Towards a wastewater energy recovery system: The utilization of humidified ammonia by a solid oxide fuel cell stack

Journal of Power Sources 450 (2020) 227608

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Towards a wastewater energy recovery system: The utilization of humidified ammonia by a solid oxide fuel cell stack Bernhard Stoeckl a, *, Michael Preininger a, Vanja Suboti�c a, Stefan Megel b, Christoph Folgner b, Christoph Hochenauer a a b

Institute of Thermal Engineering, Graz University of Technology, Inffeldgasse 25/B, 8010, Graz, Austria Fraunhofer Institute of Ceramic Technologies and Systems, Winterbergstrasse 28, 01277, Dresden, Germany

A R T I C L E I N F O

A B S T R A C T

Keywords: Solid oxide fuel cell stack Ammonia Temperature analysis Electrochemical impedance spectroscopy Scanning electron microscopy High temperature corrosion

This study presents the results of investigations on performance and durability of an ammonia-supplied MK352 solid oxide fuel cell stack with electrolyte supported cells and chromium based interconnects. The performance evaluation revealed no significant differences between ammonia and equivalent hydrogen/nitrogen gases as fuel, which was a result of the excellent ammonia conversion rates up to 99.99%. When using high ammonia flow rates, temperature measurements inside the stack revealed a temperature drop due to the endothermic ammonia decomposition of up to 18.8 K, which proceeded preferentially at the fuel inlet region. An 1000 h durability test with humidified ammonia in 80% fuel utilization condition was performed, which resulted in a stack perfor­ mance degradation rate of about 1.1%/1000 h. Tests with hydrogen/nitrogen fueled reference stacks revealed similar degradation rates during the initial 1000 h. Post-mortem analyses by scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed no significant micro-structural deterioration of the functional layers of the anode, but nitriding effects on the nickel contact meshes and chromium nitrides were found in the material structure of the interconnects. Also, an oxide layer was found between interconnect and contact meshes at the anode, which appears to be the main cause of the performance degradation.

1. Introduction Growing energy demand and the simultaneous need to reduce greenhouse emissions require the development of efficient energy con­ version and recovery systems. In this context energy-recovery systems for industrial as well as consumer applications are becoming more attractive as a way of increasing overall system efficiencies. For example, wastewater contains unused energy in form of dissolved ammonia [1]. Following the pre-treatment of wastewater, the dissolved ammonia is convertible to ammonia, which is a high-potential fuel for solid oxide fuel cells (SOFCs) [1,2]. Vacuum distillation membrane (VDM) units produce gaseous ammonia (NH3) and steam (H2O) can be converted to electrical energy in an SOFC system. Fig. 1 shows the simplified flow chart of a directly fed ammonia SOFC system down­ stream from a vacuum membrane distillation unit. The humidified ammonia gas enters the system and is directly used in an SOFC stack. As complete fuel utilization in SOFC stacks is not recommended, the exhaust gas including partly unconverted fuel is recycled and added to

the intake gas stream. The residual fuel components are subsequently oxidized in an oxycat, with the unused oxygen of the SOFC stack air supply as oxidant. The heat released in the SOFC stack is used to preheat the fuel and air stream for the cathode, as well as to provide heat for other auxiliary units of the wastewater energy recovery system. As the SOFC stack would form the core of this type of energy recovery system, in-depth investigations are needed in order to define conditions for reliable SOFC operation with ammonia-based fuels. As a carbon-free gas with high energy density and high hydrogen (H2) content, ammonia is of great interest as a fuel to SOFC researchers [3]. It is also attractive from the point of view of distribution because it is easy to liquefy and store [4,5]. As a consequence the usability of ammonia for SOFCs is frequently discussed and is the subject of an increasing number of publications. Some of these describe the utilization route of ammonia in an SOFC where the high operation temperatures favor the thermal ammonia decomposition according to Reaction (1) [6]. In a further step, the released hydrogen is utilized electrochemically and converted to steam [7,8]. The most common anode-electron con­ ducting material of commercially available SOFCs is nickel, which is

* Corresponding author. E-mail address: [email protected] (B. Stoeckl). https://doi.org/10.1016/j.jpowsour.2019.227608 Received 23 October 2019; Received in revised form 6 December 2019; Accepted 11 December 2019 Available online 20 December 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.

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Nomenclature and abbreviations Aactive a.c. AC ACV ASC CeO2 CFY CV d.c. EDX EIS ESC

FTIR GDC j LSM MEA OCV PMA SEM slpm SOFC Uf VMD YSZ

active cell area (cm2) alternating current (A) ammonia conversion (%) average cell voltage (V) anode supported cell ceria chromium iron yttrium cell voltage direct current (A) energy-dispersive X-ray spectroscopy electrochemical impedance spectroscopy electrolyte supported cell

catalytically active in the ammonia decomposition at the SOFC relevant operating temperatures. The catalytic ammonia decomposition proceeds in three steps: adsorption of ammonia on the nickel surface, N–H bond breaking, and desorption of nitrogen (N2) from the catalyst [9,10]. However, during this decomposition process high temperature corrosion in the form of nitridation may occur [11]. Lai [11] describes nitridation in an ammonia atmosphere at high temperatures (980–1090∘C) as a reaction of the metal with the atomic nitrogen. In contrast, if the tem­ perature is set too low, nickel and nitrogen form nickel nitrides in ammonia which influence an SOFC performance in a significant way [7]. However, nickel nitrides are not stable in hot (>700∘C) or reducing at­ mosphere [7,12,13]. Nevertheless, the build-up and subsequent reduc­ tion of a nitrogen sub-surface on nickel lead to formation of pores and significant deformations of the metallic material [13]. The nitridation process is described elsewhere in detail [7,14,15]. 3 1 NH3 ⇋ H2 þ N2 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ΔH025 ∘ C ¼ 46:19kJ ​ mol 2 2

