Performance of a ten-layer reversible Solid Oxide Cell stack (rSOC) under transient operation for autonomous application

Applied Energy 254 (2019) 113695

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Performance of a ten-layer reversible Solid Oxide Cell stack (rSOC) under transient operation for autonomous application

T

Michael Preiningera, , Bernhard Stoeckla, Vanja Subotića, Frank Mittmannb, Christoph Hochenauera ⁎

a b

Institute of Thermal Engineering, Graz University of Technology, Inffeldgasse 25b, 8010 Graz, Austria Sunfire GmbH, D-01237 Dresden, Germany

HIGHLIGHTS

study of an electrolyte supported solid oxide cell prototype stack. • Experimental of a 5YbSZ electrolyte is investigated with H /H O/CO/CO mixtures. • Performance mapping inside the stack showed thermal improvements. • Temperature reactions are promoted under co-electrolysis at low currents. • HTheO syngas ratio is independent of the current density applied. • 2

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ARTICLE INFO

ABSTRACT

Keywords: Reversible Solid Oxide Cell Characterization Electrolyte supported cell 10 layer stack EIS Steady state operation Transient operation

A state-of-the-art ten-layer solid oxide stack was electrochemically characterized and system-oriented experimentally investigated in reversible operation. The stack in question consists of 5YbSZ electrolyte supported planar cells promising high performance. The stack is integrated into a stackbox and is considered to be operated in an autonomous system, thus system-relevant operating conditions in terms of reversibility, inlet mixtures and temperatures were applied. A high fuel utilization, respectively reactant conversion of 80% in either mode was deployed in steady state experiments in a transient operation regime. Polarization curves were dynamically recorded and electrochemical impedance spectroscopy was performed to evaluate the performance of the stack in reversible operation feeding hydrogen and/or carbonaceous gases. Recorded temperature profiles obtained by means of thermocouples placed directly on the air electrodes showed distinct characteristics with a maximum deviation of 24.8 K in the exothermic and 14.9 K in the endothermic operating mode. The stack showed a small dependency on the applied operating temperatures of 780, 800, and 820 °C. A maximum current density of −0.7 A cm−2 was applicable under H2O electrolysis. A comparable performance was observed for co-electrolysis corroborated by current density independent syngas ratios of 9.0 and 4.0 when feeding H2/H2O/CO2-compositions of 20/70/10 and 20/60/20, respectively. Particular attention must be paid to thermal integration in the context of the implementation as a stand-alone system. The resulting operating maps related to maximum current densities, gas production and temperatures can be considered and used for simulation and design of the envisaged stand-alone system including the auxiliary power requirements.

1. Introduction Despite growing recognition of the need for alternative fuel sources, fossil fuels remain the main source of energy for liquid fuels in the mobility sector. As stocks are limited, some experts assume that the maximum annual supply of oil has already been exceeded [1]. Therefore, the price of petroleum-based products is likely to increase dramatically in the near future [2]. Due to the parallel challenges of the

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world’s growing need for energy and efforts to combat global warming, it is imperative that alternative fuel sources be developed in order to mitigate the production of greenhouse gases while maintaining a high standard of living. Additionally, the increasing use of intermittent alternative energy sources, such as wind and solar energy, means that appropriate methods of energy storage are required. Reversible solid oxide cells (rSOCs) meet both of the above mentioned requirements at high levels of efficiency. Reversible SOCs are electrochemical cells that

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

https://doi.org/10.1016/j.apenergy.2019.113695 Received 12 April 2019; Received in revised form 18 July 2019; Accepted 2 August 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature

ASC DC EC EEC EIS ESC FC FE IS GA GDC LSCF MEA MFC MFM Ni OA OCV op prod RU rSOC SOC TC tn TPB

Latin ASR F FU i n OU P Q RC l h−1 T V H EC FC

area specific resistance, cm2 Faraday’s constant, C mol−1 fuel utilization, % current density, A cm−2 number of electrons, – oxygen utilization, % power, W energy content supplied/consumed, W reactant conversion, % Volumetric flow rate, l h−1 temperature, °C or K voltage, V change in enthalpy, J mol−1 electrolysis efficiency, % fuel cell efficiency, % excess air ratio, –

Abbreviations AC AE APU

alternating current air/oxygen electrode auxiliary power unit

can be operated either in fuel-cell mode, to generate power, or in electrolysis mode, in order to store electricity in a gaseous form. Thus, they are now widely referred to as electrochemical reactors [3,4]. The operating temperatures of rSOCs can range from 600 °C to 1000 °C, depending on the structure of the mechanical cells, as well as the desired operating mode, when the cell is integrated into a system. In terms of the structure of the cell, two types of rSOCs have emerged so far: electrolyte supported cells (ESCs) and anode supported cells (ASCs). With respect to cells of industrially relevant size ESCs have a better mechanical robustness, but as a result of the thick electrolyte, they tend to have higher ohmic losses than ASCs [5]. On the other hand ASCs show higher concentration losses caused by the concentration resistance of the porous anode for the gaseous species [6]. In terms of robustness on the one hand and performance on the other hand, the anode may not be manufactured overly thick [7]. When it comes to losses, the choice of mode and the operating conditions determine the thermal management strategies required and subsequently, the efficiency of the rSOC unit [8]. By supplementing auxiliary components, an rSOC system can be realized, in which gas is converted into electricity and heat, on the one hand [9], and storable gases are generated, on the other [10,11]. Such an autonomous reversible system would prove a promising approach to mitigating greenhouse gases while maintaining current levels of comfort, by using renewable energy sources and regenerative electricity to generate hydrogen [12,13]. Furthermore, the conversion of hydrogen into electricity by means of an electrochemical reactor is almost emission-free and CO2 neutral [14]. The widespread use of rSOCs would therefore result in reduced CO2 emissions, and ultimately protect the environment from further damage, in a sustainable manner. The need for detailed research into new low-emission technologies is critical since introducing such components to the market is both difficult and time-consuming. The concept of a stand-alone reversible system based on high-temperature solid oxide cells that is capable of fuel production, storage, and synthesis is not entirely new. In recent years, much effort has been invested in modeling and simulating various concepts and many strategies have been developed [15]. An rSOC channel model was employed to determine operating conditions for design studies of rSOC systems [16]. A simulation study of different

anode supported cell direct current electrolysis cell electrical equivalent circuit electrochemical impedance spectroscopy electrolyte supported cell fuel cell fuel electrode impedance spectra gas analyzer gadolinium-doped ceria lanthanum strontium cobalt ferrite oxide membrane electrode assembly mass flow controller mass flow meter nickel oxygen analyzer open circuit voltage operation produced repeating unit reversible solid oxide cell solid oxide cell thermocouple thermoneutral triple phase boundary

system layouts and conditions to determine the optimal efficiency showed the voltage as the greatest influencing factor [17]. The latest studies present completely self-sustaining system configurations, including an optimization with regard to thermal integration and balance-of-plant (BoP) [18]. An rSOC stack model based on results from [19] 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% [20]. 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% [21]. 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 [22]. 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. Although a similar rSOC reactor was used to the one in this work, it was only operated under hydrogenbased conditions and up to 0.20 A cm−2 and −0.20 A cm−2, respectively. Stand-alone rSOC systems have been designed and their operation has been analyzed at both intermediate temperatures and under slightly exothermic conditions [23]; 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 [3]. In [24], 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 [25], nuclear energy [26,27], and other under-utilized energy sources, such as gas turbines [28,29]. Further system designs pursue the strategy of coupling rSOCs operated in electrolysis mode with additional processes, such as the dry reforming of carbon dioxide [30–32], the partial oxidation of methane [33], or the 2

