Influence of activation conditions on textural properties and performance of activated biochars for pyrolysis vapors upgrading

The main aim of the present study is to provide a comprehensive assessment of the effects of process activation conditions on the textural properties of the resulting activated carbons, which were produced from wheat straw-derived biochar through chemical activation (with K2CO3 at different pressures and mass impregnation ratios) and physical activation (with CO2 at different temperatures and pressures). For chemically activated biochars, it was found that specific surface area and pore size distribution were both only positively affected by increasing the carbonate loading. However, physically activated biochars produced at the highest pressure and lowest temperature (1.0 MPa and 700 °C) had the highest surface areas and widest pore size distributions. The materials with the most appropriate textural properties were then tested as catalysts for steam and dry reforming of the aqueous phase of pyrolysis oil. The best catalytic performance (a total gas yield of 74% and a selectivity toward H2 of almost 40%) was observed for a physically activated biochar. This good performance was ascribed to the high availability of K0 on the catalyst surface, which could effectively promote the reactions involved in the upgrading proces

Easy enrichment of graphitic nitrogen to prepare highly catalytic carbons for oxygen reduction reaction

One of the biggest challenges in producing fuel cells at affordable prices is to synthesize carbon materials selectively doped with graphitic nitrogen, as it is considered the most active nitrogen species for the oxygen reduction reaction (ORR). So far, all strategies focus on the use of nitrogen-containing carbon precursors, which limits the functionalization of commercially available carbon materials. Here, we present a post-functionalization method to boost the catalytic properties of carbon materials for the ORR by selectively enriching activated carbons with nitrogen graphitic species. A commercial high-surface area activated carbon was post-functionalized by a two-step procedure. First, the commercial carbon was mixed with different carbon/urea weight ratios and heated in air at 350 °C. Then, the functionalized materials were heat-treated at high temperature (from 700 to 1300 °C) to tailor the amount and distribution of the different nitrogen species in the resulting carbon structure. Nitrogen functionalization using a carbon to urea weight ratio of 1:2 and heat-treatment at 1100 °C led to highly selective doping in graphitic nitrogen species, which provided the tools to individually asses the catalytic activity of these nitrogen species. In addition, this study presents a low-cost and easily feasible synthesis route to improve the catalytic activity of carbon materials, leading to an onset potential of almost 0.9 V compared to reversible hydrogen electrode for ORR in an alkaline electrolyte. Moreover, this study provides significant evidence for the key role of graphitic nitrogen.The French research team thanks ANR-15-IDEX-04-LUE and the TALiSMAN project (2019-000215), financed by the European Regional Development Fund (ERDF). The authors from UA thank MICINN and ERDF (project RTI2018-095291-B-I00) for financial support

Zinc Doping Enhances the Electrocatalytic Properties of Cobalt Borides for the Hydrogen Evolution Reaction

Electrochemical water splitting requires new, low-cost cathode electrodes for the hydrogen evolution reaction to enable the worldwide implementation of electrolyzers. Cobalt borides are proposed as one of the most promising materials to overcome the limitations of the commercial electrocatalysts, but the catalytic activity still needs to be improved to be competitive. Here, we report that the introduction of zinc into cobalt boride to produce a ternary cobalt boride is an efficient route to further improve the catalytic activity towards the hydrogen evolution reaction (HER) of cobalt boride. The ternary Co-Zn-B was prepared by an easy chemical reduction method to achieve superior HER electrocatalytic performance with a lower overpotential than the homologous Co-B. The larger surface area, structural order, crystallization degree and, in particular, the different surface chemistry seem to be key factors for this improvement.This study was partly supported by the French PIA project “Lorraine Université d’Excellence,” reference ANR-15-IDEX-04-LUE and the TALiSMAN project funded by ERDF (2019-000214)

Molecular sieving of linear and branched C6 alkanes by tannin-derived carbons

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.Jimena Castro-Gutiérrez gratefully acknowledges CONACYT-SENER for the scholarship awarded (601021/438978) to support her PhD studies, resulting in the work presented herein. This study was partly funded by the French PIA project "Lorraine Université d’Excellence”, reference ANR-15-IDEX-04-LUE, and TALiSMAN project, funded by FEDER (2019-000214). Joaquín Silvestre-Albero would like to acknowledge the financial support from MINECO (PID2019-108453GB-C21) and H2020 (MSCA-RISE-2016/NanoMed Project)

Easy and Support-Free Synthesis of Bimetallic Borates for Boosting the Oxygen Evolution Reaction

The sluggish kinetics of the oxygen evolution reaction (OER) is one of the most limiting factors for the development of many “green” electrochemical devices. Expensive ruthenium and iridium oxide electrodes are often used as advanced electrocatalysts to overcome this limitation. However, these materials are rare in nature, which further limit the implementation of this kind of electrochemical device on a global scale. Compounds based on transition metals and boron have proven to be promising alternatives to commercial electrocatalysts due to their high catalytic properties and robust stability under working conditions. However, such compounds are often obtained through expensive synthetic routes that often involve the use of supports, which increases the cost of electrocatalysts. Here, we present an easy and support-free synthesis of bimetallic borates based on the introduction of transition metals into cobalt borates. Depending on the metal, different morphologies, structural order, surface chemistry, and, most importantly, electrocatalytic properties toward the OER have been obtained. Among all the transition metals, nickel is the one that most improves the catalytic activity of cobalt borate for the OER in an alkaline electrolyte. An overpotential of 230 mV, similar to that of commercial and state-of-the-art electrocatalysts, was obtained by using a support-free synthesis route for the preparation of this catalyst.This study was partly supported by the French PIA project “Lorraine Université d’Excellence”, reference ANR-15-IDEX-04-LUE, and the TALiSMAN project funded by ERDF (2019-000214). SGD thanks the Ministerio de Universidades, the European Union, and the University of Oviedo for the financial support (MU-21-UP2021-030 30267158). JQB thanks the Ministerio de Universidades, the European Union, and the University of Alicante for the financial support (MARSALAS21-21)

Nitrogen sites prevail over textural properties in N-doped carbons for the oxygen reduction reaction

Nitrogen-doped carbon-based electrodes are among the most promising alternatives to platinum-based electrodes in the cathode of fuel cells and metal-air batteries, where the oxygen reduction reaction (ORR) takes place. Among the approaches for improving ORR activity, nitrogen functionalities and well-developed textural properties have proved very effective. Nonetheless, the question of which between nitrogen active sites or textural properties are more crucial in N-doped carbon materials remains unanswered. This work proposes a comparative and critical approach through the selective functionalization of four commercial activated carbons with different textural properties. This study highlights the greater importance of N-doping in relation to the textural properties of carbon materials, and provides fundamental insights for conclusively addressing the ongoing debate within the carbon community about the significance of these two factors in the context of ORR.The ANR-15-IDEX-04-LUE project and the TALiSMAN and TALiSMAN2 projects funded by the European Regional Development Fund (ERDF) are gratefully acknowledged. The authors would also like to thank project PID2021-123079OB-I00 funded by MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe”. Finally, JQB thanks the Ministerio de Universidades, the European Union, and the University of Alicante for their financial support (MARSALAS21-21)