Electrochemical cell

Details Electrochemical cell Electrochemical cell

2001-06-22

2003-02-25

Bell, Bruce F. (Department: 1741)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S255000, C204S280000, C429S006000, C429S006000, C429S006000, C429S006000, C429S006000

Reexamination Certificate

active

06524452

ABSTRACT:

The present invention relates to electrochemical systems for the storage and delivery of electrical energy and, in particular, to apparatus for building such systems.
BACKGROUND OF THE INVENTION
Industrial electrochemical systems, such as secondary batteries, fuel cells and electrolysers, typically consist of modules which each comprise a number of repeating layered sub-assemblies clamped together to form a stack. For example, in a secondary battery of the redox flow type each sub-assembly typically consists of an electrically insulating flow-frame (i.e. a device which supports the other constituent parts of the sub-assembly and which also defines channels for the flow of electrolytes), a bipolar electrode, an ion-selective membrane or a combined membrane-electrode material and, optionally, other component layers such as meshes or electrocatalytic materials. A plurality of such sub-assemblies may be sandwiched together between suitable end-plates so as to create a plurality of electrochemical cells in series. Each cell thus comprises the positive and negative surfaces of two bipolar electrodes with an ion-selective membrane positioned therebetween so as to define separate anolyte-containing and catholyte-containing chambers within each cell, said chambers optionally comprising additional components such as meshes or electrocatalytic materials. The two electrolytes are typically supplied from two reservoirs to the cell chambers via an electrolyte circulation network. Electrochemical systems of this type are well known to a person skilled in the art.
In the manufacture of components for the creation of such assemblies there are a number of important considerations. In particular, it is desirable to suppress shunt currents within the electrolyte circulation networks. Shunt currents occur because of the conductive pathways that are created by the network of electrolyte connections linking the cell chambers. They are a particular problem for stacks which contain a large number of bipoles and their occurrence decreases the efficiency of the cell. Additionally, it is advantageous to make efficient use of all the available surface area of the electrode. In order to do this the electrolytes must be distributed evenly over the surfaces of the electrodes upon entering the cell chambers. Furthermore, in order to ensure that the fluids which are inside the stack are isolated from each other and contained successfully with minimal leakage to the outside, it is necessary for satisfactory seals to be provided between the individual components within the stack.
The occurrence of shunt currents within such cell arrays is discussed by P. G. Grimes and R. J. Bellows in a paper entitled “Shunt current control methods in electrochemical systems-applications”, appearing in Electrochemical Cell Design, R. E. White, Ed.: Plenum Publishing Corp, 1984, page 259. Typically, shunt currents are reduced by the provision of labyrinthine pathways for the electrolytes between the electrolyte circulation networks and the individual cell chambers. One method for achieving such a pathway has been to connect long-tubes between the electrolyte circulation networks and each of the individual cell chambers. However this method suffers from the disadvantage that it requires at least two seals, one at either end of the tube, which complicates the assembly procedure and can cause problems with electrolyte leakage especially since the seals must cope with pressure differentials which usually exist between the internal system and the external environment. Another method for providing a labyrinthine pathway involves forming a long groove into the surface of the flow-frame from a point in communication with the electrolyte circulation network to a point in communication with the individual cell chamber. On stacking the sub-assemblies a plate is sandwiched between successive layers so as to seal the groove and form a labyrinthine pathway for the electrolyte. This method suffers from the disadvantage that the costs of forming the grooves can be high and an extra layer, i.e. the plate, must usually be incorporated into the assembly to provide efficient sealing. This method also often requires large frame areas upon which to form the grooves. Electrolyte leakage is a particular problem in methods for controlling shunt currents which involve labyrinthine pathways for the electrolytes. Efficient fluidic sealing of the pathways is required to prevent leakage and this problem may be exacerbated by the fact that high pumping pressures are often required to push the electrolytes through the narrow pathways. Other solutions to the problem of shunt currents include electrically breaking the circuit by arranging for the flow to break up into droplets or spray or by using some form of syphon; even mechanical water wheel type structures have been proposed. Such solutions are rarely used in practice however because the mechanical and flow regimes are difficult to implement. Other solutions, rather than eliminate the shunt currents, attempt to control their effects, for example, by deliberately shunting the current through an auxiliary electronic circuit or by passing an appropriate current through the common manifold or channel interconnectors. However, these techniques do not necessarily reduce overall power loss.
It would be advantageous to provide a flow-frame, suitable for forming a sub-assembly as described above, which is a repeating structural unit within an array of electrochemical cells formed from a stack of said sub-assemblies. The flow-frame would advantageously provide a framework for supporting all the other elements of the cell array within a sealed environment together with means for providing resistance to shunt currents and means for distributing an even flow of electrolyte through the chambers of each cell.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a flow-frame for forming a sub-assembly; said sub-assembly comprising a bipolar electrode and an ion-selective membrane mounted on said flow-frame and wherein said sub-assembly may be stacked together with other such sub-assemblies to create an array of electrochemical cells, each cell thus comprising two electrode surfaces with an ion-selective membrane positioned therebetween so as to define separate anolyte-containing and catholyte-containing chambers within each cell; wherein said flow-frame is formed from an electrically insulating material and comprises
(i) a chamber-defining portion for supporting an electrode and a membrane within a defined space,
(ii) at least four manifold-defining portions which, on stacking said flow-frames, define four manifolds through which the anolyte and the catholyte are supplied to and removed from said anolyte-containing and catholyte-containing chambers,
(iii) at least two chamber entry ports for allowing the anolyte and the catholyte to flow from said manifolds into said anolyte-containing and catholyte-containing chambers, and
(iv) at least two chamber exit ports for allowing the anolyte and the catholyte to flow from said anolyte-containing and catholyte-containing chambers into said manifolds,
characterised in that one or more of the manifold-defining portions also define a pathway for the passage of the anolyte/catholyte between the manifold and the chamber entry/exit port.
Thus, in the present invention, the pathway for the passage of the anolyte/catholyte between the manifolds and the chamber entry/exit ports is formed within the manifold-defining portions of the flow-frame. The pathway may comprise grooves cut into one surface of the manifold-defining portions of the flow-frame. On stacking the flow-frames the grooves are sealed by the flat surface of the manifold-defining portion of the adjacent frame to form sealed pathways. Preferably the pathway defined within the manifold-defining portions does not allow electrolyte to travel in a straight line directly between the manifold and the chamber entry/exit ports. Preferably it causes the electrolyte to take a tortuous or labyrinthine path between the mani

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