Air Reference Electrode Research Articles

Performances of Ni−SDC Hydrogen Electrodes in Reversible Operation Between SOEC and SOFC-Modes

A reversible solid oxide cell (R-SOC), which can be operated as solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC), is a highly efficient direct energy converter between hydrogen and electricity (1). We have engaged in the R&D of high-performance, durable electrodes for the R-SOC (2-13). Recently, it was found that the durability of a double-layer H2 (DL) electrode, consisting of a SDC scaffold [samaria-doped ceria (CeO2)0.8(SmO1.5)0.2] with highly dispersed Ni0.9Co0.1 nanoparticles as the catalyst layer and a thin current collecting layer of Ni–YSZ cermet, was improved greatly by a reversible cycling operation between the SOEC and SOFC modes (12). The microstructure was observed to be stabilized by the cycling operation, i.e., lower parts of many Ni‒Co particles were anchored tightly on the SDC support and some portions were coated with SDC film, with certainty due to a strong interaction between Ni‒Co and SDC (13). However, the procedure for dispersing Ni‒Co nanoparticles on SDC (via an impregnation method) is not always suitable for large-scale fabrication. Aiming to develop a durable, practical hydrogen electrode with such a stabilized microstructure, we prepared new Ni–SDC hydrogen electrodes by a convenient protocol and examined the performances in the reversible operation.We prepared a coin-size cell with an air reference electrode (ARE) (6, 9-12):Ni–SDC H2 electrode│YSZ (0.5 mm)│SDC interlayer│LSCF–SDC O2 electrodeFor the H2 electrode, a paste was prepared by mixing Ni nanoparticles with uniform size (prepared by Noritake Co., Limited), SDC powder, and pore former. The paste was painted on YSZ electrolyte, followed by sintering. The LSCF–SDC (40 vol.% SDC) oxygen electrode was prepared on the SDC interlayer in the same manner as that described in our work (7). Test cells were operated at 800ºC by supplying humidified H2 (p[H2O] = 0.4 atm) to the H2 electrode compartment and dry O2 to the O2 electrode, irrespective of operation modes (SOEC or SOFC). The steady-state IR-free polarization curves (I–E curves) of the electrodes with area-specific ohmic resistances were measured by the current-interruption method in a three-electrode configuration. Figure 1 shows a SEM image acquired with a back-scattered electron (BSE) detector and Ni mapping by an energy-dispersive X-ray spectrometer (EDX) for the cross section of Ni–SDC electrode in pristine condition. In the BSE image, SDC is observed as light gray, Ni as dark gray, and pores as black. The distribution of Ni particles are fairly uniform in the electrode. The Ni–SDC electrode thus prepared exhibited a comparable initial performance to that of our DL-H2 electrode with Ni‒Co dispersed SDC prepared by the impregnation method (12). The durability tests of the electrodes are under progress in reversible cycling between −0.50 A cm−2 (SOEC-mode for 11 h) and 0.50 A cm−2 (SOFC-mode for 11 h). This work was supported by funds for the “Collaborative Industry-Academia-Government R&D Project for Solving Common Challenges toward Dramatically Expanded Use of Fuel Cells” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References S. D. Ebbesen, S. H. Jensen, A. Hauch, and M. B. Mogensen, Chem. Rev., 114, 10697 (2014). H. Uchida, N. Osada, and M. Watanabe, Electrochem. Solid-State Lett., 7, A500 (2004). N. Osada, H. Uchida, and M. Watanabe, J. Electrochem. Soc., 153, A816 (2006). Y. Tao, H. Nishino, S. Ashidate, H. Kokubo, M. Watanabe, and H. Uchida, Electrochim. Acta, 54, 3309 (2009). R. Nishida, P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, M. Watanabe, and H. Uchida, RSC Adv., 4, 16260 (2014). H. Uchida, P. Puengjinda, K. Miyano, K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and M. Watanabe, ECS Trans., 68 (1), 3307 (2015). K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, Electrochim. Acta, 225, 114 (2017). K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Ceram. Soc. Jon., 125, 218 (2017). P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Electrochem. Soc., 164, F889 (2017). H. Uchida, P. Puengjinda, K. Shimura, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 78 (1), 3189 (2017). H. Uchida, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 91 (1), 2379 (2019). H. Uchida, H. Nishino, P. Puengjinda, and K. Kakinuma, J. Electrochem. Soc., 167, 134516 (2020). H. Uchida, H. Nishino, and E. Da’as, ECS Trans., 103 (1), 6119 (2021). Figure 1

Performance of potentiometric oxygen sensors with LSCF electrode in lead–bismuth eutectic loop

Oxygen sensors are the major part of the active oxygen control system (OCS) to be used in the liquid lead–bismuth eutectic (LBE) system. In this paper, the electromotive force (EMF) measured by yttria stabilized zirconia (YSZ) solid electrolyte sensor with LSCF (lanthanum strontium cobalt ferrite)/air reference electrode in static oxygen saturated LBE is used to obtain relationship between EMF and temperature. The EMF output by oxygen sensors is deterministic and predictable. At the same time, the performance of oxygen sensors in flow loop of liquid LBE were also studied. EMF output curves of oxygen sensors corresponding to the operation of consuming dissolved oxygen are given. It is proved that the LSCF/air sensor can be used to monitor oxygen concentration in non-isothermal LBE systems.

