Characterization and performance study of commercially available solid oxide cell stacks for an autonomous system

Energy Conversion and Management 203 (2020) 112215

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Characterization and performance study of commercially available solid oxide cell stacks for an autonomous system

T

Michael Preininger , Bernhard Stoeckl, Vanja Subotić, Christoph Hochenauer ⁎

Institute of Thermal Engineering, Graz University of Technology, Inffeldgasse 25b, 8010 Graz, Austria

ARTICLE INFO

ABSTRACT

Keywords: Solid oxide cell System integration Stack Steady-state operation

The solid oxide cell (SOC) is a key technology for a combined generation of electricity, heat and valuable fuels in a highly efficient manner. By integrating a reversible SOC module in a compact unit, an autonomous reversible system may be realized. In order to obtain more information on the durability and reliability of SOCs, relevant stacks from different manufacturer are operated in both fuel cell and electrolysis mode under realistic operating conditions. The stacks are at relevant research and operational testing level. Thus, they are subjected to similar and comparable conditions while remain within given system boundaries. The results provide an insight for assessing the possibilities with respect to practical application under full load in fuel cell mode and efficient operation with a constantly high reactant conversion of 80% in both H2O- and co-electrolysis. The resulting operating maps can be considered and used for model evaluations and system designs. Further suggestions and proposals for improvements to be made are related to the thermal layout and the gas flow management of the experimental design in order to obtain more uniform cell performances.

1. Introduction Contemporary environmental issues are steadily increasing the demand for renewable energy and driving the adoption of alternative technologies [1]. Not only is the limited availability of fossil fuels an issue, but the greenhouse gases they generate are also a factor contributing to global climate change [2,3]. As the world moves towards replacing fossil fuels, at least a certain proportion of the global demand for energy can be met with more environmentally friendly energy sources. Among these, energy storage technologies make it possible to store and restore energy, independent of external influences; however, typical energy carriers and systems are subject to various losses in the process of converting primary energy to final energy. In this context, the fuel cell is becoming increasingly important as it presents a highly efficient technology for converting fuel energy into electrical energy. Fuel cells present an exciting opportunity to reduce total greenhouse gas emissions. Of the various types of fuel cells, the high-temperature solid oxide cell (SOC, for short) has the highest efficiency and is remarkable thanks to its outstanding fuel flexibility and use of low cost catalysts [4]. Because it can easily be operated reversibly, i.e., in electrolysis mode, in order to store electricity in gaseous form, it is now broadly referred to as an electrochemical reactor [5,6]. Reducing global CO2 emissions is critical to the continued and sustainable development

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of human society. Given that such electrochemical reactors are able to use different fuels in order to produce electricity, carbonaceous gases may also be used during electrolysis. If an SOC can be successfully integrated into an rSOC system, an autonomous reversible system will have been realized. Such an autonomous reversible system constitutes a promising approach to mitigating greenhouse gases while maintaining current comfort levels. The widespread use of rSOCs would therefore result in reduced CO2 emissions, and ultimately protect the environment from further damage, in a sustainable manner [7]. The need for detailed research into new lowemission technologies is critical since introducing such components to the market is difficult, time-consuming and costly [8]. Before such systems based on SOCs become widely commercially available, the following challenges must be overcome: 1) the performance of rSOC electrochemical reactors must be considerably improved supported by experimental tests, 2) the costs associated with their production and operation must be reduced, and 3) they must be more reliable. Most previous investigations have focused on the analysis of systems, as well as their modeling, and the development of system layouts. To date, hardly any studies have carried out comprehensive investigations into the systems’ operating strategies, nor have they considered the procedures under realistic boundary conditions. For all the reasons given above, the research and development

Corresponding author. E-mail address: [email protected] (M. Preininger).

https://doi.org/10.1016/j.enconman.2019.112215 Received 29 July 2019; Received in revised form 20 October 2019; Accepted 22 October 2019 Available online 12 November 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature

CFY CHP DC BoP EIS ESC Init GDC LSCF LSM LSMM’

Symbols Description, Unit Latin ASR F FU i kg h−1 OU RC l h−1 T V

area specific resistance, cm2 Faraday’s constant, C mol−1 fuel utilization, % current density, A cm−2 kilogram per hour oxygen utilization, % reactant conversion, % volumetric flow rate, l h−1 temperature, °C or K voltage, V air ratio, –

MFC Ni OCV Op Ref rSOC RU TC TRL YSZ

Abbreviations AC APU ASC

alternating current auxiliary power unit anode supported cell

activities from the microstructure of an SOC to the cell level up to the stack and system level have been rising accordingly in the last decade. Especially in the area of system integration and coupling the hightemperature fuel cell shows a high potential with respect to energy conversion and for use as an energy carrier. Thus, this work presents experimental investigations on electrochemical reactors in order to implement them in a technically feasible autonomous system based on solid oxide cell technology.

chromium iron yttrium combined heat and power direct current balance of plant electrochemical impedance spectroscopy electrolyte supported cell initial gadolinium-doped ceria lanthanum strontium cobalt ferrite oxide lanthanum strontium manganite lanthanum strontium manganite with additional transition metal on B-place mass flow controller nickel open circuit voltage operation reference reversible solid oxide cell repeating unit thermocouple technology readiness level yttria stabilized zirconia

Stand-alone rSOC systems have been designed and their operation has been analyzed at both intermediate temperatures and under slightly exothermic conditions [20]; in that study, which simplifies both the BoP integration and the thermal management of the stack, a roundtrip efficiency of 74% was achieved. Roundtrip efficiencies of 54% and 60% were presented at reference conditions and at 25 bar, respectively, in a study that modeled rSOC energy storage, including thermal energy storage tanks and an integrated methanation reactor [5]. In [21], the concept for a reversible hydrogen-based system for a distributed energy application of 100–200 kW was proposed, based on a self-developed, steady-state thermo-electrochemical model with a roundtrip efficiency of 60%. Another widely used approach is to combine rSOCs with other energy generation systems to create hybrid systems that utilize solar heat [22], nuclear energy [23,24], and other under-utilized energy sources, such as gas turbines [25,26]. Further system designs pursue the strategy of coupling rSOCs operated in electrolysis mode with additional processes, such as the dry reforming of carbon dioxide [27,28], direct internal reforming of methane [29], or the production of synthetic fuels with a downstream Fischer-Tropsch reactor [30]. The concepts and projects outlined above are evidence that much research is currently being undertaken in order to facilitate the integration and implementation of SOC technology into existing systems and plants. Much effort is also being directed towards the development and optimization of system design, in terms of both thermal management and auxiliary power requirements.

