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Due to the need for decarbonization in energy generation, it is necessary to develop electrocatalysts for the oxygen reduction reaction (ORR), a key process in energy generation systems such as fuel cells and metal–air batteries. Perovskite–carbon material composites have emerged as active and stable electrocatalysts for the ORR, and the interaction between both components is a crucial aspect for electrocatalytic activity. This work explores different mixing methods for composite preparation, including mortar mixing, ball milling, and hydrothermal and thermal treatments. Hydrothermal treatment combined with ball milling resulted in the most favorable electrocatalytic performance, promoting intimate and extensive contact between the perovskite and carbon material and improving electrocatalytic activity. Employing X-ray photoelectron spectroscopy (XPS), an increase in the number of M-O-C species was observed, indicating enhanced interaction between the perovskite and the carbon material due to the adopted mixing methods. This finding was further corroborated by temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) techniques. Interestingly, the ball milling method results in similar performance to the hydrothermal method in the zinc–air battery and, thus, is preferable because of the ease and straightforward scalability of the preparation process.

La, Mn and Co-based materials with diverse compositions encapsulated in N-doped carbon materials were synthesized by an in-situ sol-gel method. The as-prepared materials were physically activated with CO2 giving as a result composites with excellent performance for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The composites were characterized by different physicochemical techniques revealing the importance of porosity created by CO2 activation. This crucial step increases the number of accessible active sites such as Co-Nx-C sites, Co/MnO heterointerfaces and graphitic N groups. Moreover, the formation of La2O2CO3 was detected to enhance electrical conductivity and electrochemical performance and it is active for ORR. Although the composite containing La, Mn and Co has less concentration of the highly active sites, its small particle size favour a better distribution of these sites. The Co-containing composites were tested as air-electrodes in a Zn-air battery and compared to commercial electrocatalysts. All composites showed a better operation in terms of higher cyclability and higher energy density which is a consequence of metal nanostructures encapsulation in N-doped porous carbon shells. Interestingly, the pure Co-based composite showed an outstanding performance related to the high concentration of the Co-Nx-C active sites that provide high activity and stability.

While carbon in itself appears as simple an element as it could possibly get, the undeniable truth is that carbon materials represent a plethora of possibilities both from the perspective of their structure and their applications. While we may believe that carbon is “just another element”, one should never forget that its special ability to coordinate through different hybridizations with apparent ease grants the element properties that no other element may even match. Taking this one step further into the materials realm opens up numerous avenues in terms of materials dimensionality, surface and bulk functionalization, or degree of structural order just to mention a few examples. If these properties are translated into the properties and applications field, the results are just as impressive, with new applications and variants appearing with growingly larger frequency. This has resulted in over a million scientific papers published in the last decade in which the term “carbon” was used either in the title, abstract or keywords. When the search is narrowed down to the field “title” alone, the results drop to just over 318.000 scientific papers. These are figures that no other element in the periodic table can equal, which is a clear indicative that the story of carbon materials is still under constant evolution and development. This review will present an overview of the works published in the Frontiers in Carbon-based materials section during its 10 years of life that reflect the advancements achieved during the last decade in the field of carbon materials.

LaNi1-xCoxO3 perovskite materials were synthesized by a sol-gel method for use in electrochemical reactions which involve molecular oxygen. The metal oxides were characterized by different physicochemical techniques and it was observed that the incorporation of Co induces some changes in the surface of the materials that affect the electrocatalytic activity. The different perovskite metal oxides were mixed with carbon black (Vulcan) to improve their electrocatalytic performance. The results from TPR and TPD techniques supported the XPS interpretation suggesting that a stronger interaction between carbon material and metal oxides can be obtained by physically mixing the two materials. This interaction improves the electron transfer and enhances the catalytic activity. Among the as-prepared materials, it was observed that the electrocatalytic performance of LaNi1-xCoxO3 perovskite/Vulcan materials is higher than that of LaNiO3/Vulcan for ORR, but similar towards OER. It is claimed that Co3+ is more catalytic for ORR, while Ni3+ favors the OER. Moreover, the presence of Co stimulates the formation of chemisorbed oxygen species, which also favor the electron transfer. Metal oxides containing both cations show higher stability, being this effect more notorious for OER. LaNi0.5Co0.5O3/Vulcan seems a suitable bifunctional catalyst for both electrochemical reactions.

