Carbon felt and carbon fiber - A techno-economic assessment of felt electrodes for redox flow battery applications

Journal of Power Sources 342 (2017) 116e124

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Carbon felt and carbon fiber - A techno-economic assessment of felt electrodes for redox flow battery applications Christine Minke a, b, *, Ulrich Kunz a, b, Thomas Turek a, b a b

Clausthal University of Technology, Institute of Chemical and Electrochemical Process Engineering, Leibnizstr. 17, 38678 Clausthal-Zellerfeld, Germany Energy Research Center of Lower Saxony, Am Stollen 19A, 38640 Goslar, Germany

h i g h l i g h t s  Analysis of material and energy requirements in production of carbon felt electrode.  Production costs of different carbon felts calculated in transparent cost model.  Techno-economic comparison of conventional and biogenic carbon felt electrodes.  Correlation of market development of carbon felt and carbon fiber.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 October 2016 Received in revised form 8 December 2016 Accepted 11 December 2016

Carbon felt electrodes belong to the key components of redox flow batteries. The purpose of this technoeconomic assessment is to uncover the production costs of PAN- and rayon-based carbon felt electrodes. Raw material costs, energy demand and the impact of processability of fiber and felt are considered. This innovative, interdisciplinary approach combines deep insights into technical, ecologic and economic aspects of carbon felt and carbon fiber production. Main results of the calculation model are mass balances, cumulative energy demands (CED) and the production costs of conventional and biogenic carbon felts supplemented by market assessments considering textile and carbon fibers. © 2016 Elsevier B.V. All rights reserved.

Keywords: Felt electrode Carbon fiber Flow battery Cost analysis Techno-economic assessment

1. Introduction The finiteness of fossil resources and a growing environmental awareness lead to an extensive paradigm shift towards materials efficiency and biogenic materials. Researchers of many disciplines, e.g. energy supply, aircraft and automobile construction, focus on developing technologies and products based on sustainable sources. In parallel, a change in the energy paradigm has prompted rapid development of renewable energy technologies and thus electric energy storage systems. Against this backdrop, technology developers are facing the grand challenge to meet the urgent need for innovative energy technologies manufactured with an economic and ecologic use of raw materials and process energy. * Corresponding author. Clausthal University of Technology, Institute of Chemical and Electrochemical Process Engineering, Leibnizstr. 17, 38678 Clausthal-Zellerfeld, Germany. E-mail address: [email protected] (C. Minke). http://dx.doi.org/10.1016/j.jpowsour.2016.12.039 0378-7753/© 2016 Elsevier B.V. All rights reserved.

In this paper a techno-economic assessment of carbon felt electrodes for redox flow battery (RFB) applications is presented. In a comprehensive approach the technical, economic and ecologic potentials of felt electrodes from fossil and biogenic raw materials are analyzed. The broader development context of carbon felt and carbon fiber is presented in section 2. The application of felt electrodes is outlined on the example of the vanadium redox flow battery (VRFB). Then, the interrelation of carbon felt and carbon fiber is explained regarding the manufacturing process of carbon felts and a selection of suitable starting materials. In section 3 the methods applied in the techno-economic assessment approach are presented. The results of the comparative assessment of a conventional carbon felt made from polyacrylonitrile (PAN) and a biogenic rayon-based felt are presented and discussed in section 4, as well as their production costs and market assessment. Conclusions drawn from this comprehensive evaluation are presented in the final section 5.

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2. Development context of carbon felt and carbon fiber 2.1. Carbon felt electrodes in RFB applications Electrodes belong to the core components of RFBs. They provide active sites for the chemical reaction of redox couples dissolved in the electrolytes. Their physical structure should enable a high electric conductivity in the material and a good electrolyte flow at its surface. The energy efficiency of the battery is mainly governed by the activation and concentration polarizations determined by the electrodes [1]. Thus, suitable electrodes for VRFB applications have to meet a set of requirements: High electrical conductivity High specific surface area No participation in the redox reactions, besides providing active sites Chemical stability in highly acidic environments Electrochemical stability within a wide operation potential window The development of electrodes for VRFB has been elucidated in a recently published historical review [1]. Ever since the battery has been firstly proposed by Skyllas-Kazacos in 1986 there have been efforts to enhance the electrochemical performance of electrodes [2,3]. Carbon has been identified as the best material for use in both half-cells meeting all requirements listed above. The disadvantages of oxygen evolution and carbon degradation under high anodic potentials could be eliminated by a suitable voltage control during charging. Citation figures of 1985e2015 from the Web of Science™ plotted in Fig. 1 (a) indicate a raised interest in electrodes since 2011 [4]. Carbon felt has been the most widely used electrode type in VRFB although a variety of electrode types has been considered, e.g.