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production of synthetic fuels with a downstream Fischer–Tropsch reactor [34]. High total system efficiencies can be realized by a direct integration of SOCs into an existing system or industrial processes. In this regard it is of great importance to mention several projects, which are based on the development and demonstration of SOFC, SOEC or rSOC systems. The EU project “GrInHy” [35] has been running since 2016 and is based on an rSOC system, which is preferably operated in the electrolysis mode to produce H2 from H2O. The system is designed exclusively for H2 production – to produce 40,000 l h−1 H2, an input power of 150 kW is required. Furthermore, the system can be operated in fuel cell mode in an emergency and can generate electricity from either H2 or CH4 with an efficiency of 50%. Another EU project “Balance” [36] is pursuing a similar idea – an rSOC system, operated with H2 in SOFC mode and designed for H2 production from H2O in electrolysis operation, is to be developed to prove the feasibility of the technology mentioned. The other goal of the “Balance” project is the development of new materials that enable efficient rSOC operation even at 700 °C. Furthermore, the rSOC system is simulated numerically, taking into account the grid connection and any gas storage. The goal of another EU project, the project “REflex” [37], which started in 2018, is the development of an innovative solution for the storage of renewable energies, which is also based on the reversible SOC technology. The challenge is to achieve high efficiency, high flexibility in operation and cost optimum through improvements in the rSOC components (cells, stacks, power electronics, heat exchangers) and the system as part of the EU project “RelHy” [38], SOECs have been optimized. Within the present study, an autonomous reversible system with rSOCs was developed in order to serve as a 1 kW proof-of-concept (PoC), deriving from a 33 kW overall system [39]. This novel system layout comprises all of the components necessary for both electrolysis and fuel cell modes, although not all components are active in both modes. Under fuel cell mode, the rSOC is designed to be operated with either a common hydrogen–nitrogen mixture or a steam-reformate (H2O + CH4), containing methane, which can also be used directly for internal reforming [32]. In electrolysis mode, the system is able to operate under H2O electrolysis conditions as well as under co-electrolysis of H2O and CO2. The present system has been designed on a preliminary basis and developed for operation with high reactant conversions (RCs) 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%. Assumed stack efficiencies and parameters were used to determine the system efficiencies. In order for the system design and layout of components to be reliant on real values from experimental experiments, a state-of-the-art 10-cell stack containing high-performance cells was operated under system-relevant conditions. These are characterized by a high fuel utilization and reactant conversion in addition to moderate temperatures of 780, 800 and 820 °C relative to the maximum stack temperature of 860 °C. The experimental test program included both the characterization of the stack and repeating units, which include the contributions of two cells, since only every second interconnector was tapped via sense leads, and an extensive test matrix at selected steady-state operating points. The test sequence under stationary conditions involved one gas composition in fuel cell mode and three gas compositions in electrolysis mode; all were tested at three relevant operating temperatures. Despite the steady state operations in fuel cell mode and three electrolysis modes the stack undergoes an electrochemical characterization by means of dynamically recorded i-V curves plus electrochemical impedance spectroscopy (EIS) in steady state. These investigations were carried out in reversible operation, which focused on providing a detailed performance analysis under H2O-, co-, and CO2electrolysis, whereby the reactant and product shares were 50% in each case. As has been reported in previous studies on fuel electrode supported single cells with 4 × 4 cm−2[40–42], the performance of

H2/H2O operation is superior to that of CO/CO2 operation. Furthermore, investigations on electrolyte supported button cells [43,44] and stacks [27,45] have demonstrated that co-electrolysis performance in mixtures of H2/H2O/CO/CO2 is located between that of the oxidation of H2 and the reduction H2O, and CO oxidation and CO2 reduction, respectively, but close to carbon free H2/H2O operation. With regard to thermal management and the operating conditions applied, investigations of button and single cells are considerably easier and far less complex than investigations of the operation of an entire stack. The non-uniform temperature distribution within a stack makes the thermal management particularly difficult and thus has a great impact on the individual cell performances. While the temperature difference for single cells is in the lower single-digit range [46], the temperature spread within stacks may range up to a three-figure value [47]. In addition, the gas distribution in a stack may also result in an inhomogeneous behavior due to varying fuel flow velocities caused by varying partial pressures [48]. Furthermore, the design and configuration of the electrical contact areas can lead to additional inhomogeneities and performance differences in the individual layers of an rSOC stack [49]. A combination of all these inhomogeneities makes a stack moderately easy to handle, since the gas and gas flow distribution as well as the effectiveness of the electrical contact may vary considerably along the length of the cell as well as throughout the stack. These facts in turn influence the temperature evolution and distribution, and vice versa. Thus, temperature profiles were mapped during all the experiments. Apart from the simple handling of button and single cells with regard to temperature and concentration gradients as well as the moderate fuel utilization and reactant conversion, their output quantities are limited. Thus, the investigations carried out within this work address performance measurements on a stack with system-related purposes. The stack in question is designed in a co-flow configuration and with open oxygen electrodes, thus installed in an enclosed stackbox. The cells used have a high-performance 5YbSZ electrolyte support compared to the 3YSZ usually applied as electrolyte for those cells [50]. The fuel and air electrode is of nickel gadolinia-doped ceria (Ni-GDC) and lanthanum strontium cobalt ferrite oxide (LSCF), respectively. 2. Experimental procedure 2.1. Conceptional system approach The centerpiece and electrochemical reactor of an autonomous reversible system based on the technology of a high temperature solid oxide cell is the rSOC unit itself. Thus, the main focus of this research project was on the operation of the rSOC unit under different operating conditions, such as different operating temperatures, gas inlet compositions, and current densities. The experimental design was based on system-relevant operating conditions by means of a high fuel utilization and reactant conversion in FC and EC mode, respectively, in order to achieve a high overall system efficiency. Different boundary conditions in terms of maximum and minimum voltage and temperature were also considered for both the stack and the system. The stack limitations are pointed out in Section 2.2. The system specifications for the aspired system are presented in brief below:

• The air outlet temperature must be kept constant between 750 and 820 °C. • The temperature difference between the air in- and outlets, and between the gas in- and outlets, must not exceed 150 °C. • The temperature differences between air and gas inlets and the air and gas outlets must not exceed 100 °C. • Both a high fuel utilization (FU) (in fuel cell mode) and a high reactant conversion (RC) (in electrolysis mode) should be maintained in order to achieve high system efficiency. An FU and an RC of up to 80% are envisaged.

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The stack manufacturer’s specifications and the system’s limits allow a certain degree of leeway. To stay within the targets of the technical application sought in this study and given above, the behavior of the stack has to be identified and characterized under various operating conditions, which in turn makes it possible to create performance maps of the entire screening process. It is also important to investigate the foreseeable lifetime of the stack by determining the degradation caused by the respective operation. This may be done by comparing reference measurements carried out between each corresponding mode of operation.

ceramic plate, which distributes the force flatly. Steam was provided by an electrical steam generator supplied with deionized water. The steam was then mixed with a gas flow (either H2, N2, CO and/or CO2) before entering the fuel-side compartment of the SOC stack. All pipes downstream of the steam generator were heat-traced in order to avoid condensation. 20% H2 was maintained during electrolysis operation so as to avoid (re-) oxidation of the Ni. All gas flows (air and fuel gases) were controlled by mass flow controllers (MFCs), and the gas flow rates are given in standard liter per hour per cm2 (l h−1 cm−2). A scheme of the test set-up can be found elsewhere [52]. The stack was equipped with voltage probes on every second interconnector plate in order to monitor the individual cell voltages; thus, five cell pair voltages are obtained and collectively referred to as birepeating unit voltage (bi-RU). These as well as the entire stack voltage, were collected for all tests. The voltages of the individual cell pairs are limited to 1.3 V in fuel cell mode, while each cell pair must not exceed 3.0 V in electrolysis mode. Moreover, fuel utilizations and reactant conversions of up to 80% may be applied in FC and EC modes, respectively. In addition, eight N-type thermocouples (TCs) were led out of the stack module, whereby five were mounted inside the stack and three were arranged inside the stackbox, directly at the air leading pipes. In the latter regard one was positioned at the air inlet pipe and two were placed at the air outlet pipe in order to monitor the air outlet temperature and acquire redundant temperature data. The air outlet temperature was kept constant for all measurements since it plays a key role in maintaining both the temperature balance of the autonomous system and its resulting efficiency. Thus, the operating temperature in this work corresponds to the air outlet temperature. The maximum permissible temperature of 860 °C must not be exceeded at any measuring point at any time. The specifications and limitations of the rSOC stack considered in this study are described in more detail elsewhere [32]. The arrangement of the five TCs inside the stack is illustrated in Fig. 1(b); three of them were distributed over the length of the vertical plane at the center of the MEA as follows: at half-way up the cell (MEA mid 1/2), three-quarters of the way up the cell (MEA mid 3/4), and at the end of the cell (MEA mid 4/4). One was positioned at the center of the top cell (MEA top 1/2) and one at the center of the bottom cell (MEA bottom 1/2). The TCs within the stack were used to calculate the average stack temperature. Further thermocouples were used to