2. System concept and approach 2.1. Balance of plant In recent years, much effort has been invested in modeling and simulating various concepts [9], and many strategies, e.g. anode off-gas recirculation, have been developed [10,11]. An rSOC channel model was employed to determine operating conditions for design studies of rSOC systems [12]. A simulation study of different system layouts and conditions to determine the optimal efficiency showed the voltage as the greatest influencing factor [13]. The latest studies present completely self-sustaining system configurations, including an optimization with regard to thermal integration and balance of plant (BoP) [14]. An rSOC stack model based on results from [15] was coupled to two different balance of plant concepts with respect to the vapor treatment and the operating parameters varied achieving maximum roundtrip efficiencies close to 70% [16]. An investigation based on designed stationary operating points applied on a plant design comprising an off-gas recirculation and internal heat recovery showed a round trip efficiency of 51% [17]. A study on a rSOC system comprising ESCs, including optimizations with respect to thermal integration and BoP provided a system roundtrip efficiency of 55%–60% [18]. A numerical analysis including a simulation evaluated a complete micro-CHP power system based on experimental results in fuel cell mode obtained from hydrogen-fueled stacks consisting of the same cells as used in this paper [19]. A hydrogen based rSOC system was designed based on a simple 1D rSOC reactor model, to investigate the behavior of an rSOC reactor during transition from FC to EC mode. Though, the similar rSOC reactor as used within this work was only operated under hydrogen-based conditions and up to 0.20 A cm−2 and −0.20 A cm−2, respectively.

2.2. Proof of concept system In order to demonstrate the functionality of an independent system and to prove the feasibility of the process, a proof of concept system was designed in the power range of 1 kW, with all the components necessary for verification. Basically, the novel system concept comprises two heat recovery sections, the air and fuel heat recovery. The concept of the stand-alone reversible system is characterized by a simple layout and the implementation of a two-step vaporizer, which has yet not been invented nor implemented in any system design, for the fuel heat recovery. The approach to the process scheme for the autonomous reversible system presented in this paper is described in more detail 2

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elsewhere [31]. The preliminary configuration of the system has been designed basically on assumed efficiencies and parameters. It is initiated and developed for reversible operation featuring a high fuel utilization (FU) and reactant conversion (RC) of above 70%. In the case of electrolysis, system efficiencies of > 70% were obtained, while in fuel cell mode, efficiencies of > 55% were achieved. With the introduction of off-gas recirculation, the system efficiency was increased to over 80%. In order to determine true roundtrip system efficiency values and to identify critical design aspects, such as the thermal management and the stack operating conditions, investigations on these are carried out in a system-related and real mode of operation. Therefore, with respect to the temperature layout of the system corresponding data are required for designing the all necessary auxiliary components, especially with regard to the heat recovery sections. The temperature measurements are further used and needed to identify potentials and provide basic specifications to optimize the system concept and operational management for a longterm, efficient and reliable operation. To this end, scarcely any study is being issued towards experimental tests with regard to real and system-relevant approaches. Thus, by means of a comprehensive test procedure, this work aims to characterize SOCs stacks as well as individual cells within the stacks. The stacks are subjected to relevant and operational environments. Further purpose is to demonstrate the feasibility in realizing a self-sufficient system by providing a concrete contribution in its development. The framework, targets and importance of this work are given in the following.

Table 1 Test specifications and limitations of the stacks used for testing and characterization. Stack label

A

B

C

D

ESC CFY 126.5 10 10 0.7 1.45 80 80 2.4

ESC Crofer 22APU 127.8 10 5 0.68 1.5 80 80 9.4

ASC Crofer 22APU 84 10 10 0.6 1.45 80 70 6

ASC Crofer 22APU 80 5 5 0.6 1.6 80 80 12.5

900 750–850

860 780–830

780 650–750

820 650–800

Cross-flow

Co-flow

Co-flow

Counterflow

Cell type Interconnectors Active cell area (cm2) Number of cells Sense leads Min. voltage (V) Max. voltage (V) Max. FU (%) Max. RC (%) Clamping pressure (N cm−2) Max. temperature (°C) Operating temp. range (°C) Gas flow configuration

3. Experimental 3.1. Solid oxide cell stacks The four SOC stacks used in this study were from four different manufacturers, of which two contained ESCs and two were based on ASCs. Three stacks were comprised of ten planar cells, while one ASC stack consisted of only five planar cells. The active cell areas ranged from 80 to 127.8 cm2. The configurations of the stacks are listed in Table 1; they are described below in further detail. Fig. 1 presents photographs of the experimental test setups used to test the stacks, and indicates the individual integrated voltage probes as well as the temperature measurement points. Stack A consists of ten planar, electrolyte-supported cells and chromium-based interconnects. The cells are based on a ∼165 μm ( ± 20 μm) 10Sc1CeSZ electrolyte, with a ∼40 μm Ni/GDC fuel electrode and a 50 μm multilayer air electrode composed of a mixture of strontium doped lanthanum manganite with additional transition metal on B-place (LSMM’) and ScSZ, each of which were designed, developed and manufactured in-house by the stack manufacturer. Each cell within stack A has an active area of 126.5 cm2. The stack’s design and an exploded view of the stack is provided elsewhere [33]. The stack is implemented in a stack housing, which consists of a base plate, and contains the gas-leading in- and outlet pipes, a thick plate to decouple the stack thermally and electrically from the weights, and external air manifolds with a cross-flow configuration. The interface between the stack and its housing is sealed with mica. Thermocouples were mounted in the stack housing in order to measure the temperature at the air and gas in- and outlets, as well as the stack’s bottom and top. These are visible in Fig. 1a. The current circuit is closed by the base plate and an additional top plate. Stack B is composed of 10 planar, electrolyte-supported cells (ESCs) with an active area of 127.8 cm2 each. As supporting layer an alternative novel electrolyte (5YbSZ), compared to the conventional 3YSZ usually used within this stack [34], was used for the support of the cells promising a high level of performance [35]. The fuel and air electrode is of Ni-GDC and (La,Sr)(Cr,Fe) O3 (LSCF), respectively. The cells are interfaced with Crofer 22 APU metallic interconnectors. The stack was integrated in a stackbox, which contained all of the media carrying pipes at the bottom of the box. Thereby, the gas was distributed internally through gas channels incorporated in the stack, while the air was guided into the stackbox and, subsequently, through the stack via open oxygen electrodes. This stack uses a co-flow flow configuration.

