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1. XPS analysis.-- 2. X-ray diffraction and scanning electron microscopy.-- 3. Peak Current Analysis.-- 4. Cyclic Voltammetry in Acid Solution.-- 5. Randles-Sevčik Analysis.-- 6. Cell Voltage vs. Time Plots

An increasing number of studies focus on organic flow batteries (OFBs) as possible substitutes for the vanadium flow battery (VFB), featuring anthraquinone derivatives, such as anthraquinone-2,7-disulfonic acid (2,7-AQDS). VFBs have been postulated as a promising energy storage technology. However, the fluctuating cost of vanadium minerals and risky supply chains have hampered their implementation, while OFBs could be prepared from renewable raw materials. A critical component of flow batteries is the electrode material, which can determine the power density and energy efficiency. Yet, and in contrast to VFBs, studies on electrodes tailored for OFBs are scarce. Hence, in this work, we propose the modification of commercial carbon felts with reduced graphene oxide (rGO) and poly(ethylene glycol) for the 2,7-AQDS redox couple and to preliminarily assess its effects on the efficiency of a 2,7-AQDS/ferrocyanide flow battery. Results are compared to those of a VFB to evaluate if the benefits of the modification are transferable to OFBs. The modification of carbon felts with surface oxygen groups introduced by the presence of rGO enhanced both its hydrophilicity and surface area, favoring the catalytic activity toward VFB and OFB reactions. The results are promising, given the improved behavior of the modified electrodes. Parallels are established between the electrodes of both FB technologies.

El creciente aumento de las emisiones de CO2 y sus consecuencias ha desembocado en el desarrollo de nuevas tecnologías para paliar esta situación. Una de ellas es la electroreducción de CO2 (CO2RR), basada en la valorización del CO2 en combustibles y productos químicos de interés para la industria. Su conversión contribuye a la descarbonización del sistema energético al conseguir darle una utilidad al exceso de CO2 mientras facilita el almacenamiento de energía procedente de fuentes renovables. Para llevar a cabo la CO2RR es necesario salvar una serie de barreras tanto cinéticas como termodinámicas. El CO2 es una molécula muy estable que puede reducirse a varios compuestos. A ello hay que añadirle la existencia de una reacción competitiva, la evolución de hidrógeno. Por ende, el diseño de un catalizador activo y selectivo hacia la CO2RR puede resultar un desafío. Los catalizadores basados en cobre presentan un gran potencial ya que este metal exhibe una actividad única para la formación de hidrocarburos y alcoholes [1]. En los últimos años se han desarrollado nuevas formulaciones catalíticas basadas en dispersar átomos metálicos no nobles en una matriz de carbono dopada con N formando centros activos M-Nx. Mediante esta configuración se logra una elevada afinidad para la reducción de CO2 a bajos sobrepotenciales [2]. En el presente trabajo se ha desarrollado un conjunto de catalizadores Cu-N-C basados en materiales de carbono obtenidos a partir de diferentes precursores biomásicos codopados con nitrógeno y cobre. El comportamiento electrocatalítico para la CO2RR se ha evaluado empleando un electrodo de disco-anillo rotatorio (RRDE), una técnica poco explorada para esta reacción.

El desarrollo de la tecnología de producción de energía eléctrica a partir de fuentes renovables para reducir la emisión de gases de efecto invernadero, implica un sistema de almacenamiento acorde a sus prestaciones que evite problemas de estabilidad en la red eléctrica. Las baterías de flujo redox de vanadio (BFRV) son muy prometedoras en almacenamiento de energía debido a la rapidez de carga, gran capacidad y seguridad en la operación. Los electrodos de la BFRV, donde ocurren las reacciones químicas, juegan un papel crucial para aumentar el rendimiento del sistema y disminuir costes. Los materiales que se utilizan comercialmente como electrodos son fieltros de carbono, que se pueden modificar con diferentes estructuras metálicas, carbonosas u orgánicas para optimizar su comportamiento [1]. Diversos estudios en literatura han demostrado una mayor actividad y eficiencia de fieltros modificados con estructuras carbonosas. No obstante, los estudios en batería de flujo completa no son tantos. Por ello, en este trabajo se han investigado fieltros de carbono modificados con nanofibras de carbono y con óxido de grafeno, como electrodos de una batería de flujo redox de vanadio de 25 cm2

La reducción electroquímica de CO2 (CO2RR) destaca como una solución prometedora para la mitigación del cambio climático. Sin embargo, esta reacción presenta desafíos debido a la alta estabilidad de la molécula de CO2 y la difícil selectividad hacia diferentes productos, así como la competencia con la reacción de evolución de hidrógeno, haciendo necesario el desarrollo de catalizadores activos y selectivos. Actualmente, los catalizadores atómicamente dispersos se posicionan como una solución prometedora para modular la selectividad de esta reacción hacia CO. Entre ellos, los catalizadores M-N-C presentan sitios activos basados en átomos de un metal de transición coordinado con nitrógeno en una matriz carbonosa, lo que permite obtener catalizadores con alta porosidad, buena conductividad eléctrica y bajo coste. En cuanto a la identificación de los productos, si bien técnicas exsitu (cromatografía, espectrometría de masas), son comúnmente utilizadas, estas presentan ciertas limitaciones en términos de desfase temporal y detección de productos en concentraciones muy bajas. Para abordar este inconveniente, el uso de un electrodo rotatorio de disco anillo (RRDE) se propone como una alternativa rápida y factible para la identificación de los productos generados durante la CO2RR. En este método, los catalizadores se depositan en el disco del electrodo, donde tiene lugar la CO2RR, mientras que los productos generados son simultáneamente oxidados en el anillo. Este trabajo presenta el estudio de catalizadores Cu-N-C basados en una matriz carbonosa de xerogel de carbono dopado con nitrógeno. Se analizó la influencia del precursor de cobre y de la concentración de metal durante la síntesis de los catalizadores en su comportamiento electroquímico para la CO2RR mediante el uso de RRDE.

The electrochemical CO2 reduction reaction (CO2RR) to controllable chemicals is considered as a promising pathway to store intermittent renewable energy. Herein, a set of catalysts based on copper-nitrogen-doped carbon xerogel (Cu-N-C) are successfully developed varying the copper amount and the nature of the copper precursor, for the efficient CO2RR. The electrocatalytic performance of Cu-N-C materials is assessed by a rotating ring-disc electrode (RRDE), technique still rarely explored for CO2RR. For comparison, products are also characterized by online gas chromatography in a H-cell. The as-synthesized Cu-N-C catalysts are found to be active and highly CO selective at low overpotentials (from −0.6 to −0.8 V vs. RHE) in 0.1 M KHCO3, while H2 from the competitive water reduction appears at larger overpotentials (−0.9 V vs. RHE). The optimum copper acetate-derived catalyst containing Cu-N4 moieties exhibits a CO2-to-CO turnover frequency of 997 h−1 at −0.9 V vs. RHE with a H2/CO ratio of 1.8. These results demonstrate that RRDE configuration can be used as a feasible approach for identifying electrolysis products from CO2RR.

