Ni Atomic Ratio Research Articles

Accelerated discovery of Magnesium-based amorphous alloys through a property-driven active learning strategy

Magnesium (Mg)-based amorphous alloys hold significant potential for applications in the automotive, aerospace, and biomedical industries. However, they are limited by their smaller size compared to other amorphous alloys. A higher reduced glass transition temperature (Trg) is associated with larger sizes in Mg-based amorphous alloys. Yet, due to the vast chemical space involved, designing Mg-based amorphous alloys with higher Trg using traditional ‘trial and error’ method is a challenging endeavor. In this work, we developed a property-driven active learning strategy to customize Mg-based amorphous alloys with enhanced Trg. After just two iterations, we successfully tailored four amorphous alloys with high Trg values. Under identical experimental conditions, two of these alloys exhibited Trg values surpassing that of Mg0·65Ag0·19Cu0.06Gd0.1, the alloy with best Trg value in the reported references. SHAP analysis revealed that Trg tends to be higher when the Ag atomic ratio exceeds 0.045, the Cu atomic ratio is below 0.18, the Ni atomic ratio is below 0.025, and the Mg atomic ratio is below 0.665. Our work offers a reliable strategy for designing Mg-based amorphous alloys with higher Trg and provides valuable insights for the rational design of these alloys.

Synthesis of low cost cathode electrocatalyst Pt‐Ni/CAB using DMSO as a solvent for low temperature proton exchange membrane fuel cell application

AbstractIn the present study, low cost platinum based bimetallic electrocatalyst Pt‐Ni/CAB with varying Pt to Ni atomic ratios 3:1, 1:1, and 1:3 were successfully synthesized for oxygen reduction reaction (ORR) of the developed proton exchange membrane fuel cell (PEMFC). The solvothermal process was adopted for the synthesis of Pt‐Ni(3:1)/CAB, Pt‐Ni(1:1)/CAB, and Pt‐Ni(1:3)/CAB using dimethyl sulfoxide (DMSO) as solvent at a temperature of 190°C which is very close to the boiling point. The Pt‐Ni/CAB exhibited the highest activity for the ORR in half‐cell and single cell PEMFC performance. The electrocatalysts Pt‐Ni(1:3)/CAB appeared with smallest crystalline FCC structures having crystallite size of 8.33 nm. The transmission electron microscopy (TEM) analysis also show that the Pt‐Ni(1:3)/CAB has smallest particle size of 1.67 ± 0.45 nm. The cyclic voltammetry (CV) analysis shows Pt‐Ni(1:3)/CAB electrocatalyst offers less activation loss at ORR peak at a potential of 0.16 V as compared to Pt‐Ni(3:1)/CAB (ORR peak – 0.12) and Pt‐Ni(1:1)/CAB (ORR peak – 0.11). The synthesized Pt‐Ni(1:3)/CAB produced a maximum power density of 18.86 mW/cm2 at a maximum current density of 44.8 mA/cm2 with an open‐circuit voltage of 0.914 V at a room temperature of 33°C. The power density improved around 1.34 times when the cell temperature was raised from 33 to 70°C. The Pt‐Ni(1:3)/CAB cathode electrocatalyst could be used as economical to substitute costly commercial pure platinum based electrocatalyst.

Pd-Based Bimetallic Electrocatalysts for Hydrogen Oxidation Reaction in 0.1 M KOH Solution.

A series of carbon black-supported 7.5 wt.% Pd-2.5 wt.% M/C (M: Ag, Ca, Co, Cu, Fe, Ni, Ru, Sn, Zn) electrocatalysts, synthesized via the wet impregnation method, and reduced at 300 °C, were compared in terms of their hydrogen oxidation reaction (HOR) activity in a 0.1 M KOH solution using the thin-film rotating-disk electrode technique. Moreover, 10 wt.% Pd/C and 10 wt.% Pt/C electrocatalysts were prepared in the same manner and used as references. The 7.5 wt.% Pd-2.5 wt.% Ni/C electrocatalyst exhibited the highest HOR activity among the Pd-based electrocatalysts, although it was lower than that of the 10 wt.% Pt/C. Its activity was also found to be higher than that of Pd-Ni electrocatalysts of the same total metal loading (10 wt.%) and reduction temperature (300 °C) but of different Pd to Ni atomic ratio. It was also higher than that of 7.5 wt.% Pd-2.5 wt.% Ni/C electrocatalysts that were reduced at temperatures other than 300 °C. The superior activity of this electrocatalyst was attributed to an optimum value of the hydrogen binding energy of Pd, which was induced by the presence of Ni (electronic effect), as well as to the oxophilic character of Ni, which favors adsorption on the Ni surface of hydroxyl species that readily react with adsorbed hydrogen atoms on neighboring Pd sites in the rate-determining step.