2.2. Stack design and test facility The experimental object was a commercially available rSOC stack manufactured and provided by Sunfire (Germany). The stack was composed of ten electrolyte supported planar cells with an active area of 127.8 cm−2 each. The high-performance membrane electrode assemblies (MEAs) used each consisted of a 5YbSZ electrolyte support, a Ni-GDC fuel electrode (anode, An for short), and an LSCF ((La,Sr) (Cr,Fe) O3) air electrode (cathode, Ca for short). The MEAs are interfaced with Crofer 22 APU metallic interconnectors, subsequently referred to as the repeating unit (RU). The stack was integrated into a stackbox, in which all of the media carrying pipes are located at the bottom of the box. Fig. 1(a) shows an illustration of the stackbox integrated in the test rigrig together with the accompanying heating components, such as the heating sleeves and the top hat furnace. 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. Filtered air ( 21% O2/79 % N2) was used to supply oxygen during operation in fuel cell mode and for the removal of the oxygen generated in electrolysis mode. The temperature and quantity of the air flow are crucial for adequate thermal management of the stack due to its open oxygen electrode design. In case of highly exothermic operation, a higher volume of air flow may be used to cool the stack [51]. The gas and air inlet flow are pre-heated outside the furnace using a hightemperature tube furnace and heating sleeves. Once assembled, the stack was weight-loaded at 9.4 N cm−2 based on the active cell area of 127.8 cm2 resulting in a clamping force of 1200 N and placed inside a top hat furnace. The weights were then thermally de-coupled by a

Fig. 1. (a) View of high-temperature electrolysis facility with the major components labeled. (b) View of the 10-cell stack including markings for thermocouple positions. 4

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monitor the temperature at additional locations within the test set-up: these measured both the gas in- and outlet temperatures (outside the stackbox) as well as the furnace and condensate temperatures. The gas outflow was guided through a condenser to separate the water from the gases after the stack. Downstream from the condenser, an Advance Optima 2000 gas analyzer, with the Caldos 14 and Uras 17 modules, from ABB (Switzerland) was used to analyze the composition of the non-condensable outlet gas flow. After a certain time, both the water produced during FC operation and the residual water during EC operation were weighed in order to calculate the real reactant conversion. For reversible operation with H2/H2O-mixtures, the resulting pure hydrogen flow after condensation (100%) was detected by a mass flow meter. The results of this are comparable with the theoretically expected ones [53]. Lastly, the oxygen content in the air outflow and thus the oxygen utilization (OU) in fuel cell mode and the oxygen production in electrolysis mode were detected and measured using an oxygen analyzer from ABB. Both the gas- and air-outflow were then vented to the outside. All of the data (currents, voltages, temperatures, gas compositions) were controlled and collected through a custom control and acquisition program based on a software by Bernecker & Rainer. DC and AC characterizations were performed at both OCV and elevated current densities by means of EIS, using the external Reference 3000™-Potentiostat/Galvanostat in combination with a 30 A-booster, both from Gamry Instruments (USA). The Reference 3000™ is equipped with a multi-channel auxiliary electrometer, which makes it possible to acquire independent EIS from all five bi-RUs in the multi-cell stack. The Hybrid EIS measurement mode was used, which combines the advantages of both the common galvanostatic and potentiostatic modes [54]. Using this technique, the cell is controlled in the same manner as in galvanostatic mode, but the AC amplitude at each applied AC current can be adjusted in order to obtain a nearly constant 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 in reversible mode above 0.23 A cm−2 and below −0.23 A cm−2, a 100 A-35 V electrical load (FuelCon, Germany) was applied in FC mode, while a 100 A-30 V power supply (Delta Electronics, China) was used in EC mode. These were connected in series within the stack and in order to prevent the simultaneous electrical connection of both devices, an electric contactor was used.

DC characterizations were carried out in fuel cell mode (init 1) and steam electrolysis mode (init 2), as seen in Table 1. Performance as a function of gas composition – After the initial functional and performance tests had been completed, additional polarization curves and EIS were recorded with varying inlet fractions of H2, H2O, CO, and CO2. The concentrations of the relevant inlet species were varied in order to perform steam-, co-, and CO2-electrolysis; i.e. 50 vol% H2–50 vol % H2O, 25 vol% H2–25 vol% H2O–25 vol% CO–25 vol% CO2, and 50 vol % CO–50 vol% CO2, respectively. Performance as a function of temperature – Two different temperatures were applied, while the total gas flow rate was kept constant for all cases. The two air outlet temperatures selected were 800 and 830 °C; thus, the furnace temperature was adjusted accordingly. 800 °C corresponds to the intermediate temperature of steady-state operation, while 830 °C is the maximum air electrode exhaust temperature. Furthermore, this is the optimum temperature for the stack’s performance. Performance as a function of current density – The performance as a function of the applied current was examined in both FC and EC modes. i-V curves were recorded for all three mixtures with a current ramp of 0.008 A cm−2 min−1 (equivalent to 1 A min−1) at current densities of up to +0.32 A cm−2 ( 41 A in FC mode) and −0.32 A cm−2 ( −41 A in EC mode), corresponding to a fuel utilization and reactant conversion of 80%, respectively. Associated Electrochemical Impedance Spectra (EIS) were recorded in EC mode starting at OCV (0.00 A cm−2) up to a current density of −0.23 A cm−2, with steps of 0.05 A cm−2. The impedance measurements were initiated from 30 min after each current density was applied until the stack reached a stable condition. Performance in stationary operation – After its characterization, the stack was operated under stable conditions in both fuel cell and electrolysis modes, using the inlet gas mixtures Op-A to Op-D, given in Table 1. The fuel flow rate was adjusted at each current density in order to achieve a fuel utilization of 80% in FC mode, while the air flow rate was adapted to achieve a constant oxygen utilization of 20%. In EC mode the fuel flow rate changed according to the current densities to achieve a reactant conversion of 80% at every single steady state operation point. In contrast to the fuel cell mode, the air flow rate was kept constant during electrolysis performance assessment in order to keep the air inlet temperature predominantly uniform, which, in turn, results in an increase in the oxygen content at the air outlet as the current density increases. The data of the measurements at steady-state operating points were surveyed only after constant temperatures had been set. The relevant air outlet temperatures tested are 780, 800, and 820 °C. The stack was operated at each point and condition for as long as all of the specifications were within their limits.

2.3. Characterization Initial performance – The stack was heated up at a rate of 110 K h−1 with forming gas (N2/H2 = 95/5) at the fuel electrode and air at the air electrode. Above 600 °C, additional hydrogen was mixed into the fuel electrode’s gas flow. After the heat up phase, a leakage test was carried out according to the manufacturer’s specifications in order to prove the gas-tightness of the stack within the stackbox. Thereafter, initial AC and

2.4. Electrical equivalent circuit model for fitting and breaking down the impedance In order to determine the contributions of the impedances to the

Table 1 Experimental stack testing conditions for performance characterization for the various mixtures applied. Mixture label

init 1 init 2 A B C Op-A Op-B Op-C Op-D

Operating mode

FC EC FC/EC FC/EC FC/EC FC EC EC EC

Operating temperature

Total gas flow rate

– air outlet (°C)

(l h−1cm−2)

780/800/820/830 780/800/820/830 800/850 800/850 800/850 780/800/820 780/800/820 780/800/820 780/800/820

var. var. var. var.

