Exploration of bimetallic Pt-Pd/C nanoparticles as an electrocatalyst for oxygen reduction reaction

In this study, carbon supported Pt and Pt-Pd were synthesized as oxygen reduction reaction electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs). Pt and Pt-Pd nanoparticles have been synthesized by reduction of metal precursors in presence of NaBH₄. Various techniques such as X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) were utilized to study the prepared samples. Furthermore, electrochemical properties of the prepared samples were evaluated from cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry and electrochemical impedance spectroscopy (EIS). The results showed, the crystallite size of electrocatalysts (Pt and Pt-Pd) is below 10 nm. The higher catalytic activity was detected for Pt-Pd/C electrocatalyst for oxygen reduction reaction (ORR). In addition, it is believed that the better performance of electrocatalyst is related to the synergic effect between Pt and Pd nanoparticles, weakening of the OO bond on Pd-modified Pt nanoparticles in ORR, uniform dispersion of Pd and Pt on the carbon support and higher electrochemical active surface area (EAS) of Pt-Pd/C electrocatalyst.     

A novel humidity sensor based on NH2-MIL-125(Ti) metal organic framework with high responsiveness

Noble metal nanostructures with branched morphologies [i.e., 3-D Pt nanoflowers (NFs)] by tri-dimensionally integrating onto conductive carbon materials are proved to be an efficient and durable electrocatalysts for methanol oxidation. The well-supported 3-D Pt NFs are readily achieved by an efficient cobalt-induced/carbon-mediated galvanic reaction approach. Due to the favorable nanostructures (3-D Pt configuration allowing a facile mass transfer) and supporting effects (including framework stabilization, spatially separate feature, and improved charge transport effects), these 3-D Pt NFs manifest much higher electrocatalytic activity and stability toward methanol oxidation than that of the commercial Pt/C and Pt-based electrocatalysts.     

Oxygen Electroreduction by Single PtPd Nanocubes Encaged in Hollow Carbon Nanospheres: Improved Durability and Strong Effect of Carbon‐Shell Thickness

As a cathode reaction in fuel cells, oxygen reduction reaction (ORR) plays a critical role in determining the overall performance of a fuel cell. It is still a big challenge to find effective ways to improve the catalytic activity, efficiency, and especially stability of ORR electrocatalysts. In the present study, single nanoparticle electrocatalysis for ORR is realized for the first time by encaging PtPd nanocube in hollow and porous carbon nanosphere (PtPd@HCS). Through tuning the carbon‐shell thickness by carbonization temperature, the effects of carbon‐shell thickness on ORR catalytic performance of PtPd@HCS are systematically investigated. The PtPd@HCS calcinated at 800 °C (PtPd@HCS‐800) with the thinnest carbon shell (3.52 nm) and rich pore structure exhibits enhanced ORR catalytic activity and stability. The strategy mentioned here is expected to provide a new method to design single nanoparticle electrocatalysts for fuel cells with high catalytic performance and reduced loading of precious metals.     

Edge-enriched graphene with boron and nitrogen co-doping for enhanced oxygen reduction reaction

Carbon-based electrocatalysts for oxygen reduction reaction (ORR), especially in anion exchange membrane fuel cells (AEMFCs), have received a lot of attention because they exhibit excellent stability and are comparable to commercial Pt/C catalysts. Currently, to maximize the catalytic activity of carbon-based electrocatalysts, there are two major strategies: heteroatom doping or exposing active edge sites. However, the approach of increasing heteroatomic dopants of active edge sites has been rarely addressed. In this study, we present a simple strategy to prepare edge-enriched graphene catalysts with an increased ratio of heteroatomic dopants suitable for ORR of AEMFCs. The catalysts were prepared under harsh oxidation conditions, followed by a simple co-doping process with boron and nitrogen. The ORR activity of the catalysts was observed to be related to an increase of edge sites with heteroatomic dopants. We believe that the edge-enriched structure leads to accelerated electron transfer with enhanced oxygen adsorption.     

Bifunctional oxygen electrocatalysts for rechargeable zinc−air battery based on MXene and beyond

Oxygen electrocatalysts are of great importance for the air electrode in zinc–air batteries (ZABs). Owing to large surface area, high electrical conductivity and ease of modification, two-dimensional (2D) materials have been widely studied as oxygen electrocatalysts for the rechargable ZABs. The elaborately modified 2D materials-based electrocatalysts, usually exhibit excellent performance toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which have attracted extensive interests of worldwide researchers. Given the rapid development of bifunctional electrocatalysts toward ORR and OER, the latest progress of non-noble electrocatalysts based on layered double hydroxides (LDHs), graphene, and MXenes are intensively reviewed. The discussion ranges from fundamental structure, synthesis, electrocatalytic performance of these catalysts, as well as their applications in the rechargeable ZABs. Finally, the challenges and outlook are provided for further advancing the commercialization of rechargeable ZABs.     

