Semi-amorphous carbon nitride films derived from soluble polymeric precursors: towards multifunctional electrochemical coatings with enhanced stability
- Autores: Oleg Dubov
- Directores de la Tesis: Josep Font Capafons (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2022
- Idioma: inglés
- Tribunal Calificador de la Tesis: Carlos Alemán Llansó (presid.), Alex Fragoso Sierra (secret.), Javier Pérez Ramírez (voc.)
- Programa de doctorado: Programa de Doctorado en Nanociencia, Materiales e Ingeniería Química por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
- español
En los últimos años, la demanda generada en las nuevas industrias electroquímicas relacionades con las energías verdes han derivado en un mayor interés por distintes formas de materiales basados en carbono, ya que se han convertido en un nuevo foco en la ciencia de los electrodos. Sin embargo, a pesar de la atractiva conveniencia económica e industrial de los materiales de soporte basados en carbono, su estabilidad oxidativa es en la mayoría de los casos muy baja si se compara con la resistencia a la oxidación de metales nobles o incluso de sistemas de óxidos metálicos. Aun así, dos familias de materiales pertenecientes al grupo de los soportes basados en carbono representan una excepción a esta regla general – los carbonos vítreos y los nitruros de carbono. Los llamados carbonos vítreos, producidos mediante la pirólisis de polímeros carbonáceos baratos, poseen una estabilidad oxidativa exclusiva, junto con una alta conductividad y estabilidad química. El origen de este hecho recae en que el proceso de pirólisis, el cual deriva en la formación de carbono vítreo, requiere temperaturas notoriamente elevadas (de 1000 a 3000°C). Otro grupo de la familia de los carbonáceos, distinguidos por su elevada estabilidad oxidativa, los nitruros de carbono, exhiben una combinación de propiedades que la hacen ser prácticamente la antípoda de los carbonos vítreos. La temperatura de pirólisis en comparación con la del carbono vítreo es significativamente inferior, hallándose en el rango de los 400-600ºC. Como piedra angular de esta tesis, se formuló la hipótesis de la existencia de un nuevo miembro de la familia de los materiales carbonáceos, el nitruro de carbono vítreo, que podría ser producido mediante la pirólisis de precursores poliméricos ricos en nitrógeno, reteniendo así una gran cantidad de nitrógeno en su forma final. También se supuso que los materiales consiguientemente obtenidos combinarían las ventajas de los carbonos vítreos y los nitruros de carbono sin presentar sus inconvenientes. Este material ha sido eficientemente sintetizado y ha permitido la síntesis de varios revestimientos electroquímicamente estables y catalíticamente activos basados en él.
- català
En els darrers anys, la demanda generada a les noves indústries electroquímiques relacionades amb energies verdes ha derivat en un augment de l’interès per diferents formes de materials basats en carboni, ja que han esdevingut un nou focus a la ciència dels elèctrodes. Amb tot, malgrat l’atractiva conveniència econòmica i industrial dels materials de suport basats en carboni, la seva estabilitat oxidativa és en la majoria de casos molt inferior comparada amb la resistència a l’oxidació de metalls nobles o fins i tot de sistemes d’òxids metàl·lics. Tot i així, dues famílies de materials pertinents al grup de suports basats en carboni representen una excepció a aquesta regla general – els carbonis vitris i els nitrurs de carboni. Els anomenats carbonis vitris, produïts mitjançant la piròlisi de polímers carbonacis barats, posseeixen una estabilitat oxidativa, juntament amb una alta conductivitat i estabilitat química. El motiu principal d’aquest fet roman en que el procés de piròlisi, el qual deriva en la formació de carboni vitri, requereix temperatures notòriament elevades (de 1000 a 3000°C). Un altre grup de la família dels carbonacis, distingits per la seva elevada estabilitat oxidativa, els nitrurs de carboni, exhibeixen una combinació de propietats que la fan pràcticament l’antípoda dels carbonis vitris. La temperatura de piròlisi, en comparació a la del carboni vitri, és significativament inferior, trobant-se en el rang de 400-600°C. Com a pedra angular d’aquesta tesi, es va formular la hipòtesi de l’existència d’un nou membre de la família dels materials carbonacis, el nitrur de carboni vitri, que podria ser produït mitjançant la piròlisi de precursors polimèrics rics en nitrogen, retenint així una gran quantitat de nitrogen en la seva forma final. També es va suposar que els materials conseqüentment obtinguts combinarien els avantatges dels carbonis vitris i els nitrurs de carboni sense presentar els seus inconvenients. Aquest material va ésser eficientment sintetitzat i ha permès la síntesi de diversos revestiments electroquímicament estables i catalíticament actius basats en ell.
