Edited by: Uwe Schröder, Technische Universitat Braunschweig, Germany
Reviewed by: Annemiek Ter Heijne, Wageningen University & Research, Netherlands; Benjamin Erable, Centre National de la Recherche Scientifique (CNRS), France
This article was submitted to Bioenergy and Biofuels, a section of the journal Frontiers in Energy Research
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Novel applications of bioelectrochemical systems (BES) are emerging constantly, but the majority still lacks economic viability. Especially the use of electrochemical system components without adaptation to BES requirements causes poor exploitation of the potential system performance. The electrode material is one central component that determines BES performance. While commercial carbon fiber (CF) fabrics are commonly used, their customizability as two- or three-dimensional electrode material for BES is rarely investigated. Using pure cultures of
Bioelectrochemical systems (BES) unlock novel bioeconomic technologies by utilizing the microbial ability of extracellular electron transfer to a solid electrode (Santoro et al.,
Considerable effort has been put into the engineering of BES toward various enhanced performance parameters, which have been extensively reviewed elsewhere (Janicek et al.,
In our preliminary work, we identified carbon fiber (CF) based textile electrodes as all-rounder electrode material, since they combine all preferred features for large scale applications, i.e., large specific surface area in the (m2/g) range, good mechanical and chemical stability, customizable flexibility and porosity at μm up to cm scale; all of which enable high electrode packing densities (Morgan,
CF are derived from organic precursors with preferably high carbon yield (i.e., low amount of non-C atoms that will vanish during carbonization). The most common precursors are polyacrylonitrile (PAN) and pitch with a market share of 96 and 3%, respectively (Das et al.,
The simplified standard production processes for PAN CF woven fabrics are depicted in
Simplified general production process of woven CF fabrics (PAN based) made from either continuous multifilament rovings (1) or stretch-broken yarns (2). Stars mark the steps that are added especially for the application in BES. The parameters investigated in this study are highlighted in green (fiber type, degree of graphitization, desizing, and 2nd chemical surface activation).
The electrical properties of a carbon filament are determined by the purity and orientation of their basal planes, i.e., carbon grids along which electrons are conducted. The planes are stacked onto each other and electrons flow best in-plane, i.e., in-filament direction (Dutta,
With this study, we aimed at laying the foundation for a comprehensive customized engineering of CF woven electrodes for BES. With respect to the production process chain, we investigated possibly relevant parameters, such as yarn macrostructure (CM or SB), the filament conductivity, desizing, and 2nd surface activation, and evaluated their influence on electrode performance. To minimize biological variability during this comparative evaluation of material properties, we utilized the robust anodic electroactivity of
All textile products were denominated following international terminology standards (ASTM D123).
Fibers were subjected to a preparative physical characterization prior to evaluation as BES electrodes. The workflow was electrical resistivity measurement (
Properties of commercial PAN CF used in this study.
HTS40 | Good performance in previous |
TT | Tenax® E HTS40-F13 | 12K |
CT50 | Equivalent product to HTS40, but different processing | SGL | C T50-4.0/240-E100 | 50K |
T300 | Standard fiber and sizing | TO | Torayca® T300-40B | 3K |
CF fabrics used in this study.
SBlow | Low C-content | SGL | SIGRATEX® C W270-TW2/2/CA | SB |
SBmoderate | Moderate C-content | SGL | SIGRATEX® C W230-TW2/2/GR | SB |
SBhigh | High C-content | SGL | SIGRATEX® C W230-TW2/2/GR |
SB |
CMlow | Low C-content | SGL | SIGRATEX® C W245-TW2/2 | CM, 3K |
CMmoderate | Moderate C-content | SGL | SIGRATEX® C W230-TW2/2/GR | CM, 3K |
CMhigh | High C-content | SGL | SIGRATEX® C W230-TW2/2/GR |
CM, 3K |
T300 fabric |
Not surface activated during production, standard fiber and sizing | CT | CT200L-200 | CM, 3K |
The electrical resistivities of CF fabrics are in the same order of magnitude as the respective single CF. However, reliable standardized measurement methods do not exist on fabric level and the measurement errors tend to be high, which is why specific fabric resistivities are not given in
For the introduction of intended breaks (damaged multifilament rovings, results Effect of Fiber Type on Bacterial Current Generation), a customized indenter with sharpened grooves was fabricated in-house. Fiber strengths were assessed according to ASTM D3108 (registered method to characterize to what extent fibers are prone to filament breakage) using an in-house test rig with the following operational parameters: looping angle 180°; 5 m/min yarn throughput, test interval 5 min (tested fiber length 25 m), initial load of filament break sensors before/after indenter 5 N/12 N. The resulting fiber damage is expressed as statistical average amount of broken filaments in a random cross-section of the roving [% of x K filaments]. Fibers with filament damage exceeding 10% could not be processed further since they disassembled in the non-twisted configuration. The obtained range of fiber damage was 2.4–7.2%. Damaged fibers are referred to as “Dx” hereafter.
