Search

Materials and methods: Synthesis of C1N1.-- Synthesis of NC@Fe3C materials.-- Physicochemical characterization.-- Electrochemical characterization.-- AEMFC experiments.-- Structural Models.-- DFT computational details.-- Calculation methods for evaluating ORR activity.

Encapsulating Fe3C in carbon layers has emerged as an innovative strategy for protecting Fe3C while preserving its high oxygen reduction activity. However, fundamental questions persist regarding the active sites of encapsulated Fe3C due to the restricted accessibility of oxygen molecules to the metal sites. Herein, the intrinsic electron transfer mechanisms of Fe3C nanoparticles encapsulated in N-doped carbon materials are unveiled for oxygen reduction electrocatalysis. The precision-structured C1N1 material is used to synthesize N-doped carbons with encapsulated Fe3C, significantly enhancing catalytic activity (EONSET = 0.98 V) and achieving near-100% operational stability. In anion-exchange membrane fuel cells, an excellent peak power density of 830 mW cm−2 is reached at 60 °C. The experimental and computational results revealed that the presence of Fe3C cores dynamically triggers electron transfer to the outermost carbon layer. This phenomenon amplifies the oxygen reduction reaction performance at N sites, contributing significantly to the observed catalytic enhancement.

This study demonstrates that ammonia concentration when doping carbons using hydrothermal treatment has crucial impact on textural properties. The latter translates into an improvement of the electrochemical performance of carbon materials, when used into electrochemical capacitors, especially at high-rate performance. Nitrogen-doped carbons were synthesized by subjecting tannin to hydrothermal carbonization in ammonia solutions of varying concentrations. After carbonization and CO2 activation, the as-produced activated carbons (ACs) were tested as electrodes for electrochemical capacitors in symmetric configuration using 1 M H2SO4 as aqueous electrolyte. Interestingly, ammonia concentration during the hydrothermal synthesis step did not significantly affect the final N content (ca. 3 and 4 at. %) nor the nitrogen and oxygen functionalities on the surface. However, the use of ammonia had crucial impact on the textural properties developed by CO2 activation, and therefore on the electrochemical performance of the ACs. The best-performing N-doped AC showed specific electrode capacitance values, based on carbon material, of 212 F g−1 at 0.5 A g−1 and outstanding capacitance retention of ca. 71 % at 40 A g−1. It also showed high cycling stability, with capacitance retention of ca. 96 % after 30,000 cycles. Furthermore, this AC outperformed similar reported materials, achieving a specific energy of 4.6 W h kg−1 at 12.1 kW kg−1.

Encapsulating Fe3C in carbon layers has emerged as an innovative strategy for protecting Fe3C while preserving its high oxygen reduction activity. However, fundamental questions persist regarding the active sites of encapsulated Fe3C due to the restricted accessibility of oxygen molecules to the metal sites. Herein, the intrinsic electron transfer mechanisms of Fe3C nanoparticles encapsulated in N-doped carbon materials are unveiled for oxygen reduction electrocatalysis. The precision-structured C1N1 material is used to synthesize N-doped carbons with encapsulated Fe3C, significantly enhancing catalytic activity (EONSET = 0.98 V) and achieving near-100% operational stability. In anion-exchange membrane fuel cells, an excellent peak power density of 830 mW cm−2 is reached at 60 °C. The experimental and computational results revealed that the presence of Fe3C cores dynamically triggers electron transfer to the outermost carbon layer. This phenomenon amplifies the oxygen reduction reaction performance at N sites, contributing significantly to the observed catalytic enhancement. © 2024 Wiley-VCH GmbH.