Changes in Microstructure of NiCo-Dispersed SDC Hydrogen Electrodes for the Improvement of Durability via Reversible Cycling Operation Between SOEC and SOFC-Modes

A reversible solid oxide cell (R-SOC, operating as solid oxide electrolysis cell, SOEC and solid oxide fuel cell, SOFC) is a promising direct energy converter between hydrogen and electricity (1). We have engaged in the research and development of high-performance, durable electrodes with novel architecture for the R-SOC (2-12). Very recently, we found that the durability of a double-layer H2 (DL) electrode, consisting of a SDC scaffold [samaria-doped ceria (CeO2)0.8(SmO1.5)0.2] with highly dispersed Ni0.9Co0.1 nanoparticles as the catalyst layer (CL) and a thin current collecting layer (CCL) of Ni–YSZ cermet, was improved greatly by reversible cycling operation between the SOEC and SOFC modes (12). This presentation focuses on a stabilization of the microstructure of the CL in our hydrogen electrode via a reversible cycling operation. We prepared a coin-size cell with an air reference electrode (ARE) (6, 9-12):DL-H2 electrode│YSZ (0.5 mm)│SDC interlayer│LSCF–SDC O2 electrodeFor the DL-H2 electrode, 7.2 vol.% Ni0.9Co0.1 nanocatalyst was dispersed on SDC scaffold in the CL with addition of 10 vol.% three-dimensionally-connected Ni0.8Co0.2 (12, 13), and 60 vol.% Ni‒YSZ CCL was prepared on the CL. Test cells were operated in two operation protocols: a constant current density operation at −0.50 A cm−2 (SOEC-mode only) and a reversible cycling operation between −0.50 A cm−2 (SOEC-mode for 11 h) and 0.50 A cm−2 (SOFC-mode for 11 h) at 800ºC. Changes in the microstructure of the CL were analyzed by scanning ion microscopy (SIM) in a focused-ion beam system (FIB) in the depth direction from the CL/YSZ interface. In order to compare the actual degradation rates in the SOEC-mode for the two operation protocols, the values of IR-free potential E (vs. ARE) of each electrode and the ohmic resistance of the O2 and H2 electrode sides (R O2-side and R H2-side) during the operation at −0.50 A cm−2 are plotted in Fig. 1, while the inset shows changes in E as a function of the overall operation time. The degradation rate for R H2-side in the initial 200 h decreased as low as ca. 1/10 as a result of the cycling operation. In contrast to the negative shift in E for the SOEC-mode only, the positive shift in E at the H2 electrode in the cycling operation is clearly seen, i.e., the performance was improved. In the SIM image of pristine sample, fine Ni‒Co particles (ca. ⩽ 50 nm) are highly dispersed on the SDC scaffold. After the operation with SOEC-mode only, SDC particles are found to have agglomerated considerably, enclosing Ni‒Co particles, while fine Ni‒Co particles (ca. 50 nm) are dispersed on the surface. After the cycling operation, fine Ni‒Co particles still remain and agglomerated ones are also observed. Interestingly, the lower parts of many Ni‒Co particles are anchored tightly on the SDC support, and some portions are coated with SDC. Quantitative analyses of Ni‒Co are in progress, as a function of the distance from the interface. This work was supported by funds for the “Collaborative Industry-Academia-Government R&D Project for Solving Common Challenges toward Dramatically Expanded Use of Fuel Cells” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References S. D. Ebbesen, S. H. Jensen, A. Hauch, and M. B. Mogensen, Chem. Rev., 114, 10697 (2014). H. Uchida, N. Osada, and M. Watanabe, Electrochem. Solid-State Lett., 7, A500 (2004). N. Osada, H. Uchida, and M. Watanabe, J. Electrochem. Soc., 153, A816 (2006). Y. Tao, H. Nishino, S. Ashidate, H. Kokubo, M. Watanabe, and H. Uchida, Electrochim. Acta, 54, 3309 (2009). R. Nishida, P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, M. Watanabe, and H. Uchida, RSC Adv., 4, 16260 (2014). H. Uchida, P. Puengjinda, K. Miyano, K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and M. Watanabe, ECS Trans., 68 (1), 3307 (2015). K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, Electrochim. Acta, 225, 114 (2017). K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Ceram. Soc. Jpn., 125, 218 (2017). P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Electrochem. Soc., 164, F889 (2017). H. Uchida, P. Puengjinda, K. Shimura, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 78 (1), 3189 (2017). H. Uchida, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 91 (1), 2379 (2019). H. Uchida, H. Nishino, P. Puengjinda, and K. Kakinuma, J. Electrochem. Soc., 167, 134516 (2020). Figure 1

Depth-direction analysis of nickel depletion in a Ni–gadolinia-doped ceria hydrogen electrode after steam electrolysis operation

We have examined the initial-stage degradation of a Ni–gadolinia-doped ceria (GDC) cermet hydrogen electrode prepared on a GDC buffer layer of a stabilized-zirconia electrolyte supported cell during steam electrolysis operation at 800 °C. With use of an air reference electrode, the IR-free electrode potential E and ohmic resistance of the hydrogen electrode side (RH2-side) were recorded. After a steam electrolysis operation at −1.0 A cm−2 for 211 h, the RH2-side increased appreciably, by 36 %, while the value of E was nearly unchanged. It was found via a depth-direction analysis with use of focused ion beam-scanning ion microscopy that the remaining percentage of Ni decreased to ca. 60 % in the layer between 1 and 3 µm from the top of the GDC buffer layer, followed by a further decrease to 42 % at 0.5 µm. Since this suggests a significant cathodic polarization within the thin reaction zone, an enlargement of the reaction zone, together with a stabilization of Ni contacting with GDC, could be essential to mitigate such a degradation.