In parallel to the screening of commercial CF for suitable material properties, a desizing method was established to be applicable to all CF based electrodes. Tested desizing methods were industrial washing agents (used according to manufacturer's protocols), Soxhlet extraction (removal of sizing by organic solvents) with acetone/20 cycles, as well as pyrolysis (burning off the sizing). Pyrolysis was identified as final method of choice and subsequently applied for all CF electrodes presented here. An inert process gas was chosen, since desizing under air may alter the CF surface (Morgan,
The two CM used for surface activation experiments were C T50-4.0/240-E100 (hereafter CT50) and Tenax® E HTS40-F13 (hereafter HTS40) from two different manufacturers. The material specifications of both fibers were comparable (
HTS40 fibers had shown a good performance in previous electrode screenings (single fiber BES, not shown). Therefore, they were selected as model fibers and were further characterized by contact angle and X-ray photoelectron spectroscopy (XPS) analysis. The contact angle measurement was conducted with a K100 SF (Krüss GmbH, Germany), which is designed for single carbon fibers (the fiber is pulled out from a wetting liquid and the contact angle is calculated from the resulting force). XPS survey spectra were recorded at a pass energy of 60 eV using a Phoibos 100 analyzer with a CCD detector (SPECS Surface Nano Analysis GmbH, Germany). High-resolution spectra of the C 1s, O 1s, and N 1s peaks were recorded at a pass energy of 20 eV.
For the preculture,
The optical density at 600 nm (OD600), pH and concentrations of lactate and acetate (HPLC on Metab-AAC column, 300 × 7.8 mm, Isera GmbH, Germany; in 5 mM H2SO4 mobile phase, 0.6 ml/min, 30°C) were tracked for the whole experimental duration. Furthermore, selected bioelectrode samples were cut out and analyzed by scanning electron microscopy (SEM) with 10 kV accelerating voltage on a Zeiss DSM 982 Gemini microscope (Zeiss, Germany). The sample preparation procedure included fixation in 2.5% glutaraldehyde, storage in ethanol, and drying by hexamethyldisilazane, and sputtering with a 20 nm gold layer. Where reported, riboflavin concentrations of filtered (0.2 μm pore size) samples were analyzed spectrophotometrically in reference to a riboflavin standard (Carl Roth GmbH & Co. KG, Germany), excitation/emission 450/530 nm as reported in Lu et al. (
Single carbon rovings were configured in a loop-like structure and attached to graphite rods (grade EDM-3 (specific electrical resistivity 1.56 mΩ·cm), Ø 30 mm, Novotec, Germany) using conductive carbon cement (Leit-C, Sciences Services GmbH, Germany). Common adhesive tape delimited the desired immersed fiber length that performed as working electrode. The fiber length was normalized to a theoretical BET surface area of ~300 cm2 (calculated by considering single filaments as perfect solid cylinders), assuming that the available electrode surface area is the most crucial parameter for bacterial interaction (Chen et al.,
In deviation to the standard setup, fragile fibers of the filament break experiments (chapter Effect of Fiber Type on Bacterial Current Generation) were configured straight and stabilized by tooth picks as shown in
The working electrodes were constructed from 45 × 120 mm carbon fiber fabrics that were attached to graphite clamps. A graphite rod (grade EDM-3, Ø 30 mm, Novotec, Germany, immersed length 145 mm) was used as counter electrode. The flat-plate-type reactor consisted of two rectangular PEEK frames (10 × 148 × 55 mm) with glass walls that were pressed together along with a synthetic rubber gasket (EPDM Ø 6 mm, Hug Industrietechnik, Germany). Each of the frames held working/counter electrode and the reference electrode (
Standard T300 fabrics (
Degrees of desizing after application of varying desizing methods for woven fabrics made from CF T300 (initial sizing amount 0.992%w/w).