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carbon composites, carbon papers and graphene [5]. Carbon felt electrodes are usually made from polyacrylonitrile (PAN) fibers. SGL Carbon SE also offers a biogenic rayon-based felt electrode. The manufacturing requirements for carbon felt electrodes and the possibilities for using biogenic precursors will be considered in sections 2.2 and 2.3. However, in the present section the electrical conductivity as the main criterion of a suitable electrode will be discussed. Material data for the PAN-based felt electrode GFD5 and the rayon-based felt GFA5, both provided by SGL Carbon SE, are given in Table 1. The biogenic electrode shows higher electrical resistivity in plane and through plane. The tendency of rayon-based felt electrodes towards a higher resistivity in direct comparison to PAN-based electrodes may also be found in literature [6e8]. At the same time, it is stated in literature that the complexity of factors leading to a high performance carbon felt electrode is not yet understood [1,6,7,9e11]. There is a need for further research especially on the influence of any kind of felt activation in combination with different precursor materials. No single property may therefore be isolated to explain differences in the electrochemical performance of an electrode. In the present paper a general suitability of rayon-based carbon felt electrodes in VRFB is assumed and the focus lies on aspects of technical and economic feasibility of biogenic electrodes. 2.2. Manufacturing of carbon felt electrodes The manufacturing process of carbon felt electrodes is poorly described in literature. Information is provided by SGL Carbon SE, one of the main suppliers of felt electrodes [9,10,12]. State-of-theart reel-to-reel processes start with a so called white fiber, a polymer or biogenic fiber. A raw felt is produced in a laying and needle punching process, a textile craft for producing felts without the use of water. In the following thermal treatment, hereafter referred to as carbonization process, the felt is stabilized, carbonized and graphitized according to a specific process protocol [13e15]. Another possible manufacturing route in which carbon felt is made from pre-oxidized or black PAN fiber is not considered in this article. The first step of the thermal treatment, oxidation and stabilization of the felt, is carried out in air at a temperature range of 180e260  C. Subsequent process steps are carried out in an inert atmosphere. The partial carbonization is carried out at 320e800  C. The carbon content of the felt is raised by loss of hydrogen, oxygen and nitrogen and the degree of cross-linking of the carbon framework is increased. In the carbonization step in a temperature range of 800e1800  C the filament is nearly completely converted into carbon. In order to reach a higher carbon content of about 99% a final graphitization step at temperatures above 2000  C may be added. Carbon felts fabricated with the described methods are widely used as high-temperature insulating materials in non-oxidizing atmosphere. The main application, however, is the production of composite materials with a synthetic resin or carbon matrix. The

Table 1 Material data for SGL Carbon felt electrodes [12].

Fig. 1. Historic citation reports from Web of Science™ on (a) RFB electrodes, especially felt electrodes (Topic: “flow batter*”, Title: electrode; “graphite felt” OR “carbon felt”) and (b) precursor materials for carbon fibers (Topic: “carbon fiber” OR “carbon fibre”, Title: PAN OR polyacrylonitrile; pitch; lignin; cellulos* OR rayon); Data [4].

Fiber type Bulk density Nominal thickness Area weight Electrical resistivity through plane Electrical resistivity in plane Ash value (580  C)

Unit

GFD5

GFA5

g cm 3 mm gm 2 U mm U mm %

PAN 0.1 5 500 <5 <3 <0.20

Rayon 0.1 5 500 <15 <5 <0.15

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influence of this rapidly growing market on the niche market of felt electrodes is discussed in sections 2.3 and 4.4. Only very few manufacturers of carbon felts, such as SGL Carbon SE, a world market leader in carbon materials, offer specialized carbon felt electrodes for electrochemical applications. The requirements for felt electrodes are different from those for insulation or mechanical applications. Table 2 shows critical process steps and their complex effects on electrode properties [16]. The selection of a suitable precursor for carbon felt electrodes has a superior impact on the felt quality, because the precursor fiber builds the substructure of the felt and affects the overall processability. Precursor materials for carbon fibers will be discussed in section 2.3 in detail. In the production of the white fiber fundamental mechanical, chemical and electrical properties are set in the so called sizing process. The fiber sizing herein contains a sizing chemistry as well as physical sizing determining fineness, crimp level and fiber length. The fineness of fibers is described in the tex system. The count in the tex system is the weight of a fiber with a length of 1000 m in grams. The fineness in decitex (dtex) can be calculated from a given fiber length l in meter and a fiber mass m in grams: 10,000 m/l. The crimp level is defined as the difference between the straightened fiber length and the distance between the ends of a fiber pressed in the felt. Typical characteristics are a fineness in the range of 1.7e2.2 dtex and a fiber length between 50 and 75 mm. Subsequent web laying and needle punching methods affect mechanical, electrical and structural properties of the fabric. Important parameters in this process are: Needle type Number of needles Alignment on needle board Number of boards Puncture frequency Needling direction Line speed The final carbonization process is again crucial for all characteristics of the product. Moreover, surface properties of the felt can be tailored to application requirements. The carbonization protocol includes many parameters that may be varied in order to enhance product quality: Temperature profile Hold times Final temperature Furnace atmosphere Tensile forces To conclude, the manufacturing of carbon felt electrodes is a complex process with a variety of parameters affecting product quality. Moreover, the term quality includes specific requirements for anodes and cathodes in different types of batteries. The basic methods used to produce fiber and felt are conventional textile technologies. This means that felt electrodes may be manufactured with high levels of consistency and in large quantities. In terms of

