Search

A Pt0.3ML/Ir(111) model catalyst showed desirable properties for a polymer electrolyte fuel cell anode, including enhanced hydrogen oxidation activity and suppressed H2O2 generation.

Surface modification by carbazole dendrimers, in particular, 9-phenylcarbazole, enhances electrochemical CO2 conversion to CO on gold.

The electrochemical CO2 reduction reaction (CO2RR) activity of Ni2MnIn and Ni2MnSn Heusler alloys was investigated. This is the first report that shows the CO2RR activity of Heusler alloy particles.

The electrochemical hydrogen oxidation reaction activity of a well-defined Ir–Ru bimetallic surface alloy is closely related to Ir/Ru atomic ratios and Ir-dimer (Ir2) and/or trimer (Ir3) ensemble probability at the topmost surface.

Interlayer and surface Ir-modified Pt/Pd(111) model catalyst surfaces [Pt/Ir/Pd(111) and Ir/Pt/Pd(111)] were synthesized as surface structural models for third element-modified core–shell-type Pd@P...

Extended X-ray absorption fine structure (EXAFS) is a powerful tool to determine the local structure in Pt nanoparticles (NP) on carbon supports, active catalysts for fuel cells. Highly oriented pyrolytic graphite (HOPG) covered with Pt NP gives samples with flat surfaces that allow application of surface science techniques. However, the low concentration of Pt makes it difficult to obtain good quality EXAFS data. We have performed in situ highly sensitive BCLA-empowered Back Illuminated EXAFS (BCLA + BI-EXAFS) measurements on Pt alloy nanoparticles. We obtained high quality Pt L3-edge data. We have devised a novel analytical method (model building analysis) to determine the structure of multi-component nanoparticles from just a single absorption edge. The generation of large numbers of structural models and their comparison with EXAFS fits allows us to determine the structures of Pt-containing nanoparticles, catalysts for the oxygen reduction reaction. Our results show that PtCo, PtCoN and AuPtCoN form a Pt-shell during electrochemical dealloying and that the ORR activity is directly proportional to the Pt–Pt bond length.

Highly active and inexpensive anode materials are required for large-scale hydrogen production using alkaline water electrolysis (AWE). Here, heterolayered nanostructures of Ni-Fe hydroxides/oxides with high activity for the oxygen evolution reaction (OER) were synthesized on a 316 stainless steel (SS) substrate through constant current density electrolysis. The thicknesses, morphologies, and compositions of the nanostructures, generated through dealloying and surface oxidation of the SS elements with severe oxygen microbubble evolution, were dependent on the electrolysis time. Nanostructural analyses showed that the heterolayered Ni-Fe hydroxide/oxide nanostructures were generated during the initial stage of electrolysis, growing nanofiberlike Ni-Fe hydroxide layers with increasing electrolysis time of up to 5 h. The prolonged electrolysis resulted in densification of the nanofiber structures. The OER overpotential at 10 mA/cm

Oxygen reduction reaction (ORR) properties are investigated for the Pt/Zr/Pt(111) surfaces prepared through arc-plasma depositions of Zr and Pt on a Pt(111) substrate. The synthesized Pt(111)-shell...

We investigated the microstructures of Au-deposited PtCo@Pt core-shell nanoparticles (NPs) and discussed enhancement of the oxygen reduction reaction (ORR) properties by Au termination of low-coordination sites of the Pt-shell. The Au-deposited PtCo@Pt NPs showed an improved electrochemical structural stability, together with slight increment in increased initial, pristine area-specific activity relative to the non-Au-deposited PtCo@Pt NPs. Atomic-level microstructural characterization was performed by a back-side illumination fluorescence X-ray absorption fine structure (BI-FXAFS) method and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS). The BI-FXAFS results indicated that compressive lattice strain in the Pt-shells of the PtCo@Pt NPs was almost unchanged by the subsequent Au deposition. Furthermore, STEM-EDS mapping of Pt, Co, and Au clearly showed that the deposited Au tended to localize at low-coordination sites of the Pt-shell surface, e.g., edges and corners. The atomic-level microstructural characterization conducted in this study demonstrated that effective Au surface terminations of the Pt-shell enhance the ORR durability of Pt-based core-shell type NP catalysts.

