Electrolysis cell and electrolysis method

Abstract

The present invention provides an electrolytic cell comprising: a working electrode; a counter electrode; a liquid electrolyte in contact with a working surface of the working electrode; an acoustically transparent substrate including at least a piezoelectric substrate portion; one or more conductive electrodes connected with the piezoelectric substrate portion and configured to propagate a high frequency acoustic wave having a frequency of at least 1 MHz on the acoustically transparent substrate when electrically driven; and one or more power sources configured to (i) apply a potential between the working electrode and the counter electrode sufficient to cause an electrolytic reaction of a substance in the liquid electrolyte to produce an electrolytic reaction product near the working electrode; and (ii) electrically driving the one or more electrically conductive electrodes, where the working electrode is either located on the acoustically transparent substrate or spaced apart from the acoustically transparent substrate by the liquid electrolyte, and where propagation of the high frequency acoustic waves on the acoustically transparent substrate stimulates the liquid electrolyte during operation of the electrolytic cell, where the working electrode is located on the acoustically transparent substrate or spaced apart from the acoustically transparent substrate by the liquid electrolyte. Therefore, the production efficiency of the electrolytic reaction product is improved.

Description

Technical Field

[0001] The present invention relates to an electrolytic cell that includes a working electrode in contact with a liquid electrolyte, where the working electrode is either located on an acoustically transmissive substrate or is spaced apart from the acoustically transmissive substrate by the liquid electrolyte. During operation of the electrolytic cell, high-frequency acoustic waves propagating on the acoustically transmissive substrate stimulate the liquid electrolyte, thereby enhancing the production efficiency of electrolysis reaction products generated near the working electrode. The present invention also relates to an electrolysis method and an electrode apparatus for an electrolytic cell. The present invention is particularly applicable to the electrolysis of water to produce hydrogen, and various aspects of the present invention are conveniently disclosed in this scenario. However, the present invention is not limited thereto and also encompasses electrolytic cells and electrolysis methods for a series of electrolysis processes. Background Art

[0002] Green hydrogen (H2) produced by water electrolysis is destined to play a key role in the transition to a clean energy economy as it can provide a high-energy density carrier for renewable energy or replace H2 currently produced from fossil fuel sources. However, electrolysis currently accounts for only a small fraction of global hydrogen production, partly due to low production efficiency resulting from ohmic losses associated with the kinetic overpotential of the electrolysis system and bubble accumulation on the electrodes. Commercial water electrolysis typically uses strongly acidic or alkaline electrolytes and expensive platinum group metal (PGM) electrocatalysts to achieve the lowest onset potential for the hydrogen evolution reaction (HER). However, a high (usually several volts) overpotential is typically required to reach industrially practical current densities (e.g., 200 - 500 mA cm -2 ). In addition to low energy efficiency, electrolysis at such high overpotentials is prone to operational problems, including corrosive acid mists in acidic electrolytes and electrocatalyst instability in alkaline electrolytes.

[0003] Therefore, it is desirable to reduce the overpotential required to achieve the target current density, preferably while also utilizing neutral or near-neutral electrolytes and / or non-PGM electrocatalysts. This is particularly challenging because at neutral pH, the HER rate is significantly reduced due to the presence of hydronium ions (H3O + ) is inherently low due to the rapid consumption of ), and the consequent need for a higher overpotential to drive the thermodynamically unfavorable reduction of HO. This phenomenon can be observed as a "plateau" in the current-voltage response curve of linear sweep voltammetry (LSV) experiments.

[0004] Another difficulty in water electrolysis at any pH value is the generation and attachment of bubbles on the electrodes (H2 on the cathode and O2 on the anode). Bubbles reduce the effective surface area of the electrodes and inhibit the mass transfer of ionic species and heat transfer between the bulk electrolyte and the electrocatalyst, thereby significantly leading to electrolysis overpotential. In addition, the attachment and removal of bubbles can lead to rapid erosion of the electroactive surface of the electrodes.

[0005] Acoustic stimulation using power ultrasound (20-100kHz sound waves) has previously been used to attempt to address these problems. The observed moderate reduction in overpotential has been attributed to a variety of sonoelectrochemical mechanisms, which are largely attributed to acoustic cavitation (acoustic cavitation) caused by bulk ultrasound waves (bulkultrasound waves) propagating in the electrolyte adjacent to the electrode surface. Therefore, low-frequency and high-power ultrasonic stimulation (i.e., power ultrasound) capable of inducing cavitation is considered to be necessary to provide meaningful gain (meaningful gain). Low frequency is also generally considered to be necessary to provide an acceptable high acoustic propagation distance (acousticpropagation distance) (attenuation length) from the ultrasonic generator to the electrolyte.

[0006] In one embodiment, international patent application WO 2013 / 003499 discloses acoustic stimulation of an aqueous electrolyte in an electrolytic cell to induce acoustic cavitation in the region between the cathode and the anode. Frequencies of 20 kHz to 100 kHz are said to be suitable for cavitating the electrolyte.

[0007] However, there are many significant drawbacks to using power ultrasound to enhance electrolysis. First, the increase in power density at a given overpotential is limited, and the resulting energy efficiency is offset by the high power input required to generate the ultrasound itself (powerinput). Traditional ultrasound generators (such as bath and probe-type ultrasonic devices) are bulky and inefficient, and since the sound waves propagate through the liquid medium and associated physical equipment, only a small fraction of the applied energy enhances hydrogen production. In neutral electrolytes, the energy balance is expected to be negative, so ultrasound stimulation is typically only proposed for strongly acidic or alkaline electrolytes. Second, the acoustic cavitation generated by power ultrasound is highly erosive to the electrode surface, thus affecting the long-term stability of the electrolysis system. This problem is particularly prominent if the electrode itself is located on the vibrating surface that emits power ultrasound (i.e., the "sonotrode"), as rapid physical degradation of the electrode can be foreseen. In addition, ultrasound devices, especially those operating at Hz and KHz frequencies, co-produce high audible noise, which requires the use of ear protection equipment and limits their use in certain scenarios.

[0008] The above discussion pertains to the electrolysis reactions occurring at the cathode and anode of a water electrolysis cell (i.e., the hydrogen evolution reaction and the oxygen evolution reaction, respectively), but it should be understood that similar considerations also apply to a series of other electrolysis reactions, where performance is limited by an undesirably high overpotential.

[0009] Accordingly, there continues to be a need for new devices and methods for electrolysis cells that at least partially address one or more of the above drawbacks, or provide a viable alternative.

[0010] The citation in this document of patent literature or other matters presented as prior art should not be taken as an admission that such literature or matters were known at the priority date of any claim, or that the information they contain forms part of common general knowledge. SUMMARY OF THE INVENTION

[0011] The inventors have now found that during water electrolysis, by using acoustic waves with frequencies far higher than the power ultrasound range previously used in electrolysis reactions, the liquid electrolyte adjacent to the working surface of the working electrode is stimulated, and the rate of the hydrogen evolution reaction (HER) can be significantly increased. This is achieved by propagating appropriate high-frequency acoustic waves with a frequency of at least 1 MHz on a sound-transmitting substrate, where the working electrode is either located on the substrate or separated from the substrate by the liquid electrolyte. The high-frequency acoustic waves can be appropriately generated by electrically driving one or more conductive electrodes connected to the piezoelectric portion of the substrate, and the form of the generated waves, such as surface acoustic waves (SAW), surface-reflected bulk waves (SRBW), or Lamb waves, depends on the relative configuration of the electrodes and the substrate.

[0012] The observations show that in neutral electrolytes, the current density at the target overpotential is increased by up to 14 times. This result is both unexpected and significantly advantageous because the power input is significantly lower than that required by conventional ultrasound methods, and the acoustic frequency is too high to induce acoustic cavitation in the electrolyte. In fact, this result cannot be explained by conventional sono-electrochemical mechanisms alone, and the inventors have obtained evidence of a new effect in which the hydrogen bond network of water molecules at the electrode-electrolyte interface is disrupted by high-frequency acoustic waves propagating on the electrode surface.

[0013] Due to the absence of cavitation and the low power input, physical degradation of the working surface can be avoided or minimized, and the energy penalty associated with acoustic wave generation is reduced compared to the efficiency obtained in electrolysis. Advantageously, the method disclosed herein allows for high current densities to be achieved even when using neutral electrolytes and non-PGM electrocatalysts.

[0014] According to a first aspect, the present invention provides an electrolytic cell comprising: a working electrode; a counter electrode; a liquid electrolyte in contact with a working surface of the working electrode; an acoustically transmissive substrate comprising at least a piezoelectric substrate portion; one or more conductive electrodes connected to the piezoelectric substrate portion and configured to propagate high-frequency acoustic waves having a frequency of at least 1 MHz on the acoustically transmissive substrate when electrically driven; and one or more power supplies configured to (i) apply a potential between the working electrode and the counter electrode sufficient to cause an electrolysis reaction of a substance in the liquid electrolyte to produce an electrolysis reaction product near the working electrode; and (ii) electrically drive the one or more conductive electrodes, wherein the working electrode is either located on the acoustically transmissive substrate or spaced apart from the acoustically transmissive substrate by the liquid electrolyte, and wherein during operation of the electrolytic cell, the propagation of the high-frequency acoustic waves on the acoustically transmissive substrate stimulates the liquid electrolyte, thereby increasing the production efficiency of the electrolysis reaction product.

[0015] In some embodiments, the working electrode is located on the acoustically transmissive substrate. In some such embodiments, the working electrode is located on the piezoelectric substrate portion.

[0016] In some embodiments, the working electrode is spaced apart from the acoustically transmissive substrate by the liquid electrolyte by no more than 30 mm, or no more than 20 mm, or no more than 10 mm, such as a distance of about 5 mm or less.

[0017] In some embodiments, the frequency of the high-frequency acoustic waves is greater than 1 MHz, such as at least 1.5 MHz, or at least 2 MHz, or at least 3 MHz, or at least 5 MHz.

[0018] In some embodiments, the high-frequency acoustic waves comprise a waveform selected from surface acoustic waves (SAWs), surface reflector body waves (SRBWs), and Lamb waves. In some embodiments, the waveform is selected from SAW or SRBW.

[0019] In some embodiments, h / λ is greater than 1 / 3, or greater than 1 / 2, or greater than 1, where h is the thickness of the piezoelectric substrate portion and λ is the acoustic wavelength of the high-frequency acoustic waves when propagating on the piezoelectric substrate portion.

[0020] In some embodiments, h / λ is between 1 / 3 and 3, or between 1 / 2 and 3, or between 1 and 2, where h is the thickness of the piezoelectric substrate portion and λ is the acoustic wavelength of the high-frequency acoustic wave propagating on the piezoelectric substrate portion.

[0021] In some embodiments, after normalization by the surface area of the sound-transmitting substrate, the power input required to drive the one or more conductive electrodes is less than 20 W / cm 2 , or less than 10 W / cm 2 , or less than 5 W / cm 2 , or less than 1 W / cm 2 .

[0022] In some embodiments, the working electrode comprises an electrocatalyst for the hydrogen evolution reaction on the working surface.

[0023] In some embodiments, the working electrode comprises one or more layers of a metal composition or metal compound formed on the surface of the sound-transmitting substrate. The working electrode may comprise a layer containing at least one selected from gold, platinum, titanium, titanium dioxide, molybdenum disulfide, graphene, and aluminum at the working surface.

[0024] In some embodiments, the piezoelectric substrate portion is configured as a plate, and the high-frequency acoustic wave propagates in the plane of the plate, causing at least one surface of the plate to oscillate.

