Electrospun fibres as efficient cathodes for metal-air batteries

  • Elektrogesponnene Fasern als effiziente Kathoden für Metall-Luft-Batterien

Gehring, Markus; Eichel, Rüdiger-A. (Thesis advisor); Sauer, Dirk Uwe (Thesis advisor); Mayer, Joachim (Thesis advisor)

Aachen (2020, 2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

In this thesis, the application of electrospun polyacrylonitrile-derived carbon-fibre mats as electrodes for aqueous-alkaline metal--air batteries is investigated. Three main aims are pursued: First, a deeper understanding of the morphological properties of the fibres and their transformation during carbonisation. Secondly, establishing a method of activity evaluation that allows for an analysis in near-application scenarios. Lastly, relating structural information and activity.After outlining current challenges in the field of global energy storage, the history and working principles of the aqueous-alkaline metal--air battery systems is presented. The main reactions on the air-electrode, oxygen reduction and evolution are presented and discussed in detail. Current approaches to catalyse these kinetically challenging reactions are focussed on, considering mainly nitrogen-doped carbons and co-doped carbons using transition metal catalysts. Polyacrylonitrile is a feasible base material, because of its inherent nitrogen content and behaviour during carbonisation. It was further enhanced by adding cobalt and nickel, promising oxygen electro-catalysts. A variety of methods was employed to investigate the materials, including Raman spectroscopy, XPS, XRD, and linear sweep voltammetry. The structure and composition of polyacrylonitrile-derived carbon-fibres are discussed in wide range of carbonisation temperatures. The nitrogen content was found to be more than halved between 600 °C and 1000 °C. Also, the nitrogen bonding-types evolve from mainly pyridinic nitrogen to graphitic nitrogen. This was found to influence the electrochemical performance of the material, especially the limiting current and overpotentials of the oxygen reduction reaction. The overall activity maximum was found for samples carbonised at 800 °C.The structure of the carbon material is modified in presence of cobalt and nickel. In both cases, turbostratic carbon is formed and the nitrogen removal is shifted towards lower temperatures. The nitrogen bonding is also affected, with the metals inducing a higher content of pyridinic nitrogen. The changes affect activity in terms of oxygen reduction current densities and overpotentials. While nickel was found to influence the morphology more, cobalt made a stronger impact on electrochemical activity. Neither metal, significantly influenced the oxygen evolution activity. While the presented materials are shown to function as an air-electrode and the introduced key parameters of performance description allow for a sufficient correlation of structural properties and electrochemical activity, more research is required to enhance the performance of the electrodes to a competitive level.

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