Assessing Pt and Ni dissolution mechanism and kinetics of shape-controlled oxygen reduction nanocatalysts

An electrochemical flow cell combined with inductively coupled plasma mass spectrometry (FC-ICP-MS) is a powerful tool to understand the mechanisms of metal dissolution and to develop mitigation strategies. Herein, we quantified in situ the amount of Pt and Ni atoms dissolved from PtNi/C nanocatalysts employed to electrocatalyze the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cell cathode. The nanocatalysts feature similar crystallite size and Pt:Ni atomic ratio but different morphologies (spheres, octahedra, sponges). The FC-ICP-MS results reveal that the nanocatalyst morphology affects the dissolution rate of Pt and Ni but not the dissolution mechanism. They also provide analytical evidence that dissolution of Pt atoms is consistently accompanied by the dissolution of Ni atoms exceeding the stoichiometric composition. Furthermore, we demonstrate that ex situ acid leaching mitigates, but does not entirely prevent, the electrochemical dissolution of Ni atoms. Stabilized Pt and Ni dissolution rates were achieved after a one hour long accelerated stress test (AST). We provide evidence that the Pt dissolution rate remains constant before and after the AST. In contrast, the dissolution rate of Ni decreases by a factor of 10 following the AST. Among various nanoparticle shapes, spherical PtNi/C nanoparticles offer the best solution regarding the Pt and Ni retention compared to other nanoparticle shapes.

Intolerance of Ruddlesden-Popper La2NiO4±δ Structure to a-Site Cation Deficiency

Mixed ionic-electronic conductors derived from Ruddlesden-Popper Ln2NiO4+δ (Ln = La, Pr, Nd) attract significant attention as oxygen electrode materials for solid oxide fuel and electrolysis cells. However, literature data on the impact of A-site deficiency on the phase relationships and functional properties of Ln2NiO4+δ-based phases are contradictory. While some works report an accommodation of A-site vacancies up to x = 0.15 in La2-x NiO4±δ, others indicate difficulties in the synthesis of single-phase La2-x NiO4+δ even for low x values. The present work explores the impact of the nominal A-site deficiency on the phase composition and electrical transport properties of La2-x NiO4±δ ceramics.Ceramics with the nominal composition La2-x NiO4±δ (x = 0, 0.02, 0.05, 0.10) were synthesized by the glycine-nitrate technique and sintered at 1350-1450°C. While XRD analysis suggested the formation of single-phase materials with orthorhombic K2NiF4-type structure, the lattice parameters remained independent of composition within the experimental uncertainty. Neutron diffraction studies of x = 0 and 0.05 samples revealed that the lattice parameters, La:Ni atomic ratio (2:1) and oxygen sites occupancy remain identical for both samples, while the presence of impurity NiO phase (0.4 wt.%) was detected for La1.95NiO4±δ. SEM/EDS inspection of ceramic samples confirmed the presence of nickel-rich inclusions for x ≥ 0.02, which could not be eliminated by increasing the temperature and time of sintering. The volume fraction of inclusions increased with x. The experimental observations are supported by the atomistic simulations showing that the coexistence of cation-stoichiometric La2NiO4+δ with nickel-rich La4Ni3O10 or NiO is more favorable compared to cation-deficient La1.95NiO4±δ. Molecular dynamics simulations of La1.95NiO4±δ lattice demonstrated that the introduction of La vacancies leads formation of instability zone in the rock-salt-type layer and destabilization of the crystal lattice. The intolerance of La2NiO4+δ lattice to A-site vacancies and coexistence of La2NiO4+δ with traces of Ni-rich secondary phase in the samples with the nominal composition La2-x NiO4±δ results in an insignificant variation of electrical transport properties with x. Acknowledgements This work was developed within the scope of the project CARBOSTEAM (POCI-01-0145-FEDER-032295) funded by FEDER through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI) and by national funds through FCT/MCTES, and the project CICECO-Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020) financed by national funds through the FCT/MCTES (PIDDAC). A.B. acknowledges the PhD scholarship by the FCT (grant SFRH/BD/150704/2020). This work was partially supported by the National Science Centre, Poland, grant UMO-2020/37/B/ST8/02097.