cost potentials it can be assumed that there is no potential for cost reduction in these manufacturing steps (SGL Carbon SE, personal communication). A detailed economic consideration is presented in section 4.4. It can further be concluded that there is a strong orientation towards trends and developments in the textile industry. This relates to the utilization of different precursors for polymer and biogenic fibers. It can be assumed that only precursors and white fibers, respectively, with a good availability, appropriate price and constant quality are suitable for the production of carbon felts. Dedicated textile process parameters, such as very low tolerances, are obstacles for biogenic precursors with their natural variability. Besides this outline of the textile industry's role as a major market influencing the niche market of carbon felt electrodes, influences from the emergent industry of lightweight construction are to be considered in the sections 2.3 and 4.4 below. 2.3. Precursor materials for carbon fibers In theory, carbon fibers (CF) can be made from any fibrous starting material containing carbon in a pyrolysis process (see section 2.2). From the diversity of available polymers, biopolymers and plant fibers, a few precursor systems have become established in the CF industry and the carbon felt electrode segment, respectively. In general, the most important precursors are polyacrylonitrile (PAN) and mesophase pitch [15]. Carbon felt electrodes are made from PAN or viscose fibers [16]. These three materials are described below supplemented with the biomaterial lignin, which is a promising object of research. PAN is the dominating precursor for carbon fibers, containing 68% carbon. It was first recognized as a suitable starting material in 1961 [17]. PAN is a commodity based on fossil raw materials, i.e. crude oil and natural gas. The average yield in carbonization process is about 50%. Details on its synthesis process and market are presented in sections 4.1 and 4.4. As it is a synthetic polymer its properties can be adjusted relatively easily in the production process. The advantage of a good processability makes it the recommended precursor fiber for felt electrode manufacturing [16]. The second most important precursor, pitch, may be derived directly from petroleum and coal or from synthetic polymers. It has a high carbon content of potentially more than 80% and an equally high yield in carbonization [15]. Although it is an appropriate precursor for CF used in composite materials, constrains in the manufacturing process render it unsuitable for felt electrode production. The spun products tend to be brittle, which makes them difficult to handle in textile manufacturing processes [18]. The CF market demand is currently covered by these two standard materials. But it has been growing continuously, with a focus on lightweight and functional construction materials. Consumption estimates for PAN- and pitch-based CF and a resulting annual growth rate of 11% are shown in Table 3. In 2013 a kilogram of CF is valued with an average list price of 26 V and 35 US$, respectively [18]. As a consequence of the increasing demand for CF, alternative raw materials for low-cost fibers came into the focus of research. Worldwide research activities have been initiated in order to investigate suitable biogenic precursors that are inexpensively

Table 2 Manufacturing steps and their effects on carbon felt electrode properties [16].

Fiber oxidation Fiber sizing Web laying Needle punching Carbonization steps

Mechanical properties

Electrical resistance

X X X X

X

Purity

Thickness, porosity

Surface properties

X X X

X

X X X X

X

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Table 3 Estimated world consumption of PAN- and pitch-based carbon fibers [18].

Aircraft, aerospace Sporting goods, recreation Industrial, automotive, other Total

