1,721,056 research outputs found
Atomically Coordinated Non-Precious Metal Electrocatalysts Using Active Site Imprinted Carbon Matrix
No full textNon-precious metal catalysts generally represented as M-N-C (where M= Fe, Co, Ni etc.) have shown encouraging activity levels for different electrochemical applications involving oxygen reduction reaction (ORR) and carbon dioxide reduction reaction (CO2RR). High activities of these electrocatalysts mainly come from transition metal centres that are atomically dispersed as M-N4 active sites within a nitrogen doped carbon matrix. Because of the required pyrolytic synthesis conditions, it is quite challenging to prepare M-N-Cs that purely consist of M-N4 active sites. Classical synthesis routes often result in the formation of additional side phases such metallic nanoparticles or metal carbides, which limit the density of M-N4 sites and lead to lower catalytic activity.1 Herein, we present our work on M-N-C synthesis using an active site imprinting approach as an alternate synthetic route to address the above-mentioned issue. We show that both Mg and Zn can be used for active site imprinting. The imprinted coordination environment can be coordinated with various transition metal ions, resulting in Fe-N-C, Co-N-C and Ni-N-C catalysts containing M-N4 sites exclusively.2-4 The electrochemical performance of the synthesized catalysts is evaluated for CO2RR and ORR. Ni-N-Cs exhibit an excellent CO2 reduction activity with high CO faradic efficiency value of 95% at U= -0.5 to -0.8 VRHE (vs reversible hydrogen electrode) and a mass activity of 23 A g-1. The performance stability test carried out at -0.65 VRHE demonstrates above 92 % retention of the current density and 97 % retention of the CO selectivity after 100 h of continuous operation, reflecting the structural robustness of the Ni-N-C catalyst in CO2RR test environment. When employed as ORR catalysts, both Fe-N-C and Co-N-C deliver promising activities with half-wave potentials >0.8 VRHE in acidic electrolyte and >0.9 VRHE in alkaline electrolyte. The talk will include greater details of the structural analysis and electrochemical performance evaluation of these catalysts
Ionothermal Template Transformation as a Sustainable Route Towards Carbon Electrodes in Energy Storage and Conversion
No full textPorous carbons with tuneable functionalities and morphologies have extensively been employed as electrode materials in a variety of electrochemical energy conversion and storage systems for instance in fuel cells and electrolysers as active catalysts and catalyst supports, and in secondary batteries as anode materials. Amorphous carbons with well-developed pore structures are of particular interest due to their superior mass-transport characteristics and remarkable charge storage capacities. The salt-templating method with its advantage of combined soft and hard templating effects provides a sustainable way to synthesize nano- and mesoporous carbons with tailored porosities via in-situ ionothermal template transformation [1]. In this work, we utilized a MgCl2-based salt melt to prepare nitrogen doped carbons (N-C) with different morphologies and porosities, which were evaluated as anode materials in sodium ion batteries. Simultaneously, use of MgCl2 salt leads to the formation of Mg-N4 moieties in those carbons by means of a pyrolytic template-ion effect (active site imprinting) [2]. Porous carbon frameworks with imprinted Mg-N4 sites are interesting particularly for electrocatalysis applications as they offer an ideal platform to prepare M-N-C catalysts (where M= Co, Fe, Ni etc.) by ion-exchange reactions at low temperatures. The resultant M-N-C catalysts consist purely of M-N4 active sites and high porosity of carbon framework facilitates efficient mass-transport of reacting species.
We utilized Mg-N4 imprinted carbons to synthesize morphologically equivalent Ni-N-Cs and Co-N-Cs, containing phase pure Ni-N4 and Co-N4 sites, for electrochemical reduction of carbon dioxide (CO2RR). In electrochemical tests, Ni-N-Cs exhibited an excellent CO2 reduction activity with considerably higher CO selectivity and mass activity as compared to Co-N-C. The faradic efficiency value of Ni-N-C for CO formation was 95% at U= -0.5 to -0.8 VRHE (vs reversible hydrogen electrode) and a mass activity of 23 A g-1. The performance stability test carried out at -0.65 VRHE demonstrated above 90 % retention of the current density and CO selectivity after 100 h of continuous operation, reflecting the structural robustness of the Ni-N-C catalyst.