Manganese-based materials can catalyze the oxygen reduction reaction (ORR), although their activity is known to depend on the crystalline phases and on the concentration of surface-active species. In the present study, we have optimized these two parameters to obtain improved catalysts for ORR. A sol-gel method was used to synthesize LaMnO3-manganese oxides composites with different lanthanum-to-manganese atomic ratios. The synthesized materials, which can be described as La1-xMnOz, were tested under ORR conditions and characterized by several physicochemical techniques such as SEM, XPS, EDX or XRD. It was found that the concentration of lanthanum governs the formation of different crystal phases and determines the crystallite size. Besides, high values of x tend to increase the surface concentration of manganese and therefore to produce more active sites for ORR. Among the materials analysed, La0.6MnOz mixed with carbon black (Vulcan) showed the best electrocatalytic performance. The high tolerance to methanol makes this electrocatalyst a promising alternative to substitute Pt-based materials in alkaline electrolytes.

In this study, we synthesized MnFe2O4 solid nanospheres (MSN) calcined at different temperatures (200–500 °C) and MSN-based materials mixed with carbon black, for their use as electrocatalysts in the oxygen reduction reaction (ORR) in alkaline medium (0.1 M KOH). It was demonstrated that the calcination temperature of MSN material determined its chemical surface composition and microstructure and it had an important effect on the electrocatalytic properties for ORR, which in turn was reflected in the performance of MSN/CB-based electrocatalysts. The study revealed that the presence of Mn species plays a key role in the ORR activity. Among tested, MSN200/CB and MSN350/CB exhibited the best electrochemical performances together with outstanding stability.

Transition metal oxide-based materials are an interesting alternative to substitute noble-metal based catalyst in energy conversion devices designed for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution reactions (HER). Perovskite (ABO3) and spinel (AB2O4) oxides stand out against other structures due to the possibility of tailoring their chemical composition and, consequently, their properties. Particularly, the electrocatalytic performance of these materials depends on features such as chemical composition, crystal structure, nanostructure, cation substitution level, eg orbital filling or oxygen vacancies. However, they suffer from low electrical conductivity and surface area, which affects the catalytic response. To mitigate these drawbacks, they have been combined with carbon materials (e.g. carbon black, carbon nanotubes, activated carbon, and graphene) that positively influence the overall catalytic activity. This review provides an overview on tunable perovskites (mainly lanthanum-based) and spinels featuring 3d metal cations such as Mn, Fe, Co, Ni and Cu on octahedral sites, which are known to be active for the electrochemical energy conversion.

Zn-air batteries (ZABs) are promising electrochemical devices to store energy. Metal oxide perovskites mixed with carbon materials are highlighted as interesting materials for this application because of their appropriate bifunctional performance in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The interaction between both components of the electrocatalyst is important in the bifunctional electrocatalytic activity, and the mixing method plays an important role in this interaction. Then, different mixing methods have been studied in this work (ball-milling, mortar and manual shaking). The use of different physicochemical techniques such as temperature programmed desorption (TPD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) in the materials characterization, allows us to conclude that the mixing method strongly influences the particle size and the interaction between both components, which determine the final electrocatalytic activity. The materials prepared by ball-milling displayed the best performance. Herein, the experimental conditions were optimized to obtain electrocatalysts with enhanced electrocatalytic activity for ORR and OER. Low rotating speed, air atmosphere and low ball-milling time generate electrocatalysts with a small nanoparticle size, more homogeneous and with a higher interaction between both components, which enhances electron transfer, and consequently, the overall oxygen-involved reactions. The best electrocatalyst obtained was studied as air-electrode in a Zn-air battery and it was compared to a commercial Pt/C electrocatalyst, obtaining higher cyclability (55.2 vs. 51.7%) for 30 h, and higher energy density at 5 mA/cm2 (764 mAh/g vs. 741 mAh/g).

