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Although the efficiency of inorganic-organic perovskite solar cells (PSCs) has increased significantly owing to intermediate phase induced slow crystallization using dimethyl sulfoxide (DMSO) as a co-solvent in N2-filled glovebox, PSCs are generally fabricated in ambient air via a fast crystallization process to protect the perovskite layer from H2O or O2. In this study, we suggest a possibility of slow crystallization process for MAPbI3 perovskite crystals via intermediate phase formation by adjusting the humidity of air at RH 20%–30%. We obtained high-quality perovskite grains by implementing the slow crystallization process using an antisolvent and the ratio of PbI2:MAI:DMSO in the precursor solution at RH 20%–30%. The amount and state of the intermediate phase inside the as-deposited film were analyzed by grazing incidence wide-angle X-ray scattering (GIWAXS) measurements. The deposited perovskite layer with the pure intermediate via extremely slow crystallization showed higher absorption, fewer grain boundaries, and suppressed defect recombination losses than the fast crystallized perovskite phase. Thus, we obtained higher stability and high power conversion efficiency of up to 18.56% in a p-i-n-structured MAPbI3-based perovskite solar cell under ambient air with RH 20%–30%.

Sodium-ion batteries (SIBs) have attracted significant attention as promising replacements for lithium-ion batteries (LIBs) due to the wide availability of sodium. However, compared with LIBs, the relatively low energy density, accompanied by fast cycling and rate-performance degradation of SIBs, impedes their practical use. Exploiting oxygen redox reactions increases energy density but induces severe thermodynamic instability with concomitant irreversible capacity decay and large hysteresis. Herein, a combined experimental and theoretical understanding of the P2-type oxygen redox Na2/3[Li1/8Mn5/8Ni2/8]O2 is elucidated from three perspectives: i) electrochemical and structural characterization coupled with thermodynamic phase stability, ii) Na kinetics in terms of various physicochemical factors, and iii) charge compensation mechanism based on Ni and oxygen redox, which is compared with a binary oxide without Li substitution. The experimental results indicate that a quasi-single-phase reaction of Na2/3[Li1/8Mn5/8Ni2/8]O2 occurs during (de)intercalation. It is supported by the thermodynamic formation energies using first-principles calculations. Experimental and computational results consistently reveal improved phase stability compared with the pristine Mn-Ni binary oxide. The kinetic favorability of Na2/3[Li1/8Mn5/8Ni2/8]O2 is theoretically explained by elastic softness, lowered Na migration barrier, and phase stability. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy identifies oxygen redox (above 4.0 V) occurring after the dominant two-electron Ni2+/Ni4+ redox reactions, which is also confirmed by qualitative and quantitative electronic structure analyses. Based on these concrete considerations, the superior cycle and rate performance features of Na2/3[Li1/8Mn5/8Ni2/8]O2 compared with Na2/3[Mn6/8Ni2/8]O2 are demonstrated. This study can serve as a guide to exploit the full potential of oxygen redox properties enabling further advances in SIBs.

We, for the first time, report potassium storage in carbon-modified Li4Ti5O12 spinel (C-LTO), where the carbon coating increases the electrical conductivity from ~10−7 to ~10−1 S cm−1. C-LTO provides a high initial charge capacity of approximately 221 mAh g −1 at 0.2C (34 mA g −1) with about 77% retention for 200 cycles. Pre-potassiation of C-LTO electrode successfully improved initial Coulombic efficiency. The excellent electrode performance is further emphasized at high rate (3.2C, 544 mA g −1), with an initial capacity of 130 mAh g −1 and 70% retention for 1000 cycles. During the reaction in K cells, the incorporation of K+ ions into the cubic spinel Li4Ti5O12 induces a biphasic reaction, namely cubic rock salt K-rich K6LiTi5O12 and Li-rich Li7Ti5O12 phases accompanied by Ti4+/3+ redox, as confirmed by in-situ X-ray diffraction and ex-situ X-ray absorption analyses. The original Li4Ti5O12 spinel is recovered upon depotassiation. In addition, full cells paired with a P3-K0.5[Mn0.8Fe0.1Ni0.1]O2 cathode demonstrate the feasibility of applying the C-LTO electrode as an anode for high-rate and long-term potassium storage.

