ELECTROCHEMICAL DEVICES BASED ON SOLID OXIDE PROTON-CONDUCTING ELECTROLYTES: A REVIEW
In the last 2 years more than twenty works have been published in International journals. These works are devoted to: i) design and development of new proton conducting solid electrolytes and electrodes, ii) amperometric and potentiometric gas sensors, iii) electrolyzers for hydrogen production.
The research history in the field of protonic electrolytes is reflected in literature [1-2]. The theoretical and practical aspects, emphasizing the general principles and guidelines necessary for the design and development of future electrochemical systems based on solid oxide proton conducting electrolytes (fuel cells, electrolyzers, gas sensors, reactors), are studied in the new laboratory and part of the results is thoroughly presented and discussed in the already published works [3-4].
High-temperature proton-conducting materials are widely proposed as electrolytes for applications in electrochemical devices, including solid oxide fuel cells (SOFCs), electrolyzes, sensors, hydrogen-permeable membranes for hydrogen production or ammonia synthesis .
Having previous experience in related solid state electrochemistry topics, the last two years our main research work has been focused:
1. On the detailed analysis of thermal and chemical compatibility of cathode materials for BaCeO3 and BaZrO3-based electrolytes for solid oxide fuel cell application [4-6].
In this research work the identification of suitable cathode materials, which can be successfully used in SOFCs based on proton-conducting electrolytes during long-term operation and thermal cycling measurements was investigated. In order to better be informed about, a wide variety of materials were synthesized and their thermal and chemical compatibilities toward BaCe0.8Y0.2O3–δ and BaZr0.8Y0.2O3–δ were verified by means of detailed dilatometry and XRD analysis of calcined electrode/electrolyte mixtures, respectively. It was found that some of the studied cathode materials react to a different degree with selected electrolytes. The main attention was paid to the comparison of thermal behaviours of cathode and electrolyte system as well as the chemical compatibility between them.
Taking into account the relatively low TEC values, no significant chemical expansion and high resistance to impurity phase formation, LaNi0.6Fe0.4O3–δ and La2NiO4+δ samples can be proposed as suitable cathodes for BaCe0.8Y0.2O3−δ and BaZr0.8Y0.2O3−δ electrolytes. The layered Y0.8Ca0.2BaCo4O7+δ cobaltite has the closest TEC value with those for cerate and zirconate ceramics, however completely decomposes after treatment of Y0.8Ca0.2BaCo4O7+δ/BaCe0.8Y0.2O3−δ mixture and was stable in contact with BaZr0.8Y0.2O3−δ.
2. On the design and development of appropriate proton conducting electrolytes and electrode materials for possible application in solid oxide fuel cells and solid oxide electrolyzers [7-8].
To this aim the relationship between Y by Yb substitution on the crystal and microstructure properties as well as thermomechanical and electrical features of BaCe0.5Zr0.3Y0.2–xYbxO3−δ-based ceramics (BCZYYbx) was investigated. It was shown that the increase of Yb concentration from 0 to 0.2 leaded to decrease of: i) the unit cell parameters/volume (from 490.57 to 486.80 Å3), ii) the TEC values (from 8.5·10–6 to 7.6·10–6 K–1). However, the concentration dependences of mean grain size and total conductivity of BCZYYbx ceramics were similar, fact that allows one to conclude that the grain boundary significantly affects the transport properties of these proton-conducting electrolytes.
Moreover we demonstrated that a tape calendering method (TCM) is an effective way for the preparation of proton ceramic fuel cells with enhanced performance. More precisely the possibility of proton ceramic fuel cell (PCFC) fabrication, using TCM, has been shown for first time for Ba(Ce,Zr)O3-based proton-conducting electrolytes.
The as fabricated cell consisted of a 30 μm BaCe0.5Zr0.3Y0.2O3−δ (BCZY) electrolyte, a Y0.8Ca0.2BaCo4O7+δ cathode and the reduced 40%BCZY + 60%NiO and 45%BCZY + 55%NiO support and functional anode layers. Phase structure, thermal and electrical characterization of functional materials were also discussed in detail. It was found that the maximal power densities (Pmax) of the fabricated PCFC (174 and 308 mW cm−2 at 600 and 725 °C, respectively) were acceptable for an intermediate-temperature range of operation and comparable with those reported in literature. The BCZY electrolyte exhibited very high open circuit voltage (OCV) values (1.141 and 1.079 V at 600 and 725 °C) and, correspondingly, predominant ionic transport (average ions transport number ∼0.99 and 0.95 at 600 and 725 °C).
In our opinion, the tape calendering method can be considered as effective strategy for PCFCs fabrication, ensuring the formation of gas-tight electrolytes and acceptable electrochemical properties.