Inlet concentration fuel electrode (vol%)

0.48 0.48 0.33 0.33 0.33 (FU = 80%) (RC = 80%) (RC = 80%) (RC = 80%)

5

Air flow rate

H2

N2

H2O

CO2

CO

(l h−1cm−2)

40 25 50 25 0 60 20 20 20

60 0 0 0 0 40 0 0 0

0 75 50 25 0 0 80 70 60

0 0 0 25 50 0 0 10 20

0 0 0 25 50 0 0 0 0

2.80 0.40 1.20 1.20 1.20 var. (OU = 20%) 0.40 0.40 0.40

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overall losses, the impedance spectra were fitted using an electrical equivalent circuit model that had previously been developed for an electrolyte supported cell, as described and demonstrated in [55,45]. The model consists of an inductance, three RC equivalent circuits, a Gerischer Element, and a serial resistance. The single elements therein represent a high-frequency fuel electrode arc (quick charge transfer), a fuel electrode low-frequency arc (gas conversion), a diffusion/charge arc for the transfer at TPB (Gerischer Element) and an arc for the adsorption of oxygen on the catalyst surface. The serial resistance represents the ohmic losses, whereby the ionic conductivity of the electrolyte accounts for a major part.

along the height of the stack (i.e. MEA bottom/mid/top 1/2), the middle cell (marked as – –) shows the highest generation of heat with increasing current density, while the bottom and top cells’ temperatures increase to a lesser degree. This is explained by the higher heat losses at the ends of the stack. Looking at the temperatures along the length of the middle cell, i.e. in the flow direction, the temperature at the center of the cell (MEAs mid 1/2) shows the lowest values, while the temperature at the end of the cell (MEAs mid 4/4) is highest. To this effect, the higher the current density – and thus the H2 conversion – the greater the temperature increase and the higher the temperature gradient at the cell’s center. The recorded temperatures show a maximum deviation of 24.8 K in fuel cell mode. This is mainly caused by the temperature at the bottom of the stack and shows that the surrounding furnace environment and also the bottom plate of the top-hat furnace have the greatest thermal impact on both outer layers of the stack. Note that as a result of the exothermicity of this reaction, the furnace temperature was decreased in order to maintain a constant air outlet temperature. The centered cell pairs did not show any significant difference with regard to the voltage behavior (see Fig. 2c), while the outer layers of the stack were notably lower, due to significant thermal factors in the surrounding furnace environment. As can be seen in Fig. 2b, the temperature of RU 1–2 (marked as – –) is up to 20 °C lower than the average temperature. Compared with the voltage at the bottom, RU 1–2, the voltage of RU 9–10 at the stacks top bends downward at 0.25 A cm−2, which is consistent with the temperature progression (marked as ), which falls below the average temperature. The dispersion between the RU voltages measured was found to be 80 mV at 0.30 A cm−2, due solely to the temperature discrepancy, whereby the average cell voltage amounts to 740 mV at that point. Steam electrolysis operation respectively during charging – Fig. 3 depicts the initial performance of the same stack in electrolysis mode. Again, the absolute inlet gas flow rate was 0.48 l h−1cm−2, but with a 20% H2/80% H2O composition ratio, while the air flow rate was set to 0.40 l h−1cm−2 in order to remove the oxygen produced. As in fuel cell mode, the stack was operated at four different temperatures (with a constant air outlet temperature), which are indicated by the i-V curves in Fig. 3a. The polarization curves were recorded until one of the cell pairs reached the maximum of 3.0 V (i.e. 1.5 V per cell averaged), according to the manufacturer’s specifications. In order to meet the autonomous system’s aim of high reactant conversions of 80%, a maximum current density of −0.74 A cm−2 ( −94.5 A) was required for the chosen fuel flow rate. The oxygen content at the air outlet was 43 vol%, which corresponds to an 84% increase in the oxygen quantity. The average temperatures recorded (i.e., the mean of the five TCs in the stack), also presented in Fig. 3a (dashed lines), show an initial decrease down to a global minimum followed by an increase. These courses emerge from a

3. Results and discussion 3.1. Electrochemical performance assessment and characterization Fuel cell operation respectively during discharging – Fig. 2 shows the initial performance of the rSOC stack in fuel cell mode while feeding 0.48 l h−1cm−2 of 60% dry hydrogen in nitrogen. The air-side inlet flow was 2.80 l h−1cm−2 of dry air in order to keep the excess air ratio , which is the ratio of oxygen flow at the inlet to oxygen electrochemically converted, greater than 7–8 to ensure an adequate air supply and especially due to the need to cool the stack. Fig. 2a presents the i-V curves and the corresponding average stack temperatures of the entire stack at the relevant air outlet temperatures (780, 800, 820, and 830 °C). The polarization curves were recorded before reaching the maximum stack temperature of 860 °C or bi-RU voltages below 1.3 V (i.e. the averaged single cell voltage 0.65 V), whereas the lowest cell will reach the lower voltage limit first. Thus, this will most likely be the crucial part. As can be seen, the higher the temperature, the better the performance. The maximum current densities ranged from 0.32 A cm−2( 41 A at FU = 47%) to 0.42 A cm−2 ( 53 A at FU = 61%) at the lowest and highest temperatures examined, respectively. Due to the high air flow rate, low oxygen utilization rates of 10–15% were attained, which correspond to excess air ratios of between 8.4 and 9.5. In all cases, the average stack temperature increased by 30–50 °C during the polarization experiments as a result of the exothermic reaction of the H2 oxidation as well as the ohmic heating that occurs as a consequence of current flow (also referred to as the Joule effect). The combination of the exothermicity of the hydrogen conversion reaction and the Joule heating flattens the i-V curves as the current densities increase and thus strongly influences the stack’s performance, making it difficult to determine the ASR. More detailed results, in terms of the individual temperatures and the cell pair voltages at 800 °C, are shown in Fig. 2b and c, respectively. As can be seen in Fig. 2b, the temperatures inside the stack behave differently depending on the location of the TC, depicted in Fig. 1(b). If we compare the recorded temperature profiles in the vertical mid-plane

Fig. 2. i-V characteristics and temperature measurements for the electrolyte supported SOC stack under mixture init 1: (a) Stack voltage and average temperature over current density at four temperatures, (b) Individual temperatures at 800 °C, and (c) Individual bi-RU’s at 800 °C. 6

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Fig. 3. i-V characteristics and temperature measurements for the electrolyte supported SOC stack under mixture init 2: (a) Stack voltage and average temperature over current density at four temperatures, (b) Individual temperatures at 800 °C, and (c) Voltage of the individual bi-RU’s at 800 °C.

balance between the ohmic heating and the endothermic reaction of the H2O reduction. Below the global temperature minimum at low current densities, the thermodynamic processes outweigh the Joule heating effect. As the current density increases, the cells generate more heat due to internal resistances; therefore, the stack exhibits exothermic behavior. With respect to the temperature minima, the thermoneutral voltage Vtn is introduced, which is the voltage at which neither heating nor cooling takes place; thus, it may be used as a parameter to quantify the heating and/or cooling required by an rSOC [56]. It is defined as the reaction heat (change in enthalpy), referring to the heat produced due to current flow (charge transfer) (Vtn = H /(n F )) [57,58]. At operating temperatures of 780–830 °C, the thermoneutral voltage is equivalent to 1.286–1.288 V per cell. The current density required to reach the thermoneutral voltage increased from −0.39 A cm−2 ( −50 A at RC = 43%) to −0.55 A cm−2 ( −69.5 A at RC = 60%) with the increasing operating temperature. As expected, the thermoneutral voltage showed a linear dependency on the temperature, which is identified by a straight dot-dashed line in Fig. 3a. This line is formed by intersections of the temperatures at which the thermoneutral voltage is present at each respective temperature. The global temperature minimum also showed a linear trend, depicted by a straight dotted line in Fig. 3a. When examining the four operating temperatures, note the increasing difference between the current density at which the thermoneutral voltage is present and the current density at which the global temperature minimum is reached. It is also important to observe that there is a noticeable change in the slope of the polarization curve after the temperature reaches its minimum. The flattening of the i-V curve from this point on is caused by the

rapid increase in temperature as the current density increases and this may be relevant for steady-state operation in terms of the operational conditions applied, i.e. taking into account temperature gradients and levels, thus supplementary heat supply. The maximum temperature deviation, present at the current density where the temperatures global minimum is observed amounted to 14.9 K. As an example, Fig. 3b and c show the results obtained during the iV recording for 800 °C. The TC at the bottom of the stack recorded the lowest values. In contrast to the temperature profiles obtained in fuel cell mode (compare Fig. 2b), the stack temperature is highest at its top. As can be seen in Fig. 3b, there is a shift in the minimum of the MEA temperatures towards higher current densities, the higher the cell is located in the stack. In order to maintain a constant air outlet temperature, the furnace temperature was adjusted according to the stack temperature. As shown in Fig. 3c, similar to fuel cell mode, the bottom bi-RU 1–2 showed a temperature-related smaller performance, while the performance of RU 9–10 deteriorated at a certain point (−0.50 A cm−2 for 800 °C, shown here). This deterioration is most likely due to a non-uniform gas distribution and a resulting vertical concentration gradient. Moreover, due to a subsequent change in the partial pressures, a change in the steam content occurs, and thus also in the flow conditions. Another phenomenon that was observed (not shown) is that the higher the operating temperature, the more the top bi-RU 9–10 tends to show signs of steam starvation by changing the linearity of its i-V curve, which is also apparent in the total stack voltage at 820 °C and 830 °C in Fig. 3a.

