JP2006257480A - Hydrogen generation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hydrogen generation method where the generation efficiency of hydrogen can be improved. <P>SOLUTION: In the hydrogen generation method, a short pulse P and a tailing pulse TP following the short pulse P are alternately repeatedly applied to a pair of electrodes, thus pulse electrolysis is performed. The short pulse P is the one in which the rising and trailing are finished in a short time. The maximum voltage V(P) of the short pulse P (based on voltage V0) is higher than the maximum voltage V(TP) of the tailing pulse TP (based on voltage V0). The tailing pulse TP has a wave form which tails after the short pulse P, and in which voltage V gradually drops over a longer time than the applying time Δt(P) of the short pulse P. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for generating hydrogen by electrolysis.

  Conventionally, a hydrogen generation method for generating hydrogen by direct current electrolysis of an electrolyte containing water has been used. In the hydrogen generation method, hydrogen is generated from the surface of the cathode by applying a DC voltage to a pair of electrodes immersed in an electrolytic solution (for example, Patent Document 1).

JP 2003-257443 A

  Here, the direct current electrolysis used in the conventional hydrogen generation method will be described with reference to FIG. FIG. 9 shows a case where application of a DC voltage to the electrode pair 92 is started (FIG. 9 (1)), and a case where a predetermined time has elapsed since the start of application of DC voltage to the electrode pair 92 (FIG. 9 ( 2)) schematically showing changes in the potential inside the electrolyte 91 (hereinafter also referred to as “internal potential”) and the distribution of hydrogen ions 81 and oxygen ions 82 inside the electrolyte 91. It is.

  As shown in FIG. 9 (1), when application of a DC voltage to the electrode pair 92 is started, the distribution of hydrogen ions 81 and oxygen ions 82 in the electrolyte 91 is uniform, and the internal potential is the anode 922. It decreases linearly from the cathode 921 toward the cathode 921.

  On the other hand, as shown in FIG. 9 (2), when a predetermined time (typically about 30 ms) has elapsed since the start of application of the DC voltage to the electrode pair 92, electric current is generated in the vicinity of the cathode 921 and the anode 922. The double layers 98 and 99 are generated, and the internal potential changes greatly in the electric double layers 98 and 99, and hardly changes in the remaining portions. For this reason, in the direct current electrolysis, only the hydrogen ions 81 included in the electric double layers 98 and 99 having the width W1 are moved by the potential gradient.

  However, the conventional hydrogen generation method using direct current electrolysis has a problem that hydrogen generation efficiency is not sufficient.

  The present invention has been made to solve this problem, and an object of the present invention is to provide a hydrogen generation method capable of improving hydrogen generation efficiency.

  In order to solve the above problems, the invention of claim 1 is a hydrogen generation method for generating hydrogen by electrolysis of an electrolytic solution, and a short pulse applying step of applying a short pulse to an electrode pair immersed in the electrolytic solution And, following the short pulse applying step, a tailing pulse applying step of applying a tailing pulse whose voltage gradually decreases to the electrode pair over a longer time than the short pulse applying step, and a maximum of the short pulse The voltage is higher than the maximum voltage of the tailing pulse.

  The invention of claim 2 is characterized in that, in the hydrogen generation method according to claim 1, the current of the tailing pulse gradually decreases and the maximum current of the short pulse is larger than the maximum current of the tailing pulse. To do.

  According to a third aspect of the present invention, in the hydrogen generation method according to the first or second aspect, a current in a direction opposite to a current flowing through the electrode pair is prevented from flowing in the tailing pulse applying step. .

  According to a fourth aspect of the present invention, in the hydrogen generation method according to any one of the first to third aspects, the short pulse applying step and the tailing pulse applying step are alternately and repeatedly performed, and the tailing pulse applying step After the current flowing through the electrode pair becomes 0, the next short pulse applying step is performed.

  According to the first to fourth aspects of the invention, the hydrogen generation efficiency can be improved.

  According to the invention of claim 3, the generation efficiency of hydrogen can be further improved.

  According to the invention of claim 4, it is possible to prevent a decrease in hydrogen generation efficiency.

<1 Hydrogen generator>
FIG. 1 is a diagram showing a configuration of a hydrogen generator 1 used in a hydrogen generation method according to an embodiment of the present invention.

