Correlation between the surface characteristics of carbon supports and their electrochemical stability and performance in fuel cell cathodes

In this paper,an extensive characterisation of a range of carbon blacks(CB)with similar surface area but different surface chemistry is carried out by flow calorimetry,Raman spectroscopy,dynamic water vapour sorption,instrumental gas analysis,nitrogen adsorption/desorption and high potential chronoamperometry.Using these carbon materials as supports,Pt/CB electrocatalysts are prepared by microwave-assisted polyol-mediated synthesis in gram scale.Structural,morphological and electrochemical properties of the prepared electrocatalysts are evaluated by X-ray diffraction,transmission electron microscopy,rotating disc electrode and in situ fuel cell characterisation of the corresponding membrane-electrode assemblies.The obtained results allow to establish a relationship between surface chemistry and electrochemical properties useful for the design of Pt/C catalyst layers with high performance and stability.

Mechanisms of electrochemical magnesium(de)alloying of Mg-Sn and Mg-Pb polymorphs

Different polymorphs of Mg-Sn and Mg-Pb intermetallic compounds were prepared by high-energy mechanical alloying and then investigated as active material in magnesium batteries. Beside thermodynamically stable Mg_(2)Sn and Mg_(2)Pb crystallizing in the anti-fluorite structure, other polymorphs Mg_(~2)Sn and Mg_(~2)Pb were prepared by increasing the ball-milling time. The first dealloying process is almost complete only for the cubic polymorphs, then similar capacities are observed during the subsequent alloying and dealloying sequences.Thanks to operando X-ray diffraction, the electrochemical mechanism is revealed and shows that the cubic polymorphs Mg_(2)Sn and Mg_(2)Pb tend to preferentially form during the alloying whatever the pristine intermetallic. Weak traces of Mg_(~2)Sn and Mg_(~2)Pb are observed during the alloying, suggesting that these polymorphs act as a by-product and/or an intermediate phases of the electrochemical process. Finally, the compatibility of cubic Mg_(2)Sn and Mg_(2)Pb with Mg(TFSI)_(2)-based electrolyte is confirmed in full cell vs. a positive electrode based on the Chevrel phase Mo6S8, although limited performance is achieved. This fundamental work provides new insights in the behavior of alloy-type negative electrodes for magnesium-ion batteries.

Reduced formation of peroxide and radical species stabilises iron-based hybrid catalysts in polymer electrolyte membrane fuel cells

The incorporation of Pt into an iron-nitrogen-carbon(Fe NC)catalyst for the oxygen reduction reaction(ORR)was recently shown to enhance catalyst stability without Pt directly contributing to the ORR activity.However,the mechanistic origin of this stabilisation remained obscure.It is established herein with rotating ring disc experiments that the side product,H_(2)O_(2),which is known to damage FeNC catalysts,is suppressed by the presence of Pt.The formation of reactive oxygen species is additionally inhibited,independent of intrinsic H_(2)O_(2) formation,as determined by electron paramagnetic resonance.Transmission electron microscopy identifies an oxidised Fe-rich layer covering the Pt particles,thus explaining the inactivity of the latter towards the ORR.These insights develop understanding of Fe NC degradation mechanisms during ORR catalysis,and crucially establish the required properties of a precious metal free protective catalyst to improve Fe NC stability in acidic media.

Solar-assisted photocatalytic water splitting using defective UiO-66 solids from modulated synthesis

Metal-organic frameworks(MOFs)are attracting increasing interests as photocatalysts for solar-driven hydrogen production from water.This paper reports on a comparative study of using either acetic acid(AA)or trifluoroacetic acid(TFA)as the representative UiO-66 organic modulators for synthesizing visible light responsive UiO-66(Zr)-X(X:NH_(2) or NO_(2))photocatalysts for water splitting.The results show that photocatalytic hydrogen generation from a water/methanol mixture can be improved by varying the nature and amount of the modulator employed to prepare the different UiO-66(Zr)-X(X:NH_(2) or NO_(2))solid derivatives.UiO-66(Zr)-NH_(2) was the most active photocatalyst,followed by UiO-66(Zr)-NO_(2),both prepared with 12 equivalents of AA with respect to the organic ligand.This UiO-66(Zr)-NH_(2) solid was more active than the parent MOF in photocatalytic overall water splitting(OWS)(H_(2) and O_(2) production of 450 and 160μmol·g^(-1),respectively,in 5 h;apparent quantum yield(AQY)at 400 nm of 0.06%)in the absence of methanol and compares favourably with analogous reports.Information on the photocatalytic activity of the most active solids of both series was obtained by means of a series of techniques,including ultraviolet-visible(UV-vis)diffuse reflectance,X-ray photoelectron spectroscopy(XPS),laser flash photolysis(LFP),electron spin resonance(ESR),photoluminescence and photoelectrochemical measurements together with density functional theory(DFT)calculations.The results showed that organic acid modulators can be used to enhance the photocatalytic activity of missing linker UiO-66 defective materials in solar-powered water splitting.

