(Invited) Steam Electrolysis Using Proton-Conducting Perovskite
Authors: Matsumoto Hiroshige , Leonard Kwati , Lee Young-Sung , Okuyama Yuji , Miyazaki Kuninori 
Published: February 2020

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