Octahedral to Cuboctahedral Shape Transition in 6 nm Pt3Ni Nanocrystals for Oxygen Reduction Reaction Electrocatalysis

Pt₃Ni stands as one of the most active electrocatalysts for the oxygen reduction reaction (ORR). The activity varies with the morphology of the nanocrystals with a high activity observed for the octahedral shape where only the high density {111} crystallographic planes are exposed. Herein, the synthesis of 6 nm Pt₃Ni octahedral nanocrystals with a Pt enriched shell or cuboctahedral nanocrystals with a Ni enriched shell is described. Interestingly, the cuboctahedral nanocrystals display a six-pointed star/skeleton of platinum, which features a very uncommon atomic distribution. In the synthesis, a decrease in the oxygen partial pressure induces the transition from octahedral to cuboctahedral morphology. The octahedral and cuboctahedral nanocrystals both demonstrate high ORR activity (1.1 mA cm⁻²_Pt and 1.2 A mg⁻¹_Pt at 0.9 V vs reversible hydrogen electrode (RHE) are the highest values obtained for PtNi-20 and PtNi-15, respectively). After exposure to oxidative conditions in the acidic electrolyte, the cuboctahedral nanoparticles with a pristine Ni-rich skin show a Pt skin and retain their cuboctahedral morphology.     

Effect of Ni proportion on the performance of proton exchange membrane fuel cells using PtNi/C electrocatalysts

PtNi/C electrocatalysts were synthesised by borohydride method on functionalised carbon support. Energy-dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy and both cyclic and linear voltammetry were employed to characterise the composition, crystalline structure, morphology and catalytic properties of the PtNi/C electrocatalysts. Different Ni proportions in the PtNi/C electrocatalysts were evaluated in the cathode or anode in a H₂/air proton exchange membrane fuel cells (PEMFC) by polarisation curves. PtNi particles uniformly dispersed with different proportions of metals obtained. The increase of Ni proportion in the electrocatalyst led to materials with higher mass activity values toward the oxygen reduction reaction and a greater electrochemical-active surface area. PtNi/C electrocatalysts in the cathode presented higher mass activity values at high potential in the PEMFC. The best PEMFC performance was obtained with PtNi 13 at.% Ni (cathode) and Pt/C (anode) relative to the Pt/C (cathode and anode) with identical Pt loadings. PtNi/C electrocatalysts in PEMFC may be used as an alternative to Pt/C electrocatalyst.     

Co3O4 Hollow Polyhedrons as Bifunctional Electrocatalysts for Reduction and Evolution Reactions of Oxygen

It is very important to exploit low‐cost and efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts for the development of renewable‐energy conversion and storage techniques. Although much attention has been made to develop efficient catalysts for ORR and OER, it is still highly desired to create new bifunctional catalysts. In this study, Co₃O₄ hollow polyhedrons are synthesized as efficient bifunctional electrocatalysts for ORR and OER by simple one‐step annealing Co‐centered metal–organic frameworks (ZIF‐67). Due to the large specific surface areas and high porosity, the as‐prepared Co₃O₄ hollow polyhedrons exhibit excellent electrocatalytic activities for ORR and OER in alkaline media. Co₃O₄ hollow polyhedrons show higher peak current density (0.61 mA cm^?2) with four‐electron pathway than Co₃O₄ particles (0.39 mA cm^?2), better methanol tolerance and superior durability (82.6%) than commercial Pt/C electrocatalyst (58.6%) for ORR after 25 000 s. In addition, Co₃O₄ hollow polyhedrons also display excellent OER performances with smaller overpotential (536 mV) for 10 mA cm^?2 than Co₃O₄ particles (593 mV) and superior stability (86.5%) after 25 000 s. This facile one‐step strategy based on metal–organic frameworks self‐sacrificed templates can be used to develop the promising well‐defined porous hollow metal oxides electrode materials for energy conversion and storage technologies.     