1

Fourier-transform infrared spectroscopy gadolinia-doped ceria current density (mA cm 2) lanthanum strontium manganite membrane electrode assembly open circuit voltage (V) post-mortem analysis scanning electron microscopy standard liter per minute solid oxide fuel cell fuel utilization (%) vacuum membrane distillation yttria-stabilized zirconia

nickel nitrides. Dekker et al. [16] compared anode- and electrolyte supported cells (ASC, ESC) in an ammonia atmosphere over a period of more than 3000 h. After the initial 1000 h, the authors observed voltage degradation rates of 0.96% for the ASC (750∘C) and 0.74% for the ESC (900∘C). Previous studies of our research group [14,15] on solid oxide membrane electrolyte assemblies (MEAs) showed that while ammonia is a suitable fuel for SOFCs with nickel-based anodes, particular conditions have to be met in order to reduce the risk of high temperature corrosion through nitriding of nickel. Our studies in low fuel utilization (Uf) on anode supported MEAs confirmed the high performance potential of ammonia as fuel [14]. However, equivalent hydrogen/nitrogen mix­ tures yielded higher performance values, as ammonia decomposed incompletely and the endothermic process lowered the temperature of the cell and therefore increased the cell resistance significantly. Tem­ perature analysis showed that the fuel inlet region was the preferred location of the catalytically supported ammonia decomposition. In polarized conditions at 700∘C, significant performance degradation was observed and scanning electron microscopy (SEM) revealed the presence of nickel nitriding. Subsequent investigations adapted the conditions in order to address these problems [15]. In this study on anode and elec­ trolyte supported MEAs, the ammonia flow rate was kept low in order to increase the fuel utilization. Both cells were operated stably over a period of 100 h with 80% ammonia utilization and the negative impact of nickel nitriding and the affected area on the cell surfaces were both reduced. Scanning electron microscopy (SEM) investigations of both electrolyte and anode supported MEAs showed significantly lower micro-structural deterioration of the ESC- than of the ASC-anode. These investigations were performed on large planar SOFC-MEAs in a ceramic cell housing and were used to design suitable operation configurations

(1)

Stability studies of ammonia-fed SOFC systems are essential to demonstrate their industrial relevance and usability, and several such studies have already been published. In a stability test over 400 h with ammonia and hydrogen, Fuerte et al. [2] observed neither voltage degradation nor micro-structural damage of their microtubular cells. In contrast, Yang et al. [7] studied ammonia-purged small-scale cells with nickel/yttria-stabilized zirconia (Ni/YSZ) anodes and reported nickel nitriding with effects on cell performance and the anode micro-structures. In the temperature range between 600∘C and 700∘C, the authors observed an increase in internal resistance and a consequent reduction in cell performance when using humidified ammonia as fuel. The authors attributed the performance degradation to the formation of

Fig. 1. SOFC based wastewater energy recovery system.

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for stack or system applications. The next step is the scale-up to the stack level, and this is the content of the present study, in which the operating conditions were adapted to the intended application in a stationary wastewater energy recovery system as shown in Fig. 1. Here we present the results of electrochemical investigations on an electrolyte supported 10-cell MK352 stack with chromium iron yttrium (CFY) interconnects, developed by Fraunhofer IKTS, when purged with ammonia-based fuel mixtures. The investigations include comparative studies with equiva­ lent hydrogen/nitrogen gases and detailed analyses of the entire stack as well as the individual stack cells by means of alternating and direct current (a.c., d.c.) measurements. However, the core of the present publication is a study of constant current operation at 226 mA cm 2 and 835∘C air outlet temperature over 1000 h with 80% fuel utilization, using the product gas composition to be expected from a wastewater vacuum membrane distillation unit: 70% ammonia, 30% steam. During the observation period, voltage, current, temperature and off-gas composition were detected continuously and electrochemical imped­ ance spectra were recorded frequently. After the electrochemical testing, post-mortem analyses (PMA) of single stack cells show the ef­ fects of ammonia purging on the stack components. 2. Experimental setup 2.1. Stack structure and test rig For this study, a 10-cell solid oxide electrolyte supported stack type MK352 developed by Fraunhofer Institute for Ceramic Technologies and Systems IKTS (Germany) was used. The major components of the stack are shown in an exploded view in Fig. 2a. The membrane electrolyte assemblies of the single layers consisted of a <40 μm thick nickel/ gadolinium-doped ceria (Ni/GDC) anode, a dense, 165 μm thick scandia-doped zirconia (10Sc1CeSZ) electrolyte and a multi-layer lanthanum strontium manganite (LSM) and scandia-stabilized zirconia (ScSZ) cathode (�45 μm). The active area per cell is 127 cm2 in the MK352 design. Powder pressed chromium iron yttrium bipolar plates (also called CFY interconnects) ensured the gas separation and distri­ bution as well the electric contacting of the electrodes. These are pro­ duced by Plansee SE (Austria) [17,18]. More details on the cell as well as the stack structure can be found elsewhere [19,20]. Three thermocou­ ples were installed in the air channels of the middle cell (C05) of the stack, in the positions shown in Fig. 2a. These thermocouples (T_C05-IN, T_C05-MID, T_C05-OUT) register the temperature distribution along the fuel channel of the anode at the fifth cell. Platinum wires were welded onto each CFY interconnect in order to enable voltage and impedance measurements of the individual cells. The cells are numbered C01 to C10 from the bottom up. The stack was installed into the test rig via an interface that ensured the air and gas distribution from the supply pipes to the stack. This interface was designed and manufactured in consultation with the stack developer IKTS in order to achieve optimal conditions for the stack ex­ periments. A simplified scheme of the test rig is shown in Fig. 2b. It consists of four major units: the gas distribution, the humidification, the furnace and the off-gas analysis units. Mass flow controllers enable the precise definition of the fuel flow rate. Humidification was accom­ plished by a steam generator, which vaporizes de-ionized water at 200∘C and adds the steam stream to the fuel inlet flow. The preheating of the gas inflow from room temperature to operational temperature is essential in order to avoid thermal effects at the cells inside the stack. Too low temperature may lead to performance reduction due to the cooling of the electrolyte or even lead to mechanical damage caused by thermal stress. At the test rig, both the air and gas flows were preheated in the connecting pipes via heating sleeves. More details about the stack test rig as well as a three dimensional drawing of the stack integration can be found elsewhere [21,22].

Fig. 2. Exploded view of an MK352 stack with internal thermocouple positions (a) and simplified test rig scheme (b).

