Synthesis,studies and fuel cell performance of “core–shell” electrocatalysts for oxygen reduction reaction based on a PtNix carbon nitride “shell” and a pyrolyzed polyketone nanoball “core”

This report describes a new class of “core–shell” electrocatalysts for oxygen reduction reaction (ORR) processes for application in Proton Exchange Membrane Fuel Cells (PEMFCs). The electrocatalysts are obtained by supporting a “shell” consisting of PtNi_x alloy nanoparticles embedded into a carbon nitride matrix (indicated as PtNi_x-CN) on a “core” of pyrolyzed polyketone nanoballs, labeled ‘ST_p’. ST_ps are obtained by the sulfonation and pyrolysis of a precursor consisting of XC-72R carbon nanoparticles wrapped by polyketone (PK) fibers. The ST_ps are extensively characterized in terms of the chemical composition, thermal stability, degree of graphitization and morphology. The “core–shell” ORR electrocatalysts are prepared by the pyrolysis of precursors obtained impregnating the ST_p “cores” with a zeolitic inorganic–organic polymer electrolyte (Z-IOPE) plastic material. The electrochemical performance of the electrocatalysts in the ORR is tested “in situ” by single fuel cell tests. The interplay between the chemical composition, the degree of graphitization of both PtNi_x-CN “shell” and ST_ps “cores”, the morphology of the electrocatalysts and the fuel cell performance is elucidated. The most crucial preparation parameters for the optimization of the various features affecting the fuel cell performance of this promising class of ORR electrocatalysts are identified.     

Pt overgrowth on carbon supported PdFe seeds in the preparation of core-shell electrocatalysts for the oxygen reduction reaction

In this study, a novel core-shell structured Pd₃Fe@Pt/C electrocatalyst, which is based on Pt deposited onto carbon supported Pd₃Fe nanoparticles, is prepared for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). The carbon supported Pd₃Fe nanoparticles act as seeds to guide the growth of Pt. The formation of the core-shell structured Pd₃Fe@Pt/C is confirmed by transmission electron microscopy (TEM), X-ray diffraction (XRD) and electrochemical characterization. The higher surface area of the synthesized catalyst suggests that the utilization of Pt in the Pd₃Fe@Pt/C catalyst is higher than that in Pt/C. Furthermore, better electrocatalytic performance than that of Pt/C and Pd₃Fe/C catalyst is observed in the ORR which follows a four-electron path. Consequently, the results indicate that the Pd₃Fe@Pt/C catalyst could be used as a more economically viable alternative for the ORR of PEMFCs.     

Tuning concave PtSn nanocubes for efficient ethylene glycol and glycerol electrocatalysis

Shape-controlled synthesis of well-defined nanostructures offers a great opportunity to promote electrocatalytic performances while reducing the mass loading of noble metals. Herein, we show how morphology can effectively affect the electrocatalytic properties of nanocrystals for alcohol electrooxidation reaction, a key barrier to the application of fuel cells. We report the synthesis of a new generation of alloyed PtSn concave nanocubes (CNCs) through a facile one-pot wet-chemical method. Owing to strong synergistic effect between Pt and Sn, modified electronic structure, as well as high surface areas, the as-obtained alloyed PtSn CNCs can display outstanding electrocatalytic performances for liquid fuel electrooxidation. Impressively, the optimized Pt₄Sn₁ concave nanocubes (CNCs) can achieve a factor of 5.1 enhancements in mass activity and a factor of 5.9 enhancements in specific activity towards ethylene glycol oxidation (EGOR) in comparison with commercial Pt/C catalysts. Moreover, 4.6 and 5.3-fold enhancements in mass and specific activity were also acquired for glycerol oxidation reaction (GOR) compared to those of the commercial Pt/C, holding great promise for future application in fuel cells.     