  As shown in FIG. 1, the hydrogen generator 1 is configured such that a voltage can be applied to an electrode pair 12 immersed in an electrolyte solution 11 containing water using a pulse generator 13. The hydrogen generator 1 is an electrolyzer that generates gaseous hydrogen from the cathode 121 by electrolysis of the electrolytic solution 11.

  As the cathode 121 and the anode 122 forming the electrode pair 12, a well-known electrode used for electrolysis of the electrolyte solution 11 containing water can be employed. The cathode 121 and the anode 122 are desirably platinum electrodes, but adoption of electrodes other than platinum electrodes is not hindered.

  As the electrolytic solution 11, a known electrolytic solution used for hydrogen generation by electrolysis can be used. The electrolytic solution 11 is typically a potassium hydroxide aqueous solution, but it is not hindered to employ an electrolytic solution other than the potassium hydroxide aqueous solution.

  In the hydrogen generator 1, water that does not dissolve the electrolyte can also be used as the electrolyte 11 if a PEM electrolysis cell having electrodes formed on the surface of a proton exchange membrane (PEM) is used.

  The pulse generator 13 electrically connected to the electrode pair 12 can change the waveform (time change) of the voltage applied to the electrode pair 12 in various ways.

<2 Pulse electrolysis>
FIG. 2 shows the waveform of the voltage V applied to the electrode pair 12 (FIG. 2 (1)) and the waveform of the current I flowing into the anode 122 (flowing out of the cathode 121) in the hydrogen generation method according to the embodiment of the present invention. It is a figure which shows (FIG.2 (2)) typically. However, FIG. 2 is intended to explain the outline of the hydrogen generation method by pulse electrolysis, and is not intended to strictly reproduce the waveform actually measured when pulse electrolysis is actually performed.

  As shown in FIG. 2 (1), in the hydrogen generation method according to the embodiment of the present invention, by alternately applying the short pulse P and the tailing pulse TP following the short pulse P to the electrode pair 12, Pulse electrolysis is performed.

  The short pulse P is a short pulse that finishes rising and falling in a short time. The maximum voltage V (P) (referenced to the voltage V0) of the short pulse P is higher than the maximum voltage V (TP) (referenced to the voltage V0) of the tailing pulse TP. Further, the maximum voltage V (P) is higher than a voltage (typically about 2 V) applied to the electrode pair 12 in the conventional DC electrolysis, for example, several times to several tens of times the voltage. It has become. Here, the reference voltage V0 of the maximum voltage V (P) and the maximum voltage V (TP) is a voltage generated due to the electrode pair 12 being equivalent to a capacitor during the electrolysis, and the like. The specific voltage value is not constant. The application time Δt (P) = t2-t1 of the short pulse P is not limited to a specific value, but is about 100 ns as an example. As shown in FIG. 2 (1), it is allowed that some waveform disturbance such as ringing or overshoot accompanies the short pulse P.

  The voltage increase rate dV / dt at the start-up of the short pulse P affects the hydrogen generation efficiency, that is, the hydrogen generation rate relative to the input power (hereinafter also referred to as “hydrogen generation rate”). The hydrogen generation efficiency is maximized when the voltage increase rate dV / dt reaches an optimum value that varies depending on the hydrogen generation conditions, and decreases even when the voltage increase rate dV / dt is higher than the optimum value. (See FIG. 3). In addition, FIG. 3 is the figure which investigated and showed the change of the input electric power with respect to a voltage increase rate in case a hydrogen generation rate is 0.06 cc / s.

  The tailing pulse TP has a waveform in which the voltage V gradually decreases over a longer time than the application time Δt (P) of the short pulse P, such that the tail of the short pulse P is pulled. The application time Δt (TP) = t3-t2 of the tailing pulse TP is not limited to a specific value, but is about 60 μs as an example.

  Here, when attention is paid to the current I flowing in response to the application of the voltage V to the electrode pair 12, the current I is substantially synchronized with the application of the short pulse P to the electrode pair 12, as shown in FIG. Then, after changing in the negative direction, it changes in the positive direction, and gradually decreases toward “0” as the voltage V in the tailing pulse TP gradually decreases. Therefore, in this pulse electrolysis, the current I (> 0) effective for generating hydrogen from the cathode 121 does not flow when the short pulse P is applied, but flows when the tailing pulse TP is applied. Note that the maximum current that flows substantially in synchronization with the application of the short pulse P to the electrode pair is larger than the maximum current that flows during the application of the tailing pulse TP to the electrode pair.

