Electrical conditioning system for electrodes in an electrolysis cell

Abstract: Metal electrodes (17,20) defining a cathode and anode are positioned in an electrolyte (28) so as to comprise an electrolysis cell (18). An electrical conditioning circuit (10) includes means for generating a pulsating DC voltage signal, which is connected to the metal electrodes (17,20). Particular characteris­ tics of the pulsating signal, including in particular peak pulse voltage and duty cycle, have selected values such that hydrogen is produced from one electrode (17) and the other electrode (20) is maintained substan­ tially in a state of depassivation, such that any ox­ ide, scale or other inorganic compound pre-existing on the other electrode is removed and further such that the other electrode remains substantially clean.

Description: Technical Field This invention relates generally to the art of electrolysis,which typically involves the decomposition of an electrolyte by an electrical current, and more particularly concerns a system in which a pulsating DC current is applied to the electrodes in an electrolysis cell. Background Art In the typical operation of an electrolysis cell, an electric current is produced and hydrogen and oxygen are produced at the negative (cathode) and positive (anode) electrodes, respectively, which typically are metal plates positioned in a selected electrolyte. The hydrogen and oxygen thus produced may be captured and used as desired, or may be discarded, depending on the particular application. Many different electrolysis systems are known. Typically, the electrodes are at a different potential, resulting in a DC current between the two electrodes and hence an electric current output from the cell. In some applications, an external current is applied to the electrodes. One example of such an apparatus, which includes the use of pulsed DC current, is shown in U. S. Patent No. 3,980,053 to Horvath. Horvath's pulsed DC signal, however, is characterized by a high frequency and a high current. This apparatus is inefficient, and could be quite dangerous in actual operation. The object of the Horvath apparatus is the production of both hydrogen and oxygen. The electrode which produces the oxygen is maintained in a state of passivation. US-A-4372828 describes a process for reactivating an anode in which the operating potential of the anode of an electrolysis cell is periodically reduced for a time sufficient to cause such reactivation. The inexpensive, safe production or hydrogen is becoming increasingly desirable because hydrogen, in addition to other light hydrocarbon gases such as propane and butane, is becoming more popular as an inexpensive and clean burning source of energy. Efficiency is thus an important issue in the production of hydrogen, although most existing systems for producing hydrogen, particularly systems which operate on a relatively small scale, are quite inefficient as well as expensive. Further, such systems are often difficult to adequately monitor to maintain safety. These disadvantages, in addition to others, have resulted in a lack of emphasis and subsequent success in the commercial production of hydrogen with electrolysis technology. In a somewhat related technological area, there is also a continuing need for inexpensive and reliable methods for removing oxides or scale or other film or coating of inorganic compounds from metal plates, as well as a need for methods of polishing or otherwise preparing the surfaces of metal plates in some fashion for plating and the like. Metal plates typically develop oxide coatings or other electrically nonconductive films in use. In automobile batteries, for instance, the lead plates can become covered with an excessive amount of lead sulfate, to the point where the battery will cease operation. This is often referred to as a state of passivation, in which the electrodes become in effect "passive", i.e. no current flows between the plates. An article in Power Sources 8 (1981) pages 523-534 describes reactuating passivated electrodes using a cathodic current. A negative potential of approximately -2V was applied for a significant period of time, typically 5 minutes. Another common example of a metal oxide coating is the film of rust which covers steel plates, a condition which is undesirable in many applications. Known methods of removing such non-conductive coatings are usually somewhat cumbersome and fairly expensive. Therefore, there is a need for a reliable method of removing such coatings which is both efficient and relatively inexpensive. Besides the removal of oxides and other coatings per se, there are other related situations involving metal plates in which the surfaces thereof must be prepared in a particular way prior to use and/or further treatment. Electropolishing and pickling are but two examples of such surface preparation. Pickling, for instance, refers to a process