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Article Direct Solid Electrolysis of Dioxide: Analysis of and Processes

1, 1,2, 1 1 1,2 Severin Foit † , Lucy Dittrich †, Tobias Duyster , Izaak Vinke , Rüdiger-A. Eichel and L. G. J. (Bert) de Haart 1,*

1 Institute of and Research, Fundamental (IEK-9), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; [email protected] (S.F.); [email protected] (L.D.); [email protected] (T.D.); [email protected] (I.V.); [email protected] (R.-A.E.) 2 Institute of Physical , RWTH Aachen University, 52074 Aachen, Germany * Correspondence: [email protected] Authors with equal contribution. †  Received: 1 October 2020; Accepted: 26 October 2020; Published: 31 October 2020 

Abstract: Chemical industries rely heavily on resources for the production of carbon-based chemicals. A possible transformation towards is the usage of carbon dioxide as a source of carbon. Carbon dioxide is activated for follow-up reactions by its conversion to . This can be accomplished by electrochemical reduction in solid oxide cells. In this work, we investigate the process performance of the direct high-temperature CO2 electrolysis by current-voltage characteristics (iV) and Electrochemical Impedance (EIS) experiments. Variations of the operation parameters temperature, load, utilization, feed ratio and flow rate show the versatility of the procedure with maintaining high current of 0.75 up to 1.5 A cm 2, therefore resulting in high conversion rates. The potential of the high-temperature carbon · − dioxide electrolysis as a suitable enabler for the activation of CO2 as a chemical feedstock is therefore appointed and shown.

Keywords: carbon dioxide; solid oxide electrolysis; carbon dioxide utilization; CO2-electrolysis; high-temperature electrolysis; carbon dioxide reduction

1. Introduction The world-wide chemical industry is currently depending on fossil resources for the production of carbon based chemicals. To reach full sustainability in the sector of chemical industry, the necessity of circular carbon economy is evident. This is not possible by exploiting fossil resources. An abundant compound containing carbon is carbon dioxide (CO2). Even in a scenario of 100% generation, emissions of carbon dioxide by processes within the chemical sector is in the range of multiple hundred million metric megatons per year [1]. As CO2 has a low energetic value and is therefore rather inert, it has to be activated to be used as a starting material in value-chains of the chemical industry. Carbon monoxide (CO) is already a bulk chemical in industrial chemistry. It is used as a building block in polymers like e.g., and , but can also be transformed to a vast range of products as the carbon containing part in , a mixture of carbon monoxide and [2–7]. Hence, a sustainable conversion of carbon dioxide to carbon monoxide is enabling the processes of carbon capture and utilization (CCU) and power-to-x [8]. Using renewable energy, the electrochemical reduction of CO2 to CO is an interesting and seemingly simple process for the of the above mentioned challenges. This reaction has been examined both for low-temperature and high-temperature procedures with vast differences in performance, efficiencies and general operation parameters [9]. The high-temperature electrolysis of

Processes 2020, 8, 1390; doi:10.3390/pr8111390 www.mdpi.com/journal/processes Processes 2020, 8, 1390 2 of 14

CO2 to CO on ceramic solid oxide cells seems superior in these regards with a faradaic efficiency of 100% and overall energetic efficiencies of over 75%. This conversion has been attempted mostly as a part of syngas production by using and carbon dioxide simultaneously as starting feeds [10], which is not a direct electrolysis of carbon dioxide in particular. Concerning the electrolysis of pure carbon dioxide, a few aspects have been viewed at, however with a focus on materials [11–18]. We present the process investigation of the variation of different operation parameters temperature, load, fuel utilization, feed gas ratios and flow rates of the high-temperature solid oxide electrolysis of carbon dioxide to carbon monoxide.

2. Materials and Methods The commercially available single button cells (Elcogen®, Tallinn, Estonia) used in the following experiments consist of a cathode-supported porous NiO-8YSZ-cermet fuel cathode, a 8%-Yttria-stabilized zirconia (8YSZ) , a -doped ceria (GDC) and a porous La0.6Sr0.4CoO3-δ (LSC) anode. The cathode, electrolyte and GDC layer have a diameter of 20 mm and a totalled thickness of 300 30 µm. The anode has a diameter of 10 mm and a thickness of ± 15 5 µm, giving an active area of 0.79 cm2 ± The single-cells are installed in a two-electrode conductivity & impedance spectroscopy Probostat-setup (Norwegian Electro Cermaics® NORECS, Oslo, Norway). A gold ring is used as a sealing sample between the anodic side of the cell and the inner ceramic to separate the air and fuel side. The current collector on the fuel side consists of a and a platinum mesh. On the air side, only platinum is used for the current collection. The setup is enclosed and heated up with a furnace from the company Carbolite Gero®(Neuhausen, Germany). The heating rate is kept at 2 C min 1 throughout all the measurements to protect the materials from thermomechanical ◦ · − stress. The reduction of the fuel electrode from NiO-YSZ to Ni-YSZ to activate the catalyst properties of Nickel is carried out at 900 ◦C with a stepwise increase of hydrogen concentration in the fuel feed gas starting at 0% up to 100% with as balance gas. In this work, we have chosen the temperatures 800 and 850 ◦C for the investigation of effects of variating operation parameters. This is a suitable compromise, by which cell performance and material stability can be guaranteed. Besides, a temperature series between 700–900 ◦C in equivalent 1 1 T− /K− -steps with the volumetric fuel composition of 80:20 of CO2:CO has been performed to apply the Arrhenius equation. The supply of the H2,N2, CO2, CO (fuel side) and air (air side) is controlled by mass flow controllers (MFC, Bronkhorst Nord®, Kamen, Germany). A constant volume flow of 6 L h 1 · − is applied to both gas chambers, when not specified differently. Current-voltage characteristics (iV) and Electrochemical Impedance Spectroscopy (EIS) experiments are managed via a Vertex 5A Potentiostat/Galvanostat (Ivium Technologies®, Eindhoven, ) using a two-electrode system. The cell performance is measured consecutively twice by applying a differential current with a scan rate of 10 mA s 1 from open cell voltage (OCV) up to 1.4 V. The area-specific resistance (ASR) is · − calculated using the numerical differential of the iV curves at OCV and 350 mA cm 1. · − The process analysis of the cell is determined by AC characterization at OCV and 350 mA cm 2. · − The spectra are recorded in a frequency range from 110 kHz to 0.11 Hz with 73 measuring points and an applied amplitude of 20 mA.

