Towards 'Pt-free' Anion-Exchange Membrane Fuel Cells: Fe-Sn Carbon Nitride-Graphene 'Core-Shell' Electrocatalysts for the Oxygen Reduction Reaction

We report on the development of two new Pt-free electrocatalysts (ECs) for the oxygen reduction reaction (ORR) based on graphene nanoplatelets (GNPs). We designed the ECs with a core-shell morphology, where a GNP core support is covered by a carbon nitride (CN) shell. The proposed ECs present ORR active sites that are not associated to nanoparticles of metal/alloy/oxide, but are instead based on Fe and Sn sub-nanometric clusters bound in coordination nests formed by carbon and nitrogen ligands of the CN shell. The performance and reaction mechanism of the ECs in the ORR are evaluated in an alkaline medium by cyclic voltammetry with the thin-film rotating ring-disk approach and confirmed by measurements on gas-diffusion electrodes. The proposed GNP-supported ECs present an ORR overpotential of only ca. 70 mV higher with respect to a conventional Pt/C reference EC including a XC-72R carbon black support. These results make the reported ECs very promising for application in anion-exchange membrane fuel cells. Moreover, our methodology provides an example of a general synthesis protocol for the development of new Pt-free ECs for the ORR having ample room for further performance improvement beyond the state of the art

Synthesis and Characterization of Heterobimetallic (Pd/B) Nindigo Complexes and Comparisons to Their Homobimetallic (Pd₂, B₂) Analogues

Reactions of Nindigo-BF₂ complexes with Pd­(hfac)₂ produced mixed complexes with Nindigo binding to both a BF₂ and a Pd­(hfac) unit. These complexes are the first in which the Nindigo ligand binds two different substrates, and provide a conceptual link between previously reported bis­(BF₂) and bis­(Pd­(hfac)) complexes. The new Pd/B complexes have intense near IR absorption near 820 nm, and they undergo multiple reversible oxidations and reductions as probed by cyclic voltammetry experiments. The spectral, redox, and structural properties of these complexes are compared against those of the corresponding B₂ and Pd₂ complexes with the aid of time-dependent density functional calculations. In all cases the low-energy electronic transitions are ligand-centered π–π* transitions, but the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energiesand hence the absorption wavelength as well as the oxidation and reduction potentialsare significantly modulated by the moieties bound to the Nindigo ligand

Interplay Between Structure and Conductivity in 1-Ethyl-3-methylimidazolium tetrafluoroborate/(\u3b4-MgCl2)f Electrolytes for Magnesium Batteries

The synthesis, physicochemical properties and conductivity mechanism of a family of ionic liquid-based electrolytes for use in secondary Mg batteries are reported. The electrolytes are obtained by dissolving controlled amounts of \u3b4-MgCl2 salt into the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) which acts as a solvent. \u3b4-MgCl2 consists of an inorganic ribbon of Mg atoms covalently bonded together through bridging chlorine atoms. Due to this peculiar structural motif, with respect to the electrolytes based on conventional Mg salts, it is possible to achieve electrolytes of higher Mg concentration. Thus, concatenated anionic complexes bridged via halogen atoms are formed, improving the electrochemical performance of these materials. Electrolytes with a general formula EMImBF4/(\u3b4-MgCl2)f with f ranging from 0 to 0.117 are obtained. The composition of the obtained materials is determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The properties of these systems are investigated by means of Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and vibrational spectroscopy in both medium (MIR) and far infrared (FIR). Finally, Broadband Electrical Spectroscopy (BES) is carried out with the aim to elucidate the electrical response of the electrolytes in terms of their polarization and relaxation phenomena and to propose a conductivity mechanism. At 20 \ub0C the highest conductivity (0.007 S/cm) is observed for the electrolyte with cMg = 0.00454 molMg/kgIL

Origins, developments, and perspectives of carbon nitride-based electrocatalysts for application in low-temperature FCs

This article presents the key features and requirements of oxygen reduction reaction electrocatalysts, and provides a detailed overview of carbon nitride (CN) based electrocatalysts for low-temperature fuel cells. CN-based ECs are composed of a carbon-based matrix embedding nitrogen atoms and selected variants show very high catalytic activity towards the oxygen reduction reaction and high tolerance towards the oxidizing conditions typical at the cathodes of low-temperature fuel cells. The article presents a description of the proper nomenclature of CN based electrocatalysts, provides a detailed overview of the literature in this field to date, and describes in detail the important methods used to prepare CN electrocatalysts. The effect of nitrogen content on the properties and accessibility of the CN electrocatalysts is discussed. The concept of core-shell CN-based electrocatalysts is introduced and described in detail

Single-Ion-Conducting Nanocomposite Polymer Electrolytes for Lithium Batteries Based on Lithiated-Fluorinated-Iron Oxide and Poly(ethylene glycol) 400

A poly(ethylene glycol) 400 (PEG400) matrix doped with different amounts of a fluorinated Fe2O3-based nanofiller (LiFI) featuring a Li+-functionalised surface gives rise to nanocomposite polymer electrolytes (nCPEs) that demonstrate single-ion conduction. A family of nCPEs with general formula [PEG400/(LiFI)(y)] and y = n(Fe)/n(PEG400) ranging from 0 to 8.15 are prepared; they are characterized by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), High-Resolution Thermogravimetric Analysis (HR-TGA), Differential Scanning Calorimetry (DSC), and Fourier-transform vibrational spectroscopy in both the medium (MIR) and far (FIR) infrared. The Li+ transference number, t(Li+), is determined and Broadband Electrical Spectroscopy (BES) is used to elucidate the electrical response of the materials in terms of polarization and relaxation events. The combination of the information obtained by all the aforementioned techniques enables us to present a possible conduction mechanism for these nCPEs single-ion conducting systems