Mechanism study of Single-Step synthesis of Fe(core)@Pt(shell) nanoparticles by sonochemistry

Transition metal (TM) core-platinum (Pt) shell nanoparticles (TM@Pt NPs) are attracting a great deal of attention as highly active and durable oxygen reduction reaction (ORR) electrocatalysts of fuel cells and metal-air batteries. However, most of the reported synthesis methods of TM@Pt NPs are multistep in nature, a significant disadvantage for real applications. In this regard, our group has reported a single-step method to synthesize TM@Pt NPs for TM = Mn, Fe, Co, and Ni by using sonochemistry, namely the UPS (ultrasound-assisted polyol synthesis) method. Previously, we proposed the mechanism of the formation of these TM@Pt NPs by UPS method, but rather in a rough sense. Some details are missing and the optimal conditions have not been established. In the present work, we performed detailed studies on the formation mechanism of UPS reaction by using Fe@Pt NPs as the model system. Effects of synthesis parameters such as the nature of metal precursor, conditions of ultrasound, and temperature profile as a function of reaction time were assessed, along with the analyses of intermediates during the UPS reaction. As results, we verified our previously proposed mechanism that, under appropriate conditions, Fe core is formed through the cavitation and implosion of the solvent, induced by the ultrasound, and the Pt shell is formed by the chemical reaction between Fe core and Pt reagent, independent from the direct effect of ultrasound. In addition, we established the optimal conditions to obtain a high purity Fe@Pt NPs in a high yield (>90% based on Pt), which may enable the increase of synthesis scale of Fe@Pt NPs, a necessary step for the real application of TM@Pt NPs.     

Nitrogen‐Doped Carbon with Mesopore Confinement Efficiently Enhances the Tolerance,Sensitivity, and Stability of a Pt Catalyst for the Oxygen Reduction Reaction

Electrocatalysts for the oxygen reduction reaction (ORR) present some of the most challenging vulnerability issues reducing ORR performance and shortening their practical lifetime. Fuel crossover resistance, selective activity, and catalytic stability of ORR catalysts are still to be addressed. Here, a facile and in situ template‐free synthesis of Pt‐containing mesoporous nitrogen‐doped carbon composites (Pt‐m‐N‐C) is designed and specifically developed to overcome its drawback as an electrocatalyst for ORR, while its high activity is sustained. The as‐prepared Pt‐m‐N‐C catalyst exhibits high electrocatalytic activity, dominant four‐electron oxygen reduction pathway, superior stability, fuel crossover resistance, and selective activity to a commercial Pt/C catalyst in 0.1 m KOH aqueous solution. Such excellent performance benefits from in situ covalent incorporation of Pt nanoparticles with optimal size into N‐doped carbon support, dense active catalytic sites on surface, excellent electrical contacts between the catalytic sites and the electron‐conducting host, and a favorable mesoporous structure for the stabilization of the Pt nanoparticles by pore confinement and diffusion of oxygen molecules.     

Preparation of carbon supported Pt-P catalysts and its electrocatalytic performance for oxygen reduction

The carbon supported PtP (PtP/C) catalysts were synthesized from Pt(NO₃)₂ and phosphorus yellow at the room temperature. The content of P in the PtP/C catalysts prepared with this method is high and the average size of the PtP particles is decreased with increasing the content of P. The electrocatalytic performances of the PtP/C catalysts prepared with this method for the oxygen reduction reaction (ORR) are better than that of the commercial Pt/C catalyst. The promotion action of P for enhancing the electrocatalytic performance of the PtP/C catalyst for ORR is mainly due to that Pt and P form the alloy and then the electron density of Pt is decreased.     

Synthesis,characterization, and electrochemical performance of nitrogen-modified Pt–Fe alloy nanoparticles supported on ordered mesoporous carbons

A method has been demonstrated to synthesize nitrogen-modified Pt–Fe alloyed nanoparticles (9.2–11.3 nm) supported on ordered mesoporous carbon (Pt _x Fe_100?x N/OMC), which is fabricated by a conventional wet chemical synthesis of Pt–Fe alloyed nanoparticles and followed by carbonization of the nanoparticles with tetraethylenepentamine as nitrogen chelating agent. Among these electrocatalysts, the Pt₃₀Fe₇₀N/OMC has highly catalytic activity for the oxygen reduction reaction (ORR) with significantly enhanced methanol tolerance as well. Combining the results from X-ray diffraction and X-ray absorption spectroscopy, it can be observed that Pt metal in the Pt₃₀Fe₇₀N/OMC is present in the outer shell of Pt–Fe alloys with face-centered cubic crystalline structure. By X-ray photoelectron spectroscopy, the nitrogen-modified Pt surface of Pt₃₀Fe₇₀N/OMC exhibits significant selectivity toward the ORR in the presence of methanol. This enhancement of methanol tolerance could be attributed to the inhibition of methanol adsorption resulting from the modification of the Pt surface with nitrogen.     

