Preparation and modification of two-dimensional graphene and MoS2 nanosheets for energy and environmental applications

Tesis doctoral defendida el 28 de mayo de 2021 en la Universidad de Oviedo[EN] The main aim of this thesis is the preparation by liquid-phase methods of two-dimensional materials, such as graphene and MoS2 nanosheets, as well as the exploration of their use in energy and environmental applications. Using bulk graphite and MoS2 pieces as the starting materials, two-dimensional graphene and MoS2 nanosheets were obtained, respectively, via cathodic exfoliation in aqueous medium. Different parameters of the process were optimized. Among the different starting graphite materials used to produce graphene (graphite foil, graphite rod and highly oriented pyrolytic graphite), graphite foil shows the highest yields, while electrolytes comprising quaternary ammonium salts of intermediate size afford graphene yields close to 50 wt% in optimal conditions. Graphene obtained by this process has a low content of oxygen, a high structural quality and a good electrical conductivity. The cathodically exfoliated graphene was used as an electrode for supercapacitors in the form of a hybrid that incorporated a small amount of vertically oriented cobalt oxide nanosheets. The latter provide pseudocapacitance and act as a spacer, thus preventing the re-stacking of graphene. The hydrophobic graphene obtained by cathodic exfoliation was used as an absorbent of aqueous pollutants, such as oils and dyes. A low-density, easy-to-handle macroscopic structure was prepared by coating melamine foam with cathodic graphene. This strategy prevents the re-stacking of the nanosheets and thus a dramatic loss of specific surface area, thus facilitating the regeneration of the adsorbent. The cathodic exfoliation of bulk MoS2 was carried out using common salts, such as KCl, as the electrolyte in apropiate conditions. The resulting two-dimensional MoS2 nanosheets retained the original 2H phase of the starting material and do not undergo oxidation at any substantial extent. The obtained material was studied as catalyst for the reduction of nitroarenes and as an electrode for supercapacitors. In the latter case, a hybrid made up of the exfoliated MoS2 nanosheets and single walled carbon nanotubes, which provide electrical conductivity and act as spacer, was used. Cathodic MoS2 nanosheets were found to be very competitive for both applications in comparison with similar materials obtained through other methods. Moreover, MoS2 nanosheets were subjected to activation treatments in order to improve their catalytic activity towards the reduction of nitroarenes and organic dyes in water. Colloidal dispersions of nanosheets were first obtained by sonication of bulk MoS2 powder in isopropanol, and then treated with hydrazine to generate sulfur vacancies (active sites). The MoS2 nanosheets activated in such a way show catalytic activity values three o four times higher than those of their non-activated counterparts.[ES] El objetivo de esta tesis consiste en la preparación y procesado, mediante métodos en fase líquida, de nanoláminas bidimensionales de grafeno y MoS2 para aplicaciones energéticas y medioambientales. Utilizando como cátodo piezas de grafito y MoS2 se obtuvieron, mediante exfoliación electroquímica en medio acuoso, grafeno y nanoláminas de MoS2, respectivamente. Se optimizaron diferentes parámetros del proceso. Entre los distintos tipos de grafito de partida utilizados para la producción de grafeno (grafito flexible, barras de grafito y grafito pirolítico altamente orientado), el grafito flexible es el que condujo a mayor rendimiento en producto expandido. En cuanto al electrolito, ciertas sales de amonio cuaternario de tamaño apropiado resultaron óptimas para maximizar la expansión del electrodo y el rendimiento en grafeno, alcanzándose valores de cerca del 50%. El grafeno obtenido mediante este proceso posee un bajo grado de oxidación, una alta calidad estructural y una buena conductividad eléctrica. Se empleó el grafeno obtenido por exfoliación catódica como electrodo para supercondensadores, fabricando un híbrido mediante la adición de una pequeña cantidad de óxido de cobalto en forma de nanoláminas verticalmente orientadas. Éstas últimas proporcionan pseudocapacitancia y funcionan como espaciador, impidiendo el reapilamiento del grafeno. El alto grado de hidrofobicidad del grafeno obtenido lo hace también útil como adsorbente de contaminantes acuosos, tales como aceites y colorantes. En este caso, se recubrió una esponja de melamina con el grafeno exfoliado, evitando así el reapilamiento de las láminas y la consecuente pérdida de superficie específica, obteniéndose una estructura macroscópica de baja densidad y fácilmente manejable, lo que a su vez facilitaba su regeneración tras el uso. La exfoliación electroquímica catódica de MoS2 se llevó a cabo utilizando como electrolitos sales comunes, como el KCl, en las condiciones adecuadas. La exfoliación no provoca cambio de fase (se conserva la fase 2H) ni oxidación del material. Las nanoláminas de MoS2 obtenidas se estudiaron como catalizador para la reducción de nitroarenos y como electrodo para supercondensadores, formando híbridos con nanotubos de carbono de capa simple, que proporcionan conductividad eléctrica y actúan como espaciadores. Los materiales resultaron competitivos en ambas aplicaciones, comparando con materiales similares preparados mediante otros métodos. Además, se llevaron a cabo tratamientos de modificación (activación) de nanoláminas de MoS2 con vistas a mejorar su actividad como catalizadores en la reducción de nitroarenos y colorantes orgánicos en agua. En este caso, se obtuvieron las nanoláminas mediante sonicación de MoS2 bulk en polvo en isopropanol y se generaron vacantes de azufre (centros activos) en ellas mediante tratamiento con hidracina. Las nanoláminas activadas mostraron valores de actividad catalítica hasta 3-4 veces mayores que los del material sin tratar.Peer reviewe