3. Results

3.1. Performance

The performance of high-temperature CO2-electrolysis was studied by dc current-voltage measurements. The resulting iV curves for various CO2 partial , temperatures and flow rates are shown in Figure1a–c, respectively. Processes 2020, 8, 1390 3 of 14

Processes 2020, 8, x FOR PEER REVIEW 3 of 14

1.4 1.4

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1.2 1.2 700 °C 717 °C 734 °C

E / V / E 1.1 V / E 1.1 752 °C 771 °C 50 CO 791 °C 1.0 2 1.0 811 °C 60 CO2 70 CO 832 °C 0.9 2 853 °C 80 CO 0.9 2 874 °C 90 CO2 900 °C 0.8 0.8 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 -2 -2 |i|normalized / A·cm |i| / A·cm normalized (a) (b)

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1.0 l·h-1 E / V / E 1.1 1.5 l·h-1 2.0 l·h-1 1.0 2.5 l·h-1 3.0 l·h-1 -1 0.9 4.0 l·h 5.0 l·h-1 6.0 l·h-1 0.8 0.0 0.5 1.0 1.5 |i| / A·cm-2 normalized (c)

-1 FigureFigure 1. iViV curves curves at at various various operating operating parameters: parameters: (a) ( avariation) variation of CO of CO2:CO2:CO ratio ratio at 800 at 800°C and◦C and6 l·h 6; 1 1 L(bh) variation;(b) variation of temperature of temperature for a ratio for of a ratio CO2:CO of CO of 80:20:CO ofat 80:206 l·h −1 at; (c 6) Lvariationh ;(c) of variation flow rate of for flow a ratio rate · − 2 · − forof CO a ratio2:CO of of CO 80:202:CO at of800 80:20 °C. at 800 ◦C.

TheThe variationvariation ofof COCO22 partial pressurepressure waswas balancedbalanced byby CO.CO. ThatThat meansmeans reducingreducing COCO22 partialpartial pressurepressure waswas accompaniedaccompanied byby increasingincreasing COCO andand vicevice versa.versa. TheThe obtainedobtained areaarea specificspecific resistancesresistances (ASR)(ASR) and current current densities densities at at 1.4 1.4 V V(i1.4 (i V1.4) for V) variation for variation of CO of2 partial CO2 partial pressure are presented are presented in Figure in Figure2a (values2a (values in Table in Table A1). A1For in increasing Appendix theA). ForCO2 increasing , the CO the2 partial ASR pressure,at OCV increases the ASR atwith OCV a increases with a factor of 1.6 from 0.34 Ω cm2 at 50% CO to 0.56 Ω cm2 at 90% CO . The increase in factor of 1.6 from 0.34 Ω·cm² at 50% CO2 to· 0.56 Ω·cm² at 90%2 CO2. The· increase in ASR2 also increased ASRwith alsoeach increased 10% stepwith of CO each2 partial 10% steppressure. of CO From2 partial 50% pressure. to 60% it From only 50%increases to 60% by it 0.02 only Ω·cm², increases while by 2 2 2 0.02 Ω cm , while from 80% to 90% the increase is 0.13 Ω -cm2 . At 0.35 A cm , however, the trend is from 80%· to 90% the increase is 0.13 Ω·cm². At 0.35 A·cm· , however, the· trend− is opposite of the one oppositeobserved ofat theOCV. one With observed increasing at OCV. the With CO2 increasingpartial pressure the CO, the2 partial ASR pressure,decreases. the Analog ASR decreases.to before, Analog to before, there is a factor of 1.6, only this time from 0.42 Ω cm2 at 90% CO to 0.67 Ω cm2 at there is a factor of 1.6, only this time from 0.42 Ω·cm² at 90% CO· 2 to 0.67 Ω·cm²2 at 50% CO· 2. The 50%current CO density2. The currentat 1.4 V densityincreases at with 1.4 V increasing increases CO with2:CO increasing ratio with CO a 2very:CO similar ratio with increase a very of similar 0.08 or 2 increase of-2 0.08 or 0.09 A cm for each 10 percentage point step of partial pressure. The experimentally 0.09 A·cm for each 10 percentage· − point step of partial pressure. The experimentally determined OCV determinedis comparable OCV to the iscomparable theoretical OCV, to the which theoretical is calculated OCV, which by the is Nernst calculated equation. by the Deviations Nernst equation. are up Deviationsto 1%. are up to 1%.

Processes 2020, 8, x FOR PEER REVIEW 4 of 14 Processes 2020, 8, 1390 4 of 14

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ASR0.35A·cm-2 0.4 1 0.2 ASR 0.35A·cm-2

|i|1.4V 1 |i| 1.4V 0.0 0.0 50 60 70 80 90

p(CO2) / %

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|i|1.4 V 0.0 0.0 0 1 2 3 4 5 6 7 flow rate / l·h-1

(c)

Figure 2. (a) ASR, |i|1.4 V and OCV for various ratios of CO :CO at 800 C. (b) ASR, |i|1.4 V and OCV Figure 2. (a) ASR, |i|1.4 V and OCV for various ratios of CO22:CO at 800 ◦°C. (b) ASR, |i|1.4 V and for various temperatures for the ratio of CO :CO of 80:20. (c) ASR, |i|1.4 V and OCV for varied flow OCV for various temperatures for the ratio of CO2 2:CO of 80:20. (c) ASR, |i|1.4 V and OCV for varied rate for a CO :CO ratio of 80:20 at 800 C. 1 Two values are given because of the hysteresis. The first flow rate for a CO2 2:CO ratio of 80:20 at 800◦ °C. 1 Two values are given because of the hysteresis. The firstdenotes denotes the the forward forward scan scan and and the the second second the the backward backward scan scan of of the the iV iV curve. curve.

Processes 2020, 8, 1390 5 of 14

The variation of temperature was performed at a gas composition of 80% CO2 + 20% CO at temperatures in the range of 700–900 ◦C. The respective ASRs and i1.4 V are given in Figure2b (values in Table A2) showing decreased ASR and increased i1.4 V with increased temperature. The decrease in ASR is steeper at OCV than at 0.35 A cm 2. Moreover, the ASR increases at 700 C going from OCV to · − ◦ 0.35 A cm 2, while it decreases at 900 C. The transition from this increase to decrease is at around · − ◦ 734 ◦C, where both values of ASR are the same. The OCV follows the trend given by the Nernst equation and decreases with increasing temperature. For an investigation of the , flow rates were varied in the range of 1–6 L h 1. · − The obtained ASRs do not vary significantly, while the current at 1.4 V increases by 0.14 A cm 2 · − with increasing the flow rate from 1 L h 1 to 6 L h 1 (Figure2c, Table A3). For the OCV it can be · − · − observed that for flow rates below 3 L h 1 the initial OCV and the one measured after an iV curve · − cycle increasingly differ. At 3 L h 1 the difference is only 10 mV. However, at 1 L h 1 it is already a · − · − deviation of 30 mV.