Preparation of PtSn/C electrocatalysts using citric acid as reducing agent for direct ethanol fuel cell (DEFC)

PtSn/C electrocatalysts (Pt:Sn atomic ratios of 50:50 and 60:40) were prepared using citric acid as reducing agent, and the pH of the reaction medium was varied by the addition of OH⁻ ions. The obtained electrocatalysts were characterized by energy dispersive X-ray analysis, X-ray diffraction, and transmission electron microscopy. The electrocatalysts were tested on the direct ethanol fuel cell (DEFC) at 90 °C. The obtained PtSn/C electrocatalysts showed the presence of a face-centered cubic, Pt, and SnO₂ phases. In DEFC studies, the PtSn/C electrocatalysts showed a superior performance compared to a commercial PtSn/C and Pt/C electrocatalysts from E-TEK.     

Tuning of size and shape of Au–Pt nanocatalysts for direct methanol fuel cells

In this article, we report the precise control of the size, shape, and surface morphology of Au–Pt nanocatalysts (cubes, blocks, octahedrons, and dogbones) synthesized via a seed-mediated approach. Gold “seeds” of different aspect ratios (1–4.2), grown by a silver-assisted approach, were used as templates for high-yield production of novel Au–Pt nanocatalysts at a low temperature (40 °C). Characterization by electron microscopy (SEM, TEM, HRTEM), energy dispersive X-ray analysis, UV–Vis spectroscopy, zeta-potential (surface charge), atomic force microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma mass spectrometry were used to better understand their physico-chemical properties, preferred reactivities and underlying nanoparticle growth mechanism. A rotating disk electrode was employed to evaluate the Au–Pt nanocatalysts electrochemical performance in the oxygen reduction reaction (ORR) and the methanol oxidation reaction of direct methanol fuel cells. The results indicate the Au–Pt dogbones are partially and in some cases completely unaffected by methanol poisoning during the evaluation of the ORR. The ORR performance of the octahedron particles in the absence of MeOH is superior to that of the Au–Pt dogbones and Pt-black; however, its performance is affected by the presence of MeOH.     

Direct pyrolysis and ultrasound assisted preparation of N,S co-doped graphene/Fe3C nanocomposite as an efficient electrocatalyst for oxygen reduction and oxygen evolution reactions

Bifunctional electrocatalysts to enable efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for fabricating high performance metal–air batteries and fuel cells. Here, a defect rich nitrogen and sulfur co-doped graphene/iron carbide (NS-GR/Fe₃C) nanocomposite as an electrocatalyst for ORR and OER is demonstrated. An ink of NS-GR/Fe₃C is developed by homogeneously dispersing the catalyst in a Nafion containing solvent mixture using an ultrasonication bath (Model-DC150H; power − 150 W; frequency − 40 kHz). The ultrasonically prepared ink is used for preparing the electrode for electrochemical studies. In the case of ORR, the positive half-wave potential displayed by NS-GR/Fe₃C is 0.859 V (vs. RHE) and for the OER, onset potential is 1.489 V (vs. RHE) with enhanced current density. The optimized NS–GR/Fe₃C electrode exhibited excellent ORR/OER bifunctional activities, high methanol tolerance and excellent long-term cycling stability in an alkaline medium. The observed onset potential for NS–GR/Fe₃C electrocatalyst is comparable with the commercial noble metal catalyst, thereby revealing one of the best low-cost alternative air–cathode catalysts for the energy conversion and storage application.     