Fig. 4. Stack voltage and average stack temperature in mixtures A, B and C operated at 800 °C: (a) DC characterization sweep (b) Steady state operation.

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3.2. Electrochemical characterization in H2O-, co-, and CO2-electrolysis modes

layers showed lower temperature values, and thus an associated poorer performance. Furthermore, during endothermic operation (in electrolysis mode) the bi-RUs behaved in the same manner in terms of thermal and electrochemical behavior. As depicted in Fig. 3b, the top layer has the highest temperature and the bottom layer the lowest. The disparities between the bi-RU polarization curves are similar to those shown in Fig. 3c. In the case of all three gas mixtures, heat is produced during discharging (thus increasing the temperature), while heat is consumed during charging (decreasing the temperature). As mentioned above, this is due to the exothermicity and endothermicity of the relevant reactions in FC and EC mode, respectively. Additionally, the production/consumption of heat for CO oxidation/CO2 reduction is greater than that which occurs for H2 oxidation/H2O reduction [61]. At the same time, the temperature of the co-electrolysis mixture is similar to the voltage between the individual electrolysis operations. Table 2 also provides the ASRDC values, which were calculated from DC characterization as the slope from OCV to the voltage measured at ±0.30 A cm−2, in both modes. The lowest ASRs were obtained for both hydrogen oxidation and steam reduction (50% H2 + 50% H2O). A higher ASR was observed under carbonaceous inlet mixture operation (50% CO2 + 50% CO). The ASR generated by operation with H2/H2O/CO/CO2 lies between those of H2 oxidation and H2O reduction, and for CO and CO2, respectively, but closer to the former. Consequently, for AC characterization in electrolysis mode, the stack was brought into a stable state and operated at various operational points under stable conditions before the AC measurements were performed. Both the stack voltages and the average stack temperatures obtained under stationary conditions at the relevant operating points are depicted in Fig. 4b. As can be seen, both the stack voltage and the average stack temperatures showed the same tendency as during the DC sweeps, resulting in nearly-superimposed stack voltages, while the temperatures tend to be lower during stationary operation due to the slow transient temperature response of all parts within the test setup. Regarding the calculated ASR based on stable operation in electrolysis mode, these are less than 3.0% higher than the ASRDC values gathered from the i-V curves. This is due to the lower temperatures under steady-state operation, in which these adjust, that is, decrease, after the dwell time. Fig. 5 shows electrochemical impedance spectra for the three mixtures in the form of Nyquist plots, measured at an air outlet temperature of 800 °C in electrolysis mode. These measurements were performed at current densities up to −0.23 A cm−2, which corresponds to an RC of 60%, with steps of 0.05 A cm−2 and were conducted only after a steady state had been reached. Due to both the thermal management – in terms of maintaining a constant air outlet temperature – and the endothermic steam electrolysis reaction, the stack temperature decreases as the negative current density increases, as can be seen in Fig. 4. Thus, the IS shifts toward higher real impedances on the x-axis as a function of current applied, which is purely due to the temperature dependence of

After characterization in the conventional fuel cell and electrolysis modes, feeding the mixtures init 1 (H2/N2 = 60/40) and init 2 (H2/H2O = 20/80), respectively, the SOC stack was exposed to three different gas compositions (mixtures A – C according to Table 1) and two temperatures (800 °C, and 830 °C at the air outlet) in reversible operation. The inlet gas mixtures contained different amounts of hydrogen, steam, carbon monoxide and carbon dioxide in order to compare the stack performance during H2- and CO oxidation, and H2O- and CO2 reduction, respectively. Each of the inlet mixtures comprised 50% reactant and 50% product species at the inlet. A total gas flow rate to the fuel electrode of 0.33 l h−1 cm−2 was chosen in order to reach an FU of 80% and a RC of 80% in FC- and EC-mode, respectively, at a current density of ±0.30 A cm−2. This fairly small current density – compared to the initial characterization – was selected in order to attain a correspondingly high fuel utilizations/reactant conversions when performing impedance spectroscopy since the impedance measurement unit is limited to ±30 A ( ±0.23 A cm−2). Both the gas and air flow rate were held constant for all mixtures, in both FC and EC modes. Fig. 4 compares the performance of the three mixtures at a constant air outlet temperature of 800 °C. The i-V characteristics for the three mixtures can be seen in Fig. 4a. As can be seen and is also indicated in Table 2 the OCV for the three mixtures are nearly the same since the free energies are similar at the temperatures applied. Table 2 presents both the calculated and the measured OCVs. The temperature indicated therein corresponds to the outlet air temperature, while the average stack temperature, as shown in Fig. 4a was used for the calculation. The theoretical values were determined from a Nernst potential calculation, based on a model considering the equilibrium gas composition [59]. The maximum difference between the calculated and measured value was 9 mV, indicating a small but significant magnitude. These discrepancies between the ideal and measured OCV can be attributed to the temperature difference across the stack (as described in Fig. 3b) as well as negligible leakage rates, allowing the gas tightness of the stack to be presumed. Note that the individual i-V curves in Fig. 4a are not as close to each other in electrolysis mode as in fuel cell mode. The transition across OCV was smooth for all of the gas mixtures used, demonstrating the stack’s ability to be operated both in discharge and charge mode. Compared to the polarization curves recorded at 800 °C air outlet temperature and illustrated in Fig. 4a the performance of the rSOC stack improved by up to 6.1% when the operating temperature was increased to the aforementioned 830 °C (not shown). This improvement is mainly attributable to the increase in operating temperature of 30 °C. At this temperature, the performance of the bi-RU’s also became more uniform, although the performance of the bottom RU 1–2 was somewhat lower in both modes (3.5% in FC mode and 4.6% in EC mode), which is primarily due to lower temperatures at the bottom of the stack. More importantly, it was possible to operate the stack in reversible mode in all three cases: (A) H2O- ( ), (B) co- ( ), and (C) CO2-electrolysis ( ) are all possible. The corresponding electrochemical and chemical reactions involved are [60]: (A) the H2O reduction 1 (H2 O H2 + 2 O2 , H (800 °C) = 248.3 kJ mol 1) , (C) the CO2 elec-

Table 2 Theoretical and measured open circuit voltages and calculated ASR for the i-V characterizations shown in Fig. 4a. Mixture label (cf.