2.3. Objectives and experimental methods The most significant and important element of the proposed system is the rSOC electrochemical reactor. Its flexibility and reliability has the greatest impact on the system’s overall efficiency. Thus, tests and investigations are carried out on the basis of system guidelines. The requirements and framework for stationary operation are i) stable operation at different constant air outlet temperatures ranging from 700 to 820 °C, while keeping the temperature deviations of the gas and air flow low, ii) a constant RC of 70–80% in electrolysis operation, and iii) a consistently high FU of 80% and an oxygen utilization of 20% in fuel cell operation. The conditions under electrolysis mode are water-electrolysis with an H2O/H2 ratio of 4 as well as two different co-electrolysis constraints, namely a H2/CO2-ratio of 4 according to a simple reverse water gas shift reaction calculation and second an identical ratio of the gas inlet composition (H2O + H2/CO2) and the syngas at the outlet (H2/CO) of 4. There are numerous stacks concepts and designs with various components and different configurations developed. The technological readiness level (TRL) ranges from basic research by developing technology components (TRL 1) to a reliable use as a commercialized product (TRL 9) [32]. The stacks used within this study and considered as electrochemical reactor for the self-sufficient system are with the range of TRL 6–7. Although many companies already sell electricity generating fuel cell power systems commercially, most SOFC products are not mature yet. Especially with regard to its use as an electrolyzer or even in reversible operation. To bring the SOC technology to TRL 8–9 and to make it commercially competitive versus technologies currently being used, corresponding investigations on available and state-of-theart products need to be undertaken to point out potentials and detect improvements. Therefore, on the basis of a comparative study and analysis appropriate conclusions concerning the system-integrated operation are drawn. In addition, the operational behavior of the respective stack concept including the individual layers is identified and basic guidelines are provided.

3

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Fig. 1. Photographs of the test set ups with marking of the measuring points: (a) Stack A, (b) Stack B, (c) Stack C, (d) Stack D.

The temperatures were measured by means of five TCs inside the stack, which were placed directly on the stack’s oxygen electrodes. One TC was positioned at the center of the stack’s top and bottom layers, respectively, while the three remaining TCs were positioned at half of the stack’s length, three-quarters of the stack’s length, and on the air outlet side (=fuel outlet side) in the middle plane of the stack. The arrangement of the TCs is depicted in Fig. 1b and their locations are marked accordingly. To optimize the operational thermal management and control of this stack, another TC was placed inside the stackbox, directly beside the air inlet pipe, while two more were placed next to the air outlet pipe as a control for a redundant acquisition and monitoring. The ASC stack, C, consists of 10 planar cells and is manufactured in a cassette design. Each cassette is made of two stamped metal bipolar plates (Crofer 22 APU), which are laser-welded at their outer edges. This cassette design simplifies the manufacturing process and allows for easy scalability and assembly. The cells used within the cassette are produced by CeramTec and have an active cell area of 84 cm2. The materials used are: Ni-YSZ for the fuel electrode (0.5–1 mm), YSZ for the electrolyte (∼10 μm), pure (La,Sr) MnO3 (LSM) for the air electrode current collector, and LSM + YSZ for the functional layer of the cathode. A cross-section of such a cell, as well as further components included in this lightweight SOC stack for mobile applications can be found elsewhere [36]. This stack is placed on a gas distribution plate, which takes care of the in- and outflow of the gases on both the fuel and air sides. The cassette is designed to operate in a co-flow configuration. Between the stack and the gas distribution plate, an interface seal (mica) ensures both gas-tightness and electrical decoupling. The temperatures at the air- and gas- inlets and outlets were measured using type N thermocouples, which were mounted on the gas distribution plate and located under the stack in the gas flow field. Additional thermocouples were positioned in small holes in the top and bottom plates, in order to measure the stack’s temperature in various locations, as can be seen in Fig. 1c. With respect to operation under electrolysis mode hardly no studies on this stack’s performance as well as no comparable measurements were made at this point. The ASC stack, D, is composed of five layers, and based on a stack

design for planar cells with manifolds integrated into the interconnection plates. The piping was designed in such a way that the stack is operated in counter-flow mode. A schematic view of the stack’s design can be seen elsewhere [37]. The cells used are 10 cm×10 cm cm ASCs with an active cell area of 80 cm2 each. The cells have a Ni-YSZ fuel electrode substrate, an 8YSZ electrolyte, and an LSCF air electrode. The gas- and air-inlet and outlet temperatures are collected and measured by means of TCs integrated in the adapter plate. As depicted in Fig. 1d, another five TCs were inserted into holes in the interconnects in order to investigate the temperature profile of the stack during operation. One TC was positioned in the center of the bottom interconnect, and one in the center of the top. The three remaining TCs were positioned on the fuel inlet side (=air outlet side), at half of the stack’s length, and on the air inlet side (=fuel outlet side) at the stack’s center interconnect. As can be seen in Fig. 1, all of the stacks were equipped with voltage probes in order to measure and monitor the individual cell voltages and impedances within the stack. The wires, made of platinum, were spotwelded to the interconnects, rather than connected directly to the cell. Thus, the cell, the interconnects, and the contact between the cell and the interconnects, i.e. the entire repeating unit (RU), are taken into consideration in the measured voltages and impedances. The required clamping pressure was applied to each stack mechanically, by means of weight plates, which are thermally and electrically decoupled by a ceramic plate that distributes the force flatly (see Fig. 1). The magnitudes applied to each stack are listed in Table 1, which also details additional specifications and limitations of the stacks that were tested. This work will refer to both the anode in FC mode and the cathode in EC mode as fuel electrode; conversely, the cathode in FC mode and the anode in EC mode will be referred to as the oxygen/air electrode. 3.2. Stack test facility The test rig for stack testing and experimental facility consists of a gas distribution unit, a high temperature top-hat furnace, additional heating units, and the respective rSOC unit. All inlet gas mixtures were mixed synthetically and provided by means of mass flow controllers 4

