Electrochemical Oxidation of Ammonia over Rare Earth Oxide

Apr 14, 2015 - Anion-Exchange Membrane Fuel Cells with Improved CO2 Tolerance: Impact of Chemically Induced Bicarbonate Ion Consumption. Yu Katayama ,...
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Electrochemical Oxidation of Ammonia over Rare Earth Oxide Modified Platinum Catalysts Yu Katayama, Takeou Okanishi, Hiroki Muroyama, Toshiaki Matsui, and Koichi Eguchi* Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan ABSTRACT: The recent development of anion exchange membranes (AEMs) has increased the potential and the importance of ammonia as a fuel for anion exchange membrane fuel cells (AEMFCs). Although electrocatalysts for ammonia oxidation using Pt-based alloy catalysts have been developed recently, no obvious activity enhancement was reported. In this paper, the electrocatalytic activity of rare earth oxide (RO: CeO2, Y2O3, La2O3, and Sm2O3) modified Pt catalysts for ammonia oxidation reaction was investigated in alkaline aqueous solutions. The ammonia oxidation activity was enhanced in accordance with the amount of OHad species on the electrode surface. From various electrochemical measurements, it was revealed that the RO additive improved the supply capacity of OHad to the reactive Pt sites. Among the catalysts studied, the CeO2-modified Pt electrocatalyst exhibited the highest activity; the peak current density for ammonia oxidation was 3.5 times higher than that of the Pt catalyst. Furthermore, this activity enhancement was also observed at 60 °C. These results indicate that the addition of RO is one of the promising ways to design the high performance anode for direct ammonia AEMFCs.

1. INTRODUCTION Hydrogen is now considered as the main fuel for fuel cells, but its low volumetric energy density and difficulty in handling are main obstacles for the application in transportable devices and automobiles. To overcome this problem, hydrogen careers such as methanol and NaBH4 have been proposed as alternative fuels for fuel cell systems.1,2 Among the hydrogen carriers, ammonia is one of the promising candidates due to its low production cost, ease in liquefaction at ambient temperatures, and high energy density.3−5 Moreover, the carbon-free energy conversion system utilizing ammonia becomes an incentive for the realization of a low carbon society. However, the performance of most commonly used fuel cells, polymer electrolyte fuel cells (PEFCs) with an acidic electrolyte such as Nafion, significantly deteriorated even with a trace amount of ammonia in hydrogen fuel.6−8 In contrast, ammonia can be electrochemically oxidized in an alkaline electrolyte: The conventional alkaline fuel cell using potassium hydroxide (KOH) electrolyte was operated successfully with supplying ammonia directly.9−11 In these circumstances, anion exchange membranes (AEMs) have attracted much attention as electrolytes.12 The recent development of AEMs has gained the potential and the importance of ammonia as a fuel. Many studies on the electrochemical ammonia oxidation over a Pt electrode in an alkaline aqueous electrolyte have been reported.13,14 Gerischer and Mauerer proposed the ammonia oxidation mechanism including the production of poisoning species15 NH3(aq) → NH3,ad

© XXXX American Chemical Society

(Reaction 3)

NHx ,ad + NHy ,ad → N2Hx + y ,ad

(Reaction 4)

δ− N2Hx + y ,ad + (x + y)OHad

→ N2 + (x + y)H 2O + (x + y)δe− δ− NHad + OHad → Nad + H 2O + δe−

(Reaction 5) (Reaction 6)

where x = 1 or 2 and y = 1 or 2. This reaction mechanism has been supported by the coulometric experiment,16 differential electrochemical mass spectroscopy (DEMS),17,18 and surface-enhanced Raman spectroscopy (SERS).19,20 According to this mechanism, NH2,ad and NHad adsorbed species are produced on the Pt electrode by the sequential ammonia dehydrogenation with the aid of OHad species and subsequently combined each other to form N2Hx+y,ad (x = 1 or 2, y = 1 or 2). Finally, further dehydrogenation of N2Hx+y,ad leads to the formation of N2. On the other hand, the fully dehydrogenated adsorbate, atomic nitrogen adspecies (Nad), does not lead to N2 formation and acts as a poisoning species on the Pt electrode. Therefore, control of the relative amount of Nad species and OHad species could be the key to promote the ammonia oxidation reaction over the Pt electrode. The influence of Nad species on ammonia oxidation over the platinum group metals (Pt, Ru, Pd, Rh, and Ir) has been reported by de Vooys et al.19 They found that Nad

