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Polymer electrolyte fuel cells with alkaline anion exchange membranes (AAEMs) have gained increasing attention because of the faster reaction kinetics associated with the alkaline environment compared to acidic media. While the development of anion exchange polymer membranes is increasing, the catalytic layer structure and composition of electrodes is of paramount importance to maximize fuel cell performance. In this work, we examine the preparation procedures for electrodes by catalyst-coated substrate to be used with a well-known commercial AAEM, Fumasep® FAA-3, and a commercial ionomer of the same nature (Fumion), both from Fumatech GmbH. The anion exchange procedure, the ionomer concentration in the catalytic layer and also the effect of membrane thickness, are investigated as they are very relevant parameters conditioning the cell behavior. The best power density was achieved upon ion exchange of the ionomer by submerging the electrodes in KCl (isopropyl alcohol/water solution) for at least one hour, two exchange steps, followed by treatment in KOH for 30 min. The optimum ionomer (Fumion) concentration was found to be close to 50 wt%, with a relatively narrow interval of functioning ionomer percentages. These results provide a practical guide for electrode preparation in AAEM-based fuel cell research.

In order to decrease the cost of direct methanol fuel cells (DMFCs), allowing the market penetration of these devices, it is necessary to reduce the costs of the components, in particular the cathode catalysts based on platinum. Another issue of these fuel cells is related to methanol crossover through the polymeric membrane, which creates a mixed potential at the cathode and decreases the overall efficiency. For this reason the development of methanol-tolerant cathode catalysts that are cheaper than Pt are of great importance in this field. In this chapter a short review analysis on Pt-free catalysts, mainly based on Pd and platinum group metal-free compounds with the general formula M–N–C (M being a transition metal such as Fe or Co) for the cathode, with good methanol-tolerance characteristics, has been carried out. Preparation procedures for the catalysts and physicochemical characteristics are reported and discussed in correlation with the DMFC performance.

It has recently been established that 1-octanethiol in the electrolyte can allow iron electrodes to be discharged at higher rates. However, the effect of thiol additives on the air electrode has not yet been studied. The effect of solvated thiols on the surface positive electrode reaction is of prime importance if these are to be used in an iron-air battery. This work shows that the air-electrode catalyst is poisoned by the presence of octanethiol, with the oxygen reduction overpotential at the air electrode increasing with time of exposure to the solution and increased 1-octanethiol concentration in the range 0–0.1 mol dm−3. Post-mortem XPS analyses were performed over the used air electrodes suggesting the adsorption of sulphur species over the catalyst surface, reducing its performance. Therefore, although sulphur-based additives may be suitable for nickel-iron batteries, they are not recommended for iron-air batteries except in concentrations well below 10 × 10−3 mol dm−3.

Nanocarbon materials present sp2-carbon domains skilled for electrochemical energy conversion or storage applications. In this work, we investigate graphene oxide nanofibers (GONFs) as a recent interesting carbon material class. This material combines the filamentous morphology of the starting carbon nanofibers (CNFs) and the interlayer spacing of graphene oxide, and exhibits a domain arrangement accessible for fast transport of electrons and ions. Reduced GONFs (RGONFs) present the partial removal of basal functional groups, resulting in higher mesoporosity, turbostratic stacking, and surface chemistry less restrictive for transport phenomena. Besides, the filament morphology minimizes the severe layer restacking shown in the reduction of conventional graphene oxide sheets. The influence of the reduction temperature (140–220 °C) on the electrochemical behaviour in aqueous 0.5 M H2SO4 of RGONFs is reported. RGONFs present an improved capacitance up to 16 times higher than GONFs, ascribed to the unique structure of RGONFs containing accessible turbostratic domains and restored electronic conductivity. Hydrothermal reduction at 140 °C results in the highest capacitance as evidenced by cyclic voltammetry and electrochemical impedance spectroscopy measurements (up to 137 F·g−1). Higher temperatures lead to the removal of sulphur groups and slightly thicker graphite domains, and consequently a decrease of the capacitance.

La nanotecnología permite diseñar materiales a escala nanométrica y sus aplicaciones. El desarrollo de dichas tecnologías está revolucionando diversos campos, como es la medicina, la electrónica, la ingeniería ambiental, los procesos de producción… y un largo etcétera. Es en la segunda mitad del siglo XX cuando se comienza a experimentar y teorizar sobre estos nanomateriales, pero en los últimos 25 años se ha producido un notable incremento de la actividad de investigación debido al desarrollo de potentes técnicas instrumentales de síntesis y caracterización. Sin embargo, la inmensa mayoría de la población está siendo ajena a esta revolución. Es por ello que desde el año 2016 se viene organizando el festival de Nanociencia y Nanotecnología 10alamenos9, en el que miles de personas interaccionan con las diferentes actividades que se realizan en torno a la nanociencia y la nanotecnología. El festival pretende, de una forma amena y emocionante, acercar a todos los públicos la escala nanométrica, sus efectos y cómo este conocimiento va a cambiar nuestras vidas a través de innumerables aplicaciones y productos. Es un evento de divulgación científica que llega cada año a un número mayor de personas, desde estudiantes a público en general. Se cumple así el objetivo de informar a la ciudadanía sobre los avances científicos que se están desarrollando, para comprender el papel fundamental que el conocimiento científico supone a nuestro modo y calidad de vida.

Carbonaceous materials have always played an important role in history of electrochemical processes, from the fabrication of solid electrodes to the development of porous matrixes in the conversion and storage of energy and chemicals. This book chapter provides a general overview of emerging carbon nanostructures potentially used in electrochemistry. One-dimensional carbon nanofilaments, such as carbon nanotubes and carbon nanofibers, two-dimensional graphene, and three-dimensional materials, such as carbon gels and ordered mesoporous carbons, are reviewed. The chapter focuses on the physicochemical properties and synthesis methodologies of these carbon nanostructures together with their application to cutting-edge technologies for electrochemical energy conversion and storage, such as fuel cells, electrolyzers, supercapacitors, or batteries. The major challenges and directions for future research are also discussed to guide the design of efficient carbon-based electrodes and catalysts for electrochemical devices.