Pt–Ni alloy nanocrystals with Pt‐enriched shells were prepared by selective etching of surface Ni using sulfuric acid and hydroquinone. The changes in the electronic and geometric structure of the alloy nanoparticles at the surface were elucidated from the electrochemical surface area, the potential of zero total charge (PZTC), and relative surface roughness, which were determined from CO‐ and CO2‐displacement experiments before and after 3000 potential cycles under oxygen reduction reaction conditions. While the highest activity and durability were achieved in hydroquinone‐treated Pt–Ni, sulfuric acid‐treated one showed the lower activity and durability despite its higher surface Pt concentration and alloying level. Both PZTC and Q CO 2 /Q CO ratio (desorption charge of reductively adsorbed CO2 normalized by COad‐stripping charge) depend on surface roughness. In particular, Q CO 2 /Q CO ratio change better reflects the roughness on an atomic scale, and PZTC is also affected by the electronic modification of Pt atoms in surface layers. In this study, a comparative study is presented to find a relationship between surface structure and electrochemical properties, which reveals that surface roughness plays a critical role to improve the electrochemical performance of Pt–Ni alloy catalysts with Pt‐rich surfaces.

Changes in electronic and compositional structures of Pt-Ni electrocatalysts with 44% of Ni fraction with repeated chemical dealloying have been studied. By comparing the Pt-enriched surfaces formed using hydroquinone and sulfuric acid as a leaching agent, we found that hydroquinone generated Pt-enriched surfaces exhibit the highest oxygen reduction reaction (ORR) activity after repeating the treatment twice. In particular, it was found that while sulfuric acid causes an uncontrollable dissolution of Ni clusters, the unique selectivity of hydroquinone allows the preferential dissolution of Ni atoms alloyed with Pt. Despite its wide usage in the field, the results show that traditional acid leaching is unsuitable for Pt-Ni alloys with a high Ni content and an incomplete alloying level. We finally proved that the unique and lasting selectivity of hydroquinone enables an incompletely alloyed Pt-Ni catalyst to obtain a highly ORR active Pt shell region without an extensive loss of Ni.

Albite is one of the major constituents in the crust. We report here that albite, when subjected to hydrous cold subduction conditions, undergoes hitherto unknown breakdown into hydrated smectite, moganite, and corundum, above 2.9 GPa and 290 °C or about 90 km depth conditions, followed by subsequent breakdown of smectite into jadeite above 4.3 GPa and 435 °C or near 135 km depth. Upon the hydration into smectite, the fluid volume of the system decreases by ~14 %, whereas it increases by ~8 % upon its dehydration into jadeite. Both the hydration and dehydration depths are correlated to increases in seismicity by 93 % and 104 %, respectively, along the South Mariana trench over the past 5 years. Moreover, the formation of smectite is accompanied by the release of OH− species, which would explain the formation of moganite and expected alkalinity of the subducting fluid. Thus, we shed new insights into the mechanism of water transport and related geochemical and geophysical activities in the contemporary global subduction system., Albite is one of the major constituents in the Earth’s crust. Here, the authors report that under hydrous cold subduction conditions, albite undergoes breakdown into hydrated smectite and other phases, which release alkaline fluids into the mantle wedge.

Despite the high energy density of layered-type cathode materials for Na-ion batteries, their two-dimensional crystal structure suffers a large volume change and phase transition during Na+ de/intercalation, which often results in their poor cycling performances. Thus, a robust three-dimensional framework with minimal structural change is required for stable electrochemical sodium storage. Here, we introduce an earth-abundant element-based trigonal-type Na–Fe–F compound (Na2Fe2F7) with three-dimensionally interconnected FeF6 octahedra and three-dimensional Na+ diffusion pathways. Through combined studies using first-principles calculations and experiments, we confirm that Na2Fe2F7 delivers excellent power-capability due to large three-dimensional Na+ diffusion pathways as well as ultra-long cycling performance due to negligible structural change during Na+ de/intercalation. These results will guide new insights for material discovery for high performance rechargeable batteries.