(Invited) Improvement of Durability of NiCo-Dispersed SDC Hydrogen Electrodes via Reversible Cycling Operation Between SOEC and SOFC-Modes

A reversible solid oxide cell (R-SOC, operating as SOEC and SOFC) is a promising direct energy converter between hydrogen and electricity (1). We have engaged in the research and development of high-performance, durable electrodes with novel architecture for the R-SOC (2-12). We used an oxygen electrode consisting of a composite of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and samaria-doped ceria (CeO2)0.8(SmO1.5)0.2 (SDC) with SDC interlayer (5, 7-11) and a double-layer (DL) hydrogen electrode consisting of porous SDC with highly dispersed Ni0.9Co0.1 catalyst as a catalyst layer (CL) and a Ni-SDC or Ni-YSZ cermet attached as a current-collecting layer (CCL) (4, 7, 10-12). This presentation focuses on a significant improvement of the durability of our hydrogen electrodes via a reversible cycling operation. We prepared a coin-size cell with an air reference electrode (ARE):DL-H2 electrode│YSZ (0.5 mm)│SDC interlayer│LSCF–SDC O2 electrodeThe preparation method of the cell was similar to that described previously (7, 10, 11). For the DL-H2 electrode, 7.2 vol.% Ni0.9Co0.1 nanocatalyst was dispersed on SDC scaffold in the CL with addition of 10 vol.% three-dimensionally-connected Ni or Ni0.8Co0.2 (11, 12), and 60 vol.% Ni‒YSZ CCL was prepared on the CL. Figure 1 shows the time course of the IR-free potential E (vs. ARE) of each electrode and the ohmic resistance R ohm of the oxygen and hydrogen electrode sides (R O2-side and R H2-side) during two operation protocols: a constant current density operation at −0.50 A cm−2 (SOEC mode only) and a reversible cycling operation between −0.50 A cm−2 (SOEC mode for 11 h) and 0.50 A cm−2 (SOFC mode for 11 h). As reported previously (11, 12), the DL-hydrogen electrode showed appreciable degradation in the SOEC mode only (j = −0.50 A cm−2), specifically, an increase in the R H2-side with time. During the initial 100 h, the degradation rate of R H2-side was rapid, and the cathode potential shifted in the negative direction (increase in overpotential). On the other hand, it is striking that the durability of the DL-hydrogen electrode was improved greatly via the reversible cycling operation. Even though the actual operation time in the SOEC was nearly half that with the constant current operation, the degradation rate of R H2-side was very low. The cathode potential in the SOEC mode (red circles) shifted in the positive direction, and the anode potential (blue circles) shifted in the negative direction during the initial 400 h, i.e., the performances were improved via the cycling operation. Graves et al. have reported a remarkable improvement of the durability of an Sr-doped LaMnO3 oxygen electrode via a cycling operation (13). In contrast, our LSCF‒SDC oxygen electrode exhibited very stable operation, irrespective of the operation protocol. Thus, an improved durability of the H2 electrode via the cycling operation has been demonstrated for the first time. Research on the mechanism of the suppression of degradation of the DL-H2 electrodes is in progress in our laboratory. References S. D. Ebbesen, S. H. Jensen, A. Hauch, and M. B. Mogensen, Chem. Rev., 114, 10697 (2014).H. Uchida, N. Osada, and M. Watanabe, Electrochem. Solid-State Lett., 7, A500 (2004).N. Osada, H. Uchida, and M. Watanabe, J. Electrochem. Soc., 153, A816 (2006).H. Uchida, S. Watanabe, Y. Tao, N. Osada, and M. Watanabe, ECS Trans., 7 (1), 365 (2007).Y. Tao, H. Nishino, S. Ashidate, H. Kokubo, M. Watanabe, and H. Uchida, Electrochim. Acta, 54, 3309 (2009).R. Nishida, P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, M. Watanabe, and H. Uchida, RSC Adv., 4, 16260 (2014).H. Uchida, P. Puengjinda, K. Miyano, K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and M. Watanabe, ECS Trans., 68 (1), 3307 (2015).K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, Electrochim. Acta, 225, 114 (2017).K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Ceram. Soc. Jpn., 125, 218 (2017).P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Electrochem. Soc., 164, F889 (2017).H. Uchida, P. Puengjinda, K. Shimura, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 78 (1), 3189 (2017).H. Uchida, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 91 (1), 2379 (2019).13.C. Graves, S. D. Ebbesen, S. H. Jensen, S. B. Simonsen, and M. B. Mogensen, Nat. Mater., 14, 239 (2015). Figure 1

(Invited) Improvement of Durability of NiCo-Dispersed SDC Hydrogen Electrodes via Reversible Cycling Operation Between SOEC and SOFC-Modes