Alkaline unitized regenerative fuel cells (URFCs) could well be one of the energy sources in the next future. This technology has the potential to provide power to portable devices, transportation and stationary sectors. The bifunctional oxygen electrode plays a central role in catalytic activity and stability due to its slow kinetics under extremely oxidizing or reducing environments. With the aim to solve the principal catalytic problems at the bifunctional electrocatalysts, carbonaceous materials including carbon nanofibers (CNFs), graphitized mesoporous carbon (gCMK-3) and hydrazine-reduced graphene oxide (H-rGO) were synthesized, characterized and tested toward the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) and compared to commercial carbon black (Vulcan), graphite, and glassy carbon (GC). Physicochemical characterization was conducted by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), elemental analysis and Raman spectroscopy, meanwhile rotating ring-disk electrode (RRDE) was employed to determine the electrochemical activity and stability. Main results indicate that CNFs can act as a feasible catalyst for URFC applications.

Ordered mesoporous carbon materials (OMC) present a developed porosity, making them promising candidates for electrochemical energy-conversion devices. However, OMC undergo electrochemical oxidation causing the degradation of electrodes and exhibit low electrical conductivities due to their amorphous carbon structure. To address these issues, OMC can be graphitized by thermal treatments. Herein, the electrooxidation behavior of different OMC, prior and upon graphitization, is studied by chronoamperometry in a highly acidic electrolyte (0.5 M H2SO4). Several oxidation potentials and temperatures are used to gain insights on the electrooxidation mechanism. Potential holding tests evidenced differences in the oxidation behavior of as-synthesized and graphitized materials. Graphitization results in a strong decrease of the oxidation charge from 93 to 7% depending on the carbon properties, applied potential and temperature. Temperature plays an important role on carbon degradation of not-graphitized OMC, with a strong electrochemical capacitance loss (90%–93%) after tests at 40 °C and 60 °C. Conversely, graphitized materials present a raise of the capacitance by the creation of oxygen-functionalities as confirmed by post-mortem X-ray photoelectron spectra analysis. Additionally, graphitized materials show an increase of the apparent activation energy associated to electrooxidation (7 times higher), indicative of their improved resistance.

Graphene oxide nanomaterials, specifically reduced few-layer graphene oxide (rFLGO) sheets and reduced graphene oxide quantum dots (rGOQD), were explored as active and cost-effective counter electrodes for dye-sensitized solar cells (DSSCs). The synthesis strategy adopted in this work for these materials is based on an economic and easily scalable route of exfoliation of multi-wall carbon nanotubes (Top-Down approach). This allows the easy tailoring of rFLGO/rGOQD size and oxidation degree for their use as bulk material in counter electrodes, having a significant influence on the photovoltaic performance and stability. Despite the slower kinetics of carbon for the reduction of triiodide compared to platinum, they are characterized by low cost and promising stability. The electrochemical performance of DSSC assembled with either rFLGO or rGOQD at the counter electrode is correlated to their most important physico-chemical features. The domain size of graphene sheets had an important effect on the performance at the counter electrode, with quantum dots performing 13% higher efficiency than rFLGO, correlated with a higher density of active sites. Both graphene nanostructures exhibited a better stability behavior upon electrochemical cycling and seasoning of the cells as compared to Pt counter electrode.

The municipal solid waste (MSW) management is posing enormous environmental problems. An attractive strategy for MSW valorisation is to convert the carbon part into syngas, which may be further transformed into bio-fuels. However, some issues must be addressed, such as the low yield and the presence of sulphur compounds, which may poison the involved catalysts. The development of new desulfurization methods based on solid regenerable sorbents that can operate at high temperatures is an attractive strategy to overcome these problems. Mn oxides have been used as sulphur trapping sorbents due to their stability and high kinetics [1-2]. However, Mn oxides present unfavourable thermodynamics and stable sulfates are formed during the regeneration stage, decreasing the efficiency. In this work, the addition of Cu and/or Zn oxides has been proposed to overcome these limitations in regenerable sorbents.

Electrocatalytic materials with high efficiency for the main reactions on bifunctional oxygen electrodes, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are necessary for the development of new emerging electrochemical energy storage and conversion devices. Although some noble metals such as Pt and metal oxides (RuO2, IrO2) possess excellent activity as electrocatalysts, their high cost and instability hinder large-scale applications. Tantalum-based materials have demonstrated to be active and robust electrocatalysts for the ORR in acidic environment1. They have an excellent electrochemical stability and become active for the oxygen reactions when the surface stoichiometry is tuned2. Carbon materials can act both as support and electron conductor phase. In this work, nanocomposites based on tantalum sub-oxides on carbon black have been investigated for the ORR and OER3.

