The structure-activity relationship of Pd–Co/C electrocatalysts for oxygen reduction reaction

The Pd–Co/C alloy catalysts with an atomic ratio of 3:1 were deposited at various pH values and reduced at different temperatures for oxygen reduction reaction (ORR). The structure-activity relationship of the prepared catalysts has been elucidated. The pH values and reduction temperatures during the preparation process affect the deposition and reduction rates of Pd and Co ions significantly, and thus the degrees of alloying, surface species, and ORR activities of the Pd–Co/C catalysts are also influenced. Due to the enhancement of Co surface segregation and the formation of Co oxide on the surface, a deterioration of ORR activity for the catalysts reduced at high temperatures and high pH values is observed. The catalysts deposited at pH value of 9 and reduced at a very low temperature of 390 K have well-formed Pd–Co alloy structure, Pd-rich surface, and excellent ORR activity.     

RRDE study on Co@Pt/C core–shell nanocatalysts for the oxygen reduction reaction

Kinetics study of the oxygen reduction reaction (ORR) taking place on carbon supported Co@Pt/C core–shell nanocatalysts, were characterized by using the rotating ring-disc electrode (RRDE) technique and results compared to that of Pt/C (Etek). The supported catalyst was successfully synthesized through a simple procedure through sequential colloidal chemical reduction of Co and a galvanic replacement by Pt. X-ray diffraction results of the Co@Pt/C nanoparticles showed important structural changes which can be responsible for the observed enhanced catalytic activity. XRD Rietveld refinement confirmed the presence of metallic Pt and Co in the relative weight fraction of 12 wt% and 88 wt% respectively, as well as a Pt lattice contraction (i.e. 3.8741 Å) and Co lattice expansion (i.e. 3.6170 Å). Electrochemical results support the formation of a core–shell structure with double enhanced ORR activity and similar peroxide generation of the Co@Pt/C catalyst with respect to Pt/C (Etek).     

Electrocatalytic performance of Ni modified MnOx/C composites toward oxygen reduction reaction and their application in Zn–air battery

A series of Ni modified MnO_x/C composites were synthesized by introducing NaBH₄ to MnO₂/C aqueous suspension containing Ni(NO₃)₂. The physical properties and the activity of the composites toward the oxygen reduction reaction (ORR) were investigated via transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and the electrochemical techniques. The results show that the higher activity of the composites toward the ORR is correlated with the higher content of MnOOH species transformed from Mn(II) on the surface of the composite. The main nickel species in the composites is Ni(OH)₂, while Ni(OH)₂ shows little activity toward the ORR. However, introducing Ni(OH)₂ with proper amount into the MnO_x/C improves the distribution of the active material MnO_x, which contributes to a surface with more MnOOH. The optimal composite is of the Ni/Mn atomic ratio of 1:2 and the MnO_x loading of 28 wt.%. The maximum power density of the zinc–air battery with the optimized Ni modified MnO_x/C as the cathode catalyst reaches up to 122 mW cm⁻², much higher than the one with the MnO_x/C as the air cathode catalyst (89 mW cm⁻²), and slightly higher than those with the Pd/C and Pt/C as the cathode catalysts.     

Evaluation of carbon supported Fe–Co electrocatalyst for selective oxygen reduction to use in implantable glucose fuel cell

In this research, the activity of Fe–Co/KB (ketjenblack carbon) has been studied as a cathode catalyst for oxygen reduction reaction (ORR) in phosphate buffer saline (PBS) in presence of a solution containing low concentrations of glucose and amino acids mixture (near to physiological tissue fluid in the human body). It is worthwhile to mention that Fe–Co/KB cathode catalyst with size of 3 nm, determined by TEM, indicated an exceptional selectivity towards ORR. Results also revealed that Fe–Co/KB has a higher activity compare to 80%wt Pt/C in ORR with a superior tolerance towards poisoning agents.Further electrochemical investigations were carried out in a two-chamber implantable glucose fuel cell (IGFC) utilizing Fe–Co/KB in the cathode side. Time-dependent evaluation of cell voltage at constant current discharge 0.02 mAcm⁻² in PBS (pH = 7.4) solution containing 5 mM glucose showed only 16% loss in cathode potential; demonstrating an acceptable performance of cathode catalyst in IGFC.     