for thoroughly cleaning a metal surface, particularly steel, but other metals as well. It usually involves the dissolution of the existing oxide film or scale on the steel with mineral acids such as sulfuric, nitric, hydrochloric or hydrofluoric acids. Another somewhat related problem dealing with metal surfaces is the Protection of metal surfaces from oxidation or other deterioration through electrolysis. This is generally referred to as cathodic protection and includes the protection of pipelines and hulls of vessels. According to the present invention there is provided a method of producing hydrogen by electrically conditioning electrodes wherein the electrodes are positioned in an electrolyte, and include a cathode and an anode, the method being characterised by the steps of: forming a pulsating DC voltage signal having particular characteristics including pulse width, peak voltage, and duration between successive pulses of said signal with values selected such that when the pulsating DC voltage signal is applied to said electrodes, during the interval between the pulses the potential of the anodic portion of the electrodes is positive relative to standard hydrogen potential, and during the period of each pulse the anodic portion is made more positive than during the interval between the pulses, the anodic portion of said electrode means is maintained substantially in a state of depassivation wherein the particular metal comprising the anodic portion remains exposed, substantially no oxygen is produced off the anodic portion and the anodic portion is consumed during operation of the apparatus, and such that hydrogen is rapidly produced off the cathodic portion of the electrode means. The invention will be described now by way of example only, with particular reference to the accompanying drawings. In the drawings: Figure 1 is a simplified schematic representation of the combination of an electrode conditioning system and an electrolysis cell. Figure 2 is a simplified diagram of an electrolysis cell and related chamber for storing hydrogen. Figure 3 is an electrical schematic of the electrode conditioning system. Figure 4 is a graph showing production of hydrogen against time for one embodiment of the present invention using the circuit of Figure 3 and using aluminum electrodes in the electrolysis cell. Figure 5 is a graph showing the production of hydrogen against time for another embodiment of the present invention using the circuit of Figure 3 and using steel electrodes in the electrolysis cell. Figure 6 is a graph showing output voltage against time for the embodiment graphed in Figure 4. Figure 7 is a graph showing output current against time for the embodiment graphed in Figure 4. Figure 8 is a graph showing output voltage against time for the embodiment graphed in Figure 5. Figure 9 is a graph showing output current against time for the embodiment graphed in Figure 5. Figure 10 is a table showing results of tests conducted on one embodiment of the system of the present invention using the circuit of Figure 3 and both aluminum and steel electrodes, respectively, in the electrolysis cell. Best Mode For Carrying Out The Invention Referring to Figures 1 and 2, an electrode conditioning system is shown in a simplified form in relation to an electrolysis cell and a chamber for collecting hydrogen produced during operation of the system. The electrode conditioning circuit, shown as block 10 in Figure 1, operates from a source of AC power 12. The output of the electrode conditioning circuit 10, at lines 14 and 16, is a pulsed DC current. Line 14 is connected to a first set of plates 17 in an electrolysis cell which is generally shown at 18. Line 16 is connected to the other set of plates 20. In the embodiment shown, plates 17 comprise the cathode and plates 20 comprise the anode. In the embodiment shown, the plates 17 and 20 are four aluminum or steel rectangular plates approximately 4.3 centimeters wide, 0.1 centimeters thick and 15 centimeters long, mounted vertically within an electrolyte 28 such that approximately 13.5 centimeters of each plate is submerged. The plates are 1.1 centimeters apart. The two plates 17-17 comprising the cathode are connected in parallel by lead 24 while the two other plates 20-20 comprising the anode are connected in parallel by lead 26. The electrolyte 28 in which the plates are mounted is basically saltwater; in particular, approximately 36 grams of table salt dissolved in 1600 milliliters of tap water in the embodiment shown. Also in the circuit shown in Figure 1 are a resistor 30 and an ampmeter 32. The resistor is for balancing the circuit, if necessary, and the ampmeter is to show the amount of current in the circuit. Neither of these elements, however, are essential to the invention. Extending from the electrolysis cell 18, as shown in Figure 2, is a pipe 34 which is connected to a cylinder 36 or other