Las energías renovables son actualmente el modo más sostenible y limpio de producción de energía. Sin embargo, su naturaleza intermitente es uno de los factores limitantes para su mayor expansión e implementación en las redes de suministro eléctrico. Las baterías recargables son óptimas candidatas para el almacenamiento de dicha energía debido a su elevada eficiencia, su escalabilidad y flexibilidad1, siendo las baterías de flujo redox o las de plomo-ácido las más empleadas, junto con las baterías de ion litio, utilizadas principalmente en vehículos y dispositivos eléctricos. En las últimas décadas, la investigación en dispositivos de almacenamiento de energía se centra en desarrollar baterías de nueva generación con mayor potencia, mayor densidad energética y cuyos materiales sean seguros, económicos y abundantes. Las baterías metal-aire (MABs) pertenecen a esta nueva generación de baterías: con densidades de energía teóricas entre 1000 y 11000 Wh/kg (frente a los 250 Wh/kg aprox. de las baterías de ion litio), menores costes, y el uso de materiales abundantes, reciclables y sin los problemas de seguridad del litio 2. Las MABs generan electricidad a través de una reacción redox entre el metal (electrodo negativo) y el oxígeno del aire (que reacciona en el electrodo positivo), que, al no estar directamente almacenado en la celda, hace estas baterías mucho más ligeras. Existen diversos tipos de MABs, según sea el metal que empleen en el electrodo negativo: Zn, Al, Mg, Li, Na o Fe. De entre todos ellos, el sistema Fe-aire es uno de los más atractivos. El Fe es el cuarto elemento más abundante de la corteza terrestre, se produce en todo el mundo, su producción posee la menor huella de carbono de todos los metales, y, además, es fácilmente reciclable. Las baterías hierro-aire (FAB), con un voltaje de celda de 1,28 V, presentan hasta 764 Wh/kg de densidad de energía, así como la reversibilidad de la electroquímica del hierro, la simplicidad del diseño, y una gran durabil

The design of electrocatalytic materials with high efficiency and without noble metals for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for the development of new emerging electrochemical energy storage and conversion devices [1]. Spent bleaching earth (SBE) is a worthless oil refinery waste material containing a notable percentage of residual oil. New applications are urgent such as converting SBE into catalysts, according to the circular economy concept. In this work SBE has been used as electrocatalytic component for the reversible oxygen electrode. SBE was chemically treated to obtain based-carbon compounds. Then, cobalt, iron and/or tungsten heteropolyacid were added to the carbon-based SBE compounds to obtain different composites. The textural, structural and surface chemistry properties of the composites materials were analysed along with electrochemical characterization in a three electrode cell in alkaline medium. Electrochemical results evidenced Co/SBE catalyst proved to be a valuable bifunctional competitor for ORR and OER showing activity in both reactions (Figure 1) and a better stability than Pt in accelerated tests.

Metal-air batteries (MABs) are foreseen as a new generation device aiming to replace Li-ion batteries (LIBs). MABs present a higher theoretical energy density, lower cost, environmental friendliness, and safety. In particular, iron-based materials are cheap and easily obtained, and so they can be used for both the negative electrode and the positive electrode in MABs [1]. Here, Fe-doped carbon xerogels were synthesized in order to be tested as both positive and negative electrodes of iron-air batteries.

The growing energy worldwide requirements need of suitable facilities developments that allow the clean energy production through effective ways. In this sense, electrochemical devices, such as metal-air batteries, electrolyzers, polymeric fuel cells and regenerative fuel cells, appear as the real option to solve these energetic issues. However, the searching of active and stable oxygen electrodes toward oxygen evolution (OER) and oxygen reduction reactions (ORR) become an important challenge in performance enhancement terms. In addition, the use of noble metals like Pt, Ir and/or Ru represents a great barrier for their insertion to the market. In this study, graphenic materials (GMs) obtained from graphene oxide reduction using hydrazine (HrGO) or thiourea (SN-rGO) as reducing agents, as well as carbonaceous materials including carbon nanofibers (HC-CNF) and mesoporous carbon (g-CMK3) were synthesized, characterized and tested toward OER/ORR and compared to commercial carbon black (Vulcan) and graphite. Physicochemical characterization was conducted by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), elemental analysis and Raman spectroscopy. Besides, the catalytic performance toward ORR and OER in alkaline medium (0.1 M NaOH) was carried out by the rotating ring-disk electrode (RRDE) technique. Figure 1 indicates a profound impact of the carbonaceous structure on OER and catalyst stability. Graphenic materials mainly suffer corrosion at higher potentials than 1.0 V vs. RHE, while graphitic structures promote the oxygen evolution in basic media. Nevertheless, a possible influence of remaining metal impurities from the HC-CNF synthesis must be considered in these results.

In the present work, Pd and PdFe nanoparticles supported on CNT with and without functionalization (CNT and CNTox) were used for Formic Acid Oxidation Reaction (FAOR) in acid media. Electrocatalysts were synthesized by the borohydride reduction method with 20 wt.% metal loading. The CNTs were synthesized by the methane catalytic decomposition, and subjected to an oxidation treatment with nitric acid, named as CNTox. Themorphology, composition and structural properties were studied by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX) and X-Ray Diffraction (XRD). The FAOR was evaluated in acid media in a conventional three-electrode cell by means of cyclic voltammetry and chronoamperometry. From the steady state current density, it was found that Pd and PdFe supported at CNTox allowed improving the catalytic activity in comparison with the nonoxidized support.

We report the electrochemical properties of a novel nanocarbon material obtained by chemical oxidation and ultrasound-assisted exfoliation of fishbone carbon nanofibers (CNF). The resulting material maintains its tubular morphology and presents a characteristic interlayer spacing of graphene oxide (above 0.75 nm). Thus, this is called graphene oxide nanofibers (GONF). The new rearrangement of the accessible sp2-carbon domains makes the GONF a potential alternative for electrochemical energy conversion/storage applications, showing a developed porosity and tuneable surface chemistry. The influence of the oxidation degree of GONF on its electrochemical behaviour in 0.5 M H2SO4 is reported. Cyclic voltammetry and electrochemical impedance spectroscopy evidenced a significant increase of the capacitance for GONF, being 4–27 times higher than that obtained for pristine CNF. An optimum in the capacitance (49.2 F g−1) was obtained using an oxidation ratio (OR = KMnO4/Sample) of 6 and 60 min of sonication. The latter is ascribed to the unique structure of this material containing both graphitic and graphene oxide domains. Higher OR or longer sonication times led to a partial loss of graphitic domains and higher contribution of micropores, which worsen the fast ion/electrolyte transport. Additionally, the optimized material exhibited an improved activity for the hydrogen evolution reaction.