(Reaction 1)

δ− NH3,ad + OHad → NH 2,ad + H 2O + δe−

δ− NH 2,ad + OHad → NHad + H 2O + δe−

Received: February 19, 2015 Revised: April 13, 2015

(Reaction A

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The morphology of both Pt/PBI/MWNT and RO-modified Pt/PBI/MWNT was observed by the transmission electron microscope (TEM, JEOL, JEM-2100F). The surface of a CeO2modified Pt disk electrode was observed by a scanning electron microscope (FIB-SEM, Nvision 40, Carl Zeiss-SIINT). In addition, SEM images were used to calculate the total length of the Pt/RO interface by using the image analysis program (Avizo). 2.2. Electrochemical Measurements. For electrochemical measurements, Pt/PBI/MWNT, RO-modified Pt/PBI/ MWNT, Pt disk electrode (Hokuto denko), and RO-modified Pt disk electrode were used. Cyclic voltammograms of electrocatalysts in 1 M KOH or 1 M KOH−0.1 M NH3 solution were recorded by the following procedure. For both modified and as-prepared Pt/PBI/MWNT electrocatalysts, the obtained dispersion liquids (1.5−8.0 μL) were dropped onto the glassy carbon disk electrode (0.196 cm2). The modified Pt disk electrode was prepared by supplying a few droplets of the dispersion liquid containing RO (1.5−8.0 μL) onto the surface of a Pt disk electrode (0.196 cm2) and subsequent drying. The platinum wire and reversible hydrogen electrode (RHE) were used as counter and reference electrodes, respectively, in the conventional three-electrode cell. The electrolyte solutions were prepared by mixing 28 wt % NH3 solution (Wako Pure Chemical), KOH (Sigma−Aldrich, >85 wt %), and ultrapure water. After deoxygenation of the electrolyte solution by purging Ar, cyclic voltammetry and linear sweep voltammetry were conducted at 25−60 °C by using HSV-100 (Hokuto Denko) with a scanning rate of 20 mV s−1. The electrochemical surface area (ECSA) for each electrocatalyst was calculated from cyclic voltammograms recorded in the Ar-purged electrolyte by integrating the charge in the hydrogen adsorption/desorption region, in the range of 0.05−0.45 V vs RHE. The current density expressed was normalized by ECSA.

species served as poison regardless of metals. The electrocatalytic activity for the ammonia oxidation varied in the following order, Ru < Rh < Pd < Ir < Pt. This order in catalytic activity reflected the difference in affinity of Nad for the metal surface. However, no effective method is established to prevent the poisoning effect of the Nad species. Furthermore, the contribution of the OHad species to ammonia oxidation has not been discussed yet in detail. In this study, then, the Pt-based catalysts with high electrocatalytic activity for ammonia oxidation were developed by focusing on the enhancement of the adsorption property of the OHad species. Metal oxides are promising materials to serve as good OHad supply sources.21 Okanishi et al. reported that a SnO2-modified Pt catalyst promoted the ammonia oxidation reaction in an aqueous electrolyte.22 Other preceding studies have reported that the rare earth oxide such as CeO2 changes the OH adsorption capacity of catalysts.21,23 In addition, rare earth oxides have been used as supporting materials of electrocatalysts for polymer electrolyte fuel cells (PEMFCs), and their stability under electrochemical environments has also been confirmed.24 Accordingly, a series of rare earth oxide (RO: CeO2, Y2O3, La2O3, and Sm2O3) modified Pt catalysts were prepared, and the additive effect of RO to Pt was investigated in alkaline aqueous solutions.