3.2. Impedance Analysis Electrochemical impedance spectra were taken for the same parameter variations as discussed for the performance analysis. Additionally, the comparison at variation of current is performed. The spectra at various current densities, CO2:CO ratios, temperatures and flow rates are shown in Figure3a–d, respectively. The variation of current density in Figure3a shows increased total resistance at 700 ◦C resulting from a huge increase in high and middle frequency processes. At 900 ◦C the total resistance is first decreasing with current and starts increasing at about 0.35 A cm 2. Compared to the spectra at 700 C · − ◦ the increase in resistance is mainly caused by low frequency impedance contributions. In both cases the ohmic resistance is constant over the whole current density range. Especially the low frequency process is shifted to lower frequencies with increased current density causing the semicircle to be pushed out of the measurement range and only showing half of it in the spectra. The CO :CO ratio causes different trends at OCV and 0.35 A cm 2 as can be seen in Figure3b. 2 · − At OCV the total resistance increases strongly with increasing the CO2:CO ratio. The major contribution is from the low frequency impedance parts. At 0.35 A cm 2 the trend is opposite: an increased CO :CO · − 2 ratio causes the impedance to decrease. In this case, the ratio also influences the middle frequency region of the impedance. With increased CO2:CO ratio the low frequency process is also shifted towards higher frequencies and more into the measured frequency range. The variation of temperature in Figure3c is the first measurement to also influence the ohmic resistance. With increased temperature it is decreasing. Similarly, the polarization resistance is decreasing with increased temperature. The main contributions are from the high and middle frequency impedances. The low frequency process is shifted as well with variation of temperature. Higher temperatures shift it to higher frequencies and into the measurement range. The last process parameter that was investigated by impedance spectroscopy is the flow rate. Here, the behavior is again different at OCV compared to 0.35 A cm 2. At OCV the flow rate shows no · − significant influence on the spectra, while at 0.35 A cm 2 a slight decrease of middle and low frequency · − impedance can be observed for increased flow rate. Processes 2020, 8, 1390 6 of 14 Processes 2020, 8, x FOR PEER REVIEW 6 of 14

0.20 0.15 0.1

] ] 0.10

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W 0.05 CO :CO

-Z'' [ -Z'' -Z'' [ 0.0 2 0.00 50:50 80:20 60:40 90:10 -0.05 70:30 -0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Z' [W] Z' [W] |i| / A·cm-2 0 0.1 0.2 0.3 0.5 0.20 0.15 0.1

] 0.10

]

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-Z'' [ CO :CO -Z'' [ -Z'' 0.0 2 0.00 50:50 80:20 60:40 90:10 -0.05 70:30 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -0.1 Z' [W] 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Z' [W] |i| / A·cm-2 0 0.1 0.2 0.3 0.5 0.7 0.9 (a) (b)

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0.00 W

-Z'' -Z'' [ -1 -1 0.00 1.0 l·h 3.0 l·h -0.05 -1 -1 -Z'' -Z'' [ 1.5 l·h 4.0 l·h -0.05 -1 -1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 2.0 l·h 5.0 l·h Z' [W] 2.5 l·h-1 6.0 l·h-1 -0.10 T / °C 0.1 0.2 0.3 0.4 0.5 0.6 0.7 700 717 734 752 771 791 811 832 853 874 901 Z' [W]

0.10 0.10

]

W 0.05 0.05

0.00 ]

-Z'' -Z'' [

W -1 -1 0.00 1.0 l·h 3.0 l·h -0.05 -1 -1

-Z'' -Z'' [ 1.5 l·h 4.0 l·h 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -0.05 2.0 l·h-1 5.0 l·h-1 Z' [W] 2.5 l·h-1 6.0 l·h-1 T / °C -0.10 700 717 734 752 771 791 811 832 853 874 901 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Z' [W] (c) (d)

FigureFigure 3.3. EISEIS spectraspectra atat variousvarious operatingoperating parameters:parameters: ((aa)) variationvariation ofof currentcurrent densitydensity forfor aa ratioratio ofof 1 COCO22:CO:CO ofof 80:2080:20 atat 6 6 L l·hh −1 atat 700 700 °C◦C (top) (top) and and 900 °C◦C (bottom); (b) variationvariation ofof COCO22:CO:CO ratioratio atat 800800◦ °CC 1 · 2 andand 66L l·hh−1 atat OCV OCV (top) (top) and 0.35 AA·cmcm−2 (bottom);(bottom); ( (cc)) variation variation of temperature forfor aa ratioratio ofof CO CO22:CO:CO · 1 · 2 ofof 80:2080:20 at 6 l·h L h−1− at OCVat OCV (top) (top) and and 0.35 0.35 A·cm A−2cm (bottom)− (bottom);; (d) variation (d) variation of flow of rate flow for rate a ratio for of a ratioCO2:CO of · · 2 COof 80:202:CO at of 800 80:20 °C at at 800 OCV◦C (top) at OCV and (top) 0.35 andA·cm 0.35−2 (bottom) A cm− .(bottom). · 4. Discussion 4. Discussion 4.1. Performance 4.1. Performance The electrolysis performance can be defined as the amount of product produced per time unit. The electrolysis performance can be defined as the amount of product produced per time unit. This is given by the current or current density as the current defines the absolute converted amount of This is given by the current or current density as the current defines the absolute converted amount reactant per second according to Faraday’s law. For a high performance the current density should be of reactant per second according to Faraday’s law. For a high performance the current density should as high as possible. be as high as possible. As can be seen from the iV curves in Figure1, the current density generally increases with increasing As can be seen from the iV curves in Figure 1, the current density generally increases with the voltage. However, increasing the voltage above 1.4 V to severe and fast degradation according increasing the voltage. However, increasing the voltage above 1.4 V leads to severe and fast2 to literature [3] and our own previous studies. Compared to the experiments reaching up to 1.5 A cm− degradation according to literature [3] and our own previous studies. Compared to the experiments· at 1.4 V seen in Figure1, experiments going up to 1.8 V (even for as short as 15 min), the correspondent −2 reaching up to 1.5 A·cm at 1.4 V seen in Figure 1, experiments going2 up to 1.8 V (even for as short current density at 1.4 V was permanently reduced to below 0.1 A cm− . As measure for the performance as 15 min), the correspondent current density at 1.4 V was permanently· reduced to below 0.1 A·cm−2. it is therefore the current density at 1.4 V that is compared. As measure for the performance it is therefore the current density at 1.4 V that is compared. However, due to the non-linearity at higher current density caused by limitations it might be more suitable to perform the electrolysis at lower voltages, as the benefit of higher currents do not justify the increased power input.

