The influence of acidic edge groups on the electrochemical performance of graphene nanoflakes

Graphene nanoflakes (GNF) with lateral dimensions of ca. 30 nm and edge-terminated with carboxylic acid functionalities have been characterised and the influence of acidic functionalities on the [Fe(CN)6]3−/4− redox couple studied using cyclic voltammetry and spectroelectrochemical methods. The presence of the COOH-terminated GNF in solution as well as immobilised onto an electrode surface was found to inhibit the redox reaction, supporting the conclusion that GNF promote instability of [Fe(CN)6]3−/4− in solution. The redox reaction was also much less influenced by the presence of GNF in D2O, highlighting the role played by readily available protons in destabilising the [Fe(CN)6]3−/4− redox couple. In the presence of GNF in solution, an additional, very intense cyanide stretch IR band was observed that was attributed to the formation of a new, non-soluble species. When D2O was used as the solvent, the IR spectrum showed no evidence of a new cyano species

Electrochemical characterisation of graphene nanoflakes with functionalised edges

Graphene nanoflakes (GNF) of diameter ca. 30 nm and edge-terminated with carboxylic acid (COOH) or amide functionalities were characterised electrochemically after drop-coating onto a boron-doped diamond (BDD) electrode. In the presence of the outer-sphere redox probe ferrocenemethanol there was no discernible difference in electrochemical response between the clean BDD and GNF-modified electrodes. When ferricyanide or hydroquinone were used as redox probes there was a marked difference in response at the electrode modified with COOH-terminated GNF in comparison to the unmodified BDD and amide-terminated GNF electrode. The response of the COOH-terminated GNF electrode was highly pH dependent, with the most dramatic differences in response noted at pH < 8. This pH range coincides with partial protonation of the carboxylic acid groups as determined by titration. The acid edge groups occupy a range of bonding environments and are observed to undergo deprotonation over a pH range ca. 3.7 to 8.3. The protonation state of the GNF influences the oxidation mechanism of hydroquinone and in particular the number of solution protons involved in the reaction mechanism. The voltammetric response of ferricyanide is very inhibited by the presence of COOH-terminated GNF at pH < 8, especially in low ionic strength solution. While the protonation state of the GNF is clearly a major factor in the observed response, the exact role of the acid group in the redox process has not been firmly established. It may be that the ferricyanide species is unstable in the solution environment surrounding the GNF, where dynamic protonation equilibria are at play, perhaps through disruption to ion pairing

Impact of Cation Intercalation on the Electronic Structure of Ti3C2Tx MXenes in Sulfuric Acid

Intercalation in Ti3C2Tx MXene is essential for a diverse set of applications such as water purification, desalination, electrochemical energy storage, and sensing. The interlayer spacing between the Ti3C2Tx nanosheets can be controlled by cation intercalation; however, the impact of intercalation on the Ti3C2Tx MXene chemical and electronic structures is not well understood. Herein, we characterized the electronic structure of pristine, Li , Na , K , and Mg intercalated Ti3C2Tx MXenes dispersed initially in water and 10 mM sulfuric acid H2SO4 using X ray absorption spectroscopy XAS . The cation intercalation is found to dramatically influence the chemical environment of Ti atoms. The Ti oxidation of the MXene increases progressively upon intercalation of cations of larger sizes after drying in air, while interestingly a low Ti oxidation is observed for all intercalated MXenes after dispersion in diluted H2SO4. In situ XAS at the Ti L edge was conducted during electrochemical oxidation to probe the changes in the Ti oxidation state in the presence of different cations in H2SO4 aqueous electrolyte. By applying the sensitivity of the Ti L edge to probe the oxidation state of Ti atoms, we demonstrate that cation intercalation and H2SO4 environment significantly alter the Ti3C2Tx surface chemistr

Vibrational signature of hydrated protons confined in MXene interlayers

The hydration structure of protons has been studied for decades in bulk water and protonated clusters due to its importance but has remained elusive in planar confined environments. Two dimensional 2D transition metal carbides known as MXenes show extreme capacitance in protic electrolytes, which has attracted attention in the energy storage field. We report here that discrete vibrational modes related to protons intercalated in the 2D slits between Ti3C2Tx MXene layers can be detected using operando infrared spectroscopy. The origin of these modes, not observed for protons in bulk water, is attributed to protons with reduced coordination number in confinement based on Density Functional Theory calculations. This study therefore demonstrates a useful tool for the characterization of chemical species under 2D confinemen

In Situ X-ray Absorption Spectroscopy of Metal/Nitrogen-doped Carbons in Oxygen Electrocatalysis

Metal/nitrogen-doped carbons (M−N−C) are promising candidates as oxygen electrocatalysts due to their low cost, tunable catalytic activity and selectivity, and well-dispersed morphologies. To improve the electrocatalytic performance of such systems, it is critical to gain a detailed understanding of their structure and properties through advanced characterization. In situ X-ray absorption spectroscopy (XAS) serves as a powerful tool to probe both the active sites and structural evolution of catalytic materials under reaction conditions. In this review, we firstly provide an overview of the fundamental concepts of XAS and then comprehensively review the setup and application of in situ XAS, introducing electrochemical XAS cells, experimental methods, as well as primary functions on catalytic applications. The active sites and the structural evolution of M−N−C catalysts caused by the interplay with electric fields, electrolytes and reactants/intermediates during the oxygen evolution reaction and the oxygen reduction reaction are subsequently discussed in detail. Finally, major challenges and future opportunities in this exciting field are highlighted.</p

Overcoming Diffusion Limitation of Faradaic Processes Property Performance Relationships of 2D Conductive Metal Organic Framework Cu3 HHTP 2 for Reversible Lithium Ion Storage

Faradaic reactions including charge transfer are often accompanied with diffusion limitation inside the bulk. Conductive two dimensional frameworks 2D MOFs with a fast ion transport can combine both charge transfer and fast diffusion inside their porous structure. To study remaining diffusion limitations caused by particle morphology, different synthesis routes of Cu 2,3,6,7,10,11 hexahydroxytriphenylene Cu3 HHTP 2 , a copper based 2D MOF, are used to obtain flake and rod like MOF particles. Both morphologies are systematically characterized and evaluated for redox active Li ion storage. The redox mechanism is investigated by means of X ray absorption spectroscopy, FTIR spectroscopy and in situ XRD. Both types are compared regarding kinetic properties for Li ion storage via cyclic voltammetry and impedance spectroscopy. A significant influence of particle morphology for 2D MOFs on kinetic aspects of electrochemical Li ion storage can be observed. This study opens the path for optimization of redox active porous structures to overcome diffusion limitations of Faradaic processe

A Collaboration for Exploring Fundamental Property Performance Relationships for Electrochemical Energy Storage

This invited Team Profile was created by Jens Matthies Wrogemann. He and his collaborators at the MEET Battery Research Center, the Helmholtz Zentrum Berlin für Materialien und Energie GmbH HZB , and Paderborn University recently published a research article about property performance relationships of 2D conductive metal organic frameworks. Flake and rod like shaped particles were evaluated to investigate the impact of the particle morphology of MOFs on electrochemical Li ion storage. By optimization of the particle morphology, the diffusion limitation of the Faradaic process can be significantly reduce

Photoelectrocatalytic conversion of CO2 : application of transition metal functionalised diamond particles

The electronic properties of diamond can not only be influenced by its termination, but also by different surface functionalisations. We present the immobilisation of transition metal complexes on linker-functionalised nanodiamond particles using the concept of click-chemistry. The resulting conjugates have been characterised using various spectroscopic methods investigating the influence on the electronic structure of the particles