A reversible solid oxide cell (R-SOC, operating as SOEC and SOFC) is a promising direct energy converter between hydrogen and electricity (1). We have engaged in the research and development of high-performance, durable electrodes with novel architecture for the R-SOC (2-12). We used an oxygen electrode consisting of a composite of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and samaria-doped ceria (CeO2)0.8(SmO1.5)0.2 (SDC) with SDC interlayer (5, 7-11) and a double-layer (DL) hydrogen electrode consisting of porous SDC with highly dispersed Ni0.9Co0.1 catalyst as a catalyst layer (CL) and a Ni-SDC or Ni-YSZ cermet attached as a current-collecting layer (CCL) (4, 7, 10-12). This presentation focuses on a significant improvement of the durability of our hydrogen electrodes via a reversible cycling operation.We prepared a coin-size cell with an air reference electrode (ARE):DL-H2 electrode│YSZ (0.5 mm)│SDC interlayer│LSCF–SDC O2 electrodeThe preparation method of the cell was similar to that described previously (7, 10, 11). For the DL-H2 electrode, 7.2 vol.% Ni0.9Co0.1 nanocatalyst was dispersed on SDC scaffold in the CL with addition of 10 vol.% three-dimensionally-connected Ni or Ni0.8Co0.2 (11, 12), and 60 vol.% Ni‒YSZ CCL was prepared on the CL.Figure 1 shows the time course of the IR-free potential E (vs. ARE) of each electrode and the ohmic resistance R ohm of the oxygen and hydrogen electrode sides (R O2-side and R H2-side) during two operation protocols: a constant current density operation at −0.50 A cm−2 (SOEC mode only) and a reversible cycling operation between −0.50 A cm−2 (SOEC mode for 11 h) and 0.50 A cm−2 (SOFC mode for 11 h). As reported previously (11, 12), the DL-hydrogen electrode showed appreciable degradation in the SOEC mode only (j = −0.50 A cm−2), specifically, an increase in the R H2-side with time. During the initial 100 h, the degradation rate of R H2-side was rapid, and the cathode potential shifted in the negative direction (increase in overpotential). On the other hand, it is striking that the durability of the DL-hydrogen electrode was improved greatly via the reversible cycling operation. Even though the actual operation time in the SOEC was nearly half that with the constant current operation, the degradation rate of R H2-side was very low. The cathode potential in the SOEC mode (red circles) shifted in the positive direction, and the anode potential (blue circles) shifted in the negative direction during the initial 400 h, i.e., the performances were improved via the cycling operation. Graves et al. have reported a remarkable improvement of the durability of an Sr-doped LaMnO3 oxygen electrode via a cycling operation (13). In contrast, our LSCF‒SDC oxygen electrode exhibited very stable operation, irrespective of the operation protocol. Thus, an improved durability of the H2 electrode via the cycling operation has been demonstrated for the first time. Research on the mechanism of the suppression of degradation of the DL-H2 electrodes is in progress in our laboratory.This work was supported by funds for the “Advanced Low Carbon Technology Research and Development Program” (ALCA) from the Japan Science and Technology Agency (JST).

NO Detection By Cyclic Voltammetry with Platinum Electrodes on YSZ

Introduction Solid electrolyte sensors (SES) based on yttria stabilized zirconia (YSZ) can be used to detect traces of other components in inert gases such as Ar or N2. The O2--conducting solid electrolyte YSZ is thermodynamically highly stable and can be operated at high temperatures with stationary (potentiometry, amperometry) or dynamic methods (pulse polarization [1], cyclovoltammetry (CV) [2]). In the stationary mode, the selectivity of such sensors is limited to an extend that it is usually not possible to distinguish between individual gas components. In contrast to that, dynamic methods allow a significantly increased selectivity by using different rates of electrode reactions of the individual components in order to detect them independently in a gas mixture [2]. Dynamic electrochemical methods have been successfully used in liquids for many decades, and preliminary investigations on solid electrolyte gas sensors also show that such methods can be used to measure the concentrations of several redox-active gases, for example, oxygen and carbon monoxide [3]. The commercial solid electrolyte sensor used in this work was previously successfully tested to measure selectively different gases such as hydrogen, oxygen and water vapor [4] in nitrogen by using CV. In this work, these studies will be continued to investigate the ability of CV for selective detection of nitrogen oxides (NOx) and to elucidate the mechanisms of signal formation. Experimental setup The experimental setup shown in Fig. 1 and described in detail in [2] was used to adjust the O2 concentration (SES 1) and the measurement of the concentrations of O2 and NO in nitrogen (SES 2). Both SES of the type "Test gas, Pt|YSZ|Pt, air" contain a YSZ-tube equipped with two cylindrical platinum mesh electrodes at the inner and outer surface. The area of the inner measuring electrode was estimated by SEM to be about 3.5 cm2. A porous YSZ sintered layer was applied to join electrolyte and electrodes. Covering large parts of the reference and measuring electrode this porous layer offers an extended three-phase boundary for the oxygen transfer.SES 1 was continuously polarized during each experiment providing a constant O2 concentration in the purified carrier gas N2. At SES 2, the CVs were obtained at polarization voltages between -0.1 and -0.9 V against the Pt air reference electrode and scan rates in the range 100 ... 5000 mV/s. In each experiment, the second cycle was evaluated to obtain reproducible results. Between two consecutive cyclovoltammograms (CVM), the open circuit potential (OCP) was measured to monitor depolarization of the measuring electrode. Results and discussion The CVMs shown in Fig. 2 exhibit NO-related peaks developing in the cathodic scan direction between -0.2 and -0.7 V at scan rates between 100 and 2000 mV/s. The peak maximum shifts in the cathodic direction with increasing scan rate. The highest sensitivity was achieved at 2000 mV/s for 650 °C. At higher scan rates it decreases again. This is apparently caused by the shift of the oxygen reduction peaks from range -0.1 ... -0.3 V at lower scan rates to the cathodic direction at higher scan rates and leads to their superposition with the NO peaks. The curve shifts to higher electrolysis currents with increasing NO-concentration in the cathodic region, visible for every scan rate at U < -0.7 V correspond to the Faraday currents calculable for the NO reduction. The different peak potentials for O2 and NO enable a highly selective determination NO in O2 containing gases at scan rates between 200 – 2000 mV/s at concentrations c(NO) ≥ 10 vol.-ppm.The height of the NO peak as a suitable NO-related signal was calculated and plotted in Fig. 3 for different scan rates. This peak height rises almost linearly with the NO concentration and the curve slopes increase with the scan rate up to 2000 mV/s. Further investigations revealed that this scan rate-induced slope increase depends also on flow rate.OCP measurements at different NO concentrations and sensor temperatures (Fig. 4) indicate the known tendency of NO to decompose at hot and catalytically active platinum electrodes into N2 and O2. Therefore, in the investigated temperature range, the absolute value of the OCP decreases with increasing NO concentration, coming with rising oxygen partial pressure according to NO decomposition. Another process guiding the OCP in the NO-free carrier gas is the electronic conductivity of the solid electrolyte that increases exponentially with temperature [5].