Effects of Nickel Promotion on the Catalytic Performance of In Situ Synthesized Suspensions of Molybdenum Disulfide Nanoparticles

The study investigates the activity of in situ synthesized suspensions of nickel-promoted molybdenum disulfide particles in the hydroconversion of crude oil vacuum residues. The catalyst suspensions were prepared in situ from water-in-oil emulsions of aqueous solutions of precursors, specifically ammonium paramolybdate and nickel nitrate. The catalytic tests were carried out in a flow-type reactor at 430°C, WHSV 1 h–1, and 7 MPa hydrogen, with the Mo:Ni atomic ratio in the catalyst particles ranging from 1:0.022 to 1:1.43. The XRD of the toluene-insoluble solids (TIS) extracted from the hydrogenates identified sulfides such as MoS2, Ni3S4, and Ni3S2 in the dispersed catalyst. Increasing the nickel content in the catalyst favored its hydrogenation activity, which was indicated by an enhancement in the feed conversion, an increase in the content of paraffins and naphthenes, and a decrease in the sulfur content in the distillates and TIS derived from the hydrogenates. The conversion of high-molecular-weight feed components (resins, asphaltenes, and heavy aromatics) was enhanced as a result of the nickel promotion of the dispersed MoS2.

Investigating the nanoscale hardness/strength properties of high-entropy alloy particles using the nanoindentation technique

Particulate feedstock constitutes the building block in modern day additive manufacturing (AM) era. Cold spray (CS) is a leading process technology to adhere to the AM principle. Therefore, meeting feedstock qualities is of utmost interest to ensure the conformability and competitiveness of the developed industrial modules, including coatings, architectured components, and additively repaired devices. This research advances the understanding of nanoscale hardness/strength properties of particulate matters, specifically of an emerging material class, - high-entropy alloys (HEAs). The feasibility of determining the hardness of mechanically alloyed AlCoCrFeNix (x = 0, 1, 2.1) HEA particles was studied employing the nanoindentation technique. Mechanical properties of milled AlCoCrFeNix particles with varying Ni atomic ratio (x = 0, 1, 2.1) were investigated over different milling times ranging between 4 and 24 h. The study analyzed the impact of mounting resin, pre-determined maximum load, and indentation depth on hardness/strength properties. Results reveal that the hot mounted samples yielded greater accuracy and higher hardness values than compared to those of the cold mounted samples. Additionally, although the low-load sensitivity of AlCoCrFeNix provided consistent nano-scale hardness values across selected loads, their hardness values were found to be depth-dependent. Overall, the study concludes with a methodology for the nano-scale hardness/strength measurement of HEA particles that must account for particle size, sample preparation technique, and nanoindentation test parameters.

Effects of P:Ni Ratio on Methanol Steam Reforming on Nickel Phosphide Catalysts.