[EN] Pd, PdCo and PdFe catalysts were prepared by the impregnation method, using sodium borohydride (NaBH4) as the reducing agent and a commercial carbon material (Vulcan XC-72R) as support. The synthesized electrocatalysts were tested for the formic acid oxidation reaction (FAOR) in acid medium (H2SO4 0.5 M). The morphology, composition and particle size of the electrocatalysts were characterized by physicochemical techniques as X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and transmission electron microscopy (TEM). The catalytic activity was evaluated by cyclic voltammetry (CV) and chronoamperometry techniques (CA)., [ES] Catalizadores de Pd, PdFe y PdCo fueron preparados por el método de impregnación, usando borohidruro de sodio (NaBH4) como agente reductor y un material de carbono comercial, (Vulcan XC-72R) como soporte. Los electrocatalizadores sintetizados fueron probados para la reacción de oxidación de ácido fórmico (FAOR, por sus siglas en inglés) en medio ácido (H2SO4 0,5 M). La morfología, la composición y el tamaño de partícula de los electrocatalizadores se caracterizaron mediante técnicas físico-químicas como difracción de rayos X (XRD), microscopía electrónica de barrido con espectroscopia por energía dispersiva (SEM-EDS) y microscopia electrónica de transmisión (TEM). La actividad catalítica se evaluó mediante técnicas de voltamperometría cíclica (CV) y cronoamperometría (CA).

Oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts play an important role in energy conversion and storage devices, but highly active and robust nonprecious metal catalysts are required to address the cost and durability issues. In this work, a modified gel methodology has been used to obtain nanocomposites of perovskite CoTiO3 and N-doped reduced graphene oxide (NrGO). Moreover, the influence of the annealing extent in the synthesis has been studied. The composites show higher activity in oxygen evolution and reduction reactions in alkaline environment compared to sole ilmenite, which is related to a strong interaction between cobalt titanate and NrGO carbon matrix. The annealing duration appears as a key variable to modulate the physicochemical properties and the electrochemical behavior. The composite prepared at the largest duration (3 h) exhibit enhanced bifunctional properties for ORR and OER, with onset potentials of 0.93 V and 1.53 V vs. RHE respectively, which is mainly attributed to higher concentration of nitrogen, larger extent of defects and/or the presence of more oxidized Co species in the nanocomposite.

The World Conference on Carbon (Carbon 2018) was held on July 1–6, 2018, in Madrid, Spain (http: https://carbon2018.org/) organized by the Spanish Carbon Group. This Conference has been held for more than six decades, since the first celebrated in 1953 in Buffalo (USA). The Carbon 2018 has provided an opportunity, for a wide number of international experts, to discuss about the innovative and comprehensive overview of the latest research developments on carbon science and applications

Nitrogen doped reduced graphene oxide (NrGO) catalysts and composites of these with carbon nitride (CN) with nitrogen content as high as 19.8 wt% and 35.8 wt%, respectively, have been synthesized by thermal annealing of multilayer graphene oxide and urea at different temperatures. Nitrogen has been effectively introduced as a mix of pyridinic, pyrrolic, pyridonic and quaternary bonding configurations into the carbon lattice of graphene upon removal of CN phase over 650 °C. The electrocatalytic activity towards oxygen reduction reaction (ORR) in 0.1 M NaOH has been investigated. NrGO catalysts with less nitrogen content show better performance than composites. NrGO produced at 800 °C presents the best activity with half-wave potential of 0.76 V vs. RHE and a mean number of electrons of 3.7, which are close to a commercial Pt/C catalyst. This study concludes that a high concentration of nitrogen atoms in multilayer reduced graphene oxide does not necessarily produce a highly active electrocatalyst, suggesting that carbon in the neighbourhood of nitrogen atoms are the active sites for ORR in alkaline medium.

In the present work, the effect of different composites as electrocatalytic supports for the methanol oxidation reaction is evaluated. Two different TiCN-rGO composites, using organic compounds as linkers were synthesized. Subsequently, platinum was incorporated in the form of nanoparticles, and the electrocatalysts were characterized and compared with similar nanoparticles supported on TiCN. The electrocatalysts were characterized by XPS, TEM, XRD, Raman and FTIR spectroscopies and ICP-MS and the catalytic performance toward COads and methanol oxidations was evaluated. Results showed that the platinum nanoparticles were preferentially deposited on top of the TiCN particles rather than on the reduced graphene oxide, which allowed the nanoparticles to benefit from the TiCN electronic promoting effect. In addition, the presence of reduced graphene oxide contributes on the improvement of the morphological properties of the electrocatalyst, reducing the agglomeration degree of the particles and considerably enhancing the catalytic mass activities.

Bifunctional catalysts for oxygen electrodes, active for both the oxygen reduction and evolution reactions (ORR, OER), are highly desirable for the development of electrochemical energy conversion devices. In this work, carbon-supported tantalum based catalysts were synthesized by a microemulsion procedure from tantalum ethoxide through a fast hydrolysis reaction followed by heat treatment. The nature and composition of the different tantalates and oxides (general formula NaaTabOc, with a/b = 0–1 and c/b = 2.5–3) relied on the concentration of ethanol as well as on the annealing temperature, with Na2Ta8O21 as predominant crystal phase in all cases. High annealing temperatures resulted in a certain degree of oxygen substoichiometry, which creates defects responsible of electrocatalytic activity. The electrochemical response for the oxygen reduction and the oxygen evolution reactions in alkaline electrolyte (0.1 M NaOH) was evaluated for the first time in this type of materials. The presence of Na2Ta8O21 phase over other oxides/tantalates, a high crystallinity of such Ta-phase, and the oxygen substoichiometry are the three parameters playing a relevant role to favor ORR and OER electro-activity. The best formulations were found in a trade-off situation using 3% ethanol and annealing at 900 °C.