2. EXPERIMENTAL SECTION 2.1. Electrocatalyst Preparation. Pt/PBI/MWNT was prepared by the improved polyol method.25 In this study, the multiwall carbon nanotube (MWNT) was chosen for the support because of superior characteristics such as higher electrical conductivity,26,27 large specific surface area,28 and low electrochemical resistance29−31 compared to those of commonly used support materials, e.g., carbon black. The multiwall carbon nanotube (MWNT, Sigma-Aldrich, 0.25 g) was added to polybenzimidazole (PBI, Sato Light Industrial, 0.25 g) dissolved with N,N-dimethylacetamide (DMAc, Wako Pure Chemicals, 25 mL) and sonicated for 60 min. The mixture was filtrated and then washed with DMAc to remove excess PBI. The obtained solid (PBI/MWNT) was subsequently dried in vacuo. The deposition of Pt nanoparticles on PBI/MWNT was carried out by the reduction of H2PtCl6 (Tanaka Kikinzoku Kogyo) in an ethylene glycol−water mixture.32 First, PBI/ MWNT (1.0 g) was added to an ethylene glycol−water solvent (ethylene glycol/water = 3/2 v/v, 250 mL) and dispersed by sonication. Then, 25 mL of an ethylene glycol−water solution containing 5 mL of H2PtCl6 was added to the suspension liquid of PBI/MWNT. After stirring for 2 h at room temperature, the mixture was refluxed at 140 °C for 17 h. The solid material was collected by filtration and washed with deionized water, then dried under vacuum to obtain Pt/PBI/MWNT. Rare earth oxide (RO: CeO2, Y2O3, La2O3, and Sm2O3) modified Pt/PBI/MWNT catalysts were prepared by physically mixing Pt/PBI/MWNT and commercially available RO (Wako Pure Chemical). Each as-received RO (6.75 mg) was added to ultrapure water (Milipore MiliQ) and then ultrasonically dispersed for 2 h. Pt/PBI/MWNT (10 mg) and anion exchange ionomer solution (AS-4, Tokuyama Corp., diluted to 1 wt % solution with ethanol, 0.7 mL) were added to the resultant dispersion liquid (0.75 mL) and ultrasonically dispersed for 1 h to obtain the RO-modified Pt/PBI/ MWNT. The RO-modified Pt disk electrode was prepared by supplying an aqueous dispersion of RO containing ionomer solution using the same preparation mentioned above.

3. RESULTS AND DISCUSSION 3.1. Characterization of Electrocatalysts. Figure 1 shows Pt particle-size histograms and representative TEM images of Pt/PBI/MWNT and CeO2-modified Pt/PBI/MWNT electrocatalysts. For both samples, Pt particles were finely dispersed on the surface of MWNTs. The mean Pt particle diameter of pristine and CeO2-modified Pt/PBI/MWNT was 3.6 and 3.8 nm, respectively. In the case of CeO2-modified electrocatalyst, CeO2 particles were also successfully dispersed over the surface of MWNTs, forming a contiguous interface with Pt particles as well as MWNTs. In contrast to the fine Pt particles, the diameter of the CeO2 particle varied widely between 1 and 6 μm, which resulted from the nonuniform particle size distribution of the source CeO2 powder. Note that Pt particles were not supported directly on the CeO2 particles. Figure 2(a) shows cyclic voltammograms (CVs) of the Pt disk electrode in 1 M KOH at 25 °C. As reported previously,33,34 desorption of hydrogen (underpotential deposited hydrogen, Hupd + OH− → H2O + e−) was observed in the range of 0.05−0.45 V vs RHE. Subsequently, the double-layer capacitance was observed in the potential range of 0.45−0.70 V followed by a small pseudocapacitance that corresponded to OH adsorption at 0.70−0.90 V.35 In the case of Pt/PBI/ MWNT electrocatalyst (Figure 2(b)), the shape of the voltammogram was almost the same as that of the Pt disk electrode, indicating the electrochemically active Pt was successfully deposited on a MWNT covered with PBI. Because B

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Figure 1. Pt particle-size histograms and representative TEM images of (a) Pt/PBI/MWNT and (b) CeO2-modified Pt/PBI/MWNT electrocatalysts.