In this study, efficient electrodes for the hydrogen evolution reaction (HER) based on low-cost and metal-free carbon catalysts are presented. Phytic acid, a biosourced molecule containing carbon (C) and phosphorus (P), was found to be an excellent precursor for producing carbon materials with high P content, depending on the carbonization temperature, from 27.9 wt% at 700 °C to 7.3 wt% at 1000 °C. A green and easy route to produce P-doped carbon materials by heat treatment of this biosourced precursor without the need for additional reagents is thus proposed. We show that the conversion of P-O-type groups into P-C-type species is of paramount importance for improving the catalytic activity in HER of P-doped carbon materials. P-C-type groups appear to be the key factor in the electrocatalytic activity, reaching an onset potential of - 0.27 V. This study sheds light on the origin of the catalytic activity of P-doped carbons, in which P is expected to modify the homogenous distribution of the electron density of undoped carbons and increase their catalytic performance.

Figure S1: Pore size distributions of all PDC-T samples. Table S1: Atomic percentages (at.%) of carbon (C), oxygen (O) and phosphorus (P), obtained by XPS, and ratios of P-C bonds to total P (PT) content for all PDC-T materials. Figure S2: XPS spectra of (a) C 1s and (b) O 1s of the PDC-T series. Table S2: Literature comparison of the performance of materials similar to PDC-900 for the ORR shown in Fig. 4f.

The sluggish kinetics and high overpotential of the oxygen reduction reaction (ORR) in fuel cells and metal–air batteries are currently resolved through the use of expensive and scarce platinum-based electrocatalysts. Here, a green and straightforward synthesis method for preparing metal-free phosphorus-doped carbon (PDC) materials via carbonization of phytic acid at temperatures ranging from 700 to 1000 °C is reported. This study provides valuable insights into the effect of carbonization temperature on the surface chemistry and catalytic performance of PDCs. The PDC electrocatalysts exhibit ORR electrocatalytic activity equivalent to that of the state-of-the-art P-doped carbons and show a correlation between the content of C–P species and the electrocatalytic activity for the ORR. The study suggests that C–P-type species are responsible for the improved ORR. These findings not only demonstrate a practical and low-cost approach for preparing highly efficient metal-free electrocatalysts for the ORR, but also highlight their potential for the development of sustainable energy technologies.

Although high-surface area activated carbons used as supercapacitor (SC) electrodes are frequently produced by KOH activation, this study shows that, when aqueous electrolytes are used, CO2 activation is a better choice from the point of view of SC performance, environment and economy. Ordered mesoporous carbons (OMCs) produced by a mechanochemical synthesis method from mimosa tannin are activated with KOH to use these materials as electrodes for SCs. A comparative analysis of the same OMCs but activated with CO2 is presented to examine the effect of the activation process on materials performance. KOH-activated materials exhibit good electrochemical performance at low charging rates, reaching specific cell capacitance values of 49 F g−1 at 0.5 Ag−1, however, restricted access to microporosity and low water affinity water reduces their performance at high charging rates. In contrast, the best performing CO2-activated material can retain 81% of capacitance at 20 A g−1, compared to 25% for a KOH-activated OMC with similar properties and tested under the same conditions. A thorough review of the open literature suggests that CO2 activation would produce materials with a suitable combination of pore network connectivity and water affinity, resulting in SCs with high rate capability in an aqueous electrolyte. These conclusions were drawn by judiciously integrating the analysis of: (i) the hysteresis loop scanning of N2 adsorption-desorption isotherms; and (ii) water adsorption isotherms as tools to more accurately assess the pore network connectivity and water affinity of the materials, which are not generally considered when studying SC performance.

In this study, efficient electrodes for the hydrogen evolution reaction (HER) based on low-cost and metal-free carbon catalysts are presented. Phytic acid, a biosourced molecule containing carbon (C) and phosphorus (P), was found to be an excellent precursor for producing carbon materials with high P content, depending on the carbonization temperature, from 27.9 wt.% at 700 °C to 7.3 wt.% at 1000 °C. A green and easy route to produce P-doped carbon materials by heat treatment of this biosourced precursor without the need for additional reagents is thus proposed. We show that the conversion of P-O-type groups into P-C-type species is of paramount importance for improving the catalytic activity in HER of P-doped carbon materials. P-C-type groups appear to be the key factor in the electrocatalytic activity, reaching an onset potential of - 0.27 V. This study sheds light on the origin of the catalytic activity of P-doped carbons, in which P is expected to modify the homogenous distribution of the electron density of undoped carbons and increase their catalytic performance.