Actividad institucional divulgativa celebrada durante la "Jornada sobre la Historia de la EEAD-CSIC" realizada el 7 de junio en el Salón de Actos del Instituto Agronómico de Zaragoza perteneciente al Centro de Altos Estudios Mediterráneos (CIHEAM-IAMZ). Forma parte, junto con otras actividades divulgativas de naturaleza histórica, del II Ciclo de Exposiciones y conferencias del 75 Aniversario EEAD-CSIC (junio 2019), dentro del programa general de actividades organizadas con motivo del 75 Aniversario de la EEAD-CSIC. La conferencia, finalmente, se impartió por la autora con otro título: "Cereales, cromosomas y colchicina: Aula Dei y la citogenética en la década de 1950".

Ordered mesoporous carbon materials (OMC), obtained by the template carbonization pathway from silica templates (sacrificial method), exhibit promising features for many applications mainly due to their large surface area. Graphitization is a common approach to improve the electrical conductivity and degradation resistance. We have investigated the graphitization of two different OMC at 1500 °C and the electrochemical oxidation by potential holding at 1.4 V vs. reversible hydrogen electrode (RHE) in acidic electrolyte. Graphitization conducts to a significant reduction of the electrooxidation associated charge, between 50 and 90% depending on the carbon properties, together with a decrease of surface area of 35–48%. The materials still exhibit a large electrochemical surface area, according to electrochemical impedance and cyclic voltammetry experiments. Upon electrooxidation, the relative amount of oxygen increases according to a comparative analysis of X-ray photoelectron spectroscopy test of the electrodes before and after potential holding. The results are of interest to define strategies towards the amelioration of carbon degradation by electrooxidation in electrochemical devices.

Carbon nanofibers (CNFs) have shown great potential in multiple applications. Their versatility is derived from the possibility of tuning their physical and chemical properties. CNFs can be synthesized using two main methods: the catalytic decomposition of carbon precursors or the electrospinning and carbonization of polymers. The most appropriate method relies on the desired characteristics of the CNFs. Some of their applications include the synthesis of catalysts and catalytic supports, as electrodes for fuel cell devices, in hydrogen storage systems, and in functional nanocomposites. In this review, recent advances in the synthesis and potential applications of CNFs are examined.

The electrochemical oxidation of carbon represents a relevant limitation for the application of carbon-based materials at the electrodes of several electrochemical devices, like fuel cells, metal-air batteries, water electrolyzers or supercapacitors. Understanding the key influencing parameters is thus of paramount importance to prevent and avoid the degradation of carbon materials. In the present investigation, carbon xerogels (CXGs) as a model of synthetic amorphous carbon, were studied on the basis of different surface chemistry and structure. Electro-oxidation experiments consisted of potential holding (1.2 V vs. RHE) on electrodes prepared with CXGs. The results indicate that the presence of oxygen groups (-C-O) and a higher ordering degree hinder the carbon oxidation reaction. A sub-structural parameter considering both the amount of oxygen-free carbon atoms (C-C + C*-C-O) from X-ray photoelectron spectroscopy, and the disordering degree from Raman spectra (in terms of ID/IG) is here proposed as a new factor to evaluate the tendency of an amorphous carbon material, like xerogels, to be electrochemically oxidized.

This work reports a study of the electrical conductivity under compression of nanostructured carbon powders with different physicochemical properties (texture, chemical-surface and morphology): a commercial carbon black (Vulcan XC-72R) and synthesized ordered mesoporous carbon (OMC), carbon nanocoils (CNC) and carbon nanofibers (CNF). The electrical conductivity was determined from the sample resistance under compaction (from 0.5 to 140 MPa) using the four-probe technique. A comparison of intrinsic and grain electrical conductivities (calculated according to the percolation theory and the general effective media approximation) was performed. Additionally, samples were subjected to chemical oxidation or graphitization treatments to evaluate the effect of oxygen content and structural properties on the electrical conductivity. In spite of the physicochemical differences of carbon materials, an exponential decrease of the electrical conductivity with the oxygen amount was stated. Finally, thermal treatment of OMC at 1500 °C led to a surprising increase in its conductivity due to the graphitization of the amorphous carbon structure of the original material and the oxygen removal.

Differential electrochemical mass spectrometry (DEMS) was used to establish the difference in surface acidity in sulfuric acid solution between commercially available carbon black (Vulcan) and Vulcan which had been chemically modified with concentrated nitric acid (VulcanO). The mass signal for the m/z = 2 associated with H2 formation was observed to be influenced by functional groups at the carbon surfaces. Analysis of the mass spectra with the help of the Nernst equation allowed us to quantitatively find the difference in surface pH between the two carbon-based materials in acid media. The elevated concentration of functional groups at the VulcanO surface results in a decrease in the local acidity of the surface under these conditions (0.5 M H2SO4), and consequently the hydrogen evolution reaction (HER) is delayed.

