TiOTPPz from Dainichiseika Color & Chemicals Mfg. Co., Ltd.Reference[1] A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).[2] A. Ishihara, S. Tominaka, S. Mitsushima, H. Imai, H. Imai, O. Sugino, and K. Ota, Curr. Opin. Electrochem., 21 , 234 (2020).[3] Y. Takeuchi, K. Matsuzawa, T. Nagai, K. Ikegami, Y. Kuroda, R. Monden and A. Ishihara, Bull. Chem. Soc. Jpn., 96, 175 (2023).[4] Y. Takeuchi, K. Matsuzawa, Y. Matsuoka, K. Watanabe, T. Nagai, R. Monden and A. Ishihara, Electrochemistry, (2023) in press[5] T. Hayashi, A. Ishihara, T. Nagai, M. Arao, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima and K. Ota, Electrochim. Acta, 209, 1 (2016). Figure 1

From 2e- to 4e- pathway in the alkaline oxygen reduction reaction on Au(100): Kinetic circumvention of the volcano curve.

We report the free energy barriers for the elementary reactions in the 2e- and 4e- oxygen reduction reaction (ORR) steps on Au(100) in an alkaline solution. Due to the weak adsorption energy of O2 on Au(100), the barrier for the association channel is very low, and the 2e- pathway is clearly favored, while the barrier for the O-O dissociation channel is significantly higher at 0.5eV. Above 0.7V reversible hydrogen electrode (RHE), the association channel becomes thermodynamically unfavorable, which opens up the O-O dissociation channel, leading to the 4e- pathway. The low adsorption energy of oxygenated species on Au is now an advantage, and residue ORR current can be observed up to the 1.0-1.2V region (RHE). In contrast, the O-O dissociation barrier on Au(111) is significantly higher, at close to 0.9eV, due to coupling with surface reorganization, which explains the lower ORR activity on Au(111) than that on Au(100). In combination with the previously suggested outer sphere electron transfer to O2 for its initial adsorption, these results provide a consistent explanation for the features in the experimentally measured polarization curve for the alkaline ORR on Au(100) and demonstrate an ORR mechanism distinct from that on Pt(111). It also highlights the importance to consider the spin state of O2 in ORR and to understand the activation barriers, in addition to the adsorption energies, to account for the features observed in electrochemical measurements.

Boosting Oxygen Reduction at Pt(111)|Proton Exchange Ionomer Interfaces through Tuning the Microenvironment Water Activity.

A proton exchange ionomer is one of the most important components in membrane electrode assemblies (MEAs) of polymer electrolyte membrane fuel cells (PEMFCs). It acts as both a proton conductor and a binder for nanocatalysts and carbon supports. The structure and the wetting conditions of the MEAs have a great impact on the microenvironment at the three-phase interphases in the MEAs, which can significantly influence the electrode kinetics such as the oxygen reduction reaction (ORR) at the cathode. Herein, by using the Pt(111)|X ionomer interface as a model system (X = Nafion, Aciplex, D72), we find that higher drying temperature lowers the onset potential for sulfonate adsorption and reduces apparent ORR current, while the current wave for OHad formation drops and shifts positively. Surprisingly, the intrinsic ORR activity is higher after properly correcting the blocking effect of Pt active sites by sulfonate adsorption and the poly(tetrafluoroethylene) (PTFE) skeleton. These results are well explained by the reduced water activity at the interfaces induced by the ionomer/PTFE, according to the mixed potential effect. Implications for how to prepare MEAs with improved ORR activity are provided.

Inhibiting Metal Galvanic and Carbon Corrosion in Aluminum Alloy-Carbon Fiber Reinforced Composite Joints by Spontaneous Deposition of Diazonium Adlayers on Exposed Carbon Fibers

The spontaneous formation of a diazonium molecular adlayer on the exposed fibers of a carbon fiber reinforced polymer (CFRP) composite was investigated as a surface treatment. The purpose of the surface treatment is to reduce the extent of galvanic corrosion on a trivalent chromium process (TCP) conversion-coated aluminum alloy when the two materials are mechanically joined as is common in aircraft structures. The adlayer functions to reduce the rate of dissolved oxygen reduction on the more noble carbon thereby reducing the rate of galvanic corrosion on the more active metal alloy. The formation of the adlayer (4-nitrophenyl (NP), 4-nitroazobenzene (NAB), or 2-fluorene diazonium tetrafluoroborate (FL)) was achieved by immersing the CFRP composite specimen in a solution of 5 mM diazonium salt dissolved in acetonitrile for a specified time (1–24 h) under open circuit conditions (so-called spontaneous deposition). Cyclic voltammetry (CFRP composite) and rotating disk voltammetry (glassy carbon) were used to investigate the oxygen reduction reaction kinetics in naturally aerated 0.5 M Na2SO4 (pH 5–6). The oxygen reduction reaction current was suppressed by greater than 90% by the NP and NAB adlayers and 80% by the FL adlayer. The surface treatment (24 h immersion) with NAB significantly reduced the galvanic corrosion damage on TCP conversion-coated aluminum alloy during a 14-day continuous neutral salt spray (NSS) exposure. The mass loss (mg/cm2) and corrosion intensity (g/m2y) were reduced by over 10×. The spontaneously formed NAB adlayers are stable during the salt-spray exposure and provide better inhibition of galvanic corrosion on the aluminum alloy and resistance to carbon corrosion than do NAB adlayers formed by an electrochemically assisted process reported on previously.