[0025] In some embodiments, the piezoelectric substrate portion comprises a single crystal selected from lithium niobate, quartz, lithium tantalate, and lanthanum gallium silicate.

[0026] In some embodiments, the one or more conductive electrodes comprise a pair of separated electrodes. The pair of separated electrodes can be a pair of interdigitated electrodes. The pair of conductive electrodes can be patterned on the surface of the piezoelectric substrate portion.

[0027] In some embodiments, the liquid electrolyte is an aqueous electrolyte. In some such embodiments, the pH of the aqueous electrolyte is between 5 and 9.

[0028] In some embodiments, the electrolysis reaction product is hydrogen gas (H2) or oxygen gas (O2).

[0029] In some embodiments, the electrolytic cell is configured to flow the liquid electrolyte through the working electrode.

[0030] In some embodiments, the electrolytic cell includes an ion-permeable membrane that separates the working electrode from the counter electrode. Optionally, the electrolytic cell includes a second liquid electrolyte in contact with the counter electrode. The ion-permeable membrane can prevent substantial mixing of the liquid electrolyte in contact with the working surface of the working electrode and the second liquid electrolyte. Optionally, the electrolytic cell is configured to flow the second liquid electrolyte through the counter electrode.

[0031] In some embodiments, the electrolytic cell is an electrolytic cell for water electrolysis. Thus, the liquid electrolyte will be an aqueous electrolyte and, in some embodiments, a neutral aqueous electrolyte (pH 5-9). The working electrode can be a cathode or an anode, preferably a cathode. The electrolytic cell for water electrolysis can be configured to flow the liquid electrolyte through the working electrode. The cathode can include an electrocatalyst for the hydrogen evolution reaction on the working surface. The anode can include an electrocatalyst for the oxygen evolution reaction.

[0032] Optionally, the working electrode is a cathode, and the electrolytic cell is further configured to stimulate the liquid electrolyte in contact with the anode by propagation of high-frequency sound waves on a sound-transmitting substrate, typically a second sound-transmitting substrate. The anode can be located on the sound-transmitting substrate or spaced from the sound-transmitting substrate by the liquid electrolyte. The electrolytic cell for water electrolysis can optionally include a proton-permeable membrane that separates the cathode and the anode. Optionally, the electrolytic cell is configured to flow an aqueous catholyte through the cathode and an aqueous anolyte through the anode, where the proton-permeable membrane prevents substantial mixing of the catholyte and the anolyte.

[0033] According to a second aspect, the present invention provides an electrolysis method, which includes: contacting a liquid electrolyte with the working surface of a working electrode; propagating high-frequency sound waves with a frequency of at least 1 MHz on a sound-transmitting substrate, where the working electrode is either located on the sound-transmitting substrate or spaced from the sound-transmitting substrate by the liquid electrolyte; applying a potential between the working electrode and a counter electrode sufficient to cause an electrolysis reaction of substances in the liquid electrolyte to produce an electrolysis reaction product near the working electrode, where the propagation of the high-frequency sound waves on the sound-transmitting substrate stimulates the liquid electrolyte, thereby increasing the production efficiency of the electrolysis reaction product.

[0034] The frequency of the high-frequency acoustic wave propagating on the acoustic transmission substrate is so high that cavitation cannot be induced in the liquid electrolyte. Therefore, the liquid electrolyte is stimulated without cavitation being induced therein.

[0035] In some embodiments, the working electrode is located on the acoustic transmission substrate.

[0036] In some embodiments, the working electrode is spaced from the acoustic transmission substrate by no more than 100 mm, or no more than 30 mm, or no more than 20 mm, or no more than 10 mm, such as a distance of about 5 mm or less, through the liquid electrolyte.

[0037] In some embodiments, the electrolysis reaction product is hydrogen (H2) or oxygen (O2).

[0038] In some embodiments, the electrolysis reaction product is H2, and the potential at the working electrode is no greater than (more negative than) -2.5 V vs. RHE, or no greater than (more negative than) -1.5 V vs. RHE, or no greater than (more negative than) -1.0 V vs. RHE.

[0039] In some embodiments, the liquid electrolyte is an aqueous electrolyte with a pH between 5 and 9.

[0040] In some embodiments, the frequency of the high-frequency acoustic wave is greater than 1 MHz, such as at least 1.5 MHz, or at least 2 MHz, or at least 3 MHz, such as at least 5 MHz.

[0041] In some embodiments, the high-frequency acoustic wave comprises a waveform selected from surface acoustic waves (SAW), surface reflector body waves (SRBW), and Lamb waves. In some embodiments, the waveform is selected from SAW or SRBW.

[0042] In some embodiments, after normalizing by the surface area of the acoustic transmission substrate, the power input of the acoustic wave propagating on the substrate is less than 20 W / cm 2 or less than 10 W / cm 2 or less than 5 W / cm 2 such as less than 1 W / cm 2 .

[0043] In some embodiments, propagating the high-frequency acoustic wave on the acoustic transmission substrate includes electrically driving one or more conductive electrodes coupled to the piezoelectric substrate portion of the acoustic transmission substrate.

[0044] In some such embodiments, the one or more conductive electrodes comprise a pair of separated electrodes. The pair of separated electrodes may be an interdigitated electrode pair. The pair of conductive electrodes may be patterned on the surface of the piezoelectric substrate portion.

[0045] In some embodiments, h / λ is greater than 1 / 3, or greater than 1 / 2, or greater than 1, where h is the thickness of the piezoelectric substrate portion and λ is the acoustic wavelength of the high-frequency acoustic wave when propagating on the piezoelectric substrate portion.

[0046] In some embodiments, h / λ is between 1 / 3 and 3, or between 1 / 2 and 3, or between 1 and 2, where h is the thickness of the piezoelectric substrate portion and λ is the acoustic wavelength of the high-frequency acoustic wave when propagating on the piezoelectric substrate portion.

[0047] In some embodiments, the working electrode is located on the piezoelectric substrate portion.

[0048] In some embodiments, the piezoelectric substrate portion comprises a single crystal selected from lithium niobate, quartz, lithium tantalate, and lanthanum gallium silicate.

[0049] In some embodiments, the piezoelectric substrate portion is configured as a plate, and the high-frequency acoustic wave propagates in the plane of the plate, causing at least one surface of the plate to oscillate.

[0050] In some embodiments, the working electrode comprises an electrocatalyst for the hydrogen evolution reaction on the working surface.

[0051] In some embodiments, the working electrode comprises one or more layers of a metal composition or a metal compound formed on the surface of the acoustically transparent substrate, preferably including a layer containing at least one selected from gold, platinum, titanium, titanium dioxide, molybdenum disulfide, graphene, and aluminum at the working surface.

[0052] According to a third aspect, the present invention provides an electrode device for an electrolytic cell, the electrode device comprising: an acoustically transparent substrate comprising at least a piezoelectric substrate portion; a working electrode located on the acoustically transparent substrate, the working electrode comprising a working surface for contacting a liquid electrolyte in the electrolytic cell; and one or more conductive electrodes connected to the piezoelectric substrate portion and configured to cause a high-frequency acoustic wave with a frequency of at least 1 MHz to propagate on the acoustically transparent substrate when electrically driven.

[0053] It should be understood that various embodiments of the third aspect may include the features defined herein in the first aspect.

[0054] Unless the context otherwise requires, when the terms "comprise", "comprising", and "include" are used in the specification (including the claims), they shall be construed as specifying the stated features, integers, steps, or components but not precluding the presence of one or more other features, integers, steps, or components or groups thereof.

[0055] As used herein, terms such as "first", "second", "third", etc. associated with various features of the disclosed apparatus are arbitrarily assigned and are merely intended to distinguish between two or more such features that the apparatus may include in multiple embodiments. The terms themselves do not indicate any particular orientation or order. And it should be clearly understood that the presence of a "first" feature does not imply the presence of a "second" feature, the presence of a "second" feature does not imply the presence of a "first" feature, and so on.

[0056] Other aspects of the present invention are described in detail below in the detailed description of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Some embodiments of the present application will be described only by way of example and with reference to the accompanying drawings, in which:

[0058] Figure 1 Schematically depicted are some embodiments according to the present invention, such as an electrolytic cell prepared in Example 2, in which the working electrode is on the piezoelectric substrate portion of a sound-transmitting substrate. Insets 120 and 122 depict a laser Doppler vibrometry scan of sound waves propagating on the substrate as measured in Example 3.

[0059] Figure 2 Schematically depicted is an electrolytic cell according to some embodiments of the present invention, in which the working electrode is on the piezoelectric substrate portion of a sound-transmitting substrate.

[0060] Figure 3 Schematically depicted is an electrolytic cell according to some embodiments of the present invention, in which the working electrode is spaced from the sound-transmitting substrate by a liquid electrolyte.

[0061] Figure 4 Schematically depicted is an electrolytic cell according to some embodiments of the present invention, which is configured as a flow cell for water electrolysis, in which the working electrode and the counter electrode are each located on a piezoelectric substrate.

[0062] Figure 5 Schematically depicted is the generation and propagation of high-frequency sound waves on a sound-transmitting substrate in an embodiment in which both the conductive electrode and the working electrode are located on the piezoelectric substrate portion of the sound-transmitting substrate.

[0063] Figure 6Schematically describes the generation and propagation on a sound-transmitting substrate of high-frequency acoustic waves in an embodiment where the working electrode is on the sound-transmitting substrate but not on its piezoelectric substrate portion.

[0064] Figure 7 Schematically describes the generation and propagation on a sound-transmitting substrate of high-frequency acoustic waves in an embodiment where the working electrode is on the sound-transmitting substrate and above the conductive electrode, and a dielectric layer is between the piezoelectric substrate portion and the working electrode.

[0065] Figure 8 Schematically describes, in plan view (A) and side view (B), the generation and propagation on a piezoelectric substrate of high-frequency acoustic waves when generated by a rectangular IDT.

[0066] Figure 9 Schematically describes the generation and propagation on a sound-transmitting substrate of high-frequency acoustic waves in an embodiment where the working electrode is spaced from the sound-transmitting substrate by a liquid electrolyte.

[0067] Figure 10 Schematically describes an electrolytic cell prepared as in Example 2, showing relevant dimensions.

[0068] Figure 11 Shows linear sweep voltammograms (LSV) obtained from an embodiment of an electrolytic cell containing a neutral aqueous electrolyte according to the present invention under silent conditions and upon acoustic stimulation at different power levels (10 dBm, 15 dBm, 20 dBm), as obtained in Example 5.

[0069] Figure 12 Is an LSV that shows Figure 11 An enlargement of the low current density region.

[0070] Figure 13 Is a graph that shows the effect of acoustic stimulation on reducing the overpotential required to achieve Figure 11 Two different current densities in the LSV.

[0071] Figure 14 Describes time-sequence photographs of the growth and detachment of H2 gas bubbles on the working electrode under silent electrolysis conditions as observed in Example 6.

[0072] Figure 15 Describes time-sequence photographs of the detachment of small H2 bubbles from the working electrode under acoustic stimulation electrolysis conditions (20 dBm) as observed in Example 6.

[0073] Figure 16 Is a graph that shows, in a chronopotentiometric electrolysis experiment conducted in Example 7, the maintenance of -100 mA cm under silent conditions and acoustic stimulation conditions (15 dBm) -2The overpotential required for a current density of 6 hours.