Platinum-group-metal free catalysts have been widely studied in the last decade as an alternative to Pt, employed in fuel cells. Several carbon precursors have been investigated as carbon matrix for iron-nitrogen-carbon (Fe–N–C) catalysts, characterized by a large amount of micropores, fundamental to create Fe-Nx-C active sites. Nonetheless, it is also acknowledged that wider pores are needed to facilitate mass transfer of oxygen/water (reagent/product of fuel cells) towards/from the active sites. Organic xerogels are known as easily tunable materials, that allow doping with a variety of heteroatoms. The present manuscript presents an investigation on the use of organic xerogels combined with iron and nitrogen in a single-step to obtain Fe–N-CXG electro-catalysts for the oxygen reduction reaction (ORR) of proton exchange membrane fuel cells. Several features of both organic xerogels and catalysts on the ORR activity are assessed: the textural properties of the organic xerogel, the iron loading of the Fe–N–C catalysts and the effect of acid-leaching of the catalysts. The present study proves the feasibility of using organic xerogels as carbon precursor to obtain PGM-free catalysts in an easy manner and scalable single-step synthesis. Catalysts obtained from organic xerogels synthesized at mildly acid pHs (5.8 and 6), with a nominal iron loading of 2 wt%, and subjected to two sets of acid leaching/thermal treatments, present enhanced catalytic activity towards the ORR.

Iron-air batteries hold the potential to be a key technology for energy storage, thanks to their energy density, low cost, safety and abundance of their materials. In order to scale the technology up and optimize the cell formulations, it is key to obtain a clear understanding of how the physical-chemical properties of the electrode influence their electrochemical behaviour, in particular, the capacity loss. In this work, we propose for the first time mathematical correlations between textural and crystallographic properties of iron electrodes and their electrochemical stability. By adjusting synthesis parameters, we were able to tune pore size and volume, surface area and crystal size of iron oxides, and found that stability is highly correlated to both surface area and pore size. Large surface area and small average pore size provide electrodes with enhanced stability. We hypothesize that the cause for deactivation is the passivation of the electrodes ascribed to the formation of a non-conductive, non-reactive iron (II) hydroxide layer during discharge, which then cannot be reduced to iron again. We validate this hypothesis with electrochemical impedance spectroscopy studies, which show that, in the more stable electrodes, the charge transfer resistance in the Fe(OH)2 to Fe reduction does not significantly change after cycling, contrary to the behaviour of the less stable electrodes, corroborating our hypothesis. Furthermore, the electrode with the best properties was cycled 100 times, retaining almost 75% of its initial capacity at the end of the 100 cycles. These results are highly relevant for the future design and operation of iron-air batteries.

Figure S1.- Electrode manufacturing scheme. S2.- Mössbauer spectroscopy: Figure S2. Mössbauer effect spectra of the studied iron oxides with fitted components and distribution of doublets and sextets as explained in the manuscript. Table S1. Fitted parameters of the Mössbauer spectra using WinNormos software. S3.- Determination of crystallite size: Table S2. Relative molar fractions of iron in hematite and maghemite in %, assuming identical recoilless fractions for both oxides. S4.- Nitrogen physisorption. S5.- XPS spectra: Figure S4. XPS spectra of samples. Figure S5. XPS spectra of orbitals Fe2p and S2p of samples. Figure S6. XPS spectra of orbital O1s of samples. S6.- FESEM and TEM/STEM images of the iron oxides: Figure S7. FESEM micrographs at 50k magnification of iron oxides. Figure S8. TEM micrographs of iron oxides. S7.- Electrochemical reactions on the electrodes and electrodes deactivation: Figure S9. Detail of the processes of charge and discharge of an iron oxide electrode. Figure S10. Model applied to the discharge capacity of electrodes. Figure S11. Post-mortem XPS analyses of orbitals. S8.- Influence of the physical-chemical properties on rate capability. Figure S12. Model applied to the discharge capacity of electrodes. S9.- Additional EIS data: Figure S13. Nyquist diagrams of EIS tests after 1 and after 15 cycles of electrodes. Table S3. Optimized parameters of equivalent circuit (Figure 7 in the main text) for the electrodes in Figure S13. S9.- Post-mortem textural characterization: Figure S14. Adsorption isotherms of electrodes, before cycling and after 20 cycles. Table S4. Textural properties of the fresh and cycled electrodes.

Platinum group metal-free catalysts have been intensively investigated in the last decades as an alternative to platinum with the aim of lowering the cost of polymer electrolyte fuel cells. In particular, iron-nitrogen-carbon (Fe-N-C) has proved to be the most active towards the oxygen reduction reaction. For practical application, a hierarchical pore structure is required, with micropores favouring the creation of active sites and larger pores (meso- and macropores) facilitating the mass transport. In this work, carbon xerogels are investigated to hosting iron and nitrogen species obtained by a template-free method. The introduction of nitrogen in a one-step polymerization of urea with resorcinol and formaldehyde is investigated for the first time in this field by varying the relative content of reactants. The urea/resorcinol ratio greatly influences the pore structure of the Fe-N-C catalyst and the ORR electrocatalytic activity thereof. The ORR activity is favored for a balanced urea/resorcinol ratio where porosity is well developed and relatively high iron and nitrogen contents are incorporated to the carbon xerogel. In acid (0.5 M H2SO4), the onset potential is 0.82 V vs. RHE, with a number of exchanged electrons very close to 4 (i.e. full conversion to water) and low Tafel slope of 71 mV dec−1, for the most active catalyst of the series, possessing the best compromise of iron and nitrogen active sites. Fuel cell tests corroborate that the catalyst with the most developed porous structure shows the best performance.

El hidrógeno es el vector energético que nos permitirá realizar la transición hacia nuevas energías limpias. Para ello es necesario encontrar catalizadores activos y de bajo coste para la producción y uso del hidrógeno en tecnologías, como las pilas de combustible regenerativas en una unidad (URFCs), evitando el uso de metales nobles (Pt, Ir, Ru). En este dispositivo, la reacción de reducción de oxígeno (ORR) y la reacción de evolución de oxígeno (OER) desempeñan roles vitales en el electrodo de oxígeno [1]. Se ha realizado un estudio de materiales compuestos basados en combinaciones de Cu y Co con una matriz de carbono dopada con nitrógeno como catalizadores bifuncionales activos para OER y ORR para su uso en URFC [2]. El objetivo de este trabajo ha sido optimizar la cantidad de metales en los composites manteniendo la misma actividad y estabilidad de los electrocatalizadores.

Table S1. Raman shift (cm-1) of the G, D, D’ and D’’ from Raman spectra deconvolution and ID/IG intensity ratio. Table S2. Surface carbon, oxygen, nitrogen, cobalt and titanium contents and C/N, Co/N and Ti/N ratios calculated by XPS of the electrocatalysts and the relative content (%) of the deconvoluted peaks for N1s, Co2p and Ti2p. Figure S1. High-resolution C1s core level of NGC, Ti/NGC, Co/NGC and TiCo/NGC composites. Figure S2. LSV at 0.005 V s−1 in an O2-saturated 0.1 M NaOH solution, recorded using different electrode rotating rates indicated in the legend (rpm). (A) NGC, (B) Ti/NGC, (C) Co/NGC, (D) TiCo/NGC. Figure S3. Koutecky−Levich diagrams obtained between 0.3 and 0.56 V vs RHE for each composite. (A) NGC, (B) Ti/NGC, (C) Co/NGC, (D) TiCo/NGC. Figure S4. Koutecky−Levich diagrams obtained between 0.6 and 0.78 V vs RHE for each composite. (A) NGC, (B) Ti/NGC, (C) Co/NGC, (D) TiCo/NGC.

