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The article focuses on carbon nitride-based electrocatalysts for application in low-temperature fuel cells (FCs). Topics discussed include low-temperature fuel cells such as proton exchange membrane fuel cells (PEMFCs) and anion-exchange membrane fuel cells (AEMFCs), requirement of oxygen reduction reaction (ORR) electrocatalysts (ECs) to minimize cathode polarization losses and three dimensional (3D)-crosslinked hybrid macromolecular precursors.

Owing to the numerous benefits obtained when operating proton exchange membrane fuel cells at elevated temperature (>100 °C), the development of thermally stable proton exchange membranes that demonstrate conductivity under anhydrous conditions remains a significant goal for fuel cell technology. This paper presents composite membranes consisting of poly[2,2′-( m-phenylene)-5,5′-bibenzimidazole] (PBI4N) impregnated with a ZrO2 nanofiller of varying content (ranging from 0 to 22 wt %). The structure-property relationships of the acid-doped and undoped composite membranes have been studied using thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, wide-angle X-ray scattering, infrared spectroscopy, and broadband electrical spectroscopy. Results indicate that the level of nanofiller has a significant effect on the membrane properties. From 0 to 8 wt %, the acid uptake as well as the thermal and mechanical properties of the membrane increase. As the nanofiller level is increased from 8 to 22 wt % the opposite effect is observed. At 185 °C, the ionic conductivity of [PBI4N(ZrO2)0.231](H3PO4)13 is found to be 1.04×10−1 S cm−1. This renders membranes of this type promising candidates for use in high-temperature proton exchange membrane fuel cells. [ABSTRACT FROM AUTHOR]

The thermal, mechanical, and electric properties of hybrid membranes based on Nafion that contain a [(ZrO2)·(Ta2O5)0.119] "core–shell" nanofiller are elucidated. DSC investigations reveal the presence of four endothermic transitions between 50 and 300 °C. The DMA results indicate improved mechanical stability of the hybrid materials. The DSC and DMA results are consistent with our previous suggestion of dynamic R-SO3H…[ZrTa] cross-links in the material. These increase the thermal stability of the -SO3H groups and the temperature of thermal relaxation events occurring in hydrophobic domains of Nafion. The broadband electrical spectroscopic analysis reveals two electric relaxations associated with the material's interfacial (σIP) and bulk proton conductivities (σEP). The wet [Nafion/(ZrTa)1.042] membrane has a conductivity of 7.0 × 10-2 S cm-1 at 115 °C, while Nafion has a conductivity of 3.3 × 10-2 S cm-1 at the same temperature and humidification conditions. σEP shows VTF behavior, suggesting that the long-range conductivity is closely related to the segmental motion of the Nafion host matrix. Long-range conduction (σEP) occurs when the dynamics of the fluorocarbon matrix induces contact between different delocalization bodies (DB), which results in proton exchange processes between these DBs. [ABSTRACT FROM AUTHOR]

Abstract: In this report, three hybrid inorganic-organic proton-conducting membranes based on a novel fluorinated titania labeled TiO2F dispersed in Nafion were prepared. The mass fraction of TiO2F nanofiller ranged between 0.05 and 0.15. The water uptake and the proton exchange capacity of the membranes were determined; the membranes were further characterized by TG, DMA and FT-IR ATR investigations. Finally, the hybrid membranes were used in the fabrication of membrane-electrode assemblies (MEAs), which were tested in operating conditions as a function of the back pressure and of the hydration degree of the reagents streams. It was demonstrated that, with respect to pristine recast Nafion, at 25%RH the MEA fabricated with the membrane including a mass fraction of TiO2F equal to 0.10 yielded a higher maximum power density (0.206 W cm−2 vs. 0.121 W cm−2). Finally, it was proposed a coherent structural model of this family of hybrid membranes accounting for both the properties determined from “ex-situ” characterizations and for the performance obtained from measurements in a single fuel cell in operating conditions. [Copyright &y& Elsevier]