Synthesis and characterization of Pt-Pd alloy and core-shell bimetallic nanoparticles for direct methanol fuel cells (DMFCs): Enhanced electrocatalytic properties of well-shaped core-shell morphologies and nanostructures

In our research, we present the controlled synthesis of poly(vinylpyrrolidone) (PVP) protected Pt-Pd nanoparticles of various alloy and core-shell morphologies by modified polyol method with the assistance of AgNO₃. The Pt-Pd alloy and core-shell nanoparticles were characterized by transmission electron microscopy (TEM), high-resolution TEM, and electrochemical measurements. The comparison of electrocatalytic properties of Pd-Pt bimetallic nanoparticles was described to confirm their highest catalytic performance. Importantly, the catalytic activity of Pt-Pd alloy and core-shell nanoparticles was investigated to develop novel electrocatalysts in direct methanol fuel cells (DMFCs). The results showed that the core-shell nanoparticles with the thin nanoshells as monolayers exhibit as great nanocatalysts. The correlation among structure, size and morphology was presented in their catalytic characterization.     

Magnetically separable transition metal nanoparticles as catalysts in hydrogen generation from the hydrolysis of ammonia borane

It reviews the available reports on the preparation and use of magnetically separable transition metal nanoparticles (TMNs) as reusable catalysts for the hydrolytic dehydrogenation of ammonia borane (AB). After a short introduction, the review starts with the papers on the employment of intrinsically magnetic TMNs as catalysts for releasing H₂ gas from AB, which includes colloidal nanoparticles of intrinsically magnetic metals, TMNs in combination with materials having large surface area, and multimetallic composites containing at least one intrinsically magnetic metal together with an additional component usually acting as support or stabilizer. This is followed by a section reviewing the papers on core-shell multimetallic nanoparticles with one intrinsically magnetic metal in either core or shell used for catalyzing the hydrolysis of AB. It follows the review of papers on TMNs supported on Fe₃O₄, CoFe₂O₄, or Co₃O₄ forming magnetically separable catalysts for the same reaction. Then, a short section reviews the available reports on metal nanoparticles supported on carbon-coated iron. The last section gives a summary list of conclusions.     

A fuel cell operating between room temperature and 250 °C based on a new phosphoric acid based composite electrolyte

A phosphoric acid based composite material with core-shell microstructure has been developed to be used as a new electrolyte for fuel cells. A fuel cell based on this electrolyte can operate at room temperature indicating leaching of H₃PO₄ with liquid water is insignificant at room temperature. This will help to improve the thermal cyclability of phosphoric acid based electrolyte to make it easier for practical use. The conductivity of this H₃PO₄-based electrolyte is stable at 250 °C with addition of the hydrophilic inorganic compound BPO₄ forming a core-shell microstructure which makes it possible to run a PAFC at a temperature above 200 °C. The core-shell microstructure retains after the fuel cell measurements. A power density of 350 mW/cm² for a H₂/O₂ fuel cell has been achieved at 200 °C. The increase in operating temperature does not have significant benefit to the performance of a H₂/O₂ fuel cell. For the first time, a composite electrolyte material for phosphoric acid fuel cells which can operate in a wide range of temperature has been evaluated but certainly further investigation is required.     

Pt nanoparticles supported on titanium iron nitride nanotubes prepared as a superior electrocatalysts for methanol electrooxidation

Titanium iron nitride (Ti_0.95Fe_0.05N) supports with one-dimensional (1D) hollow and porous nanotubes(Ti_0.95Fe_0.05N NTs)are prepared by a two-step method, including hydrothermal method followed post-nitriding treatment. Pt nanoparticles (NPs) are further supported on Ti_0.95Fe_0.05N NTs for methanol electrooxidation. The experimental results reveal that the prepared material is Ti_0.95Fe_0.05N NTs with high purity, and this support is characterized by a porous tubular structure with hollow walls and large specific surface area. The X-ray photoelectron spectroscopy (XPS) pattern shows the strong interaction between the robust Ti_0.95Fe_0.05N NTs support and uniform Pt NPs catalyst. In addition, the electrochemical data demonstrate that Ti_0.95Fe_0.05N NTs loaded Pt NPs (Pt/Ti_0.95Fe_0.05N) display greatly improved activity and stability than that of Pt/C catalyst. The significantly enhanced durability of the hybrid electrocatalysts and electrochemical surface area (ECSA) preservation of the catalyst are observed after the accelerated durability test (ADT). The experimental data verify that the introducing of Fe can tune the electronic structure of Pt atoms, which contributes to the strengthened activity and stability of the Pt catalyst for the methanol oxidation reaction.     

