Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

20010024999 Case Western Reserve Univ., Dept. of Mechanical and Aerospace Engineering, Cleveland, OH USA
Gas-Evolving Effect on Mass Transfer in Rotating Electrochemical Cells Under Microgravity
Kamotani, Y., Case Western Reserve Univ., USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena
Conference; December 2000, pp. 1478-1487; In English; See also 20010024890; No Copyright; Avail: CASI; A02, Hardcopy;
A10, Microfiche
On earth, gas evolution in electrochemical reactors occurs widely in the electrochemical industry. In water electrolysis and
electrochemical machining and electrosynthesis, the gas bubbles agitate the electrolytic cells and enhance the mass transfer,
thereby improving the yield of the electrochemical cells. In fact, there are relatively few major electrochemical processes in which
evolved gases do not appear. Therefore, the design of life support systems in space requires sufficient electrochemical information
related to the behavior of bubbles generated on an electrode. For example, the processes of carbon dioxide removal, oxygen generation,
water reclamation and human waste treatment in spaceflight involve gas generation in the systems. Although much research
work has been done on electrolytic gas evolution in the presence of gravity or forced flow, much less work is available on electrolytic
gas evolution in microgravity. In the absence of gravity the conventional electrochemical cells would be inefficient as the
generated bubbles tend to stay near the electrodes. In the present work we propose rotating electrochemical cells as a means of
enhancing the mass transfer significantly and removing the generated gas bubbles effectively in microgravity. Moreover, the
design of electrochemical cells becomes relatively compact because it does not require additional pumps and separators to process
the bubble-electrolyte flow. The subject of gas evolution in rotating electrochemical cells has never been investigated. Most studies
of gas bubbles in electrolytic cells were performed in the presence of natural convection or forced convection. Forced convection
can be used to remove gas bubbles from the electrodes of an electrochemical system in microgravity but the bubbles still need
to be removed from the electrolyte somehow. by rotating the cells one can remove the bubbles from the electrodes and the electrolyte.
The main objective of the proposed work is to gain basic knowledge of bubble behavior and its effect on mass transfer in
rotating electrochemical cells under microgravity as well as normal gravity conditions. The mass transfer rates and various bubble
layer characteristics will be measured and analyzed under various conditions in rotating electrochemical cells in normal gravity
as well as during parabolic flights. Based on the results, a theoretical model will be constructed to predict the bubble-induced convection
in rotating electrochemical cells. The test section, such is shown, consists of two horizontal electrode disks, which serve
as cathode (top) and anode (bottom), and a vertical annular Lexan side wall. The anode is made of copper. The cathode is divided
into eight ring-shaped sections. The reason for the sectioned cathode is to measure local mass transfer rate. With this arrangement
only hydrogen bubbles are generated from the top cathode. The generated bubbles move toward the center along the top wall. A
tube is attached to the top plate at the center to remove the bubbles. The gap between the electrodes is variable. A cupric sulfatesulfuric
acid solution (CuSO4 - H2SO4 - H2O) is used as the test fluid. The test section is placed in a rotating system. The bubble
layer along the cathode surface is illuminated by a laser light sheet and a video camera records the bubbles. The bubbles move
in the radially inward direction as well as in the azimuthal direction due to Coriolis force. The camera and laser light source are
placed on the rotating frame. to minimize the impact of high-g on the equipment, they are placed near the center of the rotating
frame. Mirrors and a fiber optic cable are used to illuminate and take pictures of the inner test section. From this measurement
we will determine the bubble layer thickness distribution and the bubble velocities (radial components) and sizes at various locations.
The mass transfer rate will be determined from the amount of electrodeposited copper on each sectioned platinum cathode
using the anode dissolution method. Applied current density in the present experiment is a combination of the partial current density
consumed by hydrogen gas generation and that for copper electrodeposition on each sectioned cathode. In the anode dissolution
method the electrolytic cell is charged, with the polarity reversed, at a constant current density to dissolve the electrodeposited
copper on the platinum anode. The passivation of the cell occurs when the deposited copper is completely depleted on the anode.
The amount of electrodeposited copper on each sectioned platinum can then be calculated from the current density and the time
to passivate. The local mass transfer rate and partial current density for copper deposition is determined from the amount of deposition.
After the data analysis, we will construct a theoretical model to predict the average mass transfer coefficient, or the average
Sherwood number, as a function of the important dimensionless parameters. It is known that generally there are three different
mass transfer mechanisms acting at gas evolving electrodes with an intensity depending on the operating conditions: (i) bubble-induced
microconvection owing to periodic microflows in the vicinity of gas bubbles adhering to and detaching from the electrode
surface, (ii) bubble-induced macroconvection owing to buoyancy flow of a gas-liquid dispersion along the electrode, the flow is
caused by the density difference between the dispersion and that of the (nearly) bubble-free liquid, and (iii) single-phase free convection
owing to density gradients in the liquid phase near the electrode surface. We will take basically the same approach in
constructing our model. In the present work, since the solutal boundary layer is relatively thin, we have basically the microconvection
and the bubble-induced macroconvection.
Author (revised)
Gas Evolution; Electrochemical Cells; Mass Transfer; Microgravity; Bubbles; Rotation
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