3. On the sulfur and carbon tolerance of BaCeO3–BaZrO3 proton-conducting materials .
For this purpose, BaCe0.8−xZrxY0.2O3−δ-based ceramic samples (BCZYx) were prepared and their chemical stability in corrosive atmospheres containing high concentrations of H2O, CO2 and H2S was investigated. Based on both the fresh (not exposed) and the treated (exposed to corrosive atmospheres) samples, the estimation of the tolerance degree was obtained by determining the: i) phase structures, ii) unit cell parameters, iii) surface microstructures, and iv) electrical conductivities.
Fresh ceramics were found to be single-phased in the whole range of x and all the treated materials exhibited good chemical stability in the water vapor atmosphere, whereas the samples with 0 ≤ x ≤ 0.2 and 0 ≤ x ≤ 0.3 were not single-phased in pure CO2 and 10% H2S/Ar, respectively. The analysis of crystal structure and transport characteristics of the treated BCZY0.3 samples has shown a weak deviation of unit cell parameters and no degradation in electrical conductivity. For fresh BCZY0.3 the transport nature in various atmospheres was also evaluated. At 600 °C the BCZY0.3 exhibited conductivity of 2.7, 4.0, 1.7 and 3.7 mS cm−1 in air, wet air, hydrogen and wet hydrogen atmospheres, respectively. Based on the obtained results, BCZY0.3 can be considered as a perspective proton-conducting material having reasonable transport and tolerance properties.
4. On the design and development of potentiometric and amperometric sensors for gaseous components determination [10-12].
The potential application of BaCe0.7Zr0.1Y0.2O3−δ proton-conducting material as an electrolyte for hydrogen sensor operating under potentiometric and amperometric modes was also investigated. The reason of selection of BaCe0.7Zr0.1Y0.2O3−δ composition was caused by optimal combination of target properties, including acceptable stability and thermal properties of the material as well as high proton conductivity. The dense electrolyte materials were used as a base of the electrochemical cells of the sensor and then sensor’s electrochemical properties were measured under different H2-containing atmospheres and temperatures. The obtained results demonstrate the response of the hydrogen sensor in both modes of operation. Its operability was confirmed by comparing the experimental data with theoretical predictions. The sensor based on the BaCe0.7Zr0.1Y0.2O3−δ proton-conducting electrolyte can be successfully used for the detection of hydrogen content (0.1–10 vol.%) in nitrogen at the temperature range between 450 and 550 °C.
5. On the utilization of proton conducting materials in electrochemical reaction (mostly reductions of CO2 for methanol production and of N2 for ammonia synthesis) .
In order to provide information about the possibility of using proton electrolytes in electrochemical reactions for carbon dioxide of nitrogen hydrogenation (for methanol or ammonia preparation respectively), the steady-state current-overpotential characteristics of the Fe|BaCe0.5Zr0.3Y0.08Yb0.08Cu0.04O3-δ|Fe interface as a function of the gas phase composition and temperature were investigated.
To this purpose a BaCe0.5Zr0.3Y0.08Yb0.08Cu0.04O3–δ material is successfully synthesized by solid state synthesis and sintered to dense ceramics at 1400 °C. Then its crystal structure, ceramic and electrical properties are investigated. It was found that the ceramic had a high relative density (more than 90%) and acceptable proton conductivity (1.5 and 6.8 mS cm−1 at 500 and 900 °C, respectively). On the as prepared BaCe0.5Zr0.3Y0.08Yb0.08Cu0.04O3–δ electrolyte disk, three thin Fe porous layers were deposited by painting on both sides (three electrode system), which was then immersed in a tubular single-chamber continuous electrochemical reactor.
The polarization measurements were carried out in the temperature range between 500 and 700 °C and at three different H2/He wet (3% steam) compositions. The apparent anodic and cathodic charge transfer coefficients (Tafel region) are found to be: αa = αc = 0.8, while the apparent activation energy was calculated to be approximately 0.55 ± 0.05 eV. It was also found that by increasing the hydrogen concentration the current density increases, especially at higher temperature value.
6. On the comparative analysis of various synthesis methods for the preparation of Proton-conducting electrolytes based on Ba(Ce, Zr)O3 
The identifying possible synthesis ways for the formation of a proton-conducting electrolyte based on BaCeO3 and BaZrO3 solid solutions has also been thoroughly investigated in the frame of our MG project. Among different methods (solid-state reaction, citrate-nitrate combustion, oxalate co-precipitation) and their CuO-modified analogs, the citrate-nitrate combustion or solid-state reaction synthesis coupled with the introduction of 0.5 wt% CuO were found to be more appropriate strategies for the development of not only single-phase, but also high-dense BaCe0.5Zr0.3Y0.2O3–δ ceramics.
The XRD and SEM analysis, dilatometry method and hydrostatic weighing were used for the estimation of quality of ceramics. The results obtained demonstrate that the use of SSR and CNC methods modified by the introduction of 0.5 wt% CuO promotes the achievement of the necessary properties of ceramic samples (excellent densification, single-phase, and non-porous microstructure).
The research activity of our research group is still continuing this year with the fabrication of prototypes of solid oxide fuel cells, solid oxide electrolyzers and amperometric/potentiometric sensors, using the as developed proton conducting materials.
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