Abstract: Two classes of hybrid inorganic–organic proton-conducting membranes consisting of Nafion and either [(ZrO2)·(HfO2)0.25] or [(SiO2)·(HfO2)0.28] nanofiller are investigated to elucidate their relaxations and conductivity mechanism and are labeled [Nafion/(ZrHf) x ] and [Nafion/(SiHf) x ], respectively. The membranes are studied by dynamic mechanic analysis (DMA) and broadband electric spectroscopy (BES). The latter technique allows a determination of the direct current ionic conductivity (σ DC) and the proton diffusion coefficient . Pulse-field-gradient spin-echo nuclear magnetic resonance experiments (PFGSE-NMR) are carried out to determine the water self-diffusion coefficients . and are correlated to obtain insight on the conductivity mechanism of the proposed materials. Results indicate that the nanofiller particles play a major role in the proton conduction mechanism of the proposed materials. It is demonstrated that the basic [(ZrO2)·(HfO2)0.25] nanoparticles form Nafion–nanofiller dynamic cross-links with high ionic character. These cross-links improve the mechanical properties and enhance the overall proton conductivity of the membranes at low humidification levels owing to an efficient delocalization of the protons. In [Nafion/(SiHf) x ] membranes, the dynamic cross-links occur due to dipole–dipole interactions between the side groups of the Nafion host polymer and the quasi-neutral [(SiO2)·(HfO2)0.28] nanoparticles. These cross-links significantly reduce the delocalization of the protons, which decreases the overall conductivity of materials. [Copyright &y& Elsevier]

Abstract: This work reports the preparation, characterization and test in a single fuel cell of two families of hybrid inorganic-organic proton-conducting membranes, each based on Nafion and a different “core-shell” nanofiller. Nanofillers, based on either a ZrO2 “core” covered with a HfO2 “shell” (ZrHf) or a HfO2 “core” solvated by a “shell” of SiO2 nanoparticles (SiHf), are considered. The two families of membranes are labelled [Nafion/(ZrHf) x ] and [Nafion/(SiHf) x ], respectively. The morphology of the nanofillers is investigated with high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray spectroscopy (EDX) and electron diffraction (ED) measurements. The mass fractions of nanofiller x used for both families are 0.05, 0.10 or 0.15. The proton exchange capacity (PEC) and the water uptake (WU) of the hybrid membranes are determined. The thermal stability is investigated by high-resolution thermogravimetric measurements (TGA). Each membrane is used in the fabrication of a membrane-electrode assembly (MEA) that is tested in single-cell configuration under operating conditions. The polarization curves are determined by varying the activity of the water vapour (aH2O) and the back pressure of the reagent streams. A coherent model is proposed to correlate the water uptake and proton conduction of the hybrid membranes with the microscopic interactions between the Nafion host polymer and the particles of the different “core–shell” nanofillers. [Copyright &y& Elsevier]

Abstract: Many papers have focused on the thermal properties, conductivity and fuel cell performance of sulfonated poly(ether ether ketone) (SPEEK) membranes, but the electrical properties have not been extensively studied. In this work, the electric properties of SPEEK electrolytes are studied with broadband electric spectroscopy to elucidate the relationship between the degree of sulfonation and the conductivity and to explore the mechanism of long-range conductivity. SPEEK membranes exhibit two polarization phenomena that contribute to the overall conductivity: “bulk” and interfacial conductivities. The “bulk” conductivity increases with increasing degrees of sulfonation due to an increase in the concentration of charge carriers and a higher hydrophilicity that allows increased water uptake. The interfacial conductivity is the result of the accumulation of charge at the interfacial regions between the hydrophobic and hydrophilic domains of the SPEEK membranes. The bulk and interfacial conductivities can be divided into two temperature regimes: one at temperatures below 75°C that exhibits Arrhenius behaviour and the other at temperatures above 75°C that follows a Vogel–Tamman–Fulcher (VTF) trend. In the Arrhenius region, proton transport occurs primarily via a Grotthus-like mechanism where protons move between water molecules and acid groups. In the VTF region, segmental motion is critical in the long-range proton conduction process as the mean hopping distance increases along with the temperature due to loss of water. [Copyright &y& Elsevier]

Abstract: This report describes the preparation of two Pd-based carbon nitride electrocatalysts for the oxygen reduction reaction (ORR) for application in polymer electrolyte membrane fuel cells (PEMFCs). The electrocatalysts consist of multi-metallic active sites supported on a graphite-like carbon nitride (CN) matrix with a N content exceeding 13wt%. The electrochemical performance is investigated by cyclic voltammetry with the thin-film rotating disk electrode method (CV-TF-RDE) and evaluated in a single membrane electrode assembly (MEA) PEMFC. The correlation of the structural information to functional properties allows to propose a reaction mechanism and to identify the most desirable features to achieve in a CN electrocatalyst in order to obtain desired electrochemical performance in catalysis of ORR. It is established also that the CN support improves the tolerance towards the catalyst corrosion under oxidizing conditions and thus improves the catalyst durability. The stoichiometry and the morphology of Pd-based CN electrocatalysts play a crucial role in the modulation of the tolerance towards common ORR poisons such as chlorine anions and methanol. Finally, the performance of the Pd-based CN electrocatalysts in a single MEA PEMFC proved promising. [Copyright &y& Elsevier]