Three-dimensional N-doped graphene cryogels (NGC) modified with Ti and Co nanoparticles are investigated as bifunctional oxygen electrocatalysts for potential application in unitized regenerative fuel cells (URFCs). These composites have been successfully produced via a combined solvothermal, freeze-drying, and thermal treatment procedure. Crystallographic structure, surface properties, and morphology are analyzed to study the influence of each metal and the bimetallic combination in forming active sites for oxygen reduction (ORR) and oxygen evolution (OER) reactions. The intrinsic catalytic activity of the composites was determined by rotating disk electrode and further assessed in a gas diffusion electrode to test them in more realistic conditions for URFCs in 6 M KOH. Results revealed the remarkable bifunctional activity of Co/NGC and bimetallic TiCo/NGC composites. The synergistic effect of the metallic cobalt and surface atomic composition of Co3O4 distributed on the N-doped graphene cryogel in Co/NGC is responsible for the high bifunctional performance. Moreover, the partial substitution of cobalt for a less critical raw material, such as titanium, allows the electrocatalyst to maintain great bifunctional activity and stability. The nitrogen doping of the graphene cryogel structure can influence the anatase/rutile titania ratio that forms during pyrolysis. As a result, defects, oxygen vacancies, and Ti3+ species are formed, which, when combined with Co nanoparticles, contribute to the outstanding performance and stability of the TiCo/NGC composite. The excellent performance of N-doped graphene cryogel-based electrocatalysts in GDE conditions indicates their potential use in electrochemical processes, which involve gas-phase reactions.

Fig. S1. STEM (left) and TEM (right) images of Cu-N-C-Ac2 acquired using a Tecnay F30 microscope (300 kV). Fig. S2. XRD pattern of Cu-N-C-Ac2-3h obtained using the same experimental details than for Cu-N-CAc2 but with a lixiviation treatment of 3 hours. Fig. S3. N1s XPS spectra of the Cu-N-C. The material Cu-N-C-Ac1-BPT before the two-step post-treatment (acid leaching and 2nd carbonization) is also included for comparison purposes. Fig. S4. Effect of the two-step post-treatment (acid leaching and 2nd carbonization) on the nitrogen adsorption-desorption isotherms for Cu-N-C-Ac1. Fig. S5. Correlation between the Cu oxidation state and the energy position of the XANES spectrum, determined as the first maximum of the first derivative spectrum of Cu-N-C-Ac0.5 (red circle) and different copper reference compounds (black circles).Fig. S6. Variation with time of pH of the electrolyte (0.1 M KHCO3 aqueous solution) as a function of CO2 partial pressure. Balance with an inert gas for the equilibrium CO2 (aq) + H2O ↔ HCO3– + H+, pKa = 6.4, room temperature, 1 atm total pressure. Fig. S7. Oxidation of formic acid and methanol at the Pt ring of an RRDE, disc turned off, 1600 rpm. Fig. S8. Comparison of the CV in 0.1 M KHCO3 saturated with N2 (blue curve), methane oxidation voltammogram after saturation of the electrolyte with CH4 at the open circuit potential (red curve) and CH4 stripping voltammogram (Ead = 0.4 V vs. RHE, green curve) at the Pt ring of an RRDE, disc turned off, 1600 rpm. CV conditions from 0.05 to 1.6 V vs. RHE, 50 mV s-1. Room temperature. Fig. S9. CVs of the disc at 50 mV s-1 and 1600 rpm in CO2-saturated 0.1 M KHCO3 of Cu-N-C catalysts. Fig. S10. Chronoamperometric experiments at the disc for the Cu-N-C catalysts. Electrolyte: Fig. S11. CVs at the platinum ring for disc potentials reported in the legend. Table S1. Bulk chemical composition (wt. %) from EA and ICP of Cu-N-C electrocatalysts before and after the two-step post-treatment (acid leaching and 2nd

Fe−N−C catalysts are an interesting option for polymer electrolyte fuel cells due to their low cost and high activity towards the oxygen reduction reaction (ORR). Since Fe−N−C active sites are preferentially formed in the micropores of the carbon matrix, increasing the microporosity is highly appealing. In this work, carbon xerogels (CXG) were activated by physical and chemical methods to favor the formation of micropores, used as carbon matrices for Fe−N−C catalysts, and investigated for the ORR. The catalysts were characterized by solid-state techniques to determine chemical composition and pore structure. Physical activation increased microporosity up to 2-fold leading to catalysts with a larger density of active sites (more than twice iron and nitrogen uptake, pyridinic N and Nx−Fe). This entailed a higher ORR intrinsic activity determined in a 3-electrode cell (80 mV better half-wave potential). At the cathode of a fuel cell, the catalysts based on activated carbon materials showed 26 % lower power density ascribed to a more hydrophilic surface, causing a larger extent of flooding of the electrode that counterbalances the higher intrinsic activity. Interestingly, a more stable behavior was observed for the activated catalysts, with up to 2-fold better relative power density retention after 20-hour operation.

Figure S 1. SEM images of (a) CoFe/NC10% , (b) CoFe/NC20% , and (c) CoFe/NC40%. Figure S 2.TEM images of (a) CoFe/NC10% , (b) CoFe/NC20% , and (c) CoFe/NC40%. Figure S 3. Particle size distribution of the various electrocatalyst: (a) CoFe2O4, (b) CoFe/NC10% , (c) CoFe/NC20% ,(d) CoFe/NC30% , and (e) CoFe/NC40% . Table S 1. Amount of CoFe alloys and oxides in the composite samples, determined from the XRD analysis. Figure S 4. XPS high resolution spectra of (a) Fe 2p, (b) Co 2p, for all electrocatalysts. Table S 2. XPS analysis results of the electrocatalysts. Table S 3. Elemental composition of the prepared electrocatalysts. Table S 4. Specific capacitance and ECSA values for the various eletrocataysts. Table S 5. Charge transfer resistance (Rct) and solution resistance (Rs) determined from EIS measurement. Figure S 5. ECSA-normalized polarization curve.

Designing and developing affordable, high-performance, and stable electrocatalysts for oxygen evolution reaction (OER) is decisive for pragmatic water electrolysis to produce green hydrogen energy. In this work, we report cobalt and iron incorporated in phosphorus and nitrogen co-doped carbon foam (CF) derived from petroleum pitch as a promising electrocatalyst for alkaline OER. The P, N heteroatoms co-doped carbon foam (PN-CF) was first synthesized via thermo-chemical treatment of low-cost petroleum pitch in the presence of melamine (N source) and sodium hypophosphite (P source) precursors, followed by carbonization. Then, mono and bimetals of Co and Fe were impregnated into the as-prepared composite carbon foam (PN-CF) substrate, followed by further carbonization. Among the different catalysts, the bimetallic CoFe integrated with the PN-CF (CoFe@PN-CF) reveals an outstanding electrocatalytic activity (320 mV overpotential at j = 10 mA·cm-2), low Tafel slope (48 mV·dec-1), and excellent durability during OER measurement in 1 M KOH aqueous solution. The superb performance of the CoFe@PN-CF catalyst stems from the synergetic effect of the bimetals confined on phosphorus and nitrogen co-doped carbon foam support with high specific surface area, highly porous structure, and formation of graphitic domains, which enhances the electrical conductivity. This work sheds light on the potential for valorizing petroleum pitch and provides a facile synthesis approach to synthesizing a low-cost, high-performance, and durable electrocatalyst for alkaline OER.

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.