Figure 2. Cyclic voltammograms of (a) Pt disk electrode and (b) Pt/ PBI/MWNT electrocatalyst in 1 M KOH at 25 °C with a scanning rate of 20 mV s−1.

the electrochemical behavior observed was identical to the typical response of Pt, the electrochemical property of deposited Pt in Pt/PBI/MWNT was not affected by the coexisting PBI. Figure 3 shows cyclic voltammograms of the Pt disk electrode and Pt/PBI/MWNT electrocatalyst with or without the supply of 0.1 M NH3. In both electrodes, the distinct oxidation peak was observed at 0.45−0.90 V. The preceding studies have revealed that this oxidation peak was attributed to the electrochemical oxidation of NH3.13,36 Furthermore, the voltammograms for two electrocatalysts in 1 M KOH−0.1 M NH3 were almost the same. This indicates that even in the presence of NH3 the inactive ingredient of PBI does not affect the activity of Pt in the Pt/PBI/MWNT catalyst. 3.2. Effect of RO Additives on Ammonia Oxidation Behavior. Figure 4(a) shows linear sweep voltammograms of the RO-modified Pt/PBI/MWNT electrocatalysts in 1 M KOH. The voltammogram waveform remained unchanged despite the addition of RO to Pt/PBI/MWNT. In the presence of NH3 (Figure 4(b)), however, the electrochemical behavior changed dramatically depending on the presence or absence of additives. In every modified electrocatalyst, the current due to NH3 oxidation reaction increased as compared with that of pristine Pt/PBI/MWNT. In particular, the CeO2-modified electrocatalyst achieved the highest NH3 oxidation peak current density among the series of samples which was twice as high as that of Pt/PBI/MWNT. This result clearly shows that the RO additives have enhanced the activity for NH3 oxidation reaction. The peak potential of some RO-modified catalysts was shifted

to higher potential. Probably, these shifts in the peak potential reflect the intrinsic property of each RO-modified catalyst to the poisonous Nad species. On the other hand, it is important to note that the onset potential of NH3 oxidation reaction was almost the same in every modified electrocatalyst. Because the electrochemical characteristic of Pt has not changed, there will be no positive influence on the activity for the dehydrogenation reaction, which is the first step of the ammonia oxidation. The chronoamperograms of both modified and as-prepared Pt/PBI/MWNT electrocatalysts were also obtained at 0.7 V vs RHE in the presence of NH3 to confirm the additive effect of RO (Figure 5). Under polarization, the poisonous species of Nad is accumulated on the Pt surface, leading to the fast attenuation of chronoamperograms. The current density for every modified electrode after 180 s at 0.7 V was larger than that of Pt/PBI/MWNT. However, the order of activity enhancement was different from that observed in Figure 4; i.e., the activity enhancement was maximized in the CeO2modified electrocatalyst. In contrast, the Y2O3-modified electrocatalyst exhibited the highest performance in Figure 5. This behavior appears to indicate that Y2O3 is effective in weakening the poisoning by Nad species. The detail of characteristic behavior of Y2O3-modified catalyst in Figure 5 will be further studied. 3.3. Mechanism of Activity Enhancement with Additives for Ammonia Oxidation Reaction. In order to clarify the role of RO additives on the activity enhancement for NH3 oxidation, the relationship between the electrical charge C

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Figure 5. Chronoamperograms of RO-modified Pt/PBI/MWNT electrocatalysts in 1 M KOH−0.1 M NH3 at 25 °C at the potential of 0.7 V vs RHE.

