Exploring the solution space for carbon fiber based electrodes in bioelectrochemical systems

  • Untersuchung des Lösungsraums für carbonfaserbasierte Elektroden in bioelektrochemischen Systemen

Pötschke, Liesa; Blank, Lars M. (Thesis advisor); Agler-Rosenbaum, Miriam (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021


Microbial Electrochemical Technologies (MET) comprise bioeconomic technologies for areas like waste management, green chemistry, bioremediation, and many more. Their key is the ability of special - electroactive - microbes to transfer electrons to or from electrodes in bioelectrochemical systems (BES). However, poor exploitation of the microbial capabilities often impede the market introduction of MET. In this context, BES building blocks, which are often simply transferred from electrochemical systems, should be better adapted to the specific conditions in BES. This thesis contains the systematic screening of commercial woven carbon fiber (CF) fabrics for their application as electrodes in BES. All levels from the carbon filament, which is the smallest unit in a carbon fiber, to the fabric level, are covered; including considerations of three-dimensional electrode integration in BES reactors. As a first basis, the two main electroactive model organisms Geobacter sulfurreducens PCA and Shewanella oneidensis MR-1 are characterized regarding their interaction with CF fabric electrodes. A Michaelis-Menten-type kinetic links bacterial current generation with electron donor concentration in the electrolyte. The key kinetic parameters jmax (maximum current density) and kS,app (apparent half-saturation constant for the electron donor) are assessed and reveal the differing physiologies and electron transfer mechanisms of the two organisms. As a major result, the versatile physiology of S. oneidensis proves to be suitable for the detection of µm-scale electrode topology differences. The final evaluation of material characteristics is therefore performed in BES with S. oneidensis MR-1. Different test reactors are used for evaluations of single CF (Fiber BES) and fabrics (Fabric BES) to be able to assess the material characteristics as independent as possible. The results clearly show that both the underlying CF as well as fabric material parameters can be tuned to boost bacterial current generation. The major influence parameter is the fiber type (continuous multifilament, CM, or stretch-broken yarn, SB), with SB fabrics performing in the upper range of all CF fabrics tested. On the fabric level, several weave patterns are evaluated. The plain and leno weave are further explored for their application in two types of BES, which are categorized by 1) BES with undefined mixed microbial consortia and particulate electrolytes, such as wastewater treating BES, and 2) BES with pure or defined mixed cultures, with no particles in the electrolyte other than the biomass. An exemplary plain weave fabric is optimized for an application in BES type 1. This is done by varying weave parameters, which determine the fabric density (areal coverage by yarns) and thickness of the material. A trade-off between maximizing current density and material exploitation is shown. Already at low fabric densities, a considerable part of the CF material is not accessible for electroactive current generation and remains unused. The leno weave is believed to be especially suitable for BES type 2. A set of preliminary experiments highlights promising performance of such electrode materials, since they enable high electrode packing densities in stirred tank reactors due to large pores in the cm range. A leno weave fabric made from 100 % CF is presented for the first time. An envisaged application are traditional bioreactors that can be upgraded by the leno weave electrodes to BES for bioelectroproduction of specialty chemicals.