A surfactant-water-assisted mechanochemical mesostructuration method is used to produce model carbon materials with a disordered or ordered mesoporous structure (DMCs or OMCs, respectively) from a sustainable precursor, mimosa tannin. These model materials, differing only in their mesoporous structure, allow assessing the importance of the connectivity of the micro-mesopore network on the electrochemical performance of the resultant supercapacitors (SCs). Connectivity is studied through the scanning of hysteresis loops from nitrogen adsorption-desorption isotherms and, contrary to what it is suggested in the literature, order is not always beneficial for the performance of SCs. A thorough review of the open literature and comparison with our electrodes led us to conclude that CO2-activated DMCs and OMCs are among the best materials reported so far, as they exhibit excellent SC behavior, high-rate capability, and long-term stability in aqueous and organic electrolytes. It is showed that ordered mesopores improve the diffusion of the small-size ions of the aqueous electrolyte and hence favor a better performance at high charging rates, resulting in a 12% higher capacitance retention at 80 A g−1 when compared to that obtained with the disordered materials. However, the more interconnected porosity of the disordered materials allows better diffusion of large-size ions, thus improving the electrochemical performance in the organic electrolyte by 15% at 40 A g−1.

Two micro-mesoporous carbons (MMCs): a disordered mesoporous carbon (DMC) and an ordered mesoporous carbon (OMC), synthesized by an easy, low-cost, and green method are proposed as efficient hydrocarbon sieves for the separation of C6 isomers: n-hexane (nHEX), 2-methylpenthane (2 MP) and 2,2-dimethylbutane (22DMB). Their textural characterization reveals a highly interconnected pore network within the DMC, while a reverse hierarchy of ordered mesopores only accessible through narrow micropores is found in the OMC. The pore texture strongly affects their adsorption performance by kinetic and molecular sieving effects; the narrow constrictions in the OMC allow adsorption of nHEX and partially 2 MP but not 22DMB, whereas the highly connected pore network of DMC allows adsorption of the three isomers. Multi-component adsorption isotherms calculated from the single-component experimental results by ideal adsorbed solution theory (IAST) demonstrates that the OMC material has a remarkably high selectivity for the adsorption of nHEX and nHEX + 2 MP from binary and ternary mixtures, respectively. To the best of the authors’ knowledge, such behavior has never been reported so far for carbon materials. Hence, this study shows that tannin-derived MMCs have great potential to be used as an eco-friendly and low-cost alternative for the selective separation of di-branched C6 isomers.

The limited fossil fuels reserves and the increasing global warming make necessary a transition to sustainable energy production and environment-friendly technologies. However, renewable sources of energy have an intermittent nature, so the improvement of energy storage devices together with a better energy management are needed. Supercapacitors (SC) are already in use in many car models for brake energy regeneration, in Start-Stop systems (not only in electric vehicles) and in hybrid systems along with batteries. Activated mesoporous carbons are valuable materials to be used as electrodes for SCs due to their high surface area and hierarchical pore structure that facilitates the diffusion of electroactive species. Herein, an easy and green one-pot synthesis method was developed to produce mesoporous carbons through ball milling of mimosa tannin as carbon precursor, with Pluronic© F127 and water. CO2 activation carried out on selected ordered and disordered mesoporous carbons (OMCs and DMCs, respectively) allowed improving their textural properties. Activated OMCs and DMCs were tested as electrodes for SCs.

Supercapacitor (SC) devices stor energy through the interaction of electrolyte ions with the surface of the electrode, resulting in the formation of an electric double layer (EDL). The EDL mechanism leads to high power outputs but, usually, to low energy densities in comparison with batteries. Among the strategies to improve carbon-based supercapacitors performances, themodification of the surface chemistry is frequently used. The introduction of heteroatoms incarbon materials can enhance theirelectrical conductivity and/or their affinity towards the electrolyte, in addition to enabling redox reactions that provide a pseudocapacitance contribution, thus further increasing the capacitance.