for NH3 oxidation and the amount of OHad species on the electrode surface was investigated. At this point, OHad species were thought to be active reaction species in NH3 oxidation reaction, as described in the Introduction (Reaction 1−ReReaction 6). The amount of OHad species was estimated from the linear sweep voltammograms shown in Figure 4(a). According to the preceding study,37 the oxidation current at 0.75−0.85 V originated from the adsorption of OH and/or oxidation of Pt surface. In this study, the integration of this oxidation current was evaluated as a parameter related to the amount of OHad species on the electrode surface. The integration of the reduction current at 0.75−0.85 V was also calculated to make sure that the integration value of the oxidation current was not affected by the presence of O2 in the electrolyte. The NH3 oxidation charge in Figure 4(b) in the potential window of 0.45−0.85 V was estimated for evaluation of the NH3 oxidation activity. Here, the effect of catalyst structure on NH3 oxidation activity was ignored because each RO additive had a similar particle size distribution with a mean diameter of ca. 3 μm. Figure 6 summarizes the correlation between the charge thus ascribed to the OH adsorption and/or Pt surface oxidation (abbreviated as Pt oxidation charge) and NH3 oxidation charge

Figure 3. Cyclic voltammograms of (a) Pt disk electrode and (b) Pt/ PBI/MWNT electrocatalyst in 1 M KOH (blue line) and 1 M KOH− 0.1 M NH3 (red line) at 25 °C with a scanning rate of 20 mV s−1.

Figure 4. Linear sweep voltammograms of RO-modified Pt/PBI/ MWNT electrocatalysts in (a) 1 M KOH and (b) 1 M KOH−0.1 M NH3 at 25 °C with a scanning rate of 20 mV s−1. Figure 6. Correlation between Pt oxidation charge (0.75−0.85 V) and NH3 oxidation charge (0.45−0.85 V) for Pt/PBI/MWNT electrocatalysts. D

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interaction between Pt and RO additives is expected to be negligible; i.e., the change of electronic state of Pt is minimized in the modified Pt disk electrodes. Figure 8(a) shows voltammograms of modified and asprepared Pt disk electrodes in 1 M KOH. No change in

for each modified Pt/PBI/MWNT electrocatalyst. Interestingly, the Pt oxidation charge for every RO-modified electrocatalyst was higher than the unmodified electrocatalyst, indicating an increment in surface OHad species. Moreover, a distinct relationship was observed between the amount of surface OHad species and the NH3 oxidation activity; the larger the amount of OHad species, the higher the activity for the NH3 oxidation reaction became. This result strongly indicates that the amount of the surface OHad species is the key factor to enhancing the NH3 oxidation activity. Presumably, the supply of the OHad species to the active Pt sites was promoted by the addition of RO. The abundant supply of the active OHad species brings about the positive impact on the electrocatalytic activity for NH3 oxidation reaction. Figure 7 shows the relationship between the NH3 oxidation activity for each electrode and the KOH concentration in

Figure 7. KOH concentration dependency of NH3 oxidation peak current density obtained from linear sweep voltammograms in x M KOH−0.1 M NH3 at 25 °C with a scanning rate of 20 mV s−1. Figure 8. Cyclic voltammograms of Pt disk and modified Pt disk electrodes in (a) 1 M KOH and (b) 1 M KOH−0.1 M NH3 at 25 °C with a scanning rate of 20 mV s−1.