This study investigates the influence of the phosphorus-to-nickel (P:Ni) ratio on methanol steam reforming (MSR) over nickel phosphide catalysts using density functional theory (DFT) calculations. The catalytic behavior of Ni(111) and Ni12P5(001) surfaces was explored and contrasted to our previous results from research on Ni2P(001). The DFT-predicted barriers reveal that Ni(111) predominantly favors the methanol decomposition route, where methanol is converted into carbon monoxide through a stepwise pathway involving CH3OH* → CH3O* → CH2O* → CHO* → CO*. On the other hand, Ni12P5 with a P:Ni atomic ratio of 0.42 (5:12) exhibits a substantial increase in selectivity towards methanol steam reforming (MSR) relative to methanol decomposition. In this pathway, formaldehyde is transformed into CO2 through a sequence of reactions involving CH2O*→ H2COOH* → HCOOH* → HCOO* → CO2. The introduction of phosphorus into the catalyst alters the surface morphology and electronic structure, favoring the MSR pathway. However, with a further increase in the P:Ni atomic ratio to 0.5 (1:2) on Ni2P catalysts, the selectivity towards MSR decreases, resulting in a more balanced competition between methanol decomposition and MSR. These results highlight the significance of tuning the P:Ni atomic ratio in designing efficient catalysts for the selective production of CO2 through the MSR route, offering valuable insights into optimizing nickel phosphide catalysts for desired chemical transformations.

Fabrication of Pd–Ni/Au nanoshelled microsphere array for electrocatalytic oxidation of methanol

Pd–Ni/Au nanoshelled microsphere arrays with different Pd–Ni atomic ratios were successfully synthesized using the etched monolayer colloidal spheres as templates. Morphology and composition characterizations indicated that the prepared products consisted of polystyrene cores of Au and Pd–Ni metallic nanoshells with an overall diameter of 450 nm. Electrochemical measurements in an alkaline solution showed that the Pd–Ni/Au catalyst with Pd:Ni = 4:1 atomic ratio demonstrated an optimal catalytic activity with the peak current of 59.4 mA[Formula: see text], the lower oxidation peak potential and the higher stability. These results could be ascribed to the enhanced surface area which increased the number of electrocatalytic active sites, and the electronic interaction between Pd and Ni. This work provides new opportunities for the rational design of microsphere array catalysts for enhancing the catalytic performance in fuel cells.

Enhancing Resistance to Chloride Corrosion by Controlling the Morphologies of PtNi Electrocatalysts for Alkaline Seawater Hydrogen Evolution.

A solvothermal method to prepare PtNi alloys that have differing morphologies is described. By adjusting the feed ratio of Pt and Ni precursors in this process, PtNi alloys with different compositions (Pt : Ni atomic ratio from 1 : 3 to 3 : 1) and morphologies (evolution from nanobranches to nanoparticles) are generated. The prepared Pt48 Ni52 alloy, which has a composite morphology comprised of nanobranches and nanoparticles, exhibits superior activity and durability towards the hydrogen evolution reaction (HER) in seawater compared to those of commercial Pt/C catalyst and other PtNi alloys that have different compositions and morphologies. The excellent seawater HER performance of Pt48 Ni52 is ascribed to its nanobranch/nanoparticle morphology that optimally facilitates electron accumulation on Pt, which enhances resistance to chloride corrosion in seawater.

Fe3O4 core–shell catalysts supported by nickel foam for efficient heterogeneous electro-Fenton degradation of salicylic acid at neutral pH

A novel nickel foam (NF) supported core–shell catalytic material with excellent catalytic performance was successfully prepared for the degradation of salicylic acid (SA) by heterogeneous electro-Fenton (HEEF). Under the reaction conditions of SA initial concentration of 100 mg/L, catalyst dosage of 4 g/L, aeration rate of 0.1 L/min, current of 0.3 A and pH of 7, after 2 h, the removal efficiency of the progressively modified core–shell catalysts for SA and COD was as follows: NF@Fe3O4@SiO2@MgAl-LDH > NF@Fe3O4@MgAl-LDH > NF@Fe3O4 > NF@Fe3O4@SiO2. The removal efficiency of NF@Fe3O4@SiO2@MgAl-LDH for SA and COD reached 72.8 % and 52.12 %, respectively. The morphology, composition and structural characteristics of the composite catalysts were analyzed and compared by a series of characterization methods. Among them, NF@Fe3O4@SiO2@MgAl-LDH showed the largest Fe:Ni atomic content ratio, specific surface area and pore volume, with values of 1: 2.62, 7.5405 m2·g−1 and 0.010504 cm3·g−1, respectively. Radical quenching experiments, changes in H2O2 and Fe2+ ion concentrations, and X-ray photoelectron spectroscopy (XPS) analysis revealed the reaction mechanism of the HEEF process. That is, synergistic oxidation of active radicals and abundant adsorbed oxygen and lattice oxygen on the surface of the materials promoted the degradation and mineralization of SA. The excellent stability and reusability of NF@Fe3O4@SiO2@MgAl-LDH in the HEEF system were verified through five cyclic experiments with low material loss rate and sludge production.