Fe and Zn Addition Effects of Ti Oxide-Based Electrocatalyst on Catalytic Activity for Oxygen Reduction Reaction

The conventional catalyst of polymer electrolyte fuel cell (PEFC) is a platinum which is categorized as precious metal with very high-cost material, and its performance is limited in principal. From this point of view, the non-precious metal electrocatalyst should be required. We have focused and studied group 4 and 5 metal oxide-based electrocatalyst as non-platinum catalysts for the oxygen reduction reaction (ORR) because of low-cost, abundant reserves, and high stability in acidic electrolytes [1-2]. We found that titanium oxide prepared from TiOTPyzPz supported on multi-walled carbon nanotubes had superior ORR activity [3]. On the other hand, it was published as an international patent that the addition of other elements such as Fe and Ni is affected to enhance the ORR activity of Ti oxide-based electrocatalyst [4]. We have also applied for the TiOTPyzPz as a starting material with Fe, Ni, and Zn addition to enhance the ORR activity of Ti oxide-based electrocatalyst [5-6]. In this study, we have investigated the addition effects of Zn and Fe for Ti oxide-based electrocatalyst on the catalytic activity for the ORR.2,3-Dicyanopyrazine, urea, and Ti isopropoxide were dissolved in quinoline and refluxed to synthesize TiOTPyzPz. Iron acetate and zinc acetate were also added to dissolve in quinoline to obtain the Fe and Zn-added TiOTPyzPz as a starting material. These starting materials were mixed with carbon nanotube by ball-milling to prepare the precursors. These precursors were heat-treated under low oxygen partial pressure at 900 oC for 3 h to obtain the oxide-based electrocatalyst powder. The catalyst powder was dispersed into 1-propanol with Nafion solution to prepare a catalyst ink. The ink was dropped on a glassy carbon rod to use as a working electrode in electrochemical measurement.Electrochemical measurements were performed in 0.5 mol dm-3 H2SO4 at 30 oC with a conventional 3-electrode cell. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as used as a reference and counter electrode, respectively. Slow scan voltammetry (SSV) was performed at a scan rate of 5 mV s-1 from 0.2 V to 1.2 V vs. RHE under O2 and N2. The ORR current (i ORR) was determined by calculating the difference between the current under O2 and N2.Figure 1 shows the ORR polarization curves Fe and/or Zn addition to the Ti oxide-based electrocatalysts. The Fe and Zn added Ti oxide-based electrocatalyst (Fe,Zn-TiOx) was obviously the highest activity for the ORR, and it was higher ORR activity than Fe added Ti oxide-based electrocatalyst (Fe-TiOx), Zn added Ti oxide-based electrocatalyst (Zn-TiOx), and Ti oxide-based electrocatalyst (TiOx) without addition. It reveals that the addition of Fe and Zn was found to be effective for enhancing the ORR activity, and Fe addition was more effective than Zn addition for enhancing the ORR activity. XRD pattern of TiOx shows several peaks identified TiO2-Rutile and TiC0.3N0.7 while the XRD pattern of Fe-TiOx, Zn-TiOx, and Fe,Zn-TiOx also shows several peaks identified similar to TiOx. According to results from ICP analysis, the contain amount of Ti and Fe in Fe,Zn-TiOx was similar to preparation amount in precursor, but that of Zn showed below detection limit. From SEM images, Zn-TiOx and Fe,Zn-TiOx have more space in its powder than Fe-TiOx and TiOx. Based on above results, it is suggested that Zn addition affected to increased electrochemical surface area of TiOx, and Fe addition affected to form active site [7] in TiOx as similar case as Zr oxide-based electrocatalyst [8].Acknowledgement: The authors thank New Energy and Industrial Technology Development Organization (NEDO) and ENEOS Tonen General Research / Development Encouragement & Scholarship Foundation for financial support.Reference A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).A. Ishihara, S. Tominaka, S. Mitsushima, H. Imai, H. Imai, O. Sugino, and K. Ota, Curr. Opin. Electrochem., 21 , 234 (2020).S. Tominaka, A. Ishihara, T. Nagai, and K. Ota, ACS Omega, 2, 5209 (2017).K. Takahashi, T. Imai, R. Monden, Y. Wakisaka, and S. Sato, Oxygen Reduction Catalyst, Process for Producing Same, and Polymer Electrolyte Membrane Fuel Cell, WO/2013/008501.K. Matsuzawa, M. Obata, Y. Takeuchi, Y. Ohgi, K. Ikegami, T. Nagai, R. Monden and A. Ishihara, ECS Trans., 108(7), 181 (2022).K. Matsuzawa, M. Obata, Y. Takeuchi, Y. Ohgi, K. Ikegami, T. Nagai, R. Monden and A. Ishihara, ECS Trans., 108(7), 189 (2022).J-H. Kim, A. Ishihara, S. Mitsushima, N. Kamiya, and K. Ota, Acta, 52, 2492 (2007).Y. Takeuchi, K. Watanabe, K. Matsuzawa, T. Nagai, K. Ikegami, R. Monden, and A. Ishihara, Chem. Lett., 51, 927 (2022). Figure 1

Hierarchical Structure of Gold and Carbon Electrode for Bilirubin Oxidase-Biocathode.

Biofuel cells (BFCs) with enzymatic electrocatalysts have attracted significant attention, especially as power sources for wearable and implantable devices; however, the applications of BFCs are limited owing to the limited O2 supply. This can be addressed by using air-diffusion-type bilirubin oxidase (BOD) cathodes, and thus the further development of the hierarchical structure of porous electrodes with highly effective specific surface areas is critical. In this study, a porous layer of gold is deposited over magnesium-oxide-templated carbon (MgOC) to form BOD-based biocathodes for the oxygen reduction reaction (ORR). Porous gold structures are constructed via electrochemical deposition of gold via dynamic hydrogen bubble templating (DHBT). Hydrogen bubbles used as a template and controlled by the Coulomb number yield a porous gold structure during the electrochemical deposition process. The current density of the ORR catalyzed by BOD without a redox mediator on the gold-modified MgOC electrode was 1.3 times higher than that of the ORR on the MgOC electrode. Furthermore, the gold-deposited electrodes were modified with aromatic thiols containing negatively charged functional groups to improve the orientation of BOD on the electrode surface to facilitate efficient electron transfer at the heterogeneous surface, thereby achieving an ORR current of 12 mA cm-2 at pH 5 and 25 °C. These results suggest that DHBT is an efficient method for the fabrication of nanostructured electrodes that promote direct electron transfer with oxidoreductase enzymes.