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(MFCs). The quantity of gas flows within this work are given in standard liter per hour per cm2 (l h−1 cm−2). For operation with mixtures containing steam an external steam generator was used. It is piston driven and the steam is produced from deionized water and mixed with the preheated inlet gas. All gas and steam leading pipes were heat traced by means of heating sleeves in order to ensure suitable temperatures for both the gas and air inlet flow, and especially to prevent steam condensation. Further main components include a gas analyzer, a condenser for water separation after the stack, an electrical load for fuel cell operation, and a power supply for electrolysis experiments. The scheme of the experimental apparatus and the test bench can be found elsewhere [38]. For AC characterization an external impedance measurement unit is connected to the current bars and voltage probes. Electrochemical impedance spectroscopy (EIS) was performed galvanostatically with the Reference 3000TM-Potentiostat/Galvanostat in combination with a 30 A-booster both from Gamry Instruments (USA). The Reference 3000TMis equipped with a multi-channel auxiliary electrometer, which additionally enables the simultaneous independent acquisition of EIS of eight channels in the multi-cell stacks. The measurement mode used is the Hybrid EIS, which combines the advantages of both the common galvanostatic and potentiostatic mode [39]. Using this technique the cell control is as found in the galvanostatic mode, but certainly the AC amplitude at each applied AC current is changed in the manner obtaining a nearly constant desired AC voltage response. The EIS measurements were performed in a frequency range from 20 kHz to 20 mHz with 10 points per decade, while the desired AC voltage was set to 10 mV. For operation above ± 30 A, a 100 A/35 V electrical load (FuelCon, Germany) was applied in FC mode, while an external 100 A/ 30 V power supply (Delta Elektronika, Netherlands) was used in EC mode. These were connected in a series within the stack, and, in order to prevent the simultaneous electrical connection of both devices, an electric contactor was used. By convention, positive current density values correspond to operation in fuel cell mode, negative current density values represent operation in electrolysis mode in this work.

fuel utilization/reactant conversion and particular current densities in both modes. Selected operating points (full load/partial load) were investigated in order to determine the stacks’ individual efficiencies and to test load/charge changes in order to characterize the system dynamics. For the experimental investigations on operating points at constant high fuel utilizations as well as high reactant conversions, a transient test procedure was used, which was executed in the following manner. The stacks were brought up to a certain operating point, at which they achieved FU/RC rates of 80%, and were then held constant until stable conditions (temperatures and gas outlet composition) were achieved. As soon as a steady state had been reached, measured data during the following period were used for the relevant operating point. Subsequently, the inlet gas flow rates and operating conditions (both temperatures and current density) were adjusted for the next operating point, again in order to achieve FU/RC rates of 80%. The corresponding adjustments were made with gradients of 1 A min−1, while the current density was increased in steps of 0.05 A cm−2. Thus, adjusting the operating parameters took a total of six minutes and setting the boundary conditions took less than 20 min. In order to determine and compare the stacks’ performance during the steady-state experiments calculated efficiencies are used, according to Ref.[40]. 4. Experimental results and discussion 4.1. Initial performance and functionality tests Fig. 2 shows the initial i-V characteristics (solid lines) as well as the ASR values (dashed lines) of the SOC stacks, recorded at constant air outlet temperatures of 750 ± 5 °C and 835 ± 5 °C for the anode and electrolyte supported cell stacks, respectively, in order to verify the functionality and gas-tightness of the individual stacks’ test setups. For this purpose, the stacks were operated reversibly in both fuel cell and electrolysis modes. The respective polarization curves were recorded with a current increasing rate of 1 A min−1. The gas mixture applied was made up of 50% H2 + 50% H2O (Init according to Table 2), and the

3.3. Experimental methodology The stacks were subjected to equivalent and comparable experimental testing conditions, according to the test parameters stated in Table 2. The experiments undertaken included recording polarization curves, performing EIS measurements of the entire stack as well as of individual cells, and operating the stack under galvanostatic control. All of the tests were accompanied by gas analyses and temperature measurements in order to ensure adequate thermal management. In this work, the test procedure for each stack was as follows. First, an initial verification was performed, followed by electrochemical characterization with both DC and AC; finally, comprehensive and extensive tests were conducted under steady-state conditions. The stacks were operated at different air outlet temperatures, depending on their cell type. The individual stacks’ performance under steady-state conditions was determined based on whether they achieved constant a high

Fig. 2. Initial i-V curves (solid lines) and calculated ASRs (dashed lines) of the four stacks at constant air outlet temperatures of 750 °C and 835 °C, respectively, in mixture Init (50% H2 + 50% H2O).

Table 2 Experimental stack testing conditions for performance characterization while feeding various mixtures. Mixture label

Init Ref Op–A Op–B Op–C Op–D

Operating mode

FC/EC FC FC EC EC EC

Operating temperature

Total gas flow rate

Inlet concentration fuel electrode (vol%)

‘air outlet’ (°C)

(l h−1 cm−2)

H2

N2

H2O

CO2

(l h−1 cm−2)

780/800/820/830 750/850 650–820 650–820 650–820 650–820

0.48 0.39 f(i) f(i) f(i) f(i)