Oxygen reduction reaction (ORR) activity was investigated for nano-flake-like carbon-modified Pt75Ni25(111) surfaces. Surface cleaning through Ar+-sputtering and thermal annealing in an ultra-high vacuum (~ 10−8 Pa) resulted in a Pt-enriched topmost surface, i.e., a Pt(111)-skin on Pt75Ni25(111). Arc plasma deposition (APD) of graphite under 0.08 Pa N2 and in vacuum (~ 10−6 Pa) generated nitrogen-doped and non-doped nano-flake-like carbon on the Pt(111)-skin surfaces, respectively. For the latter, non-doped carbon-modified Pt(111)-skin, the area-specific initial ORR activity estimated in O2-saturated 0.1 M HClO4 decreased with increasing thickness of the deposited carbon. In contrast, the former, nitrogen-doped carbon with 2 and 6 A mass-thickness enhanced the ORR activity. The Pt 4f band energies for the nitrogen-doped Pt(111)-skin were measured by X-ray photoelectron spectroscopy (XPS) and showed the chemical shift to higher biding energy (~ 0.2 eV) compared with the corresponding bands for the non-doped and Pt(111)-skin surfaces. As for the electrochemical structural stability, a specific amount of the non-doped carbon species tends to suppress the degradation of the Pt(111)-skin under applying potential cycles. The results indicate that the surface modifications by the carbon hexagonal networks of the nano-flakes could be applicable to improve ORR performance of the practical Pt-M alloy catalysts.

Introduction Developing highly active and durable electrocatalysts for oxygen evolution reaction (OER) are required for efficient hydrogen production by polymer electrolyte membrane water electrolysis (PEMWE). While RuO2 shows the highest OER activity among pure transition metal oxides catalysts, the low durability hinders its practical application as OER catalyst for PEMWE. Recent studies demonstrated that RuO2 mixed with other metal oxides, such as IrO2 and SnO2 etc. showed higher activity and durability than pure RuO2 (1-3). When the RuO2 deposited on the other metal oxides, interfacial resistance and lattice strain which derives from the lattice mismatch should influence on the OER properties. In this study, to clarify the influence of interface between different rutile-type oxides, we fabricated RuO2/MO2/TiO2(110) (M = Ir or Sn) single crystal model catalyst and evaluated the oxygen evolution activity and durability. Experimental Nb-doped TiO2(110) (Nb: 1 wt%) single crystal substrate was etched in 20 % HF solution for 10 min and annealed at 1000 ℃ in air. After the sample transfer to the APD chamber, 2 nm-thick Ir or Sn was deposited onto the substrate at room temperature under 0.5 Pa-O2 partial pressure and annealed at 350 ℃ for 30 min under the same O2 pressure. After that, 4 nm-thick Ru was deposited on the MO2/TiO2(110) and was annealed under the same condition of the MO2 layer. Hereafter, the sample was denoted as RuO2/MO2/TiO2. Oxygen evolution activity was measured by linear sweep voltammetry (LSV) at a scan rate of 1 mV/s in N2-purged 0.1 M HClO4 at room temperature. The electrochemical impedance spectroscopy was conducted at 1.5 V vs. reversible hydrogen electrode (RHE) to measure the resistance of solution (Rs), electrodes (Re), and charge transfer for OER (Rct). The LSV curves were corrected using the measured Rs. Chronopotentiometry was conducted at 0.5 mA/cm2 for 120 min to evaluate the durability. In-plane XRD analysis was performed for estimating lattice strain in the surface RuO2 layer. Results and Discussions XRD analysis showed that both of the MO2 interlayer and RuO2 catalyst layer grow with (110) orientation on the TiO2(110) substrate. Figure 1(a) shows initial LSV curves for OER. The onset potential of RuO2/SnO2/TiO2 samples clearly shifted to lower potential side relative to the RuO2/IrO2/TiO2 and RuO2/TiO2 samples. The overpotential at 0.1 mA cm-2 of the RuO2/SnO2/TiO2 (b) was ca. 25 mV lower than the other two samples. The chronopotentiometric curves (c) showed the overpotential of RuO2/TiO2 increased by 250 mV after 120 min. On the other hand, change in the overpotential of the RuO2/SnO2/TiO2 was only 25 mV, indicating the high durability. Cole-cole plots at 1.5 V (d) revealed that both of the Re and Rct were significantly reduced owing to SnO2 interlayer, while the IrO2 layer slightly increased the Re and decreased the Rct. In-plane XRD results showed the compressive lattice strain was induced in [001] direction in RuO2 layer on TiO2 substrate and decreased by insertion of IrO2 and SnO2 layer. However, the lattice spacing in [-110] direction was almost the same for all the samples. Figure 1 (e) shows the relation between Rct and the lattice strain in [001] direction. The Rct tended to decrease with decreasing the compressive strains, suggesting the OER activity of RuO2(110) surface is related to the anisotropic strains in [001] direction. The results demonstrated that the activity and durability of RuO2/TiO2 catalysts can be improved by insertion of SnO2 layer at the catalyst/substrate interface. Acknowledgments This study was partly supported by JSPS KAKENHI Grant Number 21H01661 and Toyota Mobility Foundation Hydrogen Initiative. References R. V. Genova-Koleva, F. Alcaide, G. Álvarez, P. L. Cabot, H.-J. Grande, M. V. Martínez-Huerta and O. Miguel, Journal of Energy Chemistry, 34, 227 (2019). J. Li, X. Qin, G. Chen and G. Chen, Journal of the Taiwan Institute of Chemical Engineers, 125, 186 (2021). R. Samanta, P. Panda, R. Mishra and S. Barman, Energy & Fuels, 36, 1015 (2022). Figure 1