CO + 2 O2, H (800 °C) = 282.4 kJ mol 1) , and (B) a trolysis (CO2 combination of the two. In the latter case, referred to as co-electrolysis, the (reverse) water gas shift (WGS/rWGS) 1

(CO + H2 O

WGS

rWGS

CO2 + H2, H (800 °C) = 34.0 kJ mol

1)

[16] occurs in

addition to the individual electrolysis reactions, which makes it more complex than the single electrolysis processes. Fig. 4a also presents the average stack temperature, which was recorded along with the polarization curves. Analogous to the temperature and voltage profiles shown in Fig. 2b and c, and obtained during the initial i-V measurements in fuel cell mode, the outer

OCV (mV) Calculated a Measured

ASR ( FC mode

cm−2) EC mode

Table 1)

800 °C

830 °C

800 °C

830 °C

800 °C

830 °C

800 °C

830 °C

A Bb C

949 951 951

940 940 937

940 944 949

932 933 935

0.82 0.83 0.90

0.73 0.74 0.81

0.86 0.89 1.05

0.78 0.84 0.94

a

Based on the respective average stack temperature. The thermodynamic equilibrium composition for mixture B at the respective average stack temperature is: 25.5% H2 – 24.5% H2O – 25.5% CO2 – 24.5% CO at 800 °C air outlet temperature, for which the calculated OCV is 951 mV, and 24.9% H2 – 25.1% H2O – 24.9% CO2 – 25.1% CO at 830 °C air outlet temperature, for which the calculated OCV is 940 mV. b

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Fig. 5. Nyquist plots of the IS of the stack in electrolysis modes at 800 °C for: (a) Mixture A, (b) Mixture B, and (c) Mixture C at both OCV and elevated current densities, and (d) A comparison of the three mixtures at −0.10 A cm−2. Total gas flow rate: 0.33 l h−1 cm−2 - Air flow rate: 0.40 l h−1 cm−2.

ohmic losses. By superimposing the IS shown in Fig. 5a when supplying 50% H2 + 50% H2O (marked in blue), there is almost no change attributable to charge transfer or cathodic processes in the high- and midfrequency ranges. In contrast, the low-frequency arc, which is linked to higher gas conversion (0.02 Hz–4 Hz) increases with the current density and is the primary cause of the increase in overall resistance and losses at higher current densities. While supplying the 25%-each mixture (marked in red), similar sets of IS were obtained as shown in Fig. 5b. As can be seen from Fig. 5c, feeding 50% CO2 + 50% CO (marked in yellow/orange) resulted in a significantly higher low-frequency arc (i.e. conversion at the fuel electrode [62]) compared to that for the mixtures containing steam. Diffusion losses may not be seen in the IS; they are negligibly small due to the relatively thin electrodes in ESCs [63,64]. In order to show these process sequences even more clearly, the IS for the three mixtures at −0.10 A cm−2 and 800 °C are directly compared in Fig. 5d. The different points of intersection of the IS at high frequencies with the x-axis originate from the diverse heat demands of the respective mixtures, as mentioned above, and as can be seen in Fig. 4b in the form of the temperature profiles determined. Again, on superimposing these three IS, scarely any difference between mixture A and B is visible. Mixture C, however, has a much more pronounced low-frequency arc. The fact that the impedances move to higher

resistances at certain frequencies is basically the result of the slower kinetics of CO2-diffusion compared to that of H2O [65,66]. This also becomes apparent by collating the impedance at the same frequencies (e.g. 2 Hz, as indicated in Fig. 5d): both the real and imaginary parts are shifted upwards to higher values for CO2 reduction, indicating slower kinetics [67]. These results also reveal that the H2O reactions involved are promoted even at low currents. The results presented above are from measurements performed at relatively low temperatures, which is due to the scope of the system. In order to fulfill the full potential of the stack and reach its maximum efficiency, measurements were carried out at temperatures close to the stack’s maximum temperature of 860 °C. Fig. 6a shows a comparison of the stack voltage and the associated average stack temperature recorded under H2O electrolysis at air outlet temperatures of 800 °C and 850 °C. These were obtained in the same way as for Fig. 4b: during steady state operation, feeding mixture A before and during the EIS measurements. The average stack temperature decreased as the current density increased, due to the exothermic H2O reduction. Both current–voltage characteristics increase linearly, whereas the slope at 850 °C is more gradual due to the better conductivity of the electrolyte at higher temperatures. The corresponding Nyquist plots for the two temperatures recorded at −0.10 A cm−2 are shown in Fig. 6b. Increasing the air outlet temperature from 800 °C to 850 °C results in a

Fig. 6. Influence of the temperature under H2O electrolysis operation: (a) Stack voltage and average temperature, and (b) Nyquist plots at two air outlet temperatures (800 °C vs. 850 °C) at −0.10 A cm−2. Total gas flow rate: 0.33 l h−1 cm−2 - Air flow rate: 0.40 l h−1 cm−2. 9

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decrease of both the ohmic resistance and the total impedance when the stack was operated under the same inlet conditions. The ohmic losses decrease as the temperature rises, which is one result of the increasing ionic conductivity of both the electrolyte and oxygen-ions conducting materials. At high frequencies, the interception of the IS with the x-axis shifted from 0.52 cm2 at 800 °C down to 0.37 cm2 at 850 °C. The ASRAC, which is the intercept of the IS with the real axis at low frequencies, and this decreased from 0.83 to 0.64 cm2 when the temperature was increased from 800 °C to 850 °C. Comparing the IS at the two temperatures, it is possible to observe that the second low-frequency arc, which represents the O2−-diffusivity and surface exchange kinetics in the cathode (5–30 Hz), is significantly higher at 800 °C than at 850 °C, which is due to enhanced oxygen reduction at higher temperatures. Fig. 7 presents the data from the off-gas measurements obtained at the outlet, resulting from both the DC sweeps (Fig. 4) and AC characterization (Fig. 5) at 800 °C. The gas compositions were measured after water condensation, and are thus shown on a dry basis (volume increase or decrease due to the reduction processes is not considered). The values are plotted as a function of current density; the solid lines (i-V) were dynamically obtained during i-V scanning (Fig. 4) with a current ramp of 4 A min−1, the dashed lines (calc) indicate calculated values, and the symbols (op) represent the data obtained under stationary operation, during and prior to the AC measurements (Fig. 4b). The predictions for H2O and CO2 electrolysis were respectively based on the thermodynamics of high-temperature water and carbon dioxide electrolysis processes [68]. The gas compositions for co-electrolysis were determined on the basis of the chemical equilibrium co-electrolysis model developed by INL [69]. In Fig. 7, note the inferior agreement between the experimental data (fuel-electrode sided) obtained for mixtures B and C during fast i-V measurements (solid lines) and the results of the steady state operation (symbols). There are two possible reasons for this. First, the swift current increase of the power supply leads to nearly isothermal conditions since the temperature lags behind. Secondly, an equilibrium state has to be set in order to achieve a constant gas outflow. At the beginning of the gas analysis, it also takes some time to sweep through the long gas pipe through the condenser to the GA, which is not the case during the i-V sweeps. For straightforward H2O electrolysis, the fuel outflow (the sum of the dry hydrogen inlet flow and the produced hydrogen) was measured by means of an additional auxiliary hydrogen mass flow meter (MFM), since 100% hydrogen was detected by the GA at the outlet. Both the measured hydrogen outflow rate ( ) as well as the oxygen content ( ) on the cathode side matched perfectly with the theoretical results, as shown in Fig. 7a. The product gas compositions for co-electrolysis feeding mixture B are shown in Fig. 7b. As mentioned, the outlet gas composition at OCV was close to the thermodynamic equilibrium composition, indicating that the WGS/rWGS reaction takes place under OCV conditions and that it occurs parallel to the separate electrolysis reactions. The experimental outlet concentrations showed good agreement with the predicted values, although there was a small difference in terms of both the increasing current density and the reactant conversion.