- English
In the modern practical electrochemistry, the stability of active electrode layers is one of the most critical issues in miscellaneous industrial processes. Inevitable degradation of electrodes is especially important for catalytic transformations taking place at high oxidative potentials, such as oxygen and chlorine evolution reaction, oxygen reduction reaction, electrooxidation of organic compounds in aqueous media, etc. Industrial implementations of processes, involving electrochemical formation or consumption of aggressive oxidants, are often based on the use of noble metals as electrode materials.
It is important to emphasize that, in many cases, noble metals are not used exclusively for their oxidative stability, but play a key role in the corresponding transformation as efficient catalysts. Thus, in classical industrial electrodes they often share two roles, working simultaneously as catalysts and oxidation-resistant electrode matrices. However, even in the situations when the use of noble metals seems reasonable due to their catalytic properties, the quantity of these scarce and costly materials can often be drastically reduced. The most contemporary approach to achieve this goal is based on the implementation of the corresponding catalytic metal in the form of nanoparticles.
Nanoparticle-based catalytic systems, however, usually require an inert conducting support as stable matrix, on which the nanoparticles are deposited, thus separating the roles of the matrix material and the active component.
The integration of such systems into industrial practice, including the cases of tremendous importance for the upcoming energy transition – the industry of fuel cells and H2 production by water splitting, is drastically constrained by limited choice of appropriate conducting matrix materials.
Historically, conducting metal oxides supported over valve metals (e.g. titanium) have typically been used for harsh electrochemical processes, such as hypochlorite production. These electrodes are usually mechanically robust, limited in their possible geometrical shapes and typically are non-porous or have limited porosity – although these limitations are non-critical for the classical industries they are used for. Noble metals, in case of their implementation, are included as oxides in the mixed oxides system, TiO2/RuO2 (ORTA) anode coating being the most classical example. Non-precious catalytic metal oxide systems, based on Pb, Co, Ni and many other transition metals, have also been extensively used.
In the recent years, however, the demand generated by newborn electrochemical industries related to green energy, led to increased interest in flexible and easily shapeable electrodes with extensive porosity down to the nanometer level and corresponding high operative surface. Different forms of carbon-based materials became a new hotspot in the electrode science. The most studied supports included carbon cloth, carbon felt, carbon foams and various pyrolytic carbonaceous coatings. More sophisticated materials and composites based on carbon nanotubes and even diamond have also been investigated. The combination of physical, chemical and process parameters allows these materials to outperform massively over other possible candidates in the majority of modern-day applications.
However, in spite of appealing industrial and economic convenience of carbon-based support materials, their oxidative stability in most cases is much lower compared with the resistance to oxidation of noble metals and even metal oxide systems, which for this reason are still used while dealing with the most challenging oxidants.
Nevertheless, two families of materials belonging to the group of carbon-based supports represent an exception to this general rule – glassy carbons and carbon nitrides.
The so-called glassy (or vitreous) carbons, produced by means of pyrolysis of cheap carbonaceous polymers, possess an exclusive oxidative stability together with high conductivity and chemical inertness. Glassy carbon electrodes perform as a golden standard in research electrochemistry due to combination of stability and absence of own catalytic properties. Their industrial application, however, is limited to the applications where catalytically inactive surfaces are preferred.