Immobilization of a Molecular Copper Complex and a Carboxylate-Terminated Cocatalyst on a Metal Oxide Electrode for Enhanced Electrocatalytic Oxygen Reduction

Positioning basic/acidic groups near a molecular catalyst has become a popular strategy to modulate proton-coupled electron transfer reactions in energy conversion and storage. However, the detailed understanding of the real role played by basic/acidic groups remains lacking. Here, we report the immobilization of a copper(II) dipicolylamine catalyst bearing a phosphonic anchor (CuL) on a metal oxide electrode to investigate the activity of oxygen reduction reaction (ORR) in aqueous solution. The catalytic current of ORR by the CuL catalyst was significantly enhanced by three times as a carboxylate-terminated ligand was integrated onto the metal oxide surfaces to act as a cocatalyst. The CuL-coated metal oxide electrode with a carboxylate-terminated cocatalyst exhibits turnover frequency values higher than the one with an amine-terminated cocatalyst by 2.5-fold and the CuL-only case by 9.6-fold. Extensive experimental and theoretical studies reveal that a hydrogen bond formed between the proximal carboxylate of the cocatalyst and a Cu-OOH intermediate generated via protonation of a Cu-O2 adduct by the bulk buffer solution that aids O–O bond cleavage, thereby promoting the four-electron four-proton reduction of dioxygen into water reaching a Faradaic efficiency of 97.6%. Our finding is in stark contrast to the typical assignment on the role of basic groups near a catalyst that features proton shuttling to facilitate protonation of a metal-O2 intermediate forming a metal-OOH intermediate. This work offers a facile strategy to enhance simultaneously the activity and selectivity of surface-supported non-precious metal electrocatalysts that are important toward realizing alternative energy conversion schemes.

Case Study on the Reproducibility of Characterizing the Electrocatalytic Activity of La1−xSrxMnO3 in ORR

The electrocatalytic activity characterization of the oxygen reduction reaction (ORR) is commonly characterized using a rotating disk electrode (RDE) with linear sweep voltammetry (LSV) or cyclic voltammetry (CV) measurements. Despite the wide application of this method in the literature, its reproducibility and comparability are rarely mentioned in articles. LSV and CV are sensitive to experimental conditions, thus the reproducibility is a significant concern. In this article, the perovskite oxides La1−xSrxMnO3 (LSMOx, x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) are chosen for the case study to measure their electroactivity using the RDE method. The main elements that influence the reproducibility of the experiment are presented and the corresponding explanations as well as the means for ensuring the reproducibility are given. Through the case study of LSMOx with variations in x and calcination temperatures, the values for the kinetic current of ORR on different catalysts are compared. It is found that doping ratios above 0.1 are associated with higher ORR electroactivity, with around a 50 mV difference in ORR onset potential between x > 0.1 and x = 0.1, which is in accordance with the results of reported works, whereas there is no significant difference when x = 0.2–0.5. Calcination temperature has little influence on the electroactivity of LSMOx, resulting in only a slight advantage at lower calcination temperatures, with an ORR onset potential that is around 10 mV more positive.

Oxygen reduction reaction monitored during corrosion by means of pumped-micropipette delivery/substrate collection (Pumped-MD/SC) mode of scanning electrochemical microscopy

The key improvement of the corrosion resistance of metals requires understanding and resolving the electrochemical reactions involved at the local scale. Here, we present the use of the pumped-micropipette delivery/substrate collection (Pumped-MD/SC) mode of scanning electrochemical microscopy (SECM) to monitor the oxygen reduction reaction (ORR) during corrosion of an Aluminum (Al) / Copper (Cu) galvanic coupled electrode, a model system of copper-rich aluminum alloys. In this case, the micropipette scans the substrate electrode pumping out a controlled flow of air-saturated electrolyte solution, which allows, through convective transport, to achieve high mass-transfer rates under steady-state conditions at the substrate electrode. The most relevant parameters in pumped-MD/SC mode were studied, it was demonstrated that the ORR current is linearly dependent on the injection flow rate. In contrast, the maximum ORR current was reached regardless of the tip-substrate distance (d) within the range of L (d/ micropipette radius (a)) < 15, which represents a clear advantage in comparison with other modes of SECM. It was also proved that the ORR might take place on both Cu and Al electrodes depending on the applied potential. In addition, the pumped-MD/SC mode of SECM was successfully used for imaging large dimension surfaces with both high spatial and temporal resolution, as well as high imaging contrast thanks to the high mass-transfer rate of electroactive species achieved by forced convection. Moreover, relevant information to evaluate the effect of different corrosion inhibitors in solution such as Ce2(SO4)3 and Li2CO3 on ORR was provided by individually addressing the substrate electrode in static pumped-MD/SC mode. It was demonstrated that only Ce2(SO4)3 played the role of cathodic inhibitor through the formation of a Ce3+ oxide/hydroxide precipitate on the substrate electrode during ORR, which was confirmed by SEM and EDX analysis.

Activity Quantification of Fuel Cell Catalysts via Sequential Poisoning by Multiple Reaction Inhibitors.

The development of non-Pt or carbon-based catalysts for anion exchange membrane fuel cells (AEMFCs) requires identification of the active sites of the catalyst. Since not only metals but also carbon materials exhibit oxygen reduction reaction (ORR) activity in alkaline conditions, the contribution of carbon-based materials to ORR performance should also be thoroughly analyzed. However, the conventional CN- poisoning experiments, which are mainly used to explain the main active site of M-N-C catalysts, are limited to only qualitative discussions, having the potential to make fundamental errors. Here, we report a modified electrochemical analysis to quantitatively investigate the contribution of the metal and carbon active sites to ORR currents at a fixed potential by sequentially performing chronoamperometry with two reaction inhibitors, CN- and benzyl trimethylammonium (BTMA+). As a result, we discover how to quantify the individual contributions of two active sites (Pt nanoparticles and carbon support) of carbon-supported Pt (Pt/C) nanoparticles as a model catalyst. This study is expected to provide important clues for the active site analysis of carbon-supported non-Pt catalysts, such as M-N-C catalysts composed of heterogeneous elements.

