JPH036996B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to an electrode having a nickel metal layer containing 30% by weight of sulfur as an electrode active substance on the surface of a conductive substrate, and is particularly useful for electrolysis of aqueous alkali metal halide solutions and water electrolysis. The present invention relates to a novel electrolytic cathode exhibiting low hydrogen overvoltage. [Prior art and its problems] Conventionally, the method of producing caustic alkali by generating hydrogen through electrolysis of alkali metal halides, especially sodium chloride and potassium chloride aqueous solutions, and electrolysis of water, is well known industrially. It is widely used. However, in recent years, due to the sharp rise in energy costs, various efforts have been made to reduce the electrolytic voltage in various types of electrolysis to lower the power consumption rate. For example, in the salt electrolysis method using an ion exchange membrane, in order to reduce the electrolysis voltage, the cathode hydrogen overvoltage is reduced, the ion exchange membrane resistance is reduced, and the solution resistance and bubble resistance loss are reduced by improving the cell structure. It is being said. Among these, mild steel has been used as the cathode in the above electrolysis due to cost considerations, but since it has a hydrogen overvoltage of 400 mV under normal electrolysis conditions,
As an active cathode for hydrogen generation, a cathode in which a cathode active material is coated on a conductive substrate made of mild steel or nickel by a method such as plating, sintering, or thermal spraying, or a method for manufacturing the cathode, has been proposed. For example, Japanese Patent Application Laid-Open No. 54-62933 discloses that nickel ions can be obtained by nickel plating using a plating bath containing () a complexing agent, () ammonium or boric acid, and () a sulfur-containing and/or nitrogen compound. Method of using as a cathode; JP-A-Sho
No. 57-19388 discloses an electrolytic cathode formed by applying nickel plating to the surface of a metal substrate using a nickel plating bath containing nickel salt, thiocyanate groups, and ammonia ions; JP-A-57-113836 discloses a cathode coated with a nickel thin film containing sulfur and sulfur;
A method of using mixed metal sulfide electrocatalysts containing at least two said metals and sulfur in an electrochemical process by treating said solution with H ₂ S or a suitable sulfide, JP Publication No. 56-5994 discloses that a layer of powdered nickel or nickel-containing powder alloy is formed on a conductive substrate by a method such as plasma spraying or electroplating, and sulfurization is applied to this layer under conditions that allow sulfur to adhere to the surface. Electrolytes for electrolysis and the like are disclosed. However, the above-described cathode or manufacturing method has problems such as insufficient hydrogen overvoltage reduction effect or hydrogen overvoltage increasing over time. [Means for Solving the Problem] As a result of intensive research on active cathodes with reduced hydrogen overvoltage, the present inventors discovered that a nickel metal layer containing 30% by weight or more of sulfur as an electrode catalyst was formed on the surface of a conductive metal substrate. The present invention was achieved by discovering that an electrode having a hydrogen overvoltage has a low initial hydrogen overvoltage and can maintain a low hydrogen overvoltage over a long period of time, especially as a cathode for electrolysis. That is, the present invention is an electrode having a nickel metal layer containing 30% by weight or more of sulfur on the surface of a conductive substrate. The conductive substrate used in the present invention is not particularly limited as long as it is a metal that is durable under the environment in which it is generally used as an electrode, but metals such as mild steel and nickel, or alloys containing these as main components are generally used. preferable. Furthermore, the shape of the conductive substrate is not particularly limited, and may be a shape generally used as an electrode in an electrolytic cell, such as a flat plate,
These include net-like, punched metal, expanded metal, and sag-like shapes. The nickel metal layer of the present invention is not particularly limited as long as it contains 30% by weight or more of sulfur, and the higher the sulfur content, the more favorable the effect will be.
Taking into account the increase in electrical resistance, 30 to 80% by weight is generally sufficient, preferably 30 to 60% by weight. Manufacturing methods include, for example, a method of providing a nickel metal layer on the surface of a conductive substrate by electroplating from a sulfur-containing nickel plating bath, and a method of providing a nickel metal layer on the surface of a substrate by vacuum deposition of a mixture of nickel and sulfur or a sulfide of nickel. Alternatively, a method of forming a nickel metal layer on the surface of the conductive substrate by chemical vapor deposition (CVD) using a mixture of nickel and sulfur or sulfide of nickel is preferably employed. Among the methods for obtaining these nickel metal layers, especially when performing electroplating, a high sulfur content in the nickel metal layer can be easily obtained by using the pulse plating method in which the current is interrupted for a short time at regular intervals. It is even more suitable. Incidentally, as described in the above-mentioned JP-A-54-62933 and JP-A-57-19388, when nickel plating is applied by the DC plating method, sulfur is added to the nickel plating layer.
It is difficult to contain 30% by weight or more, and as a result, the effect of reducing the (initial) hydrogen overvoltage in the resulting cathode and the effect of suppressing the increase in the hydrogen overvoltage over time are insufficient. As the above-mentioned plating bath, for example, a sulfur-containing nickel plating bath containing rhodan nickel, nickel sulfate, nickel chloride, etc. as a main component and adjusted with nickel thiosulfate, thiourea, etc. is preferably used. The composition and plating conditions of these plating baths may be determined to be the same as or in accordance with known bath compositions and conditions so that the sulfur content in the resulting nickel plating layer is 30% by weight or more. In addition, in the pulse plating method, the time period during which the current is flowing is Ton (msec), the time during which the current is stopped is Toff (msec), and the duty cycle is defined as shown in the following formula. Duty Cycle (%) = Ton / Ton + Toff × 100 The above pulse current employed in the present invention,
Ton and duty cycle cannot be determined unconditionally because they vary depending on the composition of the plating bath, etc., but the pulse current is 0.1 of the normal electroplating current value.
~50 times, Ton is preferably 1 μsec to 1 sec, and duty cycle is preferably 1 to 80%. In addition, when obtaining the above-mentioned nickel metal layer, it may be an alloy layer containing about a few percent of cobalt, molybdenum, iron, chromium, tungsten, etc. together with nickel, or an alloy layer containing a further soluble metal or compound. An electrode in which these soluble substances are subsequently eluted is also preferable because it can further reduce the hydrogen overvoltage. [Function and Effect] As described above, the electrode of the present invention maintains a low hydrogen overvoltage for a long period of time, and has significantly superior activity and durability compared to an electrode having a nickel metal layer with a low sulfur content. ing. The reason for this is
Although it is not fully clear, it is believed that the contained sulfur plays a major role in the hydrogen evolution reaction in the nickel metal layer. For example, when examining the relationship between the crystal structure and sulfur content of a nickel metal layer created by the electroplating method using In the layer, a diffraction pattern showing the microcrystalline structure of nickel was obtained, and no change was observed even after the electrolytic test. On the other hand, in a nickel metal layer containing 30% by weight or more of sulfur, an X-ray diffraction pattern showing an amorphous structure is obtained before electrolysis, but after the electrolytic test, the amorphous diffraction pattern shows a microcrystalline structure of nickel. It had changed into a diffraction pattern. When the hydrogen overvoltage of both layers was measured in an alkaline aqueous solution, the nickel metal layer containing 30% by weight or more of sulfur showed a significantly lower hydrogen overvoltage over a long period of time. This tendency suddenly becomes noticeable when the content exceeds 30% by weight. From this, it is presumed that the electrode characteristics of the nickel metal layer in the hydrogen generation reaction are determined by the amount of sulfur contained in the nickel metal layer, regardless of the crystal structure. Incidentally, when obtaining the nickel metal layer, even if trace amounts of carbon, nitrogen, phosphorus, etc. are mixed in, the effect will not be affected. Further, the thickness of the nickel metal layer formed on the surface of the conductive substrate of the present invention is generally about 5 to 150 .mu.m, preferably about 10 to 100 .mu.m, because if it is coated too thickly, it will easily fall off. The electrode of the present invention is extremely useful as a cathode that exhibits a low hydrogen overvoltage over a long period of time, particularly in the electrolysis of aqueous alkali metal halide solutions and water electrolysis. Further, the present invention provides a good and inexpensive electrolytic cathode without using expensive platinum group metals, and is extremely advantageous industrially. Furthermore, the electrode of the present invention is considered to be usable as a hydrogen electrode for fuel cells and the like due to its effect of activating hydrogen. [Examples] Examples of the present invention will be shown below, but the present invention is not limited to these Examples. Example 1 A base material made by welding a 3mmφ Ni coated Teflon tube onto a 20mm x 40mm, 3mm thick mild steel plate.
Pretreatments such as degreasing and pickling were carried out using conventional methods. This substrate was plated under the conditions shown in Table 1 using a sulfur-containing nickel plating bath having the composition shown in Table 1. Among these, (A) and (B) are the cathodes of the present invention, and (C) and
(D) is a comparative example.