aqueous electrolytes. In this figure, the NH3 oxidation peak current density obtained from the linear sweep voltammogram in the presence of NH3 was plotted. For example, in the case of KOH concentration of 1 M, the results shown in Figure 4(b) were used. In the KOH concentration range of 0.1−1.0 M, the NH3 oxidation activity increased proportionally with an increment of KOH concentration for all samples. This result suggests that the amount of OHad species in the vicinity of the electrode surface is an important factor for the NH3 oxidation, which is consistent with the result in Figure 6. For ROmodified Pt/PBI/MWNT, the NH3 oxidation peak current density was unchanged with the KOH concentration in the concentration range of 1−4 M, whereas Pt/PBI/MWNT exhibited the linear relationship in the whole KOH concentration range studied. In the case of RO-modified Pt/ PBI/MWNT, therefore, the OHad supply capacity appeared to reach the sufficient level at 1 M KOH. These findings also support that the RO additives serves as a source of OHad. The effect of RO additives was confirmed in another series of electrodes with a different configuration. The RO-modified Pt disk electrodes were prepared and evaluated. It is noteworthy that in the modified Pt disks RO additives were coated with an anion exchange ionomer and simply fixed on the surface of bulk Pt. In other words, the modified Pt disk electrode has a simple mechanical contact of Pt/RO. Therefore, the electronic

electrochemical characteristics of Pt was observed. However, in Figure 8(b), a clear enhancement of electrocatalytic activity for NH3 oxidation was observed in both CeO2- and Y2O3-modified Pt disk electrodes. This result reveals that the RO provides the adsorption f ield of OHad species, and these adsorbates are effectively transferred to the Pt active sites to promote the NH3 oxidation reaction. Figure 9 shows the correlation between the quantity of CeO2 and NH3 oxidation peak current density at 0.67 V in the CeO2modified Pt disk electrode. The electrochemical surface area of each electrode is also listed. It is apparent that there is an optimized additive amount, e.g., the CeO2 quantity of 9.2 μg. On the other hand, the electrochemical surface area was reduced monotonously with an increment in additive amount, as a result of the coverage of the active Pt site by CeO2. The quantity of the RO additive is closely related to the total length of the Pt/RO interface. The length of the Pt/RO interface was roughly estimated from the length of outline of the CeO2 particles in SEM images. The length of the Pt/RO interface per unit area increased with the additive amount until the threshold value was attained. After the additive amount exceeded this threshold, the length of the Pt/RO interface was reduced since E

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active sites will be the main reason for the lower performance of the Pt disk electrode with CeO2 more than 9.2 μg. 3.4. Enhancement of Ammonia Oxidation Activity at High Temperature. To study the feasibility of RO-modified Pt/PBI/MWNT electrocatalysts to AEMFCs, linear sweep voltammetry was carried out in 1 M KOH−0.1 M NH3 at 60 °C, which is the typical operating temperature of AEMFCs.38,39 Figure 11 displays the results for CeO2-modified and as-

Figure 9. Correlation between NH3 oxidation current density, electrochemical surface area, and CeO2 quantity and the length of the Pt/CeO2 interface per unit area in CeO2-modified Pt disk electrode. Results of cyclic voltammetry in 1 M KOH and 1 M KOH− 0.1 M NH3 were used for calculation. Figure 11. Linear sweep voltammograms of CeO2-modified (blue line) and as-prepared (black line) Pt/PBI/MWNT electrocatalysts in 1 M KOH−0.1 M NH3 at 60 °C with a scanning rate of 20 mV s−1.

the residual additives cannot deposit effectively on the Pt surface. The SEM image shown in Figure 10 strongly supports this hypothesis. For a CeO2 amount less than 9.2 μg (Figure 10(a) and (b)), CeO2 particles were highly dispersed. On the other hand, the aggregation of CeO2 particles was confirmed for the amount more than 9.2 μg (Figure 10(c) and (d)). The overall consequence of this trend is that the NH3 oxidation activity is determined by a counterbalance of two factors; that is, a total length of the Pt/RO interface and a number of active Pt sites. Therefore, for the Pt disk electrodes with the additive amount less than 9.2 μg, the total length of the interface dominantly affected the activity by the effect of OHad supply capacity. On the other hand, the decrease in Pt