Finally, these ionothermal carbons with two different morphologies (but without any Ni or Co incorporation) were employed as the anode materials in sodium-ion batteries to evaluate the effects of carbon morphology and functionalization on sodium storage capacities. Compared to the reference carbon material, substantially higher reversible sodium storage capacities were reached with these high porosity carbons that were in the range of 300-500 mAh g-1 [3]. Although the reversible capacity was obtained only after extensive SEI formation, our results reveal the potential for much higher reversible capacities than usually observed using carbons with a tailored porosity in sodium-ion batteries. The talk will include greater details of the structural analysis and sodium storage and CO2 reduction results of these ionothermal carbons
Sol-Gel Carbonization towards Tailor-Made Boron- and Nitrogen-doped Carbon
No full textThe activation of carbon is normally proceeded in top-down or bottom-up strategies. Therein pore formation (porogenesis) occurs by reaction with steam, CO2 , KOH, or the use of acidic agents, in which chemical leaching of carbon atoms occurs. The sol-gel type synthesis of N-doped carbon in excess amounts of molten acids was presented recently, questioning the general validity of a leaching activation mechanism.[1] The protocol using inorganic salt melts (MgCl2 or ZnCl2 ) and organic precursors additionally generated N functionalities. Interestingly, it can be noticed that the imprinting cations play a crucial role towards the chemical structure of N-doped carbon framework. The coordinated geometry is wellknown from phthalocyanine, a macrocyclic Ncomplexes (MN4 -sites, where M is metal cation) which are desirable surface complexes, e.g. in electrocatalysis. The analogous phenomenon may be observed when using H3BO3 agent via sol-gel chemistry
Synthesis of Atomically Dispersed Electrocatalyst by Imprinting with Different Template Ions throughout Carbonization
No full textAtomically dispersed metal-nitrogen doped carbons (M-N-C) are
promising catalysts for the activation of small molecules such as O2
and CO2. These single atom catalysts (SAC) operate at the interface
between homogenous and heterogenous catalysts. Currently, many
examples of M-N-C are known with good oxygen reduction reaction
activity but lacking a controlled synthesis of the specific active sites of the precatalyst
Development of tailormade core-shell hard carbon materials as anode materials in sodium ion batteries
No full textThe current strong interest in electromotive mobility and the need to transition to an energy grid with sustainable energy storage has led to a renewed interest in sodium ion batteries (SIBs). Hard carbons are promising candidates for high-capacity negative electrode materials in SIBs. Their high capacities, however, are often accompanied with high irreversible capacity losses, during the initial cycles.[1]
The goal of this project is to synthesize carbon materials using different zeolite templates to obtain electrode materials that feature a defined and adjustable pore structure. A better understanding of the structure-property relationship by investigating porosity-tailored anode materials, should enable quantification and understanding of the potential of hard carbon materials for SIBs.
Furthermore, a goal is to explore whether a core-shell structure can separate sodium storage and solid electrolyte interphase formation allowing the independent investigation of storage capacity and irreversible losses.
Synthesis and Characterization of Ce-based M-N-C Catalysts
No full textIn this thesis work we prepared and characterized new cerium-based M-N-C materials. The idea to have a bigger active site imprinter like a lanthanide would have formed a bigger pocket M-Nx (with X > 4) since this could have improved the catalytic activity after the transmetallation with iron. Moreover, for Fe–N–C catalysts, one of the main degradation mechanisms that is usually discussed is the surface oxidation induced by H2O2. For this purpose, cerium is used to improve the catalytic activity and to avoid the degradation of the fuel cell since this lanthanide works like a radical scavenger which improves polymer electrolyte membrane (PEM) fuel cell durability
Core-Shell Materials as Advanced Anodes for Sodium Ion Batteries
No full textThe current imperative to shift towards an energy grid equipped with sustainable energy storage solutions has caused a renewed interest in sodium-ion batteries (SIBs). Hard carbons (HCs) are a promising option high-capacity anode materials in SIBs. Nevertheless, their elevated capacities frequently come at the cost of experiencing substantial non-reversible initial capacity losses.
Commonly, significant losses are associated with irreversible reactions, such as the creation of the solid electrolyte interphase (SEI), that occur during the initial sodium insertion in HC-materials. Intriguingly, high values of irreversible capacity are often found for samples with experimentally determined low specific surface area.[1] A more comprehensive understanding of the structure-property relations is essential for quantifying and grasping the potential of hard carbon materials in sodium-ion batteries (SIBs). Thus, the objective is to employ analytical methods to establish a link between the structure and the electrochemical attributes of HC materials. This has been a challenge, partly due to the non-stoichiometric nature of the sodium storage mechanism and the disordered structure of HCs.