Yttria-Stabilized Zirconia Aided Electrochemical Investigation on Ferric Ions in Mixed Molten Calcium and Sodium Chlorides

Electrolytic reduction of dissolved iron oxide to metal iron in molten salts with an inert anode is an alternative short route for steelmaking without CO2 emissions. A novel and simple integrated yttria-stabilized zirconia (YSZ) cell was constructed from a YSZ tube with a closed end. The YSZ tube played multiple functions, including the container for the molten salts, the solid electrolyte membrane in the O2−|YSZ|Pt|O2 (air) reference electrode (RE), and the solid electrolyte membrane between the working and counter electrodes (WE and CE). Electrochemical behavior of ferric ions (Fe3+) that were formed by dissolution of 0.5 wt pct Fe2O3 in the molten CaCl2-NaCl eutectic mixture was investigated on a Pt WE at 1273 K by various electrochemical techniques including cyclic voltammetry, linear scan voltammetry, square wave voltammetry, chronopotentiometry, chronoamperometry, and potentiostatic electrolysis. Analysis of the mechanism of electrode reactions was further assisted by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. Some electrochemical parameters were obtained, including the number of exchanged electrons and the diffusion coefficient of ferric ions in the mixed molten salts. The results from various electrochemical techniques are in good agreement with each other, and show that the electrochemical reduction of Fe3+ to Fe in the molten salt mixture could be a single three-electron transfer step and diffusion-controlled reaction that was also possibly reversible. This work may form the foundation for extraction of iron and alloys from molten salts and also provide a stable O2−|YSZ|Pt|O2 (air) RE with wide applicability for investigation on electrochemical properties of other electroactive metal oxides in molten salts.

Effect of Chromium Poisoning on Performances of Lscf-SDC Composite Oxygen Electrodes Under Reversible Operation of Solid Oxide Cells

Solid oxide cells (SOCs) are high-temperature electrochemical energy conversion and storage devices, which can be operated under solid oxide electrolysis cell (SOEC) mode to produce pure hydrogen from water vapor with high efficiency. SOCs can also be operated in reverse under solid oxide fuel cell (SOFC) mode, providing high-energy conversion efficiency to convert hydrogen fuel to electricity. Similar to SOFCs, cost-effective metallic (alloy) can be used as interconnects for planar-type SOCs operated at reduced temperatures (≤ 800 °C). However, without an effective protective coating on the Cr-containing alloy, volatile Cr-containing species are generated at high temperatures in oxidizing atmosphere. It has been recognized that several oxygen electrodes such as Sr-doped LaMnO3 (LSM) or La0.6Sr0.4Co0.2Fe0.8O3 - δ (LSCF) suffer from a serious Cr-poisoning.1–3 We have recently succeeded in improving the reversible performances of the single-layer LSCF-Ce0.8Sm0.2O1.9 (SDC) composite O2 electrode with SDC interlayer at 800 and 900 °C,4–6 compared with those of an identical-type electrode in our previous work.7 Though the degradation of the LSM or LSCF electrodes by Cr-poisoning has been studied under either SOFC or SOEC operation mode, separately, to the best of our knowledge, the Cr-poisoning have not been investigated on SOCs under simultaneously reversible operation of SOFC/SOEC modes. Here, we examined for the first time the effect of Cr-poisoning on the performances of LSCF-SDC oxygen electrodes under simultaneously cathodic and anodic polarization conditions in the presence of Fe-Cr alloy. A symmetrical cell [LSCF-SDC ǀSDC interlayerǀ ScSZ electrolyte ǀSDC interlayerǀ LSCF-SDC] was prepared.4,5 Here, we briefly describe the protocol. The SDC interlayer was deposited on both sides of the scandia-stabilized zirconia (ScSZ, (ZrO2)0.89(Sc2O3)0.10(CeO2)0.01) electrolyte by spin coating of Ce(III) 2-ethylhexanoate and Sm(III) 2-ethylhexanoate mixed solution, followed by heat-treatment at 1050 oC for 2 h. The LSCF-SDC paste (60:40 volume ratio) was tape-casted onto the SDC interlayer and heat-treated at 1050 oC for 1 h. The air reference electrode consisted of a Pt wire wound around the ScSZ disk. A commercial Fe-Cr alloy (RA446, 23–27 wt% Cr, Rolled Alloy, Canada) was used as the metallic interconnect without any protective coating. The interconnect plate (12 × 12 × 4 mm) was machined with gas channels on one side. Air was supplied to both electrodes at a flow rate of 100 mL min−1. The as-prepared symmetrical cell was sandwiched with two Fe-Cr interconnect plates, which were also acting as the current collectors. After a constant current density operation at 0.5 A cm−2 and 800 oC for 100 h, the IR-free polarization curves of both electrodes were measured. Hereinafter, the electrodes polarized anodically (O2 evolution) and cathodically (O2reduction) for 100 h will be denoted as electrode A and electrode C, respectively. Figure 1 shows the polarization curves of electrode A and electrode C with and without Fe-Cr interconnect plate at 800 oC. When operated with Pt current collectors (without any Fe-Cr alloy), very high performance of both electrode A and electrode C was observed even after 100 h (negligibly small degradation). In contrast, after operation with Fe-Cr alloy interconnects, the overpotential at both electrodes significantly increased due to Cr-poisoning. While the difference in the anodic overpotential between electrode A and C was relatively small in the O2 evolution reaction, electrode A showed much larger overpotential than that of electrode C in the O2 reduction, specifically at high current densities. This could suggest that Cr-containing oxides deposited in the pores2 might block the O2diffusion path. Analyses of the electrode morphology are under progress. This work was supported by the funds for “ALCA” from Japan Science and Technology Agency.