container. Cylinder 36 is completely filled with water. Hydrogen gas which is produced by the process described below exits from the electrolysis cell 18 through pipe 34 and is stored in cylinder 36. It should be understood that the electrolysis cell portion of the system described herein as well as the apparatus for storing the hydrogen gas produced are relatively conventional and that other structural arrangements and configurations could be used. It should be further understood that the electrolysis cell could be virtually any size, including considerably larger than that described herein. The configuration of the electrolysis cell in the embodiment shown is for purposes of illustration only. The electrode conditioning circuit shown in block 10 is connected to terminals 27, 29 of the cell shown in Figure 2 to produce the desired results. When switches 31a, 31b are closed and switches 33a, 33b are open, one terminal is positive and the other is negative. When switches 31a, 31b are open and switches 33a, 33b are closed, the opposite is true, so that the plates 17-17 and 20-20 could serve, respectively, as either the cathode or the anode in the electrolysis cell. Also, the electrolysis cell could be arranged so that opposite sides of a single plate could serve as cathode and anode, respectively, i.e. bipolar electrodes. The electrode conditioning circuit 10 is shown in more detail in Figure 3. 120 volts AC power is provided to the primary winding 38 of a center tapped transformer 40. A switch 42 controls the on-off operation of circuit 10 and a neon bulb 44 shows the operating condition. A slow-blow fuse 45 provides protection for circuit 10. The output of secondary 46 of transformer 40 is a 25 volt AC signal, which is full wave rectified by diodes 49 and 50, resulting in a pulsating DC signal of 18 volts peak. This pulsating DC signal is smoothed out by capacitor 52 which in the embodiment shown is fairly large, on the order of 0.037 farads. This signal is applied as an input to a voltage regulator comprising an operational amplifier 54 and associated feedback capacitor 56. The output of the voltage regulator is a stable 12 volt DC signal on line 58. Timing circuit 60, to which the signal on line 53 is applied, generally comprises two timer circuits in a single integrated circuit chip. In the embodiment shown, it is an NE/SE556 manufactured by Intercell Corporation, or equivalent, such as TLC 556 from Radio Shack. The output of the first timer circuit, in response to the input signal at pin 14, is a square wave, the relative timing of the square wave being determined by a series RC circuit comprising adjustable resistor 62 and capacitor 64. The square wave signal, at pin 5, in turn is shaped by a parallel RC circuit comprising adjustable resistor 66 and capacitor 68 to produce a relatively short voltage spike at each point in time when the square wave from the timer 60 at pin 5 goes positive. The voltage spike is used to control transistor 70 which, with resistor 72, forms a one-shot multivibrator which in turn, when triggered, produces an output pulse of selected duration. The width of this pulse is established by the values of adjustable resistor 74, resistor 76 and capacitor 78. In the embodiment shown, resistor 76 is 220 ohms and capacitor 78 is 0.7 microfarads. The train of pulses from capacitor 70 is applied to transistor 84 which in turn controls output transistor 86. Transistor 86 operates as an output current switch. When transistor 86 is on, a DC output of selected magnitude appears between outputs 88 and 90. Transistor 86 is controlled such that the output is a pulsed DC signal having a selected pulse repetition rate and pulse width. Resistor 92 and meter 94 are connected so as to provide a direct indication of the average current supplied to the electrolysis cell. Output points 88 and 90 are connected to an electrolysis cell, as shown in Figures 1 and 2. The peak voltage, pulse repetition rate, pulse width and the duty cycle of the DC pulse signal at outputs 88 and 90 may be varied by adjusting the various elements discussed above. In the embodiment shown, one or more of these signal characteristics, particularly peak voltage and duty cycle, are controlled, as explained in more detail below, so that application of the signal to given electrodes in a given electrolyte will result in the production of hydrogen at one electrode and the continuing depassivation of the other electrode. In the process of depassivation, oxides and other surface coverings, such as a coating of inorganic material or specific debris such as rust or other scale, are removed from the surface, and the bare metal underneath comprising the electrode is maintained substantially in an exposed state, so that the electrode continues to discharge between successive pulses. The surface of the electrode thus "dissolves", or "corrodes" instead of being stable. Typically, relatively little, if