Green and easy synthesis of P-doped carbon-based hydrogen evolution reaction electrocatalysts

7 figures, 1 table.-- Supplementary information available. © 2023. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/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.This study was partially supported by the French PIA project “Lorraine Université d’Excellence”, reference ANR-15-IDEX-04-LUE, and the TALiSMAN and TALisMAN2 projects funded by ERDF. 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).Peer reviewe

Molecular Functionalization of 2H-Phase MoS2 Nanosheets via an Electrolytic Route for Enhanced Catalytic Performance

The molecular functionalization of two-dimensional MoS2 is of practical relevance with a view to, for example, facilitating its liquid-phase processing or enhancing its performance in target applications. While derivatization of metallic 1T-phase MoS2 nanosheets has been relatively well studied, progress involving their thermodynamically stable, 2H-phase counterpart has been more limited due to the lower chemical reactivity of the latter. Here, we report a simple electrolytic strategy to functionalize 2H-phase MoS2 nanosheets with molecular groups derived from organoiodides. Upon cathodic treatment of a pre-expanded MoS2 crystal in an electrolyte containing the organoiodide, water-dispersible nanosheets derivatized with acetic acid or aniline moieties (∼0.10 molecular groups inserted per surface sulfur atom) were obtained. Analysis of the functionalization process indicated it to be enabled by the external supply of electrons from the cathodic potential, although they could also be sourced from a proper reducing agent, as well as by the presence of intrinsic defects in the 2H-phase MoS2 lattice (e.g., sulfur vacancies), where the molecular groups can bind. The acetic acid-functionalized nanosheets were tested as a non-noble metal-based catalyst for nitroarene and organic dye reduction, which is of practical utility in environmental remediation and chemical synthesis, and exhibited a markedly enhanced activity, surpassing that of other (1T- or 2H-phase) MoS2 materials and most non-noble metal catalysts previously reported for this application. The reduction kinetics (reaction order) was seen to correlate with the net electric charge of the nitroarene/dye molecules, which was ascribed to the distinct abilities of the latter to diffuse to the catalyst surface. The functionalized MoS2 catalyst also worked efficiently at realistic (i.e., high) reactant concentrations, as well as with binary and ternary mixtures of the reactants, and could be immobilized on a polymeric scaffold to expedite its manipulation and reuse.Funding by the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN), Agencia Estatal de Investigación (AEI), and the European Regional Development Fund (ERDF) through project RTI2018-100832-B-I00, as well as Plan de Ciencia, Tecnología e Innovación (PCTI) 2013-2017 del Principado de Asturias and the ERDF (project IDI/2018/000233) is gratefully acknowledged. S.G.-D. is grateful to the Spanish MINECO for his predoctoral contract (BES/2016 077830).Peer reviewe

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)

Green and easy synthesis of P-doped carbon-based hydrogen evolution reaction electrocatalysts

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.This study was partially 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)

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)

Efficient cathodic exfoliation of graphite in aqueous electrolytes towards high quality graphene for energy and environmental applications