Processes 2020, 8, 1390 7 of 14

However, due to the non-linearity at higher current density caused by transport limitations it might be more suitable to perform the electrolysis at lower voltages, as the benefit of higher currents doProcesses not justify2020, 8, x the FOR increased PEER REVIEW power input. 7 of 14 Another measure for the performance of a cell is the area specific resistance, which is a measure for Another measure for the performance of a cell is the area specific resistance, which is a measure the energy loss of the system. It includes the loss from multiple processes, which can be deconvoluted for the energy loss of the system. It includes the loss from multiple processes, which can be in impedance spectroscopy. deconvoluted in impedance spectroscopy. The analysis of the performance at different process parameters show the highest performance The analysis of the performance at different process parameters show the highest performance1 concerning the current density at 1.4 V for 90% CO2 + 10% CO, 900 ◦C and a flow rate of 6 L h− . concerning the current density at 1.4 V for 90% CO2 + 10% CO, 900 °C and a flow rate of 6 l·h· −1. Reducing the amount of CO2 results in less available reactant and this leads to earlier mass transport Reducing the amount of CO2 results in less available reactant and this leads to earlier mass transport limitations. The limitations occur mostly at higher currents when not enough reactant can be transported limitations. The limitations occur mostly at higher currents when not enough reactant can be to the cell for the conversion forced by the current to take place. The potential increases often abruptly transported to the cell for the conversion forced by the current to take place. The potential increases at a limiting current density. Regarding the fuel utilization, however, it can be seen that it does not often abruptly at a limiting current density. Regarding the fuel utilization, however, it can be seen even reach 10% for 50% CO2. Only by decreasing the flow rate the utilization can be increased up to that it does not even reach 10% for 50% CO2. Only by decreasing the flow rate the utilization can be 40% (Figure4). It must be noted though that the two measurements were conducted on di fferent cells increased up to 40% (Figure 4). It must be noted though that the two measurements were conducted and it seems they show different grades of degradation. The degradation may hinder the diffusion on different cells and it seems they show different grades of degradation. The degradation may pathways through the cell, reducing the local concentration of reactant. This explains the limiting hinder the pathways through the cell, reducing the local concentration of reactant. This current already at such low overall utilizations. explains the limiting current already at such low overall utilizations.

1.4

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1.0 l·h-1 E / V / E 1.1 1.5 l·h-1 2.0 l·h-1 1.0 2.5 l·h-1 3.0 l·h-1 -1 0.9 4.0 l·h 5.0 l·h-1 6.0 l·h-1 0.8 0.0 0.2 0.4 u / % Figure 4. Operating potential dependent on the fuel utilization, calculated according to Faraday, Figure 4. Operating potential dependent on the fuel utilization, calculated according to Faraday, for for various flow rates for a composition of 80% CO2 + 20% CO at 800 ◦C. various flow rates for a composition of 80% CO2 + 20% CO at 800 °C. 2 2 The comparison of ASR at OCV and 0.35 A cm− showed an increase from OCV to 0.35 A cm− · −2 · −2 for COThe2 concentrations comparison of of ASR 80% at and OCV lower. and For 0.35 a concentrationA·cm showed of an 90% increase CO2, however from OCV the ASRto 0.35 decreases. A·cm Observingfor CO2 concentrations a larger ASR at of OCV 80% than and under lower. load For is a typical concentration for a pronounced of 90% CO activation2, however overpotential, the ASR whichdecreases is especially. Observing dominant a larger atASR low at currents. OCV than Another under explanationload is typical for for the a behavior pronounced at low activation current densityoverpotential, is the Nernstwhich is potential. especially As dominant 90% CO at2 + low10% currents. CO is in Another the boundary explanation of the for Nernst the behavior potential, at alow decrease currentin density CO2 and is the increase Nernst inpotential. CO, like As it 90% will CO happen2 + 10% during CO is electrolysis, in the boundary causes of thethe Nernst potential, toa increasedecrease steepin CO in2 and the rangeincrease of 90%in CO, CO 2likedown it will to 80%happen CO2 during. Below electrolysis, 80% CO2, the causes increase the isNernst significantly potential lower. to incre Thease smallsteep changesin the range in CO of2 :CO90% ratioCO2 down during to electrolysis 80% CO2. B willelow then 80% cause CO2, the thermodynamicincrease is significantly voltage lower to increase. The small much changes more at in 90% CO CO2:CO2 in ratio the feedduring than electrolysis at 50% CO will2. then cause 2 the thermodynamicVarying the temperature voltage to showsincrease a similarmuch more influence at 90% on CO the2 ASRin the comparing feed than at OCV 50% to CO 0.352. A cm− . · −2 HereVarying the transition the temperature is already shows at around a similar 734 ◦influenceC. The deviation on the ASR observed comparing at around OCV to 800 0.35◦C A·cm in this. experimentHere the transition may be is explained already byat around the use 734 of a °C. diff Theerent deviation cell than observed for the composition at around 800 measurement. °C in this Itexperiment also indicates may thatbe explained probably by both the an use activation of a different overpotential cell than andfor the the composition Nernst potential measurement. are causing It thisalso decreaseindicates inthat ASR probably at low currentboth an densities.activation overpotential and the Nernst potential are causing this decrease in ASR at low current densities.

Processes 2020, 8, 1390 8 of 14

The temperature dependence measurement is also performed to gain information on the activation of the processes. Figure5 shows the Arrhenius representation of the ASR OCV. It can be observed that for temperatures above 850 ◦C the ASR remains constant. Below 850 ◦C the logarithmic of the resistance shows a slightly curved trend with recursive temperature increase. The ASR includes all the underlying processes that may occur during electrolysis. Seeing a slightly curved Arrhenius behavior indicates the domination of different processes at lower temperatures than at higher temperatures. Additionally, it may indicate an increasing degradation with increased temperature. TheProcesses series 2020 was, 8, xconducted FOR PEER REVIEW from low to high temperatures increasing the operation time of the cell9 with of 14 each temperature step. Especially the constant resistance above 850 ◦C is attributed to degradation as electrolysis were up to this time not reported in literature. Li et al. [23] investigated the influence of theProcesses LSC material 2020, 8, x FOR of the PEER anode REVIEW is reported to be unstable above this temperature [19]. 8 of 14 process parameters including temperature, however, did not show dependencies of single processes. Their obtained activation energy for the polarization resistance of around 60–70 kJ·mol−1 is larger than what we obtain (29 kJ·mol−1). It has to be noted though that they measure at different temperatures (600–700 °C) and gas compositions-0.6 employing different cell types. If only the temperature range of 700–750 °C is considered the activation energy for our polarization resistance increases to 46 kJ·mol−1, which is closer to what is reported by Li et al. showing a comparability. Other literature studies of CO-0.82 electrolysis don’t include Arrhenius analyses. Only some studies