The main reason for this lies in the fact that the pyrolysis process, leading to the formation of vitreous carbon, requires noticeably high temperatures (1000 to 3000 oC). In addition to obviously elevated cost of the material, it results in practical infeasibility of production of catalytically active composites derived therefrom – due to instability demonstrated at elevated temperatures by nearly all catalytic nanoparticles and molecular precursors to catalysts. On the other hand, application of particles or molecular compounds on the surface of preliminary fabricated vitreous carbon support results in unstable coatings due to weak adhesion typical for smooth surface of the material. Its brittleness and difficulty of mechanical processing further complicates the practical use.
Although multiple attempts to limit the pyrolysis temperature, required for glassy carbons formation, have been undertaken, it still remains far above the stability threshold of most nanoparticles, which lies in the range from 200 to 700 oC.
Another group of material of the carbonaceous family, distinguished for its high oxidative stability, carbon nitrides, exhibits the combination of properties making it nearly antipodean to vitreous carbon.
Carbon nitrides are typically produced by means of pyrolysis of melamine, dicyandiamide, urea or some other nitrogen-rich precursors. The pyrolysis temperature, in contrast with glassy carbon, is rather low, laying in the 400 to 600 oC range. Resulting material in the classic case is a light yellow powder with imperfect crystalline structure and rough surface, which cannot be directly used as a support matrix material.
As a cornerstone for the present thesis, it was hypothesized the existence of a new member of the carbonaceous materials family, glassy carbon nitride, that could be produced by means of pyrolysis of nitrogen-rich polymeric precursors and retain high nitrogen content in the final form. It was also supposed that the material thus obtained would combine the benefits of vitreous carbons and carbon nitride without having their drawbacks. Formation of smooth and mechanically robust surface was identified to be the result of polymer matrix being the precursor for the pyrolytic material, for which reason the new material was expected to form a surface with similar properties. High nitrogen content, on the other hand, was associated with low pyrolysis temperature, which was identified to be characteristic for the new material. As a result, a coating-forming product with the ability to form composites with nanoparticles and complexes with catalytically active transition metals ions, was expected.
The first publication, forming the basis of the present thesis, reports the successful obtainment of this new class of materials by means of pyrolysis of three novel triazinic polymers, also synthesized in this work. All three polymeric precursors belong to insufficiently studied class of linear triazine-based polymers. They demonstrate outstanding solubility in several organic solvents, film-forming properties and a tendency to retain high amounts of nitrogen at pyrolysis. Efficient methods for fabrication of carbon nitride coatings from N,N-dimethylacetamide solutions of the precursor polymers have been elaborated. It was demonstrated that coating materials obtained in this way possesses high mechanical robustness and requires low pyrolysis temperatures, as it have been previously expected. Finally, outstandingly high electrochemical stability of the annealed films in high anodic and cathodic polarizations was shown.
The results were patented and subsequently published in the Journal of Materials Science. The main results obtained in the publication form part of Chapter 2 of the present manuscript.
The second research project within the scope of the thesis was devoted to the fabrication of catalytically active coatings on the basis of previously developed polymer precursors. First, it was found that these water-based solutions of the precursors could successfully be produced in alkaline media. Application of water-based polymer solutions on carbon cloth substrate resulted in depositing porous carbon nitride coatings. Using precursor polymer inks containing carbon nanopowder allowed obtaining composite coatings with controlled porosity.
Secondly, coordination of Ni2+ in the aqueous media with the precursor polymers was proposed as a method for introduction of catalytically active transition metal centers into the resulting coatings. Ni-loaded coatings on carbon cloth electrodes have been demonstrated to possess excellent activity and prolonged stability as active materials for oxygen evolution reaction.
The results are under consideration in Angewandte Chemie international edition.
The last project associated with the topic of the thesis arose from the necessity to m A new class of ultra-nitrogen-rich polymers –linear imides of triazine and heterocyclic aminotriazole and aminotetrazole moieties– have been synthesized by means of condensation of cyanuric chloride with corresponding aminoheterocyclic precursors in the presence of DIPEA as strong non-nucleophilic base and then characterized. In addition, synthesis of polymeric product of 2-amino-4,6-dichloro-1,3,5-triazine self-condensation, azanediyl (2-chloro-1,3,5-triazine-4,6-diyl, has been achieved, also in the DIPEA presence. The last polymer can be viewed as partial structural analog of the first two products with triazinic amino-component.