[0074] Figure 17 Describes the Raman spectrum of the aqueous electrolyte near the electrode-electrolyte interface during a chronopotentiometric electrolysis experiment under silent conditions in Example 8. The spectrum fits 5 Gaussian peaks at set wavelengths, where the lower wavenumbers are associated with water molecules strongly bound through hydrogen bonding within a tetrahedral network structure, while the higher wavenumbers are associated with water molecules only weakly bound ("frustrated water").

[0075] Figure 18 Describes the Raman spectrum of the aqueous electrolyte near the electrode-electrolyte interface during a chronopotentiometric electrolysis experiment under acoustic stimulation (15 dBm) conditions in Example 8.

[0076] Figure 19 Is a graph that shows the fitting to Figure 17 And Figure 18 The relative peak areas of the 5 Gaussian peaks of the Raman spectrum, indicating an increase in weakly bound water under acoustic stimulation.

[0077] Figure 20 Shows the LSV obtained under silent conditions and under acoustic stimulation (20 dBm) through an electrolytic cell according to an embodiment of the present invention containing neutral, acidic, and basic aqueous electrolytes, as obtained in Example 9.

[0078] Figure 21 Shows the LSV obtained under silent conditions and under acoustic stimulation (20 dBm) through an electrolytic cell according to an embodiment of the present invention where the working electrode is separated from the sonotransparent substrate by a liquid electrolyte, as obtained in Example 10.

[0079] Figure 22 Shows the LSV obtained under silent conditions and under acoustic stimulation through an electrolytic cell according to an embodiment of the present invention where the working electrode is a nickel anode for OER and is separated from the sonotransparent substrate by an alkaline electrolyte, as obtained in Example 11.

[0080] Figure 23 Shows the LSV obtained under silent conditions and under acoustic stimulation through an electrolytic cell according to an embodiment of the present invention where the working electrode is a stainless steel anode for OER and is separated from the sonotransparent substrate by a neutral electrolyte, as obtained in Example 11.

[0081] Figure 24An electrolytic cell according to some embodiments of the present invention is schematically depicted. It is a flow electrolytic cell configured for water electrolysis, where the cathode and anode are spaced apart from a piezoelectric transducer (to enhance HER and OER) and are separated by a proton-permeable membrane.

[0082] Figure 25 An electrolytic cell according to some embodiments of the present invention is schematically depicted. It is a flow electrolytic cell configured for water electrolysis, where the cathode and anode are formed on the piezoelectric transducer (to enhance HER and OER) and are separated by a proton-permeable membrane.

[0083] Figure 26 A perspective view of a flow electrolytic cell for water electrolysis is described, as described and used in Example 12.

[0084] Figure 27 Describes in Figure 26 A side view of the flow cell described above.

[0085] Figure 28 Describes in Figure 26 The anode of the flow cell described above.

[0086] Figure 29 Shows linear sweep voltammograms (LSV) obtained during water electrolysis in a flow electrolytic cell, (a) under silent conditions and (b) under a 40 dBm (7 MHz) acoustic stimulus, as described in Example 12. Detailed Description

[0087] The present invention relates to an electrolytic cell. The electrolytic cell includes a working electrode, a counter electrode, and a liquid electrolyte that at least contacts the working surface of the working electrode. The electrolytic cell further includes an acoustically transparent substrate that includes or consists of a piezoelectric substrate portion and one or more (e.g., two) conductive electrodes connected to the piezoelectric substrate portion. The conductive electrodes are configured to cause high-frequency sound waves with a frequency of at least 1 MHz to propagate on the acoustically transparent substrate when electrically driven, for example, by an alternating current signal. The electrolytic cell further includes one or more power sources configured to (i) apply a potential between the working electrode and the counter electrode sufficient to cause an electrolysis reaction of substances in the liquid electrolyte to produce electrolysis reaction products near the working electrode; and (ii) electrically drive the one or more conductive electrodes. The working electrode is either located on the acoustically transparent substrate or spaced apart from the acoustically transparent substrate by the liquid electrolyte, i.e., the liquid electrolyte is disposed between the acoustically transparent substrate and the working electrode and contacts both.

[0088] When the electrolytic cell is operating, the propagation of the high-frequency acoustic wave on the sound-transmitting substrate stimulates the liquid electrolyte, typically near the working surface of the working electrode. The effect of this is to increase the production efficiency of the electrolysis reaction products. As used herein, the production efficiency refers to the energy efficiency of producing the electrolysis reaction products in the electrolytic cell. It can be observed that the increase in production efficiency is manifested as an increase in the production rate of the electrolysis reaction products at a given potential, and / or a decrease in the potential required to achieve a given production rate. In either case, this improvement can be observed compared to the efficiency obtained when the electrolytic cell is operated in other similar ways but the high-frequency acoustic wave does not propagate on the sound-transmitting substrate.

[0089] Reference will be made to Figure 1 and Figure 2 , to describe an electrolytic cell 100 according to some embodiments of the present invention. The electrolytic cell 100 includes a working electrode 102, a counter electrode 104, and an optional reference electrode 106, each electrode being in contact with a liquid electrolyte 110. For clarity, the cell wall 111 is omitted from Figure 1 . The electrodes are connected to a power source 112, which is configured to apply a potential between the working electrode 102 and the counter electrode 104 sufficient to cause an electrolysis reaction of the substances in the liquid electrolyte and thus produce electrolysis reaction products near the working electrode 102. The working electrode 102 can be a cathode for the hydrogen evolution reaction (HER), in which case the electrolysis reaction products include H2. In this case, the liquid electrolyte 110 can be an aqueous electrolyte of any pH, including acidic pH, neutral pH (pH 5 - 9), or basic pH. Alternatively, the working electrode 102 can be an anode for the oxygen evolution reaction (OER), thereby producing O2 as the electrolysis reaction product, or actually as the cathode or anode for any other desired electrolysis half-reaction.

[0090] The working electrode 102 is located on a piezoelectric substrate 114, which can suitably be a single crystal of lithium niobate or other piezoelectric material, and is preferably in the form of a thin plate, for example with a thickness in the range of 0.1 to 5 mm. The composition of the working electrode can depend on the desired electrolysis reaction. The working electrode can comprise a metal composition or metal compound that is in contact with the liquid electrolyte 110 at least at the working surface, such as a HER electrocatalyst. The working electrode 102 can optionally be connected to the power source 112 through a conductive contact track 115 formed of the same material as the electrode itself.

[0091] As Figure 1Schematically described, the conductive electrodes 116, such as the interdigital electrodes 116a and 116b, are also located on the surface of the piezoelectric substrate 114. The conductive electrodes are configured to cause high-frequency acoustic waves 118 with a frequency of at least (or greater than) 1 MHz to propagate on the substrate 114 when electrically driven by a power source 120, such as a radio frequency (RF) power source. The working electrode 102 is located in the propagation path of the acoustic waves. The frequency of the acoustic waves can be at least 1.5 MHz, or at least 2 MHz, or at least 3 MHz, such as at least 5 MHz, and can be observed by known techniques such as laser Doppler vibrometry. Examples in this regard can be as Figure 1 seen in the laser vibrometry surface scans 120 and 122 described in the close-up illustration, where the dashed circle 124 represents the contour of the working electrode 102 (see Example 3 for details). The form of the acoustic waves, such as surface acoustic waves (SAW), surface reflector body waves (SRBW), or Lamb waves (a form of body wave propagating in a solid substrate), will depend on the configuration of the electrodes 116 and the size of the substrate 114, which will be described in detail later. However, regardless of the form, the propagating acoustic waves will cause high-frequency oscillations on the surface of the piezoelectric substrate 114, thereby stimulating the working surface of the working electrode 102 and the adjacent liquid electrolyte 110. When the electrolytic cell 100 is operating, the effect of this stimulation is to increase the production rate of the electrolysis reaction products and / or reduce the potential required to achieve the target productivity.

[0092] Reference will be made to Figure 3 , to describe the electrolytic cell 200 according to some embodiments of the present invention. Components with similar reference numerals in the figure are the same as those in the electrolytic cell 100. However, in the electrolytic cell 200, the working electrode 202 is not located on the piezoelectric substrate 114, but is spaced apart from the piezoelectric substrate 114 by the liquid electrolyte 110, and the electrolyte is thus disposed between the surface of the substrate 114 and the working surface of the working electrode 202 and is in contact with both. The spacing is preferably small to maximize the effect of the acoustic stimulation on the electrolysis reaction occurring near the working surface of the working electrode. Therefore, the distance by which the working electrode 202 is spaced apart from the piezoelectric substrate 114 can be no more than 30 mm, no more than 20 mm, or no more than 10 mm, such as 5 mm or less.

[0093] When the electrolytic cell 200 is operating, the high-frequency acoustic waves 118 propagating on the piezoelectric substrate 114 oscillate the surface of the piezoelectric substrate 114, stimulating the adjacent liquid electrolyte 110. The effect of this is to increase the production rate of the electrolysis reaction products formed near the working electrode 202 and / or reduce the potential required to achieve the target production rate.

[0094] Reference will be made to Figure 4, an electrolytic cell 300 according to some embodiments of the present invention is described. Unless otherwise specified, the features of the electrolytic cell 300 are similar to the corresponding features of the electrolytic cell 10. The electrolytic cell 300 is a flow electrolytic cell for water electrolysis, which includes a cathode 302 and an anode 304, and each electrode is in contact with an aqueous electrolyte 310 having any suitable pH, such as an acidic pH, a neutral pH (pH 5-9), or an alkaline pH. The cathode 302 and the anode 304 may respectively include electrocatalysts suitable for promoting HER and OER. The electrodes are connected to a power source (not shown) configured to apply a potential sufficient to initiate electrolysis.

[0095] When the electrolytic cell 300 is operating, as the electrolyte 310 flows through the electrolytic cell in the direction shown by the arrow 311, H2 bubbles 326 are formed on the cathode 302, and O2 bubbles 328 are formed on the anode 304. Once flowing past the electrodes, the flowing electrolyte divides into streams 332 and 334 at the shunt joint 330. When the H2 and O2 bubbles desorb from their respective electrodes, they are transported downstream with the aqueous electrolyte 310. As long as the turbulence in the flowing electrolyte is sufficiently suppressed, the H2 bubbles 326 and the O2 bubbles 328 are selectively introduced into the streams 332 and 334 respectively, thereby avoiding or reducing the amount of mixing of the two gaseous electrolysis products. Alternatively, a membrane (not shown) may be provided between the cathode 302 and the anode 304 to prevent gas mixing.

[0096] The cathode 302 and the anode 304 are respectively located on a piezoelectric substrate 314 and a piezoelectric substrate 315. Conductive electrode pairs 316 and 317, such as interdigital electrode pairs, are also located on the surfaces of the piezoelectric substrate 314 and the piezoelectric substrate 315. The electrode pairs are configured to cause high-frequency sound waves 318 and 319 with respective acoustic frequencies of at least (or greater than) 1 MHz to propagate on their corresponding substrates 314 and 315 when electrically driven by a suitable power source. The cathode 302 is located in the propagation path of the sound wave 318, and the anode 304 is located in the propagation path of the sound wave 319. The frequency of each sound wave can be independently at least 1.5 MHz, or at least 2 MHz, or at least 3 MHz, such as at least 5 MHz, and the form of the sound waves depends on the configuration of the electrodes and the dimensions of the substrates 314 and 315. The propagating sound waves cause high-frequency oscillations on the surfaces of the piezoelectric substrate 314 and the piezoelectric substrate 315, thereby stimulating the working surfaces of the electrodes 302 and 304 and the adjacent liquid electrolyte 310. When the electrolytic cell 300 is operating, this effect is to increase the rate of water hydrolysis, and / or reduce the potential required to achieve the target rate of water electrolysis.