Ultrathin Co3O4–Pt core-shell nanoparticles coupled with three-dimensional graphene for oxygen reduction reaction

Design and synthesis of platinum catalysts within atomic level are of great significance for the practical application of fuel cells. We found that the ultrathin Co(OH)₂ nanoparticles can be converted into Co₃O₄–Co core-shell nanostructures through a thermal annealing process in reducing atmosphere, which are uniformly distributed on the surface of 3D graphene (3DG). The Co₃O₄–Co core-shell nanoparticles have been successfully transformed into Co₃O₄–Pt core-shell nanoparticles via a controlled replacement reaction. The Co₃O₄–Pt @3DG contains only a few atomic layers of Pt shell, and presents a high Pt utilization nanostructure. Besides, the 3D graphene serves as a catalysts carrier with open structure, and offers a three-dimensional molecular accessibility and conducive to mass transfer. Significantly, the optimized mass activity and specific activity of 1.018 A/mg_Pt and 2.17 mA/cm² have been achieved on Co₃O₄–Pt @3DG at 0.9 V vs RHE, which are 7.6- and 8.1- times higher than those of Pt/C (0.134 A/mg_Pt and 0.266 mA/cm²), respectively. The high activity is mainly attributed to the ultrathin core-shell structure with an ultrahigh Pt utilization, and the interaction between the near-surface Co₃O₄ and the surface Pt shell with a tensile strain to surface Pt shell, and the electrons transfer from Co to Pt.     

Pd coated one-dimensional Ag nanostructures: Controllable architecture and their electrocatalytic performance for ethanol oxidation in alkaline media

One-dimensional (1D) metal-coated Pd structures are efficient catalysts for the ethanol electro-oxidation and promising strategy for minimizing the Pd-loading toward commercialization of direct ethanol fuel cells (DEFCs). Herein, the decorated and core-shell architectures of a novel Pd coating on Ag nanowires (PdAg-NWs) are controllable by a two-step polyol method based on the galvanic replacement reaction. The integration of uniform shell with a low Pd concentration and partial hollow structure onto 1D PdAg-NWs exhibits the highest efficiency for ethanol oxidation reaction (EOR) in alkaline solution. In comparison with Pd nanoparticles (PdNPs/C), the PdAgNWs/C performes 11 times superior EOR activity, and the onset potential shifts 80 mV negatively. The presence of Ag in PdAg-NWs enhances the absorption capacity of ethanol molecules and hydroxyl ions on the active sites, and improves the catalyst tolerance to CO-like intermediates, making them a potential anodic catalyst for DEFCs.     

Structural,electrochemical and catalytic activity of Prussian blue analogues embedded with functionalized carbon for solid state battery applications

The nanoparticles of Mn_1.5[Cr(CN)₆]∙mH₂O@Ni_1.5[Cr(CN)₆]∙nH₂O core-shell prussian blue analogues (PBA) embedded with carbon additives (PBA-C) were synthesized and characterized as electrode material for solid state battery application. The impedance spectroscopy and cyclic voltametry were used to study the electrochemical properties by adding functionalized carbon in 1:1 proportion to improve the electrical performance. The value of room temperature electrical conductivity of core-shell PBA and core-shell nanoparticles mixed with vulcan carbon (PBA-C) are found to be 1.574 × 10⁻³ and 1.92 × 10⁻³ Scm⁻¹_, respectively. Using Li₇La₃Zr₂O₁₂ (LZZO) electrolyte, single cell was fabricated with PBA-C material, and studied its charging-discharging cycles, which exhibits higher current density with stable performance for 400 cycles for time slots of 400 min. The study reveals that the PBA core-shell nanoparticles mixed with carbon (PBA-C) may be a potential candidate as an electrode material in the form of a single cell using LZZO electrolyte.     