2.2. Test rig equipment For the electrochemical characterization, both alternating and direct current measurements were conducted. A Gamry Reference 3000 spec­ troscope with a 30 A booster was used to perform the electrochemical impedance measurements in order to evaluate the internal resistances of the stack and the individual cells. For the impedance measurements under load the amplitudes were set to be 4% of the d.c. applied, and the frequency range was 20 mHz–50 kHz. An ABB Advanced Optima 2000 gas analyzer with the Uras 14 and Caldos 17 modules were used to measure the hydrogen fraction and a GASMET DX-4000 Fourier-trans­ form infrared spectroscope (FTIR) identified the unconverted ammonia content. After the electrochemical tests, the micro-structures of the cells were analyzed by a Zeiss Ultra 55 field emission scanning electron mi­ croscope and an Oxford Instruments energy-dispersive X-ray spectro­ scope (EDX). 2.3. Operating conditions Vacuum membrane distillation units used in wastewater energy re­ covery systems convert the bound ammonia in liquid wastewater to gaseous ammonia and steam, which can be fed to SOFC units directly without any pretreatment. In our experiments we synthetically 3

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Table 1 Gas inlet flow rates. Inlet gas flow rate (in slpm)

Fuel type

NH3 H2O H2 N2 Air

AI

AII

ASI

ASII

HI

HII

HSI

HSII

Ref

1.671 0 0 0 32

3.342 0 0 0 60

1.671 0.716 0 0 32

3.342 1.432 0 0 60

0 0 2.507 0.836 32

0 0 5.013 1.671 60

0 0.716 2.507 0.836 32

0 1.432 5.013 1.671 60

0 0 3.27 4.94 60

reproduced the expected gas compositions delivered by a VMD unit as shown in Table 1. In experiments with AI and ASI, 1.671 standard liters per minute (slpm) of ammonia was supplied. The total flow rate was higher in the case of ASI as 0.716 slpm steam was added to the ammonia stream. HI and HSI represent the gas mixtures with the expected gas composition when the ammonia quantity of AI and ASI converts completely to hydrogen and nitrogen, according to Reaction (1). The gas mixtures with the subscript II have the same gas ratios as the corre­ sponding mixtures with the subscript I, but twice the total volume flow rate. The Ref mixture was used for reference degradation tests. The focus during the electrochemical investigations was to achieve a fuel utiliza­ tion rate of 80%. This high fuel utilization is achieved by applying current densities of 226 mA cm 2 and 452 mA cm 2 when feeding the stack with the gas mixtures with the subscripts I and II, respectively. A minimum single cell voltage of 0.68 V had to be respected in order to prevent the anode of its oxidation [23,24]. The air supply for the cath­ ode was either 32 slpm or 60 slpm, depending on the fuel mixture used. These large air flow rates were used in order to avoid elevated cathode concentration polarization. At Uf ¼ 80%, the oxygen utilization was therefore found to be roughly 15%. As the system stability is of major interest, no temperature variations were performed and the furnace temperature was controlled to keep the air outlet temperature constant at 835∘C. The heat-up and cool-down rate was limited to 2 K min 1. The j,V curve measurements were performed with a maximum gradient of 2 A min 1.

gases (according to Reaction (1)). Initially, the stack open circuit volt­ ages (OCVs) of the ammonia and hydrogen/nitrogen fuels were measured and analyzed. The stack OCVs measured were 12.23 V, 12.34 V, 12.50 V and 12.42 V when the stack was fueled with AI, HI, AII, HII, respectively. All gases containing steam yielded similar OCV values; 9.91 V was observed with both ASI and HSI, 9.94 V and 9.95 V were measured with ASII and HSII. The stack voltage is calculated as the sum of the individual cell voltages (CVs); Table 2 shows representative cell OCVs using the gases with the lower flow rates (AI, HI, ASI, HSI). Note the significant differences in the cell OCV values with the dry fuels. The OCVs of the cell C01 were the lowest of the stack when using the gas mixtures AI (1.169 V) and HI (1.168 V). In both cases the OCVs of the first four cells were lower than the average cell voltage (ACV ¼ stack voltage/10) of the stack. The differences in the OCVs point to marginal leakages of the sealant between the stack (bottom plate) and the inter­ face (gas distribution plate). As a result, with the increasing stack height the fuel dilution due to the marginal leakages decreased, and conse­ quently, the cell OCVs increased. Adding steam to the dry gases led to homogenization of the individual cell OCVs because of the alignment of the partial pressure for steam at each single cell. As can be seen in Table 2, all CVs when using ASI and HSI were 0.991 � 0.002 V. The almost identical OCVs of the ammonia- and hydrogen-based fuels hints at the stepped ammonia utilization, as the direct electrochemical oxidation of ammonia would generate significant higher voltages at open circuit conditions [27]. Further, the similarity in the OCVs points to an excellent ammonia conversion, as the partial pressure of the ammonia decomposition product gases hydrogen and nitrogen influence the cell voltage in a significant way [28]. To investigate the performance potential of ammonia as fuel for the used MK352 stack, d.c. measurements were performed to obtain polar­ ization curves. Our previous studies recommend high temperatures and high fuel utilization for directly ammonia purged SOFC systems, which was applied to the stack tests of the present study [14,15]. The j,V curves in Fig. 3a were performed with the gases shown in Tab. 1 up to a maximum fuel utilization of 80% or a minimum single CV of 0.68 V, in order to avoid oxidation of the anodes [23,24]. The curves depicted in the left part of Fig. 3a belong to the measurements with the lower ammonia or hydrogen/nitrogen flow rates. No significant differences in the j,V and j,P curve progressions were found either between AI and HI or between ASI and HSI. The ammonia measurements of the off-gas revealed excellent conversion rates with all ammonia-based gas mix­ tures. The best AC at OCV of 99.98% was observed with AI, which corresponds to the thermodynamically calculated value at 835∘C. That means, only traces of unconverted ammonia and no other species like nitrogen oxides were detected in the exhaust gas by the FTIR. Diluting AI

3. Results and discussion Sufficient ammonia conversion is one key for efficient energy con­ version in an SOFC, as the utilization of ammonia proceeds in two steps: after the initial thermally driven and catalytically supported conversion to hydrogen and nitrogen, the released hydrogen is electrochemically oxidized [2,7,25]. The determination of the ammonia conversion (AC) follows Eq. (2) where n_NH3 ;inlet and n_NH3 ;outlet are the molar flow rates of ammonia at standard conditions [26]. Thermodynamic equilibrium calculations performed with HSC Chemistry 6.0 were used to assess the AC obtained during the experiments. � � n_N H3 ;outlet AC ¼ 1 ⋅100% (2) n_N H3 ;inlet 3.1. Performance evaluation The following analyses demonstrate the comparison of the ammoniabased fuels A and AS with the gases H and HS, which are the expected gas compositions after complete conversion of the ammonia containing Table 2 Single cell voltage measurements at OCV. Gas AI HI ASI HSI

