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.     

Synthesis and structure–conductivity relationship of polystyrene-block-poly(vinyl benzyl trimethylammonium) for alkaline anion exchange membrane fuel cells

Block copolymers of polystyrene-b-poly(vinyl benzyl trimethylammonium tetrafluoroborate) (PS-b-[PVBTMA][BF₄]) were synthesized by sequential monomer addition using atom transfer radical polymerization. Membranes of the block copolymers were prepared by drop casting from dimethylformamide. Initial evaluation of the microphase separation in these PS-b-[PVBTMA][BF₄] materials via SAXS revealed the formation of spherical, cylindrical, and lamellar morphologies. Block copolymers of polystyrene-b-poly(vinyl benzyl trimethylammonium hydroxide) (PS-b-[PVBTMA][OH]) were prepared as polymeric alkaline anion exchange membranes materials by ion exchange from PS-b-[PVBTMA][BF₄] with hydroxide in order to investigate the relationship between morphology and ionic conductivity. Studies of humidity [relative humidity (RH)]-dependent conductivity at 80 °C showed that the conductivity increases with increasing humidity. Moreover, the investigation of the temperature-dependent conductivity at RH = 50, 70, and 90% showed a significant effect of grain boundaries in the membranes against the formation of continuous conductive channels, which is an important requirement for achieving high ion conductivity. © 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1751–1760, 2013     

Effects of protic ionic liquids on the oxygen reduction reaction – a key issue in the development of intermediate-temperature polymer-electrolyte fuel cells

Intermediate-temperature polymer-electrolyte fuel cells (IT-PEFCs), operated at an elevated temperature of ≈120 °C, would enable simplified system design and a potential increase in fuel cell performance compared to state-of-the-art low-temperature (LT-)PEFCs. As LT-PEFC membranes rely on the presence of water and high-temperature (HT-)PEFCs suffer from sluggish oxygen reduction reaction (ORR) kinetics, alternative materials must be developed. Promising candidates are protic ionic liquids (PILs) immobilized in, e.g., a host polymer. PILs’ properties, such as weak ion adsorption, high acidity of the proton-carrying ion, an excess of the anion precursor, and a high oxygen diffusivity and solubility, are favorable for achieving high ORR rates. Concepts proposed in the literature for incorporating PILs into MEA components are presented herein, and their utility for future IT-PEFCs is discussed.     

A novel CNT@SnO2 core–sheath nanocomposite as a stabilizing support for catalysts of proton exchange membrane fuel cells

Mesoporous SnO₂ coated carbon nanotube (CNT) core–sheath nanocomposite, CNT@SnO₂, was prepared by a hydrothermal method and proposed as a catalyst support for proton exchange membrane fuel cells (PEMFCs). The CNT@SnO₂ and its supported Pt catalyst, Pt/(CNT@SnO₂), were characterized by TEM, XRD, cyclic voltammetry, and polarization curves. The CNT@SnO₂ composite showed a much lower anodic current than the CNT, especially at high potentials, indicating the CNT@SnO₂ was more corrosion resistant. The Pt/(CNT@SnO₂) catalyst was electrochemically active and exhibited comparable activity for the oxygen reduction reaction to the CNT supported catalyst (Pt/CNT). More importantly, the long-term stability of the Pt/(CNT@SnO₂) catalyst was significantly higher than that of the Pt/CNT catalyst, which might be mainly due to the fact that the CNT@SnO₂ was more corrosion resistant and mesoporous SnO₂ was beneficial to restrict the Pt migration and aggregation. Consequently, the CNT@SnO₂ would be a promising durable catalyst support for PEMFCs.     

Synthesis of Nafion~-stabilized Pt nanoparticles to improve the durability of proton exchange membrane fuel cell

Nafion-stabilized Pt nanoparticle colloidal solution is synthesized through ethylene glycol reduction.Pt/Nafion added with carbon black as electric conduction material(labeled Pt/Nafion-XC72) shows excellent electrochemical property compared with Pt/C.After a 300-cycle discharging durability test,the cell performance of membrane electrode assembly(MEA) with the Pt/Nafion-XC72 and Pt/C catalysts indicates a 29.9% and 92.2% decrease,respectively.The charge transfer resistances of Pt/Nafion-XC72 and Pt/C increase by 27.2% and 101.9%,respectively.The remaining electrochemically active surface area of Pt is about 61.7% in Pt/Nafion-XC72 and about 38.1% in Pt/C after the durability test.The particle size of Pt/C increases from about 5.1 nm to about 10.8 nm but only from 3.6 nm to 5.8 nm in the case of Pt/Nafion-XC72.These data suggest that Pt/Nafion-XC72 as a catalyst can enhance the durability of PEMFCs compared with Pt/C.     

A model for high-surface-area porous Nafion™-bonded cathodes operating in hydrogen–oxygen proton exchange membrane fuel cells (PEMFCs)

A critical discussion of dioxygen reduction kinetics using the Tafel (for the irreversible cathode process) and the Butler–Volmer (anode process) rate equations has been used to evaluate the accuracy of “standard” modeling interpretations of experimental cell potential current (E–I) plots. The potential–current curve for what is believed to be an optimized Nafion™-bonded fuel cell cathode was analyzed. It appears to behave as a well-ordered diffusional system and shows high electrocatalyst utilization based on its electrocatalytic and gas diffusion characteristics. The electrode appears to perform as expected, without any anomalous characteristics showing any lower than expected electrocatalyst utilization. Any improvement in electrode performance, which is certainly desirable, seems to demand an improved diffusional structure, barring any potential (although unlikely) change in electrochemical kinetic characteristics.