Highly durable graphene nanoplatelets supported Pt nanocatalysts for oxygen reduction

We report graphene nanoplatelets (GNPs), which exhibit the advantages of both single-layer graphene and highly graphitic carbon, as a durable alternative support material for Pt nanoparticles for oxygen reduction in fuel cells. Pt nanoparticles are deposited on poly(diallyldimethylammonium chloride) (PDDA)-coated GNP, and characterized with transmission electron microscopy, X-ray diffraction, Raman spectra, and electrochemical tests. Pt/GNP exhibits greatly enhanced electrochemical durability (2-3 times that of Pt/CNT and commercial Etek Pt/C). These are attributed to the intrinsic high graphitization degree of GNP and the enhanced Pt-carbon interaction in Pt/GNP. If considering that GNP can be easily mass produced from graphite, GNP is a promising, low-cost, and durable electrocatalyst support for oxygen reduction in fuel cells.     

Evaluation on carbon nanocapsules for supercapacitors using a titanium cavity electrode

We synthesize carbon nanocapsules (CNCs) by a flame combustion method and evaluate their potential as the electrode material for electrochemical double layer capacitor using a titanium cavity electrode (TCE). Identical process is conducted on commercially available carbonaceous materials such as Vulcan XC72R, Black Pearl 2000 (BP2000), multi-walled carbon nanotubes (MWCNTs), and active carbon (AC1100) for comparison purposes. Images from Scanning electron microscope and Transmission electron microscope on the CNCs demonstrate irregular-shaped particles in average size of 10-20 nm with graphene layers on perimeter compassing a hollow core. Electrochemical characterizations including cyclic voltammetry (CV), current reversal chronopotentiometry (CRC), and impedance spectroscopy are carried out in 1N H₂SO₄ to determine the specific capacitance and cycle life time. Among these samples, the BP2000 still delivers the highest specific capacitance in F g⁻¹ but the CNCs demonstrate the largest value in μF cm². In addition, the CNCs exhibit impressive life time for 5000 cycles without notable degradation. Consistent results are obtained by CV, CRC, and impedance measurements, validating the TCE as a facile tool to perform reliable electrochemical evaluations.     

Mo-chelate strategy for synthesizing ultrasmall Mo2C nanoparticles embedded in carbon nanosheets for efficient hydrogen evolution

Production of hydrogen from electrochemical water splitting has been regarded as one of the most economic and sustainable techniques for green fuel production. It is significant and challengeable to develop highly efficient and low cost noble metal-free electrocatalysts. Presently, molybdenum-based electrocatalysts were regarded as potential alternatives for the hydrogen evolution reaction (HER). Here, the well-dispersed and ultrasmall Mo₂C nanoparticles (NPs) anchored on 2D carbon nanosheets were synthesized by designing chelate precursor and following pyrolysis, which was proved to be an effective approach for preparing carbon-loaded Mo₂C NPs. The as-obtained Mo₂C/C material exhibits an outstanding activity and stability in hydrogen evolution reaction (HER). It needs an overpotential of 147 mV to drive 10 mA cm⁻² and Tafel slope is 64.2 mV dec⁻¹ in alkaline medium, implying that Mo₂C/C material will be a potential noble metal-free electrocatalyst for HER. The design of Mo-chelate precursor is a feasible route to synthesize ultrafine Mo₂C and it can provide a reference for synthesizing other nanoparticles and hindering particle coalescence at high preparation temperature.     