Highly efficient, low-cost and stable bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable for the development of green electrochemical energy storage and conversion devices. Here, we report the synthesis of sulfur-doped carbon nanofibers (CNF) as support for tantalum oxide nanoparticles. Carbon nanofibers and tantalum oxides represent a durable choice for oxygen electrocatalysis, but an improvement of catalytic activity is required. Upon doping, sulfur is found to be mainly bonded to carbon as C–S–C species (more than 80%). The results show that the effective incorporation of sulfur in the support has a clear positive effect on the electroactivity of the tantalum oxide catalysts. It causes a decrease of OER/ORR overpotential of 80 mV with respect to the undoped counterpart, with special improvement in the ORR. The new catalysts have shown an interesting bifunctional behavior for the OER and ORR, as well as a good stability through time.

In recent years, renewable energies have played an essential role in the technological development to meet energy demand and replace fossil fuels. At the same time, new challenges have arisen in order to implement clean energy production. The problem to store renewable energy that is obtained intermittently in the long term has led the development of fuel cells, electrolyzers and unitized regenerative fuel cells (URFC) to use hydrogen as an energy vector. URFCs can be used for both electric power production and hydrogen generation by adapting to the energy needs of the power grid. The most recent studies using an anion exchange membrane as electrolyte have opened up a range of possibilities for synthesizing new catalysts by replacing noble metals such as Pt, Ir or Ru with cheaper transition metals in combination with N-doped carbon structures [1] This work focuses on the development of noble metal-free bifunctional electrocatalysts for the oxygen electrode of unitized regenerative alkaline fuel cell (UR-AFC), where oxygen evolution (OER) and reduction (ORR) reactions take place [2]. Composites based on non-precious metals such as Ti, Mn, Fe, Co, Ni or Zn, and nitrogen-doped carbonaceous nanostructures obtained from polydopamine have been prepared.

The development of active and inexpensive non-precious metal catalysts is a necessary and essential requirement to replace currently used Pt-based catalysts, in order to reduce the cost of polymer electrolyte fuel cells(PEFC). [1] Catalysts based on the Fe-N-C structure develop high electroactivity towards the oxygen reduction reaction (ORR), the limiting reactionin fuel cells. To increase the catalysts activity, Fe must be dispersedin acarbon material with a high surfacearea.In this context, carbon xerogels are excellent candidates, as their main properties can be easily tailored: porosity, electrical conductivity and surface chemistry. [2] The meso/macroporosity of carbon xerogels can be designed by modifying the conditions of the synthesis process, while microporosity can be generated by subsequent carbonization/activation treatments [3].In the present work, on the basis of carbon xerogels with an optimal ratio of micro/meso/macroporosity, we investigate the effect of two different methods of nitrogen-doping in the catalytic activity of Fe-N-C catalysts. Catalysts are evaluated both in half-cell and single-cell configurations and their activity is correlated to their physical-chemical features.

Electrochemical water splitting is an environmentally benign technology employed for H2 production; however, it is critically hampered by the sluggish kinetics of the oxygen evolution reaction (OER) at the positive electrode. In this work, nitrogen-doped carbon-coated CoFe electrocatalysts were synthesized via a three-step route comprising (1) hydrothermal reaction, (2) in-situ polymerization of dopamine and (3) carbonization. The effect of carbonized polydopamine on the overall physicochemical properties and electrochemical activity of CoFe catalysts was systematically studied. By controlling and optimizing the ratio of CoFe2O4 and dopamine contents, a transformation of the CoFe2O4 structure to CoFe alloy was observed. It was found that CoFe/NC30% (prepared with 30% dopamine) exhibits an excellent catalytic activity towards OER. A small overpotential of 340 mV was required to generate a current density of 10 mA cm-2 in a 1.0 M KOH electrolyte. More importantly, the CoFe/NC30% catalyst reflected exceptional durability for at least 24 h. This research sheds light on the development of affordable, highly efficient, and durable electrocatalysts for OER.

Las pilas de combustible de membrana polimérica son una de las opciones más prometedoras para la conversión fiable y eficiente de energía limpia que van a ejercer un papel fundamental en la nueva transición energética del siglo XXI. Sin embargo, uno de los mayores obstáculos que presentan estos dispositivos es la relativa baja actividad de la reacción de reducción de oxígeno (ORR) que tiene lugar en el cátodo, y el alto coste de los catalizadores basados principalmente en Pt. El objetivo de este trabajo es la elaboración de electrocatalizadores sostenibles y competitivos en la reacción de reducción de oxígeno (ORR) para una pila de combustible de membrana polimérica intercambiadora de aniones. Para ello se han preparado composites formados por una matriz carbonosa dopada con nitrógeno y utilizando el Cu como metal activo. Este tipo de catalizadores ha sido poco estudiado en la bibliografía en comparación con otros metales, que cada vez están más cotizados para otros fines y que proceden de cadenas de suministro cuestionables en países subdesarrollados.

High performance, durable and affordable electrocatalyst for Oxygen Evolution Reaction (OER) is of great importance for tenable hydrogen production via water electrolysis. Although spinel oxide-based electrocatalysts (AB2O4, A, B = metal) represent a class of promising candidates for OER, their intrinsically poor electrical conductivity impacts their electrochemical performance. To overcome these bottlenecks, heteroatom-doped carbon structures can be used to improve the conductivity, adjust the electronic structure, durability and also boost the charge transfer, leading to enhanced electrochemical water oxidation [1]. In this work, a selection of nitrogen doped carbon decorated NiFe2O4 composites derived of polydopamine for OER in alkaline medium have been investigated.

Las baterías hierro-aire son sistemas prometedores para el almacenamiento de energía, debido a su bajo coste, seguridad y elevadas energías específicas. Estas baterías constan de un electrodo negativo, donde ocurre la oxidación y reducción del hierro, y un electrodo positivo (o electrodo de aire), donde ocurren la evolución (OER) y reducción (ORR) de oxígeno. Estas reacciones, al ser lentas, requieren de la presencia de un catalizador bifuncional. Los óxidos de manganeso son catalizadores activos para estas reacciones, pero son poco conductores. En este trabajo se propone la síntesis de materiales compuestos de nanoestructuras de carbono - nanofibras de carbono (CNF), nanotubos de carbono (CNT) y óxido de grafeno reducido (rGO) - y de dióxido de manganeso (MONW), que se probaron como electrocatalizadores bifuncionales de oxígeno.

Actualmente, el dióxido de carbono (CO2) es uno de los gases de mayor contribución al calentamiento global, lo que impolica la necesidad de realizar grandes esfuerzos para descarbonizar el sector energético en particular y la economía en general. En este escenario de transición, la reacción de electro-reducción de CO2 (CO2RR) se presenta como una tecnología prometedora para la utilización de corrientes concentradas de CO2 con el fin de producir productos químicos e industriales de alto valor añadido, como alcoholes o hidrocarburos, utilizando electricidad renovable como fuente energética. Sin embargo, la alta estabilidad de la modécula de CO2 bajo condiciones ambientales implica un elevado aporte de energía para producir cantidades razonables de productos finales. Además, la CO2RR tiene lugar involucrando un complejo mecanismo de reacción con diferentes rutas paralelas y múltiples etapas de transferencia de electrones, compitiendo a su vez con la reacción de evolución de hidrógeno (HER), lo que limita el proceso. Por todo esto, el diseño de catalizadores estables, eficientes y selectivos para la reducción de CO2 es actualmente uno de los principales retos de esta tecnología. Una solución prometedora es el uso de catalizadores bimetálicos combinando un metal noble (Pt), con el fin de reducir el sobrepotencial de la reacción, y cobre (Cu), que regula la selectividad hacia la formación de hidrocarburos y alcoholes. Además, la dispersión de la fase activa en un soporte con adecuada textura y química superficial mejora la utilización del metal, reduciendo por tanto el coste de los electrodos. Este trabajo estudia el efecto del soporte carbonoso para la CO2RR en catalizadores bimetálicos CuPt/C (20% en peso de cobre y ratio atómico Cu:Pt de 95:5). Como soportes carbonosos se utilizaron xerogeles de carbono (CXG), óxidos de grafeno reducido (RGO), carbones mesoporosos ordenados (OMC) y un negro de carbono comercial (Vulcan XC-72R).