Open circuit voltage in V C01

C02

C03

C04

C05

C06

C07

C08

C09

C10

ACV

1.169 1.168 0.991 0.992

1.190 1.191 0.990 0.990

1.216 1.222 0.990 0.991

1.216 1.222 0.990 0.991

1.232 1.244 0.991 0.992

1.236 1.251 0.990 0.991

1.238 1.254 0.990 0.991

1.244 1.262 0.992 0.993

1.241 1.260 0.990 0.991

1.243 1.265 0.990 0.991

1.223 1.234 0.990 0.991

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with 0.716 slpm steam (ASI) resulted in a reduction in the AC to 99.79%. Doubling the flow rates led to ACs of 99.78% and 99.17% for AII and ASII. These values show the dependence of the flow rate on the ammonia conversion: the higher the flow rate and the flow velocity, the lower the ammonia conversion as the dwell time at the catalytic material de­ creases [6,29]. Polarizing the stack led to significant increases in the ammonia conversion as the hydrogen content and consequently the hydrogen partial pressure is decreased. At 226 mA cm 2 with AI and ASI, the ammonia conversion increased to 99.99% and 99.96%, respectively. The achieved power densities at this point were 187 mW cm 2 and 181 mW cm 2. No significant differences in the maximum power density were observed between the hydrogen- and ammonia-based gases. The gases with the higher flow rates generated 325 mW cm 2, 333 mW cm 2, and 315 mW cm 2 (AII, HII, and HSII) when applying a current density of 452 mA cm 2 (Uf ¼ 80%). With ASII the maximum current density was 435 mA cm 2 (Uf ¼ 76.2%); the first and fifth cells fell below the limiting CV of 0.68 V. As a result, the lowest power density of 307 mW cm 2 could be achieved with ASII. The AC obtained with the two gases with the higher flow rates increased to 99.86% and 99.7% at Uf ¼ 80% and Uf ¼ 76.2%, respectively. The ammonia decomposition is endothermic (see Eq. (1)), which may be advantageous for the system efficiency because less power for the air blower is necessary in order to cool the stack [8,30]. Fig. 3b shows the comparison of the measured temperatures at the middle cell of the stack (C05) at fuel utilizations of 80% and 76.2% in the case of

ASII. It is important to mention that the thermocouples were installed in the air channels and do not reflect the real temperatures of the anode quantitatively, but do show the temperature distribution of the fifth cell along the middle fuel flow channel qualitatively. The comparison of the gases with the lower ammonia flow rate is shown in the left part of Fig. 3b. No significant differences between the ammonia- and hydrogen-based gases are evident; at the fuel inlet (T_C05-IN), the dif­ ferences were 1 K and 1.5 K comparing AI with HI and ASI with HSI, respectively. The exothermic electrochemical utilization of these gases led to heating up of the fuel inlet region to 842.0∘C on average. The measured values T_C05-MID and T_C05-OUT were 841.0∘C and 833.2∘C for all fuels with the subscript I. Doubling the flow rates (subscript II) led to significant changes in the temperature distribution (right side of Fig. 3b). In general, the increase in the flow rate led to a decrease in the measured temperatures, and therefore, forces the cooling of the stack (particularly the increase in the air flow rate from 32 slpm to 60 slpm). Also, the fuel inlet region was significantly cooled when using ammonia as fuel, because of its endothermic catalytically supported decomposi­ tion at the anode surface, contact meshes and interconnects. The tem­ perature difference between AII and HII was 4.0 K, and between ASII and HSII even 18.8 K. The total flow rate significantly influences the ammonia conversion; the higher the flow rate, the lower the tempera­ ture driven ammonia conversion [14,26]. As a result of incomplete ammonia pre-conversion in the fuel heat-up zone, higher quantities of unconverted ammonia reached the fuel inlet regions of the single cells of

Fig. 3. Performance evaluation of ammonia as fuel for MK352 10-cell stack: j,V, j,P and j,AC curves (a), temperature measurements inside the stack at C05 (b) and a. c. stack impedance spectra measured with AI/ASI/HI/HSI (c) and AII/ASII/HII/HSII (d). 5