Ordered mesoporous carbons (OMCs) are produced by an easy, fast, and green surfactant-water-assisted mechanochemical mesostructuration (SWAMM) method, using only tannin, water, and Pluronic F127 as raw materials. OMCs are physically activated with CO2 to increase their surface area from ∼500 to ∼2000 m2 g–1 while maintaining the mesoporosity fairly unaltered, which allow studying the effect of the micropore size distribution on the performance of supercapacitors both in aqueous and organic electrolytes. Selected activated OMCs (AOMCs) reach maximum cell capacitance values of 37 and 27 F g–1, at 0.2 A g–1, in aqueous and organic electrolytes, respectively. High long-term stabilities over time and after continuous cycling are found for the tested AOMCs in both kinds of electrolytes. High-rate capabilities are achieved at high current densities with capacitance retentions up to 70% at 80 A g–1 and 44% at 40 A g–1 in aqueous and organic electrolytes, respectively, because of suitable pore size distribution that promotes ion diffusion into the electrodes, in particular for the material activated during 75 min.

A simple, green, one-pot mechanosynthesis method for preparing perfectly ordered mesoporous carbons (OMCs) by ball-milling is presented herein. Mimosa tannin (T) and Pluronic® F127 (P) are used as the carbon precursor and as the mesopore-directing agent, respectively. Water (W) is used to promote micelle formation and also the interaction between T and P; no crosslinker is used and drying or curing steps before carbonization are not needed either. The experimental conditions, such as the milling time, the pH of added W and the P : W ratio are optimized to obtain perfect 2D hexagonal OMCs with BET areas as high as 588 m2 g−1 after carbonization at 900 °C. Carbonization temperatures up to 1500 °C or activation allow further increasing the surface area, up to ∼1900 m2 g−1, while maintaining the ordered mesoporous structure. These materials should find relevant applications in environmental remediation strategies for oil spills, in selective CO2 adsorption from humid gases or as electrodes of supercapacitors.

In the traditional teaching of thermodynamic the subject of refrigeration is usually studied at a theoretical level, basing most of the discussion on the reverse Carnot cycle. This leaves the student with a very abstract and intangible idea. In this paper we analyze real refrigeration systems. The study of each component allows us to reinforce several subjects, not only of thermodynamics but also of chemistry, fluid dynamics and environmental impact. This analysis shows the differences in design for compression and absorption systems that carries the refrigerant through the corresponding thermodynamic cycle. Besides, we study the physical and chemical properties that characterize a substance as a refrigerant and/or absorbent. On the other hand, we deal with an issue of major importance which is the environmental consequences of currently used refrigerants, such as the ozone layer depletion or the increase in global warming. This leads us to search for systems that do not harm the environment when supplying energy to them or when emitting pollutants as a result of its operation., Habitualmente los cursos de termodinámica abordan el tema de la refrigeración de manera teórica utilizando el ciclo inverso de Carnot, esto deja al estudiante con una idea muy abstracta y poco tangible. En el presente trabajo se realiza un análisis de los sistemas reales de refrigeración, lo cual permite que, al estudiar cada componente, se refuercen y analicen diversos temas no sólo de termodinámica sino también de química, dinámica de fluidos e impacto ambiental. El análisis da a conocer las diferencias en diseño para sistemas por compresión y por absorción que llevan al refrigerante a través del ciclo termodinámico respectivo. También, se hace un estudio de las propiedades físicas y químicas que caracterizan a una sustancia como refrigerante y/o absorbente. Por otro lado, se aborda un tema de gran importancia como son las consecuencias ambientales que surgen con los refrigerantes utilizados actualmente, tales como la disminución de la capa de ozono y el aumento del efecto invernadero, lo que nos lleva a buscar sistemas que no dañen el ambiente al suministrarles energía o al emitir sustancias contaminantes como resultado de su funcionamiento.