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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.

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.

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.

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.

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

Rapid industrialization and rising living standards around the world have led to an energy crisis and severe environmental pollution caused by the excessive use of conventional fossil fuels. Preventing further climate change and reducing carbon dioxide emissions have become the most critical challenges confronting modern society. This has sparked an intense interest in discovering and designing alternative energy sources with affordable, clean, and environmentally benign features. Hydrogen energy has been recognized as a viable alternative to fossil fuels and is pivotal in neutralizing carbon emissions. H2 can be obtained using different approaches from different sources, including water, natural gas, coal, and biomass. The leading pathway for H2 production remains via steam reforming of fossil fuels. Still, this method cannot achieve the future "carbon-neutral" energy system due to its reliance on fossil fuels and high CO2 emissions. H2 production through water electrolysis driven by renewable energy is a clean and ultralow-carbon footprint route and has been regarded as the most promising approach for high-purity H2 production. Two important half-reactions occur in water electrolysis: the Hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode. Nonetheless, the sluggish kinetics and unfavorable thermodynamics of these reactions lead to enormous overpotential, which is the main challenge for mass hydrogen production. As a result, efficient electrocatalysts are required to ease the reaction energy barrier and expedite the reaction. To date, the most effective electrocatalysts for water electrolysis are primarily based on noble metals. For example, Pt-based electrocatalysts for HER and Ir- and Ru-derived electrocatalysts for OER. However, their scarcity and high cost obstruct them from sustainable use in large-scale applications. To circumvent this problem, in the last few decades, considerable efforts have been devoted t

Renewable and low-carbon hydrogen will contribute to a future climate-neutral economy as a fuel, clean energy carrier and feedstock. One of the main concerns when considering its production by the present proton exchange membrane water electrolysers (PEMWE) is the use of scarce and expensive noble metals as catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), because they contribute to increase the cost of the technology. Several strategies have been developed to overcome this drawback, such as optimising the catalyst loading in the electrodes and alloying or using alternative catalyst supports, always with the aim to maintain or even increase electrolyser performance and durability. In this review, we examine the latest developments in HER and OER catalysts intended for practical PEMWE systems, which point in the short term to the use of platinum and iridium nanoparticles highly dispersed at low loadings on conductive non-carbon supports. © 2023 Johnson Matthey.

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.

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 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.

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.

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.

High yield production of benzaldehyde in the solvent-free oxidation of benzyl alcohol by using green catalysts is highly desirable. In this work, calcium hydroxide derived from waste clamshell was used as low-cost and environmentally friendly catalyst support (CaSUP) for Pd and V nanoparticles. The physicochemical properties of the catalysts were analyzed using X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) technique, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalytic oxidation of benzyl alcohol to benzaldehyde was studied in a liquid phase reaction by using H2O2 as an oxidizing agent. The effects of catalyst loading, the molar ratio of hydrogen peroxide to benzyl alcohol, temperature and reaction duration were investigated. In the optimized conditions, Pd nanoparticles supported on clamshell-derived supports displayed excellent catalytic conversion (88%) and selectivity to benzaldehyde (89%). Furthermore, the catalyst can be effectively reused without a significant loss in its activity and selectivity. The high yield and stability can be related to the structural and basic properties of the catalyst. These results provide important insights into the benzyl alcohol oxidation process for industrial applications.

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.

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.

Hybrid organic-inorganic perovskites, MAPbX (MA: methylammonium, CH-NH; X = halogen), are active absorption materials in the new generation of solar cells. The triiodide specimen (MAPbI) remains the most widely studied perovskite due to its ability to absorb broadband light below its band gap of 1.6 eV, but its degradation in a humid atmosphere has remained a major obstacle for commercialization. Here we found that CHNHPbIBr (x = 0.0, 1.0) perovskites, prepared by ball milling, exhibit superior stability, showing no signs of degradation after several months of exposure to humid air. A synchrotron X-ray diffraction (SXRD) investigation was useful to determine some peculiar structural features that may account for the improved stability. The crystal structure was analysed to be in the I4/mcm and acentric I4cm space groups, yielding similar agreement factors. In both, the inorganic framework presents a conspicuously lower tilting effect than that observed in samples prepared by wet methods; an additional difference arises since the tetragonal structure is stable down to 140 K. The orientation of the organic MA units may play an important role in the properties, given the degrees of freedom for internal motion of MA groups within the PbX network. From the refined C/N positions, the MA units lie along the c axis with 25% probability. The structure at 140 K shows that the MA contribution along the c axis vanishes at this temperature. By contrast, MAPbIBr crystallizes in the cubic phase with the space group Pm3m, also with a larger unit-cell volume than that previously described. The absence of phase transitions down to 120 K suggests that the anion disorder prevents the localization of MA units upon cooling.

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.

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.

[EN] IrO 2 and IrRuO x (Ir:Ru 60:40 at%), supported by 50 wt% onto titania nanotubes (TNTs) and (3 at% Nb) Nb-doped titania nanotubes (Nb-TNTs), as electrocatalysts for the oxygen evolution reaction (OER), were synthesized and characterized by means of structural, surface analytical and electrochemical techniques. Nb doping of titania significantly increased the surface area of the support from 145 (TNTs) to 260 m 2 g −1 (Nb-TNTs), which was significantly higher than those of the Nb-doped titania supports previously re- ported in the literature. The surface analytical techniques showed good dispersion of the catalysts onto the supports. The X-ray photoelectron spectroscopy analyses showed that Nb was mainly in the form of Nb(IV) species, the suitable form to behave as a donor introducing free electrons to the conduction band of titania. The redox transitions of the cyclic voltammograms, in agreement with the XPS results, were found to be reversible. Despite the supported materials presented bigger crystallite sizes than the unsupported ones, the total number of active sites of the former was also higher due to their better cat- alyst dispersion. Considering the outer and the total charges of the cyclic voltammograms in the range 0.1–1.4 V, stability and electrode potentials at given current densities, the preferred catalyst was IrO 2 sup- ported on the Nb-TNTs. The electrode potentials corresponding to given current densities were between the smallest ones given in the literature despite the small oxide loading used in this work and its Nb doping, thus making the Nb-TNTs-supported IrO 2 catalyst a promising candidate for the OER. The good dispersion of IrO 2 , high specific surface area of the Nb-doped supports, accessibility of the electroactive centers, increased stability due to Nb doping and electron donor properties of the Nb(IV) oxide species were considered the main reasons for its good performance.

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.

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.

[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.

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.

Cost-efficient and sustainable electrocatalysts for oxygen evolution reaction (OER) is highly desired in the search for clean and renewable energy sources. In this study, we develop a new one-step synthesis strategy of novel composites based on Ni and molybdenum carbide embedded in N- and P-dual doped carbon matrices using mainly chitosan biopolymer as the carbon and nitrogen source, and molybdophosphoric acid (HMoP) as the P and Mo precursor. Two composites have been investigated through annealing a mixture of Ni/chitosan and HMoP with two unlike carbon matrices, melamine and graphene oxide, at a high temperature. Both composites exhibit similar multi-active sites with high electrocatalytic activity for OER in an alkaline medium, which is comparable to the IrO2 catalyst. For this study, an accurate measurement of the onset potential for O2 evolution has been used by means of a rotating ring-disk electrode (RRDE). The use of this method allows confirming a better stability in the chitosan/graphene composite. This work serves as a promising approach for the conversion of feedstock and renewable chitosan into desired OER catalysts.

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.

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.