prepared Pt/PBI/MWNT electrocatalysts. The clear difference was observed in waveform and current density between these two catalysts. In the Pt/PBI/MWNT electrocatalyst, the waveform after passing peak potential (ca. 0.65 V), the socalled diffusion tail, became gradual as compared with that at 25 °C in Figure 4. Moreover, only a 1.5-fold increase in the NH3 oxidation peak current density was confirmed with the temperature rise from 25 to 60 °C. In the case of CeO2modified Pt/PBI/MWNT electrocatalyst, however, the waveform did not change, and the NH3 oxidation peak current density increased 2.7-fold. At higher temperature, the reaction rate of NH3 oxidation is enhanced, while the molecular adsorption is suppressed at elevated temperatures because of the decrease in the Gibbs energy of adsorption. Therefore, the activity enhancement is barely expected at high temperatures when the electrochemical reaction is restricted by the amount of adsorbed species supplied to the reaction field. In the case of CeO2-modified electrocatalyst, however, this suppression of molecular adsorption, i.e., OH adsorption, was promoted significantly by the sufficient supply of OHad species from the CeO2 additive. Considering the promotion of NH3 oxidation by the addition of CeO2 at 60 °C, RO-modified Pt/PBI/MWNT electrocatalysts are expected to be applicable to direct ammonia AEMFCs.

4. CONCLUSION A series of rare earth oxide (RO: CeO2, Y2O3, La2O3, and Sm2O3) modified Pt electrocatalysts were prepared to investigate their additive effect on Pt. The NH3 oxidation peak current density increased for every RO-modified Pt catalyst; CeO2-modified Pt catalyst showed the highest activity. This activity enhancement can be explained by the promotion effect of OHad supply. The RO additive specifically worked as an adsorption field, continuously supplying OHad species to Pt

Figure 10. SEM images of Pt disk electrode surface with CeO2 amount of (a) 4.6 μg, (b) 9.2 μg, (c) 18.4 μg, and (d) 24.5 μg. F