[0097] Using acoustic stimulation in this manner facilitates the two half - reactions (HER, OER) of the water electrolysis, at least by assisting in the bubble desorption at the cathode 302 and the anode 304. However, it should be understood that the electrolytic cell 300 can alternatively be configured to propagate sound waves only in the vicinity of the cathode 302 or only in the vicinity of the anode 304.

[0098] Generation and Propagation of High - Frequency Sound Waves

[0099] The electrolytic cell according to the present invention comprises an acoustically transparent substrate, which comprises at least a piezoelectric substrate portion and one or more conductive electrodes connected to the piezoelectric substrate portion. The conductive electrodes, together with the piezoelectric substrate portion, are configured to cause high - frequency sound waves to propagate on the acoustically transparent substrate when electrically driven. The conductive electrodes and the piezoelectric substrate portion together form a piezoelectric transducer for converting an electrical signal into sound waves propagating on the substrate. The high - frequency sound waves propagate along the surface of the piezoelectric substrate portion, causing the surface to oscillate. In some embodiments where the piezoelectric substrate portion is configured as a plate, the high - frequency sound waves can propagate in the plane of the plate.

[0100] The acoustically transparent substrate can consist of the piezoelectric substrate portion. For example, Figure 5 A piezoelectric substrate 414 is described, where conductive electrodes (such as IDT electrodes) 416 and a working electrode 402 are both located on the surface of the piezoelectric substrate 414. When electrically driven, high - frequency sound waves 418 propagate on the piezoelectric substrate to stimulate the working electrode 402.

[0101] However, it is also contemplated that the acoustically transparent substrate can comprise other non - piezoelectric portions through which the high - frequency sound waves can propagate. For example, Figure 6 An acoustically transparent substrate 514 is described, which comprises a piezoelectric substrate portion 540 and a second substrate portion 542 (such as glass, silicon) that is optionally a non - piezoelectric material, and the second substrate portion 542 is adjacent to the portion 540 at a boundary 544. Conductive electrodes (such as IDT electrodes) 516 are located on the piezoelectric substrate portion 540, while the working electrode 502 is located on the second substrate portion 542. In use, high - frequency sound waves 518 are generated on the piezoelectric substrate portion 540 but propagate on the boundary 544 and the second substrate portion 542 to stimulate the working electrode 502.

[0102] Another example, Figure 7 An acoustically transparent substrate 614 is described, which comprises a piezoelectric substrate portion 640 on which conductive electrodes (such as IDT electrodes) 616 are located. A thin dielectric acoustically transparent material coating 642 is placed between the piezoelectric substrate portion 640 and the working electrode 602 to electrically isolate the functional components. In use, high - frequency sound waves 618 are generated on the piezoelectric substrate portion 640 but propagate on the acoustically transparent substrate 614 to stimulate the working electrode 602 on its surface.

[0103] The piezoelectric substrate portion can be any suitable single - crystal or polycrystalline piezoelectric material capable of generating high - frequency acoustic waves. In some embodiments, it is a single - crystal piezoelectric material. It is expected that single - crystals generate acoustic waves with a more defined waveform. A series of suitable piezoelectric materials are known for their applications in acoustic transducers used in electronic and telecommunication devices such as filters, oscillators, and transformers. In some embodiments, the piezoelectric substrate portion comprises a single - crystal selected from lithium niobate, quartz, lithium tantalate, and lanthanum gallium silicate.

[0104] The piezoelectric substrate portion can be configured as a plate (or “chip”). The piezoelectric substrate portion is preferably thin, for example, in the range of 0.1 to 5 mm or in the range of 0.1 to 1 mm in thickness. As will be explained later, the thickness affects the form of the high - frequency acoustic waves generated on the piezoelectric substrate portion and propagating through the acoustic - transparent substrate.

[0105] The one or more conductive electrodes connected to the piezoelectric substrate portion are generally formed on the surface of the piezoelectric substrate portion. The conductive electrodes can be formed by known techniques such as photolithography, chemical vapor deposition, sputtering, electron beam deposition, etc., by depositing one or more conductive compositions, such as metal compositions, in the form of one or more deposition layers on the piezoelectric substrate surface to form a desired pattern.

[0106] In principle, when electrically driven by a suitable electrical signal, such as an RF signal, a single electrode connected to the piezoelectric substrate can initiate high - frequency acoustic waves in the substrate. However, more commonly, two or more (e.g., a pair) spaced - apart conductive electrodes are connected to the piezoelectric substrate portion in a configuration and relative arrangement suitable for generating the desired waveform. In some embodiments, an interdigital electrode pair is located on the surface of the piezoelectric substrate portion, where each electrode comprises a plurality of “fingers” that are alternately interdigitated with the “fingers” of the other electrode, as Figure 1 and Figure 8 shown. This arrangement is particularly suitable for generating surface acoustic waves (SAW) or surface reflector body waves (SRBW) in the piezoelectric substrate.

[0107] In some other embodiments, the electrode pair is configured as strips and is located on the same or opposite surfaces of the piezoelectric substrate portion. This arrangement can be suitable for propagating body waves (e.g., Lamb waves) on the piezoelectric substrate. Other suitable electrode shapes can include L - shaped electrodes, one or more dot electrodes, wire electrodes, arc electrodes, or circular electrodes. When electrically driven by a power source at the resonance frequency of the piezoelectric substrate, a wide range of electrode configurations can be suitable. These can include simple electrode configurations suitable for generating Lamb waves and surface acoustic waves in a single - crystal piezoelectric substrate as disclosed in WO2015 / 054742.

[0108] In use, the high-frequency acoustic waves propagating on the acoustic substrate typically have a frequency that is too high to initiate cavitation in the aqueous electrolyte adjacent to the substrate, and thus is well above the power ultrasound range of 20 kHz to 100 kHz. Accordingly, the frequency is at least 1 MHz, and in some embodiments, the frequency of the high-frequency acoustic waves is higher than 1 MHz, such as at least 1.5 MHz, or at least 2 MHz, or at least 3 MHz or at least 5 MHz, such as about 10 MHz or higher. Suitably, the frequency can be in the range of 1 MHz to 10 GHz, or 3 MHz to 1 GHz, or 5 MHz to 100 MHz.

[0109] Figure 8 The interdigital transducer is depicted in plan view (A) and side view (B) and includes interdigital electrode pairs 716a and 716b on a piezoelectric plate substrate 714. Each electrode includes a plurality of interdigitated fingers 760a and 760b, respectively. When electrically driven with an AC signal at its resonant frequency, the IDT causes acoustic waves 718 to propagate on the plate substrate. For a rectangular IDT, the wavelength (λ) of the acoustic waves is defined by the spacing between adjacent electrode fingers on the same electrode (or four times the width of each finger). The frequency (f) of the acoustic waves is related to λ by the following equation:

[0110] λ = c / f

[0111] where c is the speed of sound in the plate medium. Accordingly, a suitable IDT can be designed to produce a target acoustic wave frequency. For any given device, the actual value of λ can be determined by laser Doppler vibrometry.

[0112] The form of the wave propagating through the acoustic plate substrate depends on the relationship between the acoustic wave wavelength (λ) and the thickness (h) of the plate, as Figure 8 shown. If h / λ is significantly greater than 1, such as greater than 2 and preferably greater than 3, the acoustic waves can be considered approximately surface acoustic waves (SAW or Rayleigh waves). SAWs propagate only on the surface of the acoustic substrate and attenuate to disappearance through the substrate thickness within a range of about 2λ from the surface, so the acoustic waves are not affected by bulk resonances or internal reflections. If h / λ is significantly less than 1, such as less than 1 / 2 and preferably less than 1 / 3, the acoustic waves can be considered approximately Lamb waves. In the intermediate region where h / λ is about 1, such as between 1 / 3 and 3, or between 1 / 2 and 2, the acoustic waves can be considered mixed waves including surface wave components and body wave components. Such waves have been classified by Rezk et al. (Adv. Mater. 2016, 28, 1970–1975) as “surface-reflected body waves” (SRBW) and are described as such herein.

[0113] The high-frequency acoustic waves propagating on the acoustically transparent substrate according to the present disclosure may include SAW, SRBW, or Lamb (bulk) waves. A piezoelectric transducer that propagates Lamb waves may be easier to fabricate and may thus be preferred in some embodiments. However, Lamb waves are considered to provide the least stimulation to the fluid in contact with the substrate. Thus, in some embodiments, the high-frequency acoustic waves propagating on the acoustically transparent substrate comprise SAW or SRBW. To achieve this, the relationship between the thickness (h) of the piezoelectric portion and the acoustic wavelength (λ) of the high-frequency acoustic waves propagating in the piezoelectric substrate portion may be such that h / λ is greater than 1 / 3, preferably greater than 1 / 2, and more preferably greater than 1.

[0114] In some embodiments, the high-frequency acoustic waves propagating on the acoustically transparent substrate comprise SRBW. To achieve this, h / λ may be between 1 / 3 and 3, such as between 1 / 2 and 3, or between 1 and 2. Advantageously, it is believed that such acoustic waves produce the highest degree of stimulation in the liquid electrolyte adjacent to the acoustically transparent substrate while still only requiring a low power input to electrically drive the conductive electrodes. The inventors have found that excellent electrolysis results are obtained when the liquid electrolyte adjacent to the working electrode is stimulated with SRBW, and attribute this result to the enhancement of the reflected body wave component by the surface wave component, with the two components acting in concert to oscillate the substrate surface and / or the working surface at maximum amplitude.

[0115] A particular advantage of the embodiments of the electrolytic cell disclosed herein is that, compared to electrolytic cells that use power ultrasonic transducers to propagate low-frequency ultrasonic waves into the electrolyte, typically for initiating cavitation, the power input required to propagate high-frequency acoustic waves on the acoustically transparent substrate is inherently lower. In some embodiments, the power input normalized by the surface area of the acoustically transparent substrate is less than 20 W / cm 2 , or less than 10 W / cm 2 , or less than 5 W / cm 2 , such as less than 1 W / cm 2 .

[0116] Despite the low power input and the short distance (attenuation length) of propagation into the electrolyte associated with high-frequency (MHz range) acoustic waves, the inventors have surprisingly found that when the working electrode is placed on or near the acoustically transparent substrate, the electrolysis production efficiency is significantly increased - thus providing a good acoustic power density at the electrode-electrolyte interface. This is in contrast to power ultrasound used in some prior art arrangements, which, despite the loss of power input, is still used to maximize the acoustic propagation distance through the electrolyte to cavitate the electrolyte.

[0117] Without wishing to be bound by any theory, the inventors propose that an important contributing factor to obtaining improved electrolysis results in an aqueous electrolyte is that the high-frequency acoustic waves propagating on the sonotransduction substrate disrupt the hydrogen bond network of water molecules in the electrolyte adjacent to the sonotransduction substrate, particularly at the working electrode - electrolyte interface. This effect has been observed and quantified by Raman spectroscopy (see Example 8). Thus, in some embodiments, the high-frequency acoustic waves increase the proportion of hindered water and / or free water in the aqueous electrolyte at the working electrode - electrolyte interface relative to the proportion of these substances under silent conditions. In some embodiments, the high-frequency acoustic waves increase the proportion of hindered water and / or free water in the aqueous electrolyte at the working electrode - electrolyte interface by at least 50%, such as at least 100%, relative to the proportion of these substances under silent conditions.