performed on analysisln(R) or steam or co-electrolysis report activation energies for separate processes. Reported by modelling studies [17,24] an activation energy of 86.50 kJ·mol−1 and 86.50 kJ·mol−1 is given for the surface reaction-1 of adsorbed CO2 to adsorbed CO and O. This would fit RRQ1 at OCV or RRQ2 or RRQ3 at 0.35 A·cm−2. As also the current density has a huge impact on RRQ2 (compare Figure 5) it is assumed that this process is connected to the triple-phase boundary reaction. In the ASROCV fitting procedure it also turned -1.2out that RRQ1 and RRQ2 tend to exchange. Leonide/Sonn et al. [20,25] report for fuel cells that the two high0.85-frequency0.9 processes0.95 are1 coupled1.05 processes including oxygen -1 -1 transport in YSZ of the fuel electrode, charge 1000·Ttransfer / K at the triple phase boundary on the fuel electrode and gas diffusion inside the fuel electrode1 functional layer. Figure 5. Arrhenius representation (ln(R) →T− −)1 of the area specific resistance for a ratio of CO22:CO of Figure 5. Arrhenius representation (ln(R)→ T ) of the area specific resistance for a ratio of CO :CO 80:20.Theof process80:20. being expressed by RRQ4 shows only a low thermal activation. In other experiments of co-electrolysis in our group the activation energy was even lower or showed negative values. Due 4.2. Process Analysis to the lowThe activation temperature energy dependence it has been measurement assigned to gas is alsodiffusion performed as reported to gain in literature information by Primdahl on the [26]activation, TheEbbesen impedance energies [27] and spectra of Leonide the inprocesses. Figure [20]. 3The wereFigure results analyzed 5 shows presented by the an Arrhenius equivalentin this study representation circuit also modelshow ofthat (ECM) the the ASR containing activationOCV. It anenergycan inductance, be of observedthis process a serial that increases resistancefor temp witheratures and increasing four above RQ 850the elements operating°C the (aASR resistor current. remains and This constant. constant-phase was also Below observed 850 element °C in theother in parallel).experimentslogarithmic The for obtainedof cothe-electrolysis resistance resistances shows in our are a plotted group.slightly against curved theirtrend respective with recursive varied temperature parameter increase. in Figures The6– 9. ASR includes all the underlying processes that may occur during electrolysis. Seeing a slightly curved Arrhenius-1 behavior indicates the domination of different-1 processes at lower temperatures than at higher temperatures. Additionally, it may indicate an increasing degradation with increased temperature.-1.5 The series was conducted from low to high-1.5 temperatures increasing the operation time of the-2 cell with each temperature step. Especially the constant-2 resistance above 850 °C is attributed to degradation as the LSC material of the anode is reported to be unstable above this temperature [19]. -2.5 -2.5

4.2. Process-3 Analysis -3

ln(R) ln(R) -3.5The impedance spectra in Figure 3 were analyzed-3.5 by an equivalent circuit model (ECM) R R containing an inductance, a serial resistanceW and four RQ elements (a resistor and constant-phaseW -4 RRQ -4 RRQ element in parallel). The obtained resistances1 are plotted against their respective varied parameter1 in RRQ RRQ Figures-4.5 6–9. 2 -4.5 2 RRQ RRQ The variation of temperature might reveal3 the most insightful analysis towards the assignment3 -5 RRQ -5 RRQ of the RQ element to physical processes. The4 resistances can be plotted in an Arrhenius diagram4 (ln(R) → 0.85T−1) and 0.9from the0.95 slope the1 activation1.05 energy can be0.85 obtained.0.9 The Arrhenius0.95 1 plots for1.05 a -1 -1 -1 -1 CO2:CO ratio of 80:201000·T are shown / K in Figure 6 and the respective activation 1000·Tenergies / Kin Table 1. For the activation energies the( resistancesa) for temperatures above 850 °C were not considered(b) due to the cell degradation falsifying the results. Figure 6. Arrhenius plot for the resistances for a ratio of CO :CO of 80:20 at 6 L−h1 1 at (a) OCV and FigureThe 6 obtained. Arrhenius activation plot for the energies resistances differ for hugelya ratio of comparing CO22:CO of 80:20 OCV at and 6 l·h 0.35 at− (A·cma) OCV−2. and Only (b ) the 2 · (0.35b) 0.35 A·cm A cm−2. − . activation of· the ohmic resistance stays the same for increased current density. With ca. 40 kJ·mol−1 it is only half of the activation energy given in literature [20,21]. There it is compared to the activation energyTable of ionic1. Activation conductivity energies in for8YSZ, the resistancesgiven as 88 R ΩkJ·mol and R−1RQ1 [22] to .R InRQ4 the for cellsa ratio used of CO in 2:COthis ofpublication, 80:20.

however, the electrolyte material is significantlyE a,thinner OCV than whatEa, 0.35 is usedA·cm−2 in literature. Since not only Process the ionic conductivity of 8YSZ but also the ionic[kJ·mol and−1 ]electronic [kJ·mol conductivities−1] of the electrodes and wires as well as the conduction across material borders influence the ohmic resistance, a reduced RΩ 40.6 ± 1.5 38.1 ± 2.0 electrolyte thickness increases the contribution of those other parts. This results in lowering the RRQ1 71.8 ± 5.4 18.5 ± 3.8 activation energy of the ohmic resistance presented here. RRQ2 36.2 ± 8.0 80.2 ± 8.4 The activation energies for the resistances RRQ1, RRQ2 and RRQ3 seem to switch in magnitude from RRQ3 36.3 ± 7.3 71.8 ± 2.4 OCV to 0.35 A·cm−2. While the thermal activation of RQ1 is significantly reduced with current, the RRQ4 10.0 ± 2.1 19.1 ± 3.5 activation of RQ2 and RQ3 are increased. Values for activation energies of single processes in CO2

Processes 2020, 8, x FOR PEER REVIEW 10 of 14

The resistance RRQ4 is also strongly influenced by the current density as can be seen in Figure 7. At low current densities it slightly decreases and then increases almost exponentially with increase current. This behavior is more pronounced at 900 °C than at 700 °C and therefore shows a similar trend as the ASR observation from dc measurements. The highly increased resistance at high current densities supports the assignment to a diffusion process as transport limitations become more and more significant with increased current density. Even though the overall reactant utilization is not reaching high values, a local depletion of reactant is possible.