The Jahn-Teller effect (JTE) is one of the most important determinators of how much stress layered cathode materials undergo during charge and discharge; however, many reports have shown that traces of superstructure exist in pristine layered materials and irreversible phase transitions occur even after eliminating the JTE. A careful consideration of the energy of cationic distortion using a Taylor expansion indicated that second-order JTE (pseudo-JTE) is more widespread than the aforementioned JTE because of the various bonding states that occur between bonding and antibonding molecular orbitals in transition-metal octahedra. As a model case, a P2-type Mn-rich cathode (Na

Mesoporous carbon derived from pyrolysis of metal–organic frameworks (MOFs) is advantageous owing to its high specific surface area, large pore volume, and versatility in both structure and composi...

Herein, we propose P′2-Kx[Ni0.05Mn0.95]O2 as a promising cathode material for high-energy-density potassium-ion batteries (KIBs). The P′2-K0.83[Ni0.05Mn0.95]O2 delivers a high discharge capacity of 155 ​mAh g−1 (52 ​mA ​g−1) as well as a high energy density of 420 ​Wh kg−1 in the voltage range of 1.5–4.3 ​V. Surprisingly, the fast K-ion migration in the P′2-K0.83[Ni0.05Mn0.95]O2 structure with a low activation barrier energy of ~271 ​meV enables achievement of high capacity at high currents, 78 ​mAh g−1 ​at 2600 ​mA ​g−1, as well as long-term cycling stability with capacity retention of ~77% after 500 cycles at 520 ​mA ​g−1. Operando synchrotron X-ray diffraction analysis reveals that P′2-K0.83[Ni0.05Mn0.95]O2 retains the P′2-phase without P′2-OP4 phase transition during charge/discharge in the voltage range of 1.5–4.3 ​V, which is abnormal compared to the other P′2-based layered cathode materials for sodium-ion batteries, being responsible for the long-term cycle stability of P′2-K0.83[Ni0.05Mn0.95]O2. First-principles calculation results indicate that the excellent electrochemical performance results from the structural stability associated with a single -phase reaction upon K+ extraction/insertion out of/into the host structure. The remarkable potassium storage capability of P′2-K0.83[Ni0.05Mn0.95]O2 makes it a promising cathode candidate for KIBs.

The 1,8-naphthalimide (NI) derivative Lumogen F Violet 570 exhibits different photoluminescence (PL) and aggregation-caused quenching properties due to its crystal polymorphism, which depends on the solvent evaporation process in tetrahydrofuran solution. In the slow drying process, molecules aggregated into an energetically more stable form (time-dependent density functional theory calculation), of which the PL peak maximum was 453 nm, corresponding to blue emission at the 365 nm excitation. However, the fast evaporation process induces an energetically less stable form, with a PL peak maximum of 508 nm, corresponding to green emission. The main difference between the two crystal structures is the alkyl conformation, as confirmed by X-ray single-crystal analysis. Due to the different alkyl conformations, NI groups aggregated into more obliquely aligned structures that emit blue PL, which plays a role in weakening the π-π interactions between molecules relative to green PL crystals. We found that the conformational stable molecular stacking induced instability in the electronic energy levels of the blue crystal compared to the green crystal.

High energy ball-milled iron sulfides with thin carbon layer coating (BM-FeS/C composites) were prepared by the simple and economical process. Ball-milled process, followed by carbon coating, reduced the particle size and increased the electrical conductivity. When employed as sodium-ion battery anodes, BM-FeS/C composites showed extremely high electrochemical performance with reversible specific capacity of 589.8 mAh·g−1 after 100 cycles at a current density of 100 mA·g−1. They also exhibited superior rate capabilities of 375.9 mAh·g−1 even at 3.2 A·g−1 and 423.6 mAh·g−1 at 1.5 A·g−1. X-ray absorption near edge structure analysis confirmed the electrochemical pathway for conversion reaction of BM-FeS/C composites.

Sodium-ion batteries (SIBs) require the development of novel anode materials due to the incompatibility of graphite anode. Herein, we explore the titanosilicate Na2Ti2O3SiO4·2H2O (STOS) as a potential anode material for SIBs. Controlling the content of crystal water in the lattice of STOS through dehydration significantly enhanced its electrochemical performances including coulombic efficiency, reversible capacity, cyclic stability and rate capability. By combining various structural analyses, the mechanism behind the enhancement was successfully clarified. Upon dehydration, the host framework is rearranged to create extra diffusion channels along the a and b axes as well as the additional vacant sodium sites previously occupied by crystal water in the original channels. Compared with the one-directional channels along the c axis in STOS, the channel of dehydrated homologue is three-dimensional with larger space finally enabling rapid sodium-ion diffusion and higher reversible capacity with the relieved lattice strains during charge/discharge. This strategy opens a new avenue and can be broadly applied to other electrode materials containing crystal water in the lattice to not only uncover their hidden potential but also suggest new realm of electrode materials for alkali ion batteries.