All three polymers demonstrate excellent solubility in aprotic dipolar solvents in the form of corresponding salts with DIPEA. In contrast with previously developed nitrogen-rich polymeric precursors, new polymers exhibited decreased tendency of nitrogen loss during thermal decomposition, thus permitting the synthesis of carbon nitride films with nearly equal atomic content of carbon and nitrogen on various substrates by liquid coating methods followed by solvent drying and annealing.
Coatings obtained by this approach have been extensively characterized using elemental analysis, TGA, TEM, XRD, FTIR, STM, UV-Vis diffuse reflectance spectroscopy, contact angles measurement and potentiometric electrochemical methods. These analytical techniques allowed classifying the coatings as partially amorphous heptazinic carbon nitride, enriched with carbon. Structural similarity to vitreous carbon materials has been demonstrated.
Electrochemical characterization of the new coatings, performed by means of voltammetry and chronoamperometry in sulphuric acid solution, demonstrated outstanding long-term stability of materials in harsh anodic polarization conditions.
In the same time, low pyrolysis temperature, required for preparation of coatings, permitted to expect the viability of synthesis of composite materials, containing nanoparticles or molecular transition-metal-based precursors as structural/catalytic and catalytic components, respectively, with carbon nitride representing the oxidatively stable and mechanically robust coating matrix.
However, intrinsically non-porous nature of coatings obtained from organic solutions of precursors represented a problem from the point of view of synthesis of catalytically active materials, where developed surface is a prerequisite for efficient practical use.
To overcome this obstacle, a modified method for coating synthesis was developed. It was noticed that two azole-based precursor polymers possess remarkable solubility in aqueous ammonia, presumably in the form of corresponding ammonium salts. Then, pyrolysis of polymeric films, produced from aqueous solutions, resulted in formation of nanoporous carbon nitride coatings.
The catalytic coating concept has been verified using carbon nanoparticles as structural and micropore-forming component, and nickel nitrate as catalytic precursor. catalytically active Ni-loaded coatings have been obtained on the basis of poly(CYA-Cl-AMTAZ) precursor polymer on carbon cloth as electrode support and characterized by SEM and XPS spectroscopy.
Microscopy of the samples, although lacks resolution, allows assuming nanometer-sized intrinsic porosity. In samples with carbon nanoparticles filler content, nanoparticles-based secondary pores structure of the size of hundreds of nanometers has been observed. XPS results allow to classify catalytic centers of the materials as nitrogen-coordinated Ni, thus assuming deep interaction between the carbon nitride matrix and the metal.
Coated carbon cloth electrode exhibits outstanding catalytic activity in oxygen evolution reaction in alkaline media with overpotentials as low as 0.26 V and Tafel slope of 61.8 mV/dec –signaling the performance of the range, typical for catalytic efficacy of platinum-group metals and their oxides. The stability of coated electrodes was exceptional, with no degradation of oxygen evolution parameters after 1000 hours of electrolysis at 100 mA/cm2.
Yet another result, obtained within the framework of the current thesis, is related to the assembly of easy and low-cost Arduino-based potentiostat device. Although initially proposed as a simple schematic for student practicum, the device in the soldered version demonstrated the ability to perform precise three-electrode potentiostatic measurements and was implemented for routine automated screening of carbon electrode properties. In addition, a simple and reliable approach to measurement of capacitances in purely potentiostatic mode has been demonstrated.
In electrochemical laboratory practice, the efficiency of massive screening of sample properties is often limited by the number of available high-end measurement units. In some applications, simple and cheap purpose-made potentiostat can serve as initial screening means, providing the quality of results, sufficient for pre-selection of efficient samples to be carefully studied on a research-grade potentiostat.
The precision and reproducibility, obtained with the proposed device, makes it suitable for industrial production of low-cost electrochemical measurement units based on the same schematic.
- español
