Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-04-12
2003-04-29
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S010000, C429S010000, C429S105000
Reexamination Certificate
active
06555267
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrochemical reactors for conducting reduction and oxidation reactions in respective positive and negative liquid electrolytes, without gas evolution at the electrodes. More specifically the invention relates to a membrane-separated, bipolar multicell electrochemical reactor for implementing a redox flow battery system.
2. Description of Related Art
Redox flow battery systems are increasingly attracting interest as efficient energy conversion systems. Among redox couple candidates, the all vanadium redox system being one of the most preferred.
Structurally, the electrochemical reactors that have been proposed for redox flow battery systems, have been derived from the electrochemical reactor structures developed for general electrolysis processes, the only adaptation having concerned the materials employed as electrodes.
Generally, the electrochemical reactors used as redox batteries are conventionally composed of a stack of bipolar plate electrode elements separated by ion exchange membranes, defining a positive electrolyte flow chamber on one side of each membrane and a negative electrolyte flow chamber on the opposite side thereof. The stack of bipolar elements is assembled together in a filter-pass arrangement between two end electrode elements.
Commonly, the elements have a frame provided with co-ordinated through holes forming inlet and outlet manifolds for the two electrolytes that are circulated in a parallel mode through the positive electrolyte flow chambers and the negative electrolyte flow chambers, respectively.
The elements are conventionally mounted and operated in a vertical position.
The assembling of a large number of bipolar elements in electrical series as required in redox batteries to reach an adequate voltage at the two ends of the battery, the positioning of innumerable gaskets for sealing the outer perimeter of each electrolyte flow chamber and the perimeter of the distinct through holes of the frames for defining the inlet and outlet manifolds for the two electrolytes and the final tightening of the filter press assembly by tie rods compressing the two end elements over the stack, are extremely delicate and time consuming operations that require particularly skilled technicians.
The parallel flow of the two electrolytes through the respective flow chambers poses serious problems in terms of minimization of so-called stray or by-pass electric currents in uninterrupted liquid veins of electrolyte, due to the fact that the electrolyte present in the manifolds offer innumerable paths for these by-pass or stray currents, driven by mutual voltage differences existing among the various bipolar elements functioning in electrical series between the two end electrodes on which the full battery voltage difference insists. By-pass or stray currents decrement the energy efficiency of the conversion system, but more seriously they cause severe corrosion phenomena on conductive parts (e.g.: carbon) because of extremely high half-cell voltages at the conductor surface.
On the other hand, the redox system require non-negligible electrolyte flow rates through the flow chambers of the reactor in order to maintain optimal half-cell reactions conditions at the electrodes and this requirement may imply the necessity of operating the bipolar electrochemical reactor at relatively high positive pressure.
Differently from conventional electrochemical processing, redox flow battery systems are intended also for uses on nonpolluting vehicles and power/weight ratio is an important parameter.
SUMMARY OF THE INVENTION
A main objective of this invention is to provide a membrane-separated bipolar multicell electrochemical reactor for half-cell reduction and oxidation reactions in respective positive and negative electrolytes, without gas evolution, with an architecture that makes it more easily assemblable by allowing to stack fully pre-assembled elements horizontally, one on top of the other, and suitable to be operated in the same horizontal orientation of the bipolar elements.
According to a fundamental aspect of the novel architecture of the invention, the multicell assembly is constituted by alternately stacking two types of pre-assembled elements, one being a bipolar electrode holding subassembly and the other being a membrane holding subassembly.
Of course, the alternate stack of elements is piled over a bottom end element and the stack is terminated by placing over the last membrane holding element a top end electrode element. The two end electrode elements are then compressed over the stack by tightening a plurality of tie rods, conventionally arranged around the perimeter of the stacked elements, according to a common practice in tightening a filter-press stack in a hydraulically sealed manner, by virtue of the gaskets operatively installed between the coupling faces of the frames of the various elements.
According to an essential aspect of the architecture of the invention, each bipolar plate electrode holding element and each ion exchange membrane separator holding element includes a substantially similar rectangular frame piece, made of an electrically nonconductive and chemically resistant material, typically of molded plastic material, having on its upper (assembly) face grooves for receiving O-ring type gasket means, and having through holes and recesses in coordinated locations disposed along two opposite sides of the rectangular frame forming, upon completion of the assembling, ducts for the separate circulation of the negative electrolyte and of the positive electrolyte through all the negative electrolyte flow chambers and all positive electrolyte flow chambers, respectively, in cascade. The negative electrolyte enters along a first side of a negative electrolyte flow chamber, flows through the chamber toward the opposite or second side thereof, exits the chamber, flows through the coordinated holes through the frame holding the electrode and through the frame holding the next membrane separator, reaching the level of the next negative electrolyte flow chamber and enters it from the same second side through which it exited from the previous negative electrolyte flow chamber and exits this next negative electrolyte flow chamber from the same first side it entered the previous negative electrolyte flow chamber, to flow through coordinated holes through the next pair of frames to the level of the next negative electrolyte flow chamber and so forth. The same flow path is arranged also for the positive electrolyte, either in a “countercurrent” or in an “equicurrent” mode through the battery.
In practice, the bipolar electrochemical reactor does not have inlet and outlet manifolds for the two electrolytes, on the contrary, the electrolytes flow through the respective flow chambers in a zigzag path, that is essentially in hydraulic series or cascade mode instead than in hydraulic parallel mode.
In this way, by-pass current may only be “driven” by a voltage difference of about one-cell voltage and becomes practically negligible and above all it does not cause any corrosion on conductive parts.
The two types of pre-assembled elements are coordinately “keyed” so as to prevent any error in correctly stacking them alternately one over the other and with a correct orientation and perfect mutual alignment to compose the bipolar battery.
Apart from the suitably shaped keying pins and sockets and the position of the through holes and of the slotted portions of communication with the flow chambers, the molded plastic frames are substantially identical for both types of elements.
Essentially, each frame has an inner flange portion, recessed from the bottom (assembly) face of the frame, that is the opposite face to the one that is provided with the grooves for accommodating O-ring gaskets around pass-through electrolyte ducting holes and around the outer seal perimeter of the chamber.
During the pre-assembling of the two types of elements, on this inner flange portion is accommodated a relatively narr
Broman Barry Michael
Zocchi Andrea
Maples John S.
Squirrel Holding Ltd.
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