【table】

[Table] When the sulfur content in the nickel metal layer obtained by this plating was investigated, (A) 32% by weight (B) 40
The weight percent (C) was 28 percent by weight, and the percent (D) was 13 percent by weight. Table 2 shows the results of electrolysis in a 30% by weight aqueous solution of caustic soda using the product thus obtained as a cathode and a platinum plate as a counter electrode.

[Table] From this result, the hydrogen overvoltage of the cathodes (A) and (B) containing 30% by weight or more of sulfur is 0.17 to 0.5% compared to the cathodes (C) and (D) with a low sulfur content.
It can be seen that the value is as low as 0.23V. Also, 3
After several months, almost no increase in the hydrogen overvoltage was observed for the cathodes (A) and (B), but the hydrogen overvoltage for the cathode (C) increased by 50 mmV, reaching almost the same value as the cathode (D). . When the sulfur content in the nickel metal layer after electrolysis was examined, no decrease in sulfur was observed in any of the cathodes. Example 2 A substrate pretreated in the same manner as in Example 1 was plated under the conditions shown in Table 3 using a sulfur-containing nickel plating bath having the composition shown in Table 3. Of these (A)
is the cathode of the present invention, and (B) is a comparative example.

【table】

[Table] When the sulfur content in the nickel metal layer obtained by this plating was examined, it was found to be (A) 35% by weight (B) 15% by weight. Table 4 shows the results of electrolysis using the product thus obtained as a cathode in the same manner as in Example 1.

[Table] From this result, the hydrogen overvoltage of the cathode (A) containing 30% by weight or more of sulfur is approximately 0.15 V lower than that of the cathode (B) with a low sulfur content. Recognize. Further, after three months, almost no increase in hydrogen overvoltage was observed in the cathode of (A), but the hydrogen overvoltage increased by 70 to 80 mV in the cathode of (B). When the sulfur in the nickel metal layer after electrolysis was examined, no decrease in sulfur was observed in any of the cathodes.

Claims (1)

[Claims] 1. An electrode having a nickel metal layer containing 30% by weight or more of sulfur on the surface of a conductive substrate.