Processes 2020, 8, x FOR PEER REVIEW 10 of 14

The resistance RRQ4 is also strongly influenced by the current density as can be seen in Figure 7. At low current densities it slightly decreases and then increases almost exponentially with increase current.0.1 This behavior is more pronounced at 900 °C than0.1 at 700 °C and therefore shows a similar

trendW as the ASR observation from dc measurements. TheW highly increased resistance at high current

densities /R supports the assignment to a diffusion process /R as transport limitations become more and Processesmore2020 significant, 8, 1390 with increased current density. Even though the overall reactant utilization is not9 of 14 reaching high values, a local depletion of reactant is possible.

RW RRQ RW RRQ 3 3 0.01 R R 0.01 R R RQ1 RQ4 RQ1 RQ4 R R RQ2 RQ2 0 0.25 0.5 0.75 1 0 0.25 0.5 0.75 1 -2 -2 |i| / A·cm |i| / A·cm 0.1 0.1 (a) (b)

W W Figure 7. Resistances with respect to current density for a ratio of CO2:CO of 80:20 at 6 l·h-1 at (a) 700

R /R /R °C and (b) 900 °C.

The flow rate is influencing the resistances RRQ2 and RRQ4 under load as can be seen in Figure 8. R R R R W RQ3 W RQ3 Below the flow rate of 3 l·h−1 both resistances increase. RRQ2 has been assigned to a process connected 0.01 R R 0.01 R R RQ1 RQ4 RQ1 RQ4 to the reaction on the fuel side, while RRQ4 has been assigned to a diffusion process. At increased RRQ RRQ current more reactant is converted. This2 reactant amount needs to be transported to the2 triple phase boundary (TPB0 ) for0.25 conversion.0.5 0.75 Decreasing1 the flow leading0 to a0.25 decreased0.5 mass0.75 transport1 is |i| / A·cm-2 |i| / A·cm-2 expected and therefore the increase of RRQ4 can be explained. The decreased mass transport, however, also reduces the local reactant(a) concentrations, which then also influences the(b )reaction itself. Thus, the increase of both resistances can be explained if they are connected/coupled. The reaction cannot occur FigureFigure 7. Resistances 7. Resistances with with respect respect to current current density density for fora ratio a ratio of CO of2:CO CO of:CO 80:20 of at 80:206 l·h-1 at ( 6a) L700h 1 at without diffusion preceding it and if diffusion is negatively affected by2 flow rate, so is the· − TPB (a) 700°C Cand and (b) (900b) 900 °C. C. reaction.◦ ◦

The0.3 flow rate is influencing the resistances RRQ2 and 0.3RRQ4 under load as can be seen in Figure 8. −1 R R R R Below the flow rate of 3 l·h both resistancesW RQ 3increase. RRQ2 has been assigned to a processW connectedRQ3

RRQ RRQ RRQ RRQ to the 0.25reaction on the fuel side, while1 RRQ4 has4 been assigned0.25 to a diffusion process. 1 At increased4 Processes 2020, 8, x FOR PEER REVIEW RRQ RRQ 11 of 14 current more reactant is converted. This2 reactant amount needs to be transported to the2 triple phase boundary0.2 (TPB) for conversion. Decreasing the flow leading0.2 to a decreased mass transport is Figure 8. Resistances with respect to the flow rate for a ratio of CO2:CO of 80:20 at 800 °C at (a) OCV expected and therefore the increase of RRQ4 can be explained. The decreased mass transport, however, and (b) 0.35 A·cm−2. also reducesW 0.15 the local reactant concentrations, which thenW 0.15 also influences the reaction itself. Thus, the increase /R of both resistances can be explained if they are connected/coupled. /R The reaction cannot occur The partial pressure dependence reveals an assignment of RRQ2, RRQ3 and RRQ4 to the fuel electrodewithout0.1 diffusion (Figure 9) preceding. Especially it the and process if diffusion RQ4 is negativelyinfluenced0.1 affectedby the partial by flow pressure. rate, so At is OCVthe TPBthe reaction. resistance increases with increasing the CO2 partial pressure, while it decreases at 0.35 A·cm−2. This 0.05 0.05 might be0.3 explained by the fact that at OCV the ac signal0.3 is in both the fuel cell and electrolysis RW RRQ RW RRQ direction. With increased CO2 partial pressure,3 the CO partial pressure is decreased causing3 the 0 RRQ RRQ 0 RRQ RRQ increase0.25 in resistance1 2 at OCV.3 The4 assignment15 6 4to diffusional0.25 processes1 2 also applies3 4 well, 15since the6 4 low RRQ RRQ concentrations of CO then cause local-1 2 depletion for CO reduction. There also the trend-1 2of RRQ2 fits in flow rate / l·h flow rate / l·h RQ4 A·cm−2 RQ4 well, as0.2 it is similar to that(a) of R only less pronounced.0.2 At 0.35 the(b) trend for R that was expected is observed. Increasing the CO2 partial pressure increases the reactant concentrations and

W W Figuredecreasing0.15 8. Resistances the diffusion with resistance respect to and the with flow it rate also for the a ratioresistance0.15 of CO R2:CORQ2, which of 80:20 is atprobably 800 ◦C atconnected (a) OCV R /R 2 /R andto the (b )reaction 0.35 A cm on− the. fuel side. 0.1 · 0.1 0.4 0.4 RW RW 0.05 0.05 0.35 R 0.35 R RQ1 RQ1 R R 0.3 RQ2 0.3 RQ2 0 0 RRQ RRQ 0.25 1 32 3 4 5 6 0.25 1 2 3 4 5 6 3 RRQ -1 -1 RRQ 4 flow rate / l·h flow rate / l·h 4 0.2 0.2 W (a) W (b) /R 0.15 /R 0.15 0.1 0.1

0.05 0.05

0 0

-0.05 -0.05 0.5 0.75 1 0.5 0.75 1

pCO pCO 2 2 (a) (b)

2 FigureFigure 9. Resistances 9. Resistances with with respect respect to the to COthe COpartial2 partial pressure pressure at 800 at 800C and °C and (a) OCV (a) OCV and (andb) 0.35(b) 0.35 A cm . 2 ◦ · − A·cm−2.