Understanding the Origin of Gas Transport Resistance in Fuel Cell Catalyst Layers

For the widespread use of proton exchange membrane fuel cells (PEMFCs), cost reduction of the stack is indispensable. For this purpose, it is effective to reduce the use of Pt and other cell components by reducing the cell area with a high-power density of the cell while reducing the use of Pt catalyst per geometric area. However, automobile manufacturers have reported that reducing the amount of Pt used per geometric area increases the resistance of gas transport in the catalyst layer, resulting in greater performance loss during high power generation.(1-3) This gas transport resistance is found to be inversely proportional to the electrochemical Pt surface area per geometric area (ECSA), and is considered to stem from near the Pt surface.(4) Previously, our group proposed a mechanism in which a high-density layer of ionomer formed at the ionomer / Pt interface is the cause of the increase in gas transport resistance at a low Pt content (low Pt area) based on the analysis of oxygen transport resistance using a microelectrode and the molecular dynamics calculation.(5-9) In this presentation, we report the results of investigating the origin of gas transport resistance in the cathode catalyst layer (CL) using CLs with a low Pt loading using Vulcan® as the catalyst support. In the case of a catalyst layer using a solid carbon support such as Vulcan®, the resistance can be divided into two categories: resistance independent of pressure and Pt surface area (Knudsen diffusion resistance in secondary pores formed among carbon particles) and resistance independent of pressure and dependent on Pt surface area (resistance near Pt surface). We modified a method of Katayama et al.,(10) which uses CO to control the ECSA of an identical catalyst layer and measures the limiting current of oxygen reduction reaction, and examined a method of separating the Knudsen diffusion resistance and the resistance near Pt using the limiting current of hydrogen oxidation reaction.(11) Furthermore, this technique was applied to catalyst layers with different ionomer-to-carbon weight ratios with a constant ionomer coverage to separate the contribution of gas transport resistance in the ionomer from that at the ionomer / Pt interface. This electrochemical analysis revealed that the gas transport resistance at the interface of ionomer and Pt surface is dominant in the range of practical ionomer-to-carbon weight ratio. The small contribution of the secondary pores was supported by a microscopic model of the porous CL. In this analysis, the microscopic secondary porous model was reconstructed from 3-dimensional scanning electron microscopy (3D-SEM) images, and the molecular and Knudsen oxygen diffusion in the reconstructed model was simulated. The combined analyses indicate that the ionomer-Pt interfacial structure is a key for the design of the cathode CL. Y. Ono, T. Mashio, S. Takaichi, A. Ohma, H. Kanesaka and K. Shinohara, ECS Trans., 28, 69 (2010). N. Nonoyama, S. Okazaki, A. Z. Weber, Y. Ikogi and T. Yoshida, Journal of The Electrochemical Society, 158, B416 (2011). T. A. Greszler, D. Caulk and P. Sinha, Journal of the Electrochemical Society, 159, F831 (2012). A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 7, 1127 (2016). R. Jinnouchi, K. Kudo, K. Kodama, N. Kitano, T. Suzuki, S. Minami, K. Shinozaki, N. Hasegawa and A. Shinohara, Nat Commun, 12, 4956 (2021). R. Jinnouchi, K. Kudo, N. Kitano and Y. Morimoto, Electrochimica Acta, 188, 767 (2016). K. Kudo, R. Jinnouchi and Y. Morimoto, Electrochimica Acta, 209, 682 (2016). T. Suzuki, K. Kudo and Y. Morimoto, Journal of Power Sources, 222, 379 (2013). T. Suzuki, H. Yamada, K. Tsusaka and Y. Morimoto, Journal of the Electrochemical Society, 165, F166 (2018). S. Katayama and S. Sugawara, Journal of Power Sources, 483, 229178 (2021). T. Schuler, A. Chowdhury, A. T. Freiberg, B. Sneed, F. B. Spingler, M. C. Tucker, K. L. More, C. J. Radke and A. Z. Weber, Journal of the Electrochemical Society, 166, F3020 (2019).

(Digital Presentation) Effect of Iron Doping on Oxygen Reduction Activity of Zirconium Oxides Containing Carbon and Nitrogen Using Pyrazine Carboxylic Acid As Platinum Alternative Cathodes for PEFC