any, oxygen is produced at the depassivated electrode, so that the electrode does not have the opportunity to "heal itself" through the formation of a surface oxide. Such action permits the use of the present invention in metal surface cleaning applications, such as surface polishing or pickling of steel or other metals. These features are discussed in more detail below, as well as the high energy and current efficiencies possible with the present invention, because of the volume of hydrogen produced with relatively low current input. Figures 4, 6 and 7 are graphs showing the results obtained from a combination comprising the electrolysis cell and the electrode conditioning system described above, using aluminum electrodes in the electrolysis cell. Figure 4 shows generally the production of hydrogen over time with respect to the electrode conditioning circuit of Figure 3, while the signal output (voltage and current against time) to the electrolysis cell is shown in Figures 6 and 7. Referring to Figure 6 in particular, the circuit of Figure 3 is arranged to produce a voltage pulse of approximately 0.7 volts peak with pulse width of 0.075 milliseconds. The time between successive pulses is 3.39 milliseconds, resulting in a duty cycle of approximately 0.02. The corresponding signal current levels are shown in Figure 7. Operationally, in response to the electrical signal of Figure 6, one set of aluminum plates is polarized as the cathode, and the other is polarized as the anode, resulting in the production of a form of aluminum oxide or aluminum hydroxide, i.e. Al₂O₃ or Al(OH)₃ at the amode and hydrogen at the cathode. During the interval between successive electrical pulses from the circuit of Figure 3, the reaction of the electrodes in the electrolysis cell continues without external electrical power being applied. The production of hydrogen and aluminum hydroxide during the interval between externally applied electrical pulses is referred to as electrolysis discharge, i.e. the electrodes are in fact "discharging". Repolarization refers to that interval of time during which the electrical pulses are applied to the electrodes, i.e. at intervals of 3.39 milliseconds in Figure 6. It has been found, as pointed out above, that the DC pulse pattern shown in Figure 6 prevents the passivation of the aluminum electrodes and assures the continuing electrolysis discharge of the cell. The surface of the electrodes is maintained essentially bare, and relatively little, if any, oxygen is produced. The peak voltage of Figure 6 is in the embodiment shown sufficient to produce Al(OH)₃. This value of peak voltage appears to be a minimum driving voltage to sustain the reaction for aluminum electrodes. At a sufficiently higher peak voltage, different aluminum oxides or hydroxides or even oxygen will be produced, which may be desirable in particular applications. However, it is advantageous in many applications to maintain the voltage and current levels relatively low so as to maximize energy efficiency and not produce oxygen. The reaction at the anode in the embodiment shown is 2Al - 6e- → 2Al³⁺, while at the cathode the reaction is 6H⁺ + 6e⁻ → 3H₂. The overall reaction is 2Al + 6H₂O → 2Al(OH)₃ + 3H₂. In this embodiment, it can thus be seen that relatively small amounts, if any, of oxygen are produced. Besides other advantages dealing with cleaning and depassivation, as discussed above, this increases the inherent safety of the apparatus. Oxygen could be produced, if desired, by increasing the driving voltage to required known levels. Similar graph information is provided for conventional high quality steel electrodes, in Figures 5, 8 and 9. Figure 5 shows production of hydrogen vs. time for the voltage conditioning signal shown in Figures 8 and 9. Referring to Figure 8 in particular, the peak voltage of the pulses is 1.2 volts, which is somewhat greater than the peak voltage for the aluminum electrode embodiment. The pulse width, at 0.89 milliseconds, is substantially wider than for the aluminum electrode embodiment, although the duty cycle of the signal is approximately the same. The peak current of the pulses is also greater, approximately 2.5 amps as opposed to 1.0 amps. The energy efficiency, however, is still up to 100% and even greater. The reaction at the anode is Fe-2e⁻ → Fe²⁺, while the reaction at the cathode is 2H⁺ + 2e⁻ → H₂. The overall reaction is Fe+2H₂0 → Fe(OH)₂+H₂. Again, very little or no oxygen is produced at the stated peak voltage levels. However, higher peak voltages will result in production of Fe(OH)₃ and at still higher levels, 0₂ will be produced. The present invention has very high energy and current efficiencies. Figure 10 is a table which shows a representative sample of results of tests on the combination of the electrolysis cell and the electrode conditioning system of Figure 3, with the output signal being in accordance with Figures 6-9. Figure 