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the stack in the case of fueling with ASII compared to AII. Consequently, higher ammonia quantities converted at the anode and led to larger temperature drops in this region. The temperature development along the middle fuel flow channel of the cell C05 again showed very similar temperatures at T_C05-MID; the temperatures measured were 835.9� 1.2∘C with all fuels. At the fuel outlet, the gases containing steam revealed slightly higher temperatures, which was due to the higher steam flow rate and consequently higher heat coefficient of the fuel. The temperature measurements shown in Fig. 3b give an idea of the ammonia conversion route in an SOFC stack. While at the lower ammonia flow rates most of the inflowing ammonia quantity is supposed to convert during the gas preheating in the connection pipes and gas distribution, at the double rate of ammonia flow residual ammonia has to be converted at the anode. The catalytically supported ammonia conversion then takes place at the fuel inlet region, where significant temperature drops were evident compared to the equivalent hydro­ gen/nitrogen gas mixtures. These results meet with our previous ex­ periments on ASC-MEAs, where significant cooling of the fuel inlet region was observed, see Ref. [14]. The numerical investigations of Appari et al. [31] demonstrate the preferential conversion of ammonia at the inlet region of a planar SOFC. Looking back to the j,V curves in Fig. 3, the temperature drop at the fuel inlet region is a potential reason for the aforementioned minor differences in the maximum performance between the ammonia-containing fuels AII, ASII and the equivalent hydrogen/nitrogen-containing fuels HII, HSII. The a.c. impedance spectra depicted shown in Fig. 3c and d were recorded at 50 mA cm 2 and 150 mA cm 2 with all investigated gases. The figures (left: lower flow rates; right: higher flow rates) confirm the earlier statements on ammonia conversion in an SOFC. The EIS spectra of the equivalent fuels are nearly superimposed, implying equal opera­ tion conditions between ammonia and the corresponding hydrogen/ nitrogen gases [29]. Note the elevated polarization resistance values when the stack was fueled with the dry gases AI, HI and AII, HII. The significant increase in the polarization resistance occurs mainly in the frequency range between 1.4 Hz and 20 mHz and is attributed to elevated electrochemical gas conversion losses [22,32]. Polarizing the stack with higher current densities (150 mA cm 2, lower diagrams) led to significant decreases in the polarization resistances when using the gases without additional steam. When comparing the impedances recorded with the dry and humidified gas mixtures, the differences are still evident in the low frequency range. These differences occurred because of the inhomogeneous steam distribution along the lengths of the cells. No steam is present at the gas inlet regions of the cells, but the steam content increases in the flow direction as more and more hydrogen is electrochemically oxidized to steam. The numerically determined two-dimensional species profile presented by Schluckner et al. [33] confirms this statement regarding the steam gradient along the flow channel as they show a steady increase of the steam content due to cell polarization along the fuel channel. Insufficient steam partial pressures lead to elevated polarization losses compared to gases that contain steam, and the higher total resistances therefore lead to steeper j,V curve gradients (see Fig. 3a) [22]. Comparing the EIS spectra in Fig. 3c and d, it appears that higher flow rates (gases with subscript II) lead to lower total resistances (real impedance value at 20 mHz) in polarized conditions because of the higher hydrogen partial pressures. On the other hand, cooling of the stack due to the higher flow rates resulted in a slight increase in the ohmic resistances of about 20 mΩ cm2. The aforementioned cooling due to ammonia fueling did not lead to a further increase in the ohmic resistance, as only parts of the MEA-active areas were affected by the endothermic ammonia decomposition; no differences in the ohmic resistances are evident between the ammoniaand hydrogen-containing gases. The total resistances measured by EIS at 150 mA cm 2 of AI, AII, ASI and ASII were 0.93 Ω cm2, 0.81 Ω cm2, 0.80 Ω cm2 and 0.66 Ω cm2. In addition to the stack voltage measurements during the polariza­ tion experiments, the individual cells’ voltages were also recorded.

Fig. 4a shows voltage measurements of the gases with the lower flow rates when applying a current density of 226 mA cm 2 in order to achieve Uf ¼ 80%. According to the j,V curves in Fig. 3a (left) the different fuels investigated resulted in minor differences in the average cell voltages; the highest and lowest ACVs were observed with HI (0.824 V) and ASI (0.803 V), respectively. However, a gradient in cell voltages over the stack height was seen with all fuel mixtures; as a rough generalization, the higher the cell, the higher the CV. According to the OCV measurements (see Table 2), the cells C01–C05 and C06–C10 had lower and higher voltages than the ACV, respectively. This gradient could be largely explained by the temperature gradient over the stack height. The temperature measured at the bottom plate of the stack was 828.4∘C and at the top plate 837.3∘C, when using AI as fuel at 835∘C air outlet temperature and at a current density of 226 mA cm 2. The fifth cell did not follow the generalization mentioned before; at the high fuel utilization of 80%, CV05 was the lowest cell voltage measured during all operation scenarios. According to the test certificate of IKTS Fraunhofer, the reduced performance output of the fifth cell is attributable to the manufacturing process of the MK352 stack. In functional testing at IKTS Fraunhofer, C05 revealed the lowest cell voltage in polarized conditions (276 mA cm 2, Uf ¼ 75%) with Ref as fuel. A.c. impedance measure­ ments at 226 mA cm2 or Uf ¼ 80% of five stack cells are depicted in Fig. 4b and c when fueling with AI and ASI. In both cases the temperature influence of the bottom cell C01 is visible, as a significant increase of the real values of the cell impedance occurred. However, the highest total cell resistances were observed at C05: 1.41 Ω cm2 and 1.23 Ω cm2 were measured with AI and ASI. C10 displayed the lowest total cell re­ sistances: 1.06 Ω cm2 and 0.97 Ω cm2. The other cells were somewhere in between. 3.2. Durability A test over 1000 h in high utilization conditions (80%) with ammonia as fuel was conducted in order to determine the performance stability during a stationary operation in system-relevant operating conditions. As humidified ammonia is the supposed product of a VMD unit, ASI was used as fuel for the following investigations. Fig. 5a shows the power density and temperature measurements during an observa­ tion period of 1000 h, where the current density was set to be constant 226 mA cm 2. The upper part of Fig. 5a shows the power density monitoring of each single cell and the average cell power density, which represents the stack power density divided by 10. From the beginning a steady decrease in the power densities was observable. Over the entire observation period, the degradation was roughly linear, with only minor fluctuations. The stack power density decreased from 181 mW cm 2 to 179 mW cm 2 which corresponds to a power degradation (ΔP/P0) of 1.1%/1000 h. Looking at the power density trends of the individual cells in Fig. 5a, it appears that some cells are not degrading with the stack degradation rate. The highest and lowest power degradation was observed at C01 and C10: the initially measured values were 179 mW cm 2 and 182 mW cm 2 and the power densities after 1000 h were 176 mW cm 2 and 181 mW cm 2 for C01 and C10, respectively. The maximum and minimum degradation rates were therefore 1.7%/1000 h and 0.5%/1000 h, respectively. All of the other cells (C02–C09) decreased with the stack degradation rate with minor deviations: 1.1�0.1%/1000 h. The bottom part of Fig. 5a shows the monitoring of the thermocouples installed in C05, and the air outlet temperature. All temperatures measured were constant during the observation time. The minor fluctuations occurred due to the temperature control and pulsa­ tion of the furnace. The trends of T_C05-IN and T_C05-MID superimpose, they were on the same level of 842�2∘C, whereas T_C05-OUT and T_AirOUT were slightly colder: 835�1∘C. Temperature measurements can help to detect several degradation effects like micro-structural damage, as the temperature in the stack would rise significantly. However, as no significant deviations in the temperature trends are visible, strong degradation effects such as the damage of the membrane or sealing can 6

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Fig. 4. Voltage measurements (a), a.c. impedance spectra with AI (b) and ASI (c) of single cells at 226 mA cm

2

(Uf ¼ 80%).