None | – | 0 |
Pyrolysis | N2 | 83.3 |
Soxhlet extraction | Acetone | 73.8 |
Ethanol | 51.4 | |
Petroleum ether | 16.0 | |
Washing agents | Rucogen DFL-200 (Rudolf Group, Germany) | 62.7 |
Sulfaton D (Bozzetto GmbH, Germany) | 57.5 | |
Tanaterge® Advance (Tanatex Chemicals, Netherlands) | 70.7 | |
Propetal140/Sulfetal 4105 (mix) (Zschimmer & Schwarz, Germany) | 44.8 |
Desizing by pyrolysis achieved the highest degree of desizing. Regarding the electrode performance in BES, the pyrolytic removal of the sizing accounted to an ~45-fold increase of jmax for a desized T300 fabric (
The two equivalent commercial PAN-based carbon fibers CT50 and HTS40 (
All surface activation methods achieved a decrease in contact angle, i.e., increase in wettability, and introduced foreign atoms into the HTS40 fiber surface (
Physical characterization of surface activated HTS40 fibers.
Not activated | 76.53 | 5.46 | 1.81 | 92.73 |
NH3 plasma | 42.46 | 5.88 | 5.92 | 88.21 |
Electrolysis | 15.68 | 15.27 | 6.64 | 78.09 |
Air plasma | 9.26 | 12.63 | 3.06 | 84.31 |
O2 plasma | 8.52 | 18.11 | 1.77 | 80.13 |
Oxygen containing functional groups were introduced into the carbon surface by all methods. In NH3 plasma treated samples, this amount was small and is unlikely related to the high-purity process gas. It was rather related to previously adsorbed atmospheric O2, which appeared mainly as C-O bonds (hydroxyl groups, since no O-C = O was observed) and, in small portions, in N-O bonds (e.g., imine-N-oxide) in the high-resolution spectra (
High-resolution XPS spectra analysis of surface activated HTS40 fibers.
Not activated | 75.73 | 13.14 | – | 3.86 | 1.4 | 0.41 | – | 2.83 | 2.83 | |
NH3 plasma | 66.77 | 15.95 | – | 5.49 | 5.92 | – | – | 0.69 | 4.51 | 0.68 |
Electrolysis | 57.45 | 12.61 | – | 8.03 | 5.74 | – | 0.9 | 4.97 | 10.3 | – |
Air plasma | 60.22 | 13.55 | 4.03 | 6.51 | 2.64 | 0.42 | – | 10.35 | 2.28 | – |
O2 plasma | 52.62 | 13.05 | 7.04 | 7.42 | 1.77 | – | – | 7.41 | 10.7 | – |
Performances of single carbon fiber electrodes CT50
Untreated CT50 and HTS40 achieved similar jmax (
For CT50 (
The two features that distinguish the less common SB from the classic CM are free filament ends (
Free filament ends and yarn twist were introduced into HTS40 fibers (12K CM) at lab scale in order to reconstruct the industrial production of stretch-broken yarns from multifilaments. Identical CM without tribological treatment served as intact control electrodes.
Current density normalized to combined filament surface in single fiber BES (HTS40 CM);
For a deeper understanding of the link between free filament ends and bacterial current generation, confocal scanning laser microscopy (CSLM) imaging of different fiber electrodes was performed (see
A set of related commercial fabric types was selected in order to verify the role of free filament ends in stretch-broken yarns and elucidate the mechanisms behind their good performance. Besides the filament macrostructure, we selected fabrics with different carbon atom content C [at.%] to vary filament conductivity, which was not possible to study at fiber level because of mechanical instability of graphitized fibers. The model electrodes were commercial SIGRATEX® fabrics made from CM or SB along with their equivalent graphitized products (
In the case of stretch-broken fabrics, a substantial increase in maximum current density jmax (by average of 100%) and OD600−max (by average of 60%) was linked to an increased filament conductivity from SBlow to SBmoderate (
Two electrode samples, which were taken from reactors sharing identical inoculum and identical starting time points, were selected for SEM imaging. The carbonized SBlow (
In CM fabrics, increased filament conductivity is not boosting jmax, and even has an adverse effect in the case of CMmoderate fabrics (
CF based textile fabrics are a popular electrode material for BES, primarily because they offer a large surface area for bacteria-electrode interactions. However, the growing CF market and new applications have created a broad spectrum of many more tunable material characteristics. Growing production capacities result in dropping prices, which may soon allow for economic electrode designs for BES (Morgan,
Pure cultures of the electroactive model organism
While SB fabrics get their sizing removed before the last production step, CM fabrics are always coated by a sizing (
Among the presented options, we recommend the use of thermal desizing as a waste-free method. Furthermore, the gaseous process medium leaves a clean fabric surface without the need for extensive washing steps. The thermal desizing can easily be scaled and implemented into existing process chains (technically, it is not much different from the equipment for carbonization). In a broader context, the thermal method could be used for a combined desizing and chemical surface activation by simply adapting the process gas. This can be O2 or ambient air, or as shown by Wang et al. (
According to our results, the effect of surface activation cannot be generalized and depends on the fiber. This is not surprising in the light of the great spectrum of commercial products and their material properties. Although identical treatments were applied to the almost equivalent fibers HTS40 and CT50, the effects were only marginal in HTS40, but strong in CT50 CF. In this case, the increase of j12h for pre-treated fibers could be correlated with an increase in hydrophilicity. The same effects on fiber surface and j12h were achieved by increasing the C-content (