A breakthrough utilizing an anionic redox reaction (O2−/On−) for charge compensation has led to the development of high‐energy cathode materials in sodium‐ion batteries. However, its reaction results in a large voltage hysteresis due to the structural degradation arising from an oxygen loss. Herein, an interesting P2‐type Mn‐based compound exhibits a distinct two‐phase behavior preserving a high‐potential anionic redox (≈4.2 V vs Na+/Na) even during the subsequent cycling. Through a systematic series of experimental characterizations and theoretical calculations, the anionic redox reaction originating from O 2p‐electron and the reversible unmixing of Na‐rich and Na‐poor phases are confirmed in detail. In light of the combined study, a critical role of the anion‐redox‐induced two‐phase reaction in the positive‐negative point of view is demonstrated, suggesting a rational design principle considering the phase separation and lattice mismatch. Furthermore, these results provide an exciting approach for utilizing the high‐voltage feature in Mn‐based layered cathode materials that are charge‐compensated by an anionic redox reaction., P2‐type Na0.60Li0.20Mn0.80O2 undergoes a reversible two‐phase reaction at high voltage (≈4.2V vs Na+/Na), in which oxygen ion contributes the charge compensation. The detailed reaction mechanism is thoroughly investigated through experimental and theoretical analyses including in situ XRD and XAS observations. The combined study provides a rational material design for the reversible two‐phase reaction.

When magnetic metal films are oxidized, in many cases, their saturation magnetization values decrease. The importance of high magnetization is well-known because it is directly related to the maximum energy product. Thus, prevention of oxidation in magnetic metal films via capping is important not only for studying the magnetism in magnetic metal films but also for developing new packaging technology for such films. In this research, we successfully grow epitaxial (110) Fe films on (0001) Al2O3 substrates by using radio-frequency (RF) magnetron sputtering. We capped 10-nm-thick Mo layers on the Fe films to prevent oxidation. By varying the thickness of the films, we systematically observed the changes in both the coercivity and the saturation magnetization. Especially, when the film’s thickness was below 8.5 nm, the coercivity of the film started to decrease. We believe the drastic change in the coercivity appeared in the Fe films when the film’s thickness approached the critical domain size for a magnetic domain transition.

Carbon-supported Pt and PtNi nanoparticles (NPs) were synthesized using a borohydride reduction method. Structural properties were studied by synchrotron X-ray diffraction (XRD) and the size/shape of the NPs was determined by transmission electron microscope (TEM). X-ray absorption spectroscopy with its two amendments; X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), has been employed to investigate the local electronic/atomic structure surrounding the Pt and Ni atoms. XANES results, at Pt L 3 -edge and Ni K-edge, have shown fractional oxidation of Pt and Ni atoms. The Pt3Ni1NPs have exhibited a lower bond distance of Pt–Ni shell and higher coordination number of Pt–Ni shells, indicating the alloy formation between Pt and Ni. We further have demonstrated that the Pt and PtNi NPs can serve as effective photocatalysts towards the degradation of water pollutant dye (methyl orange (MO)). By considering the interband charge-transfer of Pt (5d →6sp), a tentative mechanism is proposed to understand the photocatalytic degradation of MO dye molecules by Pt/PtNi NPs under the light irradiation.

Scanning transmission X-ray microscopy (STXM) with ~30-nm spatial resolution revealed at annealing effect on the metal-organic interface of an Al layer with a poly (3-hexylthiophene) (P3HT) organic semiconducting layer with and without phenyl-C61-butyric acid methyl ester (PCBM). In the case of a P3HT/Al layer with a high-tensile-strain organic over-layer, the overall interface changed to a zagged morphology with Al nano pillars after thermal annealing. On the contrary, a P3HT:PCBM/Al layer with a less-tensile-strain organic over-layer showed a smooth interface except for a few nano pillars. These interfacial morphology changes were related to the initial strain status and to the relaxation process and the phase separation of P3HT crystals relating to the PCBM. Grazing-incident wide-angle X-ray scattering (GI-WAXS) measurements were also conducted to examine the residual strain and the crystalline properties of P3HT in the presence of PCBM.

