No abstract

F uel cells (FCs) operating at low temperatures (T < 200 °C)show several very attractive features, including: (a) relatively simple assembly; (b) good compatibility with the environment; and (c) very high efficiency with respect to internal combustion engines. However, the full potential of low-temperature FCs can only be achieved by addressing a number of crucial issues involved in their operation. One of the most important bottlenecks is represented by the slow kinetics of the oxygen reduction reaction (ORR).1 Typical examples of low-temperature fuel cells include proton exchange membrane fuel cells (PEMFCs) and anion-exchange membrane fuel cells (AEMFCs).1 To achieve energy conversion efficiencies and power densities compatible with applications, all these devices require suitable ORR electrocatalysts (ECs) to minimize cathode polarization losses.Ideally, ORR ECs should possess the following features: a) Active sites capable of the highest turnover frequency at the lowest overpotentials; b) A large active surface area, maximizing the number of active sites; c) A morphology that facilitates the efficient transport of reactants and products to and from the active sites; d) High electron conductivity, to minimize ohmic losses; e) A high dielectric environment to aid the ion exchange processes between the active sites and the ion-conducting membrane; f) High stability under operating conditions, to achieve high durability.To comply with these requirements, state-of-the-art ORR ECs consist of Pt-nanocrystals supported on conductive carbon nanoparticles (NPs) that possess a spherical morphology and a large surface area 2 (indicated as "Pt/C ref. ECs"). These systems are characterized by a high dispersion of the ORR active sites, which are easily accessible by the reactants. Their performance is comparable to that of pristine "Pt-black" ECs, but with a reduced loading of precious metal. However, the large-scale rollout of FC technology employing Pt/C ref. ECs is hindered by their insufficient durability and very high costs."Carbon nitride-based electrocatalysts" (CN-based ECs) have shown great promise to address the issues outlined above. CN-based ECs are composed of a carbon-based matrix embedding nitrogen atoms. The matrix coordinates metal-based species, including: (a) NPs of metals (e.g., Pt, Pd), metal alloys (e.g., PtNi x , PdCo y Ni z ), oxides (e.g., Fe 3 O 4 ) or carbides (e.g., FeC x ); or (b) coordination complexes of single metal atoms (e.g., Fe, Co).3 There are two main driving forces behind the development of CN-based ECs. CN-based ECs including Pt-group elements (PGMs) show very high ORR activity and a remarkable tolerance towards the oxidizing conditions typical at the cathodes of low-temperature FCs. 4 The carbon nitride (CN) matrix embeds the various inorganic NPs in "nitrogen coordination nests". 5These strong interactions inhibit the main mechanisms involved in the long-term degradation of typical Pt/C ref. ECs, especially particle agglomeration and particle detachment from the support.

No abstract

We demonstrate that the true hydroxide conductivity in an e-beam grafted poly(ethylene-co-tetrafluoroethylene) [ETFE] anion exchange membrane (AEM) is as high as 132 mS cm(-1) at 80 °C and 95% RH, comparable to a proton exchange membrane, but with very much less water present in the film. To understand this behaviour we studied ion transport of hydroxide, carbonate, bicarbonate and chloride, as well as water uptake and distribution. Water uptake of the AEM in water vapor is an order of magnitude lower than when submerged in liquid water. In addition (19)F pulse field gradient spin echo NMR indicates that there is little tortuosity in the ionic pathways through the film. A complete analysis of the IR spectrum of the AEM and the analyses of water absorption using FT-IR led to conclusion that the fluorinated backbone chains do not interact with water and that two types of water domains exist within the membrane. The reduction in conductivity was measured during exposure of the OH(-) form of the AEM to air at 95% RH and was seen to be much slower than the reaction of CO2 with OH(-) as the amount of water in the film determines its ionic conductivity and at relative wet RHs its re-organization is slow.

