JPS5834183A - Electrolyzing method of steam - Google Patents

Abstract

PURPOSE:To produce inexpensive hydrogen by electrolyzing steam by the use of electrodes having the structure wherein the surface of a solid electrolyte is coated with noble metals. CONSTITUTION:Paste of noble metals (e.g.; Ag) is coated on the interface of a solid electrolyte (e.g.; ZrO2 stabilized by Y2O3) expressed by the formula and after drying, the paste is sintered at 600-800 deg.C in dry N2, whereby an electrode structural body coated with a noble metal electrode 2 is produced. When steam is electrolyzed by using a steam electrolyzing cell consisting of such electrode structural body, the temp. indicating oxygen ion conductivity shifts to the lower temp. side as compared to such structural body having no electrodes of noble metals. More specifically, the above-described steam electrolyzing cell electrolyzes steam sufficiently at low temp. of 350-450 deg.C, whereby H2 is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a steam electrolysis method for producing high-quality and inexpensive hydrogen by baking a noble metal as an electrode on a solid electrolyte interface.

Conventionally, various methods for producing hydrogen from water have been proposed, but the mainstream is water electrolysis. Recently, due to the high cost of electrolytic hydrogen, steam electrolysis has also been considered.

However, the current drawback is that it requires a high temperature of about 1000° C. for operation.

The object of the present invention is to provide a steam electrolysis method which eliminates the above-mentioned drawbacks, operates at low temperatures, and produces hydrogen at low cost.

That is, the steam electrolysis method of the present invention uses the general formula AOm-BO
A steam electrolytic cell is used, which includes an electrode having a structure in which the surface of a solid electrolyte represented by n is coated with M. FIG. 1 and FIG. 2 show the structure of an electrode and a steam electrolytic cell according to the present invention. In Figures 1 and 2] is a solid electrolyte, 2 is an electrode, 3 is an alumina tube, 4 is a stainless steel tube, 5 and 6 are lead wires, 7°8 is a thermocouple, 8 is a brass stopper, 9
is the 0-ring. 10 is a pipe that is directly connected to the system that can change the oxygen partial pressure and water vapor partial pressure. Reference numeral 11 denotes a pipe directly connected to a system that can change the oxygen partial pressure.

It has already been reported that the above-mentioned solid electrolyte exhibits good oxygen ion conductivity. The present invention was achieved by discovering that the temperature shifts to the lower temperature side compared to that without noble metal electrodes. This will be specifically explained using the following examples.

Example 1 Figure 3 shows the relationship between temperature and electromotive force when Ag is used as the electrode of zirconia ZrO2,2 stabilized with yttria Y2O3 as the solid electrolyte in Figures 1 and 2.
Shown below. 12 is the result when electrodes are used, and 13 is the result when no electrodes are used. The solid line in the figure is the electromotive force determined from equation (1). However, P(jt = 159 Torr(0
2), PO2== 8. OTorr, (02). Further, η is l. That is, in an oxygen ion transfer solid electrolyte, when there is a difference in oxygen concentration on both sides of the electrolyte, oxygen moves from the side with a higher oxygen partial pressure to the side with a lower oxygen partial pressure, and an electromotive force shown in equation (1) is generated during this time. By the way, the temperature at which the electromotive force can be measured is significantly shifted to the lower temperature side when there is an electrode than when there is no electrode. This can be considered as follows. When oxygen moves from a higher partial pressure to a lower partial pressure, oxygen molecules first dissociate and adsorb onto the electrode surface, become atomic oxygen, and diffuse through the electrode. At this time, a metal oxide is formed between the oxygen atom and the metal atom, and the thermal stability of this metal oxide is thought to determine the performance of the electrode. According to this idea, the temperature of 30°C at which the electromotive force can be measured as in this example is AgtO
decomposition temperature.

The silver electrode was prepared by the following method. The results in Figure 3 are (m
(1).

(The same results as in Figure 3 were obtained with the electrodes obtained by the IQ method.

(1) Silver paste (Ag fine powder made into a slurry with surfactant + ethanol + cellulose solution, etc.) is applied to the solid electrolyte interface, and after drying, it is heated to 600°C in dry N2.
Sintering was performed at ~800'C for approximately 3-6 hours.

(ii) After silver deposition in vacuum, at 60°C, in a hydrogen stream for 2
Sintered for hours.

(tri) A mixed solution of silver nitrate ammonia solution and Polmarine was prepared, a solid electrolyte was immersed in the prepared solution, silver was precipitated, and then washed with water and sintered at 600° C. for 2 hours in a hydrogen stream.