Abstract: This report describes a new family of bimetal carbon nitride (CN) electrocatalysts for the oxygen reduction reaction (ORR) for application in polymer electrolyte membrane fuel cells (PEMFCs). The materials, which bear active sites based on Pt and Rh, are prepared with a three-step protocol consisting of: (1) the synthesis of a homogeneous zeolitic inorganic–organic polymer electrolyte (Z-IOPE) precursor; (2) the pyrolysis of the precursor at a temperature T f =400, 500, 600, 700 or 900°C; (3) the activation, yielding the final product. It is found that T f has a major effect on the structure and the electrochemical properties of these electrocatalysts, as determined from a wide array of independent characterization techniques including: high-resolution thermogravimetry (HR-TGA); Fourier-transform medium infrared (FT-MIR), Fourier-transform far infrared (FT-FIR) and confocal micro-Raman spectroscopies; powder X-ray diffraction (powder XRD); X-ray photoelectron spectroscopy (XPS); and electrochemical investigations carried out with the cyclic voltammetry thin-film rotating disk electrode (CV-TF-RDE) method. All the information is integrated to propose a comprehensive model correlating the effect of T f on the structure of the materials with the corresponding electrochemical performance. The best results in the ORR are obtained with materials prepared at 600≤ T f ≤700°C. These systems present the optimal compromise between the size of the metal-rich nanoparticles, the degree of graphitization of the carbon nitride support and the concentration of ligands blocking the active sites. The best Pt-Rh electrocatalysts exhibit an ORR activity very similar to the Pt/C reference. In addition, the peculiar structure of the proposed materials, which are characterized by active sites supported on the carbon nitride bulky materials, results in a better tolerance toward typical contaminants in the ORR process such as chloride anions with respect to the Pt/C reference. [Copyright &y& Elsevier]

Abstract: In this report, a new nanofiller consisting of silica “cores” bearing fluoroalkyl surface functionalities is synthesized and adopted in the preparation of a series of hybrid inorganic–organic proton conducting membranes based on Nafion. The hybrid materials are obtained by a solvent-casting procedure and include between 0 and 10wt.% of nanofiller. The resulting systems are extensively characterized by Thermogravimetry (TG), Modulated Differential Scanning Calorimetry (MDSC) and Dynamic Mechanical Analysis (DMA), showing that the hybrid materials are stable up to 240°C and that their overall thermal and mechanical properties are affected both by the polar groups on the surface of the silica “cores” and by the fluoroalkyl surface functionalities of the nanofiller. The electric properties of the hybrid materials are investigated by broadband dielectric spectroscopy (BDS). It is shown that proton conductivity of the materials is not compromised by the lower water uptake arising from the hydrophobic character of the nanofiller. With respect to a pristine Nafion recast membrane, the hybrid material characterized by 5wt.% of nanofiller, [Nafion/(Si80F)0.7], shows the highest conductivity in all the investigated temperature range (5≤ T ≤155°C). Indeed, [Nafion/(Si80F)0.7] features the lowest water uptake and presents a conductivity of 0.083Scm−1 at 135°C. This result is consistent with the good performance of the membrane in single fuel cell tests. [ABSTRACT FROM AUTHOR]

Abstract: This report describes the development and the optimization of new synthesis routes yielding electrocatalysts for the oxygen reduction reaction (ORR) aimed at application in proton exchange membrane fuel cells (PEMFCs). The preparation protocols consist in the synthesis of two groups of hybrid inorganic–organic precursors, characterized by a different concentration of nitrogen, which subsequently undergo a high-temperature pyrolysis in inert atmosphere, washing and activation. The resulting materials show a well-controlled stoichiometry. The nitrogen incorporated in the support transforms the matrix into a supramolecular ligand, and stabilizes the electrocatalyst by coordinating the active metal clusters. The latter are composed of an “active metal” such as Pt or Pd, combined with one or more “co-catalyst” elements such as Au, Fe, Co and Ni. An extensive characterization of the carbon nitride electrocatalysts under the chemical, structural, morphological and electrochemical points of view is described, together with their use in membrane electrode assemblies (MEAs) tested in single fuel cells under operative conditions. Results indicated that the best electrocatalysts are those characterized by a “core–shell” morphology. These systems consist of metal carbon nitride materials with a low concentration of nitrogen (shell) supported on electronically conductive graphite nanoparticles (core). Promising results were obtained both in terms of ORR overpotential (η) and of mass activity (A m). Indeed, η resulted up to ∼30mV lower with respect to reference Pt-based systems, and an A m equal to 0.3–0.4g of Pd or Pt to achieve 1kW was reached. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the preparation of a new bimetal electrocatalyst for the oxygen reduction reaction (ORR), which is one of the most important bottlenecks in the operation of polymer electrolyte membrane fuel cells (PEMFCs), is described. This material was synthesized through a pyrolysis process of a zeolitic inorganic–organic polymer electrolyte (Z-IOPE-like) precursor, followed by suitable washing and activation procedures of the product. The electrocatalyst, whose active sites consist of platinum and rhodium, was: (a) extensively characterized from the chemical, structural, morphological and electrochemical points of view and (b) used to prepare a membrane-electrode assembly (MEA) which was tested under operative conditions in a single-cell configuration. It was observed that, with respect to a reference material based on supported platinum, rhodium did not compromise the performance of the electrocatalyst in the ORR. This behaviour was interpreted in the framework of a general model concerning the enhancement of ORR performance in bimetal systems supported on carbon nitrides. Finally, the material shows a slightly better tolerance toward a few common contaminants for the ORR such as methanol and chloride anions, typical of direct methanol fuel cells (DMFCs) and vehicular applications, respectively. [Copyright &y& Elsevier]