We report new insights in dissolution mechanisms of nickel in PtNi bimetallic nanoparticles (NPs) to develop active and durable oxygen reduction catalysts for fuel cells. Leaching out nickel by using acidic aqueous solution has been regarded as one of the most efficient chemical treatments to obtain a platinum-rich surface, which has shown both increased activity and stability during oxygen reduction reaction. In this work, we introduce a new approach using hydroquinone dissolved in ethanol to leach out nickel from PtNi NPs. The degree of alloying level is followed by X-ray photoelectron and absorption spectroscopies. Electrochemical measurements including potential cycling under oxygen reduction conditions allow us to investigate the dissolution behavior of nickel, depending on the chemical systems, and assess the relationship with electrochemical activity and stability. From comparative studies regarding the traditional acid treatment and the hydroquinone method introduced in this article, it is reveale...

Tuning the properties of transition-metal oxides by controlling oxygen content has value in many applications. $T\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ oxides, which can vary their crystal structure as oxygen content changes, are promising materials for reversible manipulation, since they do not create defects in random fashion. Real-time x-ray scattering is used to monitor the topotactic phase transition and its reversibility in SrFe${}_{0.8}$Co${}_{0.2}$O${}_{3\ensuremath{-}x}$. Reversible, redox-driven topotactic transformations at low temperature and atmospheric pressure indicate that these materials may be of use in electrochemical devices, such as solid oxide fuel cells.

We report KFe(SO4)2 as a novel cathode material for potassium-ion batteries. The synthesis of phase-pure KFe(SO4)2 is confirmed through Rietveld refinement of X-ray diffraction data, and the ion diffusion path and possible positions of K+ ions in the crystal structure are verified using bond-valence sum analysis. In addition, the theoretical average voltage, ion diffusion paths, and associated activation barrier energies obtained by first-principles calculations predict feasibility of the KFe(SO4)2 as a robust cathode for potassium-ion batteries. . Experimentally, the average working voltage of KFe(SO4)2 is approximately 3.3 V (vs. K+/K) assisted by Fe3+/2+ redox pair, and the resulting specific capacity (94 mAh g−1) obtained at C/20 approaches to the theoretical capacity, 94 mAh g−1. In addition, the capacity retention of KFe(SO4)2 at 2C was approximately 80% of the initial capacity after 300 cycles, with a Coulombic efficiency of over 99%. X-ray absorption near-edge structure and operando synchrotron X-ray diffraction analyses reveals the reversible change of the crystal structure with a minimal volume difference (1.83%) by occurrence of Fe3+/2+ redox reaction during dis/charge. The demonstrated excellent structural stability of KFe(SO4)2 endorses feasibility of the KFe(SO4)2 as a sustainable cathode for potassium-ion batteries.

The optical glow of ceramics that becomes established during the constant state of flash, known as Stage III in flash sintering experiments, is investigated. The specimen temperature in this state is obtained from in situ experiments at the Pohang Light Source II. The measurements of the specimen temperature agree very well with the predictions from the black body radiation model. The optical emission spectrum from the specimen is measured from the visible into the deep infrared, and compared with black body radiation that would have been expected from Joule heating. It is concluded that the specimens radiate by electroluminescence, which is ascribed to electron–hole recombination of excitons. The phenomenon is likely the same as discovered by Nernst at the turn of the twentieth century.

Core-shell structure nanoparticles have been the subject of many studies over the past few years and continue to be studied as electrocatalysts for fuel cells. Therefore, many excellent core-shell catalysts have been fabricated, but few studies have reported the real application of these catalysts in a practical device actual application. In this paper, we demonstrate the use of platinum (Pt)-exoskeleton structure nanoparticles as cathode catalysts with high stability and remarkable Pt mass activity and report the outstanding performance of these materials when used in membrane-electrode assemblies (MEAs) within a polymer electrolyte membrane fuel cell. The stability and degradation characteristics of these materials were also investigated in single cells in an accelerated degradation test using load cycling, which is similar to the drive cycle of a polymer electrolyte membrane fuel cell used in vehicles. The MEAs with Pt-exoskeleton structure catalysts showed enhanced performance throughout the single cell test and exhibited improved degradation ability that differed from that of a commercial Pt/C catalyst.