Effect of Microstructure on Performances of Hydrogen and Oxygen Electrodes for Reversible SOEC/SOFC

Solid oxide fuel cells (SOFCs) are, in principle, able to be operated in reverse as solid oxide electrolysis cells (SOECs) to generate pure hydrogen from water vapor with the highest efficiency. Since the pioneering work in late 1960s (1) and HOT-ELLY (hot-electrolysis) project in 1980s (2), the materials and system for the SOECs have been extensively investigated (3). We have engaged in the research and development of high-performance electrodes with novel architecture for the SOEC/SOFC reversible system (4-9). The essential factors to improve the electrode performance are a high electrocatalytic activity and an extended effective reaction zone (ERZ: effective transport of gas, ions, and electrons). In the present work, we demonstrate that the microstructure of both hydrogen and oxygen electrodes plays an important role in reducing the ohmic and polarization losses under reversible SOFC/SOEC operations. Figure 1 shows cross-sectional SEM images of double-layer (DL) hydrogen electrodes (6) prepared on YSZ electrolyte. The DL electrodes consisted of a mixed conducting samaria-doped ceria, (CeO2)0.8(SmO1.5)0.2 (SDC), with highly dispersed Ni0.9Co0.1 catalysts (8 vol%-Ni0.9Co0.1) as a catalyst layer (CL, thickness = 14 to 16 μm), on top of which a Ni-SDC cermet (60 vol%-Ni) was attached as a current collecting layer (CCL) with a thickness of 5 to 20 μm. A SEM image of the single CL without CCL is also shown for comparison in Fig. 1 (a). The porosity of the CCL has to be high to increase the gas-diffusion rate. The CCLs formed on the CL were found to be uniform in the thickness and the apparent porosity was 69 to 70%, irrespective of the thickness. A test cell was prepared with the following configuration: DL H2 electrode│YSZ (0.5 mm)│SDC interlayer│O2electrode The oxygen electrode employed was a composite of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) with 40 vol% SDC. A detailed analysis of the oxygen electrode microstructure is given elsewhere (8). As a Pt/air reference electrode, a Pt wire was wrapped along the lateral of the YSZ electrolyte disk and fixed with Pt paste. Figure 2 shows steady-state IR-free I−E curves of the DL hydrogen electrodes, which were measured by the current interruption method, in the reversible SOFC/SOEC operation mode with H2 + H2O (p[H2O] = 0.4 atm) at 800ºC. The thickness of the CCL showed a significant effect in reducing the polarization loss. The test cell with 5 μm-CCL exhibited the highest performance in both SOFC/SOEC operations, i.e., the IR-free cathode potential at j = −0.5 A cm−2(SOEC operation) was −1.16 V vs. air reference electrode, which corresponds to an overpotential of −0.20 V. We also expect that the thin CCL with high electronic conductivity can act as a micrometer-size current collector to minimize the ohmic loss between the separator (interconnect) and the CL in stacks. The ohmic resistance of the test cells R cell was indeed smaller than that without the CCL, and reached the minimum value with the 5 μm-CCL. This suggests that the whole Ni0.9Co0.1/SDC of the catalyst layer can work effectively. A further increase in the CCL thickness (≥ 10 μm) increased the R cell (additional ohmic resistance of the cermet). The gas diffusion rate through the CCL is presumably slowed down with increasing the CCL thickness. In summary, the 5 μm-CCL provides the best balance in enlarging the ERZ by an effective transport of gases (H2O and H2) and an effective conductive network for electrons and oxide ions. This work was supported by the funds for “Advanced Low Carbon Technology Research and Development Program” (ALCA) from Japan Science and Technology Agency (JST).