Preparation of porous TiNi-Ti alloy by diffusion sintering method and study of its composition, structure and martensitic transformations

The study demonstrates a method for controlling not only the phase composition but also the atomic composition of TiNi matrix in porous TiNi-Ti alloys developed for biomedical uses as implants. The alloys were obtained from TiNi powder which was sintered with Ti powder added at as much as 0–10 at%. The structure, phase and chemical composition of the produced TiNi-Ti alloys was investigated with respect to the amount of Ti added into the material. It is shown that in the sintered product containing 5 at% and more of Ti additive, the composition of its TiNi matrix becomes close to equiatomic (with Ti:Ni atomic ratio ~1), and the excessive Ti precipitates as secondary phases Ti2Ni and Ti3Ni4. In parallel, with increase in Ti additive from 0–10 at%, the structure of the precipitating Ti2Ni type phases changes its morphology from separate spherical or pyramidal precipitates to large dendritic formations. The direct martensitic transformation from austenite to martensite in all the samples was found to proceed in two stages and through the R-phase (B2→R→B19′). Thermoresistive analysis demonstrated that TiNi-Ti samples with 5 and more at% of Ti had their characteristic starting temperature of martensite transition stabilizing at ~57 °C (330 K). This implies that the sample with 5 at% of Ti additive exhibited desired martensite transition temperatures, while containing a minimum concentration of secondary-phase precipitates in its matrix which deteriorate its properties. Thus, for the first time, we show that a very simple preparation approach based on sintering powders of TiNi and Ti is capable of producing porous TiNi-Ti alloys with properties optimized for fabricating bone implants.

Nanocolumnar Pt:Ni Alloy Thin Films by High Pressure Sputtering for Oxygen Reduction Reaction

Self-supported nanocolumnar Pt:Ni thin films (TFs) with varying Pt:Ni atomic ratios and Pt mass loadings were produced on a microporous layer (MPL)-like surface composed of carbon particles by high pressure sputtering and examined as oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. Cauliflower-like microstructures were observed from scanning electron microscopy imaging. Various Pt:Ni atomic ratios were obtained by simply changing the relative deposition power between Pt and Ni source and investigated by X-ray diffraction and quartz crystal microbalance analysis. Electrochemical characterization of the Pt:Ni-TF/MPL-like-layer/glassy-carbon samples was conducted through benchtop cyclic voltammetry and rotating disk electrode measurements. The electrochemically active surface area (ECSA) was found to be between 22–42 m2 g−1 for different Pt:Ni atomic ratios. Lower Pt mass loadings exhibited a higher ECSA and the catalytic activity of all Pt:Ni ratios increased with the increase in Pt mass loading. The ORR activity of the Pt:Ni-TFs increased in the order of 3:1 < 1:1 < 1:3 with exhibiting a specific activity of 1781 μA cm−2 and mass activity of 0.66 A mg−1 for the Ni-rich film with 1:3 ratio. The catalytic performance of Pt:Ni-TFs were higher than traditional high surface area carbon supported Pt nanoparticles, elemental Pt nanorods, and Pt-Ni nanorods.

(Invited) Pt Alloy Octahedral Nanoparticle Catalysts from Screening Studies to Fuel Cell Measurements