50 60 60 20 20 20

0 40 40 0 0 0

50 0 0 80 70 60

0 0 0 0 10 20

1.20 1.20 f(i) 0.40 0.40 0.40

5

Air flow rate

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stacks operated reversibly in the range of −0.35 to + 0.35 A cm−2. The fuel utilization and reactant conversion were set to 80% for fuel cell and electrolysis modes, respectively. The air flow rate at the air electrode was 1.20 l h−1 cm−2 in fuel cell mode, which corresponds to an oxygen utilization (OU) of 25%. In electrolysis mode, the air stream flow removes the oxygen produced, resulting in oxygen-enriched air. Stacks A and B (both comprising ESCs) exhibited almost the same iV behavior in both modes, whereas the lightweight ASC stack, C, achieved a similar operational voltage in fuel cell mode, but its i-V curve in electrolysis operation bends up strongly compared to the others. The five-layer ASC stack, D, produced a much flatter curve in FC mode, as well as a slightly lower trend in EC mode. Fig. 2 also presents the calculated ASRDC values. These ASR values are determined from the i-V curves as the slope from OCV to the operational voltage at a current density of + 0.20 A cm−2 in fuel cell mode and −0.20 A cm−2 in electrolysis mode. All of the ASR values are calculated from their respective i-V curves with a correction to the fuel utilization/reactant conversion according to Ref.[41]. The ASR values for the four stacks, in alphabetical order, amounted to 0.57, 0.58, 0.68, and 0.35 cm2 in fuel cell mode and 0.83, 0.82, 1.09, and 0.52 cm2 in electrolysis mode. The five layer ASC stack, D, however, exhibited the best performance overall, with the lowest ASR values at an FU of 80% and a power output density of 0.28 W cm−2 in fuel cell mode. In electrolysis mode at an RC of 80%, a power consumption density of 0.40 W cm−2 was determined. Taken as performance parameters, stack D’s power density value shows an increase of 17% in fuel cell operation, and a 4% lower required power input in electrolysis operation compared to the ESC stacks A and B. Stack C’s performance was found to be considerably lower in both modes. A more detailed analysis of the individual stacks’ behavior during reverse operation, while feeding gas mixture Init containing 50% H2 + 50% H2O (cf. Table 2) is presented in Fig. 3. This performance comparison is important since, in this initial check, the most important characteristic of each stack is the ability to operate reversibly. In Fig. 3, each diagram presents the RU voltages of the individual stacks, as well as the temperature profiles at an air outlet temperature of 835 °C for the ESC stacks (Fig. 3a and b) and 750 °C for the ASC stacks (Fig. 3c and d). It is possible to see that stacks A, B, and D exhibited very homogeneous cell behavior within the stack, whereas two of those stacks revealed

slightly worse performance from the bottom cell, which is a common and well-known problem in such stacks – caused by either or both poorly contacted cells and lower temperatures at the bottom of the stack. Considering the performance of stack D, in addition to the poor performance of the bottom cell, a large scattering of the cells within the stack was observed. After this verification of the functionality of the stacks’ experimental set-up and gas tightness, reference measurements were carried out by means of both DC and AC characterization, using mixture Ref, as specified in Table 2. The mixture supplied to the fuel electrodes consisted of 60%H2 + 40%N2, while air was provided to the oxygen electrode. The air outlet temperature was held constant at 850 °C for stacks A and B and 750 °C for stacks C and D. Initial AC characterizations were performed for each stack and the individual RUs within each stack, at identical conditions and under the same operating parameters. The same measurements were conducted after each operational step. Fig. 4 presents the results from the DC characterizations of the initial reference measurements. Fig. 4a shows the i-V curves of the single RUs within stacks A and B, while Fig. 4b presents the polarization curves of the single cells within stacks C and D. Note that the curves have different vertical axes. Again, comparing the performances of the individual voltages, stacks A, B, and D exhibited very homogeneous cell behavior. Stacks B and D revealed slightly worse performance in the bottom cell. Stack C, which stands out in this work with its lightweight design and easy assembly using stamped metal interconnector plates produced a large scattering between the cells within the stack, in addition to the much worse performance of the bottom cell. Fig. 4c presents the average cell voltage of each of the four stacks superimposed on one another in order to demonstrate the similar performances of the ESC stacks, somewhat lesser performance of stack C compared to those, and the superior performance of the five cell stack, D. As stated above, AC characterization was carried out by means of EIS. In this manner, mixture Ref (cf. Table 2) was used to screen the stacks and the RUs within the stacks in order to investigate the origin of the variations in cell performance that are apparent in the single i-V curves presented in Fig. 3 and Fig. 4a and b. Fig. 5 shows the results obtained from the EIS measurements. The diagrams depict the impedance spectra of the individual RUs as Nyquist plots recorded at + 0.10 A cm−2. Up to eight channels (which is the maximum of the

Fig. 3. Initial i-V curves of the RUs (solid lines) and recorded temperatures (dashed lines) of (a) stack A at 835 °C, (b) stack B at 835 °C, (c) stack C at 750 °C, and (d) stack D at 750 °C while fed with mixture Init: 50% H2 + 50% H2O, 0.36 l h−1 cm−2 gas – 1.20 l h−1 cm−2 air). 6

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Fig. 4. DC characteristics of the individual RUs in (a) stack A (left-hand ordinate) at 850 °C and stack B (right-hand ordinate) at 850 °C, (b) stack C (left-hand ordinate) at 750 °C and stack D (right-hand ordinate) at 750 °C, and (c) a direct comparison of the average cell voltage of the four SOC stacks under mixture Ref. Total gas flow rate: 0.39 l h−1 cm−2 (60%H2+40% N2) – Air flow rate: 1.20 l h−1 cm−2.

impedance measurement unit) were recorded simultaneously. Thus, for stacks A and C, which each have 10 sensing leads, two cells were excluded (cf. Fig. 5a and c). The regions which are of primary interest are: the central area of the stack, as it is usually the hottest, as well as the outer layers of the stack, as they are critical with respect to both the temperature and contacting. Thus, the cells that were excluded were the third and the third-last cell (i.e. No 3 and No 8). Looking at stack A, the IS in Fig. 5a reveal that both the bottom and top cells show a slightly higher ohmic resistance (intercept of the IS with the real axis at low frequencies). The center cells exhibited homogeneous behavior throughout, although some showed artifacts at high frequencies. Such distortions and similar spectra shapes at high frequencies have also been observed elsewhere [42]. This is a common feature when one is determining the impedance of electrodes in high temperature fuel cells, since low impedances and low phase angles are obtained at high frequencies. Thus, choosing an appropriate standard resistor is essential in first place [43]. However, since such artifacts were not observed for all RUs or stacks, equipment-induced errors can be excluded. According to that, such an artifact tail may also arise as a result of electromagnetic coupling between the current and voltage sensing leads [44]. It can be reduced by wiring the current and voltage measurement leads [45]. Nevertheless, although the best case wiring to reduce the contribution of mutual induction was used for all experiments, corresponding artifacts were obtained. This further means that they are the result of the experimental test set-up. For stacks B and D, both of which are