Las celdas solares sensibilizadas con colorante (DSSCs), o celdas de Grätzel, son dispositivos fotovoltaicos prometedores para la generación de energía eléctrica, debido a su bajo coste y su simplicidad de fabricación. Las DSSCs están formadas por un fotoánodo basado en una película de colorante y óxido de titanio, un electrolito que contiene pares redox yoduro/triyoduro y un contraelectrodo basado en una película delgada de platino. El rol del contraelectrodo es catalizar la reacción de reducción de yodo a yoduro del electrolito. Los materiales carbonosos se han postulado como un sustituto prometedor del platino, gracias a su elevada área superficial y conductividad eléctrica, lo que permitiría reducir el coste de fabricación de las DSSC. En este trabajo se han investigado xerogeles de carbono (CXGs) dopados con nitrógeno (N-CXG) o azufre (S-CXG) como contraelectrodos de DSSCs y se ha evaluado su comportamiento electroquímico.

La reacción elctroquímica de reducción de dióxido de carbono (CO2RR) representa una tecnología atractiva de almacenamiento de energía renovable (intermitente) en combustibles químicos y otras materias primas. En este proceso, el CO2 es reducido en la superficie de un catalizador y, por ende, la naturaleza de la fase activa resulta clave en la actividad/selectividad de la reacción. Entre los diferentes sistemas catalíticos estudiados, aquellos basados en cobre han despertado un gran interés por su tendencia única para producir hidrocarburos y alcoholes. Sin embargo, la mayoría de los electrodos reportados presentan una baja eficiencia energética y selectividad, lo que desalienta su implementación a nivel industrial. Recientemente, la alteración del entorno químico del cobre con un metal noble se ha propuesto como una estrategia prometedora para reducir el sobrepotencial de reacción y favorecer la CO2RR., Este trabajo se centra en la reducción electroquímica de CO2 usando catalizadores CuPt soportados en un negro de carbono comercial (CuPt/C, 20% en peso de cobre). Los catalizadores se sintetizaron variando el pH y con diferentes relaciones atómicas Cu:Pt, con el fin de estudiar su influencia en la actividad y selectividad hacia la CO2RR.

Las pilas de combustible regenerativas en celda unidad (URFC) están llamadas a tener un papel importante en el diseño de nuevos planes energéticos que permitan abandonar el uso de combustibles fósiles. Sin embargo, esta tecnología se encuentra limitada por la baja cinética de las reacciones que ocurren en el electrodo de oxígeno (reacciones de evolución (OER) y reducción (ORR) de oxígeno) y del alto coste de los catalizadores empleados en su operación. Una forma de incrementar la cinética de estas reacciones es el uso de electrolitos alcalinos. Este tipo de medio disminuye la corrosión de los materiales empleados como catalizadores, permitiendo el uso de metales no nobles, así como soportes basados en carbono. El objetivo de este trabajo es la síntesis de materiales nanoestructurados, basados en criogeles de grafeno dopados con nitrógeno y nanopartículas metálicas de Ti y Co, y el estudio de su actividad como catalizadores para la ORR y la OER en medio alcalino.

Ceria-iron oxide mesoporous materials with Fe:Ce molar ratio of 5:5 and 9:1 were synthesized by hydrothermal method using CTAB as a template and subsequently modified with NiO (molar ratio Ni:Fe = 1:2) by incipient wetness impregnation technique. In order to increase the electro-capacitive properties and reduce the intrinsic impedance of the metal oxides, the samples were consecutively modified by reduction in hydrogen to obtain highly dispersed Ni–Fe alloys into ceria matrix. By exploiting the high permeability of carbon inside ferrous alloys, the metal phase has been further modified into ferrous carbides and metal alloys encapsulated within carbon nanofibers. For this purpose, a reaction, already widely studied for the production of hydrogen, was used, that is the decomposition of methanol vapors. In fact, this decomposition, in addition to producing syn-gas and methane, changes the catalysts in use through a chemical vapor deposition-carbon coating process. This fact, has been used by us to demonstrate how the newly obtained metal-carbon nanocomposites can be used for electro-catalytic purposes. The modified phases of the two molar ratios of the Fe–Ni–Ce catalysts were tested in the Oxygen Evolution Reaction (OER) in an alkaline environment (1 M KOH), showing a satisfactory and progressive increase in activity and a surprising decrease in the overpotential at 10 mA/cm2 of current density. The morphological, textural and physicochemical properties of the samples were characterized in details by XRD, N2-physisorption, TG-TPO, TEM, EDX, FTIR, XPS, Raman and Moessbauer spectroscopies.

Las reacciones de reducción y de evolución de oxígeno (ORR, OER) son de especial interés en el desarrollo de dispositivos energéticos como pilas de combustible regenerativas o baterías metal-aire. El diseño de catalizadores bifuncionales capaces de catalizar eficientemente ambas reacciones es un tema de gran actualidad. Algunos óxidos metálicos de los grupos IV y V de la tabla periódica, como el tantalio, son catalíticamente activos y además presentan una muy buena estabilidad, pero su conductividad eléctrica y su área superficial son generalmente muy bajas. En este trabajo se ha investigado la utilización de nanofibras de carbono como soporte de óxidos de tantalio para mejorar dichas características.

Iron–air systems are a very promising technology with the potential to become one of the cheapest and safest energy storage solutions of the future. However, iron anodes still face some challenges like passivation, resulting in loss of capacity, due to the formation of nonconductive species during cycling as well as the hydrogen evolution reaction, a parasitic reaction interfering with the charging of the electrode. In the present work these two issues are addressed: Sulfur-modified mesoporous iron oxides are obtained and used as hot-pressed negative electrodes for alkaline iron–air batteries. Iron electrodes present average capacity values between 400 and 500 mA h g Fe–1 for ∼100 h of operation, the S-modified iron oxides being the most stable ones. An exponential deactivation model fitting the discharge capacity of the different electrodes compared to the number of cycles was proposed. According to the model, the best of the electrodes loses less than 0.5% of its capacity per cycle. Furthermore, doubling the charge and discharge rates allows increasing both the discharge capacity and the Coulumbic efficiency, though at the expense of stability. This manuscript proves that the proper distribution of sulfur on the surface of the iron oxide is fundamental to suppress the HER and passivation, enhancing the stability of the electrode. These properties were further corroborated in long test-runs which comprised more than 400 h of charging and discharging.

