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Project Description and Objectives: |
The process of hydrogen evolution during alkaline electrolysis is governed by mass conversion, growth of hydrogen bubbles and removal of hydrogen. Two mechanisms are decisive: Dissolved hydrogen is carried off from the cathode surface by diffusion and convection, and gas bubbles are transported into a two-phase flow. Bubbles lead to an increase of the electrical resistance. They grow closely packed on the cathode surface and raise the resistance remarkably in a thin layer of the electrolyte. Gas voidage in the bulk is smaller than in the bubble layer, when forced convection assures a removal of the bubbles.
References:
[1] Riegel, H.; Mitrovic, J.; Stephan, K.; Determination of the Concentration of Dissolved Hydrogen in Electrolytic Solutions. Poster Presentation: 2. Technologies of Hydrogen Production, Hydrogen ´96
[2] Kreysa, G.; Kuhn, M.: Modelling of Gas Evolving Electrolysis Cells. I. The Gas Voidage Problem. J. Appl. Electrochem. 15 (1985) 517-526
[3] Maxwell, J. C.: A Treatise on Electricity and Magnetism. 2nd ed. Vol. 1 p. 435, Oxford: Clarendon Press 1881
[4] Bruggemann, D.A.G.: Berechnung verschiedener Physikalischer Konstanten von heterogenen Substanzen. Analen der Physik 24 (1935) 636-679
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Technical Goals: |
In the project, the influence of gas bubble growth and two-phase flow on electrolytical production of hydrogen was investigated. Sensors and measuring techniques were developed for the determination of the void-fraction distribution and the concentration of dissolved hydrogen in forced flow [1]. With increasing electrolyte velocity void fraction and resistance decrease. However a higher velocity requires a higher pumping power which reduces the efficiency of the process. Obviously, there exists a maximum of effiency at a certain gas voidage distribution. The mean velocity at this distribution was determined as a function of current density. Some representative results on void fraction are presented in the following.
The volumetric void fraction of hydrogen and its distribution in forced convection flow was determined by measuring the electrical resistivity of the two-phase-mixture flowing up in a vertical channel of rectangular cross section. The flow sheet of the experimental set-up is presented in Fig. 1. It consists of two closed circuits for hydrogen and oxygen. The gases were evolved on ten electrode-pairs mounted in the upper section of the electrolyzer. Cathodes and anodes were made of nickel, the size of which is 4 x 2 cm2. The channel was divided vertially by a diaphragm into two subchannels. The electrolyte was pumped through the elecrolyzer at velocities up to 1.0 m/s. At the end of the channel the two two-phase flows streamed into reservoirs were the fluids were purged from gas. After that, the electrolyte passed a heat exchanger, wherein the temperature could be regulated in a range from 200C to 600C. The whole electrolyte volume amounted to about 100 l.
At the upper end of the channel, above the last electrode pair, nine platin-electrodes (Pt-El.) were mounted perpendicular to the cathode surface, Fig. 2. The electrical current through the last electrode pair consists of a direct current superimposed by an alternate current of small amplitude. An BOP (Bipolar Operational Amplifier) served as current source. The frequency of the alternate current corresponds to the frequency of an external alternating voltage, supplied by an oscillator of a Lock-In Amplifier. The amplifier was used to measure the AC-voltage between neighboured platinum-electrodes at the frequency of the oscillator. The frequency of 1.1 kHz was high enough to avoid polarizing effects on the electrodes. With this technique the electrical resistance of the fluid was measured as a function of distance. The location of the platinum-electrodes (diameter 100 * 10-6m) was determined from the resistance without gas bubbles.
As is well known, the resistance of the electrolyte depends on the void fraction. Several relations corrrelate the resistance and the void fraction . Kreysa and Kuhn [2] examined the different relations and recommended those derived by Bruggemann [3] and Maxwell [4]. Since these equations differ only slightly from each other, we used the Maxwell equation where REl represents the electrical resistance with and R0,El without gas bubbles.
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