A bipolar cell for advanced alkaline water electrolysis

Int. 1. HydrogenEnergy,Vol.7, No. 9, pp. 703-710.1982. Printedin Great Britain.

0360-3199/82/090703--08$03.00/0 PergamonPressLtd. (~)1982InternationalAssociationfor HydrogenEnergy.

A BIPOLAR CELL FOR ADVANCED ALKALINE W A T E R ELECTROLYSIS J. DIVISEKand H. SCHMITZ Institute of Applied Physical Chemistry, Chemistry Department, Nuclear Research Centre, Juelich, Federal Republic of Germany (Received for publication 7 December 1981) Abstract--A new NiO diaphragm has been developed as a substitute for the conventional asbestos diaphragm used in alkaline water electrolysis. The NiO diaphragm has very promising corrosion and resistance properties and has demonstrated its good performance in long-time tests over a few thousand hours. For the production of highly active nickel electrodes, a galvanic method for the deposition of the activated Ni/Zn alloy was applied. By combining this new diaphragm and the nickel electrodes as construction elements, a bipolar cell in a sandwich arrangement was built, providing a very low cell voltage. With the described construction, further improvements of the anode should lead, in the future, to a cell which will operate at a higher heating value voltage and a current density of 500 mAcm -2. INTRODUCTION The best developed method for hydrogen production from water is at present direct electrolytic water decomposition using KOH solution as the electrolyte. All the other processes are still far away from technical realisation [1]. In order to compare the energy balance of alkaline water electrolysis with those of the thermodynamically favoured processes, the electrolysis efficiency has to be increased. The economic analysis results in the following technical conditions. (i) The energy efficiency should reach the value of r/= 1, where r/= VUHv/Vob, with a higher heating value voltage VHHv= 1.50V between 100 and 150°C [2] and the achieved voltage Vob~.A current output of 100% is assumed. (ii) Excluding noble metals with respect to costs, only steel and nickel can be used as the electrode and construction materials. This implies a maximum working temperature of 130-140°C with respect to sufficiently minimized corrosion. (iii) The current density should not be lower than 400-500 mA c m -2. These conditions approach economically those already calculated for water vapour electrolysis in molten hydroxides at 350°C [3]. In this paper, a bipolar cell that can fulfil the mentioned requirements is described. COMPONENTS OF THE CELL Diaphragm A porous metallic nickel body was turned into an oxide structure by thermal treatment in air and used as a NiO diaphragm [4]. The stability of this diaphragm was successfully tested during 3500 h in 40% KOH at 110°C. This production method yields diaphragms with excellent corrosion resistance in hot KOH up to 140°C and very low ohmic diaphragm resistance values between 50 and 90 mg2 c m 2 at 25°C in 10 M KOH. Dis703

advantages are caused by the two-step process involved [5] and by the fact that the oxidation has to be carried out under careful control so that interconnected metallic residual patches will not remain in the oxide structure (Figs. l a and 2a). In order to remove the residual metal patches, a relatively long oxidation treatment at 1100-1200°C is necessary. This creates a risk of brittleness. Therefore, during diaphragm production, metal powder is directly sludged and cold-pressed on the carrying nickel net and subsequently oxidized at 9001000°C. By this procedure, the sintering step is omitted. During the oxidation 2Ni + O2---~2 NiO the in situ-formed oxide grows through and penetrates the entire layer and at the same time fixes itself onto the net. This binding is mechanically very stable. The formation of metallic connections and residual metallic patches is avoided (Figs. l b and 2b) as the diffusion path of oxygen is in the order of the particle size (12 ~tm). The oxidation time can vary between 10 and 20 min.

(a)

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704

J. DIVISEK AND H. SCHMITZ

Fig. 2. (a) Micrograph of the diaphragm with the remaining metal patches. (b) Micrograph of the metal-free diaphragm structure.

The quality and adhesiveness of the oxide layer formed are the same as those obtained when using the usual sintering method and the diaphragm can be produced at strongly decreased temperatures. This in situ-method can also be applied in diaphragm production on the basis of other oxides or mixed oxides. NiO was chosen mainly for economical reasons. The stability of such thermally produced oxides is much better in alkaline medium than that of the anodically formed oxide layers. The oxides did not show any measurable corrosion in alkaline medium (see Table 1). The corrosion measurements should be extended and include higher temperatures and longer periods of time.