High performance, durable and inexpensive electrocatalyst for oxygen evolution reaction (OER) is of great importance for tenable hydrogen production via water electrolysis. Although spinel oxides (AB2O4, A, B = metal) represent a class of promising candidates for OER, their intrinsically poor electrical conductivity impacts their electrochemical performance. Herein, we employed a facile approach to transform an intrinsically low active NiFe2O4 into nitrogen-doped carbon decorated Ni3Fe@Fe3O4 catalyst with improved activity and stability for alkaline OER. Initially, a pristine NiFe2O4 octahedron-like structure was synthesized by a hydrothermal route. Then, series electrocatalysts were prepared by incorporating the pristine NiFe2O4 with different dopamine concentrations via in-situ polymerizations of dopamine followed by carbonization. The morphology, crystalline structure, and chemical composition of the catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma (ICP). The OER electrocatalysis performance was measured in a standard three-electrode system. The effect of the carbonized dopamine on the electrocatalytic activity and structure of the NiFe2O4 precursor was systematically investigated. Among several NiFe electrocatalysts, the one with 10 wt% of dopamine (NiFe/NC10%) exhibited a relatively higher catalytic activity for OER tested in 1.0 M KOH; unveiled low overpotential (350 mV at 10 mAcm−2 current density), a low Tafel slope (56 mVdec−1), low charge transfer resistance, relatively higher electrochemically active surface area. Most prominently, it remained stable for at least 12 h. This work provides a new perspective for functionalizing metal oxides and affords a facile synthesis approach, low-cost, high-performance, and robust electrocatalyst for alkaline OER electrodes.

The stainless steel mesh (SSM) has received remarkable attention for hydrogen and oxygen evolution reactions. It was demonstrated that the SSM exhibits admirable performance towards oxygen evolution reaction (OER) electrocatalysis, while its catalytic activity for hydrogen evolution reaction (HER) remains quite low. This obstructs the utilization of SSM-based catalysts for sustainable complete water electrolysis. In this study, a facile hydrothermal route followed by a phosphorization process was adopted to transform commercially available SSM materials into high-performance and stable electrocatalysts for alkaline HER. We report an interconnected NiCoP-CoP sponge-like structure on SSM substrate without polymer binder. Benefiting from the 3D construction with high exposed surface area, close contact between electroactive species and conductive surface, and facilitated infiltration of electrolyte, the as-prepared NiCoP@SSM electrocatalyst brought an improved catalytic activity for HER, required a low overpotential of 138 mV to derive a current density of 10 mAcm−2 in 1.0 M KOH aqueous solution. The high performance of the NiCoP@SSM catalyst has also unveiled fast reaction kinetics (presents a small Tafel slope of 74 mV/dec), a relatively large electrochemical active surface area (ECSA), and small charge transfer resistance. Furthermore, the NiCoP@SSM electrode also presented excellent stability during long-term measurements, making it one of the most encouraging HER electrodes to date. This research study paves the way for the development of HER-active electrocatalysts made from SSMs that are commercially available, low-cost, and highly active.

In this work, carbon xerogels (CXGs) doped with nitrogen or sulfur have been investigated as DSSC counter electrodes. CXGs have been prepared by a sol–gel method from resorcinol and formaldehyde and subsequent carbonization. Nitrogen doping has been carried out by introducing melamine into the synthesis process along with resorcinol and formaldehyde, while sulfur has been incorporated by direct reaction of the carbon material with elemental sulfur. The counter electrodes for DSSCs have been prepared by airbrushing on conductive glass (fluorine-doped tin oxide, FTO), and their electrochemical behavior has been evaluated, observing that the introduction of heteroatoms such as nitrogen or sulfur leads to an improvement in efficiency compared to the undoped material thanks to a decrease in charge transfer resistance.

En los últimos años, la reducción de CO2 por vía electroquímica ha emergido como una estrategia para la obtención de compuestos de alto valor añadido (alcoholes o hidrocarburos) con aplicaciones energéticas o como moléculas plataforma para la síntesis de otros productos químicos. Si bien el CO2 es una fuente de carbono abundante, la alta estabilidad de este compuesto dificulta su conversión en otros productos, ya que implica un aporte de energía significativo. Además, la reacción de electro-reducción de CO2 (CO2RR) tiene lugar a partir de un mecanismo de reacción complejo con diferentes rutas paralelas y con múltiples etapas de transferencia de electrones. En este contexto, la utilización de catalizadores activos y selectivos que aumenten la eficiencia de la reacción y modulen la formación del producto deseado resulta primordial. Los sistemas catalíticos basados en cobre han atraído mucha atención por su tendencia única a producir hidrocarburos y alcoholes, aunque requieren de altos sobrepotenciales para activar la molécula de CO2. Una solución prometedora es el uso de catalizadores bimetálicos entre este elemento y un metal noble (Pt), de forma que el Cu regule la selectividad y el Pt reduzca el sobrepotencial. En este trabajo se estudia el efecto del ratio atómico Cu:Pt en el comportamiento electroquímico para la CO2RR de catalizadores bimetálicos soportados sobre negro de carbono (CuPt/C, 20 % de cobre en peso). La síntesis consistió en la preparación de nanopartículas de Cu/C por impregnación y reducción con NaBH4. Posteriormente, el Pt se incorporó mediante desplazamiento galvánico, generando partículas bimetálicas CuPt/C [1]. Los electrocatalizadores CuPt/C fueron analizados mediante diversas técnicas (XRD, ICP, TEM), confirmando cargas metálicas similares a las nominales, así como la correcta deposición de las nanopartículas en la superficie del material carbonoso. Por otro lado, la actividad de los catalizadores hacia la CO2RR se evaluó por voltamperometría

Active and durable electrocatalysts for the oxygen evolution reaction (OER), capable of replacing noble metal catalysts, are required to develop efficient and competitive devices within the frame of the water electrolysis technology for hydrogen production. In this work, we have investigated tantalum based catalysts supported on carbon nanofibers (CNF) for the first time. The effect of CNF characteristics and the catalyst annealing temperature on the electrochemical response for the OER have been analyzed in alkaline environment using a rotating ring disc electrode (RRDE). The best OER activity and oxygen efficiency were found with a highly graphitic CNF, despite its lower surface area, synthesized at 700 °C, and upon a catalyst annealing temperature of 800 °C. The ordering degree of carbon nanofibers favors the production of oxygen in combination with a low oxygen content in tantalum oxides. The most active catalyst exhibited also an excellent durability.

Biomass from agricultural by-products is gaining increasing interest as cheap and abundant precursor in the development of active materials for efficient and environmentally friendly devices like fuel cells. Herein, we investigated iron and nitrogen co-doped nanoporous carbons derived from aronia, peach stones and coal tar pitch/furfural as electrocatalysts for the electrochemical oxygen reduction reaction (ORR) in alkaline media. Urea was used as nitrogen precursor and two annealing steps with intermediate acid leaching served to activate the catalysts. Within the series, the peach stone-derived catalyst exhibited a catalytic activity for the ORR close to the benchmark Pt/C, with a 60 mV dec−1 Tafel slope upon the incorporation of 0.57 wt% Fe and proper combination of N-Fe species (20%) with pyridinic/pyridonic moieties (49%). We concluded that the microporosity and a certain content of meso/macro-pores of the activated carbon, together with the creation of graphitic domains result in a high relative amount of Fe-N4 and nitrogen functionalities, which determine the electrocatalytic performance.