  In order to prevent a current (hereinafter also referred to as “reverse current”) RC that flows substantially synchronously with the application of the short pulse P, as shown in FIG. 4, between the pulse generator 13 and the electrode pair 12. It is desirable to insert a diode D. As the diode D, a diode having a short reverse recovery time represented by a SiC Schottky diode can be preferably used. Due to such a reverse current prevention measure by inserting the diode D, a reverse current RC flowing in the opposite direction to the current effective to generate hydrogen from the cathode 121 flowing through the electrode pair 12 during application of the tailing pulse TP flows. Therefore, it is possible to suppress a significant disturbance in the waveform of the short pulse P, and to improve the voltage increase rate dV / dt when the short pulse P rises (see FIG. 5). FIG. 5 (1) shows an actually measured waveform (voltage increase rate 287 V / μs) at the time of starting up the short pulse P when reverse current prevention measures are taken, and FIG. 5 (2) shows reverse current prevention. It is an actually measured waveform (voltage increase rate 81 V / μs) when the short pulse P is started up when no countermeasure is taken. These are all test data with NaOH 1N solution.

  In this pulse electrolysis, the short pulse P and the tailing pulse TP are alternately and repeatedly applied to the electrode pair 12. After the current I flowing during the application of the tailing pulse TP becomes “0”, The short pulse P is applied. This is because if the next short pulse P is applied before the current I becomes “0”, the waveform of the applied short pulse P is significantly disturbed, leading to a decrease in hydrogen generation efficiency. . On the other hand, in order to increase the hydrogen generation rate by increasing the input power to the system related to pulse electrolysis, the time Δt () from when the current I becomes “0” until the next short pulse P is applied. It is desirable to shorten W) = t4-t3, and most desirably Δt (W) = 0.

  Here, the pulse electrolysis will be described with reference to FIG. FIG. 6 shows a case where the application of the short pulse P to the electrode pair 12 is started (FIG. 6 (1)) and a time when a predetermined time has elapsed after the application of the short pulse P to the electrode pair 12 is started (FIG. 6). 6 (2)) schematically shows changes in potential inside the electrolytic solution 11 (hereinafter also referred to as “internal potential”) and distributions of hydrogen ions 21 and oxygen ions 22 inside the electrolytic solution 11. FIG.

  As shown in FIG. 6A, when the application of the short pulse P to the electrode pair 12 is started, the distribution of the hydrogen ions 21 and the oxygen ions 22 in the electrolyte is uniform, and the internal potential is the anode 122. It decreases linearly from the cathode toward the cathode 121.

  On the other hand, as shown in FIG. 6 (2), when a predetermined time (for example, 100 ns) has elapsed since the application of the short pulse P to the electrode pair 12 has elapsed, the vicinity of the cathode 121 and the anode 122 (hereinafter referred to as 48) and 49), the internal potential changes greatly, and the remaining portion hardly changes. For this reason, in this pulse electrolysis, only the hydrogen ions 21 contained in the pseudo-electric double layers 48 and 49 having the width W2 are moved by the potential gradient.

  However, the maximum voltage V (P) of the short pulse P is higher than the voltage applied to the electrode pair 12 in the direct current electrolysis, and the application time Δt (P) of the short pulse P is the electrode in the direct current electrolysis. Since the time from the start of application of the DC voltage to the pair 12 to the generation of the electric double layers 98, 99 is shorter, the width W2 of the pseudo electric double layers 48, 49 is the electric double layers 98, 99 generated by DC electrolysis. It becomes thicker than the width W1. For this reason, in this pulse electrolysis, it is possible to move a larger amount of hydrogen ions inside the electrolyte 11 at a higher speed during the application of the short pulse P than in the DC electrolysis. A large amount of hydrogen is efficiently generated by reducing the generated hydrogen ions 21 during the application of the tailing pulse TP.

  In the following, a voltage is applied to two rectangular platinum electrode pairs immersed in a 1N potassium hydroxide aqueous solution at a distance of 45 mm while varying the waveform, and the input power and the hydrogen generation rate are applied. The example which investigated the relationship with is described.