The use of carbon-supported platinum catalysts as cathode of present polymer electrolyte fuel cells (PEFCs) have some problems such as high cost and low amount of resources of platinum. Therefore, we have attempted to develop substitute of platinum based on group 4 transition metal oxides. We found that stable oxygen vacancies on the surface and lattice distortion by doping foreign elements of these metal oxides could act as active sites of oxygen reduction reaction (ORR) [1]. However, although zirconium oxides show some ORR activity, the onset potential of the ORR still remains low. Then, we focused on the improvement of the ORR activity by an addition of iron ions to zirconium oxides according to the research of P. Madkikar et. al. [2]. In this study, we used pyrazine carboxylic acid as ligand of metal ions to prepare zirconium-iron complexes including carbon and nitrogen as precursors. The precursors were heat-treated under low oxygen pressure to obtain the iron-added zirconium oxide-based catalysts, which are designated as ZrOx-Fe. We investigated the correlation between the ORR activity and physical characteristics such as crystalline phase.ZrOx-Fe catalysts were synthesized according to the previous patent [3]. We prepared zirconium complexes solutions by mixing zirconium butoxide and acetic acid in acetylacetone as a solvent. On the other hand, we also prepared iron complexes solutions by mixing iron acetic acid and pyrazine carboxylic acid in mixed solvents of water, ethanol and acetic acid. The amount of the iron addition was changed in atomic ratios of 0, 0.004, 0.01, 0.1, and 0.2 against Zr atoms. The iron atomic ratio of 0.004 was selected according to P. Madkikar’s results, because Fe atomic ratio of 0.0036 caused the highest ORR activity in the report [2]. After mixing of two solutions, the solutions were stirring several days, and the precursors were obtained by evaporation and vacuum drying. These precursors were heat-treated under argon including 2 % hydrogen and 0.05 % oxygen for low oxygen pressure oxidation, and finally ZrOx-Fe catalysts were obtained. To investigate the effect of Fe addition to zirconium oxides, we also synthesized iron-based catalyst without Zr in the same way. Electrochemical measurements were conducted in the solution of 0.5 M H2SO4 at 30 oC. In this study, we used the ORR onset potential as the parameter of the ORR activity, which was defined as the potential when the ORR current (i ORR) was 0.01 ampere per a mass of the whole catalysts.Among the ZrOx-Fe catalysts prepared in this study, ZrOx-Fe with the Fe atomic ratio of 0.1 showed the highest E ORR, which was designated as ZrOx-Fe(0.1). Therefore, we compared with ZrOx and FeOx prepared by the same way as well as conventional catalysts. Fig. 1 shows the ORR polarization curves of the ZrOx, ZrOx / CNT [1], ZrOx-Fe / KB [2], FeOx and ZrOx-Fe(0.1) catalysts. As shown in Fig.1, the ZrOx-Fe(0.1) catalyst had highest ORR onset potential among these catalysts. Comparing with ZrOx, FeOx and ZrOx-Fe(0.1), the Fe addition into zirconium oxide was essential to increase the ORR onset potential. In addition, the onset potential of the ZrOx-Fe(0.1) was higher than that of the ZrOx-Fe / KB [2] and our previous ZrOx / CNT [1]. The onset potential of the ZrOx-Fe(0.1) was exceeded 0.9 V that we have never been able to reach with zirconium oxides. Therefore, the addition of iron to zirconium oxides improved drastically the quality of the active sites of the ORR.Next, we examined crystalline phase of all ZrOx-Fe catalysts with XRD. All ZrOx-Fe catalysts were consisted of ZrO2 monoclinic or tetragonal phase. We calculated monoclinic rate in each catalyst using the equation in the literature [4]. The change of atomic ratio of Fe caused the different monoclinic rate. A positive correlation between the ORR onset potentials and the monoclinic rates of the ZrOx-Fe was obtained. Thus, the ORR onset potential increased with increasing the monoclinic rate in the catalysts, suggesting that the monoclinic phase formed the active sites of the ORR.AcknowledgementThe authors thank the financial support of the New Energy and Industrial Technology Development Organization (NEDO), JSPS grants-in-aid for scientific research, and ENEOS Tonen General Sekiyu Research / Development Encouragement & Scholarship Foundation.References(1) A. Ishihara et. al., Int. J. Hydro. Energy, 45, 5438 (2019).(2) P. Madkikar et. al., J. Electrochem. Soc., 166, F3032 (2019).(3) Showa Denko Corp., R. Monden et. al., Oxygen reduction catalysts, the methods of these, and polymer electrolyte fuel cells, JP 2013-240785, 2013-12-05.(4) H. Toraya et. al., J. Am. Ceram. Soc., 67, C119 (1984). Figure 1

Operando 57 Fe Mössbauer Spectroscopy of Fe-N-C Catalysts during Oxygen Reduction Reaction

Proton exchange fuel cells (PEFCs) are a clean technology for efficient conversion of chemical into electrical energy and are specifically promising for the decarbonization of heavy duty vehicles [1]. Currently, the drawback of PEFCs is the high cost of Pt-based catalysts used for cathode and anode, which hinders their commercialization. [2] The rapid development of FeNCs holds promise for replacing Pt-based catalysts for the oxygen reduction reaction (ORR). The nature and characterization of the FeNC active sites is a challenging subject of research, and the exact structure of intrinsic active center for FeNC catalysts is still under debate. [3-6] 57Fe Mössbauer Spectroscopy is powerful in obtaining knowledge of iron sites, with respect to structural composition, electronic states as well as magnetic environment [3,7-9]. To solve the debate, 57Fe Mössbauer experiments were carried out under ex situ, in situ, or operando conditions to identify iron signatures and their changes induced by different conditions. On the basis of our in situ results of three differently prepared catalysts, two transitions between the oxygenated and deoxygenated state were found and assigned to sites involved in the direct and indirect ORR. [10-11] In order to gain an in-depth understanding of active sites operando conditions (thus during ORR) were performed for the FeNC catalyst that exhibited the strongest change during in situ testing. One iron signature (D4) gets exclusively formed under ORR conditions and its intensity scales with the ORR current. Together with density functional theory calculations the overall set of data enables us to make important conclusions on the ORR mechanism on FeNC catalysts.Literature:[1] M. K. Debe, Nature. 486, 2012, 43−51.[2] C. Sealy, Mater.Today.11, 2008, 65.[3] S. Wagner, H. Auerbach, et al. Angew. Chem. 131.31, 2019, 10596-10602.[4] A. Zitolo, V. Goellner, et al. Nat. Mater. 14.9, 2015, 937.[5] X.,Li, C. Cao, et al. Chem. 6, 2020, 3440–3454.[6] J. Li, M. T. Sougrati, et al. Nat. Catal. 4, 2021, 10–19.[7] U.I. Kramm, M. Lefèvre, et al. J. Am. Chem. Soc. 136, 2014, 978-985.[8] U.I. Kramm, J. Herranz, et al. Phys. Chem.Chem.Phys. 14, 2012,11673-11688.[9] U.I. Kramm, L. Ni, et al. Adv. Mater.31.31, 2019, 1805623.[10] L. Ni, C. Gallenkamp, et al. Adv. Energy Sustainability Res. 2, 2021, 2000064.[11] L. Ni, P. Theis,et al. Electrochim. Acta. 395, 2021, 139200.