[0118] As used herein, free water refers to water molecules that do not participate in the hydrogen bond network. The proportion of free water can be calculated by (i) measuring the Raman spectrum of the aqueous electrolyte; (ii) deconvolving the Raman spectrum in the range of 3000 - 3800 cm-1 into at least four Gaussian peaks, including the highest frequency peak at approximately 3624 cm-1 corresponding to free water; and (iii) calculating the proportion of the peak area of the Gaussian peak at 3624 cm-1 to the total area of all Gaussian peaks. In some embodiments, five Gaussian peaks at approximately 3055 cm-1, 3230 cm-1, 3392 cm-1, 3520 cm-1, and 3624 cm-1 can be deconvolved.

[0119] As used herein, hindered water refers to free water (as defined above) and water weakly bound within disrupted three-fold coordination structures. The proportion of hindered water can be calculated by (i) measuring the Raman spectrum of the aqueous electrolyte; (ii) deconvolving the Raman spectrum in the range of 3000 - 3800 cm-1 into at least four Gaussian peaks, including the two highest frequency peaks at approximately 3624 cm-1 and 3520 cm-1; and (iii) calculating the proportion of the combined peak area of the Gaussian peaks at 3624 cm-1 and 3520 cm-1 to the total area of all Gaussian peaks. In some embodiments, five Gaussian peaks at approximately 3055 cm-1, 3230 cm-1, 3392 cm-1, 3520 cm-1, and 3624 cm-1 can be deconvolved.

[0120] Working electrode

[0121] The electrolytic cell according to the present application includes a working electrode, which is either located on the acoustic substrate or spaced apart from the acoustic substrate by the liquid electrolyte. As used herein, the working electrode is the electrode in the electrolytic cell where the faradaic reaction responsible for generating the electrolysis reaction products (as disclosed herein, the production efficiency is improved) occurs.

[0122] The working electrode can be suitably a cathode or an anode, and the composition of the working electrode can depend on the desired electrolysis reaction. For example, the working electrode can include an electrocatalyst for the electrolytic half-reaction occurring on the working electrode, where the electrocatalyst is present at least on the working surface. Suitable electrode compositions can include metal compositions or metal compounds, such as metal oxides or metal sulfides. If the working electrode acts as a cathode in water electrolysis, the working electrode can include a HER electrocatalyst. Suitable HER electrocatalysts include, but are not limited to, gold, platinum, titanium, titanium dioxide, molybdenum disulfide, graphene, and aluminum. In some other embodiments, the working electrode can include an OER electrocatalyst.

[0123] In some embodiments, the working electrode is located on the acoustic substrate, as disclosed herein with reference to Figure 1 , 2 , 4, 5, 6, and 7. In embodiments where the acoustic substrate is a plate and the high-frequency wave includes SBRW or Lamb wave, the working electrode can be located on either surface of the plate.

[0124] In some embodiments, the working electrode is on the same surface as the conductive electrode. In use, the high-frequency acoustic wave propagating on the acoustic substrate can propagate through and thus oscillate the working surface of the working electrode, directly stimulating the liquid electrolyte at the electrode-electrolyte interface. The inventors have demonstrated that with this arrangement, the electrolysis efficiency can be significantly improved. For example, when performing water electrolysis in a neutral electrolyte, the current density at the target overpotential can be increased by up to 14 times (see Example 5). Without wishing to be bound by any theory, it is believed that multiple mechanisms can contribute to the improvement of the electrolysis efficiency in such electrolytic cells, including (i) locally generating active species, such as H + or H3O + ions (see Example 5) through the relevant electromechanical field mechanism; (ii) locally disrupting the intermolecular structure of the liquid electrolyte, such as the hydrogen bond network in an aqueous electrolyte, thereby generating weakly bound or free water molecules ("hindered water", see Example 8); (iii) enhancing the detachment of bubbles from the working surface at a smaller bubble size due to local convective flow (see Example 6); (iv) improving the mass transfer between the electrode-electrolyte interface and the bulk electrolyte.

[0125] The working electrode may be present on the surface of the acoustic substrate as one or more layers. As will be appreciated, the thickness of the working electrode is preferably such that the high-frequency acoustic wave can propagate through the working electrode, causing its surface to oscillate. In some embodiments, the thickness of the working electrode is less than 1 mm, or less than 100 μm, or less than 10 μm, such as less than 1 μm.

[0126] The working electrode may be formed on the surface of the acoustic substrate by depositing one or more metal compositions or metal compounds on the surface of the piezoelectric substrate in the form of one or more deposited layers using known techniques such as photolithography, chemical vapour deposition, sputtering, etc.

[0127] Although excellent results have been obtained when the working electrode is located on the acoustic substrate, such an arrangement is not a prerequisite for obtaining enhanced electrolysis efficiency. Thus, in some embodiments, the working electrode is spaced apart from the acoustic substrate by the liquid electrolyte, such as as disclosed herein with reference to Figure 3 The liquid electrolyte may thus be disposed between and in contact with the surface of the acoustic substrate and the working surface of the working electrode.

[0128] The inventors have demonstrated that the electrolysis efficiency is significantly improved when using a working electrode spaced apart from the surface of the acoustic substrate (see Examples 10 and 11). Without wishing to be bound by any theory, it is believed that at least some of the mechanisms described herein (for the case where the working electrode is located on the acoustic substrate) still apply when the working electrode is spaced apart from the acoustic substrate by the liquid electrolyte.

[0129] The spacing distance between the working electrode and the acoustic substrate is preferably small so as to maximize the effect of the acoustic stimulation on the electrolysis reaction occurring near the working surface of the working electrode. When a high-frequency acoustic wave, such as a SAW, propagates on an acoustic substrate in contact with a liquid, the liquid is stimulated by the oscillating surface to propagate a corresponding acoustic beam (or acoustic jet) into the liquid, and the acoustic beam (or acoustic jet) attenuates as it propagates further in the liquid. The attenuation distance of the acoustic beam is inversely proportional to the frequency of the high-frequency acoustic wave. Using a measure of the attenuation distance (X s, the position of the maximum velocity within the acoustic jet), the distance of the acoustic jet propagated into water by SAW on the lithium niobate chip has been estimated to be: for frequencies of 20 and 54 MHz, approximately 10 - 17 mm and 6 mm respectively (Dentry et al, Physical Review E89, 013203, 2014); at 10 MHz, it is expected to be approximately 20 mm; and at lower MHz frequencies, the attenuation distance is expected to be greater. In some embodiments, the working surface of the working electrode is located within the attenuation distance at the relevant acoustic frequency determined by X s within the attenuation distance at the relevant acoustic frequency determined by X

[0130] In some embodiments, the working electrode is spaced apart from the acoustic transmission substrate by no more than 100 mm, or no more than 30 mm, or no more than 20 mm, or no more than 10 mm, such as a distance of about 5 mm or less, through the liquid electrolyte.

[0131] Figure 9 A piezoelectric substrate 814 provided with a conductive electrode (such as an IDT electrode) 816 is described. The working electrode 802 optionally formed on the support substrate 862 is spaced apart from the piezoelectric substrate 814 by a distance d, and a liquid electrolyte 810 is disposed between the surface of the piezoelectric substrate 814 and the working surface of the working electrode 802. During use, high-frequency acoustic waves 818 propagate on the piezoelectric substrate 814, thereby directly stimulating the liquid electrolyte in contact with the piezoelectric substrate. Although the working electrode 802 is spaced from the surface of the piezoelectric substrate 814, the stimulation of the liquid electrolyte can still improve the electrolysis efficiency during the electrolysis reaction. To maximize the enhancement degree, the distance d can be minimized as much as possible so that the liquid electrolyte 810 is stimulated near the working surface of the working electrode 802. In some embodiments, d does not exceed 100 mm, or does not exceed 30 mm, or does not exceed 20 mm, or does not exceed 10 mm, such as about 5 mm or less.

[0132] The working electrode can be formed on the surface of the support substrate by depositing one or more metal compositions or metal compounds in the form of one or more layers of deposition layers on the surface of the piezoelectric substrate, using known techniques such as photolithography, chemical vapor deposition, sputtering, etc. However, when the working electrode is spaced from the acoustic transmission substrate, a wide range of working electrode configurations may be suitable because it is not necessary for the propagating high-frequency acoustic waves to pass through the working surface of the working electrode. For example, the working electrode can be a rod-shaped or plate-shaped electrode held in the electrolyte and maintaining an appropriate distance from the acoustic transmission substrate.

[0133] Other electrodes

[0134] The electrolytic cell according to the present invention includes a counter electrode. As used herein, a counter electrode is an electrode in the electrolytic cell where a Faraday reaction occurs to balance the charge of the Faraday reaction occurring at the working electrode. If the working electrode is a cathode, the counter electrode is an anode (and vice versa). The electrolytic cell can be a two - electrode system, i.e., the only electrodes are the working and counter electrodes. Alternatively, the electrolytic cell can further include other electrodes, such as a reference electrode.

[0135] The counter electrode can be of a conventional design in the electrolytic cell and perform a conventional function. However, it is contemplated that the counter electrode can also be located on a sound - transmitting substrate or be spaced from the sound - transmitting substrate by the liquid electrolyte. The sound - transmitting substrate can be the same as or different from the sound - transmitting substrate that acoustically enhances the electrolytic reaction near the working electrode. Advantageously, by subjecting both electrodes to acoustic stimulation, the overall performance of the electrolytic cell can be improved. For example, this can be particularly useful in water electrolysis, where bubbles are generated on the surface of each electrode.

[0136] Liquid electrolyte

[0137] The electrolytic cell according to the present invention includes a liquid electrolyte in contact with the working surface of the working electrode. The same liquid electrolyte can also be in contact with the counter electrode. However, it is understood that this is not necessarily the case. For example, separate anolyte and catholyte separated by an ion - permeable membrane can be used in the electrolytic cell.

[0138] The liquid electrolyte can be any suitable composition, including aqueous electrolytes, organic electrolytes (organic solvent carriers), and ionic liquid electrolytes. In some embodiments, the liquid electrolyte is an aqueous electrolyte. The aqueous electrolyte can have any suitable pH for the desired electrolytic reaction, including acidic pH (e.g., pH < 5), neutral pH (e.g., pH 5 - 9), or basic pH (pH > 9). The inventors have demonstrated enhancement during water electrolysis in each of these electrolytes (see Example 9). In some embodiments, the pH of the aqueous electrolyte is between 5 and 9, for example, between 6 and 8. At this pH value, the enhancement of water electrolysis is of particular interest because it was previously thought to be challenging to achieve a commercially significant current density in neutral electrolytes.

[0139] The liquid electrolyte includes one or more substances that will undergo an electrolytic reaction near the working electrolyte to produce the desired electrolytic reaction products. In water electrolysis, these substances are water and its ionic forms (H + , H3O + , OH - ). However, in other electrolytic reactions, it is understood that the reactant substances can be one or more molecular or ionic substances present in, e.g., dissolved in, the liquid electrolyte.

[0140] Power supply

[0141] The electrolytic cell according to the present invention comprises one or more power supplies configured to (i) apply a potential between the working electrode and the counter electrode sufficient to cause an electrolytic reaction of a substance in the liquid electrolyte to produce an electrolytic reaction product near the working electrode; and (ii) electrically drive the one or more conductive electrodes such that high-frequency sound waves with a frequency of at least 1 MHz propagate on the sound-transmitting substrate. In some embodiments, the power supplies for these functions are separate. However, it will be understood that they may be integrated into a single power supply device.