This was due to the change in temperature under real conditions, which was not considered in the model. Feeding mixture C, the CO production rates (80% at 30 A and RC = 60%; marked as ) were close to the theoretical values, proving that the stack is capable of performing CO2 electrolysis, as shown in Fig. 7c. For a more detailed analysis of the resistances caused by the individual processes and how they contribute to the total resistance, a breakdown of the impedances is provided in Figs. 8a–c for mixtures AC, respectively. Above each diagram, the measured stack voltages and average stack temperatures for the relevant measurement points at the time the impedance measurements were performed under stable conditions are presented. Looking at Fig. 8, it is possible to see that the total resistance (both ohmic and polarization) increases again with the increasing negative current density in electrolysis mode and is highest for CO2 electrolysis. In terms of the ohmic resistance, which increases slightly with the increasing negative current density, basically due to the temperature decrease evident in Fig. 8a–c in the profile above, the difference is attributable to the differential heat dissipation of each mixture fed. For the individual polarization resistances, it was observed of all fed mixtures that the conversion resistance of the fuel electrode decreases, while the resistances of the air electrode ( AEhigh and AElow marked in yellow and dark blue, respectively, in Fig. 8a–c, below) increase when a current is applied in EC mode. The relative change seems to be related to the compositions, e.g. mixtures C > B > A . With regard to the lowfrequency impedance representing the adsorption of oxygen on the catalyst’s surface (air side), a gradual transition to the conversion process at the fuel electrode takes place since the O2-adsorption process appears at similar low frequencies under the measurement conditions [70]. The charge transfer (FEhigh ) at the fuel electrode, visible at high frequencies, remained practically constant for all of the current densities tested, with marginally higher values during CO2 electrolysis; although for all compositions the contribution of the charge transfer process to the total resistance is minor compared to all other individual resistances. To this end, the largest share of the total resistance, as well as the distinct differences between the mixtures, can be attributed to the fuel electrode-sided conversion resistance and the air electrode resistance, which represent the diffusion process visible at high frequencies, and are therefore discussed in the following. During H2O electrolysis the change in both the conversion and diffusion resistance is insignificant during charging, while for the CO2-containing mixtures, there is a noticeable decrease in conversion and increase in diffusion resistance. The increase in both the ohmic resistance and the electrochemical electrode resistances at the air electrode cannot be explained by either the increased diffusion resistance or the asymmetric conversion resistance in electrolysis mode [71]. In fact, the oxygen content at the air electrode increases linearly from 20.9 vol% to 29.6 vol% as the current density increases. The electrical conductivity of LSCF decreases with temperature and oxygen partial pressure [72]. As a result, the increase in the air electrode resistances may be caused by an increased partial oxygen pressure. Compared to H2O electrolysis, the sharper increase in ohmic and air electrode-side resistances as the current density Fig. 7. Product gas compositions (dry based) determined theoretically (- - -), measured during DC characterization (—), and steady-state operation ( , , Δ, , ): (a) Mixture A/800 °C, (b) Mixture B/800 °C, and (c) Mixture C/800 °C.

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Fig. 8. Break down of the resistances for the IS of the stack shown in Fig. 5 in electrolysis modes at 800 °C for (a) Mixture A, (b) Mixture B, and (c) Mixture C at both OCV and elevated current densities, and (d) a comparison of the three mixtures at −0.10 A cm−2. Total gas flow rate: 0.33 l h−1 cm−2 - Air flow rate: 0.40 l h−1 cm−2.

increases during co-electrolysis (see Fig. 8b) and the even stronger increase for CO2 electrolysis (see Fig. 8c), can be attributed to the greater temperature difference due to the above-mentioned mixture-dependent dissipation of heat. For H2O electrolysis (see Fig. 8a), the fuel electrode conversion polarization resistance (also referred to as the gas diffusion resistance, marked in red) remains almost at the same level after polarization, while the conversion resistance of the carbonaceous mixtures decreases with an increasing negative current. When adding up the fuel sided resistances (charge transfer (FEhigh marked in light-blue) and conversion resistances (Conversion marked in red)) shown in Fig. 8, it becomes apparent that these remain nearly constant – and even decrease slightly – throughout both increasing polarization and the three mixtures. The conversion resistance at the Ni/GDC electrode is somewhat higher in absolute terms for CO2 and co-electrolysis compared to H2O electrolysis; however, this is caused by the higher molar weight of CO2 compared to H2O [73]. One phenomenon that is particularly striking is that the air sided polarization results in a remarkable increase in the resistances of the air electrode (diffusion and adsorption of oxygen) when the current density is increased and the inlet gas composition is changed (mixtures C > B > A ). The increase with increasing current density is moderate, however, compared to that of the oft-used Ni/YSZ electrodes [74,75]. In comparison to YSZ, GDC has a higher electronic/ ionic conductivity. As a result, the adsorbed oxygen ions can more readily enter the electrolyte lattice at the conductor interface and are thus better able to store O 2 [76]. Thus, one can suppose that the GDC catalyst plays an important role in allowing the electrochemical reactions taking place. The GDC surface seems to make a great contribution to the oxygen adsorption process, whereby the Ni is even likely to be substituted in decently active regions. The assumption that there is a different mechanism behind the electrochemical reaction in Ni/GDC electrodes is supported by similar observations made during a direct comparison of diverse fuel electrodes [70]. A further advantage of the electrode used is its thickness from the use of electrolyte supported cells, which subsequently reduces the likelihood of carbon deposition and is beneficial for co-electrolysis. When the temperature decrease in the stack was neglected, while its magnitude is dependent on the current density caused by the thermal management (constant air outlet), there was only a small difference in performance in terms of the losses resulting for H2O and co-electrolysis, while for CO2 electrolysis, a slight loss of performance was observed. For steam and co-electrolysis, the total resistance from OCV to −0.23 A cm−2 increased by less than 10%, while for the electrolysis of pure carbon dioxide, that growth amounted to 20%. The ohmic resistance had the main share of the total resistance and evolved in relation to the temperature change, which is evident for electrolyte supported cells. In comparison, for fuel electrode supported cells, large

differences between steam, co-, and CO2 electrolysis have been observed [77,78]. This can be explained primarily by the higher resistance of the Ni/YSZ electrode and also the slower diffusion of adsorbed species through the support structure, thus resulting in a higher concentration resistance. For ESCs, it is generally true that the exchange of reactants/products proceeds more easily due to the smaller anode thickness. This stack’s more or less comparable performance for steam, co-, and CO2 electrolysis suggests that electrolyte supported cells and stacks are highly suitable for electrolysis operation and may even be preferred over electrode supported cells. 3.3. Stack performance under a transient operating regime After DC and AC characterization, the stack was operated at three different air outlet temperatures (780, 800, and 820 °C) under steadystate conditions at a constant high fuel utilization, respectively reactant conversion and various current densities in both modes. The stationary operations were examined by means of a transient operating procedure. In this context, the regime was performed as follows. The stack was brought up to a specific operating point and held constant until stable conditions (temperatures and gas outlet composition) were achieved. As soon as a steady-state had been reached, measurements were carried out at the relevant operating point. Subsequently, the inlet gas flow rates and operating conditions (temperatures and current density) were adjusted for the next operating point. The corresponding adaptations were made with gradients of 0.008 A cm−2 min−1 (equivalent to 1 A min−1). The current density was increased in steps of 0.05 A cm−2 so that adjusting the operating parameters took a total of 6 minutes, and settling the boundary conditions took less than 20 minutes. The results obtained in the course of this operating regime at system-relevant conditions are presented in the following paragraph. The testing conditions are summarized in Table 1, wherein the mixtures are labeled Op-A to Op-D. Both the FU and RC were set to 80% in fuel cell and electrolysis mode, respectively. In fuel cell mode, dry hydrogen in nitrogen (Op-A) was used, while in EC mode operation conditions in steam electrolysis (Op-B) and co-electrolysis (Op-C and Op-D) were tested. The configurations and operating conditions were specified and classified for the system design for reversible operation, as mentioned in Section 2.1. In order to comply with the manufacturer’s specified fuel utilizations and reactant conversions of 80% in the respective modes, the inlet flow rates were adjusted according to the corresponding current densities. In addition, for fuel cell operation, the air flow rate was adjusted concurrently with the fuel flow rate in order to achieve an OU of 20%. By contrast a constant air flow rate was applied throughout for electrolysis operation, resulting in an increase in the oxygen content as the current intensity increased. The O2 content in the air-sided off-gas was up to 43% (see Fig. 9a). This oxygen-enriched air flow may be 11

