Effect of supports on activity and stability of Pt–Pd catalysts for oxygen reduction reaction in proton exchange membrane fuel cells

The platinum–palladium alloy (Pt–Pd) catalysts were prepared on various supports including Vulcan XC72, Hicon Black (HB), multiwalled carbon nanotubes (MWCNTs), and titanium dioxide (TiO₂) by a combined approach of impregnation and seeding using NaBH₄ reduction at low temperature. Their oxygen reduction reaction (ORR) activities in single proton exchange membrane fuel cell (PEMFC) under a H₂/O₂ environment and their stability in an acid electrolyte (0.5 M H₂SO₄) were tested and compared with the Vulcan XC72-supported Pt (Pt/C) catalysts. The presence of the Pd metal as well as different types of supports affected the ORR activity in H₂/O₂ environment and stability in the acid electrolyte. Overall, the HB-supported Pt–Pd (Pt–Pd/HB) catalysts provided the highest current density at 0.6 V under a H₂/O₂ environment, while the MWCNT-supported Pt–Pd (Pt–Pd/MWCNT) catalyst provided the best stability in an acid electrolyte.     

AuPt core–shell nanocatalysts with bulk Pt activity

In the presented work, the evaluation of an unsupported AuPt core–shell catalyst for the oxygen reduction reaction is introduced. Applying only basic chemicals in an upscalable synthesis route, it is demonstrated that uniform, flat, and complete Pt layers around a spherical Au core are obtained. The electrocatalytic measurements show that the surface area specific activity of the AuPt core–shell catalyst towards the important oxygen reduction reaction equals the one of polycrystalline bulk Pt. To our knowledge, this is the first time that the unfavorable particle size effect of Pt nanoparticles could be by-passed for a nanoscale catalyst.     

Aqueous treatment of single-walled carbon nanotubes for preparation of Pt–Fe core–shell alloy using galvanic exchange reaction: Selective catalytic activity towards oxygen reduction over methanol oxidation

We have demonstrated a new, cost effective synthesis of single-walled carbon nanotube supported Pt–Fe core–shell alloy catalyst (Pt–Fe/SWNT) for the direct methanol fuel cell using galvanic exchange reaction. The Pt–Fe/SWNTs have shown much larger Pt active surface area (150 m²/g-Pt) than the commercial catalyst (54 m²/g-Pt). Furthermore, four-fold enhancement of catalytic activity of the Pt–Fe/SWNTs for oxygen reduction reaction (ORR) has been observed. This catalyst has also demonstrated its tolerance to methanol in ORR.     

Metal–nitrogen coordination moieties in carbon for effective electrocatalytic reduction of oxygen

Oxygen reduction reaction is a critical process at the cathode of proton-exchange membrane fuel cells and metal–air batteries. Carbon-based single metal atom nanocomposites have emerged as effective alternatives to state-of-the-art platinum catalysts, in which the electrocatalytic activity is attributed largely to the formation of metal–nitrogen coordination moieties (MN_x) within the carbon matrix. In this review, we summarize recent progress in the studies of metal and nitrogen codoped carbon as single-atom catalysts toward oxygen reduction reaction within the context of the atomic configuration of the MN_x active sites and topologic characteristics of the carbon skeletons and include a perspective of the design and engineering of the nanocomposites for further enhancement of the electrocatalytic activity.     

Chemically dealloyed Pt–Au–Cu ternary electrocatalysts with enhanced stability in electrochemical oxygen reduction

We report simple synthesis of ternary Pt–Au–Cu catalysts consisting of active Pt-rich shell and Pt transition-metal alloy core for use as highly active and durable electrocatalysts in oxygen reduction reactions. The ternary Pt–Au–Cu catalysts were synthesized by chemical coreduction followed by thermal treatment and chemical dealloying. During synthesis, thermal treatment formed metal particles into high-degree alloys, and chemical dealloying led to selective dissolution of soluble Cu species from the outer surface layer of the thermally treated alloy particles, resulting in Pt-based alloys@Pt-rich surface core–shell configuration. Compared with a commercial Pt/C catalyst, our Pt_1?xAu _x Cu₃/C-AT catalysts exhibited approximately 2.4-fold enhanced performance in oxygen reduction reactions. Among the catalysts employed in this work, Pt_0.97Au_0.3Cu₃/C-AT showed the highest performance in terms of mass activity, specific activity, and electrochemically active surface area loss with negligible change during 10,000 potential cycles. The synthesis details, electrochemical characteristics, oxygen reduction reaction performance, and durability of the chemically dealloyed ternary Pt–Au–Cu catalysts are presented and discussed.     

