1. Proton conductivity in solid oxides
50 years ago Forrat [1] supposed that some complex solid oxides, namely AlLa1-xMxO3 (M = Ca, Sr, Ba) had proton conductivity at high temperatures (600 – 1000oC). Later, Takahashi demonstrated the presence of proton conductivity in solid oxides like SrZrO3 [2]. Since then, many complex solid oxides with proton conductivity had been discovered and studied [3]. Proton carriers appear in the oxides having oxygen vacancies due to interaction of the oxide with water vapor of the gaseous phase. An oxygen ion of the water molecule occupies the vacancy and the protons localize at the oxygen ions. The protons can jump from one oxygen ion to another one thus providing charge transfer.
Furthermore, it was stated that all “proton conductors” present also oxygen conductivity. It was offered to call them "co-ionic electrolytes". Each charge carrier "q" is characterized by a conductivity provided by transfer of this charge carrier, sq. The ratio of a partial conductivity to the total conductivity, s, is called "transfer number", tq. Peculiarities of a charge transport in co-ionic electrolytes were considered in [4].
A lot of complex perovskites possess proton conductivity.
2. Application of solid oxide proton electrolytes
From the very beginning it was considered that these types of conductors could be a basis for different electrochemical devices. The most important of them are: solid oxide fuel cells (SOFCs), solid oxide electrolyzers (SOEs) and sensors.
2.1. Solid oxide fuel cells
Interest to proton conductors is primarily due to the fact that in the solid oxide fuel cell based on proton electrolytes (SOFC(H+)), complete utilization of fuel is attainable if this fuel is hydrogen and impossible in SOFC based on the oxygen-conductive electrolyte (SOFC(О2–)). Taking into account that in practice the fuel utilization factor (FUF) in SOFC (О2–) never exceeds 85 %, possibility to increase the efficiency factor under other equal conditions only due to the increased FUF looks rather attractive.
On the other hand, thermodynamic analysis shows that an average electromotive force (EMF) in the SOFC(H+) is significantly higher than that in the SOFC(O2-), Fig 1a [5]. On the whole, the electrical efficiency of the former is by 15-20% higher than of the latter [6].
Figure 1. EMF distribution along the SOFCs (a); dependence of SOFC(H+) efficiency on temperature and relative power (b).
2.2. Solid oxide electrolyzer
An idea to use electrochemical devices based on solid oxides for hydrogen production by means of steam decomposition appeared in the late 1960s [7]. In the late 1970s Rohr [8] performed series of experiments with electrolysis cells and stacks and showed that they have acceptable stability during ca. 8000 h. Since then the base of the electrolysis cells was yttria-stabilized zirconia (YSZ) having oxygen ion conductivity. It was demonstrated that a high temperature solid oxide electrolyzer (SOE) based on an oxygen ion solid oxide electrolyte had acceptable characteristics when it worked at 1.1V [9], which corresponded to electrical power inputs 2.63 kWh/m3 H2. In this case it is necessary to supply the SOE electrochemical section and the vaporizer with additional heat (about 0.5 kWh/m3 H2 each), but both electrical power inputs and total (electrical + heat) inputs are significantly lower than in the low temperature electrolyzer.
In the beginning of 1980s it was proposed to use them for steam electrolysis [10]. Since then a number of experiments were performed with the cells based on proton electrolytes. Recently, the interest to the application of solid oxide proton electrolytes for steam electrolyzers is growing [11]. The solid oxide electrolyzer based on proton electrolyte (SOE(H+)) allows producing pure hydrogen whereas hydrogen produced by the SOE(O2-) contains at least some per cent of steam. A theoretical model of a high temperature electrolyzer based on solid oxide co-ionic electrolyte was developed in [12].
2.3. Sensors
Sensors on the base of based on solid oxide proton electrolytes allows detecting hydrogen in mixtures with inert gases and nitrogen. Very recently amperometric hydrogen sensors based on proton-conducting solid oxide electrolytes of La0.95Sr0.05YO3 and CaZr0,9Sc0.1O3 compositions are prepared and investigated in the Laboratory of Electrochemical Devices based on Solid Oxide Proton-Conducting Electrolytes at the IHTE [13].
The report on this type of sensors is presented in this conference by Gorbova et.al.
[1] Forrat F., Dauge G., Trecoux P., Danner G., Christen M., C.R. Acad. Sci. (Paris) 259 (1964) 2813
[2] Takahashi T., Iwahara H., Solid State Ionics, 17 (1980) 243
[3] T. Schober, Solid State Ionics, 162-163 (2003) 277.
[4] A. Demin, E. Gorbova, M. Glumov, P. Tsiakaras, Ionics 11 (2005) 289.
[5] A. Demin, P. Tsiakaras, Internat. J. Hydrogen Energy, 26 (2001) 1103.
[6] A. Demin, P. Tsiakaras, A. Sobyanin, S. Hramova, Solid State Ionics 152-153 (2002) 555.
[7] H.S. Spacil, J.T. Tedmon, J. Electrochem. Soc. 116 (1969) 1618.
[8] F. J. Rohr, in: T., Takahashi, A., Kozawa, (Eds.), Applications of Solid Electrolytes, JEC Press, Cleveland, OH, p. 196.
[9] A.K. Demin, B.L.Kuzin, A.S. Lipilin, Soviet Electrochim. 23 (1987) 1258.
[10] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359.
[11] J. W. Phair, S.P.S. Badwal, Ionics 12 (2006) 103.
[12] E. Gorbova, A. Demin, P Tsiakaras, Journal of Power Sources, 171 (2007) 205
[13] A. Kalyakin, G. Fadeyev, A. Demin, E. Gorbova, A. Brouzgou, A. Volkov, P. Tsiakaras Electrochimica Acta, 141 (2014) 120.