Evolution of the Porosity and Adsorption Properties of Pyrolytic Carbons During Activation

Cervini, Luca and Griffin, John and Barrow, Nathan S. (2021) Evolution of the Porosity and Adsorption Properties of Pyrolytic Carbons During Activation. PhD thesis, Lancaster University.

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Abstract

Activated carbons are industrially relevant materials because of a broad range of applications, such as catalyst supports and supercapacitor electrode materials. The performance of supercapacitors depends on the properties of the carbon pores; that is the pore sizes, volumes, connectivity, and the ability to capture electrolyte species. The characterization of these aspects is a challenging task because no single technique can fully describe activated carbons. Despite sample-specific features, most activated carbons show distorted graphite-like features on the nanometre scale, the void between which constitutes the micropores. The macroscopic structure is amorphous. Pyrolytic PEEK-Derived Carbons (PDCs) generally contain a broad pore size distribution ranging from subnanometre pores up to a few nanometres, and the average pore size can easily be tuned during synthesis. This thesis aims to characterize PDCs by combining a range of complementary techniques, mainly NMR, XRD, and GS. Insight was gained into the nanometre scale structure of the pores, the macroscopic connectivity of the pores, and the electrolyte adsorption properties of those pores. The influence on the appearance of the NMR spectrum of sample parameters such as the particle size, the burn-off and the spatial pore distribution, as well as solvent parameters such as viscosity and polarity, were investigated. The quantification by NMR of the ex-pore and in-pore populations can be biased by exchange-averaging of these two environments into a broad peak. This phenomenon was more pronounced in small particles because of shorter diffusion paths, in low viscosity and polar solvents because of weaker attraction to the hydrophobic pore surface, and in samples with higher degrees of activation due to faster diffusion kinetics in bigger pores. Steam, CO2 and KOH activation conditions were used at different temperatures (700 – 900 °C) and reaction times (minutes to hours) to assess the effect on the structure. Lower temperatures yielded rougher and less stable pore walls. The pores were poorly connected, and the pore sizes inhomogeneously distributed within the particle, leading to distinct NMR environments. Conversely, activation at 900 °C provided a stable, homogeneous and well-connected structure. The adsorption of water and alkali ions in solution constitutes the first NMRbased experimental insight into how pore properties affect the total adsorption strength of the carbon. Large pores were found to contain higher concentrations of ions than small pores, at equilibrium. The ratio of adsorbed ions relative to free ions depended on the nature of the ion and its solvation shell. The adsorption mechanism of sodium was further characterized under an applied potential using a custom designed and 3D printed cell employed in an ex-situ electrode charging protocol, benchmarked against a commercial Swagelok cell. The self-discharge kinetics were quantified and found to consist of at least two contributions: a fast charge redistribution and a slow faradaic process. The in-pore population was then observed with NMR after ion-redistribution was complete, and found to qualitatively agree with the literature, although more experiments are required to identify sample specific features. This thesis has shown the importance of multi-technique characterisation as well as careful control over activation conditions. The tunability of pore structures in carbon and subsequent interpretation of adsorbed ions in NMR spectra is relevant for many industrial fields. Demonstration of a 3D printed ex-situ electrochemical cell for NMR will help to accelerate studies of carbons in the area of battery materials.