[0142] The power supply for powering the working electrode and the counter electrode may be a conventional power supply for an electrolytic cell, such as a water electrolytic cell. This power supply is typically a DC power supply. Optionally, the power supply may be from a photovoltaic solar cell or other renewable energy source. The power supply is capable of providing and typically maintaining the potential required between the working electrode and the counter electrode when current passes through the electrolytic cell. For a water electrolytic cell, in accordance with the principles disclosed herein, the potential at the working electrode is advantageously reduced, but in certain embodiments (in the case of the cathode), it may still be greater than (more negative than) -1.5 V relative to the RHE (RHE is the reversible hydrogen electrode). However, in some embodiments, the potential at the working electrode (in the case of the cathode) is not greater than (more negative than) -1.5 V relative to the RHE, or not greater than (more negative than) -1 V relative to the RHE.

[0143] The power supply for electrically driving the conductive electrodes is typically an AC power supply with a frequency equal to the resonance frequency of the electrode connected to the sound-transmitting substrate.

[0144] Flow electrolytic cell for water electrolysis

[0145] In some embodiments, the electrolytic cell is a flow electrolytic cell for water electrolysis. An embodiment of such has been disclosed herein, i.e., the electrolytic cell 300 described with reference to Figure 4 the electrolytic cell 300 described.

[0146] Reference will be made to Figure 24The flow electrolytic cell 1000 for water electrolysis as described in other embodiments of the present invention. The electrolytic cell 1000 includes a cathode 1002 and an anode 1004 connected to a power source (not shown), and the power source is configured to apply a potential sufficient to electrolytically decompose water between the electrodes, thereby generating electrolysis reaction products near the cathode 1002 (i.e., H2) and near the anode 1004 (i.e., O2). Thus, the cathode 1002 may include an electrocatalyst for the HER, and the anode 1004 may include an electrocatalyst for the OER. The electrodes may have any suitable configuration, for example, they may independently be plate-shaped, perforated, or porous.

[0147] The cathode chamber 1052 and the anode chamber 1054 are separated by a proton exchange membrane 1056 (e.g., made of an ionomer such as Nafion) or other semi-permeable membrane that prevents significant mixing of the electrolytes present in each chamber while facilitating the passage of ionic species required to maintain charge neutrality within the electrolytic cell. In use, separate aqueous electrolyte streams flow through the cathode chamber 1052 (catholyte 1062) and the anode chamber 1054 (anolyte 1064), respectively. The catholyte 1062 and the anolyte 1064 may be aqueous electrolytes of any suitable pH value, such as acidic pH, neutral pH (pH 5 - 9), or basic pH, and their compositions may be the same or different, preferably the same. Thus, the H2 formed on the cathode 1002 flows out of the electrolytic cell with the catholyte outlet stream 1072, and the O2 formed on the anode 1004 flows out of the electrolytic cell with the anolyte outlet stream 1074. Preferably, the electrolyte flow direction is upward, so that the bubbles detached from the electrodes rise upward along the electrolyte flow direction. Due to the vertical flow direction and the membrane 1056 that prevents significant mixing of the anolyte and the catholyte, the mixing of the product gases is avoided or minimized.

[0148] The electrolytic cell 1000 also includes piezoelectric substrates 1012 and 1014, and electrodes and a power source (not shown) configured to propagate high-frequency acoustic waves on the piezoelectric substrates 1012 and 1014 as described herein. Optionally, as shown, the piezoelectric substrates may form part of the outer wall of the flow electrolytic cell. The cathode 1002 is spaced from the piezoelectric substrate 1012 by a small distance, and the catholyte 1052 is present in the gap. Similarly, the anode 1004 is spaced from the piezoelectric substrate 1014 by a small distance, and the anolyte 1054 is present in the gap. For both electrodes, the distance between the electrode and the piezoelectric substrate is preferably not more than 10 mm, or not more than 5 mm, or not more than 1 mm.

[0149] In use, high-frequency acoustic waves propagate on piezoelectric substrates 1012 and 1014, thereby stimulating the liquid electrolyte between the piezoelectric substrates and the electrodes. This improves the electrolysis efficiency of the HER and the OER, and increases the yield of the overall water splitting reaction according to the principles disclosed herein.

[0150] Reference will be made Figure 25 to the flow electrolytic cell 1100 for water electrolysis described in other embodiments of the present application. The flow electrolytic cell 1100 is similar to the flow electrolytic cell 1000, and the components with similar numbers in the two electrolytic cells are the same. However, in the electrolytic cell 1100, the cathode 1002 is directly formed on the surface of the piezoelectric substrate 1012, and the anode 1004 is directly formed on the surface of the piezoelectric substrate 1014. Thus, according to the principles disclosed herein, the high-frequency acoustic waves propagating on the piezoelectric substrate directly stimulate (oscillate) the working surface of the electrodes, thereby stimulating the aqueous electrolyte at the electrode-electrolyte interface.

[0151] Electrolysis method

[0152] The present invention further relates to an electrolysis method. The method includes bringing a liquid electrolyte into contact with the working surface of a working electrode, and propagating high-frequency acoustic waves having a frequency of at least 1 MHz on a sound-transmitting substrate. The working electrode is either located on the sound-transmitting substrate or is spaced apart from the sound-transmitting substrate by the liquid electrolyte. A potential sufficient to cause an electrolysis reaction of substances in the liquid electrolyte to produce electrolysis reaction products near the working electrode is applied between the working electrode and the counter electrode. The propagation of the high-frequency acoustic waves on the sound-transmitting substrate stimulates the liquid electrolyte, thereby increasing the production efficiency of the electrolysis reaction products.

[0153] In some embodiments, propagating the high-frequency acoustic waves on the sound-transmitting substrate includes electrically driving one or more conductive electrodes connected to the piezoelectric substrate portion of the sound-transmitting substrate.

[0154] In some embodiments, the electrolysis method is a water electrolysis method to produce H2 as an electrolysis reaction product. Thus, the liquid electrolyte is an aqueous electrolyte, and in some embodiments, the aqueous electrolyte can be a neutral electrolyte (pH value of 5 to 9). In some such embodiments, the potential at the cathode is not greater than (more negative than) -2.5 V relative to RHE, or not greater than (more negative than) -1.5 V relative to RHE, for example not greater than (more negative than) -1.0 V relative to RHE. The potential (i.e., potential difference) applied between the cathode and the anode must be at least 1.23 V (for a 100% efficient system), but in practice it will be higher. However, due to the acoustic stimulation, the potential difference can be advantageously reduced. In some embodiments, the potential applied between the cathode and the anode is thus less than 2 V.

[0155] It is understood that other relevant features of the method are generally as described herein in the context of the electrolytic cell of the present invention.

[0156] Electrode device for an electrolytic cell

[0157] The present invention further relates to an electrode device for an electrolytic cell. The electrode device comprises an acoustically transparent substrate, which at least comprises a piezoelectric substrate portion and a working electrode on the acoustically transparent substrate. The working electrode comprises a working surface for contacting a liquid electrolyte in the electrolytic cell. One or more conductive electrodes, which are connected to the piezoelectric substrate portion and are configured to propagate high-frequency acoustic waves with a frequency of at least 1 MHz on the acoustically transparent substrate when electrically driven.

[0158] The electrode device is suitable for use in an embodiment of an electrolytic cell as described hereinbefore, wherein the working electrode is located on the acoustically transparent substrate. Thus, it is understood that other relevant features of the electrode device are generally as described herein in the context of the electrolytic cell of the present invention.

[0159] Embodiments

[0160] This application is described with reference to the following embodiments. It should be understood that the embodiments are for illustrative purposes and not for limiting the invention described herein.

[0161] Example 1. Preparation of an acoustic wave generator and working electrode on a piezoelectric substrate

[0162] An acoustic wave generator is fabricated by lithographically patterning interdigital transducer (IDT) electrodes, which consist of 30 pairs of finger bars, the finger bars being titanium layers and gold layers with thicknesses of 10 nm and 200 nm, and an aperture size of 5.6 mm, formed on a single-crystal piezoelectric (128° Y rotated, X-propagating lithium niobate; LiNbO3) substrate of 28×47×0.5 mm (Roditi Ltd., London, UK). The pitch of the finger bars on each electrode, i.e., the distance between adjacent finger bars on the same electrode, is 0.4 mm.

[0163] Then, on the substrate within the IDT aperture width range, a circular working electrode (WE, 0.36 cm in diameter) of 0.1 cm was prepared by a patterning process using the same materials (a 10-nm-thick titanium layer and a 200-nm-thick gold layer), and it was positioned on the wave propagation path 4 mm away from the IDT. 2

[0164] ​The thickness (h) of the piezoelectric substrate is 0.5 mm, and the wavelength (λ, equal to the pitch) of the acoustic wave excited by the IDT is 0.4 mm. Therefore, the value of h / λ is approximately 1.2. The high-frequency acoustic wave is thus expected to be SRBW. The resonance frequency of the IDT is expected to be 10 MHz (based on the sound velocity in the lithium niobate substrate, approximately 3980 m / s).

[0165] Under the conditions of Cu Kα radiation, 40 mA current, and 40 kV voltage The 2θ scanning range is 20° - 90°, the step size is 0.01°, and the scanning rate is 2.6° min -1 , powder X-ray diffraction (Bruker D8 General Area Detector Diffraction System, GADDS, Preston, VIC, Australia, Bruker Pty. Ltd.) is used to determine the crystal phase of the sputter-coated gold electrode layer. The X-ray diffraction (XRD) pattern shows that it is a polycrystalline structure with a {311} oriented plane, manifested as a strong (311) diffraction peak, accompanied by weaker (111), (200), and (220) peaks.

[0166] Example 2. Preparation of an electrochemical cell

[0167] As Figure 1An electrochemical cell is schematically described as follows. A rectangular electrolyte chamber (with internal dimensions of 9 mm × 20.5 mm) of 13.0 mm × 24.5 mm was constructed by splicing wall panels laser-cut from a 2-mm thick glass plate with glass glue (Nano470TM, Varsity Lakes, QLD, Australia, SafeLight Technologies) and thoroughly cleaning. Then, the bottom of the chamber was connected to the piezoelectric substrate with a patterned transducer and cathode (prepared as in Example 1) using silicone rubber (Parfix All Purpose, Clear; Bunnings, VIC, Australia) with a thickness of approximately 0.5 mm and dried at 50 °C for 3 hours. The IDT was retained outside and the WE was retained inside the resulting chamber. Subsequently, the chamber was covered with a custom 3D-printed lid in which a counter electrode (CE) and a reference electrode (RE) were installed. The CE was a 0.5-mm diameter helical platinum wire (temper-annealed, 99.95%, Advent Research Materials Ltd., Oxford, UK), and the RE was Ag / AgCl in 1 M KCl (CH Instruments Inc., Austin, TX, USA). The chamber was thoroughly cleaned by rinsing with a large amount of MilliQ water (18.2 MΩ·cm, Merck Millipore, Bayswater, VIC, Australia). The relevant dimensions of the electrolytic cell are as Figure 10 shown.

[0168] Example 3. Generation and characterization of acoustic waves

[0169] To enable the acoustic wave to propagate along and through the surface of the LiNbO3 substrate, the IDT was connected to a radio frequency (RF) source including a signal generator (SML01, Rhode & Schwarz GMbbH & Co. KG, Munich, Germany) and an amplifier (ZHL-5W-1+, Mini-Circuits, Brooklyn, NY, USA). Subsequently, an AC electrical signal at the resonance frequency (10 MHz) was applied to the transducer. The shape of the generated acoustic wave was imaged by a laser Doppler vibrometer (LDV; UHF-120, Polytec Inc., Irvine, CA).