A strategy for easy synthesis of carbon supported Co@Pt core–shell configuration as highly active catalyst for oxygen reduction reaction

An oxygen-mediated galvanic battery reaction strategy has been developed to one-step synthesize carbon-supported Co@Pt core–shell nanostructures. Relying on this strategy, a structural evolution of 3-D Pt-on-Co bimetallic nanodendrites into Co@Pt core–shell configuration is readily achieved in our study. These well-supported and low-Pt-content nanostructures show superior electrocatalytic activities to oxygen reduction reaction. Especially, the supported Co@Pt core–shell electrocatalyst for oxygen reduction reaction shows a high activity with the maximal Pt-mass activity of 465 mA mg⁻¹ Pt at 0.9 V (vs. RHE). The present investigation clearly demonstrates that the design and synthesis of the core–shell nanostructures is a viable route for building Pt-based electrocatalysts with optimized utilization efficiency and higher cost performance.     

Preparation of a high performance Pt–Co/C electrocatalyst for oxygen reduction in PEM fuel cell via a combined process of impregnation and seeding

The preparation of a Pt–Co/C electrocatalyst for the oxygen reduction reaction in PEM fuel cells was achieved via a combined process of impregnation and seeding. The effects of initial pH of the precursor solution and Pt loading were all found to have a significant effect on both the electrocatalyst morphology and the cell performance when tested in a single PEM fuel cell. The optimum condition found for preparing the Pt–Co/C electrocatalyst was from an initial precursor solution pH of 2 at the metal loading of 23.6–30.3% (w/w). The Pt–Co/C electrocatalysts, formed under these optimal conditions, tested in a single PEM fuel cell with the carbon sub-layer, gave a cell performance of 772 mA/cm² or 460 mW/cm² at 0.6 V in a H₂/O₂ system. An electron pathway of oxygen reduction on the prepared Pt–Co/C electrocatalyst was also determined using a rotating disk electrode.     

The synthesis of Fe/N–C@CNFs and its electrochemical performance toward oxygen reduction reaction

The one-dimensional filamentous carbon nanofibers hold great promise to substitute noble catalysts ascribing to the excellent physicochemical properties, environmentally friendly, easy to prepare, etc. Synthesizing non-noble catalysts with outstanding electro-catalytic activity for oxygen reduction reaction and excellent durability and application in the field of commercialization still exist lots of challenges. Herein, we report a facile synthesis of carbon nanofibers coated with iron doping nitrogen-carbon (Fe/N–C@CNFs) derived from carbon nanofibers coated with polyaniline (PANI@CNFs) via chemical vapor deposition and heat-treatment, which exhibit an outstanding catalytic activity toward oxygen reduction reaction. In detail, the Fe/N–C@CNFs exhibit onset potential of 0.99 V (vs RHE) and half-wave potential of 0.80 V (vs RHE) in 0.1 M KOH solution, indicating superior electrochemical activity for oxygen reduction reaction (ORR). Meantime, the transferred electron number of oxygen reduction reaction was 3.77, suggesting a nearly 4e⁻ transferring process with little intermediate product (H₂O₂). Moreover, the relative current value of carbon nanofibers coated with nitrogen-carbon film (N–C@CNFs) and Fe/N–C@CNFs maintain 89.6% and 88.2% respectively after 40,000 s, exhibiting good stability and durability. This facile and easy method could provide inspiration for synthesizing carbon nanofibers-based (CNFs-based) oxygen reduction reaction catalysts with excellent catalytic activity and good stability.     