In proton exchange membrane fuel cells (PEMFCs) hydrogen is oxidized at the anode and oxygen is reduced at the cathode, where Pt-based catalysts represent the state of the art. Due to the sluggish oxygen reduction reaction (ORR) kinetics, Pt loadings per vehicles are still rather high when compared to platinum-group metal (PGM) loadings in combustion engine cars ( ~ 5 gPGM/vehicle in light-duty internal combustion engine vehicles),1 impacting the cost of fuel cell electric vehicles.Octahedral PtNi nanoparticles have been reported to achieve extremely high ORR mass activity in rotating disk electrode (RDE) experiments,2 which would allow for a significant decrease of Pt loadings in fuel cells. In addition, the introduction of a third metal (X) as a surface dopant has been recently shown to have beneficial effects on the RDE performance, enhancing both activity and stability.3 However, despite these promising steps toward shape-stable PtNiX octahedral nanoparticles, the morphological stability and the performance in membrane electrode assembly (MEA)-based fuel cell measurements still needs to be improved to match and surpass the state of the art Pt and de-alloyed Pt alloy catalysts.4 In this contribution, we will show the results of our recent investigations that were aimed at improving the performance of PtNi based octahedral nanoparticle catalysts towards integration in low Pt loading cathodes for PEMFCs. These include exploration of different dopants, Pt loadings and carbon supports. Preliminary screening by RDE identified promising dopants (i.e. Mo4 and other transition metals) and Pt:Ni atomic ratios. The activity and stability of the PtNiX octahedral nanoparticles were then investigated in a comparative study by using different carbon supports, including high surface area porous carbon. Finally, selected catalysts were investigated in MEA-based fuel cells and their activity compared with state-of-the art reference Pt and PtNi alloy materials, measured under the same conditions. References A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 2016, 7, 1127-1137.P. Strasser, Science, 2015, 349, 379-380.X. Q. Huang, Z. P. Zhao, L. Cao, Y. Chen, E. B. Zhu, Z. Y. Lin, M. F. Li, A. M. Yan, A. Zettl, Y. M. Wang, X. F. Duan, T. Mueller and Y. Huang, Science, 2015, 348, 1230-1234.F. Dionigi, C. C. Weber, M. Primbs, M. Gocyla, A. M. Bonastre, C. Spöri, H. Schmies, E. Hornberger, S. Kühl, J. Drnec, M. Heggen, J. Sharman, R. E. Dunin-Borkowski and P. Strasser, Nano Lett, 2019, 19, 6876-6885. Acknowledgements The GAIA project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 826097. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research.

Activity and sulfur resistance of co-impregnated bimetallic PdNi/γ-Al2O3 catalysts during hydrogenation of styrene

Pyrolysis gasoline (PyGas) is an unstable byproduct of the pyrolysis of naphtha and other hydrocarbons for the production of olefins. PyGas is stabilized by hydrogenation at mild conditions. The hydrogenation of styrene to ethylbenzene is considered a model test reaction for studying the selectivity of the catalysts because it has the slowest rate of conversion. The catalytic activity and selectivity of two bimetallic co-impregnated Pd–Ni catalysts supported on γ-alumina were assessed and compared to those of a commercial monometallic Pd catalyst (AXENS LD265). The laboratory-prepared catalysts had different metal content and Pd:Ni atomic ratios (1:1 and 1:7). Catalytic tests of selective hydrogenation of styrene to ethylbenzene were made in a batch stirred tank reactor and in a continuous fixed bed trickle bed reactor. The sulfur resistance of the bimetallic catalysts in semicontinuous condition, was assessed by means of the same operational conditions using thiophene as a model poison compound. The support, Pd–Ni co-impregnated catalysts and LD265 commercial catalysts were further characterized by N2 chemisorption, EPMA, OM, ICP elemental analysis, temperature programmed reduction, XPS and X-Ray diffraction. The results indicated the presence of different metal species: Pd0, Pdδ+, Ni0 and NiO. The lab-prepared bimetallic catalysts were found to be active for the selective hydrogenation of styrene (both in the batch and continuous system). During the continuous evaluations, the commercial LD265 catalyst had an intermediate activity level that lied between the values corresponding to the Pd–Ni bimetallic catalysts: PdNi(1:1) $$ \gg $$ LD265 > PdNi(1:7). During the poison free semicontinuous evaluations, the pattern of conversion as a function of contact time of co-impregnated Pd–Ni catalysts are quite similar to that of commercial LD265 catalyst, though the catalytic activity is slightly better for PdNi(1:1). After 90 min of contact time the order of conversion was: PdNi(1:1) > LD265 > PdNi(1:7). On the other hand, during the poison semicontinuous tests (with 300 pp of thiophene), the initial reaction rates of the co-impregnated catalysts decreased, pointing a poisoning of the active sites by thiophene. PdNi(1:1) had higher initial reaction rate than PdNi(1:7) during poison free or poisoned in both conditions. PdNi(1:7) was more sulfur resistant than PdNi(1:1). The higher activity of the PdNi(1:1) catalyst was attributed to the presence of Pd0 and Pdn+/Nim+ species that favored respectively, the homolytic cleavage of the H–H bond and the adsorption of styrene. The higher sulfur resistance of the PdNi(1:7) catalyst would be associated with the higher Cl/Al surface atomic ratio in this catalyst (electronic effect) and the presence of electrodeficient species of Pd (Pdxδ+OyClz) that prevented the adsorption of thiophene by steric hindrance (geometrical effect of the large species) or by electronic effects.