Introduction To achieve prolonged lifetime of polymer electrolyte fuel cell (PEFC), hydrogen-peroxide (H2O2)-induced chemical degradation of polymer electrolyte membrane (PEM) is a crucial problem. In general, oxygen (O2) crossover from cathode to anode through the PEM yields H2O2 generation on Pt/C anode catalyst surface through 2-electron pathway of O2 reduction reaction. Thus, mitigation of H2O2 generation on Pt/C surface is a key to suppress the PEM degradation. Trogadas and Ramani reported tungsten trioxide (WO3) addition to Pt/C suppressed H2O2 generation.1 The results suggest that the cooperation of Pt nanoparticles and WO3 located near-by should be effective for suppression of H2O2 generation. However, influence of the oxidation states of tungsten oxide (WO x ) on H2O2 generation at the Pt surface is not fully resolved. Furthermore, it was reported that, although fully oxidized WO3 is stable in acid solution (pH < 2),2 sub-oxides of tungsten (WO x ) could dissolve in Pt-WO x electrochemical system.3 Thus, electrochemical dissolution behaviors of WO x located on Pt substrate surface should be investigated in detail. In this study, we prepare Pt(111) surface that modified by WO x having different oxidation states, and investigated the electrochemical H2O2 generations. Experimental Pt(111) single crystal substrate surface was cleaned by repeated cycles of Ar+ sputtering and annealing in an ultrahigh vacuum (UHV; ~10-8Pa). Subsequently, WO x was deposited on the cleaned Pt(111) via an arc-plasma deposition (APD) method using a W target under partial pressures of O2 (p(O2) = 1×10-1 or 1×10-3 Pa) at ca. 298 K. Deposition amounts of the WO x was estimated to be ca. 1.5 µg/(cm2 of substrate) by using a quartz crystal microbalance installed in the UHV chamber. Then, WO x -modified Pt(111) were annealed at 703 K for 10 min. in UHV. Hereafter, the fabricated samples are referred to as p(O2)-WO x /Pt(111). X-ray photoelectron spectroscopy (XPS) was conducted for the as-fabricated samples in situ in UHV. H2O2 generation amounts were evaluated by the tip generation/substrate collection mode of scanning electrochemical microscope (SECM)4 in O2-saturated 0.1 M HClO4. Generated H2O2 was detected by a Pt tip micro-electrode (diameter: ca. 20 µm, tip potential: 1.26 V vs. RHE) positioned ca. 50 µm at surface normal of the sample electrodes, with sweeping sample electrode potential (E S) at sweep rate of 2 mV/s in negative going direction. Thereafter, potential cycles (PCs: 0.05–1.0 V vs. RHE, sweep rate: 100 mV/s, 100 cycles) were applied to the sample surfaces; then, H2O2 generation was re-evaluated. Finally, the PCs-applied sample surfaces were re-introduced to the UHV chamber to perform XPS measurements. Results and Discussion XP spectra of W4f bands for the as-fabricated and PCs-applied sample surfaces are summarized in Figure. 1 (A). Considering peak areas of deconvoluted components (solid lines), the oxidation states of WO x as-fabricated can be judged mainly to be W6+ and Wx+ (x < 4) for 10-1 Pa-WO x /Pt(111) (a) and 10-3 Pa-WO x /Pt(111) (c), respectively. Figure. 1 (B) shows SECM-estimated H2O2 generations of the WO x /Pt(111) surfaces before (as-fabricated) and after the 100 PCs. As for the as-fabricated surfaces (solid lines), H2O2 detection current (i T) of the Pt tip micro-electrode normalized by respective sample electrode current (i S) is lower for 10-1 Pa-WO x /Pt(111) than 10-3 Pa-WO x /Pt(111), yet normalized i T/|i S|-values for both surfaces are smaller than that of clean Pt(111). The results show that surface modification of Pt(111) by the small amount of WO x , particularly with higher oxidation states, effectively suppressed H2O2 generations. However, the suppression effect degraded by the 100 PCs application for both the electrode surfaces (dotted lines). Compared of W4f bands for the PCs-applied surfaces (Figure. 1 (A) (b, d)) with corresponding as-fabricated ones (a, c), the bands shifted to negative- and positive-binding-energies for 10-1 Pa-WO x /Pt(111) and 10-3 Pa-WO x /Pt(111), respectively, indicating respective reduction and oxidation of the surface WO x by the PCs. In addition, W4f bands for both surfaces decreased in intensity by the PCs application, suggesting decrease in surface amount of the WO x , probably through electrochemical dissolution during the PCs. The PCs-induced changes in oxidation states and amounts of surface WO x should correlate to the degraded H2O2 generation suppressions. Acknowledgement This study was supported by the new energy and industrial technology development organization (NEDO) of Japan, JSPS KAKENHI Grant Number JP21H01645, and JST SPRING Grant Number JPMJSP2114. References P. Trogadas and V. Ramani, J. Electrochem. Soc., 155, B696–B703 (2008). M. Anik and K. Osseo-Asare, J. Electrochem. Soc., 149, B224 (2002). A. J. Martín, A. M. Chaparro, and L. Daza, J. Power Sources, 196, 4187–4192 (2011). C. M. Sánchez-Sánchez and A. J. Bard, Anal. Chem., 81, 8094–8100 (2009). Figure 1