In the pursuit of good performing, low cost, and scalable anion-exchange membranes (AEM), a series of blended electrolytes based on cross-linked quaternized poly epichlorohydrin (qPECH) and polyvinylidene fluoride (PvDF) were prepared to evaluate their suitability for AEM fuel cell application. The thermo-mechanical and swelling analyses revealed that the blending of these two macromolecules produces robust and heat-resistant microphase-segregated membranes with good dimensional stability. By varying the blend ratio, the ion-exchange capacity (IEC) and transport properties of the resulting membrane can be easily adjusted and optimized with clear impact on its electrochemical performance. At 67:33 wt % blend ratio, high hydroxide conductivity (i.e., 56.3 mS cm–1 at 80 °C) and quite reasonable alkaline stability were achieved. The single H2–O2 fuel cell using the qP-67 membrane yielded a beginning-of-life maximum power density of 32 mW cm–2 and an open circuit voltage (OCV) of 1.03 V at 50 °C without optimization. These preliminary results demonstrate that qPECH/PvDF blended membranes can be potentially applied in AEMFCs.

Bifunctional oxygen electrocatalysts are essential in the development of low-temperature unitized regenerative fuel cells (URFCs), as a promising alternative for storing energy via hydrogen. TiO2, as a semiconductor material, is commonly not established as an active electrocatalyst for oxygen reduction and oxygen evolution due to its poor electrical conductivity and low reactivity. Here, we demonstrated that composites composed of TiO2 and N-doped graphene can be active in oxygen reduction and evolution reactions in an alkaline environment. Combination factors such anatase/rutile interaction, N-doping graphene, and the presence of Ti3+/Ti–N species raise the active sites and improve the electrochemical activity. Our results may afford an opportunity to develop a non-noble and promising electrocatalyst in energy storage technology.

The deployment of hydrogen as an energy carrier is found to be a vital alternative fuel for the future. It is expected that water electrolysis, powered by renewable energy sources, be able to scale‐up hydrogen production. However, the reaction kinetic of oxygen evolution reaction (OER) is a sluggish process, which predominantly limits the efficiency of water electrolysis. This review recapitulates the recent progress and efforts made in the design and development of two selected earth-abundant bimetallic electrocatalysts (NiCo and CoFe) for alkaline OER. Each bimetal electrocatalyst is thoroughly outlined and discussed in five sub-sections, including bimetal (oxy) hydroxides, Layered double hydroxides (LDHs) structures, oxides, composites, alloy and nanostructured electrocatalysts, and assembled with heteroatoms. Furthermore, a brief introduction to an in situ/operando characterization techniques and advantages for monitoring the structure of the electrocatalysts is provided. Finally, a summary outlining the challenges and conceivable approaches to advance OER performance is highlighted and discussed.

En los últimos años, las energías renovables han cobrado un papel esencial en el desarrollo tecnológico para satisfacer la demanda energética creciente y sustituir a los combustibles fósiles. Al mismo tiempo, han surgido nuevos retos para poder implementar estas vías de obtención de energía limpia, la incógnita de cómo almacenar a largo plazo la energía renovable que se obtiene de manera intermitente ha favorecido el desarrollo de pilas de combustible, electrolizadores o pilas regenerativas en una unidad (URFC) para utilizar el hidrógeno como un vector energético. Estas últimas se pueden utilizar tanto para la producción de energía eléctrica como para la generación de hidrógeno adaptándose a las necesidades energéticas de la red eléctrica. Los estudios más recientes utilizando una membrana de intercambio aniónico como electrolito, han abierto un abanico de posibilidades a la hora de sintetizar nuevos catalizadores sustituyendo los metales nobles, como el Pt, Ir o Ru, por otros metales de transición más baratos combinándolos con materiales de carbono, como nanotubos o grafeno, óxidos metálicos o estructuras organometálicas (MOFs). Este trabajo se centra en el desarrollo de electrocatalizadores bifuncionales libres de metales nobles, para el electrodo de oxígeno de una URFC alcalina, donde tienen lugar las reacciones de evolución (OER) y reducción (ORR) de oxígeno, las cuales poseen una cinética de reacción lenta. Se han preparado composites basados en metales no preciosos de Co, Fe, Mn, Ni y Zn y nanoestructuras carbonosas dopadas con nitrógeno, obtenidas a partir de precursores poliméricos.

Actualmente, uno de los mayores retos en la tecnología de pilas de combustible de electrolito polimérico (PEFCs) es la disminución del coste de los sistemas catalíticos empleados en el ánodo y en el cátodo. Una de las estrategias más exploradas es la de sustituir los catalizadores basados en Pt por otros basados en metales de transición coordinados con nitrógeno y carbono (Me-N-C, donde Me = Fe, Co y/o Mn), siendo los catalizadores Fe-N-C los más activos para la reacción de reducción de oxígeno (ORR), la reacción limitante en las pilas de combustible. Generalmente, estos catalizadores emplean negros de carbono microporosos como matrices carbonosas. En este trabajo se han sintetizado diferentes catalizadores de hierro usando como estructura carbonosa los xerogeles de carbono, previamente dopados con nitrógeno (N-CXG), con el fin de usar una matriz carbonosa con una mezcla adecuada de microporos y meso/macroporos, necesarios para una correcta difusión de especies en la pila de combustible.

La reducción de CO2 por vía electroquímica es una alternativa novedosa con altas expectativas de cara a la reutilización de corrientes concentradas de CO2, con el fin de obtener productos de alto valor añadido (como alcoholes e hidrocarburos) con aplicaciones energéticas o para la síntesis química. Sin embargo, uno de los principales inconvenientes de la reacción de reducción de CO2 (CO2RR) es la alta estabilidad de esta molécula bajo condiciones ambientales, lo que implica la necesidad de utilizar una gran cantidad de energía para obtener cantidades razonables de dichos productos. Además, la reducción de CO2 viene inevitablemente acompañada de la evolución de hidrógeno cuando se lleva a cabo en electrolitos acuosos. En este contexto, la utilización de catalizadores altamente activos y selectivos hacia los compuestos de interés se torna esencial para aumentar la eficiencia de la reacción. Entre los diferentes sistemas catalíticos, los catalizadores basados en cobre son los más prometedores para la electro-reducción de CO2 debido a la actividad única de este metal a la formación de hidrocarburos, alcoholes y aldehídos. Sin embargo, se requieren altos sobrepotenciales para alcanzar densidades de corriente sustanciales. Una posible solución es el uso de catalizadores bimetálicos usando cobre y un metal noble (por ejemplo, Pt). De este modo, la presencia del metal noble es de utilidad para disminuir el sobrepotencial de la reacción mientras que el cobre permite modular la selectividad a compuestos derivados de la reducción de CO2. Además, el uso de un soporte adecuado con alta área superficial permite dispersar la fase activa (normalmente en forma de nanopartículas), mejorando la utilización del metal y la distribución del CO2 en la superficie del catalizador y, por tanto, favoreciendo el proceso y disminuyendo el coste del electrodo. Por ello, el uso de nanopartículas metálicas soportadas en materiales de carbono representa una vía prometedora para la electro-reducción

Las baterías acuosas metal-aire son una tecnología prometedora tanto para electromovilidad como para almacenamiento de energía estacionario, debido a su bajo coste, su elevada energía específica y la capacidad de usar materiales abundantes y seguros. En el electrodo positivo de estos sistemas ocurre, durante la descarga, la reducción del oxígeno ambiental (ORR) y, durante la carga, la reacción de evolución de oxígeno (OER), reacciones ambas de cinética lenta que requieren del uso de electrocatalizadores. En este trabajo, se han sintetizado nanofilamentos de dióxido de manganeso (MONW), conocidos por su reversibilidad y durabilidad como catalizadores para las reacciones de reducción (ORR) y evolución de oxígeno (OER). Además, dichos nanofilamentos se han modificado con nanopartículas de hierro (Fe-MONW), de elevada actividad para la OER y/o níquel (Ni-MONW), activo para la ORR.

