Influence of Fluorine Doping of Activated Carbon Fibers on Their Water Vapor Adsorption Characteristics.

The characterization of fluorinated carbon fibers by water sorption has been broadly investigated in this work. In brief, a pitch-based activated carbon fiber (ACF) was submitted to a fluorination process under different conditions of partial pressure (F2:N2 ratio) and temperature. This led to samples with varied fluorine content and C-F type bonding. The effect of the fluorination treatment on the textural properties of the ACF was studied by means of nitrogen and carbon dioxide adsorption at -196 and 0°C, respectively, while the changes induced in the surface chemistry of the materials were analyzed by XPS. Also, the affinity and stability of the materials toward water was evaluated by single and cycling isotherms. The obtained results show that a mild fluorination not only can preserve most of the textural properties of the parent ACF, but enhance the water uptake at the first stages of the water sorption process, together with a shift in the upswing of the water isotherms toward lower relative humidities. This indicates that fluorination under certain conditions can actually enhance the surface hydrophilicity of carbon materials with specific properties. On the contrary, higher partial pressures led to highly fluorinated fibers with lower porosity and more hydrophobic character. Moreover, they presented a lower chemical stability as demonstrated by a change in the shape of the water isotherms after two consecutive measurements. The kinetics of water sorption in the ACFs provided further insights into the different sorption phenomena involved. Hence, water sorption can definitely help to tailor the water affinity, stability and performance of fluorinated porous carbon materials under humid conditions.

Adenine nucleobase directed supramolecular architectures based on ferrimagnetic heptanuclear copper(II) entities and benzenecarboxylate anions.

Two planar organic anions, benzoate and benzene-1,4-dicarboxylate (terephthalate), have been selected as potential π-stacking intercalators among ferrimagnetic [Cu7(µ-adeninato)6(µ3-OH)6(µ-H2O)6]2+ heptameric discrete entities. The resulting supramolecular architecture is highly dependent on the negative charge density distribution, mainly located in the carboxylate groups of the organic anions. In this sense, the benzoate anion, with just one carboxylate group, does not allow its intercalation between the adeninato ligands as it would imply a high steric hindrance among the heptameric entities. As a consequence, these benzoate anions are located inside the voids of the crystal structure reducing the accessible volume of compound [Cu7(µ-adeninato)6(µ3-OH)6(µ-H2O)6](benzoate)2·~17H2O (1). On the contrary, the terephthalate anion, containing two carboxylate groups at opposite sites, adopts a π-stacking sandwich arrangement between two adeninato ligands that affords the porous open structure of formula [Cu7(µ-adeninato)6(µ3-OH)6(µ-H2O)6](terephthalate)·nH2O (2a, 2b; n: 12 and 24, respectively). In addition to that, the less directional nature of the π-stacking interactions in comparison to the complementary hydrogen bonding based supramolecular metal-organic frameworks (SMOFs), suits them with a flexible architecture able to reversibly adsorb/desorb water (up to a 25-30% at 20 °C) altogether with the expansion/shrinkage of the crystal structure. The bridging adeninato and hydroxido ligands are effective magnetic exchange mediators to provide a ST = 5/2 ferrimagnetic state for the heptanuclear entity.

Boosting the Oxygen Reduction Electrocatalytic Performance of Nonprecious Metal Nanocarbons via Triple Boundary Engineering Using Protic Ionic Liquids.

The oxygen reduction reaction (ORR) in aqueous media plays a critical role in sustainable and clean energy technologies such as polymer electrolyte membrane and alkaline fuel cells. In this work, we present a new concept to improve the ORR performance by engineering the interface reaction at the electrocatalyst/electrolyte/oxygen triple-phase boundary using a protic and hydrophobic ionic liquid and demonstrate the wide and general applicability of this concept to several Pt-free catalysts. Two catalysts, Fe-N codoped and metal-free N-doped carbon electrocatalysts, are used as a proof of concept. The ionic liquid layer grafted at the nanocarbon surface creates a water-equilibrated secondary reaction medium with a higher O2 affinity toward oxygen adsorption, promoting the diffusion toward the catalytic active site, while its protic character provides sufficient H+/H3O+ conductivity, and the hydrophobic nature prevents the resulting reaction product water from accumulating and blocking the interface. Our strategy brings obvious improvements in the ORR performance in both acid and alkaline electrolytes, while the catalytic activity of FeNC-nanocarbon outperforms commercial Pt-C in alkaline electrolytes. We believe that this research will pave new routes toward the development of high-performance ORR catalysts free of noble metals via careful interface engineering at the triple point.