Electrochemical performance of carbon gels with variable surface chemistry and physics

This study describes the electrochemical characterization of N-doped carbon xerogels in the form of microspheres and of carbon aerogels with varied porosities and surface oxygen complexes. The interfacial capacitance of N-doped carbon xerogels decreased with increased micropore surface area as determined by N2 adsorption at −196 °C. The interfacial capacitance showed a good correlation with the areal NXPS concentration, and the best correlation with the areal concentration of pyrrolic or pyridonic nitrogen functionalities. The gravimetric capacitance decreased with greater xerogel microsphere diameter. The interfacial capacitance of carbon aerogels increased with higher percentage of porosity as determined from particle and true densities. The interfacial capacitance showed a linear relationship with the areal oxygen concentration and with the areal concentrations of CO- and CO2-evolving groups.

A "Know-How vs. Know-What" Approach in the Teaching-Learning of Competences in Physical Chemistry

The methodological approach a teacher uses in the competence teaching-learning process determines the way students learn. Knowledge can be acquired from a series of perspectives, mainly: “know-what” (concept), where facts and descriptions of (natural or social) phenomena are pursued; “know-how” (procedure), where methods and procedures for their application are described; and “know-why” (competence), where general principles and laws that explain both the facts and their applications are sought. As all the three cases are interconnected, the boundaries between them are not fully clear and their application uses shared elements. In any case, the depth of student’s acquired competences will be directly affected by the teaching-learning perspective, traditionally aiming to a “know-why” approach for full competence acquisition. In this work, we discuss a suitable teaching-learning methodology for evaluating whether a “know-how”, “know-what” or combined approach seems better for enhancing competence learning in students. We exemplify the method using a selection of formative activities from the Physical Chemistry area in the Grades of Chemistry and Chemical Engineering.

Assessment of Competences in the Physical Chemistry Area: Use of the Department Teaching Portfolio

Competences have become a standard learning outcome in present university education within the European Higher Education Area (EHEA). In this regard, updated tools for their assessment have turned out essential in this new teaching-learning paradigm. Among them, one of the most promising tools is the “learner´s portfolio”, which is based on the gathering and evaluation of a range of evidences from the student, which provides a wider and more realistic view of his/her competence acquisition. Its appropriate use as a formative (continuous) assessment instrument allows a deeper appraisal of student´s learning, provided it does not end up as another summative (final) evaluation tool. In this contribution we propose the use of the portfolio as a unifying assessment tool within a university department (Physical Chemistry), exemplifying how the portfolio could yield both personalized student reports and averaged area reports on competence acquisition. A proposed stepwise protocol is given to organize the individual competence reports and estimate the global competence level following a bottom-up approach (i.e. ranging from the class group, subject, grade, and academic course).

Quantum Dot-Sensitized Solar Cells Based on Directly Adsorbed Zinc Copper Indium Sulfide Colloids

Heavy metal-based quantum dots (QDs) have demonstrated to behave as efficient sensitizers in QD-sensitized solar cells (QDSSCs), as attested by the countless works and encouraging efficiencies reported so far. However, their intrinsic toxicity has arisen as a major issue for the prospects of commercialization. Here, we examine the potential of environmentally friendly zinc copper indium sulfide (ZCIS) QDs for the fabrication of liquid-junction QDSSCs by means of photoelectrochemical measurements. A straightforward approach to directly adsorb ZCIS QDs on TiO2 from a colloidal dispersion is presented. Incident photon-to-current efficiency (IPCE) spectra of sensitized photoanodes show a marked dependence on the adsorption time, with longer times leading to poorer performances. Cyclic voltammograms point to a blockage of the channels of the mesoporous TiO2 film by the agglomeration of QDs as the main reason for the decrease in efficiency. Photoanodes were also submitted to the ZnS treatment. Its effects on electron recombination with the electrolyte are analyzed through electrochemical impedance spectroscopy and photopotential measurements. The corresponding results bring out the role of the ZnS coating as a barrier layer preventing electron leakage toward the electrolyte, as argued in other QD-sensitized systems. The beneficial effect of the ZnS coating is ultimately reflected on the power conversion efficiency of complete devices, reaching values of 2 %. In a more general vein, through these findings, we aim to call the attention to the potentiality of this quaternary alloy, virtually unexplored as a light harvester for sensitized devices.

Binderless thin films of zeolite-templated carbon electrodes useful for electrochemical microcapacitors with ultrahigh rate performance

Controlled nanozeolite deposits are prepared by electrochemical techniques on a macroporous carbon support and binderless thin film electrodes of zeolite-templated carbon are synthesized using the deposits as templates. The obtained film electrodes exhibit extremely high area capacitance (10–12 mF cm−2) and ultrahigh rate capability in a thin film capacitor.