To address the challenges mentioned above, our approach is to explore whether a core-shell structure can separate sodium storage and SEI-formation. This way, we can investigate and fine-tune storage capacity and irreversible losses, independently. The strategy involves the synthesis of various porous carbon structures to serve as the core material and their combination with sodium-conductive structures to core-shell materials. Herein, we will present different synthesis routes towards tailor-made carbon core materials. Moreover, different coatings concepts will be introduced, and the electrochemical performance of the core and core-shell materials compared. To elucidate the storage mechanism, the results of advanced analytical methods such as operando NMR and SAXS will be presented. Generally, these core-shell anodes promise to enable high capacities accompanied with low irreversible losses due to optimized SEI-formation
ZIF-derived Atomically Dispersed Non-precious Metal Catalysts (M-N-C) for Electrochemical Energy Applications
No full textZeolitic imidazolate frameworks (ZIFs) which are a subtype of metal organic frameworks (MOFs) have been extensively used to prepare catalyst materials for a variety of electrochemical reactions for energy conversion and storage applications. Most notable examples of ZIFs used for that purpose include ZIF-8 and ZIF-67 etc. Particularly ZIF-8 with its high surface area, defined pore structure and tunable particle size is widely utilized as a platform material to prepare so-called metal- and nitrogen-doped carbon (M-N-C) catalysts with M= Co, Fe, Ni, Zn etc., which are an emerging class of catalyst materials and consist of nitrogen-doped porous carbon matrix hosting atomically distributed active metal sites.[1, 2] The active sites in M-N-Cs ideally have M-N4 coordination resembling to metal centres in macromolecules such as porphyrins and phthalocyanines.[3] Most representative examples of M-N-Cs include Fe-N-Cs, Co-N-Cs and Ni-N-Cs etc. which are showing promising activities for a variety of electrochemical reactions e.g. oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR) and hydrogen evolution reaction (HER). The structures of M-N-C catalysts are quite complex and require a fine balance between morphological, electronic, and chemical properties to reach optimal electrocatalytic activities. In this talk, I will present our activities on (i) preparation of phase-pure M-N-C catalysts derived from ZIF-8 via active-site imprinting [4, 5] and highlight the benefits of our strategy to achieve high density of active sites and enhanced electrochemical performance levels [6] and (ii) feasibility of using gas physisorption techniques as a new approach to quantify active sites in M-N-Cs. The challenges of maximizing active site utilization and eliminating unfavourable mass-transport characteristics faced by ZIF-8 derived M-N-Cs in electrochemical energy devices e.g. fuel cells will also be briefly discussed
Developing Core-Shell Carbon Materials to Link Porosity Features to Sodium Storage Capacities
No full textPorous carbon materials play an important role for energy storage and conversion. One (re-)emerging research field is the ability of porous carbons to store sodium metal ions. Current results shows that internal pores – hence, pores which are not accessible for the electrolyte – allow to store large amounts of sodium at low potentials, yielding high energy sodium-ion battery (SIB) anodes.
The common synthesis approach to gain carbons with internal pores involves the pyrolysis of a non-graphitizing precursor, resulting in a so-called hard carbon (HC). However, HC-materials frequently show substantial non-reversible initial capacity losses. Commonly, significant losses are associated with the creation of the solid electrolyte interphase (SEI) on the carbon’s surface that occurs during the initial sodium insertion. Intriguingly, large irreversible capacities are often found for samples with experimentally determined low specific surface area.[2] A more comprehensive understanding of the structure-property relations is essential for quantifying and grasping the potential of carbon materials in SIBs. However, the typical synthesis methods do not allow to individually tune the storage properties – mainly connected to the internal properties of the carbons – and the SEI-formation – primarily related to the surface properties. Hence, the objective of the present work is to develop a synthesis route which tackles this challenge.
Herein, the main approach is to develop tailor-made core-shell carbon materials consisting of a highly porous carbon core and a quasi-non-porous carbon shell. For the core, two strategies are pursued: A) microporous carbon materials with varied porosity, however, similar chemistry, and B) microporous carbons with tuneable chemistry, but similar porosity. Approach A involves the selection of commercially available activated carbons (ACs). Strategy B is based on the modification of the chemical composition (i.e., amount and type of N-sites) of zeolitic imidazolate framework (ZIF-8) derived carbons. In both cases, the shell is realized by chemical vapour deposition (CVD). Different analytical methods, e.g., powder XRD, gas physisorption (N2, Ar, CO2), XPS, and SAXS are used to thoroughly characterize
morphological and chemical features of the core as well as of the core-shell carbons. These features are linked to the electrochemical characteristics of the materials.
After CVD-coating, all materials show a significant reduction in detectable surface area (up to a factor of up to 190x) by N2-physisorption. The coating technique is successfully applied to a range of AC-materials, enabling to link porosity features to Na-storage behavior. For the best performing AC-based material, the reversible capacity is increased from ~140 mAhg-1 to ~400 mAhg-1 while irreversible capacity is decreased from ~640 mAhg-1 to ~90 mAhg-1.
The results of the coated ZIF-derived carbon reveal that a higher nitrogen content leads to a greater capacity in the sloping region, but to a lower capacity in the plateau region of the voltage profile.
Generally, core-shell carbon anodes promise to enable high capacities accompanied with low irreversible losses
The Influence of Interface Modification on Gassing of Sodium Ion Battery Anodes
No full textThis flash talk will briefly introduce the DEMS method[3,4] and highlight a recent piece of our research from the DIALYSORB project. Fitting to the main topic of the conference, interfaces, we will present and the measurement technique to understand the side reactions occurring at this interface and recent data on anode materials with designed interface and interphase
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