equipped with five sensing leads, complete EIS data acquisition was carried out for the five individual bi-repeating units within stack B (Fig. 5b) and all five RUs in stack D (Fig. 5d). Both stacks exhibited consistent and uniform behavior among the RUs within the stack, except the bottom layer. In stack B, which consists of ten cells and is installed within a stackbox, the IS of the bottom bi-RU is only horizontally shifted towards the right, i.e., to higher real resistance values, when the spectra are superimposed; however, it is congruent with the other bi-layers. The reason for this higher ohmic resistance is a much lower temperature in the lower region of the stackbox. This lower temperature is most likely due to the experimental setup: heat is transported via the gas and air in- and outlet pipes. Additionally, and the main cause of this temperature difference, the use of a top-hat furnace means that much heat is lost from the bottom of the test rig. This influence of the heat losses through the bottom of the furnace is not as great or visible for stack A, which is positioned slightly higher in the oven. In stack D, the increase of the overall resistance for the bottom RU is mainly caused by an increased high frequency are, visible between 20 kHz and 20 Hz. The worse behavior of the bottom layer in stack D may have been caused by a lower temperature at the stack’s bottom and/or increased contact resistance between the cathode and the interconnector, as a result of the mechanical deflection of the base plate. Assessing the individual IS of the RUs within stack C (Fig. 5c), no consistent intercept of the IS with the real axis at high frequencies (i.e.

Fig. 5. AC characteristics for the four SOC stacks under mixture Ref at + 0.10 A cm−2: (a) stack A, (b) stack B, (c) stack C, and (d) stack D. Total gas flow rate: 0.39 l h−1 cm−2 (60%H2+40%N2) – Air flow rate: 1.20 l h−1 cm−2. 7

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20 kHz) was visible; rather, there were are noticeable inductive artifacts, caused by the electrical wiring. Fig. 5c also depicts the extremely heterogeneous and non-uniform electrochemical behavior of the RUs. The bottom RU shows significantly higher impedance arcs at low and middle frequencies compared to the other cells; the high frequency arc vanishes completely, which may have been caused by the disproportionately higher contact resistance in the stack.

relevant current density, and consisted of 21% O2/79% N2. The oxygen utilization was set to 20%. The resulting excess air ratio , which is the ratio of oxygen flow at the inlet to oxygen electrochemically converted, amounted to 6.1, ensuring both an adequate oxygen supply and, importantly, fulfilling the amount required to effectively cool the stack in FC mode. In contrast, during electrolysis operation, a constant air flow rate was applied throughout, which resulted in an increase in oxygen content as the current intensity increased. Each stack was held constant under these conditions until a steady state had been reached. Subsequently, the inlet flow rate was adjusted according to the next current density, before the current density was increased. The stacks were exposed to four different mixtures (Op-A – Op-D according to Table 2). The first was used in fuel cell operation, while the other three were fed while the stacks were used for electrolysis. The first mixture, consisting of 60%H2+40%N2 (Op-A), was selected in order to operate the stacks in fuel cell mode. This is the purpose for which the stacks were built, and they are known to operate safely and efficiently in this mode. For electrolysis operation, the three mixtures used were composed of steam (H2O electrolysis; Op-B) or, in co-electrolysis, steam and carbon dioxide (Op-C and Op-D); see Table 2. During

4.2. Stack performance under a transient operating regime After the initial characterization and reference measurements had been carried out, the stacks were operated at different air outlet temperatures under steady-state conditions at a constant high conversion and various current densities. These investigations of both transient and stationary operation were performed as described in Section 3.3. The fuel utilization in FC and reactant conversion in EC mode were set to a considerably high levels of 80%. In order to assess the stacks’ performance, boundaries, and limits, the fuel flow rates were adjusted based on to the relevant current density in order to achieve a fuel utilization/ reactant conversion of 80%. The air flow rate was adjusted based on the

Fig. 6. Comparison of the performance of the four stacks in (a) fuel cell mode feeding mixture Op-A at a constant of FU = 80%, and (b) in H2O electrolysis mode feeding mixture Op-B at a constant of RC = 80% (70% for stack C). (c) Hydrogen and Oxygen measurements at the outlet during H2O electrolysis (Op-B) operation. (d) Product gas composition and syngas ratio during co-electrolysis (Op-C and Op-D).

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co-electrolysis, the H2O/CO2 ratios at the inlet were 7 and 3 for the mixtures Op-C and Op-D, respectively. A concentration of 20% hydrogen was maintained at the inlet during all electrolysis operations in order to avoid the oxidation of the metallic phase. Fig. 6 shows the results extracted from the time-dependent profiles recorded during both transient and steady state operation in both fuel cell and H2O electrolysis modes. The operating points presented in these graphs were chosen at temperatures of 800 °C for the ESC and 750 °C for the ASC stacks. Although the stacks equipped with ESCs perform optimally at much higher temperatures, in these experiments, they were only operated at temperatures up to 820 °C (air outlet temperature; not shown) in order to meet the system requirements of keeping the overall efficiency high. For comparison, the temperature chosen for the ASC stacks is located at the lowest level of the temperature range for the system. In addition, 750 °C is the maximum temperature at which stack C can be operated safely. Fig. 6a compares the four stacks at the abovementioned temperatures, in fuel cell mode, while feeding mixture Op-A, with their respective current densities and fuel flow rates adjusted to ensure a constant FU of 80%. Stacks A and B exhibited comparable performance at current densities up to 0.32 A cm−2. Stack C also settles around the same performance level as the ESC stacks. However, stack C reached a maximum current density of 0.50 A cm−2, which is due to the fact that it was operated at its upper temperature limit. Finally, stack D outperformed all of the other stacks in fuel cell operation, with a maximum current density of 0.85 A cm−2 and a constant FU of 80%. For the sake of completeness, it should be noted that stack D was operated up to a maximum current density of 1.05 A cm−2 ( 84 A) at an air outlet temperature of 780 °C, which is shown in Fig. 7. The power density recorded at this operation point was 0.69 W cm−2 at a fuel cell efficiency FC of 52%. Fig. 6b presents a comparison of the extracted results of all four stacks for H2O reduction during steady state operation at high constant reactant conversion rates at the same temperatures that were described for fuel cell mode. The performance of the two ESC stacks was similar, with maximum power densities of 0.61 W cm−2 and 1.00 W cm−2 for stacks A and B, respectively. The limiting factor for stack A was the top cell of the stack; this finding confirms the observation and resulting conclusion drawn from Fig. 5a regarding the performance limitations of that stack. Comparable, but slightly worse performance, was achieved with stack C, which reached a maximum power consumption density of 0.78 W cm−2. Note that stack C was operated at its upper temperature