Fig. 5. 1000 h stability study with ASI as fuel: Power density and temperature monitoring (a), a.c. impedance measurements of C01, C06, C10 (b) and comparison of power degradation rates of MK352 stacks fueled with Ref (blue) and ASI (red) (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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be excluded. The off-gas was analyzed continuously as well. The H2 content in the off-gas was 14.71�0.15 vol% during the entire test period. The continuity of the hydrogen content and temperatures show that external influences on the degradation can be excluded, as no conspicuous behavior was observed. Electrochemical impedance measurements help to define degrada­ tion phenomena during SOFC operation. Fig. 5b show the EIS spectra of the cells C01, C06 and C10 recorded initially (black markers) and after 1000 h (red markers). These are cells with the maximum, an average and the minimum degradation rates, respectively. After 1000 h operation on ASI, the real values of the cell impedance increased significantly, indi­ cating an increase in the ohmic resistance of the cell. The highest in­ crease of 34 mΩ cm2/1000 h in the total resistance was observed at C01. In contrast, the total resistance of the cells C06 and C10 increased by 29 mΩ cm2/1000 h and 23 mΩ cm2/1000 h. These results agree with the power density measurements, as the smallest degradation rate was observed in C10. The average increase of total cell resistances of the MK352 stack was 32 mΩ cm2/1000 h. In order to qualify the degradation behavior of the humidified ammonia fueled MK352 stack, two 10-cell MK352 were tested simul­ taneously at the stack developer’s laboratory. The reference tests were performed with Ref-gas, the applied current density was 276 mA cm 2, the fuel utilization was 75% and the test bench furnace was temperature controlled by the air outlet temperature (835∘C). Fig. 5c shows the comparison of the degradation rates of the MK352 stacks fueled with ASI (red trends) and Ref (blue trends) over the observation period of 1000 h. All stacks exhibited similar degradation behavior: after an initial decrease the degradation rates increased to a maximum. Subsequently, the degradation rates leveled off, or even underwent a slight decrease with increasing operation time. It appeared that the stack operated with ASI did not lead to elevated degradation rates during the initial 1000 h, as similar degradation rates were observed with Ref and ASI. The operation with humidified ammonia resulted in a degradation rate of 1.1%/1000 h, which was situated between the two Ref-gas supplied stacks (MK352ref 1: 1.3%/1000 h; MK352ref 2: 0.9%/1000 h). To conclude, the similarity in the degradation rates shows the very good

usability of humidified ammonia as fuel for an MK352 stack. Further­ more, the degradation rate may even decrease with the time, as Megel et al. [20] reported about an 20,000 h MK352 duration test with an overall degradation rate of only 0.7%/1000 h. 3.3. Post-mortem analyses After the operation on ammonia the furnace was cooled down and the individual stack components were analyzed by means of SEM and EDX. Our previous experiments on large planar SOFC-MEAs assembled in a ceramic cell housing revealed significant changes and impairments of the nickel components due to high temperature corrosion in form of nitridation, see Refs. [14,15]. In order to evaluate the micro-structural changes after the 1000 h operation with 80% utilized ASI gas, the stack was broken up and the single components were analyzed in detail. Fig. 6 presents post-mortem scanning electron microscopy analyses of the anode-relevant parts of the second and seventh cells of the stack. These cells were chosen to be representative cells, as their degradation rates match the stack degradation rate of 1.1%/1000 h. The SEM images depicted in Fig. 6a and c shows cross-sectional and surface views of the anodes at the fuel inlet and outlet region. Comparing the micro-structure of the fuel inlet and outlet region, no significant differences in the Ni parts are recognizable. The Ni grains do not seem to be influenced by the ammonia fueling, no hints of nitriding effects due to ammonia’s catalytic supported decomposition are verifiable. Note the three anode layers with the different Ni and GDC contents in Fig. 6a and c. The closer to the electrolyte, the higher the GDC content and the smaller the Ni parts and content, respectively. However, the anode components are well distributed and exhibit therefore no hints to a degradation forcing phenomena here. Nickel meshes ensured the electrical contact and current distribution between the bipolar plates and the anodes in the MK352 stack. In Fig. 6b and d the two layers of Ni meshes are shown. The fine mesh was situated at the bipolar plates and the coarser meshes were used to contact the MEA anodes. The impact of the catalytic ammonia conversion on nickel material is shown impressively in Fig. 6b. This figure shows the Ni

Fig. 6. Backscattered scanning electron microscopy images of anodes (a,c) and nickel contact meshes (b,d) of the fuel inlet (a,b) and fuel outlet region (c,d) taken from the second cell. 8

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Journal of Power Sources 450 (2020) 227608

meshes surface (left) and a cross sectional view (right) of the meshes installed at the fuel inlet region. Compared to the nickel wires at the fuel outlet region depicted in Fig. 6d, the material structure of the wires significantly changed at the fuel inlet. The formation of microscopic pores and the slight increase in volume are indicative of nitridation processes in a hot ammonia atmosphere [7,11,13]. Ammonia is decomposed on the catalyst surface and the N–H bond cleaves. The atomic nitrogen binds with nickel and its subsequent reduction in the hot or reducing atmosphere causes the mentioned impairments of the nickel parts [7,13]. In the right part of Fig. 6b, the cross-sectional view of the wires, the presence of gas enclosures and pores is demonstrated. The originally solid material is perforated and therefore weakened. As a result, the material expands and stress cracks appear at the grain boundaries of the nickel wires. The mesh wires of the fuel outlet region shown in Fig. 6d were in a very different condition than the wires taken from the fuel inlet. No significant material deterioration was evident; no microscopic pores or cracks were found here. The material of the wires was still solid, and not significantly different from its initial state. After our experiments with the same normalized ammonia flow rates with MEAs installed in a ceramic cell housing [15], a similar kind of nickel material deterioration was found, in fact to a greater extent despite a much shorter period of ammonia fueling (100 h vs. 1000 h). The inflowing gas was preheated in metallic, stainless steel pipes at the stack test rig but in ceramic pipes at the single cell test rig. As stainless steel supports the ammonia conversion to hydrogen and nitrogen catalyti­ cally, higher ammonia quantities are already decomposed when entering the flow chambers of the anode compared to the preheating in ceramic material [34]. The impact of direct ammonia fueling on the CFY bipolar plates is shown in Fig. 7. CFY stands for chromium iron yttrium, which is the main component of the bipolar plates used in the MK352 stack [18]. EDX mapping was used to detect the element distribution near the fuel gas channels. The images in Fig. 7 were taken from the gas outlet region of the seventh cell. The images recorded from the gas inlet region are not presented, as they showed similar element distribution, and, therefore, it seems reasonable to conclude that the micro-structural changes of the bipolar plates were evenly distributed along the fuel channels. The four images show the SEM image and the element distributions of chromium (Cr), nitrogen (N), and iron (Fe). Superimposing the maps of Cr and N, the patterns of intensities are very similar, indicating that the released nitrogen from the ammonia conversion process binds to chromium and forms chromium nitrides (CrN, Cr2N). The formation of chromium nitride is possible at temperatures higher than 700∘C and the shares of CrN and Cr2N are temperature dependent [35]. Further, the EDX map­ ping did not reveal any accordance between nitrogen and iron. For comparison see Ref. [36], where EDX maps of CFY interconnects are shown after purging CFY samples for 600 h with 3 vol% moistened H2/N2 (80/20 vol%) at 875∘C. In contrast to the EDX maps revealed after the SOFC experiments with ammonia of the present study, Folgner et al. [36] found regions of accordance in intensity of the elements chromium, nitrogen and iron, indicating formation of an Fe–Cr–N phase in contrast to the nitridation of chromium only when using ammonia as fuel. Also the impacted depth was significantly lower; the Fe–Cr–N phase was found only at the surface of the bipolar plate (<25 μm), the chromium

