Electrocatalytic applications of pyrolytic carbons derived from sugarcane kraft lignin

Reactive carbons from Kraft lignin pyrolysis: Stabilization of peroxyl radicals at carbon/silica interface

Electrochemical preparation of metal–melanin functionalized graphite surfaces

Aryl (Ar) modified carbon electrodes are of importance for a range of sensing and electrocatalysis applications. However the majority of the modification methods result in a thick polymer layer. The commonly applied method using electrochemical reduction of diazonium salt (to form Ar˙) leads to polymerisation at very low surface coverages. We report an alternative approach using a methodology based on the pre-adsorption of the anthraquinone-2-diazonium salt (2-AQN+2BF−4) onto an edge plane pyrolytic graphite electrode to form a thin unreacted sub-monolayer film. After transfer to a buffer solution containing no diazonium salt, the adsorbed material then thermally decomposes at room temperature and completes the modification procedure. When the surface coverage of anthraquinonyl groups is below ca. 2 × 10−10 mol cm−2, non-broadened voltammetric peak signals were observed for the electro-reduction of the surface bound 2-AQ groups in sodium hydroxide buffer solution. This near-ideal voltammetric response reflects the mode of attachment which is likely viaester linkages formed between reaction of carbocation intermediates (Ar+) and carboxylate groups present on the edge plane sites regions of the graphite electrode. The desired thin sub-monolayer has significant importance in creating tailor made interfaces. It not only provides molecular level control over the surface layer, but also provides physical insight into the origins of the observed non-ideal behaviour of the redox modified surfaces.

As a highly abundant renewable carbon source, lignin can be converted to a variety of advanced carbon materials with tailorable properties through slow pyrolysis. In this study, slow pyrolysis of kraft lignin, for the first time, was investigated with a commercial pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) system through evolved gas analysis-MS (EGA-MS) and heart-cutting-GC–MS (HC-GC–MS) analyses. These analyses allow recovery and examination of the multiphased gas products generated from thermal decomposition of lignin during slow pyrolysis at a controlled heating rate over a long time course, thus making it possible to link operation conditions, pyrolysis chemistry, and carbon material properties. The overall product distributions, including volatiles and solid products, were quantitatively tracked at different heating rates (2, 20, and 40 °C/min) and different temperature regions (100–200, 200–300, and 300–600 °C). Solid residues were further characterized using a suite of analytical tools, in correlation with the investigation of formation mechanisms of volatiles to reveal the reaction chemistry of lignin during slow pyrolysis and to determine the morphology, pore structure, and interfacial chemical properties. This study provides critical insights into the slow pyrolysis chemistry of lignin and the properties of the resulting carbon material. These results will facilitate a better design and control of the lignin slow pyrolysis process for synthesizing functional carbon materials.

Scanning electrochemical cell microscopy (SECCM) has been applied for nanoscale (electro)activity mapping in a range of electrochemical systems but so far has almost exclusively been performed in controlled-potential (amperometric/voltammetric) modes. Herein, we consider the use of SECCM operated in a controlled-current (galvanostatic or chronopotentiometric) mode, to synchronously obtain spatially resolved electrode potential (i.e., electrochemical activity) and topographical "maps". This technique is first applied, as proof of concept, to study the electrochemically reversible [Ru(NH3)6]3+/2+ electron transfer process at a glassy carbon electrode surface, where the experimental data are in good agreement with well-established chronopotentiometric theory under quasi-radial diffusion conditions. The [Ru(NH3)6]3+/2+ process has also been imaged at "aged" highly ordered pyrolytic graphite (HOPG), where apparently enhanced electrochemical activity is measured at the edge plane relative to the basal plane surface, consistent with potentiostatic measurements. Finally, chronopotentiometric SECCM has been employed to benchmark a promising electrocatalytic system, the hydrogen evolution reaction (HER) at molybdenum disulfide (MoS2), where higher electrocatalytic activity (i.e., lower overpotential at a current density of 2 mA cm-2) is observed at the edge plane compared to the basal plane surface. These results are in excellent agreement with previous controlled-potential SECCM studies, confirming the viability of the technique and thereby opening up new possibilities for the use of chronopotentiometric methods for quantitative electroanalysis at the nanoscale, with promising applications in energy storage (battery) studies, electrocatalyst benchmarking, and corrosion research.