10 also shows corresponding calculations for energy efficiency and current efficiency, using the obtained values for current and voltage. The known formula for current efficiency (CE), in percent, relative to hydrogen production is: where H is in milliliters for a particular time interval (t) and I av is in amperes. Further, from Perry's Chemical Engineering Handbook, it is known that 325 BTU/cuft is the maximum fuel value of hydrogen. This is equivalent to 0.00336 watt hr/ml of hydrogen. Thus, energy efficiency (EE), in percent, is: As shown in Figure 10, energy efficiencies for the present invention can be 100% or even much higher in particular circumstances. Other circuit configurations, using other metals as electrodes and other electrolysis cell configurations, may have somewhat different efficiencies. In the above-identified embodiments, one primary purpose is to produce high quality hydrogen at a high energy efficiency and in a relatively safe manner. These embodiments have applications which include, among others, a portable fuel cell, in which hydrogen gas is the energy source, or as a means for producing hydrogen directly at a remote site, which eliminates the need to transport hydrogen to the site. In addition to the production of hydrogen, the invention can be used as a relatively low cost means of producing different metal hydroxides or oxides, such as aluminum hydroxide, depending upon the metal used for the electrodes. Such hydroxides/oxides have a number of different commercial uses. In other applications involving the present invention, energy or current efficiency per se may not be the most important factor. For instance, in applications involving surface treatment of metals, including specifically the removal of metal oxides or inorganic compound or other scale or debris, such as rust, from plates, or in pickling steel or other metal, or in the reconditioning of batteries by removing lead sulfate from the plates, or in metal deplating, or in electropolishing or etching to clean and prepare a metal surface, especially for further treatment, such as plating, factors other than high energy efficiency are quite important. The principles of the present invention can be used to provide a fast, inexpensive and convenient means to accomplish the above functions. This is done by adjusting the one or more characteristics of the signal, i.e. the duty cycle, peak voltage, and pulse repetition rate to suit the particular application. With respect to the duty cycle, for instance, a normal range of adjustment would be between somewhat below 2% to 10%, with the higher duty cycle resulting in lower efficiencies. However, in certain applications, such as pickling steel and reconditioning lead-acid batteries, the duty cycle will typically be higher, i.e. 35%, with a possible range of 30%-40%. Still further, the principles of the present invention can be used to provide cathodic protection for pipelines and boats, by adjusting the current to a level sufficient to stop or significantly reduce corrosion of the cathodic element. Although a preferred embodiment of the present invention has been disclosed herein for illustration, it should be understood that various changes, modifications and substitutions may be incorporated in such embodiment.

Claims: A method of producing hydrogen by electrically conditioning electrodes wherein the electrodes (17, 20) are positioned in an electrolyte, and include a cathode (17) and an anode (10), the method being characterised by the steps of: forming a pulsating DC voltage signal having particular characteristics including pulse width, peak voltage, and duration between successive pulses of said signal with values selected such that when the pulsating DC voltage signal is applied to said electrodes, during the interval between the pulses the potential of the anodic portion of the electrodes is positive relative to standard hydrogen potential, and during the period of each pulse the anodic portion is made more positive than during the interval between the pulses, the anodic portion of said electrode means is maintained substantially in a state of depassivation wherein the particular metal comprising the anodic portion remains exposed, substantially no oxygen is produced off the anodic portion and the anodic portion is consumed during operation of the apparatus, and such that hydrogen is rapidly produced off the cathodic portion of the electrode means. A method according to claim 1 wherein said particular characteristics of the signal includes the peak pulse voltage and duty cycle of the signal. A method according to claim 2, wherein the particular characteristics includes the pulse repetition rate.

Inventors: TIMEWELL RICHARD R, TIMEWELL RICHARD R.
Applicants: DYNAMOTIVE ELECTROCHEM CORP, DYNAMOTIVE ELECTROCHEM CORPORATION
Application year: 1988
Main classification: C25B1
IPC codes: C25B11, C25B1, H01M10, H01M8, C25F1