nitrides however, were found in deeper regions, as can be seen in Fig. 7a. The atomic nitrogen from ammonia seems to diffuse into deeper regions of the interconnects and binds to chromium. Li et al. [35] describe the reaction processes between chromium and nitrogen in hot ammonia atmosphere by means of a diffusion model: after the initial adsorption and reaction of nitrogen and chromium on the surface (thin film of CrN), the atomic nitrogen diffuses further into the chromium particles and forms Cr2N as an intermediate product because of the shortage of additional nitrogen atoms. However, the formation of chromium ni­ trides can be ruled out as the process driving degradation, because the stacks fueled with humidified ammonia and hydrogen/nitrogen had comparable degradation rates. During the 1000 h test at 226 mA cm 2 and 80% utilization of ASI, the EIS measurements revealed a steady increase in the ohmic resis­ tance. When hydrogen is used as the main fuel component, the oxidation of the interconnect materials is the primary process driving performance degradation [36]. Therefore, we investigated the oxidation state of the interconnects and the boundary-nickel layers. In the original state, a nickel layer is situated on the CFY interconnects, in order to ensure sufficient electrical contact between the nickel contact meshes and the

Fig. 8. Backscattered SEM image (a) and EDX spectra (b) of the Ni contact layer, oxide layer and CFY interconnect.

Fig. 7. EDX mapping of the fuel outlet region of the CFY bipolar plate of C07 after the direct operation with ammonia based fuels. 9

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bipolar plates. The SEM images in Fig. 8 confirm the formation of an oxide layer on the CFY material. Backscattered SEM enable the locali­ zation of different materials, which appear in backscattered SEM images in varied gray tones, as can be seen in Fig. 8a. Fig. 8b shows the EDX spectra of the marked spots in Fig. 8a in order to detect the elements present in the single layers. Spot 1 is on the nickel contact layer which ensures the electrical contact to the nickel mesh and consequently to the anode. EDX detected nickel as the dominant component in this layer. Underneath the contact layer a dark gray layer is visible, which mainly consists of chromium oxide, based on the EDX spectra of spot 2. The light gray area around Spot 4 shows the interconnect material that consists mainly of chromium. The slightly darker regions are the nitrided re­ gions, see spot/EDX spectra 3. To estimate the oxide layer thickness, an array of 20 single SEM images was taken from gas inlet and outlet region of the seventh layer. The mean value of the arithmetic average of the oxide layer thickness of gas inlet and outlet was 5.7 μm. It is well known that oxide layers increase the resistance of a electric contact, and therefore, the aforementioned increase in the ohmic resistance during the 1000 h testing with ASI at 226 mA cm 2 (Uf ¼ 80%) can be attributed to the formation of the chromium-oxide layer, similar to when hydrogen-based fuels are used as fuel for a stack with CFY interconnects [36]. In their study, Folgner et al. [36] investigated the oxidization behavior of various interconnect materials in reducing atmosphere and obtained a median oxide thickness of 6.2 μm on their CFY plates when purging them with H2/H2O ¼ 50/50 vol% for 1000 h. The small dif­ ference of 0.5 μm can be attributed to the different operation tempera­ tures: 875∘C vs. 835∘C. However, the similarity in the oxide layer thickness confirms that the oxidation process was not influenced by using ammonia instead of hydrogen as fuel. The results of the post-mortem analyses link the degradation processes to connecting and gas separating parts of the stack, but not to the electrochemically active parts. This interpretation is supported by our previous investigations on single ESC-MEAs in a ceramic cell housing where no voltage degradation effects were observed, although micro-structural deteriorations appeared [15].