Example 2 Ittria Y! is used as the solid electrolyte l in FIGS. 1 and 2. Zirconia Zr stabilized with Oi 02.2
FIG. 4 shows the relationship between temperature and electromotive force when Pt is used as the electrode. Curve 13 is the case when a Pt electrode is used,
Curve 14 is the case without electrodes. The solid line is the electromotive force determined from equation (1) as explained in Example 1. In the case of a Pt electrode, the temperature at which the electromotive force can be measured is around 560° C., which, as explained in Example 1, is close to the decomposition temperature of PtO. The Pt electrode was created by the method (1) as described in Example 1, replacing the Ag paste with a Pi paste.

Example 3 The relationship between temperature and electromotive force when using CaO-stabilized zirconia Zr0z, 2 and Ag as the solid electrolyte in Figures 1 and 2 is shown in Figure 5. . The solid line is obtained from equation (1), curve 15 is the case when an Ag electrode is used, and curve 16 is the case without the electrode. The electromotive force can be measured from around 300°C.

Example 4 FIG. 6 shows the relationship between temperature and electromotive force when Ag is used as the electrode of ZrO2,2 stabilized with MgO as the solid electrolyte of 1 in FIGS. 1 and 2.

Curve 17 is the case with the electrode, and curve 18 is the case without the electrode. The solid line is the electromotive force calculated from equation (1).

The electromotive force can be measured from around 300°C.

As explained in Example 1.3.4, even if the solid electrolyte is different, the electromotive force can be measured at almost the same temperature at the silver electrode. is considered to be fully possible in the low temperature range. Furthermore, as explained in equation (1), the fact that the electromotive force can be measured means that oxygen moves from the side where the oxygen partial pressure is higher to the side where the oxygen partial pressure is lower. This is shown in Example 5.

Example 5 In FIGS. 1 and 2, zirconia Zr0z stabilized with yttria Y2O3 was used as the solid electrolyte 1, Ag was used as the electrode 2, and PO'2 = 10-2 to 102.
Torr (02), Po2 = 10' Torr (0
When setting 2), 1 diffuses to the side where the oxygen partial pressure is low.
FIG. 7 shows the relationship between the amount of oxygen per second and the voltage applied at both ends. 20 is 400℃, PO & = 50 Torr
(Ox), 21 is 350℃, PO'2 = 59
Torr (02), 22 is 400℃, PO'2-
This is the result when 1×10”Torr (02).p
When the oa is around 50 Torr, oxygen will diffuse by simply shorting both ends. Further, when POa is 10 −2 Torr, it is necessary to apply a voltage to diffuse oxygen.

From this, it can be seen that this steam electrolyzer can be easily operated by applying a voltage to both ends or by increasing the partial pressure ratio of 02. Moreover, as shown in Example 1, oxygen diffuses sufficiently even in the low temperature range where electromotive force can be measured.

As shown in the examples above, a steam electrolyzer in which a silver electrode is attached to an oxygen ion transfer solid electrolyte can be used in a low temperature range (350° to
It is suggested that water can be sufficiently decomposed and hydrogen can be obtained even at 450°C. This will be explained by the following example. Table 1 shows the water vapor partial pressure of 20~20 using the steam electrolyzer according to the present invention.
These are the results of steam decomposition at 30 Torr.

According to Table 1, as expected in Example 2, the Pt electrode requires a temperature of 700° C. or higher to decompose water with 100% current efficiency. On the other hand, an Ag electrode can decompose water with 100% current efficiency even at a low temperature of 400°C.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of the electrode, Figure 2 is a cross-sectional view of the steam electrolyzer, and Figure 3 is the PO of Ag1ZrO22Y20111 Ag.
'2 = 159 Torr (02), Po2 = 8 T
orr (02), Figure 4 is a diagram showing the relationship between electromotive force and temperature for Pt1Zro2Y20slPt (conditions are the same as Figure 3), Figure 5
The figure shows the relationship between the electromotive force and temperature of Ag l Zr (h-CaOIAg (the conditions are the same as in Figure 3). Figure 6 shows the relationship between the electromotive force and temperature of Ag l Zr (h-CaOIAg).
g l A diagram showing the relationship between the electromotive force and temperature of Zr02-MgOIAg (the conditions are the same as in Fig. 3), and Fig. 7 shows the relationship between the electromotive force and temperature of Zr02-MgOIAg.
FIG. 3 is a diagram showing the relationship between the amount of oxygen diffused by ZrO+ Y20B1 Ag and the applied voltage. Humidity 0) 4th mountain one! 7L (@go) -427- O71 sword C floor (v)

Claims (1)

[Claims] A solid represented by the general formula AOm-BOn (where A is Zrs, B is a generic term for Hg, Ca5Y, and rare earth elements, and m and n are numerical values determined by the thickness values of A and B, respectively. The same applies hereinafter). The surface of the electrolyte is M (where M is Ru5Rhx Pd, Ag, Irs
It is a general term for Ptq Au. same as below. ) Steam electrolysis method using an electrode with a structure coated with