Electrochemical determination of the surface composition of Pd–Pt core–shell nanoparticles

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.     

Platinum–titanium intermetallic nanoparticle catalysts for oxygen reduction reaction with enhanced activity and durability

A facile method to prepare Pt–Ti intermetallic nanoparticles supported on carbon was developed. Starting from a commercial Pt/C catalyst, TiO₂ layers were formed on the Pt/C then thermal annealing under a reducing condition successfully produced intermetallic Pt–Ti nanoparticles with an average size of 4.2 nm. The intermetallic Pt–Ti/C showed enhanced activity and durability for oxygen reduction reaction due to the change in electronic structure and less aggregation.     

Fe–N/C nanofiber electrocatalysts with improved activity and stability for oxygen reduction in alkaline and acid solutions

Fe–N/C nanofiber (Fe–N/CNF) electrocatalysts were prepared by impregnating electrospun polyacrylonitrile nanofibers with iron nitrate (Fe(NO₃)₃) solution and subsequent heat treatment, exhibiting improved activity and stability during oxygen reduction reaction (ORR) both in 0.1 M KOH (pH?=?13) and 0.5 M H₂SO₄ (pH?=?0) electrolyte solutions. Higher treatment temperature and NH₃ atmosphere were preferred by the Fe–N/CNF catalysts, and especially the concentration of Fe(NO₃)₃ solution exerted great effects on the surface morphology, structure, and thus electrocatalytic performance of the catalysts. The Fe–N/CNFs prepared using 0.5 wt% Fe(NO₃)₃ solution showed relatively higher ORR activity in alkaline and acid solutions and better stability especially in 0.5 M H₂SO₄ solution than the catalyst without Fe, probably because Fe could promote the graphitization of the polymer-converted carbon species, enhancing the resistance to electrochemical oxidation and thus the stability of the Fe–N/CNF catalysts.     

Effect of acid treatment of Corich core–Ptrich shell/C electrocatalyst on oxygen reduction reaction

Co_{rich core}–Pt_{rich shell}/C prepared by thermal decomposition and chemical reduction methods were treated by 20% H₂SO₄ aqueous solution and used as the electrocatalysts for the oxygen reduction reaction (ORR). The particle size range of Co_{rich core}–Pt_{rich shell} (molar ratio of 0.92:1) on carbon powder support decreased from 3–8 to 1–6 nm when the time for the electrocatalysts immersed and treated with 20% H₂SO₄ aqueous solution increased from 0 to 4 h. Using Co_{rich core}–Pt_{rich shell} (molar ratio of 0.92:1)/C treated with 20% H₂SO₄ from 0 to 4 h as the working electrode, the open circuit potential of ORR in 0.5 M HClO₄ aqueous solution increased from 0.9995 to 1.0155 V, and the current density, mass activity, and specific activity at the overpotential of 0.1 V increased from 0.619 mA cm^?2, 6.184 A g^?1, and 18.614 μA cm^?2 to 0.912 mA cm^?2, 15.544 A g^?1, and 23.413 μA cm^?2, respectively.     

Pt–Fe cathode catalysts to improve the oxygen reduction reaction and methanol tolerance in direct methanol fuel cells

High surface area carbon-supported Pt and bimetallic Pt–Fe catalysts are investigated for the oxygen electro-reduction reaction (ORR) in low-temperature direct methanol fuel cells (60 °C). The electrocatalysts are prepared using a combination of colloidal and incipient wetness methods allowing the synthesis of carbon-supported bimetallic nanoparticles with a particle size of about 2–3 nm. These materials are studied in terms of structure, morphology and composition using X-ray diffraction, X-ray fluorescence and transmission electron microscopy techniques. The electrocatalytic behaviour of these catalysts for ORR is investigated by employing the rotating disc technique. An enhancement of the ORR is observed with the bimetallic Pt–Fe catalyst in the oxygen-saturated electrolyte solution, with and without methanol. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

Effect of de-alloying of Pt–Ni bimetallic nanoparticles on the oxygen reduction reaction

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.     