CO tolerant PtRu–MoOx nanoparticles supported on carbon nanofibers for direct methanol fuel cells

Novel nanostructured catalysts based on PtRu–MoO_x nanoparticles supported on carbon nanofibers have been investigated for CO and methanol electrooxidation. Carbon nanofibers are prepared by thermocatalytic decomposition of methane (NF), and functionalized with HNO₃ (NF.F). Electrocatalysts are obtained using a two-step procedure: (1) Pt and Ru are incorporated on the carbon substrates (Vulcan XC 72R, NF and NF.F), and (2) Mo is loaded on the PtRu/C samples. Differential electrochemical mass spectrometry (DEMS) analyses establish that the incorporation of Mo increases significantly the CO tolerance than respective binary counterparts. The nature of the carbon support affects considerably the stabilization of MoO_x nanoparticles and also the performance in methanol electrooxidation. Accordingly, a significant increase of methanol oxidation is obtained in PtRu–MoO_x nanoparticles supported on non-functionalized carbon nanofiber, in parallel with a large reduction of the Pt amount in comparison with binary counterparts and commercial catalyst.     

Electrodeposition preparation of Pt–HxWO3 composite and its catalytic activity toward oxygen reduction reaction

Platinum–hydrogen tungsten bronze (Pt–H_xWO₃) was prepared on glass carbon electrode by potentiostat in 0.1 mM H₂PtCl₆ + 4 mM Na₂WO₄ + 2 M H₂SO₄. Its surface morphology, structure and activity toward oxygen reduction reaction were studied with scan electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy, and linear sweeping voltammetry. It is found that platinum and hydrogen tungsten bronze can be co-deposited together on glassy carbon and the activity of platinum toward oxygen reduction can be improved significantly by H_xWO₃. Furthermore, the activity of Pt–H_xWO₃ toward oxygen reduction is hardly influenced by methanol.     

Carbon supported MnOx–Co3O4 as cathode catalyst for oxygen reduction reaction in alkaline media

The electroreduction of oxygen of MnO_x–Co₃O₄/C was firstly studied in alkaline media. The MnO_x–Co₃O₄/C showed better electrocatalytic activity towards ORR than MnO_x/C and Co₃O₄/C. Compared to Pt/C, MnO_x–Co₃O₄/C showed better methanol tolerance and durability in alkaline solution. Thus, the MnO_x–Co₃O₄/C catalyst had potential for applications in metal–air batteries and alkaline fuel cells.     

Proton conductivity and fuel cell performance of organic–inorganic hybrid membrane based on poly(methyl methacrylate)/silica

Organic–inorganic hybrid membranes based on poly(methyl methacrylate) (PMMA)/silica have been synthesized using a sol-gel technique for use in polymer electrolyte fuel cells (PEFCs). The properties of these membranes were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and thermogravimetric analysis. The results indicate that these membranes are formed through hydrogen bonds between the carbonyl group of PMMA and the uncondensed alcohol functional groups of the inorganic clusters. The proton conductivity of these membranes is on the order of 10⁻¹ S cm⁻¹, and the 60PMMA–30SiO₂–10P₂O₅ membrane displays the highest proton conductivity of 3.85 × 10⁻¹ S cm⁻¹ at 90 °C and 50% RH. The performance of a fuel cell using these membranes was tested. A maximum power density of 370 mW cm⁻² is obtained at 80 °C, and the current density at 0.4 V remains almost unchanged during the 100-h test time under the test conditions. This class of hybrid membranes is an extremely promising material for use in PEFCs.     