limit as well as a constant RC rate of only 70% since the upper voltage limits for electrolysis had already been reached at this point. At the same temperature (750 °C), stack D showed the best performance of all: it reached an RC of 80% at a much lower voltage setting, which also means that it consumed less power, and as a further consequence, exhibited higher electrolysis efficiency. Fig. 6c presents the net hydrogen outflow rates measured on the fuel electrode outlet and the oxygen contents measured on the air electrode outlet during H2O electrolysis. As can be seen, the hydrogen production rates and oxygen contents are located on the same curves. Stack D’s maximum net outflow rate of hydrogen was 0.47 l h−1 cm−2 and the maximum oxygen content was 50 vol% at a current density of −1.10 A cm−2 ( −88 A) at an air outlet temperature of 750 °C. Fig. 6d shows the product gas concentrations and syngas ratios during the transient operation experiments at a constant RC of 80%. Note that co-electrolysis with inlet mixtures Op-C and Op-D was not carried out with stack A, and stack C was subjected to an RC of only 70% (as described above). As the figure shows, both the H2 and the CO concentrations of the outlet gas flow (y-axis; left hand side) increased with the current density, which is due to the stacks increasing temperature; in contrast, the air outlet temperatures were equal for all operating points. Nonetheless, the resulting syngas-ratio (y-axis; right hand side) remained fairly constant at 4.0 for inlet mixture Op-C and 9.0 for inlet mixture Op-D. The slightly increasing trend observable for stack D is due to its rather small inlet, which results in outlet gas flows at slightly lower current densities. In order to study the influence of the operating temperature, which is assigned to the air outlet temperature, the applied current density, and the gas inlet mixture, in Fig. 7, the results of the steady-state operation points of stack D are analyzed in more detail. Fig. 7a shows the calculated fuel efficiencies and power densities during operation in fuel cell mode under dry hydrogen in nitrogen (Op-A), as a function of current density, at various temperature levels. Obviously, the higher the temperature, the higher the power density that can be attained. The decrease in the fuel cell’s efficiency that is seen with increasing current density can be attributed to the gradual decrease of the power density, while the chemical energy content of the fuel flow used increases linearly. 4.3. Performance comparison and considerations Comparing the results of these investigations and the observations

Fig. 7. Performance of stack D in (a) Fuel cell mode feeding mixture Op-A (FU = 80%), and (b) Electrolysis operation feeding mixture Op-B–Op-D (RC = 80%).

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made for the four stacks with different concepts in terms of operating temperature, cell and stack design, type of cell support, and flow configuration, it is useful to look at their overall efficiency as well as their maximum power input and output. The findings of these equivalent experiments, conducted on commercially available stacks provide a basis to evaluate the proposed autonomous system, and, subsequently, to make decisions about which stack should be used. Additionally, these findings and the descriptions of the operating conditions used may be employed as inputs in future studies in order to define, design, build and operate the auxiliary components and parts of similar systems. The following sections will present and discuss the performance of each stack in terms of its classification, as well as certain operating parameters, including temperature, current density, type of interconnect, and gas inlet concentration. All of the results presented from this point on were obtained under constantly high (80%) fuel utilization in fuel cell mode and reactant conversion in electrolysis mode, which is important to keep in mind when considering the statements below. Performance as a function of classification – In terms of the type of cell that would be desirable for use within an SOC unit for an autonomous system, at low current densities, the power densities attained from ASCs were larger those of ESCs during operation in fuel cell mode. When the current densities were increased, the difference in the power densities of ASCs and ESCs gradually increased. Accordingly, the ASC stacks exhibited much better performance than the ESC stacks, in particular when it comes to operation at higher current densities. Conversely, in electrolysis operation, the power density consumed at the same current density for ASC stack D was lower than that of the ESC stacks. At 750 °C ASC stack C exhibited similar performance to the ESC stacks, which were exposed to 800 °C, during electrolysis operation. However, this is due to the fact that stack C was operated at its upper temperature limit, thus optimum thermal conditions, while, for the ESC stacks, the applied 800 °C is far from their optimum operating temperature. Performance as a function of temperature – Fig. 8 presents an example of the trend of the effect of current density for both fuel cell and electrolysis operation. The diagrams depict average cell voltage curves of equal current densities as a function of temperature (y-axis left hand side over x-axis below). As expected, the stack voltages decrease and the power densities increase (cf. Fig. 7a) in fuel cell mode as the operating temperature increases. In electrolysis mode (cf. Fig. 7b), the stack voltages decrease, as does the higher the temperature, which leads to a reduction in power consumption. Increasing the operating temperature leads to an increased thermal energy demand. On the other