Thin layer vs. nanoparticles: Effect of SnO2 addition to PtRhNi nanoframes for ethanol oxidation reaction

When designing catalysts for direct ethanol fuel cell applications (DEFC), four main parameters must be considered: shape, structure, size, and chemical composition. According to this knowledge, it is assumed that polyhedral hollow Pt-based nanoframes, with the addition of Rh and SnO₂ with a size below 50 nm, could be a promising nanocatalysts for the anode of DEFC. In this work, two different PtRhNi/SnO₂ nanoframes-based catalysts are obtained. First consists of PtRhNi nanoframes covered with small, about 3 nm, SnO₂ nanoparticles (PtRhNi/SnO₂ NPs); and second is the PtRhNi nanoframes covered with a thin and incomplete SnO₂ layer (PtRhNi/SnO₂ TL). Both nanocatalysts were tested toward ethanol oxidation reaction (EOR) and show higher activity in comparison to PtRhNi nanoframes without SnO₂ (PtRhNi NFs) addition and commercially used Pt nanoparticles. Especially, the electrochemical durability and stability of obtained nanocatalysts were tested. It was shown that both PtRhNi/SnO₂ nanoframes-based catalysts develop similar mass and specific activity, as well as nearly the same onset potential, but their stability is significantly different. It turns out, that catalyst based on PtRhNi nanoframes covered with a thin SnO₂ layer is susceptible to degradation, while the catalyst consisting of PtRhNi nanoframes covered with SnO₂ nanoparticles is much more durable and could be used as an efficient catalyst toward EOR.     

Noncovalently functionalized graphitic mesoporous carbon as a stable support of Pt nanoparticles for oxygen reduction

We report a durable electrocatalyst support, highly graphitized mesoporous carbon (GMPC), for oxygen reduction in polymer electrolyte membrane (PEM) fuel cells. GMPC is prepared through graphitizing the self-assembled soft-template mesoporous carbon (MPC) under high temperature. Heat-treatment at 2800 °C greatly improves the degree of graphitization while most of the mesoporous structures and the specific surface area of MPC are retained. GMPC is then noncovalently functionalized with poly(diallyldimethylammonium chloride) (PDDA) and loaded with Pt nanoparticles by reducing Pt precursor (H₂PtCl₆) in ethylene glycol. Pt nanoparticles of ∼3.0 nm in diameter are uniformly dispersed on GMPC. Compared to Pt supported on Vulcan XC-72 carbon black (Pt/XC-72), Pt/GMPC exhibits a higher mass activity towards oxygen reduction reaction (ORR) and the mass activity retention (in percentage) is improved by a factor of ∼2 after 44 h accelerated degradation test under the potential step (1.4-0.85 V) electrochemical stressing condition which focuses on support corrosion. The enhanced activity and durability of Pt/GMPC are attributed to the graphitic structure of GMPC which is more resistant to corrosion. These findings demonstrate that GMPC is a promising oxygen reduction electrocatalyst support for PEM fuel cells. The approach reported in this work provides a facile, eco-friendly promising strategy for synthesizing stable metal nanoparticles on hydrophobic support materials.     

Ultrafine iron oxide nanoparticles supported on N-doped carbon black as an oxygen reduction reaction catalyst

An oxygen reduction reaction (ORR) catalyst comprising ultrafine iron oxide nanoparticles supported on N-doped Vulcan carbon (FeO_1.4/N-C) was prepared via a two-step method. X-ray photoelectron spectroscopy revealed the iron oxide nanoparticles comprised Fe₂O₃ and FeO phases with a combined average oxidation state of 2.8. The FeO_1.4/N-C catalyst produced an ORR onset potential of −0.056 V and a half-wave potential of −0.190 V in alkaline media, which was comparable to that of commercial Pt/C catalyst. Moreover, FeO_1.4/N-C had higher methanol tolerance than Pt/C catalyst and thus affords a promising non-precious metal ORR catalyst for fuel cells.     