[0170] Figure 1 (Insets 120, 122) show the laser Doppler vibrometry scan results of the acoustic wave (20 dBm) propagating through the WE (shown with a dashed contour 124). The vibrometry scan results are consistent with the propagation of surface reflected body waves on the piezoelectric chip.

[0171] Example 4. Preparation of an electrolyte

[0172] A first neutral electrolyte solution containing 0.1 M sodium phosphate buffer was prepared by dissolving 0.31 g of sodium dihydrogen phosphate (NaH2PO4, 99.0%; Merck Millipore, Baywater, VIC, Australia) and 1.09 g of disodium hydrogen phosphate (Na2HPO4, 99.0%; Sigma-Aldrich Pty. Ltd., Castle Hill, NSW, Australia) in 100 mL of pure water (sterile filtered molecular biology reagent, Sigma-Aldrich Pty. Ltd., Castle Hill, NSW, Australia).

[0173] A second neutral electrolyte was prepared by dissolving 0.1 M potassium chloride (KCl, 99.0%, Sigma-Aldrich Pty. Ltd., Castle Hill, NSW, Australia) in pure water (sterile filtered molecular biology reagent, Sigma-Aldrich Pty. Ltd., Castle Hill, NSW, Australia).

[0174] An acidic electrolyte of 0.5 M H2SO4 was prepared by diluting 2.72 mL of sulfuric acid (H2SO4, 98%, Thermo Fisher Scientific, Taren Point, NSW, Australia) in 97.28 mL of pure water (sterile filtered molecular biology reagent, Sigma-Aldrich Pty. Ltd., Castle Hill, NSW, Australia).

[0175] An alkaline electrolyte was prepared by dissolving 1 M potassium hydroxide (KOH, 99.0%, Sigma-Aldrich Pty. Ltd., Castle Hill, NSW, Australia) in pure water (sterile filtered molecular biology reagent, Sigma-Aldrich Pty. Ltd., Castle Hill, NSW, Australia).

[0176] The pH of the electrolyte was measured using a three-point calibrated pH meter (InLab Ultra-Micro-ISM, Sigma-Aldrich Pty. Ltd., Castle Hill, NSW, Australia).

[0177] Example 5. Electrochemical measurements in a neutral electrolyte

[0178] The electrochemical performance of the electrochemical cell in Example 2 in the electrolysis reaction was evaluated using a three - electrode setup, which used (i) the Pt wire CE; (ii) Ag / AgCl in 1M KCl RE; (iii) the polycrystalline gold WE prepared on the piezoelectric substrate as in Example 1. The electrolyte volume was 1 mL. Before the electrochemical study, nitrogen was bubbled through the electrolyte for 20 mins to remove dissolved oxygen. Each electrode was connected to a potentiostat (VSP - 128, BioLogic, Seyssinet - Pariset, France). All experiments were carried out at ambient temperature (about 25 °C), and the software (v11.25, BioLogic, Seyssinet - Pariset, France) was used to analyze the results. Linear sweep voltammetry (LSV) measurements were performed at a scan rate of 10 mV s -1 . The measured or applied potential was converted to a reversible hydrogen electrode (RHE) value by Equation (1):

[0179] E RHE = E (Ag / AgCl) + 0.197 V+ pH×0.059 V (1)

[0180] and iR - correction was used;

[0181] E iR = E RHE - iR u (2)

[0182] where i is the measured current density and R u is the electrolyte resistance (the ionic electrolyte resistance between the tip of the reference electrode and the working electrode surface). The electrolyte resistance (R u ) of 0.1M sodium phosphate electrolyte (pH 7.2) was determined by electrochemical impedance spectroscopy (EIS; 100 kHz–0.1 Hz, 10 mV amplitude) under silent conditions and under acoustic stimulation at different power levels (10 dBm, 15 dBm, 20 dBm). The EIS was collected at three constant voltages after the starting potential (i.e., the potential greater than that providing - 10 mA cm -2 ; as described by Anantharaj et al. in ChemElectroChem, 7, 2297 - 2308, 2020) and was fitted using software (v11.25, BioLogic, Seyssinet - Pariset, France). The uncompensated resistance R u values obtained from the fitted Nyquist plot are shown in Table 1 below.

[0183] Table 1.

[0184] Conditions <![CDATA[Uncompensated resistor R u (Ω)]]> n (number of experiments for fitting) Silent 205.2±2.6 3 10 dBm 186.5±4.7 3 15 dBm 124.6±7.0 3 20 dBm 86.5±4.1 4

[0185] These uncompensated Rs u values were then averaged for each condition and applied to subsequent LSV experiments using Equation (2) (Anantharaj et al, Energy Environ. Sci., 11, 744 - 771, 2018): 100% iR correction was performed at a current density of -10 mA cm -2 , and 80% iR correction was performed when exceeding -10 mA cm -2 .

[0186] Linear sweep voltammograms (LSVs) obtained using a 0.1 M sodium phosphate electrolyte (pH 7.2) under silent conditions and acoustic stimulations at different power levels (10 dBm, 15 dBm, 20 dBm) are shown in Figure 11 and Figure 12 ([[]] Figure 12 is Figure 11 an enlarged view of the low current density region). Figure 1 The inset of plots the Tafel slope for each condition, showing that the Tafel slope decreases as the acoustic power increases. Apparently, the high-frequency acoustic stimulation reduces the overpotential, thus increasing the hydrogen production rate, and the degree of influence depends on the power. The Tafel plot shows that the Tafel slope decreases significantly from 480 mV dec -1 under silent conditions to 205, 201, and 184 mV dec -1 at powers of 10 dBm, 15 dBm, and 20 dBm, respectively, corresponding to powers of 10, 15, and 20 dBm.

[0187] As shown in Figure 13 , at current densities of -10 and -100 mA cm -2 , the overpotential is thus reduced by up to 0.52 V and 1.37 V, respectively. At an applied power of 20 dBm (0.3 W), an overpotential of only 400 mV is required to reach a current density of -10 mA cm -2 . This is approximately half of the overpotential required for a Pt electrode under neutral conditions (Strmcnik et al, Nat. Chem.;2013, 5, 300–306;Shinagawa et al, ChemElectroChem 2014, 1, 1497–1507), indicating that under neutral conditions, the electrochemical performance of the acoustically stimulated gold electrode is even better than that of the Pt electrode. At an overpotential of -1 V, the current density almost increases to 14 times the original value under acoustic stimulation (from -11.8 mA cm -2Increased to -164.4 mA cm -2 ), thus reaching an industrially relevant range.

[0188] The overpotential (η total ) observed in HER is generally considered to consist of three parts, as shown in Eq. (1):

[0189] η total = η act + η Ω + η conc (3)

[0190] Where η act is the activation overpotential that controls the reaction kinetics, η act is the ohmic overpotential corresponding to the resistance of the electrolytic cell, and η conc is the overpotential related to mass transfer limitations in the system.

[0191] At low current densities (when the back-and-forth diffusion limitation of ions in the bulk electrolyte is minimal) and before the onset of bubble nucleation, η act is expected to be the main contributing factor. Therefore, the low current density region as shown in Figure 12 helps to understand the effect of high-frequency acoustic stimulation on η act .

[0192] In the absence of sound, a plateau (starting from about -400 mV) was observed in the current density-potential response curve. This plateau is a known result of the depletion of available H + and H3O + ions in the electrolyte, and these substances can only be replenished when water (H2O) molecules dissociate at higher applied potentials. Therefore, the disappearance of the plateau under high-frequency acoustic stimulation is attributed to the local generation of H + or H3O + ions at the electrode / electrolyte interface, as previously reported by Rezk et al. in Phys. Chem. Lett. 2020, 11, 4655 - 4661. Without wishing to be bound by any theory, it is believed that the large evanescent electric field associated with the electromechanical coupling of the acoustic wave with the piezoelectric substrate exceeds the threshold intensity of the autoionization of pure water, at least in the highly polarized region defined by the nanoscale amplitude of the high-frequency acoustic wave.

[0193] At high current densities, due to the accumulation of bubbles on the electrode surface, η Ω and η conc are expected to be the main contributing factors to the total observed overpotential η total . Without wishing to be bound by any theory, as shown in Figure 11As shown, at high current densities, the observed effect of high-frequency acoustic stimulation is attributed to acoustic streaming (i.e., bulk liquid recirculation), which overcomes diffusion mass transfer limitations by compressing the diffusion layer and promotes the separation and removal of bubbles that would otherwise be trapped at the working surface. Thus, this mechanism has a beneficial effect on η Ω and η conc both.

[0194] The effect of the acoustic stimulation on η Ω can also be clearly seen from the value of the uncompensated electrolyte resistance R u obtained in the EIS experiment, as shown in Table 1. As the acoustic power increases, R u decreases (up to 58% at 20 dBm compared to the silent condition), which means faster charge transfer and higher Faradaic efficiency.

[0195] Example 6. Bubble imaging

[0196] To study the effect of high-frequency acoustic stimulation on bubble generation, chronopotentiometric electrolysis experiments were carried out under silent conditions and high-frequency acoustic stimulation at different power levels (10 dBm, 15 dBm, 20 dBm). At the same time, a high-speed camera (SA5, Tokyo, Japan, Photron Ltd) with a magnifying lens (K2 Objective CF-4, Barrington, NJ, USA, Edmund Optics Inc.) was used to capture images of the generated bubbles at a frame rate of 10,000 frames per second, and the electrode surface was observed through the glass wall of the electrolytic cell. The same electrolytic cell and electrolyte (0.1 M sodium phosphate; pH 7.2) as in Example 5 were used, and a potentiometer (Model 1440 and v17.02 software; CH Instruments, Inc., TX, USA) was used to keep the current density at -30 mA cm -2 .

[0197] As Figure 14 is representatively visible in the time-series photographs at 0, 0.02 s, 0.04 s, and 0.08 s, the scale bar in the figure represents a length of 500 μm. Under silent conditions, the formed bubbles grow and coalesce. After reaching a critical size of about 450 μm, their buoyancy overcomes the adhesion force, and the bubbles detach from the electrode surface. In contrast, as Figure 15 shown, when the electrode surface is subjected to high-frequency acoustic waves at 20 dBm, the bubbles only grow to about 40 μm and then detach from the surface without coalescence. Without wishing to be bound by any theory, it is believed that this detachment is due to the drag force exerted on the bubbles by the flow convection induced by the acoustic stimulation, and the suppression of bubble accumulation on the electrode surface reduces the contribution of the resistance (η Ω ) and the diffusion barrier (η conc ) to the overpotential.

[0198] Example 7. Long-term stability of sonostimulation electrolysis

[0199] To study the effect of high-frequency acoustic stimulation on long-term electrolysis performance, chronopotentiometric electrolysis experiments were carried out under silent conditions and high-frequency acoustic stimulation (15 dBm). The experiment was carried out at a current density of -100 mA cm -2 for 6 hours of reaction process. The same electrolytic cell and electrolyte (0.1 M sodium phosphate; pH 7.2) as in Example 5 were used, and the results are as Figure 16 shown. Under acoustic stimulation, the reaction showed excellent stability at an overpotential of about -2.5 V. Therefore, a significantly reduced overpotential was obtained throughout the experimental process compared to the equivalent reaction carried out under silent conditions.

[0200] In another experiment using the same experimental setup as in Example 4, LSV experiments (0–2 V) were carried out in 0.1 M potassium chloride neutral electrolyte under silent and acoustic stimulation (20 dBm) conditions. After the experiment, the surface of the working electrode was imaged using a scanning electron microscope (SEM; Quanta200ESEM, USA, OR, Hillsboro, FEI). After electrolysis under silent conditions, pits appeared on the surface, while after electrolysis under acoustic stimulation, there was little or no obvious damage. These results are consistent with the damage expected when large H2 bubbles accumulate and detach on the electrode surface.