The most important diaphragm characteristics are summarized in Table 1. The corresponding pore size distribution of this diaphragm is shown in Fig. 3. Diaphragm manufacture causes no problems at present. Using simple laboratory equipment, diaphragms with a surface area of 400 cm 2 have been produced. It is well known that NiO is a p-semiconductor with Ni 2+ vacancies formed by a nickel deficit which can be expressed as Nil-~O. When oxidizing nickel in air at higher temperatures, typical compositions of the type Ni0.99670 are achieved [6]. On the other hand, the NiO powder, which was produced in air at 1000°C from INCO-nickel powder, had a composition of the type

Table 1. Characteristics of the NiO diaphragm Thickness (mm)

Surface resistance at 25°C in 10 M KOH (mf~ cm2)

Gas purity (%HJO2)

Main pore diameter (l~m)

Corrosion rate (10 M KOH, 120°C)

0.4-0.5

50-90

0.1-0.3

2

Not measurable after 1000 h

A BIPOLAR CELL FOR A D V A N C E D ALKALINE WATER ELECTROLYSIS

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706

J. DIVISEK AND H. SCHMITZ

NIO0.9887. This would imply an oxygen deficit or remaining surplus metal in the compound. Considering the manufacturing conditions, oxygen deficit is very unlikely to occur. The produced NiO diaphragm had the same composition. Its electric conductance as a function of temperature is shown in Fig. 4 with the oxygen partial pressure equal to that of air. The temperature dependence has the familiar shape of a logarithmic function [7, 8] and the resistance magnitude is typical for a NiO with Ni 2+ vacancies [8]. It can therefore be assumed that the diaphragm consists of an ordinary NiO with small amounts of remaining surplus metal. Increasing the oxidation period in order to remove the remaining metal is not suggested as the carrying net can become brittle. This diaphragm is not attacked by K O H and hydrogen at ll0°C during more than 3000 h or at 120°C during (a) _

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more than 1000 h operation time. No short-circuit risk occurred in long-time experiments up to 2700 h at 110°C in a cell with a sandwich electrode arrangement. It seems, however, that this NiO diaphragm is not suited to withstand temperatures above 200°C as the produced hydrogen possibly reduces it to metallic nickel. For application at higher temperatures diaphragms made from other materials were tested.

Electrodes Active electrodes were obtained on a Raney nickel base by the cathodic deposition of a Ni/Zn alloy on metal gauze or directly on the NiO diaphragm and subsequent activation was achieved by leaching the zinc in hot potassium hydroxide [5]. The relevant electrochemical properties of these electrodes have already been described [9]. Current-voltage characteristics of

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A BIPOLAR CELL FOR ADVANCED ALKALINE WATER ELECTROLYSIS the cathode and anode are shown in Fig. 5. The unusually small slopes b of the current-voltage relation for the hydrogen evaluation at the cathode with a b-value of 55 mV decade -1 indicate a different mechanism of cathodic hydrogen evolution rather than the usual 'slow discharge-fast recombination' obtained at room temperature on nickel in alkaline medium with o~= 0.5. The determined value for a: amounts to ca 1.5, which means that the electrodes have changed the hydrogen evolution mechanism and now show a catalytic effect which cannot be caused by the increase of the area only. In principle, with te of 1.5, at least three variants of the mechanism are possible. In order to determine the most probable variant, additional measurements with instationary methods and investigations concerning coverage of the surface with hydrogen are necessary. This work is in progress. The slope b of 5 5 m V d e c a d e -1 is extensively temperature-independent and always larger than expected for an tr of 1.5 in the temperature range between 25 and 120°C. Usually, one would expect at 25°C a b-value of 39 mV and at 120°C a b-value of 52 mV. The observed b of 55 mV can be explained by the porous structure of the electrodes. The ratio Q between the corrected electrode thickness 1 and the theoretical maximal penetration depth of electric current 3. equals Q = 1/3. = 0.05 at 25°C [9]. Therefore, the increase of the Tafel slopes b is to be expected in a region between b =2.303 × 2RT/3F and b= 2.303 x 4 R T / 3 F [10]. The curves should lie close to the lower boundary because of Q = 0.05. This agrees with the experimental results. The shape of the current-voltage relation for the anode in the oxygen regions is, as expected, more complicated, with two different slopes in the range of low and higher current densities. The curves at 25°C with a double tangent of 60 or 120 mV decade -1 (re = 1.0 or t r = 0.5), respectively, correspond with the already known double mechanism occurring on smooth nickel [11]: (i) At current densities below 100 m A cm -2 the ratedetermining step is

CONSTRUCTION CHARACTERISTICS OF THE CELL In constructing the cell, special attention has to be given to minimize the electrode distance and the influence of the developed gases on the cell voltage. On the basis of the model and comparative calculation it can be shown that the sandwich construction of the cell (Fig. 6) is well suited for this double purpose. Fig. 7 shows for different current densities two types of curves, where the voltage is plotted as function of the distance between the electrodes and diaphragm (10M KOH, 100°C, ambient pressure). In the first case (Fig. 7a), two activated sheet electrodes with a potential minimum are presented. This version has been modelled mathematically several times [12]. The magnitude of the potential minimum at constant current density follows, among others, a function increasing with the third root of the electrode height. This is a great disadvantage when increasing the scale of the cell. On the other hand, for comparable decrease of the distance between porous electrodes (Fig. 7b), the developed gases are not disturbing. The activated expanded metal gauze electrodes behave in a similar way, but are not as effective. Furthermore, it is very important that the poor contacts between the sandwich electrodes and the bipolar plate do not produce the similar potential losses as those gained by the sandwich construction and the application of porous electrodes. Pressure contacts between porous electrodes and the dipolar separation plate lead to ten times higher voltage losses on the anodic side, caused by the formation of oxide layers, as is the case on the cathodic side. As these voltage losses were much too high, pressure contacts were generally avoided and sufficient current distribution in the electrodes was ensured by the use of diagonally-conducting bars, expanded