In the past decade, there has been significant research on carbon-transition metal oxides (TMO) hybrid structures and their applications as novel catalysts and sensors[1-3]. Some of these research efforts include i) development of alternatives to platinum-based electrocatalysts to generate hydrogen and ii) design of hybrid nanofiber-based sensors having a high surface to volume ratio. Carbon-TMO-based structures are strong candidates to fulfill these needs owing to their catalytic and sensing performance. However, typical synthesis processes of these structures, such as hydrothermal deposition of TMO on carbon, or combined pyrolysis of carbon and TMO precursors, are challenging to control and reproduce.Addressing this controllability issue, we recently developed a highly reproducible process at the wafer level to synthesize carbon-TMO structures, consisting of a suspended Glassy Carbon Wire (GCW) coated with WO3-x[4-5]. A monolithic carbon structure featuring a suspended GCW of known diameter and length is microfabricated by first laying a SU-8 fiber on top of a photopatterned SU-8 scaffold, using the nearfield-electrospinning technique, followed by pyrolysis. The WO3-x coating is deposited via Localized Chemical Vapor deposition (LCVD), activated by controlling the temperature profile on the suspended GCW through Joule heating. The length and thickness of the coating are adjusted by manipulating the current through the GCW and monitoring its voltage. The featured suspended GCW is ideal for understanding and describing the LCVD process of different TMOs on carbon. However, a single microstructure is not suitable for a large catalysis process. Therefore, to exploit the catalytic capability of the developed material, a multi-wire carbon-TMO is crucial.Here we report the LCVD technique's development to coat carbon nanofibers (CNF) mats with WO3-x for large-scale catalysis. We prepare the CNFs mats using the far-field electrospinning technique of Polyacrylonitrile/Dimethylformamide (PAN/DMF) solution followed by stabilization and carbonization. We have studied in detail the Joule heating of the CNFs mat, and we have developed a model allowing us to predict its temperature at a certain value of power dissipation used for a controlled chemical vapor deposition process. The deposited WO3-x films at the CNF mats are uniform, and the stoichiometry of the coating can be tuned by a post oxidation process or annealing in an inert atmosphere, yielding different catalytic behavior.[1] R. Wu, J. Zhang, Y. Shi, D. Liu, B. Zhang, Metallic WO2-carbon mesoporous nanowires as highly efficient electrocatalysts for hydrogen evolution reaction, J. Am. Chem. Soc. 137 (22) (2015) 6983-6986.[2] J. Chen, D. Yu, W. Liao, M. Zheng, L. Xiao, H. Zhu, M. Zhang, M. Du, J. Yao, WO3-x nanoplates grown on carbon nanofibers for an efficient electrocatalytic hydrogen evolution reaction, ACS Appl. Mater. Interfaces 8 (28) (2016) 18132-18139[3]Y. Lim, S. Kim, Y.M. Kwon, J.M. Baik, H. Shin, A highly sensitive gas-sensing platform based on a metal-oxide nanowire forest grown on a suspended carbon nanowire fabricated at a wafer level, Sens. Actuators B Chem. 260 (2018) 55-62.[4] Cisquella-Serra, A., Gamero-Castaño, M., Ferrer-Argemi, L., Wardini, J. & Madou, M. Controlled joule-heating of suspended glassy carbon wires for localized chemical vapor deposition. Carbon 156, 329–338 (2020)[5] L. Ferrer-Argemi, E.S. Aliabadi, A. Cisquella-Serra, A. Salazar, M. Madou, J. Lee, Size dependent electrical and thermal conductivities of electro-mechanically-spun glassy carbon wires, Carbon 130 (2018) 87-93.

Due to the abundance and the electrochemical versatility of carbon, it is becoming an increasingly popular material for electrocatalytic applications. Whether it is used as a support material, as a catalyst, or even as a bifunctional catalyst, insights into the reaction processes are of fundamental value. In this light, we employ electrochemical scanning tunneling microscopy (EC-STM) to in-situ evaluate electrode surfaces' behavior and identify the nature of the active sites.[1] The experimental distinction between inactive and active sites of a catalytic system can be achieved by comparing the noise level of surface sites in the EC-STM signal while a reaction is ‘Off’ or ‘On’, respectively. The tunneling current will be stable under both conditions if the scanning tip is positioned over an inactive site. Over an active site, reactions occurring within the tunneling gap will influence the EC-STM signal, which can be observed as locally confined noise features superimposed on the surface morphology.Here, we examine highly ordered pyrolytic graphite (HOPG) in alkaline and acidic media as a model system for carbon-based structures. In an alkaline medium, we compare the activity of specific surface sites under oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) conditions (Figure 1a).[2] In both cases, predominantly steps and defects are active. However, in the case of the OER, the terraces also play a role. For the hydrogen evolution reaction (HER) in acidic media, it was possible to identify individual active sites on the ‘honeycomb’ structured surface with down to atomic resolution (Figure 1b).[3] Apart from HOPG, the technique will be demonstrated for metal-organic frameworks, another class of promising catalysts for ORR and OER.

Bulk-scale syntheses of sp2 nanocarbon have typically been generated by extensive chemical oxidation to yield graphite oxide from graphite, followed by a reductive step. Materials generated via harsh random processes lose desirable physical characteristics. Loss of sp2 conjugation inhibits long-range electronic transport and the potential for electronic band manipulation. Here, we present a nanopatterned holey graphene material electronically hybridized with metal-containing nanoparticles. Oxidative plasma etching of highly ordered pyrolytic graphite via previously developed covalent organic framework (COF)-5-templated patterning yields bulk-scale materials for electrocatalytic applications and fundamental investigations into band structure engineering of nanocomposites. We establish a broad ability (Ag, Au, Cu, and Ni) to grow metal-containing nanoparticles in patterned holes in a metal precursor-dependent manner without a reducing agent. Graphene nanoparticle compounds (GNCs) show metal-contingent changes in the valence band structure. Density functional theory investigations reveal preferences for uncharged metal states, metal contributions to the valence band, and embedding of nanoparticles over surface incorporation. Ni-GNCs show activity for oxygen evolution reaction in alkaline media (1 M KOH). Electrocatalytic activity exceeds 10,000 mA/mg of Ni, shows stability for 2 h of continuous operation, and is kinetically consistent via a Tafel slope with Ni(OH)2-based catalysis.