Hydrogen/Chlorine Regenerative Fuel Cell

Regenerative fuel cells for energy storage are well known, commonly using hydrogen and oxygen as the reacting gases. These cells have several disadvantages, the principal one being that the high overvoltage for the oxygen reaction in either direction, charge or discharge, limits the energy storage efficiency to about 55 to 60% compared to energy storage efficiency of secondary batteries of about 70%. The high overvoltage also causes severe corrosion of catalysts and support materials limiting the life of the cathode. This corrosion may well require separate fuel cells for charge and discharge.

An important feature of the present invention is a regenerative cell using hydrogen and chlorine as the reactive gases. With appropriate catalysts, the lower overvoltage for chlorine makes the cell substantially more reversible and minimizes corrosion problems. Energy efficiencies can be expected in the 70% range.

According to the invention, the fuel cell is made up of electrodes having catalyst coatings suitable for the hydrogen and chlorine reaction and having as an electrolyte an aqueous solution of a conductive salt or acid that readily absorbs the gaseous hydrogen chloride resulting from the cell discharge. This makes possible the use of the same cell for charge and discharge without any excessive overvoltages to affect catalyst stability or to produce excessive corrosion. A suitable conductive salt or acid is selected for the electrolyte so as to control the solubility of the chlorine, thereby minimizing a direct chemical reaction with the hydrogen and producing a maximum electrochemical reaction with the hydrogen.

The foregoing and other objects, features, and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof as illustrated in the accompanying drawing.

The single FIGURE is a diagrammatic sectional view through a fuel cell embodying the invention.

The cell to which the invention is applied is shown diagrammatically in the drawing. As shown, the cell includes a matrix 2 which is porous and which supports the electrolyte. This latter is an aqueous solution of an acid capable of dissolving or adsorbing the hydrogen chloride gas produced by the fuel cell discharge. This may be an aqueous solution of hydrogen chloride gas in water and this solution becomes stronger with cell discharge and weaker with cell recharge as will become apparent.

Instead of an acid electrolyte, it may be desirable to utilize a conductive salt dissolved in water. For example, a chloride salt, such as sodium chloride, may be preferred to the acid solution. These salts serve to control better, in some instances, the solubility of the chlorine (Cl.sub.2) in the electrolyte by the formation of Cl.sub.3.sup.-, Cl.sub.5.sup.- in solution; and, in this way, minimizes concentration polarization losses at the chlorine electrode while avoiding excess chlorine solubility which would favor self-discharge of the cell due to direct chemical reaction of the dissolved Cl.sub.2 with hydrogen (H.sub.2). These salts also serve to improve the conductivity of the electrolyte, reducing ohmic losses in the cell.

On opposite sides of the matrix are the electrodes 4 and 6 which are a porous gas diffusion material, as for example powdered and compressed carbon or graphite formed into a thin plate, or alternatively, a carbon or graphite paper using a fibrous form of the graphite as for example in U.S. Pat. No. 3,972,735. The same material may be used for both electrodes or for example the chlorine electrode, the cathode may be a titanium screen or other material having the desired porosity and capable of supporting the catalyst layer. On the face of the hydrogen electrode 4, the anode, is a catalyst layer 8 preferably platinum supported on carbon black placed thereon to function with the anode as a gas diffusion electrode. These catalyst layers may be formed and used as described in U.S. Pat. No. 4,028,274.

The chlorine electrode 6, the cathode, also has a catalyst layer 10 on the side against the matrix and this catalyst is preferably ruthenium oxide suitably applied so as to form a gas diffusion cathode.

As an alternative to the graphite electrode the cathode 6 may be, as above stated, a titanium screen on which the catalyst is placed. The prime purpose of the electrode is to support the catalyst and allow a passage of the reactive gas through the electrode to contact the electrolyte and to prevent escape of the fluid electrolyte from the matrix through the electrode. Thus the size of the screen, that is, the voids or spaces between the wires of the screen must be selected to accomplish the necessary results.

Against the outside surfaces of the electrodes are chambers 12 and 14, the chamber 12 being supplied with hydrogen under pressure and the chamber 14, against the cathode being supplied with chlorine gas under pressure. These gases may be supplied in any suitable manner, for example a hydrogen tank 16 and a chlorine tank 18 may be provided; and gas under pressure from these tanks is delivered through conduits 20 and 22 to the hydrogen and chlorine chambers, respectively.

Although they may not be needed during cell discharge, these conduits have pressure pumps 24 and 26, respectively, for pumping the respective gases into the tanks under pressure from the chambers during cell recharging when as will be apparent these gases are evolved from the electrodes during the charging process.

It is also desirable to have a tank 28 for the aqueous electrolyte. This tank communicates with the matrix by way of a conduit 30 and a pump 32 controls the desired flow of electrolyte into and out of the cell during discharge and recharge, respectively. Desirably the electrolyte will be circulated through a loop 33 during both charge and discharge of the cell. Because of the heat evolved during discharge, suitable heat exchangers may be necessary to cool the electrolyte. Also heaters may be necessary to maintain the gases in gaseous condition during expansion into the chambers during the discharge process.

For best efficiency, it is desirable to optimize the heat transfer from the heat exchangers to the heaters to minimize heat losses and thereby increase the total cell efficiency. Thus a suitable heat exchange fluid could be circulated between the heat exchangers (the coolers) for the electrolyte and the heaters for the gases.

The opposite may be necessary during recharge. That is to say, heaters for heating the aqueous electrolyte and heat exchangers or coolers for the evolved gases may be necessary. Again, for best efficiency, there would be desirably a heat transfer from the coolers to the heaters. It may also be necessary to have pressure control valves 34 and 36 on supply conduits 38 and 40 leading from the gas storage tanks to the cell chambers for controlling the gas pressures in the chambers during the discharge of the cell. A similar pressure control valve 42 may be necessary on a supply conduit 44 from tank 28 to the matrix. These devices are not a part of the present invention and need no detailed description; their use in other types of regenerative cells is well known.

The cell reaction for this cell during charge is ##EQU1## On discharge the reaction is ##EQU2##

Obviously in charging or recharging the reaction occurs through the input of electrical energy from a source with a basic voltage of 1.36 volts. During discharge the cell produces electrical energy at the same basic voltage. Although a simple fuel cell has been described it will be apparent that a stack of cells may be assembled to produce the desired voltage for effective use. This stacking of cells is well known in the art.

The cell described is a relatively simple cell to produce and the necessary materials, in the amount needed are relatively inexpensive per cell so that the total cost per kilowatt hour is within a practical amount.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that other various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.