Chemistry: electrical current producing apparatus – product – and – Process of cell operation
Reexamination Certificate
2001-11-30
2004-02-17
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Process of cell operation
C429S051000, C429S101000, C429S105000
Reexamination Certificate
active
06692862
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally 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 the operation of a membrane-separated, bipolar multicell electrochemical reactor implementing a redox flow battery system, although it may be useful also for different systems.
2. Description of Related Art
Redox flow battery systems are increasingly attracting interest as efficient energy storage systems. Among redox couple candidates, the all vanadium redox system is 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 coordinated 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 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 abnormally high half-cell voltages at the conductor surface.
On the other hand, the redox system require nonnegligible 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 pressures.
A different architecture, object of the prior patent application PCT/IT99/00195 of the same applicant, contemplates alternately stacking a bipolar electrode holding subassembly and a membrane holding subassembly, laying them horizontally.
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. The battery may be operated with the piled elements laying horizontally.
In the above noted architecture, 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 it does not cause any corrosion on conductive parts.
Pitting corrosion is not the only consequent of by-pass currents.
By-pass currents lower the overall efficiency of the charging and discharging processes because by-pass currents represent parasitic discharge mechanisms of the flow redox battery.
A typical way of using flow redox battery systems is to accumulate energy by transforming electrical energy into chemical energy during periods of excess electrical power generating capabilities (for example solar energy conversion during daylight hours or excess electrical power capabilities during night time hours in power generation plants) and to deliver accumulated energy in the form of electrical power when required by a load circuit.
Often, in the normal daily cycling of a flow redox battery system there may be prolonged periods of inactivity that is periods when the battery is not charging nor supplying electrical power to an external load circuit. During these idle periods, the pumps that circulate the positive electrolyte and the negative electrolyte through the cell are switched off to save energy and the electrolyte in the battery remain still.
In these conditions, the volumes of electrolytes, contained in the respective compartments of the cells composing the battery stack, supports the by-pass currents that typically are practically entirely confined within the electrolyte battery stack and therefore tends to slowly decrement their state of charging.
As a consequence, if electrical energy is required by the utilizer circuits, the system may take several minutes of “start-up” before becoming ready to provide the appropriate output voltage, a condition that is attained upon a complete refreshing of the electrolytes in the compartments of the battery stack upon resuming their forced circulation by switching on the respective pumps.
This phenomenon may impose the presence of auxiliary battery systems for providing the electrical power necessary to operate the electrolyte pumps at least during the “start-up” period when the output voltage of the battery may have dropped to an insufficient level because of the intervening discharge of the electrolytes volumes retained in the respective compartments during a prot
Alejandro R
Connolly Bove & Lodge & Hutz LLP
Hume Larry J.
Kalafut Stephen
Squirrel Holdings Ltd.
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