Pt/CNTs电催化剂制备的UV-Vis、FTIR和XRD光谱分析

采用在乙二醇溶液中添加十二烷基硫酸钠(SDS)作为稳定剂的调变乙二醇还原法,制备了高分散的碳纳米管(CNTs)负载Pt电催化剂Pt/CNTs。利用紫外-可见(UV-Vis)、傅里叶变换红外(FTIR)和X射线衍射(XRD)光谱研究了催化剂的制备过程和结构,考察了Pt/CNTs制备过程中SDS的添加对其结构和甲醇电催化氧化活性的影响。结果表明,在乙二醇溶液中PtCl2-₆与SDS形成了配合物,PtCl2-₆能够被乙二醇完全还原;超声处理后的CNTs表面接上了含氧基团,有利于Pt粒子的吸附,催化剂上不残留有SDS;Pt/CNTs电催化剂具有典型的面心立方结构,添加SDS制备的Pt/CNTs-2电催化剂Pt高度分散,粒径更小,达4.5 nm。循环伏安(CV)测试结果表明,添加SDS制备的Pt/CNTs-2电催化剂比传统乙二醇还原法制备的Pt/CNTs-1具有更高的甲醇电催化氧化活性。     

Near neutrality of an oxygen molecule adsorbed on a Pt(111) surface

The charge state of paramagnetic or nonmagnetic O2 adsorbed on a Pt(111) surface is analyzed using density functional theory. We find no significant charge transfer between Pt and the two adsorbed molecular precursors, suggesting these oxygen reduction reaction (ORR) intermediates are nearly neutral, and changes in magnetic moment come from self adjustment of O2 spin-orbital occupations. Our findings support a greatly simplified model of electrocatalyzed ORR, and also point to more subtle pictures of adsorbates or impurities interacting with crystal than literal integer charge transfers.     

Composite Nanoarchitectonics with iron nickel bimetallic nanoparticles and carbon composite for efficient electrocatalysis in hydrogen evolution reaction

The poor efficiency and stability of cost-effective metal compounds are major hurdles to substitute expensive metal-based nanomaterials for the hydrogen evolution reaction (HER). As a result, new concepts and tactics for developing electrocatalysts based on earth-abundant elements must be developed. We present iron-nickel alloy nanoparticles that are supported with carbon (FeNi@C) to improve HER performance in alkaline conditions. FeNi particle was supported on Trimesic acid (TMA) based carbon. In particular, the high conductivity of the carbon and a large number of catalytically active sites in the FeNi demonstrated a synergistic effect, making the hybrid structure a good choice for HER catalyst. Moreover, the physicochemical interaction between the carbon and FeNi metal enhanced the electrocatalytic performance and resulted in achieving 10 mA/cm² current density at 190 mV overpotential with 15 h chronopotential cycling, proving the possibility for replacing costly Pt-based catalysts.     

SiO2 stabilized Pt/C cathode catalyst for proton exchange membrane fuel cells

This paper describes the preparation of SiO₂ stabilized Pt/C catalyst (SiO₂/Pt/C) by the hydrolysis of alkoxysilane, and examines the possibility that the SiO₂/Pt/C is used as a durable cathode catalyst for proton exchange membrane fuel cells (PEMFCs). TEM and XRD results revealed that the hydrolysis of alkoxysilane did not significantly change the morphology and crystalline structure of Pt particles. The SiO₂/Pt/C catalyst exhibited higher durability than the Pt/C one, due to the facts that the silica layers covered were beneficial for reducing the Pt aggregation and dissolution as well as increasing the corrosion resistance of supports, although the benefit of silica covering was lower than the case of Pt/CNT catalyst. Also, it was observed that the activity of the SiO₂/Pt/C catalyst for the oxygen reduction reaction was somewhat reduced compared to the Pt/C one after the silica covering. This reduction was partially due to the low oxygen kinetics as revealed by the rotating-disk-electrode measurement. Silica covering by hydrolysis of only 3-aminopropyl trimethoxysilane is able to achieve a good balance between the durability and activity, leading to SiO₂/Pt/C as a promising cathode catalyst for PEMFCs.     

Direct methanol fuel cells: The influence of methanol on the reduction of oxygen over Pt/C catalysts with different particle sizes

Two Pt/C catalysts with different particle sizes (Pt/C: 2.5 nm, Pt/C-700Ar: 5.1 nm) were investigated by applying a half-cell configuration —rotating disk electrode (RDE) technique in H₂SO₄ aqueous solutions in the absence of or in the presence of methanol with different concentrations. Pt/C catalyst exhibited higher mass activity in H₂SO₄ aqueous solution without methanol and slightly lower mass activity in H₂SO₄ plus 0.1 mol/L CH₃OH in comparison with that of Pt/C-700Ar catalyst. On the contrary,single direct methanol fuel cell (DMFC) tests showed that Pt/C exhibited higheroxygen reduction reaction (ORR) activity and better cell performance, mainly due to the different kinds of electrolyte properties. Furthermore, it suggested that a better single DMFC performance could be obtained with a smaller particle size Pt-based cathode catalyst. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 — 18, 2004.