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(13) Suzuki, S.; Muroyama, H.; Matsui, T.; Eguchi, K. Fundamental Studies on Direct Ammonia Fuel Cell Employing Anion Exchange Membrane. J. Power Sources 2012, 208, 257−262. (14) Vidal-Iglesias, F. J.; Solla-Gullón, J.; Rodriguez, P.; Herrero, E.; Montiel, V.; Feliu, J. M.; Aldaz, A. Shape-dependent Electrocatalysis: Ammonia Oxidation on Platinum Nanoparticles with Preferential (1 0 0) Surfaces. Electrochem. Commun. 2004, 6, 1080−1084. (15) Gerischer, H.; Mauerer, A. Untersuchungen zur Anodischen Oxidation von Ammoniak an Platin Electroden. J. Electroanal. Chem. 1970, 25, 421−433. (16) Gootzen, J. F. E.; Wonders, A. H.; Visscher, W.; van Santen, R. A.; van Veen, J. A. R. A DEMS and Cyclic Voltammetry Study of NH3 Oxidation on Platinized Platinum. Electrochim. Acta 1998, 43, 1851− 1861. (17) Vidal-Iglesias, F. J.; Solla-Gullón, J.; Feliu, J. M.; Baltruschat, H.; Aldaz, A. DEMS study of Ammonia Oxidation on Platinum Basal Planes. J. Electroanal. Chem. 2006, 588, 331−338. (18) de Vooys, A. C. A.; Koper, M. T. M.; van Santen, R. A.; van Veen, J. A. R. The Role of Adsorbates in the Electrochemical Oxidation of Ammonia on Noble and Transition Metal Electrodes. J. Electroanal. Chem. 2001, 506, 127−137. (19) de Vooys, A. C. A.; Mrozek, M. F.; Koper, M. T. M.; van Santen, R. A.; van Veen, J. A. R.; Weaver, M. J. The Nature of Chemisorbates Formed from Ammonia on Gold and Palladium Electrodes as Discerned from Surface-Enhanced Raman Spectroscopy. Electrochem. Commun. 2001, 3, 293−298. (20) Vidal-Iglesians, F. J.; Solla-Gullón, J.; Perez, A.; Aldaz, A. Evidence by SERS of Azide Anion Participation in Ammonia Electrooxidation in Alkaline Medium on Nanostructured Pt Electrodes. Electrochem. Commun. 2006, 8, 102−106. (21) Boehm, H. P. Acidic and Basic Properties of Hydroxylated Metal Oxide Surfaces. Discuss. Faraday Soc. 1971, 52, 264−275. (22) Okanishi, T.; Katayama, Y.; Muroyama, H.; Matsui, T.; Eguchi, K. Electrochim. Acta 2015. (23) Morterra, C.; Bolis, V.; Magnacca, G. Surface Characterization of Modified Aluminas. Part 4.Surface Hydration and Lewis Acidity of CeO2−Al2O3 Systems. J. Chem. Soc., Faraday Trans. 1996, 92, 1991−1999. (24) Sharma, S.; Pollet, B. G. Support Materials for PEMFC and DMFC ElectrocatalystsA review. J. Power Sources 2012, 208, 96− 119. (25) Wiley, B.; Sun, Y. G.; Mayers, B.; Xia, Y. N. Shape-Controlled Synthesis of Metal Nanostructures: The Case of Silver. Chem.Eur. J. 2005, 11, 454−463. (26) Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Zhou, Z.; Sun, G.; Xin, Q. Preparation and Characterization of Multiwalled Carbon NanotubeSupported Platinum for Cathode Catalysts of Direct Methanol Fuel Cells. J. Phys. Chem. B 2003, 107, 6292−6299. (27) Tian, Z. Q.; Jiang, S. P.; Liang, Y. M.; Shen, P. K. Synthesis and Characterization of Platinum Catalysts on Multiwalled Carbon Nanotubes by Intermittent Microwave Irradiation for Fuel Cell Applications. J. Phys. Chem. B 2006, 110, 5343−5350. (28) Xing, Y. Synthesis and Electrochemical Characterization of Uniformly-Dispersed High Loading Pt Nanoparticles on Sonochemically-Treated Carbon Nanotubes. J. Phys. Chem. B 2004, 108, 19255− 19259. (29) Li, L.; Xing, Y. Electrochemical Durability of Carbon Nanotubes at 80 °C. J. Power Sources 2008, 178, 75−79. (30) Kongkanand, A.; Kuwabata, S.; Girishkumar, G.; Kamat, P. Single-Wall Carbon Nanotubes Supported Platinum Nanoparticles with Improved Electrocatalytic Activity for Oxygen Reduction Reaction. Langmuir 2006, 22, 2392−2396. (31) Shao, Y.; Yin, G.; Gao, Y.; Shi, P. Durability Study of Pt/C and Pt/CNTs Catalysts under Simulated PEM Fuel Cell Conditions. J. Electrochem. Soc. 2006, 153, A1093−A1097. (32) Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Zhenhua; Sun, G.; Xin, Q. Preparation and Characterization of Multiwalled Carbon NanotubeSupported Platinum for Cathode Catalysts of Direct Methanol Fuel Cells. J. Phys. Chem. B 2003, 107, 6292−6299.

active sites. This enhancement of OHad supply was prominent in electrolytes with KOH concentration less than 1 M. Moreover, this efficacy of additive was also confirmed at 60 °C, which is the typical operating temperature of AEMFCs. Throughout this study, it was clarified that the enhancement of adsorption of OHad species was an important development strategy for the realization of highly active catalysts for ammonia oxidation reaction.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-75-383-2519. Fax: +81-75-383-2520. E-mail: [email protected] (K. Eguchi). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). We thank Tokuyama Corporation for the supply of anion exchange membrane (A201) and anion exchange ionomer (AS4).

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b01710 J. Phys. Chem. C XXXX, XXX, XXX−XXX