Effects of the Composition and the Particle Size of Octahedral Pt-Ni Nanoparticles on Their Durability

Carbon-supported octahedral Pt-Ni nanoparticles (oct-Pt-Ni/C) have been expected to be one of the most promising cathode catalysts for polymer electrolyte membrane fuel cells because of their extremely high catalytic activities for oxygen reduction reaction. While the effects of the Pt-Ni composition and the particle size on the activity have been widely studied to achieve the highest activity, their effects on the durability have been hardly examined. Here, we synthesized oct-Pt-Ni/C with different Pt-Ni compositions and sizes by changing concentrations of the precursors and the protecting agents, respectively. Then, their degradation behaviors were systematically examined by using an RDE-based accelerated durability test (ADT). Although the oct-Pt-Ni/C exhibited extremely high activities (630–3400 A g−1-Pt) with a volcano-like dependency against the Ni atomic ratio, all these ORR activities rapidly decreased in the beginning to approach that of Pt/C (400 A g−1-Pt). After the ADT, all the oct-Pt-Ni/C lost their octahedral shape through Ni leaching. There was no significant dependency of the degradation rate on the Pt-Ni composition and the particle size for the oct-Pt-Ni/C with the sizes smaller than 7 nm, which are needed to exhibit at least decent performances in a fuel cell. Therefore, further efforts are needed to develop highly durable oct-Pt-Ni/C.

Ternary IrO2-Pt-Ni deposits prepared by galvanic replacement as bifunctional oxygen catalysts

Abstract Bifunctional oxygen catalysts with IrO2 and Pt active components have been prepared by means of galvanic partial replacement of Ni layers electrodeposited on glassy carbon substrates (Ni/GC). The resulting IrO2Pt(Ni)/GC electrodes have a homogeneous and compact morphology (SEM), while surface electrochemistry and spectroscopic measurements (EDS, XPS) confirm the ternary bulk structure (EDS: 56.39% Ir – 7.60% Pt – 36.01% Ni atomic ratio) and the existence of a IrO2-Pt thin skin on the outermost layers of the deposits (XPS: Ir÷Pt atomic ratio of 21.7). IrO2Pt(Ni)/GC electrodes were evaluated as bifunctional electrocatalysts for oxygen evolution (OER) and oxygen reduction (ORR) reactions in acid, by means of steady-state current-potential measurements and linear sweep voltammetry at a Rotating Disc Electrode (RDE) respectively. The OER overpotential required for a current density (per substrate geometric area) of 1 (η1) and 10 mA cm‐−2 (η10) on the prepared electrodes was 235 and 302 mV respectively, which is in the range of some typical thermally produced IrO2-based DSAs. Their intrinsic OER catalytic activity (as estimated by normalization of the recorded currents at an overpotential of 260 mV by the oxides charge) was found to be 0.29 mA mC−1 comparable or better to other thermally produced IrO2 and Pt-IrO2 systems. Koutecky-Levich analysis shows that ORR proceeds on the ternary catalysts via the four electrons pathway. The overall ORR catalytic activity (0.4 mA cm−2 at +0.8 V vs. SHE) is found to be in the range of that recorded on nanoparticle catalysts, while a suppression of the intrinsic activity compared to bulk Pt is observed most probably due to the presence of IrO2, the latter being in line with similar non-alloyed Pt-IrO2 systems reported in the literature.