\u201cCore-shell\u201d carbon nitride electrocatalysts for the oxygen reduction reaction (ORR) based on graphene and related materials for application in low-temperature fuel cells

Fuel cells (FCs) are a family of advanced energy conversion systems characterized by an outstanding energy conversion efficiency, up to two-three times as high as typical internal combustion engines (ICEs). Furthermore, FCs show a number of other attractive features including a simple engineering of the power plant and a high compatibility with the environment. In particular, FCs operating at a low temperature (T<200\ub0C) are typically characterized by a very high energy and power density. As a consequence, they are very promising candidates to provide power to a number of applications, ranging from portable electronic devices to light-duty vehicles. one important bottleneck in the operation of FCs functioning at a low temperature is the sluggishness of the ORR. Suitable ORR electrocatalysts (ECs) are needed to achieve a level of permormance compatible with applications. This contribution describes the preparation of innovative ORR ECs on the basis of the unique protocols developed in our laboratory. The proposed ECs are characterized by "core-shell" morphology. In detail, the "core" consists of sheets/nanoplatelets of graphene and related materials. The latter are covered by a carbon nitride "shell", which coordinates nanoparticles bearinf the ORR active sites through "nitrogen coordiantion nests". The chemical composition of the proposed "core-shell" ECs is determined by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis. The structure of the materials is studied by powder X-ray diffraction (XRD); the morphology is inspected by high-resolution scanning electron microscopy (HR-SEM) and high-resolution transmission electron microscopy (HR-TEM). The electrochemical performance, reaction mechanism and selectivity of the ECs in the ORR is evaluated "ex situ" by cyclic voltammetry with the rotating ring-disk electrode (CV-TF-RRDE) method. Finally, the most promising ECs are used to fabricate membrane-electrode assemblies (MEAs), which are tested in single fuel cell in operating conditions

Anion Exchange Membranes: Correlation between Physicochemical Properties and Anion Conductivity By Broadband Electrical Spectroscopy

In recent years, anion exchange membrane fuel cells (AEMFCs) have been extensively studied owing to significant advantages over their proton exchange membrane fuel cell (PEMFC) counterparts [1]. The possibility to adopt electrocatalysts that do not comprise of precious metals as well as diminished poisoning effects are among the most relevant reasons for which AEMFCs are believed to be advantageous. However, AEMFCs do suffer from some drawbacks, specifically concerned with the anion exchange membrane (AEM) which is responsible for the selective migration of OH- anions from the cathode to the anode, and is one of the most critical components of the entire AEMFC. In particular, with respect to the proton exchange membranes used in PEMFCs, AEMs typically exhibit a lower ionic conductivity and an inferior chemical stability, the latter typically associated with the degradation of anion-exchange functionalities. For these reasons, it is very important to elucidate the details of the complex interplay between the nanostructure and the ion conductivity mechanism of the AEMs. Over the last 30 years it has been demonstrated that conductivity in ion-conducting materials occurs via a number of different processes including: (a) the migration of ions between coordination sites [2-5]; and (b) the diffusion of conformational states of the host matrix (segmental motion). [2-5]. In ion-conducting membranes the long-range charge migration is often correlated with the dielectric relaxation modes of the polymeric chains; the latter are typically associated with the fluctuation of: a) the main backbone chain bearing permanent dipole moments; b) side chains; or c) functional groups involved in ion-dipole interactions. The key technique to investigate the interplay between structure and conductivity of ion-conducting materials is Broadband Electrical Spectroscopy (BES). Here we present several case studies of AEMs paying particular attention to their thermal stability and the thermomechanical properties. BES is then adopted to study the electrical response of each material in terms of polarizations and relaxation phenomena. The results allow us to: (a) suggest a comprehensive model capable to rationalize the long-range charge transfer mechanism in AEMs; and (b) clarify how the chemical composition and nanostructure of the materials is influencing the coordination of mobile species. Acknowledgements The authors thank the StrategicProject \u201cFrom materials for Membrane electrode Assemblies to electric Energy conversion and SToRAge devices\u201d (MAESTRA) of the University of Padova for funding this activity. References [1] Polymer Electrolytes: Fundamentals and Applications; Sequeira, C.; Santos, D., Eds.; Woodhead Publishing Limited, Oxford, 2010. [2] Di Noto, V. J. Phys. Chem. B, 104 (2000) 10116. [3] Di Noto, V.; Vittadello, M.; Lavina, S.; Fauri, M.; Biscazzo, S. J. Phys. Chem. B, 105 (2001) 4584. [4] Di Noto, V. J. Phys. Chem. B, 106 (2002) 11139. [5] Di Noto, V.; Vittadello, M.; Greenbaum, S. G.; Suarez, S.; Kano, K.; Furukawa, T. J. Phys. Chem. B, 108 (2004) 18832

Nanocomposite membranes based on polybenzimidazole and ZrO2 for high-temperature proton exchange membrane fuel cells

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 ](H3 PO4 )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