Electrochemical devices based on perovskites-type proton conducting oxides, are gaining wide interest as promising green technologies for hydrogen production and seperation. Interest in this class of materials stems from their lower activation energy for proton transport (typically 0.3-0.6 eV) relative to that for oxide ion conduction, thus making them suitable electrolyes for steam electrolysis cell operation at intermediate temperature. Despite the advances in proton-conducting electrolytes, their development still lag behind the oxygen-ion counterparts. The most prominent limitation being, conductivity, chemical stability, incomplete hydration of the electrolytes, as well as the grain boundary resistance. In the present contribution, starting from SrZr0.5Ce0.4Y0.1O3-δ, we examined the effects of Ba substitution for Sr and the doping level of Y on the lattice structure, electrical properties, proton content as well as chemical stability of the following compositions A(Zr5/9Ce4/9)1-x Y x O3- δ , (A= Ba, Sr and x=0.1, 0.2). Thermo-gravimetric analyses revealed almost similar proton concentration in both SZCY541 and SZCY(54)8/9-2 with a constant Zr/Ce fraction of 5:4. Both samples showed almost comparable proton conductivities in humidified 1% H2 hydrogen. BZCY(54)8/9-2 shows a maximum proton limit of 13.3 mol % compared to only 6.03 mol % for SZCY(54)8/9-2 and retained the highest proton conductivity of 1.44 × 10-2 Scm-1 in wet 1 % H2 at 600 oC (Fig. 1.) as well as excellent chemical stability under 80 % steam for 200 hours. Ba at A-site provides higher proton mobility than Sr. It is also evident from the data that the bulk and grain boundary contributions on the conductivity were strongly dependent on the A-site host, with the grain boundary response dominating the impedance spectra of the BZCY system relative to the SZCY. However the major difference in conductivity among the compositions originates mostly from the differences in proton concentration levels as well as the magnitude of the grain boundary contribution. This paper discusses these aspects based on experimental results as well as steam electrolysis using SZCY541 and BZCY(54)8/9-2 as electrolytes. Acknowledgement This work was supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier”(Funding agency: JST) and by World Premium International Research Center Initiative (WPI), MEXT Japan. Figure 1
Proton-conducting oxides have been studied as intermediate-temperature electrolyte materials for fuel cells and steam electrolysis. These materials can operate at intermediate temperatures because the activation energy for proton conductivity is lower than that of oxide ion conductivity. When proton conducting oxides are used as the electrolyte for fuel cells and steam electrolysis, nickel or its composite with electrolyte materials, so-called Ni cermet, are used as the fuel-side electrode, and transition-metal-containing perovskite such as (La,Sr)(Co,Fe)O3 (LSCF) and (La,Sr)MnO3 (LSM), are used for the air-side electrodes. The electrode materials contain Ni, Fe, Co, Mn, and so forth, which possibly react with the electrolyte oxides during processing and operation of the fuel cells and electrolysers. A possible concern is that the diffusion of these transition metals will degrade the electrical conductivity of the electrolyte. Shimura et al. reported a significant decrease in the electrical conductivity of BaCe0.9Y0.1O3-δ-based (BCY) proton conductors on partially substituting Fe, Mn and Co for Ce [1]. In this study, transition metal doping of perovskite type oxide have been performed, i.e., BaCe0.85Y0.1 M 0.05O3-δ, BaZr0.85Y0.1 M 0.05O3-δ, SrCe0.85Y0.1 M 0.05O3-δ and SrZr0.85Y0.1 M 0.05O3-δ which are referred to hereafter as BCYM, BZYM, SCYM and BZYM, respectively (M = Co, Fe, Mn and Ni), and their electrical conduction properties investigated to understand the effect of introducing transition metals to the proton conductor oxides. (Ba,Sr)(Ce,Zr)0.9-x Y0.1 Mx O3-δ were prepared by a solid-state reaction method. BaCO3, SrCO3, CeO2, ZrO2, Y2O3 and transition metal oxides were appropriately weighed, mixed, and calcined at 1200-1300 °C for 10 h in air, ball-milled, pressed into pellets, and sintered at 1400-1700°C for 10 h in air. Phase identification was carried out by X-ray diffraction. The conductivity was measured by a four-terminal AC impedance method. The electromotive force of gas concentration cells was measured in the temperature range from 873 to 1073 K. The electrical conductivity change was observed by introducing transition metals. These results suggest that the introduction of transition metals causes the change in the conductivity