Abstract: In this report, we are presenting studies of the effect of M x O y nanopowders on the thermal, mechanical and electrical properties of [Nafion/(M x O y ) n ] membranes with M=Ti, Zr, Hf, Ta and W and n =5wt%. Five homogeneous membranes with thicknesses ranging from 170 to 350μm were studied. The thermal transitions characterizing [Nafion/(M x O y ) n ] materials were investigated by modulated differential scanning calorimetry (MDSC). The mechanical parameters and relaxation processes were studied on temperature by dynamical mechanical analyses (DMA). Broadband dielectric spectroscopy (BDS) was used to study the conductivity mechanism and dielectric relaxation events in bulk materials. DMA investigations showed two distinct relaxation events. The first relaxation phenomenon, detected at about 19°C, was attributed to the mechanical β-relaxation mode of Nafion. The second event, revealed in the temperature range 100–135°C, was assigned to the mechanical α-relaxation mode of Nafion. The electric response of membranes was studied by BDS measurements in the frequency and temperature range respectively of 40Hz–10MHz and 5–135°C. Real and imaginary components of permittivity (ɛ*(ω)= ɛ′ (ω)−iɛ″(ω)) and conductivity spectra (σ*(ω)= σ′(ω)+iσ″(ω)) were analyzed. Conductivity spectra allowed us to accurately determine the σ dc of membranes at 100% RH as a function of T. Two relaxation peaks were detected in the ɛ*(ω) profiles. The low frequency relaxation event was attributed to the α-relaxation mode of fluorocarbon chains of Nafion. The high frequency relaxation peak corresponds to the β-relaxation of acid side groups. The results allowed us to conclude that M x O y influences: (a) the relaxations of both the hydrophobic and the hydrophilic domains of Nafion polymer host; (b) the thermal stability range of conductivity (SRC) and the σ dc of membranes. In conclusion, nanofillers affect the macromolecular dynamics of Nafion-based polymer host owing to the formation of dynamic cross-links, R–SO3H⋯M x O y ⋯HSO3–R, in hydrophilic polar cages. The membranes doped with HfO2 and WO3 oxoclusters present a stability range of conductivity of 5°C≤ T ≤135°C and give rise to σ dc values of respectively 2.8×10−2 and 2.5×10−2 Scm−1 at 135°C and 100% RH. These latter conductivity values make the nanocomposite membranes based on HfO2 and WO3 oxoclusters very promising materials for the application in polymer electrolyte fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). [Copyright &y& Elsevier]

The in situ identification of fuel-cell material parameters is crucial both for guiding the research for advanced functionalized materials and for fitting multiphysics models, which can be used in fuel cell performance evaluation and optimization. However, this identification still remains challenging when dealing with direct measurements. This paper presents a method for achieving this aim by stochastic optimization. Such techniques have been applied to the analysis of fuel cells for ten years, but typically to specific problems and by means of semi-empirical models, with an increased number of articles published in the last years. We present an original formulation that makes use of an accurate zero-dimensional multi-physical model of a polymer electrolyte membrane fuel cell and of two cooperating stochastic algorithms, particle swarm optimization and differential evolution, to extract multiple material parameters (exchange current density, mass transfer coefficient, diffusivity, conductivity, activation barriers …) from the experimental data of polarization curves (i.e. in situ measurements) under some controlled temperature, gas back pressure and humidification. The method is suitable for application in other fields where fitting of multiphysics nonlinear models is involved. [ABSTRACT FROM AUTHOR]