Deposited Raney Ni-catalyst ~ NiO-diaphragm

OH(ads) + O H - ~ O(ads) + H20 + e. (ii) At current densities above 100 m A cm -2 the system starts oxidizing water H20 --> OH(ads) + H + + e which then becomes the rate-determining step. The difference between our results and those of Sato and Okamoto [11] lies only in the calculated formal exchange current densities for the separate intermediate steps. They are about 100 times larger for our electrodes. It can be therefore assumed that with a given electrode structure there is no catalytic effect on the anode in contrast to the cathode. The formal increase in the anodic current density is caused exclusively by the enlarged active electrode surface. With simple laboratory equipment, electrodes with a geometric working area of 300 cm 2 have been successfully produced.

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Bipolar currentcollector Fig. 6. Schematic view of the electrolysis cell.

708

J. DIVISEK AND H. SCHMITZ

(a)

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metal gauze or welded joints. Similar solutions of con- In Fig. 10, curve 1 was obtained with pressure contacts struction problems were described for chlor-alkali elec- between the bipolar plate and electrodes, curve 2 without those contacts and with activated expanded metal trolysis cells by Bergner and Hannesen [13]. The bipolar electrolysis cell units with geometric gauze electrodes, and curve 3 with porous electrodes working surface areas of the single electrodes of 30 cm 2 in direct contact with the diaphragm arranged in a were constructed as shown in Fig. 8. They were con- sandwich configuration. nected together in a removable manner, unlike the common filter press system, which can also be used. CONCLUSIONS Detailed comparisons between the system preferred by us and the filter press principle have been made recently (i) As a suitable substitute for the conventional asbestos by Bergner and Hannesen [13]. diaphragm, an oxide diaphragm, for example from NiO, can be applied. Fig. 9 shows the performance of the described system by means of the current-voltage curve of one cell unit. (ii) The method of cathodic deposition of an activated

A BIPOLAR CELL FOR ADVANCED ALKALINE WATER ELECTROLYSIS

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alloy, for example on a Ni/Zn base, allows simple construction of a bipolar cell for an advanced alkaline water electrolysis. The diaphragm and electrodes are stable for more than 3000 h of operation time at ll0°C. The anode has to be improved further so that the catalytic effect of the semiconducting oxides with spinel or Perovskit structures can be reached.

(iii) However, with the present Ni anode the cell voltage has already been significantly decreased by the described cell construction. This achievement has to be maintained when the anode is improved further. It must be remembered that the achieved gain in cell voltage is again lost if improved anodes lead to an inferior cell construction.

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710

J. DIVISEK AND H. SCHMITZ

<1 o

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Acknowledgement--We wish to acknowledge valuable discussions with Prof. Dr. H. W. Niirnberg, Director of Institute of Applied Physical Chemistry, in the preparation of this paper. 5. REFERENCES 1. H. W. Niirnberg, J. Divisek and B. D. Struck, Modern electrolytic procedures for the production of hydrogen from water. In A Perspective on Adaptive Nuclear Energy Evolutions, Towards a World of Neutron Abundance. IASSA, Laxenburg, (in press). 2. R. L. LeRoy, Ch. T. Bowen and D. J. LeRoy, J. electrochem. Soc. 127, 1954 (1980). 3. J. Divisek, J. Mergel and H. F. Niessen, Int. J. Hydrogen Energy 5, 151 (1980). 4. J. Divisek, J. Mergel and H. Schmitz, Hydrogen energy progress, improvements of water electrolysis in alkaline

6. 7. 8. 9. 10. 11. 12. 13.

media at intermediate temperatures. In Hydrogen Energy Progress, (T. N. Veziro~lu, K. Fueki and T. Ohta, eds.) Vol. 1, p. 209. Pergamon Press, New York (1980). J. Divisek, H. Schmitz and J. Mergel, Chemie-lngr-Tech. 52, 465 (1980). Y. Shihomura, I. Tsubohava and M. Kojima, J. phys. Soc. Japan 9, 521 (1954). S. P. Mitoff, J. chem. Phys. 35 882 (1961). K. Hauffe and A. L. Vierk, Z. phys. Chemie 196, 160 (1950). J. Divisek and H. Schmitz, Extended Abstr. of the 32nd ISE Meeting, Vol. II, p. 1101. September (1981). A. Winsel, Z. Elektrochem. 66, 287 (1962). N. Sato and G. Okamoto, Electrochim. Acta 10, 495 (1965). W. Thiele and M. Schleiff, Chem. Tech. 31,623 (1979). D. Bergner and K. Hannesen, Chemie-lngr-Tech. 52, 413 (1980).