The effect of chemical surface oxidation of the carbon black Vulcan XC-72R on the capacitance, tolerance to corrosion and electrochemical activity toward the hydrogen evolution reaction (HER) has been studied in 0.1 M NaHCO3. Acid treatments with HNO3 or a HNO3H2SO4 mixture resulted in a progressive introduction of oxygen-containing groups which led to a strong increase of the capacitance and a higher tendency to carbon corrosion. In contrast, an inhibition of the hydrogen evolution current (at potentials more negative than −1.0 V) was observed for oxidized samples in comparison to the un-treated material. HER was also tested in the presence of dissolved CO2 to study the influence of the surface chemistry on the CO2 electroreduction. An inhibition of the H2 evolution current was evidenced in the CO2 saturated electrolyte due to the adsorption of species from CO2 reduction. A strong hydrogen current decrease (65–78%), and thus a higher tendency to adsorb (CO2)red species, was obtained on the original Vulcan and the HNO3-treated samples, in comparison to the carbon oxidized with HNO3H2SO4 (15%), which could be related to the highest content of basic groups of the last carbon.

Chemically tuned inorganic–organic hybrid halide perovskites based on bromide and chloride anions CH3NH3Pb(Br1–xClx)3 have been crystallized and investigated by synchrotron X-ray diffraction (SXRD), scanning electron microscopy, and UV–vis spectroscopy. CH3NH3PbBr3 and CH3NH3PbCl3 experience successive phase transitions upon cooling, which are suppressed for intermediate compositions probably due to compositional disorder. For CH3NH3PbCl3, a transient phase, formerly described as tetragonal, was identified at 167.5 K; the analysis of SXRD data demonstrated that it is indeed orthorhombic, with space group Pnma, and a ≈ √2ap; b ≈ 2ap; c ≈ √2ap (ap is the ideal cubic perovskite unit-cell parameter). The band gap engineering brought about by the chemical management of CH3NH3Pb(Br1–xClx)3 perovskites can be controllably tuned: the gap progressively increases with the concentration of Cl ions from 2.2 to 2.9 eV, and shows a concomitant variation with the unit-cell parameters of the cubic phases at 295 K. This study provides an improved understanding of the structural and optical properties of the mixed CH3NH3Pb(Br1–xClx)3 perovskites.

The electrochemical reduction of CO2 represents a promising alternative for obtaining a variety of useful products, with important applications in chemical industry or for power generation. However, finding an effective electrode for the activation and further reduction of CO2 continues being a key challenge. In addition, the reaction mechanism of the process and the involved intermediates are not fully clear as most works analyze the electrolysis products after CO2 reduction reaction, sampling the compounds in gas collectors or liquid solvents. With the main aim of generating knowledge, in situ techniques have been also used for the identification of the electrolysis products from CO2 reduction. Although these techniques are powerful tools for the elucidation of the reaction pathways, they are not commercial or recently introduced in the market and not-easy to use and, hence, these methods have not been widely employed. In the current chapter a review of the most commonly in situ techniques used for studying the CO2 electroreduction is addressed, paying special attention to the electrodes involved in the reaction. Furthermore, an explanation of the main experimental specifications is also described for an easier understanding of the method and a comparison of the in situ techniques, summarizing their advantages and disadvantages, is displayed.

Developing efficient, durable, and low cost catalysts based on earth-abundant elements for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for renewable energy conversion and energy storage devices. We report herein a simple one-pot procedure for the synthesis of non-precious metals N-doped graphene composites employing urea as nitrogen source, and their application as bifunctional catalysts for both the ORR and OER in alkaline environment. In this study, the effects of the addition of Ti and Co on the structure and performance of the N-doped graphene composites are investigated. The incorporation of Ti leads to a composite with both anatase and rutile TiO2 crystalline phases as well as Ti3+ species stabilized upon hybridization with N-doped reduced graphene oxide. The ORR onset potential for the Ti-based composite is 0.85 V (vs. RHE) and the number of electrons transferred is 3.5, showing superior stability than Pt/C after accelerated potential cycling in alkaline solution. However, this composite shows low activity and stability for the OER. On the other hand, the composite consisting of metallic Co and Co3O4 nanocrystals grown on N-doped reduced graphene oxide exhibits the best performance as bifunctional catalyst, with ORR and OER onset potentials of 0.95 V and 1.51 V (vs. RHE), respectively, and a number of electrons transferred of 3.6 (ORR). The results reveal the presence of important structural features such as metallic Co as the predominant crystalline component, amorphous Co3O4 phase and the coordination of Co-N-doped graphene. All of them seem to be fundamental for the high activity and stability towards ORR and OER.

Nanocatalyst performance towards the ethanol oxidation reaction (EOR) for fuel cell applications can be enhanced by exploiting the benefits of structural and compositional sensitivity and control. We describe the synthesis and characterization of cubic Pt-Sn alloyed nanoparticles consisting of (100) faceted Pt-Sn nanoparticles with a core-shell structure, with a Pt-rich core and a Sn-rich shell. Stability tests indicated that this catalyst preserves its morphology and remains well-dispersed on the carbon support after 5,000 potential cycles, while a cubic (pure) Pt catalyst exhibited severe agglomeration of the nanoparticles after the same stability testing protocol. Analysis of the elemental distribution in the nanoparticles by STEM-EELS indicated that Sn dissolves from the outer part of the shell after potential cycling, forming a ~0.5 nm Pt skin. Finally, the activity of this bimetallic core-shell nanostructured electrocatalyst towards the EOR was investigated, exhibiting currents about three times higher than those of unshaped Pt-Sn nanoparticles and six times higher than Pt nanocubes.