Synthesis of nano-tube CoNi/N–C complex and its catalytic properties for highly efficient oxygen reduction

Electrochemical energy storage systems such as fuel cells and metal air cells can be used as clean energy. In these systems, an essential reaction on the cathode is the oxygen reduction reaction (ORR). As ORR catalyst, non noble metal based catalyst is an important substitute for commercial Pt/C catalyst due to its rich reserves, low cost, good stability and high catalytic activity. Herein, CoNi alloy supported on nitrogen doped carbon, CoNi/N–C nanotubes, prepared by hydrothermal method and high-temperature pyrolysis method, shows excellent ORR catalytic activity and stability in 0.1 M KOH solution. In particular, the obtained CoNi/N-C-800 demonstrates the highest ORR activity of the prepared samples, with a half wave potential of 0.81V, which was equivalent to the commercially available Pt/C (0.82V). At the same time, it exhibits approximate 4e^- pathway with a comparable electron transfer number to the commercial Pt/C, and is much higher tolerant in methanol than the latter. Co and Ni alloying can induce the internal electron interaction of the catalyst, thus exposing more active sites. Furthermore, nano-tube CoNi exhibits appropriate size and hollow geometry, and its large surface area and strong conductivity improve its catalytic activity. The results may possibly provide a new impetus to the rational design of non noble metal based nanocomposite catalysts. Moreover, it is also of great significance to improve the performance of electrocatalysis and energy storage applications.     

Phase stability and conductivity of Ba1−ySryCe1−xYxO3−δ solid oxide fuel cell electrolyte

The structure, phase stability, and electrical properties of BaCe_1−xY_xO_3−δ (x = 0-0.4) in humidity air and CO₂ atmosphere are investigated. XRD results indicate that the BaCe_0.9Y_0.1O_3−δ sample has a symmetric cubic structure, and its phase changes to tetragonal as the Y³⁺ doping amount increases to 20 mol%. The conductivity of BaCe_1−xY_xO_3−δ increases with temperature, and it depends on the amount of yttrium doping and the atmosphere. BaCe_0.8Y_0.2O_3−δ exhibits the highest conductivity of 0.026 S cm⁻¹ at 750 °C. The activation energy for conductivity depends on yttrium doping amount and temperature. The conductivity of BaCe_0.8Y_0.2O_3−δ is 0.025 S cm⁻¹ in CO₂ atmosphere at 750 °C which is 3.8% lower than that in air due to reactions with CO₂ and BaCO₃ and the CeO₂ impure phases formed. The structure of BaCe_0.8Y_0.2O_3−δ is unstable in water and decomposes to Ba(OH)₂ and CeO₂ phases. It is found that the activation energy of samples in CO₂ atmosphere is higher than that of sample in air. Sr-doped Ba_1−ySr_yCe_0.8Y_0.2O_3−δ (y = 0-0.2) is prepared to improve the phase stability of BaCe_0.8Y_0.2O_3−δ in water. The conductivity of Ba_0.9Sr_0.1Ce_0.8Y_0.2O_3−δ is 0.023 S cm⁻¹ at 750 °C which was 11% lower than that of BaCe_0.8Y_0.2O_3−δ, however, the phase stability of Ba_0.9Sr_0.1Ce_0.8Y_0.2O_3−δ is much better than that of BaCe_0.8Y_0.2O_3−δ in water.     

Fabrication and evaluation of Nafion nanocomposite membrane based on ZrO2–TiO2 binary nanoparticles as fuel cell MEA

Synthesis and characterization of nanocomposite membranes for proton exchange membrane fuel cell (PEMFC) operating at different temperatures and humidity were investigated in this study. Recast Nafion composite membrane with ZrO₂ and TiO₂ nanoparticles with 75 nm in mean size diameter, prepared for PEM fuel cells. Nafion/TiO₂ composite membranes have been also fabricated by in-situ sol–gel method. However, fine particles of the ZrO₂ were synthesized and Nafion/ZrO₂ composite membrane were produced by blending a 5% (w/w) Nafion-water dispersion with the inorganic compound. All nanocomposite membranes demonstrated higher water retention in comparison with unmodified membranes. Proton conductivity increased with increasing ZrO₂ content while TiO₂ additive (with mean size of 25 nm) enhanced water retention. Subsequently, structures of the membranes were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) as well as X-Ray Diffraction (XRD). In addition, water uptake and proton conductivity of the modified membranes were also measured. The nanocomposite membrane was tested in a 25 cm² commercial single cell at the temperature range of 80–110 °C and in humidified H₂/O₂ under different relative humidity (RH) conditions. The membrane electrode assembly (MEA) prepared from Nafion/TiO₂, ZrO₂ presented highest PEM fuel cell performance in respect of I–V polarization under condition of 110 °C, 0.6 V and 30% RH and 1 atm.     