Stress-Affected ORR on Stainless Steel

Oxygen reduction reaction (ORR) is an important cathodic reaction that often drives galvanic corrosion. In atmospheric environments, this reaction is responsible for accelerated corrosion from enhanced transport associated with discontinuous electrolytes. In load bearing structures, galvanic couples are subject to mechanical stresses that often necessitates these dissimilar metal unions. Greater understanding of mechanical stress effects on fundamental reactions of galvanic corrosion, such as ORR, is important from a forecasting perspective for improved prediction of component lifetimes and service requirements. Furthermore, extension of this behavior to confined, concentrated electrolytes is important to understand stress effects in atmospheric environments.This work reports the impact of tensile stress upon oxygen reduction reaction (ORR) for stainless steel fastener materials. Chronoamperometry was used to observe and quantify the influence of tensile stress profiles on ORR under NaCl droplet electrolytes. NaCl was utilized as a seawater proxy at concentrations of 0.6 M NaCl and 4.6 M NaCl, the latter corresponding to 80% relative humidity, which is near the deliquescence point for NaCl. The ORR current was shown to increase with tensile stress and displayed reversible behavior with simple stress profiles. Deoxygenated experiments were performed to isolate ORR contributions from oxide dynamics. Details of current transients during strain ramping are used to demonstrate these features are distinct from the ORR current shift observed during static tensile stress. Polarization experiments were performed during static tensile holds in the elastic and plastic regime for greater insight of stress effects on reduction kinetics. EIS and Mott-Schottky experiments were conducted to gain insight into the oxide behavior.

Impact of Anion Exchange Ionomers on the Electrocatalytic Performance for the Oxygen Reduction Reaction of B-N Co-doped Carbon Quantum Dots on Activated Carbon.

Composite electrocatalytic electrodes made from B-N co-doped carbon quantum dots (CQD) and various anion exchange ionomers (AEI) are studied for the oxygen reduction reaction (ORR) in alkaline solutions. The quantity and positions of dopants in CQD, prepared by hydrothermal synthesis, are analyzed by various spectroscopies, including 11B NMR spectroscopy that evidenced boronic acid at edge sites. The AEI are synthesized with various backbones, including more hydrophilic polysulfone, hydrophobic poly(alkylene biphenyl), and poly(2,6-dimethyl-1,4-phenylene oxide) with intermediate hydrophilicity; the functional groups are trimethylammonium moieties grafted on long (LC) or short (SC) side chains. The CQD/AEI ink is drop-casted on activated carbon paper, and the samples are fixed on a rotating disk electrode and studied in three-electrode configuration in oxygen-saturated 0.1 M KOH. The onset potentials are among the best in the literature (Eonset ≈ 0.94 V vs RHE). The highest electrocatalytic activity is observed for electrodes containing AEI with long side chains; the sample containing PPO LC attains excellent ORR currents approaching that of benchmark Pt/C cloth. The electrocatalytic performances are discussed in view of the many relevant AEI parameters, including hydrophilicity, oxygen permeability, catalyst dispersivity, and ionic conductivity.

Effect of Nitrogen Doping and Oxygen Vacancy on the Oxygen Reduction Reaction on the Tetragonal Zirconia(101) Surface

In this study, we investigated the role of defects in promoting the oxygen reduction reaction (ORR) of a zirconia-based cathode, which is motivated by the need for a stable and active electrocatalyst for proton exchange membrane fuel cells. A defective catalyst was modeled by a tetragonal ZrO2(101) slab introduced with oxygen vacancies and nitrogen dopants, and the structure of the ORR intermediates was exhaustively computed from first principles. As is typical of oxides, various adsorption structures with slightly different energies were obtained. Therefore, we obtained the free energy profile for the ORR and established that the energy barrier of the rate-determining step, or the removal of OH*, is comparable to (or only slightly lower than) that of the pristine surfaces, indicating that defects play only a minor role in enhancing the ORR activity. The measured enhancement in the ORR current could be ascribed to the enhanced carrier supply, which may be attributed to the formation of a gap state or change in the potential profile, or to other factors so far overlooked. The results of the first-principles study provide a significant insight for experimentalists to develop optimal cathode materials.

(Digital Presentation) Effects of Foreign Elements Added Oxide-Based Electrocatalyst for Oxygen Reduction Reaction as Non-Precious Metel Cathodes

On Dec. 2020, new MIRAI of fuel cell vehicle (FCV) from Toyota started to sell in Japan. However, the price of new model is almost same as previous model. For wide commercialization of FCV, it should be required for more development. One of the barriers of innovative system for the FCV is conventional platinum-based electrocatalyst. The cost of Pt is very expensive due to its very poor resources. Moreover, the conventional Pt-based electrocatalyst cannot reach enough performance for next generation FCV that is proposed in new roadmap released from NEDO, Japan. From this point of view, the non-precious metal electrocatalyst should be required for the breakthrough against above issue. We have focused and studied group 4 and 5 metal oxide-based electrocatalyst as non-platinum catalysts for the oxygen reduction reaction (ORR) because of low-cost, abundant reserves, and high stability in acidic electrolytes [1-2]. We found that titanium oxide prepared from TiOTPyzPz supported on multi-walled carbon nanotubes had superior ORR activity [3]. On the other hand, it was published as an international patent that the addition of other elements such as Fe and Ni is affected to enhance the ORR activity of Ti oxide-based electrocatalyst [4]. In this study, we have investigated to apply for the TiOTPyzPz as a starting material with and without Fe, Ni, and Zn addition to enhance the ORR activity of Ti oxide-based electrocatalyst.2,3-Dicyanopyrazine, urea, and Ti isopropoxide were dissolved in quinoline and refluxed to synthesize TiOTPyzPz. Iron acetate, Nickel acetate, and Zinc acetate were also added to dissolve in quinoline to obtain the Fe, Ni, and Zn-added TiOTPyzPz as a starting material. The molar ratio of Ti:Fe:Ni:Zn was set to constant. These starting materials were mixed with carbon nanotube by ball-milling to prepare the precursors. These precursors were heat-treated under low oxygen partial pressure for 3 h to obtain titanium oxide-based catalysts. The catalyst powder was dispersed into 1-propanol with Nafion solution to prepare a catalyst ink. The ink was dropped on a glassy carbon rod, and dried for an hour to use as a working electrode in electrochemical measurement.Electrochemical measurements were performed in 0.5 mol dm-3 H2SO4 at 30 oC with a conventional 3-electrode cell. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as used as a reference and counter electrode, respectively. Slow scan voltammetry (SSV) was performed at a scan rate of 5 mV s-1 from 0.2 V to 1.2 V vs. RHE under O2 and N2. The ORR current (i ORR) was determined by calculating the difference between the current under O2 and N2.Figure 1 shows the effect of Fe, Ni, and Zn addition to TiOTPyzPz as a starting material on the ORR polarization curves of the titanium oxide-based catalysts. The vertical axis is based on catalyst weight. Ti oxid-based electrocatalyst prepared from Fe, Ni, and Zn addition to TiOTPyzPz (TiOx-Fe, Ni, Zn) resulted in the increase in the onset potential for the ORR and ORR current compared to that without addition (TiOx), revealing that the addition of Fe, Ni, and Zn was found to be effective in improving the ORR activity. XRD pattern of TiOx shows several peaks identified TiO2-Rutile, TiO2-Anatanse and TiO2-Brookite while the XRD pattern of TiOx-Fe, Ni, Zn also shows several peaks identified similar to TiOx. In the case of XRD pattern of TiOx-Fe, Ni, Zn, the peak identified compounds relating to additional elements such as Fe, Ni and Zn does not detect strongly. It is suggested that the addition of Fe, Ni and Zn to TiOTPyzPz conduce to the distortion of crystal phase of TiO2 [5] and it affected to suppress the crystal growth of Ti oxide-based electrocatalyst. These facts are possibly contributed to enhance the ORR activity of TiOx-Fe, Ni, Zn compared to that of TiOx.Acknowledgement: The authors thank New Energy and Industrial Technology Development Organization (NEDO) and ENEOS Tonen General Research / Development Encouragement & Scholarship Foundation for financial support.Reference[1] A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).[2] A. Ishihara, S. Tominaka, S. Mitsushima, H. Imai, H. Imai, O. Sugino, and K. Ota, Curr. Opin. Electrochem., 21 , 234 (2020).[3] S. Tominaka, A. Ishihara, T. Nagai, and K. Ota, ACS Omega, 2, 5209 (2017).[4] K. Takahashi, T. Imai, R. Monden, Y. Wakisaka, and S. Sato, Oxygen Reduction Catalyst, Process for Producing Same, and Polymer Electrolyte Membrane Fuel Cell, WO/2013/008501.[5] Y. Yamamoto, S. Kasamatsu, and O. Sugino, J. Phys. Chem. C, 123, 19486 (2019). Figure 1