The mechanism of CO and HCOOH electrooxidation in an acidic solution on carbon-supported Au–Pd core–shell nanoparticles was investigated by differential electrochemical mass spectrometry and in situ Fourier transform infrared (FTIR) spectroscopy. Analysis performed in nanostructures with 1.3 ± 0.1 nm (CS1) and 9.9 ± 1.1 nm (CS10) Pd shells provides compelling evidence that the mechanism of adsorbed CO (COads) oxidation is affected by structural and electronic effects introduced by the Au cores. In the case of CS10, a band associated with adsorbed OH species (OHads) is observed in the potential range of CO oxidation. This feature is not detected in the case of CS1, suggesting that the reaction follows an alternative mechanism involving COOHads species. The faradaic charge associated with COads oxidation as well as the Stark slope measured from FTIR indicates that the overall affinity and orbital coupling of CO to Pd are weaker for CS1 shells. FTIR spectroscopy also revealed the presence of HCOOads intermediate species only in the case of CS1. This observation allowed us to conclude that the higher activity of CS10 toward this reaction is due to a fast HCOOads oxidation step, probably involving OHads, to generate CO2. Density functional theory calculations are used to estimate the contributions of the so-called ligand and strain effects on the local density of states of the Pd d-band. The calculations strongly suggest that the key parameter contributing to the change in mechanism is the effective lattice strain.

In the present work, a wide variety of electrocatalysts, involving different metals, synthesis routes and oxidation states, have been prepared and the response towards the CO2 reduction on carbon-supported established. In particular, catalysts based on non-noble metals (Cu, Co, Fe and Ni) supported on carbon black (Vulcan XC-72R) were tested in the electrochemical reduction of CO2. Amorphous oxides of magnetite/maghemite and crystalline CuO and Cu2O oxides obtained by different methodologies were also studied. Previously, the efficiency of the catalysts for the hydrogen formation was evaluated in 0.1 M NaHCO3 since it takes place as a competitive reaction during CO2 reduction in aqueous electrolytes. By comparison of the voltammograms in the absence and presence of CO2, an inhibition of the hydrogen evolution was showed for all (metal and oxide-based) catalysts due to the adsorption of species from CO2 reduction (CO2,red). Interestingly, the CO2,red formation on Co and Fe-surfaces operating under these conditions has not been stated before. A strong current decrease, and consequently, a higher adsorbate amount, was observed on copper-oxides (75–85%) vs. the metal catalyst (45%).

This work reports a study of the electrocatalytic properties and stability of nanostructured carbon electrodes by on-line differential electrochemical mass spectrometry (DEMS). A wide electrochemical characterization in 0.1 M NaHCO3 involving anodic (carbon corrosion and oxygen evolution reaction, OER) and cathodic (CO2 reduction and hydrogen evolution reaction, HER) key reactions in energy-conversion devices was performed. DEMS studies showed that the faradaic current from 1.3 to 1.6 V vs. RHE of carbons was only associated to CO2 formation by corrosion and not to OER, the stability being improved by the graphitic character of carbon. H2 evolution was also enhanced with graphitic carbon nanofilaments, even though metal traces might positively influence their catalytic activity. By comparison of the faradaic currents and the H2 formation signals, in the absence and presence of CO2, a high inhibition of the HER was established for all carbon electrodes due to the species adsorption from CO2 reduction.

The electrooxidation of ethanol and carbon monoxide on Pt nanoparticles (NPs) supported on carbon nanofibers (CNFs) and carbon blacks (oxidized and non-oxidized) is studied. Quite similar characteristics are found for Pt NPs, including particle size and crystal phases, regardless of the nature of carbon support. Cyclic voltammetries and current-transient curves show higher ethanol oxidation current density for Pt/CNF than for Pt/Vulcan. In situ spectroelectrochemical techniques including Fourier transform infrared spectroscopy (FTIR) and differential electrochemical mass spectrometry (DEMS), are employed in order to identify adsorbed reaction intermediates and products. Pt/CNF results in a lower CO poisoning of the NP surface compared to Pt/Vulcan, allowing higher amount of free Pt active sites for the oxidation of ethanol which leads to the formation of acetic acid during the reaction.

Durability and limited catalytic activity are key impediments to the commercialization of polymer electrolyte fuel cells. Carbon materials employed as catalyst support can be doped with different heteroatoms, like nitrogen, to improve both catalytic activity and durability. Carbon xerogels are nanoporous carbons that can be easily synthesized in order to obtain N-doped materials. In the present work, we introduced melamine as a carbon xerogel precursor together with resorcinol for an effective in-situ N doping (3–4 wt % N). Pt nanoparticles were supported on nitrogen-doped carbon xerogels and their activity for the oxygen reduction reaction (ORR) was evaluated in acid media along with their stability. Results provide new evidences of the type of N groups aiding the activity of Pt for the ORR and of a remarkable stability for N-doped carbon-supported Pt catalysts, providing appropriate physico-chemical features.

Low temperature fuel cells technologies are currently shifting very fast from fundamental research to real growth. The development of electrocatalysts plays a vital role in the electrocatalytic reactions involved in these devices, because the catalyst determines the overall reaction efficiency, durability and cost. This article review progress in the research and development of electrocatalysts for low temperature fuel cells technologies, with especial attention in the contribution of our research teams over the last 15 years or so. The intensive research efforts in reducing or replacing Pt-based electrodes in fuel cells have been focus in the use of carbon nanomaterials as electrocatalytic supports, including carbon nanostructures tailored by surface modification or building in particular dopants/defects. Recent research effort has also led to the use of electronic conductivity noncarbon support materials. In addition, carbon-composite materials are proving to be a robust, inexpensive and active electrocatalysts, where the synergetic effect between the carbon nanomaterials and the ceramic or polymer nanostructures can lead to a superior electrocatalytic performance and durability for low temperature fuel cells. Perspectives on these catalysts and possible pathways to address current remaining challenges are also discussed.