The application of scanning electrochemical microscopy to the discovery of Pd–W electrocatalysts for the oxygen reduction reaction that demonstrate high activity, stability, and methanol tolerance

An array of Pd–W alloys was fabricated, and the electrocatalytic activity of the alloys for the oxygen reduction reaction (ORR) in acidic media was screened by scanning electrochemical microscopy. The Pd_0.7W_0.3 showed the highest activity for the ORR, close to that for Pd_0.8Co_0.2 and Pt. A Pd–W electrocatalyst loaded on carbon black was formed by the NaBH₄-reduction method, exhibiting high activity and stability, suggesting that it is a good candidate for the proton exchange membrane fuel cell cathode.     

Hydrogen oxidation reaction: Evidences of different electrocatalytic activity between α and β Pd–H

The hydrogen oxidation reaction (hor) was studied in steady state conditions on a palladium thin film electrode supported on a gold rotating disc. The electrode surface was characterized by cyclic voltammetry and SEM observation. The hydrogen absorption process was analysed by open circuit potential decay in an acid solution saturated with hydrogen and the kinetic measurements were carried out potentiostatically in the same solution. The results obtained show a marked change on the dependence of the current density on overpotential due to the transition between the α and β phases of the Pd–H system. These results were correlated with appropriate kinetic expressions and the corresponding electrocatalytic activity of both phases was estimated.     

Preparation and surface characterization of Pt–Au/C cathode catalysts with ceria modification for oxygen reduction reaction

Alloy catalysts of Pt₅₀Au₅₀/Ce_xC with various Ce additions (x) were prepared for the oxygen reduction reaction (ORR). The characterization of the alloy structures, surface species, and electro-catalytic activities of prepared alloy catalysts were performed by XRD, temperature-programmed reduction (TPR), and rotating disc electrode (RDE) technique, respectively. The ORR activity of Pt₅₀Au₅₀/C alloy catalyst with a promotion of 15% CeO₂ was enhanced significantly in comparison to the commercial Pt/C catalyst within the mixed kinetic-diffusion control region. The addition of CeO₂ decreased the particle sizes, increased the dispersion and enhanced the surface segregation of Pt which resulting in an alloy surface with a moderate oxophilicity on alloy catalysts.     

Synthesis,characterization and electrocatalytic activity for ethanol oxidation of carbon supported Pt,Pt–Rh,Pt–SnO2 and Pt–Rh–SnO2 nanoclusters

Nanoclusters of Pt, Pt–Rh, Pt–SnO₂ and Pt–Rh–SnO₂ were successfully synthesized by polyol method and deposited on high-area carbon. HRTEM and XRD analysis revealed two phases in the ternary Pt–Rh–SnO₂/C catalyst: solid solution of Rh in Pt and SnO₂. The activity of Pt–Rh–SnO₂/C for ethanol oxidation was found to be much higher than Pt/C and Pt–Rh/C and also superior to Pt–SnO₂/C. Quasi steady-state measurements at various temperatures (30–60 °C), ethanol concentrations (0.01–1 M) and H₂SO₄ concentrations (0.02–0.5 M) showed that Pt–Rh–SnO₂/C is about 20 times more active than Pt/C in the potential range of interest for the fuel cell application.     

Completely green synthesis of colloid Adams' catalyst α-PtO2 nanocrystals and derivative Pt nanocrystals with high activity and stability for oxygen reduction

We report a first solution strategy for controlled synthesis of Adams' catalyst (i.e., α-PtO(2)) by a facile and totally green approach using H(2)PtCl(6) and water as reactants. The prepared α-PtO(2) nanocrystals (NCs) are ultrasmall in size and have very "clean" surfaces, which can be reduced to Pt NCs easily in ethanol under ambient conditions. Such Adams' catalysts have been applied as electrocatalysts beyond the field of heterogeneous catalysis. Noticeably, the water-only synthesized α-PtO(2) NCs and their derivative Pt NCs all exhibit much higher oxygen reduction reaction (ORR) activities and stabilities than that of the state-of-art Pt/C electrocatalysts. This study provides an example on the organics-free synthesis of α-PtO(2) and Pt NCs as promising cathode catalysts for fuel cell applications and, particularly, this simple, straightforward method may open a new way for the synthesis of other "clean" functional nanomaterials.     