Las baterías de flujo rédox (RFB) se han postulado en los últimos años como unas de las tecnologías más prometedoras para el almacenamiento de energía a gran escala debido a su diseño modular que favorece el escalado y el diseño separado de energía y potencia. Los electrodos de las RFB contribuyen al rendimiento de la batería condicionando la densidad de potencia y la eficiencia energética, pues es sobre la superficie electroactiva de éstos, y gracias a su capacidad de transporte electrónico, donde se lleva a cabo la reacción electrocatalítica. Además, el uso de especies de vanadio en los dos electrolitos presenta unas ventajas únicas como son: la eliminación de la contaminación cruzada debida a la difusión de iones a través de la membrana, un tiempo de respuesta corto y una vida útil prolongada. En este trabajo se ha llevado a cabo la modificación de fieltros comerciales de carbono de poliacrilonitrilo (PAN) carbonizado (GFD 4.6 IW1) suministrados por Sigracell, con el fin de mejorar sus propiedades como electrodos, que actualmente son los más empleados a gran escala en la fabricación de baterías de flujo redox. Para ello se ha diseñado un método de modificación de la superficie del fieltro con óxido de grafeno (GO) y polietilenglicol (PEG), seguido de una pirolisis en atmósfera inerte. Se ha estudiado la influencia del contenido en GO mediante una deposición progresiva de distintas cantidades de GO y PEG. El objetivo de la modificación con GO y PEG es el de obtener una superficie funcionalizada con oxígeno gracias al poder oxidante del PEG y aumentar la actividad catalítica de los fieltros hacia las reacciones de reducción y oxidación del vanadio, por acción del óxido de grafeno.

This review paper presents the most recent research progress on carbon-based composite electrocatalysts for the oxygen evolution reaction (OER), which are of interest for application in low temperature water electrolyzers for hydrogen production. The reviewed materials are primarily investigated as active and stable replacements aimed at lowering the cost of the metal electrocatalysts in liquid alkaline electrolyzers as well as potential electrocatalysts for an emerging technology like alkaline exchange membrane (AEM) electrolyzers. Low temperature electrolyzer technologies are first briefly introduced and the challenges thereof are presented. The non-carbon electrocatalysts are briefly overviewed, with an emphasis on the modes of action of different active phases. The main part of the review focuses on the role of carbon–metal compound active phase interfaces with an emphasis on the synergistic and additive effects. The procedures of carbon oxidative pretreatment and an overview of metal-free carbon catalysts for OER are presented. Then, the successful synthesis protocols of composite materials are presented with a discussion on the specific catalytic activity of carbon composites with metal hydroxides/oxyhydroxides/oxides, chalcogenides, nitrides and phosphides. Finally, a summary and outlook on carbon-based composites for low temperature water electrolysis are presented.

The development of active and low-cost electrocatalysts for the oxygen reduction reaction (ORR) is crucial for the sustainable commercialization of fuel cell technologies. In this study, we have synthetized Me/Mo2C (Me = Fe, Co, Cu)-based composites embedded in N- and P-dual doped carbon by means of inexpensive industrial materials, such as melamine and chitosan, as C and N sources, and the heteropolyacid H3PMo12O40 as P and Mo precursor. The effect of the transition metal (Fe, Co, and Cu) on the ORR in alkaline medium has been investigated. The physicochemical properties of the electrocatalysts were performed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). Activity towards ORR was carried out in a three-electrode cell using a ring-disk electrode in 0.1M NaOH. The results obtained clearly show the important role played by each transition metal (Fe, Co, and Cu) in the electrochemical activity. Among them, Fe gives rise to the best performing composite in carrying out the oxygen reduction reaction. The formation Fe3C/Mo2C species embedded in N- and P-dual doped carbon seems to be the determining role in the increase of the ORR performance.

[EN] The interest in hydrogen technologies has grown in recent years, mainly because an economy based on hydrogen can help to solve important challenges related to the global economy of the future: energy security and climate change. Taking advantage of this momentum, more and more countries are implementing a growing number of policies related to hydrogen. Indeed, the European Hydrogen Strategy establishes hydrogen as essential drivers for the total decarbonization of the current energy system in order to achieve the EU’s commitment related to carbon neutrality by 2050. However, the successful development of the hydrogen technologies requires the collaboration of the public and private sectors to accelerate its deployment and make more competitive its implementation at large-scale. The research groups that take part of the line of work dedicated to hydrogen technologies, within the CSIC Interdisciplinary Thematic Platform PTI Mobility 2030, work in this regard, developing their investigations in several important areas related to the hydrogen technologies such as hydrogen generation, storage, distribution and uses., [ES] El interés por las tecnologías del hidrógeno ha crecido en los últimos años, principalmente porque una economía basada en el hidrógeno puede dar respuesta a los grandes desafíos de la economía global del futuro: seguridad energética y cambio climático. Aprovechando este impulso, cada vez son más los países que están implementando un número creciente de políticas en favor del hidrógeno. Prueba de ello es la Estrategia Europea del Hidrógeno que establece al hidrógeno como un elemento esencial en la descarbonización total del actual sistema energético para alcanzar el compromiso de la UE con la neutralidad de carbono en 2050. No obstante, el desarrollo exitoso de las tecnologías del hidrógeno requiere que todos los actores, incluidos los sectores público y privado, aumenten sus esfuerzos para acelerar su despliegue y hacer que su implantación a gran escala resulte competitiva. Los grupos de investigación que forman parte del área de trabajo de tecnologías del hidrógeno, dentro de la Plataforma Temática Interdisciplinar PTI Mobility 2030 del CSIC, trabajan en este sentido, desarrollando su labor en áreas tan diversas como la generación, el almacenamiento, la distribución y los usos del hidrógeno.

[EN] The application of batteries coupled to renewable energy conversion systems in cities will greatly help to overcome some of the technological challenges for grid electrification and transport, such as difficult accessibility to charging points, and cost, by improving their technical capabilities and making the energy system globally more sustainable. Furthermore, its implementation will have an immediate effect on citizens’ health due to the reduction of greenhouse gas emissions into the atmosphere, as well as other pollutants. Below is a review of recent efforts to develop new advanced technologies for future sustainable batteries. We will also highlight current battery recycling strategies that are being implemented towards a zero-carbon future based on the concept of the circular economy., [ES] La aplicación de baterías acopladas a sistemas de conversión de energía renovable en las ciudades ayudará en gran medida a superar algunos de los retos tecnológicos para la electrificación de la red y el transporte, como la difícil accesibilidad a puntos de carga, y de coste, mejorando sus capacidades técnicas y haciendo que el sistema energético a nivel global sea más sostenible. Además, su aplicación tendrá un efecto inmediato en la salud de los ciudadanos debido a la disminución de las emisiones de gases de efecto invernadero a la atmósfera, así como otros contaminantes. A continuación, presentamos una revisión de los esfuerzos recientes para desarrollar nuevas tecnologías avanzados para las futuras baterías sostenibles. También destacaremos las estrategias actuales de reciclado de baterías que se aplican hacia un futuro con cero emisiones de carbono y basado en el concepto de economía circular.