Facile synthesis of efficient core-shell structured iron-based carbon catalyst for oxygen reduction reaction

Controlled synthesis of efficient core-shell non-precious metal catalysts for oxygen reduction reaction (ORR) is undoubtedly crucial but challenging for the extensive application of fuel cells and metal-air batteries. Herein, we prepared a core-shell structured Fe/FeC_x nanoparticles and porous carbon composited catalyst (Fe/FeC_x@NC) via a facile two-step heat treatment strategy. The Fe/FeC_x@NC-800_?0.5 prepared with secondary anneal at 800 °C for 0.5 h exhibits superior ORR performance to the commercial Pt/C in terms of comparable onset potential, higher half-wave potential, and outstanding long-term durability in alkaline media. Through combining the physical and electrochemical characterizations of Fe/FeC_x@NC-T_?t with different anneal temperature and precursors, the outstanding ORR performance of Fe/FeC_x@NC-800_?0.5 is caused by the synergistic effect between Fe/FeC_x core and enriched pyridinic N- and graphitic N-doped carbon shell as well as porous carbon with large specific surface area. The structure-activity relationship of core-shell structured Fe–N–C catalysts for ORR provides directions for the development of advanced nonprecious metals catalysts.     

Review on core-shell structured cathode for intermediate temperature solid oxide fuel cells

Lowing the operating temperature can greatly promote the commercialization of solid oxide fuel cells (SOFCs), however it also results in a significant increase in cell impedance, which is the bottleneck for the development of intermediate temperature SOFCs (IT-SOFCs). Major hurdles in developing conventional single-phase cathode materials for IT-SOFCs are poor electrochemical performance or durability. The investigation of new cathode materials or the optimization of the existing cathodes is imminent for the development of IT-SOFCs. Among them, core-shell structured cathode can combine the advantages of multiple components, and has been demonstrated with excellent oxygen reduction reaction (ORR) catalytic activity and long-term stability. This review summarizes the recent research progress on core-shell structured cathode for enhanced electrochemical performance, long-term stability, CO₂ tolerance and Cr tolerance. Furthermore, the future directions are discussed from a perspective of materials design, preparation and characterization. Core-shell structured cathodes are expected to play an increasingly critical role in the commercialization of IT-SOFCs.     

Sorption-enhanced steam reforming of glycerol over NiCuCaAl catalysts for producing fuel-cell grade hydrogen

The concentration of CO in the high-purity hydrogen from sorption-enhanced steam reforming (SESR) processes is usually too high to be directly used in fuel cells. Herein, we report a production of fuel-cell grade H₂ with <30 ppm CO through SESR of glycerol (SESRG), a by-product of biodiesel manufacture. High purity H₂ can be produced by employing a catalyst-sorbent hybrid material composed of Ni as catalyst, CaO as CO₂ sorbent and Ca₁₂Al₁₄O₃₃ as spacer. By introducing copper as promoter, the performance of the bi-functional catalyst could be modified to produce a 97.15 vol% purity of H₂ with 28 ppm CO. With an optimized Ni/Cu ratio, the 7.5Ni–7.5Cu catalyst shows the excellent stability for producing about 97% H₂ with <30 ppm CO for ten cycles. The characterizations and model reaction tests indicate that copper can affect CO, CO₂ hydrogenation and water gas shift reaction to adjust the performance of SESRG reaction. The results presented here show the promise of tuning the catalyst composition for achieving high quality H₂ through SESR processes.     

Supercapacitor behavior of carbon-manganese oxides nanocomposites synthesized by carbon arc

The Mn-C-O composites were synthesized by the electric-arc discharge method. The composite materials were obtained by spraying of graphite electrode with the addition of MnO₂. The morphology of Mn-C-O composites formed during electric-arc spraying of metal-carbon electrodes in various buffer gases (N₂ and He) and the effect of their subsequent annealing in an oxygen-containing atmosphere was studied. It was experimentally determined that MnO_x (MnO, Mn₃O₄) nanoparticles are mainly formed in N₂ atmosphere, and Mn₇C₃ carbide nanoparticles are formed in He atmosphere. This phenomenon is explained by different cooling rates of the formed composites. With further annealing of materials, partial oxidation of nanoparticles and graphitization of the carbon matrix occur due to the thermal effect of the oxidation reaction. According to the study of electrochemical activity of materials in the 1 M KOH aqueous electrolyte, the materials with a higher MnO content and a higher degree of soot graphitization have the highest electrochemical capacity of 135 Fg⁻¹.