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subsequently be used for further processing. Fig. 9a and b provide examples of chronological sequences obtained during steady-state operation in fuel cell and electrolysis modes, respectively. The upper graph shows the behavior of the voltages as being dependent on a change in the current density. The central plots present the in- and outlet gas flow rates as well as the gas and oxygen contents measured at the outlet. Finally, the bottom charts depict the corresponding temperature profiles. For operation in fuel cell mode, feeding mixture Op-A, both varying gas and air flow rates were applied for each current density in order to achieve a FU of 80% and an OU of 20% at every operation point. The profiles shown in Fig. 9a were obtained at a constant air outlet temperature of 800 °C, as can be seen in the bottom graph (marked in blue). Secondly, the higher the current density, the higher the stack temperature since the hydrogen reaction is highly exothermic. In order to maintain a constant temperature at the outlet, less external heat – provided by the furnace – is required. The dips and swells in the voltages and concentrations, clearly visible during the transitions (between operating points) can be explained by temporary increases in the flow rates. As can be seen in the central graph of Fig. 9a, both the hydrogen and oxygen concentrations go to the same level after balancing at a steady-state and they reflect the desired FU and OU at each operating point. The stack performance in electrolysis mode was also tested by means of this transient operation procedure. Fig. 9b shows the time courses using the example of H2O electrolysis operation, feeding mixture Op-B at 820 °C. For operation during charging, the gas flow rates were adjusted for a desired RC of 80%, while the air flow rate was kept constant. Thus, as with the air outlet temperature, the temperature at the air inlet was also held constant. In contrast, the oxygen content at the outlet naturally increases with the current density. The hydrogen

outflow rate for H2O electrolysis is presented in red in the central plot of Fig. 9b. Due to its metrological acquisition by means of an MFM, it is subject to fluctuations and occasional drops. Except for the deflections, the measurement quality is extremely good according to the terms of deviations and uncertainty and agrees with the theoretical predictions. From the measurement data obtained after the transient procedure, the corresponding operating points were extracted at each current density. However, the values were only measured once they had settled and were almost constant. The results of these measurements, all obtained by subjecting the stack to a high fuel utilization/reactant conversion, are discussed in the following paragraphs. The plots in Fig. 10 show the calculated efficiency and power (consumption) density of the steady-state experiments. The fuel cell’s efficiency was determined by considering the electric power output in relation to the energy content of the hydrogen flow used ( FC = Pout , el /Qfuel used ). In this calculation, one can refer to Qfuel used , since a large part of the off-gas in the system is to be recirculated. Assuming that no recirculation takes place, Qfuel tot would have to be considered, whereby the efficiency would be lower as a result. Analogously, the electrolysis efficiency refers to the ratio of the energy content in the stack outlet flow to the electric power input ( EC = Qfuel prod/ Pin, el ) ([57]). Both the external energy demands of the furnace and the energy needed to evaporate the water are neglected; hence, EC can be greater than unity. Stationary operation as a function of mode – Fig. 10a depicts the fuel cell’s efficiency and the electric power output density of the extracted data as a function of the current density, under fuel cell operation with a constant FU of 80% or a constant RC of 80% during discharging. The maximum current density achieved was 0.32 A cm−2 ( 40.9 A), while a stack efficiency of up to 68% was achieved. Fig. 10b shows the

Fig. 9. Time course of voltages, current density, flow rates, gas/oxygen contents, and temperatures over time during transient operation complying 80% fuel utilization operated under (a) Fuel cell mode feeding mixture Op-A at 800 °C, and 80% reactant conversion (b) H2O electrolysis feeding mixture Op-B at 820 °C. 12

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Fig. 10. Performance of the stack under steady-state operation as a function of current density, operating temperature, and inlet mixture: (a) Fuel cell mode feeding mixture Op-A (FU = 80%), (b) H2O electrolysis operation feeding mixture Op-B (RC = 80%), and (c) Comparison of inlet composition feeding mixture Op-B–Op-D (RC = 80%).

electrolysis efficiency and electric power input density determined from the measured values obtained under H2O electrolysis (H2/H2O/CO2=20/80/0) over current density. It was possible to operate the stack up to a maximum current density of −0.70 A cm−2 ( −89.5 A) before the maximum stack temperature or cell pair voltages were reached, yielding a hydrogen output of 0.30 l h−1 cm−2. Steady-state operation as a function of temperature – As can be seen in Fig. 10a, the measured values obtained during discharging are very close together at all three temperatures. The stack performance under the chosen parameters was nearly independent of temperature. By contrast, for electrolysis operation, the tests showed a small dependency on the tight temperature range. Electrolysis operation as a function of gas inlet concentration – Fig. 10c presents the stack performance during steady-state electrolysis operation while feeding the inlet 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 performance was thoroughly comparable in all of the electrolysis modes tested. Operating the stack in co-electrolysis mode, the independece of the current density to the syngas ratio (H2/CO) is supported by near-constant ratios of 9.0 for mixture Op-C and 4.0 for mixture Op-D, measured at the outlet.

obtain more homogeneous temperature profiles and consequently uniform cell performances. The subsequent comprehensive steady state experiments with respect to the systems operating strategies and procedures under realistic boundary conditions specified and given by the planner provide guidelines for optimized operation of the envisaged fully autonomous rSOC system. 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 conversion of 80% in both H2O- and co-electrolysis. The resulting operating maps related to maximum current densities, gas production and temperatures can be considered and used in system simulation and design. Furthermore, the outcomes are also directed at the development and optimization of system designs in terms of thermal management and auxiliary power requirements. 4. Conclusion In this study, a reversible Solid Oxide Cell stack was operated and characterized in both fuel cell and electrolysis modes. The stack prototype consists of ten electrolyte supported planar cells. The cells had an active area of 127.8 cm−2, and the stack was designed with open oxygen electrodes and integrated into a stackbox. The stack and the individual bi-repeating units within the stack were electrochemically characterized during operation under real operating conditions, while it was fed with H2/N2 as well as H2/H2O/CO/CO2-mixtures. The characterization was performed by carrying out both direct and alternating current measurements, which were supported by temperature measurements and gas analyses. The i-V curves showed great continuity across OCV, demonstrating the stack’s reversible operation capabilities, and proving its applicability for reversible operation in conditions with practical relevance. Based on a comparison of the area specific ratio values and the gas outlet compositions, this study clearly demonstrates that CO is formed in co-electrolysis mode due to both CO2 reduction and the reverse water–gas shift reaction, which occurs parallel to the individual electrolysis reactions. Due to the dominant ohmic resistance combined with the thin electrodes and the high operating temperature of electrolyte supported cells, the decrease in the activation and diffusion resistances is low. Additionally, the electrochemical impedance spectroscopy measurements revealed that most of the overall resistance can be attributed to gas conversion losses. The second part of this work consisted of a detailed examination of the stack in both fuel cell and electrolysis modes at system relevant conditions. In accordance with the manufacturer’s recommendations, the stack was operated at a high fuel utilization/reactant conversion of 80% under various conditions and considering taking the impact of the different gas mixtures used into consideration, measurements of the operating temperatures and volume flow were performed. The stack performance during stationary operation was examined by means of a transient operating procedure. The results observed during these

3.4. Usability of the experimental outcomes The experiments conducted, evaluated and analyzed within this study are intended for the design of a stand-alone system based on rSOCs, as mentioned earlier in Section 2.1. In order to obtain the concept and layout with the highest level of system performance all auxiliary components need to be optimally designed and economically operated. The operating conditions were thus selected in accordance with this principle. The results observed in the preliminary study under initial conditions give the stack manufacturer an insight into the performance and potential of the newly introduced electrolyte for the support of the cells. The operational investigations carried out with various mixtures including carbonaceous gases or subsequently conducted fractions of these in reversible mode are of great importance and interest with regard to the technical usability of the novel electrolyte support. The reversible DC operation and electrochemical AC characterization and the subsequent analysis of the measurement data revealed the limits and potentials of the cells, cell configuration, stack assembly and the stackbox concept. The post mortem analysis of the stack, cells, interconnectors and additional components, such as the mica sealing and the stackbox itself led to a considerable improvement of the integration in the stackbox in respect of the sealing and the bottom of the stackbox. Further suggestions and proposals for improvements to be made are related to the thermal layout of the experimental design in order to 13

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experiments indicated fuel cell efficiencies of up to 68%. In steady-state electrolysis operation, the net hydrogen flow was 0.30 l h−1cm−2 at its maximum in H2O electrolysis. In co-electrolysis, syngas ratios of 9.0 and 4.0 were achieved while feeding H2/H2O/CO2-compositions of 20/70/10 and 20/60/20, respectively.

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