This report describes the electrochemical behavior of a family of “core‐shell” electrocatalysts consisting of a carbon nitride (CN) “shell” matrix and a “core” of conducting carbon nanoparticles (NPs). The CN “shell” matrix embeds PdCoNi alloy NPs and covers homogeneously the carbon “core”. The chemical composition of the materials is determined by inductively‐coupled plasma atomic emission spectroscopy (ICP‐AES) and microanalysis; the structure is studied by powder X‐ray diffraction (powder XRD); the morphology is investigated by high‐resolution transmission electron microscopy (HR‐TEM). The surface activity and structure are probed by CO stripping. The oxygen reduction reaction (ORR) kinetics, reaction mechanism, and tolerance towards contamination from chloride anions are evaluated by cyclic voltammetry with the thin‐film rotating ring‐disk electrode (CV‐TF‐RRDE) method. The effect of N concentration in the matrix (which forms “coordination nests” for the Pd‐based alloy NPs bearing the active sites) on the ORR performance of the electrocatalysts is described. Results show that N atoms: 1) influence the evolution of the structure of the materials during the preparation processes, and 2) interact with alloy NPs, affecting the bifunctional and electronic ORR mechanisms of active sites and the adsorption/desorption processes of oxygen molecules and contaminants. Finally, the best PdCoNi electrocatalyst shows a higher surface activity in the ORR at 0.9 V vs. RHE with respect to the Pt‐based reference (388 μA cmPd‐2 vs. 153 μA cmPt‐2).

No abstract

The thermal, mechanical, and electric properties of hybrid membranes based on Nafion that contain a [(ZrO(2))·(Ta(2)O(5))(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-SO(3)H···[ZrTa] cross-links in the material. These increase the thermal stability of the -SO(3)H 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 (σ(IP)) and bulk proton conductivities (σ(EP)). 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. σ(EP) shows VTF behavior, suggesting that the long-range conductivity is closely related to the segmental motion of the Nafion host matrix. Long-range conduction (σ(EP)) occurs when the dynamics of the fluorocarbon matrix induces contact between different delocalization bodies (DB), which results in proton exchange processes between these DBs.

In this report, the preparation of Fe-carbon nitride (CN)-based electrocatalysts (ECs) with a “core-shell” morphology for the oxygen reduction reaction (ORR) is described. The ECs consist of spherical XC-72R carbon nanoparticles, the “cores”, that are covered by a CN matrix, the “shell”, that embeds Fe species in “coordination nests”. The latter consist of hollow cavities in the CN matrix, whose internal surface is covered by N- and C-ligands able to stabilize alloy nanoparticles or active sites. Two families of CN-based ECs are prepared, which are grouped on the basis of the concentration of N atoms in the CN “shell”. Each group comprises of both a “pristine” and an “activated” EC; the latter is obtained from the “pristine” EC by a suitable series of treatments (A) devised to improve the ORR performance. The chemical composition of the CN-based ECs is determined by Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and microanalysis. The thermal stability under both inert and oxidizing atmospheres is gauged by High-Resolution Thermogravimetric Analysis (HR-TGA). The structure is probed by powder X-ray diffraction, and the morphology is inspected by Scanning Electron Microscopy (SEM) and High-Resolution Transmission Electron Microscopy (HR-TEM). The surface area of the CN-based ECs is determined by nitrogen physisorption techniques, and the surface composition is probed by X-ray Photoelectron Spectroscopy (XPS). The electrochemical performance and reaction mechanism of the CN-based ECs in the ORR is investigated in both acid and alkaline environments by cyclic voltammetry with the Thin-Film Rotating Ring-Disk Electrode setup (CV-TF-RRDE). The influence of the preparation parameters and of the treatments on the physicochemical properties, the ORR performance, and reaction mechanism is studied in detail. In the alkaline environment the FeFe2-CNl900/CA“core-shell” EC shows a remarkable ORR onset potential of 0.908 V vs. RHE which, with respect to the value of 0.946 V vs. RHE of the Pt/C ref., classifies the proposed materials as very promising “Platinum Group Metal-free” ECs for the ORR