hand, the cell voltage, and thus the electrical power output in fuel cell mode, increases, while in electrolysis mode, the cell voltage, thus the electrical energy supply, decreases. Performance as a function of current density – Fig. 8 also illustrates the effect of the operating temperature on the cell voltage as a function of the current density (y-axis right hand side over x-axis top). The curves of the same air outlet temperature represent the average cell voltage at various current densities, at a consistently high fuel utilization/reactant conversion of 80%. During fuel cell operation, a short rise at low current densities is observable in Fig. 8a at temperatures higher than 750 °C. The initially lower average voltage can be ascribed to the weaker performance of two of the five cells, showing large fluctuations due to the small inlet flow rate applied, which resulted at low current densities under 0.30 A cm−2 to comply with 80% fuel utilization. From 0.30 A cm−2 upwards, a gradual decrease in voltage can be seen, as the current density increases, but the FU remains constant at 80%. The current density dependent curves of the same temperature in the upper part of Fig. 8b were obtained during H2O electrolysis with a constant RC of 80% and exhibit linear behavior. Performance with respect to the interconnects used – The thin metal plates that were used as interconnects in both stacks B and C are made from Crofer 22 APU, which makes the stacks lightweight, but also fragile and sensitive to thermal distortion and stresses. While the use of these interconnects had a negative impact on the homogeneity of stack C’s cell behavior, no such problems or abnormalities were witnessed in stack B in this regard. This may be related to the fact that the clamping force used to seal stack B was more than 1.5 times what was necessary. Neither stack A, consisting of interconnects made from CFY and produced via a powder metallurgical process route, nor stack D, comprising solid interconnects made from Crofer 22 APU, exhibited any negative characteristics with regard to their behavior or their performance. Though the use of these types of interconnects makes the stacks quite cumbersome and heavy, those factors are not significant for the development of a stationary application. Performance as a function of gas inlet concentration – Fig. 7b presents the stack’s performance during steady-state electrolysis operation while the inlet was fed with H2/H2O/CO2 mixtures of 20/80/0 (Op-B), 20/ 70/10 (Op-C), and 20/60/20 (Op-D). The maximum current density applied decreased as the CO2 fraction increased, and the power input required also increased slightly. However, at similar power consumption densities and electrolysis efficiencies, the stack’s performance was fairly constant in all of the electrolysis modes tested. Operating the

Fig. 8. Average cell voltage as a function of temperature at various current densities (left hand side) and current density for various temperatures (right hand side) of stack D in (a) Fuel cell mode feeding mixture Op-A (FU = 80%) and (b) H2O-electrolysis operation feeding mixture Op-B (RC = 80%).

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stack in co-electrolysis mode, the independency of the current density on the syngas ratio (H2/CO) is supported by near-constant ratios of 9.0 for mixture Op-C and of 4.0 for mixture Op-D, measured at the outlet.

•

5. Conclusions The safe and efficient operation of an autonomous system based on solid oxide cells (SOCs) is a challenging and interdisciplinary field of research. As the SOC unit is the key component in such a system it defines the control strategy that is utilized in terms of both thermal management and fuel utilization/reactant conversion at full and partial loads/power. For that purpose experimental investigations on four SOC stacks, which have varying approaches and designs regarding materials, components and flow configuration are presented. The stacks are operated reversibly under similar and comparable conditions, and analysis of the characteristics and behavior of individual stack repeating units and a comparisons of these are carried out. Additionally, experimental tests of the stacks intended to act as an electrochemical reactor in an autonomous system. The experimental methods applied are DC and AC characterization as well as a steady-state operation at a consistently high fuel utilization/reactant conversion (FU/RC) under galvanostatic control. The latter experiments are examined by means of a transient regime, increasing the volume flow rates according to the current density in order to ensure a continuously high FU/RC. At the same time, the air outlet temperature, as the controlling parameter, was held constant at various levels between 750–820 °C. Based on the experiments carried out, the operational behavior and potentials of each stack design can be derived accordingly. Furthermore, the issue whether one of the stacks with the technology maturities ‘prototype’, ‘low-volume series’ or ‘small-scale production’ with the range of TRL 6–7 fits for system integration is clarified. Finally, recommendations and suggestions to minimize the losses and to optimize the operation of a stack as part of an self-sufficient system are addressed. All conclusions are summarized below:

• • •

•

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

• A comparison of the individual layers within a stack shows partially

• • •

• • •

system needs. While the ASC stacks are limited to an air outlet temperature of 780 °C at the given conditions to stay within the stacks limits, operating the ESC stacks, at low temperatures increase the electrochemical losses. Temperature profiles and distributions are derived for designing and optimizing the heat recovery sections in the proposed system. Furthermore, the parameters and values that are obtained may be used to validate models and simulation approaches. Regarding the alignment and direction of air and gas flow, co-flow configuration is favored as the temperature distribution as well as the associated resulting current density and gas composition contribution are most balanced and evenly distributed over the active cell area. With respect to the materials of the layers and the complementary stack components, such as interconnects and seals used, both conventional and state-of-the-art types are used. Among these, a newly introduced electrolyte for the support of the ESCs performed showed an improved and significantly better performance. Improvements with respect to the thickness and material of the cell layers should be targeted towards an optimum operation at the specified temperature range. This further will result in lower electrochemical losses and an overall stack resistance, whereby with regard to system integration a high system efficiency at economically viable power densities is achieved. However, consistent durability, stability and robustness needs to be considered. During electrolysis operation the SOC unit may be operated in a slightly exothermic regime for a good balance of the required heat for compensation of the thermal demand of the endothermic electrolysis processes.

Acknowledgments

high disparities of the gradients of the temperature and, thus of the pressure, and FU/RC deriving therefrom along the cells as well as perpendicular to the horizontal stack plane throughout the stack. In addition, improvements of the thermal environment as well as the contact situation with regard to the bottom layer are observed. In reversible operation the recorded polarization curves of all stacks evolved smoothly when switching from discharging to charging operation, demonstrating that the stacks can be operated in either mode. Constant FUs of 80% are achieved over a wide range of current density, namely 0.32 A cm−2 ( 40.9 A) providing a power density of 0.23 W cm−2 at 800 and 820 °C with the ESC stacks and 1.05 A cm−2 ( 84 A) offering 0.69 W cm−2 at 780 °C with the ASC stack D. In electrolysis operation constant high RCs of 80% are obtained, except stack C provides only an RC of 70%. However, the ESC stacks reach a current density of −0.70 A cm−2 ( -89.5 A) with a power consumption density of 0.98 W cm−2, while the ASC stack D operates up to −1.25 A cm−2 ( −100.0 A) and consumes 1.67 W cm−2 at 780 °C. The resulting syngas-ratios for the given requirements during coelectrolysis were 9 for the simple reverse water gas shift constrain (H2/CO2 = 4), and 4 for the envisaged equality of the gas inlet composition and the resulted syngas outlet-ratio. The investigations show that temperature deviations between gas or air in- and outlet of <150 K and between coincide gas and air flow of < 100 K are realizable. This also contributes to avoid or at least reduce both thermal and mechanical stresses within the stack. With respect to the operating temperature, the envisaged temperature range of the stacks is not sufficiently compatible with that of the

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