N-Doped Colloid Imprinted Carbons As Promising ORR Catalysts for Alkaline Applications

The oxygen reduction reaction (ORR) has long been of interest in relation to its many energy applications and interesting multi-pathway mechanisms. The ORR is a key reaction in a range of electrochemical energy conversion and storage devices, such as hydrogen fuel cells and metal-air batteries, respectively. These devices are expected to play an ever-increasing role in the global transition to net zero emissions. Metal-air batteries, such as Zn/air batteries, can operate at room temperature, use recyclable materials, are environmentally friendly, and are preferred in relation to consumer safety than batteries relying on organic solvents and reactive electrode species.1 High rates of the ORR are crucial for the development of high performance Zn/air batteries and catalysts play a significant role, with lowering of the catalyst cost also of increasing importance.The costs of conventional Pt-based ORR catalysts are high. Therefore, metal-free carbon-based ORR electrocatalysts are viewed as increasingly promising alternatives, especially as they are lower in cost due to the availability of the precursor materials. Carbon is also a good electrical conductor and support material, is chemically stable, and can have large surface areas. However, the ORR kinetics are sluggish on carbon and chemical and physical modifications are required to enhance its activity. In typical Zn/air batteries, the ORR occurs at the three-phase boundary (TPB) formed between the solid electrode, liquid electrolyte, and gaseous oxygen. The porosity of the catalyst layer, the wettability of the catalyst/electrolyte interface, and the gas permeability and hydrophobicity of the gas diffusion layer (GDL) are thus also important, significantly influencing the cathode performance and durability. The catalyst layer (CL) must therefore be constructed with both a high-performance catalyst and an optimized TPB length to provide high performance without compromising durability.In the current work, we have doped nitrogen into the lattice of a family of nanoporous colloid imprinted carbon (CIC) powders to increase its ORR activity. The CICs are unique for their versatility in terms of pore size control and ease of surface functionalization.2 Pore sizes in the range of 12 to 100 nm were examined and their effect on the ORR activity and mass transport limitations were investigated. To carry out N-doping, the CICs were exposed to ammonia at 800 ˚C for 7 hr. Catalyst inks were then prepared by mixing the CICs with a binder in an isopropyl alcohol/water solution. Aliquots of the ink were drop-casted on the disc of an RRDE system, or were spray coated or drop-casted on a GDL to determine the ORR performance in an in-house Zn/air battery testing cell, with the N-doped CIC catalyst layer sandwiched between an electrolyte chamber and a graphite current collector. In this setup, O2 gas was flowed through the pores in the GDL to the catalyst/electrolyte interface, a Zn wire installed in the electrolyte chamber was used as the reference electrode, and a Ni sponge was used as the counter electrode.The RRDE experiments showed that, after N doping of the CIC powders, the production of peroxide decreased significantly and the ORR onset potential increased to a very respectable value of ca. 0.9 V vs RHE, indicating the successful activation of the ORR. Electron transfer numbers were found to be greater than 3.5, indicating that either a direct or pseudo- 4 electron transfer ORR pathway is dominant. In agreement with the literature, the ORR currents in the kinetic regions increased linearly with mass loading, expected to be proportional to the total active N-doped CIC surface area.3 The N-doped CIC samples retained excellent performance up to a loading of 0.350 μg/cm2 without losing mechanical stability.Similar N-doped CICs and binders of different hydrophobicity were tested in the Zn/air battery testing system. Electrodes made with a hydrophobic NCS microporous layer (MPL) showed much better ORR performance and durability than hydrophilic NCS MPLs. Although electrodes made using hydrophobic polytetrafluoroethylene (PTFE) as the binder in the catalyst layer showed a similar initial performance to those made using hydrophilic Nafion binders, the PTFE based electrodes exhibited better durability. Furthermore, the temperature and pressure used during electrode fabrication were also found to have a significant impact on the binder distribution and ORR performance. Once optimized, a very good correlation was obtained between the N-doped CIC catalyst performance in the RRDE setup and in the Zn/air battery testing system. References J. Pan et al., Adv. Sci., 5, 1700691 (2018).X. Li et al., ACS Appl. Mater. Interfaces, 10, 2130–2142 (2018).N. Gavrilov et al., J. Power Sources, 220, 306–316 (2012).