Preparation and characterization of Nd2−xSrxCoO4+δ cathodes for intermediate-temperature solid oxide fuel cell

K₂NiF₄-type structural Nd_2−xSr_xCoO_4+δ (x = 0.8, 1.0, 1.2) was synthesized and evaluated as cathodes for intermediate-temperature solid oxide fuel cell (IT-SOFC). The crystal structure, thermal expansion, electrical conductivity and electrochemical properties were investigated by X-ray diffraction, dilatometry, DC four-probe method, AC impedance and polarization techniques. It is found that the electrochemical properties were remarkably improved with the increasing of Sr in the experiment range. Nd_0.8Sr_1.2CoO_4+δ showed the highest electrical conductivity of 212 S cm⁻¹ at 800 °C, the lowest polarization resistance and cathodic overpotential, 0.40 Ωcm² at 700 °C and 35.6 mV at a current density of 0.1 A cm⁻² at 700 °C, respectively. The chemical compatibility experiment revealed that Nd_0.8Sr_1.2CoO_4+δ cathode was chemically stable with the SDC electrolyte. The thermal expansion coefficient also increased with the Sr content.     

Superior electrochemical performance and oxygen reduction kinetics of layered perovskite PrBaxCo2O5+δ (x = 0.90–1.0) oxides as cathode materials for intermediate-temperature solid oxide fuel cells

The layered perovskite PrBa_xCo₂O_5+δ (PB_xCO, x = 0.90–1.0) oxides have been synthesized by a solid-state reaction technique, and evaluated as the potential cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Room temperature X-ray diffraction patterns show the orthorhombic structures which double the lattice parameters from the perovskite cell parameter as a ≈ a_p, b ≈ a_p and c ≈ 2a_p (a_p is the cell parameter of the primitive perovskite) in the Pmmm space group. There is a good chemical compatibility between the PB_xCO cathode and the Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte at 1000 °C. The electrical conductivity and thermal expansion coefficient of PB_xCO are improved due to the increased amount of electronic holes originated from the Ba-deficiency. The results demonstrate the high electrochemical performance of PB_xCO cathodes, as evidenced by the super low polarization resistances (R_p) over the intermediate temperature range. The lowest R_p value, 0.042 Ω cm², and the cathodic overpotential, −15 mV at a current density of −25 mA cm⁻², are obtained in the PrBa_0.94Co₂O_5+δ cathode at 600 °C in air, which thus allow to be used as a highly promising cathode for IT-SOFCs. A CGO electrolyte fuel cell with the PrBa_0.94Co₂O_5+δ cathode presents the attractive peak power density of ∼1.0 W cm⁻² at 700 °C. Furthermore, the oxygen reduction kinetics of the PrBa_0.94Co₂O_5+δ cathode is also studied, and the rate-limiting steps for oxygen reduction reaction are determined at different temperatures.     

Co-deficient PrBaCo2−xO6−δ perovskites as cathode materials for intermediate-temperature solid oxide fuel cells: Enhanced electrochemical performance and oxygen reduction kinetics