Introduction Nanoparticles of Pt as well as Pt-based alloys are widely used as cathode catalyst materials for proton exchange membrane fuel cells (PEMFC). However, electrochemical stability of the materials is rather low under practical operating conditions of PEMFC cathode, resulting in severe deactivation of oxygen reduction reaction (ORR). Therefore, further material’s developments are required for next-generation PEMFC cathode catalysts, i.e., more enhanced ORR durability with low platinum group metal (PGM) usage. High entropy alloys (HEAs), defined as single phase solid solutions of five or more elements in equal composition ratios, are known as thermodynamically stable, in comparison to conventional binary alloys. Furthermore, complex atomic-level local structures bring about unique electronic as well as (electro-)chemical properties that originating from lattice strains induced by specific local structures and/or so-called sluggish diffusion of the constituent elements. [1] However, to our best knowledge, no study has been made for ORR properties of Pt alloying with non-PGM Cantor alloy (fcc structure HEA with equi-atomic ratio of Cr-Mn-Fe-Co-Ni [2]) elements in strong acid condition. In this study, we synthesized lattice stacking structures of Pt/HEA(hkl) (hkl = (111), (110), (100)) through arc-plasma deposition (APD) of the Cantor alloy layer on Pt(hkl) substrate surfaces, followed by deposition of the surface Pt layer in ~10-7 Pa. Then, we performed cross-sectional STEM-EDS observations for Pt/HEA/Pt(hkl) stacking structures with atomic-level resolution and evaluated the ORR properties (initial activity and structural stability). Experimental An APD target of Cr-Mn-Fe-Co-Ni (Cantor alloy) was fabricated by sintering of equal quantity corresponding elements. 10 ML(monolayer)-thick (1 ML = ca. 0.3 nm) Cantor alloy layer (as HEA) was vacuum-deposited by APD on surface cleaned Pt(hkl) substrate surfaces at 300 K, and subsequently annealed in vacuum at 773 K for 30 minutes. Then, 4ML-thick Pt layer was deposited on the pre-deposited Cantor alloy layer at 300 K and annealed at 623 K. The samples thus prepared are designated as Pt/HEA/Pt(hkl). The atomic-level micro-structures and chemical bonding states of Pt/HEA/Pt(hkl) surfaces were characterized by cross-sectional STEM-EDS, RHEED, XPS etc. CV and LSV with the RDE method were conducted in N2-purged and O2-saturated 0.1 M HClO4. ORR activity was evaluated from j k values at 0.9 V vs. RHE by using Koutecky-Levich equation and structural stability (ORR durability) was discussed based upon activity transitions during applying the potential cycles (PCs) of 0.6(3s)‐1.0(3s) V vs. RHE in O2 saturated 0.1 M HClO4 at room temperature. Results and Discussion Atomically-resolved, cross-sectional HAADF-STEM images for Pt/HEA/Pt(hkl) (a) and EDS line profiles of elemental distributions at corresponding yellow arrows (b) are presented in Figure 1. As clearly shown in (a), irrespective of the Pt surface indices, (hkl), HEA (Cantor alloy) and surface Pt layers are epitaxially grown on the substrates. By contrast, elemental distributions in each surface normal (b) seriously depend on the substrate Pt lattice indices. Notably, severe thermal diffusion of the constituent elements including Pt is confirmed by (a) and (b) for both Pt/HEA/Pt(110) and (100). Figure 2 summarizes electrochemical results. 4ML-thick-Pt/10ML-thick-Co/Pt(hkl) that prepared under the same preparation condition of Pt/HEA/Pt(hkl), and clean Pt(hkl) (light blue and gray, respectively) are also shown as references. As shown in the figure, the Pt/HEA and Pt/Co fabricated on Pt(111) substrate (top) show quite similar CV characteristics (shrink in hydrogen adsorption charges (0 – 0.3 V) and higher potential shifts in oxygen-related species adsorption (0.6 – 1.0 V)), in comparison to clean Pt(111), and almost the same initial ORR activity. Meanwhile, the Pt/HEA fabricated on Pt(110) and (100) substrates show more reduced hydrogen absorption charges, compared with corresponding Pt/Co samples. Particularly, distinctive redox features for clean Pt(110) at 0.12 V and for Pt(100) at 0.35 V are absent for corresponding CVs, suggesting specific topmost surface atomic-structures might be formed in the electrolyte, that probably resulting from significant electronic interactions between surface Pt and HEA constituent elements (Cr, Mn, Fe, Co, Ni) and/or local strain of the surface Pt layer induced by distorted Pt-HEA lattices located nearby. One might notice that Pt/HEA/Pt(110) reveals remarkable ORR activity enhancement compared with corresponding Pt/Co/Pt(110), while the activities for Pt/HEA and corresponding Pt/Co surfaces fabricated on Pt(100) are almost the same value. At the meeting, correlations between surface atomic-level micro-structures of Pt/HEA/Pt(hkl) and ORR properties will be discussed in detail. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan and JST SPRING, Grant Number JPMJJSP2114. Reference [1] J. Yeh, JOM, 65, 1759 (2013). [2] B. Cantor et al., Mater. Sci. Eng. A, 375, 213 (2004). Figure 1