Regulating electronic structure of porous nickel nitride nanosheet arrays by cerium doping for energy-saving hydrogen production

Water electrolysis for energy-efficient H_(2)production coupled with hydrazine oxidation reaction(HzOR)is prevailing,while the sluggish electrocatalysts are strongly hindering its scalable application.Herein,we schemed a novel porous Ce-doped Ni3N nanosheet arrays grown on nickel foam(Ce-Ni3N/NF)as a remarkable bifunctional catalyst for both hydrogen evolution reaction and HzOR.Significantly,the overall hydrazine splitting system can achieve low cell voltages of 0.156 and 0.671 V at 10 and 400 mA·cm^(−2),and the system is remarkably stable to operate over 100 h continuous test at the high-current-density of 400 mA·cm^(−2).Various characterizations prove that the porous nanosheet arrays expose more active sites,and more excellent diffusion kinetics and lower charge-transfer resistance,therefore boosting catalytic performance.Furthermore,density functional theory calculation reveals that the incorporation of Ce can effectively optimize the free energy of hydrogen adsorption and promote intrinsic catalytic activity of Ni_(3)N.

Fatigue phenomena in thin ferroelectric films stimulated by repeated switching of the polarization

With the use of techniques of Sawyer
Tower and Mertz method regularities of the origin of "fatigue"in thin ferroelectric films of lead titanate and lead zirconate titanate have been investigated in the fields above coercive.We propose a model to explain the experimentally observed switching-dependent fatigue processes by the motion of charged dislocations in an applied external alternating electric field.

Well-Defined Fluorinated Copolymers:Current Status and Future Perspectives

Well-defined fluoropolymers exhibit unique properties such as excellent oil and water repellency,satisfactory thermal stability,a low refractive index,and low surface energy.The origin of these properties is attributed to the presence of a strong electronegative and low polarizable fluorine atom in the backbone of such polymers,which leads to a strong C−F bond(with a high bond dissociation energy of 485 kJ mol−1).Because of these features,these polymers have found applications as functional coatings,thermoplastics,biomedical items,separators,and binders for Li ion batteries,fuel cell membranes,piezoelectric devices,high-quality wires and cables,and so forth.Usually,fluoropolymers are synthesized by the conventional radical(co)polymerization of fluoroalkenes,which leads to the production of(co)polymers with an ill-defined end group,uncontrolled molar mass,and high dispersity values.In the last two decades,significant developments of various reversible deactivation radical polymerization(RDRP)techniques have helped the design of macromolecular architectures(including block,graft,star,and dendrimers)on demand.However,for relevant new applications,well-defined fluoropolymers with controlled macromolecular architectures(e.g.,block copolymers as thermoplastic elastomers and electroactive polymers or graft copolymers for fuel cell membranes)are required.Several RDRP methods,developed in the last two decades,have paved the way for the synthesis of(co)polymers with well-defined molar mass,dispersity,chain end-functionality,and macromolecular architectures.Some of these RDRP techniques have been successfully employed for the synthesis of well-defined fluorinated copolymers.These techniques include iodine-transfer polymerization(ITP),reversible addition−fragmentation chain-transfer(RAFT)polymerization,organometallic-mediated radical polymerization(OMRP),and,to a lesser extent,nitroxide-mediated polymerization(NMP).Impressive control of the molar mass parameters of the fluoropolymers synthesized via these techniques also encouraged the researchers to combine these techniques with other postpolymerization strategies,leading to innovative novel polymeric materials.Thus,synthesized well-defined fluoropolymers exhibited a unique combination of properties(such as excellent weather resistance,high thermal/chemical/aging resistance,morphological versatility,and a low dielectric constant/flammability/refractive index).These led to the application of such developed materials in various high-technology applications such as high-performance elastomers,coatings for marine antifouling applications,fluorinated surfactants,fuel cell membranes,and gel polymer electrolytes for Li ion batteries.Newer advances in the field of polymer synthesis techniques are the need of the hour in order to synthesize more advanced fluorinated materials which may change the use of such polymers in engineering and biomedical fields in the current century.However,it should be kept in mind that success in this regard shall heavily depend on a deeper understanding of the polymerization process and structure−activity relationships.