Co-deficient PrBaCo_2?xO_6?δ perovskites (x = 0, 0.02, 0.06 and 0.1) are synthesized by a solid-state reaction, and the effects of Co-deficiency on the crystal structure, oxygen nonstoichiometry and electrochemical properties are investigated. The PrBaCo_2?xO_6?δ samples have an orthorhombic layered perovskite structure with double c axis. The degree of oxygen nonstoichiometry increases with decreasing Co content (0 ≤ x ≤ 0.06) and then slightly decreases at x = 0.1. All the samples exhibit the electrical conductivity values of >300 S cm^?1 in the temperature range of 100–800 °C in air, which match well the requirement of cathode. With significantly enhanced electrochemical performance and good chemical compatibility between PrBaCo_2?xO_6?δ and CGO, this system of Co-deficient perovskite is promising cathode material for IT-SOFCs. Among all these components, PrBaCo_1.94O_6?δ gives lowest polarization resistance of 0.059 Ω cm² at 700 °C in air. When tested as cathode in fuel cell, the anode-supported Ni-YSZ|YSZ|CGO|PrBaCo_1.94O_6?δ cell delivers a maximum peak power density of 889 mW cm^?2 at 650 °C, which is higher than that of PrBaCoO_6?δ cathode-based cell (764 mW cm^?2). The oxygen reduction kinetics at the PrBaCo_1.94O_6?δ cathode interface is also explored, and the rate-limiting steps for oxygen reduction reaction are determined.     

Fabrication of bimetallic Pt–M (M = Fe,Co, and Ni) nanoparticle/carbon nanotube electrocatalysts for direct methanol fuel cells

The electrochemical activities of three bimetallic Pt–M (M = Fe, Co, and Ni) catalysts in methanol oxidation have been investigated. An efficient approach including chemical oxidation of carbon nanotubes (CNTs), two-step refluxing, and subsequent hydrogen reduction was used to thoroughly disperse bimetallic nanopartilces on the oxidized CNTs. Three catalysts with a similar Pt:M atomic ratio, Pt–Fe (75:25), Pt–Co (75:25), and Pt–Ni (72:28), were prepared for the investigation of methanol oxidation. The Pt–M nanoparticles with an average size of 5–10 nm are uniform and cover the surface of CNTs. Cyclic voltammetry showed that the three pairs of catalysts were electrochemically active in the methanol oxidation. On the basis of the experimental results, the Pt–Co/CNT catalyst has better electrochemical activity, antipoisoning ability, and long-term cycleability than the other electrocatalysts, which can be justified by the bifunctional mechanism of bimetallic catalysts. The satisfactory results shed some light on how the use of Pt–Co/CNT composite could be a promising electrocatalyst for high-performance direct methanol fuel cell applications.     

Electrodeposition of Pt–Ru and Pt–Ru–Ni nanoclusters on multi-walled carbon nanotubes for direct methanol fuel cell

A full-electrochemical method is developed to deposit three dimension structure (3D) flowerlike platinum-ruthenium (PtRu) and platinum-ruthenium-nickel (PtRuNi) alloy nanoparticle clusters on multi-walled carbon nanotubes (MWCNTs) through a three-step process. The structure and elemental composition of the PtRu/MWCNTs and PtRuNi/MWCNTs catalysts are characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray polycrystalline diffraction (XRD), IRIS advantage inductively coupled plasma atomic emission spectroscopy (ICP-AES), and X-ray photoelectron spectroscopy (XPS). The presence of Pt(0), Ru(0), Ni(0), Ni(OH)₂, NiOOH, RuO₂ and NiO is deduced from XPS data. Electrocatalytic properties of the resulting PtRu/MWCNTs and PtRuNi/MWCNTs nanocomposites for oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) are investigated. Compared with the Pt/MWCNTs, PtNi/MWCNTs and PtRu/MWCNTs electrodes, an enhanced electrocatalytic activity and an appreciably improved resistance to CO poisoning are observed for the PtRuNi/MWCNTs electrode, which are attributed to the synergetic effect of bifunctional catalysis, three dimension structure, and oxygen functional groups which generated after electrochemical activation treatment on MWCNTs surface. The effect of electrodeposition conditions for the metal complexes on the composition and performance of the alloy nanoparticle clusters is also investigated. The optimized ratios for PtRu and PtRuNi alloy nanoparticle clusters are 8:2 and 8:1:1, respectively, in this experiment condition. The PtRuNi catalyst thus prepared exhibits excellent performance in the direct methanol fuel cells (DMFCs). The enhanced activity of the catalyst is surely throwing some light on the research and development of effective DMFCs catalysts.