The oxygen reduction reaction (ORR) has been studied on single crystals of the ternary alloys Pt45Pd45Co10 and Pt77Pd8Co15 in O2-saturated 0.1 M HClO4 using a hanging meniscus rotating disk electrode. Specific activity for the ORR depends markedly on the composition and surface structure of the single crystal electrodes. The activity on Pt45Pd45Co10(hkl) increases in the order Pt45Pd45Co10(100) ≈ Pt45Pd45Co10(111)

We investigated electrochemical stability of the NiFe-hydroxide/oxide-catalyst-layer covered stainless-steel(SS)-anode (NiFe-HyOx/SS) for alkaline water electrolysis. The NiFe-HyOx catalyst layer was synthesized through constant current density electrolysis of 30 mA/cm2 in 1 M KOH solution of a 316 SS plate at 75 ℃ for 5 h. The initial overpotential of oxygen evolution reaction (OER) was estimated to be ca. 270 mV at 100 mA/cm2 and the potential kept almost constant during applying the 20,000 potential cycles (PCs) of 0.5 and 1.8 V vs. reversible hydrogen electrode in 7 M KOH at 20 ℃. Scanning transmission electron microscopic observations conducted before and after the PCs loading revealed that the 50 nm-thick, nanofiber-like NiFe-HyOx catalyst layer remained unchanged in structure and in thickness, while the ca. 850 nm-thick, relatively dense NiFe-(hydro)oxide interlayer was generated under the catalyst layer. The results suggest that the superior OER property of the NiFe-HyOx/SS anode is originated from the surface catalyst layer.

We evaluated the electrochemical CO2 reduction reaction (ECR) on low-index Au single crystal surfaces (Au(111), (100), and (110); Au(hkl)) and discussed the surface-atomic-arrangement-dependence of...

Extended X-ray absorption fine structure (EXAFS) is a powerful tool to determine the local structure in Pt nanoparticles (NP) on carbon supports, active catalysts for fuel cells. Highly oriented pyrolytic graphite (HOPG) covered with Pt NP gives samples with flat surfaces that allow application of surface science techniques. However, the low concentration of Pt makes it difficult to obtain good quality EXAFS data. We have performed in situ highly sensitive BCLA-empowered Back Illuminated EXAFS (BCLA + BI-EXAFS) measurements on Pt alloy nanoparticles. We obtained high quality Pt L