Different amounts of Pt atoms were deposited onto the surface of Pd nanoparticles supported on carbon black by hydroquinone reduction method in anhydrous ethanol. Here, we surveyed electrochemical probing of surface compositions of Pd–Pt surface alloys. They were calculated from hydrogen desorption, carbon monoxide adlayer oxidation, and reduced carbon dioxide oxidation charges. The surface composition of Pt drastically increased up to Pt[0.3]/Pd/C (23.1 at.% of Pt) and then approached that of pure Pt with the moderate rate of increase. Keywords: Carbon dioxide, Core–shell, Oxygen reduction reaction, Electrocatalyst, Fuel cell

To investigate the enhancement of the oxygen reduction reaction (ORR) activity on a carbon-supported PtAu alloy nanoparticle catalyst, the catalyst was subject to heat treatment at 423 K under a CO or Ar atmosphere. The surface composition was analyzed by XPS and the composition was shown to increase from 66 ± 2% (PtAu-AP) to 74 ± 2% (PtAu-CO) after heat treatment under a CO atmosphere, which was confirmed by electrochemical techniques, while the bulk composition was invariant at 55%. For the oxygen reduction reaction (ORR), the mass activity of PtAu-CO increased by 75.6% (33.2 A/gPt) when compared to that of PtAu-AP (18.9 A/gPt). Since the increase in EAS was only 15.8%, it was concluded that the specific activity was enhanced by 52.6% due to surface Pt segregation after heat treatment under a CO atmosphere. The enhanced specific activity was attributed to the reduced OH adsorption energy which was characterized by measuring the potential of total zero charge. The weaker OH adsorption was resulted from the higher Pt/Au ratio at the surface layers.

The hydrogen oxidation reaction activity of sub-monolayer Pt-shell/Pd-core nanoparticles was investigated at a Pt surface concen-trationof0∼5.7%.Thenanoparticleswerepreparedusingacolloidalapproach.Thecombinationofelectrochemicalmeasurementsandhighresolution-X-rayphotoelectronspectroscopyrevealedasignificantdecreaseinoxidizedPdatoms(23.4%)withaPtsurfaceconcentration of 1.7% compared with that of Pd/C. X-ray absorption spectroscopy of the Pt LIII edge suggested preferred Pt deposi-tion, which led to more oxidized Pt atoms during Pt shell growth. The exchange current density of the hydrogen oxidation reactionon the electrocatalyst with a Pt surface concentration of 4.9% was 3.5 times higher than that on Pd/C and was comparable with thaton Pt surfaces. The changes in hydrogen oxidation reaction apparent enthalpy due to Pt shell growth suggested a rate determiningstep (RDS) change (Volmer reaction →Tafel reaction) at a Pt surface concentration of 1.7%.© 2012 The Electrochemical Society. [DOI: 10.1149/2.058301jes] All rights reserved.Manuscript submitted July 31, 2012; revised manuscript received October 24, 2012. Published November 7, 2012.

Highly dispersed Pd nanoparticles were prepared by borohydride reduction of Pd(acac)(2) in 1,2-propanediol at an elevated temperature. They were uniformly dispersed on carbon black without significant aggregation. X-ray diffraction showed that carbons from the Pd precursor dissolved in Pd, increasing its lattice parameter. A modified reduction process was tested to remove the carbon impurities. Carbon removal greatly enhanced catalytic activity toward the oxygen reduction reaction. It also generated an inconsistency between the electronic modifications obtained from X-ray photoelectron spectroscopy and the electrochemical method. CO displacement measurements showed that the formation of Pd-C bonds decreased the work function of the surface Pd atoms.