La estrategia de aprendizaje en la que se ha centrado este trabajo es el método POGIL (Process Oriented Guided Inquiry Learning), el cual está orientado a mejorar el trabajo del alumnado en grupo haciendo uso de diferentes roles participativos. Este método de carácter cooperativo se aplicó en diferentes asignaturas impartidas en la Facultad de Ciencias, tanto de grado como de postgrado, obteniendo resultados que reflejan la aceptación del método POGIL por parte del alumnado y una mejora en la adquisición de los conceptos en comparación con las clases tradicionales. En esta experiencia se ha planteado el desafío de implementar este método en clases en formato no presencial (online), ya que la situación sanitaria acaecida ha limitado la asistencia del alumnado. A partir de las encuestas realizadas por el alumnado y comparando con los resultados de las encuestas de años anteriores, se puede afirmar que este método se ha aplicado de manera satisfactoria tanto en clases presenciales como en clases online. Adicionalmente, se observó que el contraste entre clases POGIL online y las clases expositivas clásicas es aún mayor que en años anteriores cuando se aplicaba el método POGIL en clases presenciales.

Copper ferrite nanospheres (CFNS) were prepared by a solvothermal method and they were calcined at different temperatures ranging from 200−600 °C, resulting in CFNS-based materials with different both surface chemistry and structural features, which play a crucial role in the electrocatalytic activity of carbon black-containing CFNS catalysts towards the oxygen reduction reaction (ORR). The study was addressed to elucidate which kind of crystalline phases, as well as surface chemistry of CFNS were responsible for enhancing the electrocatalytic activity in ORR. The pristine CFNS and CFNS200 (calcined at 200 °C) exhibited cubic spinel, cuprite and copper as main crystalline phases. In the case of CFNS400, it also displays the cubic phase along with a new phase (tenorite). Apart from the tenorite, CFNS600 exhibited hematite, and the cubic phase of spinel shifted to the tetragonal phase. Regarding the electrocatalytic activity in ORR, the CFNS revealed poor activities compared to those of carbon black-containing CFNS catalysts. The high-performing catalyst was CFNS400/Vulcan in terms of current density and onset potential along with its good stability. This improvement may be assigned to both the chemical composition and structure.

En la presente Tesis Doctoral se centra en la síntesis de óxidos con estructura perovskita y espinela para las reacciones electroquímicas de reducción de oxígeno (ORR), evolución de oxígeno (OER) y evolución de hidrógeno (HER). Debido a que los óxidos metálicos poseen una baja conductividad eléctrica y baja área superficial que limitan sus aplicaciones catalíticas se mezclan con materiales de carbón para mitigar estas dos desventajas. Además, los materiales de carbón juegan un papel fundamental en las reacciones catalíticas favoreciéndolas debido a un efecto sinérgico con los óxidos metálicos. Los óxidos tipo perovskita LaB1-xB'xO3 (donde B es Mn o Ni y B' es Co) se sintetizaron a través de un método sol-gel. Tal como se puede observar en la formulación en este tipo de perovskitas se realiza una sustitución parcial del catión B con otro metal de transición 3d con el fin de modificar las propiedades fisicoquímicas y de este modo mejorar la actividad catalítica. Los materiales obtenidos presentan una buena actividad catalítica tanto en la ORR como en la OER. Del conjunto de materiales obtenidos se ha observado que los materiales. LaMn1-xCoxO3/ Vulcan presentan una buena actividad catalítica en la ORR, mientras que los materiales LaNi1-xCoxO3/ Vulcan tienen un mejor comportamiento en la OER. Además los materiales presentan una buena estabilidad y en el caso de la ORR una buena tolerancia al metanol. Además, con el fin de incrementar el número de sitios catalíticos en superficie se ha modificado la relación atómica de lantano y manganeso durante la síntesis y se ha obtenido materiales con una buena actividad catalítica debido al aumento de sitios catalíticos de Mn en la superficie debido a la formación de nuevas fases cristalinas catalíticamente activas. Los óxidos con estructura tipo espinela CuFe2O4 se han sintetizado mediante un método solvotermal con el fin de obtener nanopartículas para la ORR. Además, con el fin de mejorar las propiedades físicas, el óxido posteri