5. Conclusions Taking the increased degradation into account, the optimal operating temperature of a direct carbon dioxide electrolysis on solid oxide cells would be 800–850 °C. The topic of degradation will and has to be addressed in detail in future publications of our group. The highest performance can then be obtained for 80–90% CO2 balanced with CO. At 1.4 V, also the current density is the highest. However, it is more beneficial to stay at around 1.1 V as the increased current density above that voltage does not follow a linear increase with voltage. Therefore, the increase in performance comes at a high cost in additionally needed power. The general trend of our outcomes will be observed in on-going and future long-term degradation experiments. The variation of the feed gas composition CO2:CO starting at a ratio of 90:10 with increments of 10% to 50:50 shows the simulated gradient of a gas flow through channels over a larger cell. With these results for operation at load (0.35 A·cm−2), the lack of a critical drop in performance regarding ASR and polarization resistance indicates, that an operation with both a high starting concentration of CO2 and a high fuel utilization above 40% is possible. The dominant part of the decrease in performance is identified as the process of diffusion. With a distinct design of gas flux within stacks and system, the possibility of high conversion rates is given even for elevated feeds of CO2:CO ratios. The highest reactant utilization can be reached for low flow rates. With suitable mass flow controllers, lower rates than 1 l·h−1 are possible and probably increase the utilization even further.

Processes 2020, 8, 1390 10 of 14

The variation of temperature might reveal the most insightful analysis towards the assignment of the RQ element to physical processes. The resistances can be plotted in an Arrhenius diagram (ln(R) T 1) and from the slope the activation energy can be obtained. The Arrhenius plots for a CO :CO → − 2 ratio of 80:20 are shown in Figure6 and the respective activation energies in Table1. For the activation energies the resistances for temperatures above 850 ◦C were not considered due to the cell degradation falsifying the results.

Table 1. Activation energies for the resistances RΩ and RRQ1 to RRQ4 for a ratio of CO2:CO of 80:20.

1 2 1 Process Ea, OCV [kJ mol− ]Ea, 0.35 A cm− [kJ mol− ] · · · R 40.6 1.5 38.1 2.0 Ω ± ± R 71.8 5.4 18.5 3.8 RQ1 ± ± R 36.2 8.0 80.2 8.4 RQ2 ± ± R 36.3 7.3 71.8 2.4 RQ3 ± ± R 10.0 2.1 19.1 3.5 RQ4 ± ±

The obtained activation energies differ hugely comparing OCV and 0.35 A cm 2. Only the · − activation of the ohmic resistance stays the same for increased current density. With ca. 40 kJ mol 1 it · − is only half of the activation energy given in literature [20,21]. There it is compared to the activation energy of ionic conductivity in 8YSZ, given as 88 kJ mol 1 [22]. In the cells used in this publication, · − however, the electrolyte material is significantly thinner than what is used in literature. Since not only the ionic conductivity of 8YSZ but also the ionic and electronic conductivities of the electrodes and wires as well as the conduction across material borders influence the ohmic resistance, a reduced electrolyte thickness increases the contribution of those other parts. This results in lowering the activation energy of the ohmic resistance presented here. The activation energies for the resistances RRQ1,RRQ2 and RRQ3 seem to switch in magnitude from OCV to 0.35 A cm 2. While the thermal activation of RQ1 is significantly reduced with current, · − the activation of RQ2 and RQ3 are increased. Values for activation energies of single processes in CO2 electrolysis were up to this time not reported in literature. Li et al. [23] investigated the influence of process parameters including temperature, however, did not show dependencies of single processes. Their obtained activation energy for the polarization resistance of around 60–70 kJ mol 1 is larger than · − what we obtain (29 kJ mol 1). It has to be noted though that they measure at different temperatures · − (600–700 ◦C) and gas compositions employing different cell types. If only the temperature range of 700–750 C is considered the activation energy for our polarization resistance increases to 46 kJ mol 1, ◦ · − which is closer to what is reported by Li et al. showing a comparability. Other literature studies of CO2 electrolysis don’t include Arrhenius analyses. Only some studies performed on fuel cell analysis or steam or co-electrolysis report activation energies for separate processes. Reported by modelling studies [17,24] an activation energy of 86.50 kJ mol 1 · − and 86.50 kJ mol 1 is given for the surface reaction of adsorbed CO to adsorbed CO and O. · − 2 This would fit R at OCV or R or R at 0.35 A cm 2. As also the current density has a RQ1 RQ2 RQ3 · − huge impact on RRQ2 (compare Figure5) it is assumed that this process is connected to the triple-phase boundary reaction. In the fitting procedure it also turned out that RRQ1 and RRQ2 tend to exchange. Leonide/Sonn et al. [20,25] report for fuel cells that the two high-frequency processes are coupled processes including oxygen transport in YSZ of the fuel electrode, charge transfer at the triple phase boundary on the fuel electrode and gas diffusion inside the fuel electrode functional layer. The process being expressed by RRQ4 shows only a low thermal activation. In other experiments of co-electrolysis in our group the activation energy was even lower or showed negative values. Due to the low activation energy it has been assigned to gas diffusion as reported in literature by Primdahl [26], Ebbesen [27] and Leonide [20]. The results presented in this study also show that the activation energy of this process increases with increasing the operating current. This was also observed in other experiments for co-electrolysis in our group. Processes 2020, 8, 1390 11 of 14

The resistance RRQ4 is also strongly influenced by the current density as can be seen in Figure7. At low current densities it slightly decreases and then increases almost exponentially with increase current. This behavior is more pronounced at 900 ◦C than at 700 ◦C and therefore shows a similar trend as the ASR observation from dc measurements. The highly increased resistance at high current densities supports the assignment to a diffusion process as transport limitations become more and more significant with increased current density. Even though the overall reactant utilization is not reaching high values, a local depletion of reactant is possible. The flow rate is influencing the resistances RRQ2 and RRQ4 under load as can be seen in Figure8. Below the flow rate of 3 L h 1 both resistances increase. R has been assigned to a process connected · − RQ2 to the reaction on the fuel side, while RRQ4 has been assigned to a diffusion process. At increased current more reactant is converted. This reactant amount needs to be transported to the triple phase boundary (TPB) for conversion. Decreasing the flow leading to a decreased mass transport is expected and therefore the increase of RRQ4 can be explained. The decreased mass transport, however, also reduces the local reactant concentrations, which then also influences the reaction itself. Thus, the increase of both resistances can be explained if they are connected/coupled. The reaction cannot occur without diffusion preceding it and if diffusion is negatively affected by flow rate, so is the TPB reaction. The partial pressure dependence reveals an assignment of RRQ2,RRQ3 and RRQ4 to the fuel electrode (Figure9). Especially the process RQ4 is influenced by the partial pressure. At OCV the resistance increases with increasing the CO partial pressure, while it decreases at 0.35 A cm 2. 2 · − This might be explained by the fact that at OCV the ac signal is in both the fuel cell and electrolysis direction. With increased CO2 partial pressure, the CO partial pressure is decreased causing the increase in resistance at OCV. The assignment to diffusional processes also applies well, since the low concentrations of CO then cause local depletion for CO reduction. There also the trend of RRQ2 fits in well, as it is similar to that of R only less pronounced. At 0.35 A cm 2 the trend for R that was RQ4 · − RQ4 expected is observed. Increasing the CO2 partial pressure increases the reactant concentrations and decreasing the diffusion resistance and with it also the resistance RRQ2, which is probably connected to the reaction on the fuel side.