of the proton conducting electrolytes in most cases, and that the change in the conductivity depends on the sort of transition metals. This means that the impact of introducing transition metals to the electrical conduction properties depends on the oxide. The above two cases suggests that either A-site (Sr or Ba) or B-site (Ce or Zr) possibly governs for the impact of conductivity is sensitive to the transition metals. Acknowledgement This work was supported by the Cross-ministerial Strategic Innovation Promotion Program (SIP), International Institute for Carbon Neutral Research (I2CNER) and World Premium International Research Center Initiative (WPI), Japan. [1] T. Shimura, H. Tanaka, H. Matsumoto, T. Yogo, Solid State Ionics, 176 (2005) 2945–2950
Water electrolysis is an important technology for producing renewable hydrogen and work for energy conversion between the electricity and hydrogen in combination with hydrogen fuel cells. Steam electrolysis is characterized by low electrolysis voltage in comparison with other methods. Proton conduction in alkali earth cerates, zirconates and their solid solutions is useful for steam electrolysis, as well as hydrogen fuel cells, since it is less temperature-dependent than oxide ion conduction in zirconia and ceria based electrolytes. This aspect enables us to operate steam electrolysis at intermediate temperatures and hence to reduce the cost for production and operation. In addition, as schematically illustrated in Fig. 1, steam electrolysis using proton conducting electrolyte requires steam in the anode compartment, being a benefit of proton conductor cells because hydrogen generated at the cathode will already be separated from steam. This paper demonstrates conductivity and stability of some perovskite-type proton conducting metal oxides and the intermediate temperature operation of steam electrolysis. We also discusses several challenges of the proton conductor cells particularly for the electrolysis mode of operation When SrZr0.5Ce0.4Y0.1O3-δ (denoted below as SZCY541) is used as the electrolyte for the steam electrolysis, electrolysis voltage as low as 1.2 V has been attained for the current density of 0.1 A/cm2at 600°C, using 1% hydrogen and 1% oxygen as the cathode and anode gases, respectively. The voltage is equivalent to 1.4 V for air/pure hydrogen gas atmospheres, corresponding to around 90% of energy efficiency (HHV; vaporization of water considered for required energy).Starting from SZCY541 used in the above mentioned experiment, we examined the effects of Ba substitution for Sr and the doping level of Y on the lattice structure, electrical properties, proton content as well as chemical stability. In the examination keeping the ratio of Zr/Ce to be 5/4, Ba(Zr5/9Ce4/9)0.8Y0.2O3-δ, BZCY(54)8/92, has been found to show high proton conductivity: proton conductivity of 1.44 × 10-2 Scm-1 in wet 1 % H2 at 600°C. By use of this electrolyte, higher steam electrolysis performance can be obtained compared to the SZCY541 case shown above. There are still several challenges for the proton conductor cell to be applied for steam electrolysis. One is the stability of anode in steam. Several transition metal containing perovskites, such as Sm0.5Sr0.5CoO3 (SSC55) can be used as the anode with acceptable electrode performance, but these materials are not sufficiently durable against high steam concentration at intermediate temperature, e.g. 600°C, and SSC55 anode time dependently deteriorates. Another issue is the inter-diffusion of transition metal species from the electrodes to the electrolyte causing the reduction of the proton conductivity. We have examined the impact of several transition metals by introducing them as a part of the B-site component of AB 0.9Y0.1O3-δ (A=Ba, Sr; B=Ce, Zr) to find the proton conductivity is more or less reduced by the transition metal incorporation. Electronic leakage is another crucial problem of the proton conductor steam electrolysis, i.e., electrolysis rates deviates lower from that based on Faraday’s low of electrolysis. This is due to a partial electronic current flow originated from electron hole staying in the perovskite in oxidative atmosphere, anode environment in the electrolysis case, and penetrating the cathode on applying high current density. These challenges as well as possible solutions for some will be explained in this talk. Acknowledgement This work was supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), "energy carrier" (Funding agency: JST) and by World Premium International Research Center Initiative (WPI), MEXT Japan. Figure 1
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