2012 In t

2018 In t

Average annual growth rate 2012 to 2018 In %

8740 8700 23,630 41,070

18,360 10,250 48,100 76,710

13.2 2.8 12.6 11.0

available from cellulose- and lignin-containing plants [19]. A historic citation report on precursor materials for carbon fibers shows a rise of interest within the recent years (Fig. 1(b)). The figures indicate efforts to improve the supply of low-cost CF from biogenic sources. Cellulose-based precursors have recently been rediscovered, driven by the paradigm change towards biogenic materials and the need for low-cost CF. The first cellulosic CF served as filaments in electric lamps, patented by Edison in 1880 [20]. Substantial research and development activities were made in the 1950se1970s, but the PAN-based fiber drove the rayon-based CF from the market. Cellulosic precursors have carbon contents of about 44% leading to a low actual carbonization yield of about 25%. Another drawback is the natural variability of fiber properties. With regard to a change towards biogenic materials, cellulose has the indisputable advantage of being the most abundant biopolymer in the world [19,21]. A detailed consideration of the manufacturing process and market of rayon-based carbon felt electrodes is presented in sections 4.2 and 4.4. Lignin is wood's second most abundant biopolymer after cellulose. It is a by-product in the cellulose pulping process (Kraft process). The main application is thermal utilization for process energy recovery. Together with a carbon content exceeding 60% and a high yield in carbonization, lignin is a promising precursor for low-cost CF [15,19]. An appropriate manufacturing process was first patented in 1969 [22]. Great research efforts towards lignin-derived CF have been made in recent years with a focus on automotive applications, whilst a suitability for use in felt electrode production has not been considered so far [23,24]. 3. Methods 3.1. System definition In the presented techno-economic approach material and energy requirements in the production of carbon felt electrodes from fossil and biogenic resources are compared. The cradle-to-gate

approach is illustrated in Fig. 2. The considered process chain starts with the precursor fiber production from fossil and biogenic raw materials, respectively. The subsequent textile processing involves conventional technologies, such as web laying and needle punching (dry felting). Finally, the raw felt is converted to carbon felt in a carbonization process. The standard precursor from fossil resources is PAN, which is analyzed in section 4.1. In comparison an electrode made of the biogenic precursor cellulose is evaluated in section 4.2. A brief outline of the methodology is presented below. In a first step, a mass balance is calculated for the production of a kilogram of carbon felt. Subsequently, the cumulative energy demand (CED) for the production of a kilogram of product is calculated. The CED involves energy in feedstock, as well as process energy according to the approach of the life cycle inventory database ecoinvent. Energy contents of fossil fuels and electric energy applied in this work are listed in Table 4. The inferior calorific value Hi specifies the reaction enthalpy during complete combustion. The difference between superior and inferior calorific value corresponds to the vaporization enthalpy of the respective water vapor at 25  C (2442 kJ kg 1). The total CED is defined as the sum of the energy in feedstock and the process energy demand for carbonization, the process energy demand for dry felting and the CED for the white fiber. The latter is calculated in dependence of each precursor. The energy demands for carbonization and felting are assumed to be equal for both kinds of carbon felt, respectively. 3.2. Process energy assumptions The process energy required in the textile process is assumed with 0.3 MJ kg 1 and in consequence neglected in the overall balance. This value is based on an electric energy demand of 0.084 kWh kg 1 for a needle punching machine processing felts of 96 kg m 3 density and 6e11 mm thickness [25]. Data regarding process energy in the carbonization process are very inconsistent in literature. Figures depend on kind and scope of considerations and system boundaries set in each study. Values, usually referred to as energy intensity of PAN-based carbon fiber production, are 247 MJ kg 1 (CF processing and assembly) [26], 186 MJ kg 1 (CF and sizing), 364 MJ kg 1 (CF production) [27], 170 MJ kg 1 (CF production step) and 459 MJ kg 1 (CF production) [23]. The last value correlates to an energy demand for the production of car parts from carbon fiber reinforced plastics of 460 MJ kg 1 and is therefore neglected in further consideration [28]. The assumption used in this study is an average value of 265 MJ process energy for carbonization of a kilogram of carbon felt.

Table 4 Inferior calorific values Hi for fuels and conversion factor for electric energy.

Fig. 2. System description of cradle-to-gate carbon felt manufacturing process.

Source of energy

Hi

Unit

Natural gas Crude oil Coal Electricity

41.0 42.8 30.0 3.6

MJ MJ MJ MJ

m 3 kg 1 kg 1 kWh

1

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heat treatment (Data: [30]).

3.3. Cost model and market assessment The cost assessment of carbon felt electrodes is carried out in accordance with the system modules described in Fig. 2. Thus, total costs are defined as the sum of costs of the white fiber, the textile processing and the carbonization process. Costs of white fibers and key intermediates in the production of those are taken from a 2013 industry report [29]. All estimates are based on an exchange rate of 0.76 V per US$. Information on production costs of felt electrodes are identified as proprietary or confidential. Thus, the costs of each production step are modeled on the basis of statistical data from comparable processes. These are the manufacturing of technical textiles and industrial heat treatment [30]. The cost structures of both industries are illustrated in Fig. 3. The data reveal a share of materials costs in textile processing and in the more energyintensive heat treatment of 41% and 31%, respectively. The final production cost model illustrated in Fig. 4 is based on the ratio of materials and processing costs along the process chain. The costs of precursor fibers are weighted with a factor f identified in the mass balance of felt production. Thus, the calculated materials costs are processed sequentially in the calculation of costs of the textile and carbonization processes. Finally, the resulting production costs of carbon felt electrodes are obtained (section 4.4.). Note that production costs are total costs consisting of materials and processing costs of carbon felts.