<Relationship between input power and hydrogen generation rate>
FIG. 7 is a diagram showing the relationship between input power (horizontal axis) and hydrogen generation rate (vertical axis). In FIG. 7, the solid line and the dotted line indicate the upper and lower limits of the relationship when a DC voltage is applied to the platinum electrode pair, respectively, and the one-dot broken line is the ideal when all the input power is used for generation of hydrogen. The relationship. In FIG. 7, plots P1, P2, and P5 are actual measurement results in Examples 1, 2, and 5 when no reverse current prevention measures are taken, and plots P3, P4, and P6 are reverse current preventions. It is an actual measurement result in Examples 3, 4 and 6 when measures are taken. It should be noted that the frequency of the short pulse P, the maximum voltage V (P) of the short pulse P, the maximum voltage V (TP) of the tailing pulse TP, and the voltage increase rate dV / dt at the start of the short pulse P in Examples 1-6. Are listed in FIG. Note that the “Remarks” column in FIG. 8 corresponds to the primary side voltage when the voltage generated by the pulse generator 13 is boosted by the step-up transformer. As shown in FIG. 8, the optimum dV / dt value when the pulse reverse current is suppressed is improved by an order of magnitude compared to FIG.

  As is apparent from FIG. 7, in Examples 1 to 4 of the present invention, the hydrogen generation efficiency, that is, the hydrogen generation rate relative to the input power can be significantly increased as compared with the direct current electrolysis. In particular, in Examples 3, 4, and 6 (plots P3, P4, and P6), the hydrogen generation efficiency can be further remarkably increased by the countermeasure against reverse current.

It is a figure which shows the structure of the hydrogen generator 1 used with the hydrogen generation method which concerns on embodiment of this invention. FIG. 4 is a diagram schematically showing waveforms of a voltage V applied to an electrode pair 12 and a current I flowing into an anode 122 in the hydrogen generation method according to the embodiment of the present invention. It is the figure which investigated and showed experimentally the change with respect to input voltage with respect to a voltage increase rate in case a hydrogen generation rate is 0.06 cc / s. It is a figure explaining the reverse current prevention measure by insertion of a diode. It is a figure which shows the change of the waveform of the short pulse P by the presence or absence of a reverse current prevention measure. It is a figure explaining the pulse electrolysis used with the hydrogen generation method concerning a 1st embodiment of the present invention. It is a figure which shows the relationship between input electric power (horizontal axis) and hydrogen generation speed (vertical axis). Lists the frequency of the short pulse P, the maximum voltage V (P) of the short pulse P, the maximum voltage V (TP) of the tailing pulse TP, and the voltage increase rate dV / dt at the start of the short pulse P in Examples 1 to 6. FIG. It is a figure explaining the direct current electrolysis used with the conventional hydrogen generating method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Hydrogen generator 11 Electrolytic solution 121 Cathode 122 Anode 12 Electrode pair 13 Pulse generator 21 Oxygen ion 22 Hydrogen ion P Short pulse TP Tailing pulse D Diode

Claims (4)

A hydrogen generation method for generating hydrogen by electrolysis of an electrolyte solution,
A short pulse applying step of applying a short pulse to the electrode pair immersed in the electrolytic solution;
Following the short pulse applying step, a tailing pulse applying step of applying a tailing pulse whose voltage gradually decreases to the electrode pair over a longer time than the short pulse applying step;
With
The method for generating hydrogen, wherein the maximum voltage of the short pulse is higher than the maximum voltage of the tailing pulse. The hydrogen generation method according to claim 1,
As the current of the tailing pulse gradually decreases,
The method for generating hydrogen, wherein the maximum current of the short pulse is larger than the maximum current of the tailing pulse. In the hydrogen generation method according to claim 1 or 2,
A method for generating hydrogen, characterized in that in the tailing pulse applying step, a current in a direction opposite to a current flowing in the electrode pair is prevented from flowing. In the hydrogen generating method according to any one of claims 1 to 3,
While alternately executing the short pulse application step and the tailing pulse application step,
A hydrogen generation method comprising: performing a next short pulse applying step after a current flowing through the electrode pair becomes 0 in the tailing pulse applying step.

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