Polarization properties of La 0.6Sr 0.4Co 0.2Fe 0.8O 3-based double layer-type oxygen electrodes for reversible SOFCs

We have developed double layer-type (catalyst layer/current collecting layer) oxygen electrodes (DLE) for reversible SOFCs. As the catalyst layer (cathode for SOFC and anode for steam electrolysis) interfaced with a samaria-doped ceria [(CeO 2) 0.8(SmO 1.5) 0.2, SDC] interlayer/YSZ solid electrolyte, mixed conducting La 0.6Sr 0.4Co 0.2Fe 0.8O 3 (LSCF) and SDC particles were employed. The current collecting porous LSCF layer was formed on the catalyst layer. By controlling the SDC content, as well as the thickness and porosity of the catalyst layer, the gas diffusion rate and the conduction networks for electrons and oxide ions were optimized, resulting in a marked reduction of the overpotential. The LSCF + SDC/LSCF DLE exhibited higher performance than single-layer electrodes of LSCF + SDC or LSCF; the IR-free anode potential vs. an air reference electrode was 0.12 V (corresponding to an overpotential of 0.08 V) at 0.5 A cm −2 and 900 °C under an atmosphere of O 2 (1 atm).

Au–oxide composites as HC-sensitive electrode material for mixed potential gas sensors

In potentiometric zirconia-based gas sensors, gold electrodes show a high sensitivity for hydrocarbons (HC) when the measurements take place in nonequilibrated oxygen containing gas mixtures at temperatures <700 °C. That behaviour explained by mixed potential theory is not stable and depends strongly on preparation and particularly on measuring conditions. To clarify the correlation between the potential behaviour of electrodes and their catalytic activity for the hydrocarbon combustion, we investigated composite electrodes Au–oxide/YSZ with Ga 2O 3, Y 2O 3, Al 2O 3, ZrO 2, In 2O 3 and Nb 2O 5 as oxides and 70 mass percent gold in C 3H 6–O 2 gas mixtures using a platinum air reference electrode. Gold composite electrodes using Nb 2O 5, Ga 2O 3, In 2O 3 as oxides show, as compared with pure gold, an enhanced propylene sensitivity and a time-independent characteristic at high concentrations of C 3H 6. Gold composite electrodes with Y 2O 3, Al 2O 3, ZrO 2 tend to the equilibrium behaviour. The catalytic activity of the Au-composites for the HC combustion correlates very well with the cell voltage response.

Gold-composite electrodes for hydrocarbon sensors based on YSZ solid electrolyte

In potentiometric zirconia based sensors gold electrodes show a high sensitivity for hydrocarbons (HC's) when the measurements are carried out in non equilibrated oxygen containing gas mixtures at temperatures <700 °C. This behaviour explained by mixed potential theory is not stable and depends strongly on preparation and particularly on measuring conditions. To modify the electrode behaviour composites consisting of gold and gallium oxide were investigated. Gold pastes with different amount of Ga2O3 were prepared and screen printed on YSZ pellets. After sintering at defined temperatures between 900 and 950 °C the cells were tested regarding the electrode behaviour in a C3H6, O2 gas mixture using a platinum air reference electrode. These composite electrodes show as compared with pure gold an enhanced sensitivity at low propylene concentrations and a time-independent characteristic at high concentrations of C3H6. The optimal composition is found to be at 20 mass-% Ga2O3. This electrode can be treated in reducing gases at temperatures 850 °C without changing its characteristics.

High‐Performance Electrode for Medium‐Temperature Solid Oxide Fuel Cells: Effects of Composition and Microstructures on Performance of Ceria‐Based Anodes

Polarization properties of ceria‐based anodes dispersed with nanometer‐sized Ru catalysts, which we developed for medium‐temperature solid oxide fuel cells, were greatly improved by controlling the composition and microstructure. Among samaria‐doped ceria (SDC) anodes with compositions of (0 ≦ x ≦ 0.4) a SDC anode with x = 0.2 was found to exhibit the maximum current density at a given overpotential at temperatures of 800 to 1000 °C, when operating under a hydrogen atmosphere. This high current density is a direct result of improved conductivities of both oxide ions (σion) and electrons (σe). Attaching a very thin film of SDC onto a yttria‐stabilized zirconia electrolyte before coating the Ru‐dispersed SDC layer appreciably lowered the anodic overpotential for the SDC. The current densities on the improved Ru‐SDC anode at a potential of −0.9 V vs an air reference electrode were 0.8 and 1.4 A/cm2 at 800 and 900 °C, respectively.

Development of Dissolved Hydrogen Sensors Based on Yttria‐Stabilized Zirconia Solid Electrolyte with Noble Metal Electrodes

A hydrogen sensor has been developed for in situ measurements of the concentration of hydrogen in aqueous solutions at elevated temperatures. The sensor was based on an electrochemical cell employing an yttria‐stabilized zirconia (9% ) solid electrolyte, a (in air) reference electrode, and a noble metal working electrode (Pt or Pd) covered with a polytetrafluorene‐ethyl membrane. The response of the sensor to hydrogen has been examined in gas mixtures and aqueous solutions at temperatures as high as 300°C, and for hydrogen concentrations ranging from 0.001 to 100% or to mol/kg, respectively. The sensor displayed rapid responses to changes in concentration of hydrogen in both gas mixtures and aqueous solutions. Linear relationships with Nernstian slopes (2.303RT/2F) were observed between the measured potential of the sensor and the logarithm of the hydrogen concentration. Oxygen in solution had little effect on the potential of the sensor when the concentration of oxygen was lower than that of hydrogen, while there was a significant change in the potential with oxygen concentration in gas mixtures. The potential of the sensor was not affected by the pH of the solution.