Introduction To develop highly active and durable cathode electro-catalysts for polymer electrolyte fuel cell (PEFCs), core-shell type nanoparticles (NPs) have been actively studied. It is well known that oxygen reduction reaction (ORR) properties of the core-shell NPs greatly depend on the core-materials (transition metal elements (M) and/or Pt-M alloys). Particularly, deactivation of ORR under the PEFC operating conditions (low pH, electrochemical potential fluctuations) through elution of M is serious issue. Therefore, developments of highly-stable core materials under the operating conditions have been needed for practical applications of the core-shell type NPs. Ham et al. reported that early transition metal carbides, such as TaCx and HfCx, show high thermal stability and high corrosion resistance under the PEFC operating condition [1]. In this study, we prepared Pt/TaCx NPs by using arc-plasma deposition (APD) of TaCx followed by electron-beam deposition of Pt in ultra-high vacuum (UHV; ~10-8Pa) and investigated the ORR properties (initial activity and electrochemical stability) of the Pt/TaCx NPs. Experimental Highly-oriented pyrolytic graphite (HOPG) was used as a substrate. First, HOPG was scraped by Scotch tape in air and cleaned by annealing for 30 min in UHV. TaCx NPs are synthesized by using APD (Advanced RICO; APS-1) of Ta at substrate temperatures of 1173K or 1273K under CH4 partial pressure of 0.05 Pa. Subsequently, Pt was deposited on the TaCx NPs/HOPG at 873K by electron-beam deposition in UHV. As a reference, Pt/Ta NPs was fabricated through APD of Ta followed by the electron-beam deposition of Pt at 873K in UHV. The prepared Pt/TaCx and Pt/Ta NPs samples were transferred from UHV to electrochemical (EC) setup without air exposure to avoid sample oxidation and contamination. Cyclic voltammetry (CV) was performed in N2-purged 0.1 M HClO4. After that, linear sweep voltammetry (LSV) for ORR was conducted by using rotating disc electrode (RDE) method in O2-saturated 0.1 M HClO4. ORR activity was evaluated as j k values at 0.9 V vs. RHE: the j k value was estimated from Koutecky-Levich plots and electrochemical surface area (ECSA) of Pt. The electrochemical stabilities of the samples were discussed based on change in the j k values during applying square-wave potential cycles (PCs) between 0.6 (3 s) and 1.0 V (3 s) in O2-saturated 0.1 M HClO4 at a room temperature. Crystal structures, surface morphologies, and micro-structures of the prepared NPs samples were observed by XRD, scanning tunneling microscopy (STM), and scanning transmission electron microscope (STEM) combined with energy dispersive X-ray spectrometry (EDS), respectively. Results and Discussion STM images of the Pt/Ta and Pt/TaCx (1173K & 1273K) NPs were shown in Fig. 1(a). The NPs with average diameter of ca. 6 nm are synthesized on the HOPG substrate, irrespective of the Ta deposition conditions (with or without CH4 partial pressure during APD). Fig. 1(b) shows CVs for the corresponding NPs. Hydrogen (QH; 0.05 ~ 0.35 V) and OH-related (QOH; 0.6 ~ 1.0 V) charges for both the Pt/TaCx NPs are higher than that for the Pt/Ta NPs. The evaluated specific ORR activities (j k / ECSA) of the samples are summarized in Fig. 1(c). Filled and hatched bars are the activity before and after 10k PCs of each samples, respectively. One might notice that both Pt/TaCx NPs exhibited much higher activity than the Pt/Ta NPs even after the 10k PCs. Especially, the Pt/TaCx - 1173K shows highest ORR activity and durability among the samples: the retention rate after 10k PCs was ca. 85% of the initial activity. The results clearly reveal that, as for the core materials, tantalum carbide is effective for enhancing ORR activity as well as durability of the core-shell type Pt-based NPs. Acknowledgement This study was supported by the new energy and industrial technology development organization (NEDO) of Japan. References [1] D.J.Ham et al., Energies, 2(2009),873. Fig. 1 (a)STM images (100 nm × 100 nm), (b)CVs of the Pt/TaCx model catalysts prepared under CH 4 partial pressure of 0.05 Pa at substrate temperature of 1173K(orange) and 1273K(green). The Pt/Ta model catalyst (black) prepared in UHV at the substrate temperature of 873K is shown as a reference. (c) Specific ORR activity of the samples Figure 1

We investigated the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity of Co and Pt bimetallic systems (n-monolayer (ML)-Co/Pt(111); n = 0.13–2.0) in 0.1 M KOH. For the ORR, the 0.25ML-Co/Pt(111) substrate showed the greatest activity enhancement: 1.7 times of that of clean Pt(111). The ORR activity enhancements were sensitive for the submonolayer-thick Co: the 0.25ML-Co/Pt(111) showed highest activity (1.7-fold vs. clean Pt(111)) and the activity steeply decreased above the Co thickness of 0.5 ML. For the OER, the activity increased with an increasing Co thickness and the 2ML-Co/Pt(111) substrate was the most active. This study suggests that the tuning of the near-surface nanostructures (e.g., Pt-Co composition ratio and chemical state of the Co-(hydro)oxide) is important for achieving high ORR and OER activity in Pt-3d-transition-metal alloy electrocatalysts under alkaline conditions.

We investigated atomically resolved interface microstructures of vacuum-deposited Pt on SnO2(hkl) (hkl = 110, 101, 111; Pt/SnO2(hkl)) substrate surfaces. The Pt/SnO2(hkl) samples were prepared by electron-beam deposition of a 1.6-nm-thick Pt layer on SnO2(hkl) at room temperature under ultrahigh vacuum (∼10−8 Pa) followed by thermal annealing. Cross-sectional structural analysis was conducted by scanning transmission electron microscopy combined with x-ray energy dispersive spectroscopy and x-ray photoelectron spectroscopy. The Pt/SnO2(101) and Pt/SnO2(111) samples clearly showed Pt(111) epitaxial growth on the SnO2(101) and (111) substrate surfaces, whereas an islandlike Pt growth was observed on SnO2(110). Increasing the annealing temperature of the samples from 473 to 673 K resulted in alloying of the deposited Pt with Sn atoms from the substrate in the case of Pt/SnO2(110). In comparison, for Pt/SnO2(101) and Pt/SnO2(111), the interfaces were atomically steep, and no Pt–Sn alloys were generated even on 673 K annealing. The results clearly showed that the morphologies of the deposited Pt and the thermal diffusion behaviors of the Pt and substrate Sn atoms, i.e., the Pt/SnO2 interface microstructure, are determined by the lattice matching of the deposited Pt and SnO2 substrate interface.