Following the promising electrochemical investigations, postmortem analyses of the relevant stack parts were carried out using SEM and EDX. The functional layers of the MEAs did not exhibit any degradation phenomena. Ni as well as GDC were distributed well and no negative impact of ammonia fueling was found there. In contrast, the Ni contact meshes revealed nitridation effects, manifesting as microscopic pores, gas enclosures, stress cracks and slight increases in the wires’ diameters. EDX detected formation of chromium nitrides in the CFY bipolar plates; the catalytic ammonia conversion at the CFY in­ terconnects led to nitrogen diffusion into the chromium and subse­ quently to the formation of chromium nitrides. The main fuel-sided process promoting performance degradation, however, was the forma­ tion of an oxide layer between the interconnects and the contact layers, which builds-up when using humidified ammonia as well as humidified hydrogen-based fuels. Acknowledgment This work was supported by the Austrian Research Promotion Agency (FFG) [grant number 858839]. References [1] G. Cinti, U. Desideri, SOFC fuelled with reformed urea, Applied Energy 154 (2015) 242–253, https://doi.org/10.1016/j.apenergy.2015.04.126, 03062619. [2] A. Fuerte, R.X. Valenzuela, M.J. Escudero, L. Daza, Ammonia as efficient fuel for SOFC, Journal of Power Sources 192 (1) (2009) 170–174, https://doi.org/ 10.1016/j.jpowsour.2008.11.037, 03787753. [3] N. Maffei, L. Pelletier, J.P. Charland, A. McFarlan, An ammonia fuel cell using a mixed ionic and electronic conducting electrolyte, Journal of Power Sources 162 (1) (2006) 165–167, https://doi.org/10.1016/j.jpowsour.2006.06.056, 03787753. [4] M. Ni, D.Y. Leung, M.K. Leung, Electrochemical modeling of ammonia-fed solid oxide fuel cells based on proton conducting electrolyte, Journal of Power Sources 183 (2) (2008) 687–692, https://doi.org/10.1016/j.jpowsour.2008.05.018, 03787753. [5] Q. Ma, J. Ma, S. Zhou, R. Yan, J. Gao, G. Meng, A high-performance ammoniafueled SOFC based on a YSZ thin-film electrolyte, Journal of Power Sources 164 (1) (2007) 86–89, https://doi.org/10.1016/j.jpowsour.2006.09.093, 03787753. [6] A.F.S. Molouk, J. Yang, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi, Electrochemical and catalytic behavior of Ni-based cermet anode for ammoniafueled SOFCs, ECS Transactions 68 (1) (2015) 2751–2762, https://doi.org/ 10.1149/06801.2751ecst, 1938-6737. [7] J. Yang, A.F.S. Molouk, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi, A stability study of Ni/Yttria-Stabilized zirconia anode for direct ammonia solid oxide fuel cells, ACS applied materials & interfaces 7 (51) (2015), https://doi.org/10.1021/ acsami.5b11122, 28701–28707, ISSN 1944-8252. [8] G. Cinti, U. Desideri, D. Penchini, G. Discepoli, Experimental analysis of SOFC fuelled by ammonia, Fuel Cells 14 (2) (2014) 221–230, https://doi.org/10.1002/ fuce.201300276. [9] A.F.S. Molouk, J. Yang, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi, Comparative study on ammonia oxidation over Ni-based cermet anodes for solid oxide fuel cells, Journal of Power Sources 305 (2016) 72–79, https://doi.org/ 10.1016/j.jpowsour.2015.11.085, 03787753. [10] M. Kishimoto, N. Furukawa, T. Kume, H. Iwai, H. Yoshida, Formulation of ammonia decomposition rate in Ni-YSZ anode of solid oxide fuel cells, International Journal of Hydrogen Energy 42 (4) (2017) 2370–2380, https://doi. org/10.1016/j.ijhydene.2016.11.183, 03603199. [11] G.Y. Lai, High-temperature Corrosion and Materials Applications, ASM International, Ohio, 2007. [12] H.A. Wriedt, The N-Ni (Nitrogen-Nickel) system, Bulletin of Alloy Phase Diagrams 6 (6) (1985) 558–563, https://doi.org/10.1007/BF02887159, 0197-0216. [13] A.-M. Alexander, J.S.J. Hargreaves, C. Mitchell, The reduction of various nitrides under hydrogen: Ni3N, Cu3N, Zn3N2 and Ta3N5, Topics in Catalysis 55 (14–15) (2012) 1046–1053, https://doi.org/10.1007/s11244-012-9890-3, 1022-5528. [14] B. Stoeckl, V. Suboti�c, M. Preininger, M. Schwaiger, N. Evic, H. Schroettner, C. Hochenauer, Characterization and performance evaluation of ammonia as fuel for solid oxide fuel cells with Ni/YSZ anodes, Electrochimica Acta 298 (2019) 874–883, https://doi.org/10.1016/j.electacta.2018.12.065, 00134686. [15] B. Stoeckl, M. Preininger, V. Suboti�c, C. Gaber, M. Seidl, P. Sommersacher, H. Schroettner, C. Hochenauer, High utilization of humidified ammonia and methane in solid oxide fuel cells: an experimental study of performance and stability, Journal of The Electrochemical Society 166 (12) (2019), https://doi.org/ 10.1149/2.0781912jes. F774–F783, ISSN 0013-4651. [16] N.J.J. Dekker, G. Rietveld, Highly efficient conversion of ammonia in electricity by solid oxide fuel cells, Journal of Fuel Cell Science and Technology 3 (4) (2006) 499, https://doi.org/10.1115/1.2349536. [17] C. Bienert, M. Brandner, S. Skrabs, A. Venskutonis, L.S. Sigl, S. Megel, W. Becker, N. Trofimenko, M. Kusnezoff, A. Michaelis, CFY-stack technology: the next design,

4. Conclusion Ammonia seems to be a promising fuel for SOFCs, because it can be recovered from wastewater, among other sources. This is confirmed by the experiments on a 10-cell MK352 SOFC stack with CFY interconnects. The inflowing ammonia was converted to hydrogen and nitrogen with conversion rates close to the thermodynamic limits: at open circuit conditions up to 99.98% and in polarized conditions at 226 mA cm 2 (Uf ¼ 80%) up to 99.99% as a result of the reduced hydrogen partial pressure at this operation point. Temperature analyses at high fuel uti­ lization demonstrate the impact of ammonia on the temperature distri­ bution obtained from temperature measurements at the fifth stack cell. Significant cooling by as much as 18.8 K due to the endothermic ammonia conversion was observed at high ammonia flow rates. The cooling effect, however, is limited to the fuel inlet region, and, therefore no significant increases in the ohmic resistances of the stack were observed. An 1000 h durability test with humidified ammonia in 80% fuel utilized conditions was conducted in order to evaluate the stability of the MK352 stack. The individual cell power densities and consequently the entire stack power density decreased quasi-linearly over time. The degradation rate of the individual layers varied from 0.5%/1000 h (top layer) to 1.7%/1000 h (bottom layer). The stack degradation rate as the median degradation rate of the individual layers amounted to 1.1% per 1000 h. A.c. impedance measurements identified the increase in the ohmic resistance as the main reason for the power reduction during the evaluation period. Comparative tests under hydrogen/nitrogen condi­ tions conducted on identical MK352 stacks resulted in comparable degradation rates and demonstrate the excellent usability of humidified ammonia for ESC stacks with CFY interconnects. 10

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