Water adsorption in hydrophilic zeolites: experiment and simulation

We have measured experimental adsorption isotherms of water in zeolite LTA4A, and studied the regeneration process by performing subsequent adsorption cycles after degassing at different temperatures. We observed incomplete desorption at low temperatures, and cation rearrangement at successive adsorption cycles. We also developed a new molecular simulation force field able to reproduce experimental adsorption isotherms in the range of temperatures between 273 K and 374 K. Small deviations observed at high pressures are attributed to the change in the water dipole moment at high loadings. The force field correctly describes the preferential adsorption sites of water at different pressures. We tested the influence of the zeolite structure, framework flexibility, and cation mobility when considering adsorption and diffusion of water. Finally, we performed checks on force field transferability between different hydrophilic zeolite types, concluding that classical, non-polarizable water force fields are not transferable.

Clay-supported graphene materials: application to hydrogen storage

The present work refers to clay–graphene nanomaterials prepared by a green way using caramel from sucrose and two types of natural clays (montmorillonite and sepiolite) as precursors, with the aim of evaluating their potential use in hydrogen storage. The impregnation of the clay substrates by caramel in aqueous media, followed by a thermal treatment in the absence of oxygen of these clay–caramel intermediates gives rise to graphene-like materials, which remain strongly bound to the silicate support. The nature of the resulting materials was characterized by different techniques such as XRD, Raman spectroscopy and TEM, as well as by adsorption isotherms of N2, CO2 and H2O. These carbon–clay nanocomposites can act as adsorbents for hydrogen storage, achieving, at 298 K and 20 MPa, over 0.1 wt% of hydrogen adsorption excess related to the total mass of the system, and a maximum value close to 0.4 wt% of hydrogen specifically related to the carbon mass. The very high isosteric heat for hydrogen sorption determined from adsorption isotherms at different temperatures (14.5 kJ mol−1) fits well with the theoretical values available for hydrogen storage on materials that show a strong stabilization of the H2 molecule upon adsorption.

Controlled rippling of graphene via irradiation and applied strain modify its mechanical properties: a nanoindentation simulation study

Ripples, present in free standing graphene, have an important influence in the mechanical behavior of this two-dimensional material. In this work we show through nanoindentation simulations, how out-of-plane displacements can be modified by strain resulting in softening of the membrane under compression and stiffening under tension. Irradiation also induces changes in the mechanical properties of graphene. Interestingly, compressed samples, irradiated at low doses are stiffened by the irradiation while samples under tensile strain do not show significant changes in their mechanical properties. These simulations indicate that vacancies, produced by the energetic ions, cannot be the ones directly responsible for this behavior. However, changes in roughness induced by the momentum transferred from the energetic ions to the membrane, can explain these differences. These results provide an alternative explanation to recent experimental observations of stiffening of graphene under low dose irradiation, as well as paths to tailor the mechanical properties of this material via applied strain and irradiation.

Surface structured platinum electrodes for the electrochemical reduction of carbon dioxide in imidazolium based ionic liquids

The direct CO2 electrochemical reduction on model platinum single crystal electrodes Pt(hkl) is studied in [C2mim+][NTf2−], a suitable room temperature ionic liquid (RTIL) medium due to its moderate viscosity, high CO2 solubility and conductivity. Single crystal electrodes represent the most convenient type of surface structured electrodes for studying the impact of RTIL ion adsorption on relevant electrocatalytic reactions, such as surface sensitive electrochemical CO2 reduction. We propose here based on cyclic voltammetry and in situ electrolysis measurements, for the first time, the formation of a stable adduct [C2mimH–CO2−] by a radical–radical coupling after the simultaneous reduction of CO2 and [C2mim+]. It means between the CO2 radical anion and the radical formed from the reduction of the cation [C2mim+] before forming the corresponding electrogenerated carbene. This is confirmed by the voltammetric study of a model imidazolium-2-carboxylate compound formed following the carbene pathway. The formation of that stable adduct [C2mimH–CO2−] blocks CO2 reduction after a single electron transfer and inhibits CO2 and imidazolium dimerization reactions. However, the electrochemical reduction of CO2 under those conditions provokes the electrochemical cathodic degradation of the imidazolium based RTIL. This important limitation in CO2 recycling by direct electrochemical reduction is overcome by adding a strong acid, [H+][NTf2−], into solution. Then, protons become preferentially adsorbed on the electrode surface by displacing the imidazolium cations and inhibiting their electrochemical reduction. This fact allows the surface sensitive electro-synthesis of HCOOH from CO2 reduction in [C2mim+][NTf2−], with Pt(110) being the most active electrode studied.