Iron and cobalt materials have attracted enormous interest as low-cost alternatives to noble-metal catalysts able to catalyse oxygen reduction reaction. Here, the effect of the heteropolyacid H3PMo12O40 (HPMo) into the structure of new composites formed by Fe, Co and reduced-graphene oxide/chitosan (rGOCS) has been investigated. Chitosan was used as a nitrogen precursor for nanocarbon, while reduced graphene oxide was introduced to tune the electrical conductivity. The physicochemical properties of electrocatalysts were performed by Raman, XRD, XPS and TEM. Activity toward ORR was carried out in a three-electrode electrochemical cell using a rotating disk electrode (RDE). The molybdophosphoric acid incorporation into the structure of Fe/rGOCS composites resulted in an increase of the activity of the oxygen reduction reaction in alkaline medium. Furthermore, the replacement of iron by cobalt yielded in a great improvement of the activity and stability, which may open new avenues for the design of nanomaterials utilizing HPMo/reduced-graphene oxide/chitosan composites.

In this work, Pd–Ni catalysts supported on carbon nanofibers were synthesized, with metal contents and Pd:Ni atomic ratios close to 25 wt.% and 1:2, respectively. Previously, the carbon nanofibers were chemically treated, in order to create surface oxygen and/or nitrogen groups. The synthesized catalysts displayed low crystallinity degree and high dispersion on carbon supports, especially in those with surface functional groups. Oxygen reduction reaction (ORR) was studied by rotating ring-disk electrode (RRDE) techniques. When the kinetic current is normalized by the mass of Pd present in the electrode, higher activities were obtained for the synthesized materials in comparison with the activity observed for a commercial Pd/C E-TEK catalyst. Some differences are reported for the different materials under study, mainly dependent on the presence of oxygen surface groups on the carbon support. In light of the results, we can propose the synthesized catalysts as possible candidates for cathodes in alkaline direct methanol fuel cells.

Graphitized ordered mesoporous carbon CMK-3-R8 supported PtCrCo electrocatalysts were synthetized by the borohydride reduction method (BM) and then submitted to a reductive thermal treatment (TT) at 300, 500 and 700 °C with the aim to improve their catalytic activity toward the oxygen reduction reaction (ORR) and the methanol tolerance. Atomic composition and total metal loading were determined by energy dispersive X-ray (EDX) analysis and the values obtained were close to the nominal ones, i.e. 20 wt.% PtCrCO/CMK-3. Electrochemical measurements were carried out using rotating disk electrode (RDE) technique in oxygen-saturated sulfuric acid solution in absence and presence of dissolved methanol. Besides other species and parameters, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis revealed the formation of an ordered PtCr alloy phase and the surface enrichment by CrO species with the rise of the annealing temperature. The last, in conjunction with the absence of Pt sites with (110) orientation, enhances the catalytic activity toward the ORR and the methanol tolerance. Kinetic parameters are reported and analyzed in terms of the physicochemical properties of the catalysts obtained by the X-ray techniques.

The graphitisation of ordered mesoporous carbons (CMK-3) was carried out by thermal treatments under different conditions of temperature and heating rate. The electrochemical characterization in acidic medium of the graphitised CMK-3 showed that such a thermal treatment is effective to decrease the carbon oxidation rate (corrosion) while preserving a good porosity in terms of capacitance. Besides, the graphitisation degree and, in consequence the electrochemical corrosion resistance, can be modulated by an appropriate choice of thermal treatment conditions, where the heating rate and temperature appear as critical parameters. Under certain graphitisation conditions, the carbon corrosion of CMK-3 can be minimized showing lower rates compared to a commercial carbon black (Vulcan XC-72R). Palladium nanoparticles were supported on the most promising graphitised CMK-3 and Vulcan using a method based on impregnation and reduction with borohydride, resulting in suitable metal crystallite sizes. The electrocatalytic activity towards the oxidation of carbon monoxide and formic acid were assessed in aqueous sulphuric acid electrolyte for application at the anode of direct formic acid fuel cells (DFAFCs). The best activity results were obtained for the Pd catalyst supported on a graphitised CMK-3.

Pt-Sn catalysts supported on carbon, with different Pt-Sn atomic ratios (3:1, 1:1, 1:3), were prepared through the formic acid method (FAM) and supported on different carbon supports (nanofibers and carbon blacks) to study their behavior toward CO stripping and ethanol oxidation reaction (EOR). PtSn/C catalysts and supports were physicochemically characterized by XRD, EDX, XPS, TEM and TPD. Good particle dispersion onto the carbon support, similar particle sizes (around 4-5 nm) and the presence of tin oxides were observed in all cases. Sn insertion favored the development of the Pt3Sn1 phase and the presence of higher oxidation states of both metals in the catalyst. Ethanol and adsorbed CO oxidation was studied at these materials both in acid and alkaline media, by linear sweep voltammetry and chronoamperometry. Higher EOR current densities were obtained with the increase in the amount of Sn in the samples, been those materials supported on CNF the ones with the best CO tolerance and catalytic activity toward this reaction. As expected, a great improvement of the EOR activity was found in alkaline media compare with sulfuric acid media, providing good expectative for these materials as catalysts for alkaline direct ethanol fuel cells.

Palladium–nickel catalysts supported on carbon blacks containing similar metal contents (25 wt. %) and Pd:Ni atomic ratios (1:1 and 1:2) were synthesized. Carbon supports were chemically treated to create different oxygen and nitrogen groups on surface. X ray patterns revealed a weak alloying between Pd and Ni, whereas crystallite sizes were between 2.1 and 3.2 nm, being consistent with the values detected by transmission electron microscopy (TEM). CO strippings demonstrated a higher poisoning tolerance of the Pd–Ni catalysts than that observed for a Pd catalyst supported on Vulcan carbon black. The methanol oxidation on Pd–Ni catalysts suggested that presence of Ni increase the activity of the materials, considering that Pd/CB exhibited the lowest methanol oxidation current densities. Nevertheless, no effects associated to the presence of surface functional groups on carbon supports were observed in the performance of this reaction.