Synthesis and Evaluation of PtNi Electrocatalysts for CO and Methanol Oxidation in Low Temperature Fuel Cells

Pt(Ni)/C and PtRu(Ni)/C catalysts were synthesized by electroless deposition of Ni on a carbon dispersion followed by sequenced Pt deposition and spontaneous deposition of Ru species. The structural analyses of the catalysts with 88:12 and 98:2 Pt:Ni atomic ratios pointed out to the formation of small hexagonal Ni crystallites covered by thin cubic Pt surface structures with no evidence about PtNi alloy formation. The onset potentials for CO oxidation on Pt(Ni)/C and PtRu(Ni)/C were about 0.10 and 0.24 V more negative than those of Pt/C, thus indicating their better CO tolerance. The surface Ru species appeared to have the major effect by facilitating the CO removal by the bifunctional mechanism. The onset potential for the methanol oxidation reaction (MOR) of Pt(Ni)/C was about 0.15 V lower than that of Pt/C. The mass and specific activities together with the exchange current densities of the Pt(Ni)/C catalysts were also higher than those of Pt/C, making in evidence their higher activity in front of the MOR. The Tafel slopes for the MOR on Pt(Ni)/C suggested different reaction mechanism than on Pt/C. The electronic (ligand) effect of Ni on Pt was considered the main reason to explain the higher activity of Pt(Ni)/C in front of the CO oxidation and the MOR.

Dry reforming of methane over Pt-Ni/CeO2 catalysts: Effect of the metal composition on the stability

A series of Pt-Ni/CeO2 catalysts with different Pt:Ni atomic ratios were synthetized through incipient wetness impregnation over CeO2 prepared by precipitation. Catalysts were characterized by XRD, EDS-SEM, N2-Adsorption, TEM-STEM, H2-TPR, CO-DRIFTS and O2-TPO. Samples were studied in the Dry Reforming of Methane (DRM) reaction. The catalytic activity of nickel-ceria catalysts was improved by addition of platinum at any composition, and the behavior of bimetallic Pt-Ni/CeO2 catalysts resembled to that exhibited by the platinum-ceria sample. Stability results showed that bimetallic catalysts presented a steady behavior, with no deactivation after 24 h on-stream and enhanced anti-coke properties by the presence of platinum, while the H2/CO ratio at all metal compositions was superior compared to that of monometallic samples. Nickel not only contributes to the methane activation, but also to the hydrogen production. The enhancement of the catalytic properties of Pt-Ni/CeO2 catalysts was explained through a synergic effect between the two metals which impacted in the metal dispersion, the reduction properties and the stabilization of the surface structure of the active ensembles.

MWCNT-COOH supported PtSnNi electrocatalysts for direct ethanol fuel cells: Low Pt content, selectivity and chemical stability

PtSnNi electrocatalysts (60: 40: 40 mass ratio) supported on Vulcan® XC-72 (Cabot) carbon and COOH-functionalized multiwalled carbon nanotubes (Cheaptubes®) with 15% of metal loading were prepared. The nanoparticles size of 2–3 nm for both supports was estimated by HRTEM. In the direct ethanol fuel cell experiments, PtSnNi/C presents 50 mA cm−2 reaching the maximum power density (MPD) of 12 mW cm−2 and decreasing at higher currents, while PtSnNi/MWCNT-COOH obtains similar values of MPD (60 mA cm−2), but keeping the best performance. By GC (gas chromatography) technique, it was possible to observe that the electrocatalyst supported on MWCNT-COOH favored the ethanol oxidation to acetaldehyde and acetic acid, although the material supported on Vulcan® XC-72 carbon presented almost 100% of selectivity for acetaldehyde. This behavior was maintained also when the current of 0.1 A was applied for 80 min. For the PtSnNi/C electrocatalyst, the selectivity to only acetaldehyde can be related to Sn and Ni dissolution process that can become the electrocatalytic activity similar to Pt/C, decreasing the power density as observed in our experiments. Established by EDS analysis, after 80 min of polarization, the Ni and Sn relative atomic ratio was lower on the catalytic anodic layer of PtSnNi/C than on PtSnNi/MWCNT-COOH.