4. Results and discussion 4.1. Production of conventional carbon felt electrode The manufacturing of a PAN-based carbon felt consists of the following steps; the preparation of the precursors and white fiber, the textile processing of the raw felt from the acrylic fiber and a final carbonization step. Important process steps and key intermediates are presented in a flow chart in Fig. 5(a). A detailed consideration is presented in the following. The cradle-to-gate approach starts with fossil fuels (not the production of those) from which the chemicals are synthesized. Considerations of energy and materials consumption in the manufacturing process of acrylic fibers are based on literature data [31e33]. The mass balance for the production of a kilogram of PANbased carbon felt is calculated backwards. 2 kg of acrylic fiber are required considering a yield in carbonization of 50% [34]. The amount of PAN precursor is equal as high. PAN results from a polymerization reaction of acrylonitrile (AN) with a co-monomer. In this study the co-monomer is assumed to be methyl methacrylate (MMA), whereas a detailed consideration of its preparation from fossil sources is beyond the scope of this analysis. It is further assumed that 2 kg of PAN are prepared in a solution polymerization of 1.9 kg of AN with 0.1 kg of MMA. AN is primarily made in the Sohio process from propylene and ammonia. Propylene is a product of crude oil refining, whereas the second feedstock ammonia is produced in the Haber-Bosch process. A production of 1.90 kg AN requires 0.91 kg ammonia and 2.24 kg propylene. The CED of a kilogram of PAN-based carbon felt is calculated regarding the whole process chain. Assuming the use of fossil fuels as feedstock and for the generation of process energy leads to a coupled mass and energy balance. Fuel rates for all intermediates and process energies from literature are listed in Table 5. Together with the heating values from Table 4 the CED for each process step is calculated. Thus, 1 kg of white fiber requires 116.0 m3 natural gas, 24.1 kg crude oil, 11.1 kg coal and 6.4 kWh of electricity equal to a total of 158 MJ. Considering the yield in carbonization of 50%, the CED for 2 kg of acrylic fiber amounts to 316 MJ. Adding 265 MJ kg 1 for the carbonization process respectively, the total CED for PANbased carbon felt is 581 MJ kg 1. The composition of the CED is illustrated in Fig. 5(b). 4.2. Production of biogenic carbon felt electrode

Fig. 3. Cost structure of industrial processes of manufacturing of technical textiles and industrial.

The considered biogenic carbon felt electrode is made on the basis of cellulose, which is the most abundant biopolymer in the

Fig. 4. Model for calculation of production cost of 1 kg of carbon felt.

C. Minke et al. / Journal of Power Sources 342 (2017) 116e124

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Fig. 5. Simplified flow chart with material and process energy input and composition of CED for the production of a kilogram of PAN-based carbon felt ((a) and (b)) and rayon-based carbon felt ((c) and (d)).

Table 5 CED and fuel rates for the production of acrylic fibers and intermediates. CED In MJ kg Ammonia Propylene Acrylonitrile MMA Acrylic fibers a b

41.41 66.50 100.77 125.72 157.68

1

Natural gas In m3

Crude oil In kg

Coal In kg

Electri-city In kWh

Energy in feedstock In MJ

Literature

1.010 1.160 0.038 1.800 0.944

e 0.430 0.001 0.930 0.033

e e 0.014 0.260 0.344

e 0.150 0.111 1.200 1.450

e e 98.35a e 102.02b

[29,30] [27,29] [29] [27] [29]

CED of required quantity of Ammonia and Propylene. CED of required quantity of Acrylonitrile and MMA.

world (see section 2.3). The white fiber produced from cellulose is known as viscose or rayon. The process chain from wood to cellulose to viscose fibers and the subsequent textile and thermal

processing is illustrated in a flow chart in Fig. 5(c). It is to be noted that this consideration is based on a highly integrated manufacturing process characterized by a close linkage of