The oxygen reduction reaction (ORR) activity and electrochemical stability of well-defined n monolayer (ML)-Pt/PtxNi100−x(111) (n = 2 and 4; x = 75, 50, and 25) model electrocatalyst surfaces were investigated in this study. The initial activity of the as-prepared two-monolayered Pt-covered PtxNi100−x(111) substrates (2ML-Pt/PtxNi100−x(111)) increased with increasing Ni composition in the PtxNi100−x(111) substrate. In particular, 2ML-Pt/Pt25Ni75(111) showed the initial activity that was 25 times higher than that of clean Pt(111) although the higher Ni composition resulted in destabilization of the catalyst upon the application of potential cycles (PCs). As for 4ML-Pt/PtxNi100−x(111), activity enhancements were insensitive to alloy composition and thicker Pt shell layers stabilized the catalyst against PC applications. In particular, the activities of 4ML-Pt/Pt50Ni50(111) and 4ML-Pt/Pt25Ni75(111) gradually increased during 1000 PCs probably because of the PC-induced mono-atomic heights and nanometer-size islands that had (110) and (100) steps introduced into the topmost (111) terraces. Thus, the simultaneous tuning of core–alloy composition and Pt shell thickness is vital for developing practical, highly efficient Pt-based alloy ORR electrocatalysts.

We demonstrate highly enhanced ORR activity and electrochemical stability of Pt/Ir(111) model core-shell catalysts prepared by molecular beam epitaxy (MBE) in ultra-high vacuum (UHV). Reflection high-energy electron diffraction patterns for the surfaces show that Pt grew epitaxially on the clean Ir(111) substrate and the corresponding scanning tunneling microscope images collected in UHV reveal atomically flat terraces with 50–80 nm widths at a substrate temperature of 673 K. In contrast, the corresponding surfaces prepared at a substrate temperature of 303 K show island-like topmost surface structures. The two-monolayer (ML)-thick Pt grown on Ir(111) (Pt2ML/Ir(111)) surfaces, prepared at substrate temperatures of 303 K and 673 K, show ca. 6 and 24 times higher ORR activities than clean Pt(111), respectively. The anomalous activity enhancement for the latter surface prepared at 673 K is probably caused by homogeneous surface strain acting on the Pt shells that is derived from the 2.2% lattice mismatch between the Pt and Ir. The preparation-temperature–dependent ORR activity suggests that the activity can be dominated by the topmost surface and interface structures of the Pt shell–Ir(111) bimetallic system. Furthermore, while the initial ORR activity of pristine surfaces decreases with increasing Pt shell thickness, the stability during room temperature potential cycling between 0.6 and 1.0 V in a 0.1 M HClO4 solution was greatly enhanced above three ML thickness; the Pt4ML/Ir(111) surface prepared at 673 K retained 6.5 times higher ORR activity than Pt(111), even after 5000 potential cycles. The ORR activity and electrochemical stabilities for the Pt/Ir(111) bimetallic surfaces are the highest among the MBE-prepared Pt/M(111) (M = Ir, Pd, Au) systems reported to date. The results obtained in this study show that Pt/Ir core–shell nanostructures are potential candidates for highly active and durable ORR catalysts.

We investigated oxygen reduction reaction (ORR) activity and electrochemical stability of Pt/Pd(111) model electrocatalysts having well-defined topmost and sub-surface structures. The Pt/Pd(111) bimetallic surfaces were prepared by vacuum-deposition of Pt on clean Pd(111) at substrate temperatures of x ( x -Pt y nm /Pd(111), where x = 573 K or 673 K, y = 0.6 or 1.2) in ultra-high vacuum (UHV). Reflection high-energy electron diffraction patterns and UHV scanning tunneling microscopy images revealed that Pt grew epitaxially on the Pd(111) substrate under the aforementioned Pt deposition conditions. High substrate temperatures resulted in thermal diffusion of deposited Pt with substrate Pd atoms. The Pt atomic ratio at the topmost surface of 573K-Pt 0.6nm /Pd(111) is estimated to be 95% using He-ion scattering spectroscopy (ISS), which is 8% greater than that of 673K-Pt 0.6nm /Pd(111). ORR activities of the 573K-Pt 0.6nm /Pd(111) and 673K-Pt 0.6nm /Pd(111) surfaces were 6.3 and 3.6 times higher than that of clean Pt(111), respectively, indicating that the activity is sensitive to the topmost surface Pt atomic ratios. Moreover, electrochemical stability of 573K-Pt 0.6nm /Pd(111) evaluated under potential cycle loadings (0.6 V–1.0 V) is better than that of 673K-Pt 0.6nm /Pd(111). Depth profiles of the surfaces judged by corresponding ISS spectra suggest that the stability stems not only from the topmost Pt atomic ratios but also effective Pt-shell thickness determined by thermal diffusion of Pt and Pd. Furthermore, an increase in Pt-thickness from 0.6 nm to 1.2 nm improved the electrochemical stability: 573K-Pt 1.2nm /Pd(111) retained 5 times more activity vs. clean Pt(111) even after 2000 potential cycles, at which the activity of 573K-Pt 0.6nm /Pd(111) was the same as that for initial activity of clean Pt(111). The results obtained in this study demonstrate that atomic-level structures Pt-Pd bimetallic alloy surfaces determine the ORR activity and electrochemical stability of practical Pd@Pt core–shell catalysts.