The electrochemical exfoliation of graphite in aqueous medium stands out as an attractive and scalable approach for the production of graphenes for different applications, due to its simplicity, cost-effectiveness and environmental friendliness [1]. In particular, cathodic exfoliation in water should allow access to high quality and non-oxidized graphene flakes, but this possibility has been limited by a poor intercalation ability of aqueous cations [2]. Here, we demonstrate that with a proper choice of starting graphite and electrolyte, high quality graphene flakes can be obtained in substantial yields via cathodic delamination in water.Funding by the Spanish Ministerio de Economia y Competitividad (MINECO) and the European Regional Development Fund (ERDF) through project MAT2015-69844-R and by the Spanish Ministerio de Ciencia, Innovacion y Universidades and ERDF through project RTI2018-100832-B-I00 is gratefully acknowledged. S.G-D. and J.M.M. are grateful to MINECO and the Spanish Ministerio de Educacion, Cultura y Deporte (MECD), respectively, for their pre-doctoral contracts (BES/2016 077830 and FPU14/00792, respectively

Advanced design of metal nanoclusters and single atoms embedded in C1N1-derived carbon materials for ORR, HER, and OER

5 figures.-- Supplementary information available.Single atoms and nanoclusters of Fe, Ni, Co, Cu, and Mn are systematically designed and embedded in a well-defined C1N1-type material that has internal cavities of ≈0.6 nm based on four N atoms. These N atoms serve as perfect anchoring points for the nucleation of small nanoclusters of different metal natures through the creation of metal-nitrogen (TM-N4) bonds. After pyrolysis at 800 °C, TM@CNx-type structures are obtained, where TM is the transition metal and x < 1. Fe@CNx and Co@CNx are the most promising for oxygen reduction reaction and hydrogen evolution reaction, respectively, with a Pt-like performance, and Ni@CNx is the most active for oxygen evolution reaction (OER) with an EOER of 1.59 V versus RHE, far outperforming the commercial IrO2 (EOER = 1.72 V). This systematic and benchmarking study can serve as a basis for the future design of advanced multi-functional electrocatalysts by modulating and combining the metallic nature of nanoclusters and single atoms.ANR-15-IDEX-04-LUE and the TALiSMAN and TALiSMAN2 projects, financed by the European Regional Development Fund (ERDF), are gratefully acknowledged. J.Q.B. thanks the Ministerio de Universidades, the European Union, and the University of Alicante for the financial support (MARSALAS21-21). S.G.D. thanks the Ministerio de Universidades, the European Union, and the University of Oviedo for the financial support (MU-21-UP2021-030 30267158).Peer reviewe

An aqueous cathodic delamination route towards high quality graphene flakes for oil sorption and electrochemical charge storage applications

The electrochemical exfoliation of graphite in aqueous medium stands out as an attractive, scalable approach for the production of graphenes for different applications, due to its simplicity, cost-effectiveness and environmental friendliness. In particular, cathodic exfoliation in water should allow access to high quality, non-oxidized graphene flakes, as it avoids the intrinsic oxidizing conditions that typically plague the anodic route, but this possibility has been limited by a poor intercalation ability of aqueous cations. Here, we demonstrate that with a proper choice of starting graphite and electrolyte, high quality graphene flakes can be obtained in substantial yields via cathodic delamination in water. Graphites having some pre-expanded edges and interlayer voids (e.g., graphite foil) were found to be critical for a successful exfoliation. Large differences in the efficiency of a range of aqueous quaternary ammonium-based electrolytes were observed, quantitatively compared and rationalized on the basis of their chemical structure. Graphene yields up to 40–50 wt% were attained with the most efficient cations (tetrapropylammonium and hexyltrimethylammonium). Hydrophobic sponges made up of cathodic graphene-coated melamine foam exhibited a notable capacity towards the sorption of oils and organic solvents from water with good re-usability. Hybrids comprised of cathodically exfoliated graphite and a small amount of vertically oriented cobalt oxide nanosheets displayed good electrochemical charge storage behavior. Overall, the ability to access graphene flakes in considerable yields by the aqueous cathodic route disclosed here should raise the prospects of cathodic exfoliation as a competitive method for the industrial manufacturing of high quality graphene for practical applications.Funding by the Spanish Ministerio de Economía y Competitividad (MINECO) and the European Regional Development Fund (ERDF) through project MAT2015-69844-R and by the Spanish Ministerio de Ciencia, Innovación y Universidades and ERDF through project RTI2018-100832-B-I00 is gratefully acknowledged. Partial funding by Plan de Ciencia, Tecnología e Innovación (PCTI) 2013-2017 del Principado de Asturias and the ERDF through project IDI/2018/000233 is also aknowledged. S.G-D. and J.M.M. are grateful to MINECO and the Spanish Ministerio de Educación, Cultura y Deporte (MECD), respectively, for their pre-doctoral contracts (BES/2016 077830 and FPU14/00792, respectively).Peer reviewe