This report presents the interplay between the synthesis parameters, physicochemical properties, and electrochemical performance of core-shell low-Pt electrocatalysts (ECs) for the oxygen reduction reaction (ORR) based on Pt x Ni active sites stabilized on a carbon nitride shell. The impact of the pyrolysis temperature (T f), of the support core (H) composition and of an electrochemical dealloying-activation step on the EC morphology and on the accessibility and stability of the active sites are studied in detail. Three supports are employed based on carbon nanoparticles and/or graphene platelets. The ORR performance of activated ECs measured by cyclic voltammetry with the thin-film rotating ring-disk electrode approach is strongly affected by T f and H. The best performing ECs are tested in single proton exchange membrane fuel cells under operating conditions. The simultaneous presence of graphene and carbon in H improves the dispersion of active sites, resulting in a vastly improved mass activity and durability in comparison with a benchmark state-of-the-art Pt/C EC. [Display omitted] • Hierarchical core-shell Pt–Ni ORR electrocatalysts (ECs) are obtained. • Interplay between synthesis, properties, and performance of ECs is discussed. • Hierarchical Pt–Ni ECs exhibit mass activities surpassing DOE targets. • Hierarchical Pt–Ni ECs are more durable than Pt/C reference ECs. [ABSTRACT FROM AUTHOR]

Polybenzimidazole (PBI) has becomea popular polymer of choicefor the preparation of membranes for potential use in high-temperatureproton exchange membrane polymer fuel cells. Phosphoric acid-dopedcomposite membranes of poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole](PBI4N) impregnated with hafnium oxide nanofiller with varying contentlevels (0–18 wt %) have been prepared. The structure–propertyrelationships of both the undoped and acid-doped composite membranesare studied using thermogravimetric analysis, modulated differentialscanning calorimetry, dynamic mechanical analysis, wide-angle X-rayscattering, infrared spectroscopy, and broadband electrical spectroscopy.Results indicate that the presence of nanofiller improves the thermaland mechanical properties of the undoped membranes and facilitatesa greater level of acid uptake. The degree of acid dissociation withinthe acid-doped membranes is found to increase with increasing nanofillercontent. This results in a conductivity, at 215 °C and a nanofillerlevel x≥ 0.04, of 9.0 × 10–2S cm–1for [PBI4N(HfO2)x](H3PO4)y. This renders nanocomposite membranes of this type as goodcandidates for use in high temperature proton exchange membrane fuelcells (HT-PEMFCs). [ABSTRACT FROM AUTHOR]

Abstract: This paper reports a FT-ATR-IR spectroscopic study on proton conducting poly(2,5-benzimidazole) (ABPBI) membranes doped with orthophosphoric acid. The analysis of the vibrational profiles is a good diagnostic tool to help understand the interactions occurring between the phosphoric acid and the polymer membranes. The experimental data show evidence that an acid-base proton exchange reaction has occurred between the imidazole moieties in the polymer chain and phosphoric acid to produce dihydrogen phosphate ions and protonated imidazolium cations in ABPBIn+. Vibrational modes associated with the dihydrogen phosphate ions are evident in the FT-IR spectra at lower doping levels and then become partially masked by the large amount of free phosphoric acid at high acid concentrations. Several bands in the FT-IR and FT-Raman spectra attributed to mixed modes containing varying contributions from NH bending motions exhibit high frequency shifts upon protonation the imidazole moieties. The correlatively assigned experimental vibrational bands were compared with calculated normal modes for small molecule models. The optimized geometry of the benzimidazolium dimer suggests that protonation of ABPBI results in a perturbation of the extended conjugated π system and allows rotation of the benzimidazole monomer units along the polymer chain. The results described here provide insight into the roles of phosphoric acid and ABPBI in the conduction mechanism of polybenzimidazole systems. [Copyright &y& Elsevier]

Abstract: An interesting new material based on a Nafion® membrane doped with a sulfated-zirconia filler is presented. This filler is unique in that the filler itself can contribute to the proton conductivity due to the presence of acidic functionalities on the surface of the filler. The presence of the filler in the membrane results in the deprotonation of Nafion®’s acid moieties as indicated by the absence of the acid mode at 1475cm−1 in the FTIR spectrum. Spectra from DSC, DMA and broadband electric spectroscopy (BES) show the presence of several molecular transitions, two of which are detected in the BES permittivity profiles. The membrane exhibits a reasonably high conductivity (3×10−3 Scm−1 at 120°C) even in completely dry conditions, which makes it a promising material for an anhydrous fuel cell. The conductivity behaviour exhibits a mix of Arrhenius and VTF behaviours and is closely tied to the dielectric relaxations. [Copyright &y& Elsevier]