Iron atom–nanoparticles for interactional enhancing the electrocatalytic reaction activity in Li-S batteries

The undesirable shuttle effect and sluggish redox kinetics of polysulfides seriously result in low sulfur utilization and poor capacity retention. Here, an integrated strategy is proposed by rational designing multifunctional architecture to manipulate the redox kinetics of polysulfides, specifically, by employing iron atoms (Fe-As) and iron-species nanoparticles (Fe-NPs) co-embedded nitrogen-doped carbon nanotube (Fe-NCNT) as catalyst and host for sulfur. The synergistic cooperation of Fe-As and Fe-NPs provides efficient active sites to facilitate the diffusion, strengthen the affinities, and promote the conversion reactions for polysulfides. Furthermore, the NCNT not only offers practical Li⁺ transport pathways but also immobilize the polysulfides effectively. Benefiting from these merits, the Fe-NCNT/S electrodes exhibit high initial specific capacity of 1502.6 mAh/g at 0.1 C, outstanding rate performance (830 mAh/g at 2 C), and good cycling performance (597.8 mAh/g after 500 cycles with an ultralow capacity fading rate of 0.069% per cycle). This work features the distinct interaction of iron atom-nanoparticles on facilitating immobilization-diffusion-transformation process of polysulfides, and it also expected to pave the way for the application in practical Li-S batteries.     

Metal-organic-framework-derived formation of Co–N-doped carbon materials for efficient oxygen reduction reaction

Non-precious metal nitrogen-doped carbonaceous materials have attracted tremendous attention in the field of electrochemical energy storage and conversion. Herein, we report the designed synthesis of a novel series of Co-N-C nanocomposites and their evaluation of electrochemical properties. Novel yolkshell structured Co nanoparticles@polymer materials are fabricated from the facile coating polymer strategy on the surface of ZIF-67. After calcination in nitrogen atmosphere, the Co–N–C nanocomposites in which cobalt metal nanoparticles are embedded in the highly porous and graphitic carbon matrix are successfully achieved. The cobalt nanoparticles containing cobalt metal crystallites with an oxidized shell and/or smaller(or amorphous) cobalt-oxide deposits appear on the surface of graphitic carbons. The prepared Co–N–C nanoparticles showed favorable electrocatalytic activity for oxygen reduction reactions,which is attributed to its high graphitic degree, large surface area and the large amount existence of Co–N active sites.     

Structural characterization of the epitaxially grown core–shell ZnTe/ZnMgTe nanowires

We report the method of the epitaxial growth of the core–shell ZnTe/ZnMgTe nanowires. The morphology and the crystal structure of several samples grown in different processes have been studied by scanning electron microscopy, high resolution transmission electron microscopy and X-ray diffraction methods. It was shown that the ZnMgTe shell growth was clearly epitaxial with a good crystal quality. The average lattice spacing of the ZnTe cores and ZnMgTe shells have been calculated and Mg content in the shells has been estimated. It was documented that growing the shell lattice mismatched to the core induces the strain in the core. The model of the strain creation mechanism has been proposed. The presence of a shell with a larger energy gap than that of the core results in a strong emission in the spectral region near the band edge.     

Electrochemical preparation and characterization of magnetic core–shell nanowires for biomedical applications

Magnetic CoNi@Au core–shell nanorods have been electrochemically synthesized, characterized and functionalized to test their inherent cytotoxicity in order to assess their potential use for biomedical applications. The initially electrodeposited CoNi nanorods have been covered with a gold layer by means of galvanic displacement to minimize the nanowires toxicity and their aggregation, and favour the functionalization. The presence of a gold layer on the nanorod surface slightly modifies the magnetic behaviour of the as-deposited nanorods, maintaining their soft-magnetic behaviour and high magnetization of saturation. The complete covering of the nanorods with the gold shell favours a good functionalization with a layer of (11-Mercaptoundecyl)hexa(ethylene glycol) molecules, in order to create a hydrophilic coating to avoid the aggregation of nanorods, keeping them in suspension and give them stability in biological media. The presence of the organic layer incorporated was detected by means of electrochemical probe experiments. A cytotoxicity test of functionalized core–shell nanorods, carried out with adherent HeLa cells, showed that cell viability was higher than 80% for amounts of nanorods up to 10 μg mL^− 1. These results make functionalized nanorods promising vehicles for targeted drug delivery in medicine, which gives a complementary property to the magnetic nanoparticles.