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production processes of pulp, viscose fibers, process chemicals and energy [35,36]. Before analyzing this integrated process, the individual standard pulp and fiber processes are described below. The cradle-to-gate assessment starts with wood, whereas its production is beyond the scope of the system. Cellulose is produced from wood, here European beech wood, in a pulping process called Kraft process. The required quality of so called dissolving grade pulp (in contrast to paper grade pulp) is constituted by a very high content of alpha-cellulose (>90%) after removal of the wood ingredients lignin, resin and large amounts of hemicellulose [35]. The most significant fabrication method of fibers from pulp is the viscose process, patented in 1892 [37,38]. In this process, the pulp is first alkalized in caustic soda, followed by a depolymerization and xanthation step, using carbon disulfide. After filtration, degassing and ageing, the cellulose xanthate solution or viscose is extruded into a spinning bath containing sulphuric acid, sodium sulphate and zinc sulphate. Literature data on mass and energy balances for pulp and fiber processes are ambiguous. This is caused by great differences in production plants concerning energy efficiency, rates of recovery and recycling and the degree of integration of adjacent processes. For orientation, consumption data collected by the European Commission is cited below [39]. A kilogram of viscose fibers requires a kilogram of pulp. Essential chemicals are sulphuric acid and caustic soda, required in amounts of 0.6e1 kg and 0.5e0.7 kg per kilogram of fibers, respectively. In addition, approximately 9 g of carbon disulfide are required. In the considered production process with a high degree of integration, most chemicals are recycled in the process or at least in form of process energy. The mass balance concerning the main raw material, wood, is calculated backwards. With an assumed yield in carbonization of 25%, 4 kg of rayon felt are required for the production of a kilogram of carbon felt [34]. As the beech wood is used in its entirety, a total amount of 10 kg of wood are required, of which 40% are cellulose and 60% contribute to process heat and power generation [36]. Process energy for the production of a kilogram of viscose fiber in an integrated pulp and fiber process is cited from a comprehensive analysis by Shen and Patel [35]. Thus, a kilogram of fibers requires 38 MJ of energy from renewable sources, primarily wood (36 MJ), and 31 MJ of energy from non-renewable sources. Adding the feedstock energy in fiber of 15 MJ per kilogram of beech wood or cellulose, the CED for viscose fibers is 84 MJ kg 1. The CED of a kilogram of rayon-based carbon felt consists of the CED for the production of the required amount of 4 kg of white fibers (336 MJ kg 1) and the CED of the carbonization process (265 MJ kg 1) as described in section 3. In conclusion, the total CED calculates to 601 MJ kg 1. Its composition is illustrated in Fig. 5(d). 4.3. Comparative assessment of production processes Focusing on the energy balances of the products, the analysis reveals that the biogenic fiber has a significantly lower CED of 84 MJ kg 1 compared to the acrylic fiber with 158 MJ kg 1. Similar figures can be found in an environmental assessment of textiles by Laursen et al. [40]. Laursen et al. equally calculate the CED of acrylic fibers at 158 MJ kg 1 including 112 MJ kg 1 for the preparation of AN and 46 MJ kg 1 for polymerization and spinning processes. For comparison, in the present study the CED for the preparation of AN amounts to 102 MJ kg 1 and the process energy for polymerization and spinning is 56 MJ kg 1. Considering the viscose fiber, Laursen et al. calculate a total CED of 71 MJ kg 1. They assume inputs of renewable energy of 36 MJ kg 1 and non-renewable energy of 35 MJ kg 1. In the present study 53 MJ kg 1 energy input from renewable sources and 31 MJ kg 1 from non-renewable sources are