The relationship between enhanced BES startup and a hydrophilic electrode surface has also been observed by Guo et al. (
Finally, a closer look reveals that j12h of the CT50 was promoted to the level of an untreated HTS40 after surface activation (
In a perfect graphite crystal, the electrical conductivity along the planes is 104 fold higher than perpendicular to the planes (Dutta,
Moving from single fiber to fabric level, we could further elaborate on the role of free filament ends in SB (chapter Effect of Filament Conductivity and Fiber Type on Bacterial Current Generation). With this yarn type, an increased filament conductivity was responded by an enhanced bacterial current generation (
This consideration is supported by a fast initial planktonic growth compared to a slower current increase in the moderately graphitized fabric SBmoderate (
Despite the high jmax of graphitized SB fabrics, the adverse effect of elevated C-content on startup current j12h cannot be neglected. Since the effect was independent of the fiber type, it can be related to the increased purity of the CF surface, i.e., its increased hydrophobicity (
Carbon fiber woven fabrics are a versatile electrode material that can be customized with a high reproducibility. Their excellent trade-off between good mechanical stability, flexibility, electrical conductivity, and material exploitation makes them suitable materials for the wide variety of novel BES applications. This study shows that the potential of carbon fiber fabrics can be comprehensively exploited and there are several factors to be customized for the specific BES application. In the light of the diversity of CF and thereof derived products, we recommend that BES studies using such electrodes report the specific CF material properties and the applied treatments such as desizing and surface activation. This way, research results may be translated to a comprehensive adaptation of process parameters and push forward the industrial implementation of BES-oriented CF product lines. With the above results, we focused on identifying the relevant fiber material properties and found graphitized, stretch-broken fiber based woven fabrics to be interesting for stirred pure culture BES. For low-tech applications in highly competitive technological fields, such as wastewater treatment, the use of less costly fabrics based on continuous multifilament rovings is recommended. For these fabrics, we point out that a proper removal of the sizing is crucial for efficient bacteria-electrode interaction and should be verified by, e.g., TGA analysis. We also looked into different chemical surface activation methods in order to increase the biocompatibility of the carbon fiber surface. The most suitable methods are such that introduce both nitrogen and oxygen containing residues and increase the hydrophilicity of the carbon surface. We could not confirm an impact of surface activation on maximum current density, but only on the reduction of startup time. However, the effect depended strongly on the fiber and we found that there are commercial fibers that already come with sufficiently biocompatible surfaces. We concluded that a surface activation is of limited use for pure culture BES lacking microbial competition during startup. Nevertheless, it may be useful for mixed culture BES to enhance selection of highly electroactive biofilm members.
All relevant data are included in this manuscript and the corresponding
LP coordinated the study, designed, conducted and analyzed all BES experiments, and prepared the manuscript. PH coordinated and performed the fiber and fabric material selection, supervised and analyzed their physical characterization experiments, and co-prepared the manuscript. SS coordinated and supervised the desizing, surface activation experiments and revised the manuscript. VR conducted SEM sample preparation and analyses in chapter Effect of Filament Conductivity and Fiber Type on Bacterial Current Generation and Revised the Manuscript. TG advised on all CF work, discussed results, and revised the manuscript. LB discussed the work and revised the manuscript. MR conceived the work, advised on the experimental plan, discussed experiments, and revised the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We are very thankful to Almut Schwenke (SGL Carbon, Germany), who provided CF materials, discussed their properties, provided graphitization services, and revised the manuscript. We are also grateful to Sebastian Wittig (Diener electronic GmbH & Co. KG, Germany) for providing plasma activation services and expertise regarding surface activation. Furthermore, we wish to thank Myong-Hun Jung, Tobias Bolz, and Felicitas Schmitz for their great contribution to the experimental work. We also thank Jürgen Klimke (CARBO-TEX GmbH, Germany), who provided T300 fabrics and was always available for helpful advice.
The Supplementary Material for this article can be found online at:
bioelectrochemical system
coulombic efficiency
carbon fiber
continuous multifilament roving
microbial fuel cell
polyacrylonitrile
stretch-broken yarn
thermogravimetric analysis
X-ray photoelectron spectroscopy.