Introduction Pt–M (M = Fe, Co, Ni, Cu, Pd, etc.) alloy and Pt/M core–shell nanoparticles have been intensively studied as oxygen reduction reaction (ORR) catalysts for polymer electrolyte fuel cells (PEFCs). As for the core–shell catalysts, the core elements easily dissolve in the electrolyte under the PEFCs operating conditions, leading to degradation of the catalytic activity. To control dissolution behavior of the core elements, Kuttiyiel et al. proposed the structure of Pt–shell/nitride–core nanostructures and demonstrated that nitride core is effective not only for enhancing ORR activity but also for improving durability [1]. For developing the practical Pt–M catalysts, it is significant to clarify a role of the nitrogen atoms located at the core–shell interface. In this study, we fabricate model catalyst structures that composed of Pt(111) epitaxial layers on low–energy N2–beam irradiated Pt25Ni75(111) substrate surfaces by using molecular beam epitaxy (MBE) and discuss the ORR activity and the durability. Experimental Model catalyst sample surfaces are fabricated as follows. First, Pt25Ni75(111) single crystal substrate was cleaned by Ar+ sputtering and annealing at 1173 K in ultra–high vacuum (UHV). Next, neutralized N2–beam (≤ 100eV) was irradiated onto the cleaned Pt25Ni75(111) at room temperature by using low–energy neutralized nitrogen ion beam gun. Then, 3ML–thick–Pt was deposited on the N2–beam irradiated Pt25Ni75(111) substrate by an electron–beam evaporation method at room temperature, followed by annealing at 673 K for 10 minutes in UHV to flatten the topmost surfaces. The resulting surface structure was verified by scanning tunneling microscopy (STM). The sample thus fabricated was transferred to electrochemical evaluation systems set in a N2–purged glove box without air exposure [2]. CV and LSV measurements were conducted in N2–purged and O2–saturated 0.1 M HClO4, respectively, and ORR activity was evaluated from j k values at 0.9 V vs. RHE by using Koutecky–Levich equation. The electrochemical stability of the sample was investigated by applying potential cycles between 0.6 V (3s) and 1.0 V (3s) in O2–saturated 0.1 M HClO4. Results and Discussion UHV–STM images of the as–prepared 3ML–Pt/Pt25Ni75(111) and 3ML–Pt/100eV–N2–beam irradiated Pt25Ni75(111) (3ML–Pt/N2–beam–Pt25Ni75(111)) surfaces are shown in Fig. 1(a). The line profiles of white arrows are depicted below the corresponding STM images. The STM images clearly show that atomically flat surfaces can be obtained through the UHV–process, irrespective of the 100eV–N2–beam irradiation. Furthermore, the both surfaces exhibited moiré–like height modulations (line profiles) that derived from lattice mismatches between the topmost Pt–shell layers and corresponding substrates. The results suggest that compressive surface strain should be induced for both the shell layers. Corresponding CV curves of the 3ML–Pt/Pt25Ni75(111) (blue), 3ML–Pt/N2–beam–Pt25Ni75(111) (red) and Pt(111) (black–dashed) are summarized in Fig. 1(b). Compared with the clean Pt(111), the total Hupd charges for the 3ML–thick–Pt surfaces considerably decreased and the onset of Oads positively shifted. Such CV features are common for highly–active Pt skin–type surfaces [2, 3]. Furthermore, the oxidation of the Pt shell layers for the latter N2–beam irradiated sample is suppressed in comparison to the former. ORR activities evaluated before and after 1000 potential cycles loading are summarized in Fig. 1(c). As for the initial activity enhancement factor, the 3ML–Pt/Pt25Ni75(111) andthe N2–beam irradiated one reveals ×15 and ×8 vs. Pt(111), respectively. The results suggest that the nitrogen atoms located at the Pt–shell/Pt25Ni75(111) interface should release the surface strain of the Pt–shell layers, leading to decrease in pristine ORR activity. In contrast, N2–beam irradiated sample is relatively stable against the potential cycle loading, indicating that the interface nitrogen atoms dominate not only activity but also durability of the Pt– shell/nitride–core type catalysts. At the PRiME 2016, I will discuss ORR activity and durability for various–thick Pt shells prepared on N2–beam–Pt25Ni75(111). Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan. References [1] K. A. Kuttiyiel et al., Nano Energy, 13, 442 (2015). [2] T. Wadayama et al., Electrochem. Commun., 12, 1112 (2010). [3] V. R. Stamenkovic et al., Science, 315, 493 (2007). Figure 1