assumed. In the underlying model by Shen and Patel an energy credit from by-products of 14 MJ kg 1 is taken into account. Thus the resulting CED for viscose fibers is reduced to 70 MJ kg 1 as well [35]. In the present study credits from by-products are not taken into consideration in order to retain the comparability of the different products and processes. The advantage of a low energy demand for the biogenic white fiber is invalid, when regarding the very low yield in carbonization of 25%. The yield of PAN-based fibers is twice as high. This difference in the carbon yield is taken into account only for the mass balance of felt production, as any further information on possible effects on parameters of the carbonization process are missing. This leads to a demand of 4 kg of viscose and 2 kg of acrylic fiber for the production of a kilogram of carbon felt, respectively. The CED for the biogenic raw felt amounts to 336 MJ kg 1, whereas the PANbased raw felt has a slightly lower CED of 316 MJ kg 1. With respect to the above mentioned potential in the energy balance of the biogenic fiber, the difference can be neglected. The same applies to the total CED of 601 and 581 MJ kg 1 for the rayon- and PANbased carbon felt, respectively. The global warming potential (GWP) of the two kinds of fiber can be calculated based on their energy balances. For the PAN fiber the GWP amounts to 18 kg CO2 eq. per kilogram of fiber. This value is calculated using the greenhouse gas emission factors provided by the U.S. Environmental Protection Agency [41]. However, the fossil carbon emissions of the rayon fiber are as low as 1.2 kg CO2 eq. per kilogram of fiber. With respect to the biogenic carbon embedded in the product ( 1.5 kg CO2 eq.), a nearly zero GWP is calculated for the fiber [35]. In conclusion, no significant difference in the energetic evaluation in terms of CED for both kinds of carbon felt can be determined. The results show, that the conventional carbon felt electrode is fully based on fossil fuels, whereas the biogenic product allows a holistic use of the raw material wood in an appropriate production process. The biogenic fiber clearly is the more sustainable product. For the carbon felt a detailed consideration of the sources of carbonization energy, accounting for 45% of total CED would be necessary to assess the ecologic impact of the final product. At present, a nonrenewable energy feed is assumed. 4.4. Cost and market assessment In the following a cost breakdown for both kinds of carbon felt considered is presented and discussed. Basic assumptions are described in section 3. Cost of white fiber and key intermediates are taken from a 2013 industry report [29]. Thus, the price of acrylic staple fiber is 2.10 V kg 1, based on an AN price of 1.40 V kg 1, a propylene price of 1.00 V kg 1 and an oil price of 0.70 V kg 1. The viscose staple fiber is rated significantly lower at 1.70 V kg 1, based on a price for wood pulp of 0.70 V kg 1. In general, for a cost assessment of new materials and technologies, economies of scale have to be taken into account. It should therefore be noted that above mentioned price estimates are based on large sales volumes. These are hardly applicable to the present market demand and manufacturing situation and in consequence may be perceived as a cost perspective for carbon felt electrodes. In order to give a more differentiated view of cost figures, cost ranges indicated in a recent review on low-cost CF are cited below [42]. Thus, textile grade PAN precursor costs about 1.70e5.00 V kg 1, which is equal to a range of 80%e240% of the abovementioned price of acrylic textile fibers. Considering quality requirements in felt electrode manufacturing (see section 2.2) as well as relatively low quantities at present a higher price for precursor fibers is realistic. Input data for the production cost model are costs ranges for PAN fibers of 1.70 V kg 1 to 5.00 V kg 1 and for rayon

C. Minke et al. / Journal of Power Sources 342 (2017) 116e124

Fig. 6. Calculated production costs of carbon felt electrodes depending on precursor fiber costs.

fibers of 1.40 V kg 1 to 4.10 V kg 1, respectively. The precursor demand f identified in the mass balance is 2 kg per kg of CF for acrylic fibers and 4 kg per kg of CF for rayon fibers. Considering the area weight of 500 g m 2 for both felt types, production costs for PAN-based electrodes amount to 39.30 V m 2 at present with prospects for a lower limit of 13.40 V m 2 at very low fiber costs. At the same time the biogenic felt electrodes show production costs of 64.50 V m 2, whilst their lower limit is 22.00 V m 2. The results illustrated in Fig. 6 match with literature data on carbon felt electrodes in VRFB applications. Costs for carbon felt electrodes of 5 mm thickness are assumed with 55 V m 2 at present and 35 V m 2 and 16 V m 2 in a near term and optimistic forecast, respectively (based on the 2012 exchange rate of 0.78 V per US$) [43]. To further compare the economic perspectives of carbon felts from different precursors, market developments of the basic fibers and their key ingredients are assessed. World fiber production is expected to grow at 3.7% per year within the next ten years. The world production of cellulosic fibers is about 4 million metric tons per year and increases rapidly. The projected growth rate of 5% per year is primarily driven by growth in China and South Asia. Limited production growth for biogenic material combined with a rapidly increasing demand for man-made cellulosic fibers are expected to put upwards pressure on viscose in the long term [44,45]. Meanwhile, the acrylic fiber market is expected to stagnate at a maximum growth rate of 0.4%. The present world production of 1.7 million metric tons per year of PAN-fiber consumes about 30% of the AN world production. Considering a consistent overcapacity in the AN production, of currently about 30%, margins for AN are likely to be depressed in near future [46]. This could possibly lead to a moderate cost reduction for PAN-based CF. The above mentioned dramatic annual growth rate of the CF market of 11% (Table 3) will be supportive for a price reduction of carbon fibers and felts. Target costs of CF in automotive applications of approximately 10 V kg 1 give orientation for an expected economic development [42]. 5. Conclusions The presented techno-economic assessment reveals significant ecologic advantages in the manufacturing process of carbon felt or carbon fiber from cellulose, which is a well available biopolymer. The study examines how one third of the CED for biogenic carbon felt can already be covered from renewable sources. In particular the holistic use of the main raw material wood, converted to viscose and energy from biomass, has to be emphasized. The total CED for both kinds of carbon felt is calculated equally to about 600 MJ kg 1.