5. Conclusions Taking the increased degradation into account, the optimal operating temperature of a direct carbon dioxide electrolysis on solid oxide cells would be 800–850 ◦C. The topic of degradation will and has to be addressed in detail in future publications of our group. The highest performance can then be obtained for 80–90% CO2 balanced with CO. At 1.4 V, also the current density is the highest. However, it is more beneficial to stay at around 1.1 V as the increased current density above that voltage does not follow a linear increase with voltage. Therefore, the increase in performance comes at a high cost in additionally needed power. The general trend of our outcomes will be observed in on-going and future long-term degradation experiments. The variation of the feed gas composition CO2:CO starting at a ratio of 90:10 with increments of 10% to 50:50 shows the simulated gradient of a gas flow through channels over a larger cell. With these results for operation at load (0.35 A cm 2), the lack of a critical drop in performance regarding ASR · − and polarization resistance indicates, that an operation with both a high starting concentration of CO2 and a high fuel utilization above 40% is possible. The dominant part of the decrease in performance is identified as the process of diffusion. With a distinct design of gas flux within stacks and system, the possibility of high conversion rates is given even for elevated feeds of CO2:CO ratios. The highest reactant utilization can be reached for low flow rates. With suitable mass flow controllers, lower rates than 1 L h 1 are possible and probably increase the utilization even further. · − These parameters describe superior efficiencies for the electrochemical reaction of CO2 to CO. Our results give an extensive outlook for scaling carbon dioxide electrolysis into stacks and systems. Especially regarding the moderate increase from 0.44 to 0.47 Ω cm2 of ASR at 0.35 A cm 2 with the decrease in · · − flow rate, the fuel utilization is improved significantly resulting in elevated space-time velocities and Processes 2020, 8, 1390 12 of 14 yields. With a suitable chemical engineering of the electrochemical system, desired high fuel utilization combined with low degrading operation conditions are achievable. Overall, our study explicitly exhibits the electrochemical trends of the variation of all important parameters for the running process of pure CO2-electrolysis. Our work puts a stronger focus on CO2 as a useable base chemical without having water involved in the process. Thereby, it allows the contemplation of electrolysis for the production of carbon monoxide by diversifying from co-electrolysis and pure water electrolysis coupled with water-gas shift reaction. Therefore, the potential of the conversion of carbon dioxide to carbon monoxide via solid oxide electrolysis to be a key enabling technology for the transformation of chemical industry from fossil resources to sustainable sources of carbon is disclosed.

Author Contributions: L.D., S.F. and T.D. conceived and designed the experiments; L.D. and T.D. performed the experiments; L.D., S.F. and T.D. analyzed the data; L.D., S.F. and T.D. wrote the paper, S.F., L.D., I.V., R.-A.E. and L.G.J.d.H. reviewed and edited the manuscript; S.F., I.V. and L.G.J.d.H. supervised the work; R.-A.E. and L.G.J.d.H. were responsible for funding acquisition. All authors have read and agreed to the published version of the manuscript. Funding: The authors gratefully acknowledge funding by the German Federal Ministry of Education and Research (BMBF) within the Kopernikus Project P2X: Flexible use of renewable resources–research, validation and implementation of ‘Power-to-X’ concepts and the iNEW Project: incubator sustainable and renewable value chains. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Appendix A

Table A1. ASR, |i|1.4 V and OCV for various ratios of CO2:CO at 800 ◦C.

ASR ASR 2 |i| OCV OCV CO :CO OCV 0.35 A cm− 1.4 V exp theo 2 [Ω cm2] [Ω cm2·] [A cm 2] [V] [V] · · · − 50:50 0.34 0.67/1.05 1 0.58/0.50 1 0.95 0.94 60:40 0.36 0.54/0.91 1 0.66/0.56 1 0.93 0.93 70:30 0.39 0.51/0.82 1 0.74/0.66 1 0.91 0.91 80:20 0.43 0.46/0.70 1 0.83/0.75 1 0.89 0.88 90:10 0.56 0.42/0.61 1 0.92/0.83 1 0.85 0.84 1 Two values are given because of the hysteresis. The first denotes the forward scan and the second the backward scan of the iV curve.

Table A2. ASR, |i|1.4 V and OCV for various temperatures for the ratio of CO2:CO of 80:20.

ASR ASR 2 |i| OCV OCV T[ C] OCV 0.35 A cm− 1.4 V exp theo ◦ [Ω cm2] [Ω cm2·] [A cm 2] [V] [V] · · · − 700 0.53 0.60 0.74 0.94 0.94 717 0.47 0.51 0.85 0.93 0.93 734 0.44 0.44 0.95 0.92 0.92 752 0.41 0.39 1.03 0.91 0.91 771 0.41 0.39 1.07/1.03 1 0.90 0.90 791 0.37 0.35 1.14/1.07 1 0.89 0.89 811 0.36 0.33 1.21/1.17 1 0.88 0.88 832 0.35 0.30 1.28/1.24 1 0.87 0.86 853 0.33 0.30 1.32 0.86 0.85 874 0.33 0.29 1.43 0.84 0.84 900 0.34 0.29 1.50 0.83 0.83 1 Two values are given because of the hysteresis. The first denotes the forward scan and the second the backward scan of the iV curve. Processes 2020, 8, 1390 13 of 14

Table A3. ASR, |i|1.4 V and OCV for varied flow rate for a CO2:CO ratio of 80:20 at 800 ◦C.

2 Flow Rate ASROCV ASR0.35 A cm− |i|1.4 V OCVexp OCVtheo [l h 1] [Ω cm2] [Ω cm2·] [A cm 2] [V] [V] · − · · · − 1.0 0.44 0.47 1.11 0.87/0.90 1 0.88 1.5 0.44 0.46 1.12 0.87/0.89 1 0.88 2.0 0.43 0.46 1.14 0.87/0.89 1 0.88 2.5 0.42 0.46 1.17 0.87/0.88 1 0.88 3.0 0.43 0.45 1.19 0.87/0.88 1 0.88 4.0 0.42 0.44 1.22 0.87/0.87 1 0.88 5.0 0.42 0.44 1.23 0.87/0.87 1 0.88 6.0 0.42 0.44 1.25 0.87/0.87 1 0.88 1 Two values are given because of the hysteresis. The first denotes the forward scan and the second the backward scan of the iV curve.

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