Las estrategias de aprendizaje tradicionales donde generalmente el profesor tiene el papel principal, podrían no alcanzar las necesidades educativas del alumnado (Guzmán, 2011), así como las competencias planteadas dentro del Espacio Europeo de Educación Superior (EEES). Desde este ámbito se pretenden realizar cambios en las metodologías de enseñanza-aprendizaje para que el alumnado adquiera un papel principal. En este sentido, el método POGIL (Process Oriented Guided Inquiry Learning) ha demostrado ser un método efectivo de enseñanza orientado a mejorar el trabajo del alumnado en el aula a partir de diferentes roles participativos. Este método de carácter cooperativo se aplicó en diferentes asignaturas impartidas en la Facultad de Ciencias, tanto de grado (Oceanografía Química u Operaciones Básicas de Laboratorio), como de postgrado (Polimeros Conductores en el Máster de Ciencia de Materiales). También se realizó una actividad POGIL en el Master en Profesorado de Educación Secundaria para ver el funcionamiento en grupos con mayor diversidad del alumnado. Las encuestas realizadas a los estudiantes reflejan una gran aceptación del método POGIL. Los resultados obtenidos de las pruebas escritas han mostrado que el método POGIL ayuda al alumnado a una mejor adquisición de los conceptos en comparación a los adquiridos en una clase magistral.

LaMn1−xCoxO3 perovskites were synthesized by a modified sol-gel method which incorporates EDTA. These materials’ electrochemical activity towards both oxygen reduction (ORR) and oxygen evolution reactions (OER) was studied. The cobalt substitution level determines some physicochemical properties and, particularly, the surface concentration of Co and Mn’s different oxidation states. As a result, the electroactivity of perovskite materials can be tuned using their composition. The presence of cobalt at low concentration influences the catalytic activity positively, and better bifunctionality is attained. As in other perovskites, their low electrical conductivity limits their applicability in electrochemical devices. It was found that the electrochemical performance improved significantly by physically mixing with a mortar the active materials with two different carbon black materials. The existence of a synergistic effect between the electroactive component and the carbon material was interpreted in light of the strong carbon–oxygen–metal interaction. Some mixed samples are promising electrocatalysts towards both ORR and OER.

The chemical composition of a LaMnO3 perovskite was modified sequentially by an improved sol–gel method to include cobalt centers in some B sites formerly occupied by Mn. In this way, a representative set of materials of general formula LaMn1-xCoxO3 was obtained whose composition extends from LaMnO3 to LaCoO3. These perovskites, as promising materials for oxygen reduction or oxygen evolution reactions, were characterized by several imaging (SEM), spectroscopic (XPS, EDX) and diffraction (XRD) techniques to elucidate their structure and to demonstrate the existence of composition differences between the catalytic surface and the bulk material. Specifically, it was found that lanthanum ions prevail at the surface of the catalyst but high cobalt-substitution levels stimulate the surface enrichment in B cations in their respective higher oxidation states (Mn4+ and Co3+ against Mn3+ and Co2+). This phenomenon opens the possibility of tuning their electrocatalytic properties and to synthesize suitable materials for electrochemical reactions involving molecular oxygen.

The development of electrocatalysts based on the doping of copper over cobalt spinel supported on a microporous activated carbon has been studied. Both copper–cobalt and cobalt spinel nanoparticles were synthesized using a silica-template method. Hybrid materials consisting of an activated carbon (AC), cobalt oxide (Co3O4), and copper-doped cobalt oxide (CuCo2O4) nanoparticles, were obtained by dry mixing technique and evaluated as electrocatalysts in alkaline media for hydrogen evolution reaction. Physical mixtures containing 5, 10, and 20 wt.% of Co3O4 or CuCo2O4 with a highly microporous activated carbon were prepared and characterized by XRD, TEM, XPS, physical adsorption of gases, and electrochemical techniques. The electrochemical tests revealed that the electrodes containing copper as the dopant cation result in a lower overpotential and higher current density for the hydrogen evolution reaction.

The one-pot hydrolytic hydrogenation of cellulose (HHC) with heterogeneous catalysts is an interesting method for the synthesis of fuels and chemicals from a renewable resource like lignocellulosic biomass. Supported metal catalysts are interesting for this application because they can contain the required active sites for the two catalytic steps of the HHC reaction (hydrolysis and hydrogenation). In this work, Ru catalysts have been prepared using a commercial carbon black that has been modified by sulfonation and oxidation treatments with H2SO4 and (NH4)S2O8, respectively, in order to create acidic surface sites. The correlation between the catalysts’ properties and catalytic activity has been addressed after detailed catalyst characterization. The prepared catalysts are active for cellulose conversion, being that prepared with the carbon black treated with sulfuric acid the most selective to sorbitol (above 40%). This good behavior can be mainly explained by the suitable porous structure and surface chemistry of the carbon support together with the low content of residual chlorine.