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The conventional PAN-based carbon felt is thereby entirely produced from fossil fuels. This has effects on cost as well as technical aspects. Considering the cost and market assessment the precursor PAN is dependent on the crude oil price. It is therefore subject to many influences in a widely spread network of industries. In the presented cost assessment, PAN-based carbon felt is rated at about 40 V kg 1, whereas costs of a rayon-based felt amount to 65 V kg 1. The market demand for biogenic fibers in general is expected to increase sharply. Together with the naturally limited production growth for biomaterials a possible upwards pressure on cellulosic fibers may occur in the long term. However, initially a dramatic annual growth rate of 11% in the CF industry results in a substantial need for low-cost carbon fibers. Thus, economies of scale may also be applicable for carbon felt production. Resulting lower limits for the production costs are identified at 22 V kg 1 and 14 V kg 1 for rayon-based and PAN-based carbon felts, respectively. Finally, considering technical aspects of different felt electrodes, the obstacles of biogenic fibers in textile processing have to be taken into account. Dedicated parameters, in particular narrow tolerances, have to be met. In general, due to their natural variability, it is more difficult to configure desired properties of biogenic products than of polymer-based products. This also applies to the electrochemical performance of the considered final product, the carbon felt electrode for VRFB application. Experimental investigations reveal a higher through plane resistivity of rayon-based electrodes compared to PAN-based electrodes. It is however important to underline the actual need for research on the understanding of influences on the electrochemical performance, especially all kinds of felt activation. References [1] K.J. Kim, M. Park, Y. Kim, J.H. Kim, S.X. Dou, M. Skyllas-Kazacos, J. Mat, Chem. A 3 (2015) 16913e16933. [2] M. Skyllas-Kazacos, M. Rychcik, R. Robins, US4786567A, (1986). [3] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, A.G. Fane, M.A. Green, J. Electrochem. Soc. 133 (1986) 1057e1058. [4] Web of Science™, https://www.webofknowledge.com, accessed October 12, 2016. [5] M.H. Chakrabarti, N.P. Brandon, S.A. Hajimolana, F. Tariq, V. Yufit, M.A. Hashim, M.A. Hussain, C.T.J. Low, P.V. Aravind, J. Power Sources 253 (2014) 150e166. [6] S. Zhong, C. Padeste, M. Kazacos, M. Skyllas-Kazacos, J. Power Sources 45 (1993) 29e41. € rfler, A. Davydov, S. Wo €hner, A. Hirschvogel, [7] R. Schweiss, T. Oelsner, F. Do Edinburgh, United Kingdom, in: The International Flow Battery Forum, 2011, pp. 44e45. [8] R. Schweiss, R. Schmitt, O. Oettinger, Shanghai, PR China, in: Carbon, 2011. [9] R. Schweiss, J. Power Sources 278 (2015) 308e313. € €hner, D. Schneider, M. Kucher, O. Ottinger, [10] R. Schweiss, S. Wo Munich, Germany, in: The International Flow Battery Forum, 2012, pp. 36e37. [11] R.E.G. Smith, T.J. Davies, N.B. de Baynes, R.J. Nichols, J. Electroanal. Chem. 747 (2015) 29e38. [12] SGLCarbon, Sigracet and Sigracell. Components for Flow Batteries. New € SGL Carbon SE, 2012. Markets. Doc. no. 062012/0.12NA, [13] M. Heine, D. Kompalik, US5853429, (1998). [14] M.S.A. Rahaman, A.F. Ismail, A. Mustafa, Polym. Degrad. Stab. 92 (2007) 1421e1432. [15] X. Huang, Materials 2 (2009) 2369e2403. € € hner, M. Kucher, O. Ottinger, [16] R. Schweiss, S. Wo D. Schneider, C. Rüdiger, Hamburg, Germany, in: The International Flow Battery Forum, 2014, pp. 38e39. [17] A. Shindo, Report No. 317, Government Industrial Research Institute, Osaka, Japan, 1961. €lin, K. Yokose, Chemical Economics Handbook, Carbon Fibers [18] S.N. Bizzari, T. Ka (542.4000), IHS Chemical, 2013. €rl, M.R. Buchmeiser, Angew. Chem. [19] E. Frank, L.M. Steudle, D. Ingildeev, J.M. Spo Int. 53 (2014) 5262e5298. [20] T.A. Edison, US223898, (1880). [21] A.G. Dumanlı, A.H. Windle, J. Mater. Sci. 47 (2012) 4236e4250. [22] S. Otani, Y. Fukuoka, B. Igarashi, K. Sasaki, US3461082, (1969). [23] S. Das, Int. J. Life Cycle Assess. 16 (2011) 268e282. [24] D.A. Baker, N.C. Gallego, F.S. Baker, J. Appl. Polym. Sci. 124 (2012) 227e234. [25] C. Guidry, Modified Comparative Life Cycle Assessment of End-of-life Options

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