Recent developments in Pt–Co catalysts for proton-exchange membrane fuel cells

Development of Pt-based oxygen reduction reaction catalysts with high efficiency and high durability is central to the application of proton-exchange membrane fuel cell systems. Pt–Co bimetallic catalysts have drawn extensive attention owing to their capability of delivering high performance and long lifetime for fuel cell applications including light-duty and heavy-duty vehicles. However, further improvements in durability and performance are needed to meet market requirements. To fully exploit the potential of Pt–Co catalysts, new insights into the relationship between catalyst properties and fuel cell performance and durability are needed, and more effective methods to tailor the features of Pt–Co catalysts need to be developed. This review provides a summary and perspective on recent efforts, including work on customizing the Pt shell and Pt:Co ratio, tailoring the crystal structure, and improving carbon support properties, with a particular emphasis on mechanisms leading to enhancement of mass activity, power density, and durability in membrane electrode assembly testing.     

A simple in situ characterization technique for the onset of the chemical degradation of PEM fuel cells’ fluorinated membranes

The onset of the chemical degradation of the fluorinated PEM fuel cells’ membranes is characterized using an in situ novel technique. It is based upon measuring the pH of the water drained out from the cathode and the anode compartments using a flow pH meter connected to these outlets. It was found that the acidity of water increases significantly as the load increases if the cell operates at low temperature–low relative humidity (RH) condition after it was working at high temperature–high RH condition previously. Degradation rates were calculated from the pH measurements.     

A computational mapping of the R–NHC coupling pathway – the key process in the evolution of Pd/NHC catalytic systems

C–C coupling reactions are of great importance in metal-catalyzed synthetic transformations. Reductive elimination of two carbon centers is the key stage, which takes place in the metal coordination sphere. In the present study, we provide a detailed analysis of nonclassical R–NHC coupling in the model (NHC)Pdⁱⁱ(Ph)(X)(Solv) complex, which is a representative intermediate of the Mizoroki–Heck and cross-coupling reactions. This C–C bond formation stage proceeds as Ph ligand movement and insertion into the Pd–NHC bond, rather than classical C–C coupling. Based on the analysis by the quantum theory of atoms in molecules (QTAIM) of the reaction path structures, the atomic rearrangements and alterations in the electronic system during the R–NHC coupling process were characterized in detail.     

Beyond the hype surrounding biofuel cells: What's the future of enzymatic fuel cells?

After a short comparison of biofuel cells based on enzymes and microorganisms, several important developments and applications of enzymatic fuel cells (EFCs) are discussed. This discussion emphasizes how to evaluate the performance of EFCs, and highlights the influence of temperature and how it must be carefully considered for practical use of EFCs as power sources. Some of the latest and most important innovations in EFC design using buckypapers and redox nanoparticles are briefly reviewed.     

The stereochemistry of glycolipids. A key for understanding membrane functions?

A Commentary on the paper ”Studies on liquid‐crystalline glycosides" by Volkmar Vill, Thomas Böcker, Joachim Thiem and Fred Fischer. First published in Liquid Crystals, 6, 349‐356 (1989).     

Reduction in the operating temperature of solid oxide fuel cells—potential use in transport applications

The development of solid oxide fuel cells (SOFC) offers new perspectives, in particular as auxiliary power units for vehicle applications. The elaboration of thin electrolyte layers is the main challenge in order to reduce their operating temperature. A brief review of the deposition techniques and of the potential electrolytes is presented. A relatively new technique, Atomic Layer Deposition (ALD), allows to produce thin, dense and homogeneous layers, i.e. zirconia or zirconia-based thin layers can be deposited on different substrates. The interest of elaborating bi- or multi-layer electrolytes is outlined.     

Electron transfer mechanisms,characteristics and applications of biological cathode microbial fuel cells – A mini review

Since the microbial fuel cells (MFCs) research in the laboratory has reached an unprecedented success, it has raised a research upsurge internationally in recent years. However, compared with laboratory studies, the widespread applications of the conventional MFCs were restrained by the limitations of high cost and low efficiency. This stimulates researchers to overcome the obstacles. In this condition, bio-cathodes attracted their great interests. This paper is a brief review about the experimental progress of bio-cathodes in microbial fuel cells with an emphasis on the classification according to the final electron acceptors and the comparison with the traditional abiotic cathode MFCs. Bio-cathodes are feasible in removing nutrient in wastewater treatment and being used as biosensors in bioremediation. Presently, tremendous efforts are being made in investigating appropriate electrodes and dominant strains to achieve the effective practical applications.     

Development of methanol–air fuel cells with membrane materials based on new sulfonated polyheteroarylenes

New proton-conducting membranes were synthesized from sulfonated polynaphthoyleneimide (SPNI) and polytriazole (SPTA), which are of interest for use in portable methanol fuel cells. The membrane electrode assembly (MEA) based on SPNI and SPTA showed power and voltage-current characteristics comparable to those of MEA based on Nafion®-117. The direct and reverse polarization curves coincided almost completely in shape, indicating that the obtained characteristics are stable. At a voltage of 0.3 V and a temperature of 40°С, the current density and power density reached 68 mA cm^–2 and 20.5 mW cm^–2, respectively.     

Effect of ultrasonic treatment of Nafion® solution on the performance of fuel cells

The effect of preliminary high-power ultrasonic treatment of Nafion® alcohol solutions on the properties of membranes and characteristics of membrane-electrode assemblies of membrane-based fuel cells has been explored. The changes in the microstructure of Nafion® membranes upon ultrasonic treatment of their solutions lead to an increase in their proton conductivity, gas permeability and the membrane-based fuel cells power by almost 10%.