To understand the difference in degradation characteristics between carbon-supported platinum (Pt/C) and platinum–nickel alloy (Pt1Ni1/C) cathode catalysts in membrane electrode assemblies (MEAs) of a polymer electrolyte membrane fuel cell (PEMFC), constant current operation of MEA in a single cell was conducted for 1100 h. A significant change in cell potential for the Pt1Ni1/C MEA was observed throughout the test. High-resolution transmission electron microscopy showed that sintering and detachment of metal particles in the Pt1Ni1/C catalyst occurred more sparingly than in the Pt/C catalyst. Instead, X-ray photoelectron spectroscopy element mapping revealed dissolution of Ni atoms in the Pt1Ni1 catalysts even when the Pt1Ni1/C catalyst used in the MEA was well synthesized.

Carbon-supported Pt nanoparticles with various loading of Pt are prepared by a novel seed-mediated growth method using hydroquinone (HQ) assisted selective deposition. The structural and morphological dependence of the Pt nanoparticles on the catalytic activity in oxygen reduction reaction (ORR) is investigated. The analysis of core-level X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) cannot elucidate the origin of the large enhancement of the ORR activity. In contrast, electrochemical measurements of the CO-displacement charge and the concomitant CO adlayer oxidation behavior prove that the selective deposition of additional Pt contributed to the decrease of surface defects and to the increase of the onset potential for OH adsorption.

Polarization curves of membrane electrode assemblies (MEAs) containing carbon-supported platinum (Pt/C) and platinum–nickel alloy (Pt 1 Ni 1 /C) as cathode catalysts were obtained for durability test as a function of time over 1100 h at constant current. Charge transfer resistance was measured using electrochemical impedance spectroscopy and postmortem analysis such as X-ray diffraction and high-resolution transmission electron microscopy was conducted in order to elucidate the degradation factors of each MEA. Our results demonstrate that the reduced performance of MEAs containing Pt 1 Ni 1 /C as a cathode catalyst was due to decreased oxygen reduction reaction caused by the corrosion of Ni, whereas that of MEAs containing Pt/C was because of reduced electrochemical surface area induced by increased Pt particle size.

The PtRu alloying degree was controlled by mixing various amounts of water in the solvent while the nominal Pt/Ru ratio was remained at the value of one, and the relationship between the alloying degree and electrochemical activities was examined in this study. X-ray diffraction analysis showed that the fraction of the alloyed Ru ranged from approximately 18% for 100% water solvent to 86% for 0% water solvent. In X-ray photoelectron spectroscopy (XPS), Pt and Ru metallic components increased with the PtRu alloying degree. Electrochemical activities for CO oxidation and methanol oxidation were enhanced as the PtRu alloying degree became higher. Electrochemical stability was also investigated via potential cycling in a methanol-contained electrolyte solution. After the potential cycling process, the catalysts were analyzed by transmission electron microscopy, XPS, XPS element mapping. Possible reasons for the difference in the electrochemical stability of the catalysts were discussed.

Carbon-supported Pt–Ni alloy nanoparticles with various compositions were prepared by a borohydride reduction method in anhydrous ethanol solvent. Here, we surveyed effect of thermally induced de-alloying on activity of the oxygen reduction reaction (ORR). Especially, changes in surface and bulk structures were investigated through electrochemical and spectroscopic measurements. The activity of as-prepared Pt–Ni alloy nanoparticles showed a monotonous dependence on Pt content. However, heat-treatment induced the phase separation between Pt and NiO and the resultant enhancement in ORR activity without significant increase in surface Pt concentration. Keywords: Pt–Ni, Oxygen reduction reaction, Fuel cell, Electrocatalyst

The degradation behavior of a membrane-electrode assembly (MEA) was investigated in accelerated degradation tests under constant voltage (0.8 V and 0.7 V) and load cycling (from open circuit voltage to 0.35 V) conditions. Changes in the structural and electrochemical characteristics of MEA after the durability tests give information as to the degradation mechanism of MEAs. The results of cyclic voltammogram and postmortem analysis by X-ray diffraction and high resolution-transmission electron microscopy indicate that the cathode catalyst layers of the MEAs showed no extreme degradation under constant voltage mode, whereas MEAs under repetition of load cycling mode showed very severe degradation after 280 h. However, the single cell performance of the MEA under repetition of load cycling mode was higher than under constant voltage mode. In addition, although the Pt band in the membrane of the MEA under repetition of load cycling mode was observed by field emission scanning electron microscopy, it did not affect the ohmic resistance.