High‐Performance Electrode for Medium‐Temperature Operating Solid Oxide Fuel Cells: Polarization Property of Ceria‐Based Anode with Highly Dispersed Ruthenium Catalysts in Gas

Polarization properties of catalyzed anode, which we developed for medium‐temperature operating solid oxide fuel cells, were examined in a synthetic gas containing and . The overpotential was greatly decreased by loading only a small amount of Ru microcrystals onto the mixed conducting oxide, samaria‐doped ceria (SDC), anode layer in both synthetic gas and wet hydrogen atmospheres. It was found that the Ru‐SDC anode exhibited the highest electrocatalytic activities at an optimum oxygen partial pressure in the fuel gas due to high conductivities of both electrons and oxide ions in the SDC. The maximum current density on the Ru‐SDC anode exceeded 1 A/cm2 at all the operating temperatures between 800 and 1000°C at the potential of −0.9 V vs. air reference electrode, when 0.1 mg/cm2 of Ru microcatalysts were loaded on the SDC surface and the appropriate fuel gas was used.

Potentiometric determination of the thermodynamic stability of SrORuO 2 system

A partial phase diagram for SrORuO 2 system in the temperature range 1023 to 1473K under 1 atmosphere of oxygen was determined for the purpose of galvanic cell studies. The c.m.f. of the galvanic cells using 15 wt.% CaO-stabilized ZrO 2 (15 CSZ) solid electrolyte and air ( P O 2 = 0.21 atm)/Pt as the reference electrode yielded the following least-squares expressions for the test electrodes SrO/Sr 2RuO 4/Ru (I), Sr 2RuO 4/Sr 3Ru 2O 7/Ru (II) and Sr 3Ru 2O 7/SrRuO 3/Ru (III) over the temperature ranges 1044 to 1235, 926 to 1164 and 991 to 1282K respectively: 1. E ( I) = 1009.25−0.40411 T±3.37 mV (1) 2. E ( II) = 1060.67−0.46896 T±3.26 mV (2) 3. E ( III) = 1010.50−0.43449 T±3.78 mV (3) These e.m.f. results, after correcting for the standard state of oxygen in the air reference electrode, were combined with the ΔG f o(RuO 2) from literature and the standard Gibbs energies of formation, ΔG f,ox o , of Sr 2RuO 4, Sr 3Ru 2O 7 and SrRuO 3 were determined.

Thermodynamic stability of CaRuO 3

The e.m.f. of the galvanic cell Pt, CaO, CaRuO 3, Ru|15 CSZ|O 2 ( P O2 = 0.21 atm), Pt was studied over the range 971–1312 K using 15wt.%CaO-stabilized ZrO 2 (15 CSZ) as the solid electrolyte. This study yielded the least-squares expression E (1) = 754.16−0.36659 T±1.70 mV. After correcting for the standard state of oxygen in the air reference electrode and by combining these results with the standard Gibbs energy data on RuO 2 from the literature, the standard Gibbs energy of formation ΔG f, ox o of CaRuO 3 from CaO and RuO 2 was determined to be ΔG f, ox o (CaRuO 3(s)) = 14396−44.221 T±1905 J mol −1.

Thermodynamic stabilities of NiTeO 3 and Ni 3TeO 6 by solid oxide electrolyte e.m.f. method

The e.m.f. of the galvanic cells Pt,C,Te(l),NiTeO 3,NiO/15 YSZ/O 2 ( P o 2 = 0.21 atm),Pt and Pt,C,NiTeO 3,Ni 3TeO 6,NiO/15 YSZ/O 2 ( P o 2 = 0.21 atm),Pt (where 15 YSZ=15 mass% yttria-stabilized zirconia) was measured over the ranges 833–1104 K and 624–964 K respectively, and could be represented by the least-squares expressions E (1)±1.48 (mV) = 888.72 − 0.504277 (K) and E (II) ±4.21 (mV) = 895.26 − 0.81543 T (K). After correcting for the standard state of oxygen in the air reference electrode, and by combining with the standard Gibbs energies of formation of NiO and TeO 2 from the literature, the following expressions could be derived for the Δ G ° f of NiTeO 3 and Ni 3TeO 6: Δ G f °(NiTeO 3) ± 2.03 (kJ mol −1) = −577.30 + 0.26692 T (K) and Δ G ° f(Ni 3TeO 6)±2.54 (kJ mol −1) = −1218.66 + 0.58837 T (K).

An Ionized Air Reference Electrode: III . Use with Solid Polymer Electrolyte

An ionized air reference electrode that uses a column of ionized air to establish electrical contact with an electrolyte was used to measure the ohmic voltage profile resulting from the flow of an applied current through a strip of Nafion 425 ion exchange membrane. The voltage profile thus obtained was used to calculate the conductivity of the reinforced Nafion solid polymer electrolyte and yielded a value consistent with the value reported in the literature. It was concluded that the ionized air reference electrode may be useful for use with solid polymer electrolyte systems.