Three-dimensionally ordered mesoporous Co3O4 decorated with Mg as bifunctional oxygen electrocatalysts for high-performance zinc-air batteries

Developing low-cost and high-efficiency bifunctional catalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is critical to expedite the widespread implementation of rechargeable zinc-air batteries. Herein, a unique Mg-decorated three-dimensionally ordered mesoporous (3DOM) Co3O4 electrocatalyst is engineered and evaluated as cathodic material for zinc-air batteries. The modulation of electronic structure and bonding configuration of Co sites through coordination with substituted Mg atoms effectively enhance the interaction with oxygen species and, therefore, the ORR/OER activity. Meanwhile, the substitution of Co2+ with Mg2+ creates abundant, more catalytically active octahedral sites (Co3+) in 3DOM-MgxCo3−xO4. Moreover, the tailored 3D interpenetrating porous structure endows the electrocatalyst with large diffusion channels for oxygen species and highly accessible active sites. The as-prepared catalyst retains 99% and 98% of its initial ORR and OER current, respectively, after 16 h under chronoamperometric measurement. The zinc-air battery assembled with 3DOM-MgxCo3−xO4 exhibits a high power density of 253 mW cm−2 and long-term cyclability over 236 h, outperforming the commercial noble-metal based catalysts in terms of performance and stability. This work offers a straightforward and promising design strategy for development of robust bifunctional electrocatalysts toward practical applications of zinc-air batteries.

Noble Metal-Added Titanate Nanosheets for PEFC Cathode

A large overpotential of oxygen reduction reaction (ORR) at a cathode in polymer electrolyte fuel cells (PEFCs) is one of the serious problems to decrease the energy conversion efficiency. Nørskov’s group reported that even Pt which is currently used as an ORR catalyst cannot reach the theoretical oxygen electrode potential of 1.23 V at 298 K [1]. On the other hand, Yamamoto et al. recently suggested that noble metal doped titanium dioxides (TiO2) can reach the theoretical potential by changing the absorption energies of the ORR intermediates according to the first-principle calculations [2]. However, the first-principle calculations did not consider the continuous electron supply to proceed the ORR. It is difficult to form the sufficient electron conduction path from supports to active sites on the TiO2 because TiO2 has low conductivity due to a large band gap. Therefore, we focused on the formation of the active sites on the TiO2 surface by doping noble metal such as Rh and Pd and the electron conduction path from carbon supports to the active sites by preparing the titanate nanosheets.Na2Ti3O7 was synthesized as precursor of anionic titanate nanosheets([Ti3O7]2- ;TiNS) according to the previous paper [3]. Na2Ti3O7 was treated through ion-exchange reaction with hydrochloric acid and tetramethylammonium aqueous solution, finally TiNS were obtained. A TEM observation revealed that the formation of very large sheet-like materials with a size of approximately upwards of ten micrometers, resulting that TiNS were completely synthesized.Next, we focused on the formation of the electron conduction path through nanosheets from supports using the electron tunneling effect. When the nanosheets were directly supported on the glassy carbon electrode, the ORR activity was the same as that of GC, and almost no ORR current flowed even low potential region. Then, vapor grown carbon fibers (VGCFs) as purchased, with immersion of mixed acid (MA; H2SO4 and HNO3) at 170 oC for 24 h and PDDA (Poly(diallyl dimethylammonium chloride)) as polycation were attempted to use as supports. The TiNS were immersed in the distilled water dispersed with the treated VGCFs to be supported by them. These catalysts were designated as TiNS/VGCF, TiNS/VGCF(MA), and TiNS/VGCF(MA-PDDA).The ORR current of TiNS/VGCF(MA-PDDA) was larger than that of TiNS/VGCF, TiNS/VGCF(MA). In addition, the ORR onset potential of the TiNS/VGCF(MA-PDDA) increased by 0.3 V compared to that of TiNS/VGCF. These results suggest that surface treatment of VGCFs was effective to form the electron conduction path from VGCF to active sites of nanosheet surface. The electrostatic interaction between VGCF surface positively charged by PDDA and anionic TiNS contributed the formation of the electron conduction path. However, according to the TEM observation, TiNS were partially contacted with VGCF and some TiNS were agglomerated. It is necessary to obtain higher ORR activity that a control of size of TiNS and contact with supports such as VGCF.On the other hand, we attempted to dope Rh and Pd to TiNS through solid-state reaction. Rh-,Pd-doped TiNS (designated as Rh-TiNS and Pd-TiNS) were synthesized the same way as previous paper [3]. XRD patterns of the precursors revealed that Rh2O3 or PdO were existed with Na2Ti3O7 above the addition of 5 atomic %. Rh-TiNS and Pd-TiNS were supported by VGCF(MA-PDDA) after Rh-TiNS and Pd-TiNS were treated TMAOH aqueous solution. The ORR activities of the samples with the addition of 5at% Rh or 10at% Pd were higher than those of the samples which Rh2O3 or PdO were not observed in XRD patterns. The ORR active sites were estimated by Rh2O3 or PdO deposited on the surface of the TiNS. This result showed that an amount of doped Rh or Pd was limited to Na2Ti3O7. The limitation of doped Rh or Pd was 5at% or 10at%, respectively. Therefore, the doped amounts of noble metals such as Rh or Pd to TiNS should be appropriately controlled.AcknowledgementThe authors thank the New Energy and Industrial Technology Development Organization (NEDO) for financial support. This work was partly supported by TOKYO CITY UNIVERSITY Interdisciplinary Research Center for Nano Science and Technology for instrumental analysis.Reference(1) A. Kulkarni et al., Chem. Rev., 118, 2302 (2018).(2) Y. Yamamoto et al., J. Phys. Chem. C., 123, 19486 (2019).(3) W. Soontornchaiyakul et al., RSC Adv., 7, 21790 (2017). Figure 1