[0201] Example 8. Raman study of the electrolyte-electrode interface

[0202] An in-situ Raman spectrometer (LabRAM HR Evolution, France, SAS, Horiba Scientific) was used to study the working electrode-electrolyte interface at an excitation wavelength of 633 nm (acquisition range 2800–3900 cm-1), and then a 10× objective lens and a 1800 gr / mm grating were used. All spectra were calibrated relative to a silicon wafer at 520 cm-1. Therefore, using the same experimental setup as in Example 4 and a neutral electrolyte (0.1 M sodium phosphate; pH 7.2), chronopotentiometric electrolysis experiments were carried out at low current densities (-1 mA cm 2 or -8 mA cm 2 ) to minimize the interference of bubble formation on spectral analysis. Raman spectra in the range of 3000–3800 cm Figure 17 were obtained under silent conditions ( Figure 18 ) and high-frequency acoustic stimulation (15 dBm) ( -1 ), and this range corresponds to the hydrogen bond structure of the electrolyte near the electrode-electrolyte interface.

[0203] Five Gaussian peaks are at about 3055 cm-1 , 3230 cm -1 , 3392 cm -1 , 3520 cm -1 and 3624 cm -1 were deconvoluted at, where the lower wavenumbers (<3400 cm -1 ) are associated with water molecules strongly bound through hydrogen bonding within the tetrahedral network structure, and the higher wavenumbers (>3400 cm -1 ) are associated with water molecules weakly bound within the trigonal coordination structure. The peak at 3624 cm -1 corresponds to "free" water molecules that do not participate in the hydrogen bond network (Holzammer et al, J. Phys. Chem. B 2019, 123, 2354–2361). Figure 19 The relative peak areas of five peaks in four experiments (-1 mA cm -2 or -8 mA cm -2 ; silent or 15 dBm acoustic stimulation) were plotted. Under silent conditions, as the current density increased, the weakly coordinated molecules (peak 4 and peak 5) increased moderately, indicating that the electrolysis reaction inherently affected the water structure to some extent. Under acoustic stimulation, at low overpotentials (-1 mA cm -2 ) and at overpotentials before significant bubble formation (-8 mA cm -2 ), significantly higher concentrations of weakly coordinated molecules were observed. Notably, compared to the same current density under silent conditions, when under 15 dBm acoustic stimulation at -8 mA cm -2 , the concentrations of "free" water molecules (peak 5) and "hindered water" (peak 4 and peak 5) increased by 61% and 120%, respectively.

[0204] Without wishing to be bound by any theory, it is proposed that the disruption of the water structure caused by high-frequency acoustic stimulation plays an important role in the improved HER performance described herein. Compared to strongly bound water molecules, free water molecules can be more easily adsorbed onto the catalytic sites on the electrode, where the activation energy for water dissociation is expected to decrease in the order of tetrahedrally coordinated water > trigonally coordinated water > "free" water. This reduces the η act of the electrolytic cell, making the electrolysis process more efficient.

[0205] When the same experiment was conducted using deionized water as the electrolyte instead of 0.1M sodium phosphate electrolyte, similar Raman spectroscopic results were observed. The results indicate that under high-frequency acoustic stimulation, the increase in "hindered water" (peaks 4 and 5) and "free" water (peak 5) at the electrode-electrolyte interface is not an artifact caused by sodium ions (when using sodium phosphate buffer electrolyte). In fact, the disruption of the intermolecular water network is more significant in deionized water, suggesting that the increase in the ionic strength of the electrolyte may shield the acoustic-induced electric field. Therefore, the proportion of free water increases from less than 5% to more than 10% (15 dBm acoustic wave, current density for electrolysis is 2 mA / cm -2 ). This highlights the unique advantage of the method disclosed herein when seeking to use low molar concentration, especially neutral electrolytes.

[0206] Example 9. Electrochemical measurements in acidic and alkaline electrolytes

[0207] The method of Example 5 was used to study the effect of high-frequency acoustic stimulation on the electrolysis of water in electrolytes of different pH values. Figure 20 Linear sweep voltammograms (LSVs) obtained using acidic (0.5M H2SO4), basic (0.1M KOH), and neutral (0.1M sodium phosphate; pH 7.2) electrolytes are shown under silent conditions and under acoustic stimulation at a power level of 20 dBm.

[0208] As expected, under silent conditions, the neutral electrolyte requires a significantly higher overpotential compared to the acidic and basic electrolytes. High-frequency acoustic stimulation is effective in reducing the overpotential of all three electrolytes, such that the overpotential of the neutral electrolyte under acoustic stimulation is lower than that of the acidic or basic electrolyte under silent conditions. The largest effects are observed for the acidic and neutral electrolytes.

[0209] Example 10. Working electrode in close proximity to a transducer

[0210] As Figure 3 The electrochemical cell described schematically was fabricated in a manner similar to Example 2, except that the working electrode was a 2 mm diameter gold (Au) rod electrode (CHI101, USA, TX, Austin, CH Instruments Inc.) instead of the WE patterned on the piezoelectric substrate. The electroactive working surface of the electrode is only the circular bottom and the inner rod, rather than the side of the rod. The configuration of the piezoelectric substrate and IDT, as well as the Pt wire CE and Ag / AgCl in 1M KCl RE, remained the same as described in Example 2. The three electrodes were fixed by a custom 3D printed lid and immersed in 1 mL of neutral (0.1M sodium phosphate; pH 7.2) electrolyte. The end of each electrode was approximately 0.5 cm from the piezoelectric substrate.

[0211] Linear sweep voltammograms were recorded using a potentiostat (VSP-128; BioLogic, Seyssinet-Pariset, France) in a manner similar to Example 5, either under silent conditions or under acoustic stimulation (20 dBm). The LSV curves plotted with a 30% iR correction factor are as Figure 21 shown. The results show that, although there is a short displacement of the surface of the working electrode from the piezoelectric substrate, the propagation of high-frequency acoustic waves on the piezoelectric surface reduces the overpotential required (thereby increasing the rate of the hydrogen evolution reaction).

[0212] Example 11. High-frequency sonostimulation of the oxygen evolution reaction

[0213] A 10 MHz acoustic wave generator similar to that described in Example 1 (but omitting the gold electrode) was fabricated. The IDT electrodes consisted of 55 pairs of finger electrodes with an aperture size of 12.2 mm on a single crystal piezoelectric (128° Y rotated, X propagating lithium niobate; LiNbO3) substrate (Roditi Ltd., London, UK) measuring 21.8 × 16 × 0.5 mm. The pitch of the fingers on each electrode, i.e., the distance between adjacent fingers on the same electrode, was 0.4 mm. This device was used to apply acoustic stimulation (thereby enhancing OER) to the anode in an electrochemical flow cell fabricated according to the design of electrolytic cell 1000 described herein with reference to Figure 24 The flow cell walls were made of acrylic, with the acoustic wave generator serving as one of the cell walls. The anode (i.e., the working electrode in this experiment) was positioned near the surface of the single crystal piezoelectric substrate, with 10 MHz acoustic waves (SRBW waveform) propagating on the substrate surface, and the spacing between the anode and the substrate (through the electrolyte in use) was 0.5 to 3 mm. The working area of the anode was approximately 0.5 × 0.8 cm 2 , and the anode thickness was approximately 0.5 mm. Nickel anodes and stainless steel mesh anodes were used in different experiments. A commercially available graphite counter electrode (cathode; 1 × 1 × 0.5 cm) was placed at the other end of the electrolytic cell, less than 3 cm from the anode. The reference electrode was placed in the middle of the electrolytic cell. The volume of the electrolytic cell was approximately 5 mL, and the flow rate of the electrolyte could be controlled from 0 (static experiment) to 100 mL / min.

[0214] The electrochemical performance of the electrolytic cell in the water splitting reaction was tested using an alkaline electrolyte (1 M KOH) or a neutral electrolyte (0.1 M sodium phosphate buffer). Each electrode was connected to a potentiostat (VSP-128, BioLogic, Seyssinet-Pariset, France). All experiments were carried out at room temperature (approximately 25 °C), and the results were analyzed using EC- Analysis was performed using software (v11.25, BioLogic, Seyssinet-Pariset, France) as described in Example 5.

[0215] Figure 22 Linear sweep voltammograms (LSV) obtained with the nickel anode and alkaline electrolyte (flow rate 10 mL / min) under silent and acoustic stimulation conditions are shown. The current density corresponds to the rate of oxygen production through the OER. Under high-frequency acoustic stimulation, the overpotential required to generate a current density of 10 mA / cm 2 decreases from 2.41 V to 1.99 V (versus RHE), a decrease of 420 mV. At the same overpotential of 2.5 V versus RHE, under high-frequency acoustic stimulation, the current density increases from 11.8 mA / cm 2 to 28.4 mA / cm 2 (a 2.4-fold increase).

[0216] Figure 22 LSVs obtained with the stainless-steel anode and neutral electrolyte (flow rate 10 mL / min) under silent and acoustic stimulation conditions are shown. Under high-frequency acoustic stimulation, the overpotential required to generate a current density of 10 mA / cm 2 decreases from 2.44 V (versus RHE) to 1.90 V, a decrease of 540 mV. Under high-frequency acoustic stimulation, the overpotential required to generate a current density of 25 mA / cm 2 decreases from 3.28 V (versus RHE) to 2.39 V, a decrease of 890 mV.

[0217] Example 12. Flow-through electrolytic cell

[0218] According to the design of electrolytic cell 1000 described herein, a flow electrolytic cell for water electrolysis is fabricated with reference to Figure 24 . The specific configuration is shown in Figure 26 , Figure 27 and Figure 28 and shows the cathode electrolyte inlet 1062, the cathode electrolyte and H2 outlet 1072, the piezoelectric transducer 1012 for cathode stimulation (including a piezoelectric substrate and interdigital electrodes and configured to propagate high-frequency surface SRBW at 7 MHz), the electrical contacts 1080 for the transducer, the stainless-steel cathode 1002 (with its protruding electrical contacts shown), the stainless-steel anode 1004 (with its protruding electrical contacts shown). The electrolytic cell is symmetric, so the components related to the anode have the same configuration as the components related to the cathode shown. The membrane 1056 is a proton-permeable Nafion membrane. The electrodes are separated from the piezoelectric transducer 1012 by gaskets that maintain a spacing distance of 0.5 mm, and the electrolytic cell is bolted together through bolt holes 1059. Figure 28An anode 1004 is shown, where only the portion shown by the rectangle 1004a is in contact with the anolyte. Thus, the working surfaces of the two electrodes comprise a series of strips arranged in front of the piezoelectric transducer.

[0219] The electrochemical performance of the electrolytic cell in the water splitting reaction was tested using a neutral electrolyte (0.1 M neutral sodium phosphate (PBS) buffer), which flowed at a rate of 2 mL / min (split into catholyte and anolyte). Each electrode was connected to a potentiostat (VSP-128, BioLogic, Seyssinet-Pariset, France). All experiments were carried out at ambient temperature (about 25 °C), and the experimental results were analyzed using EC- software (v11.25, BioLogic, Seyssinet-Pariset, France), as described in Example 5.

[0220] Figure 29 Linear sweep voltammograms (LSVs) obtained under (a) silent conditions and (b) acoustic stimulation at 40 dBm (7 MHz) are shown, with the acoustic wave acting on the two electrodes. As a result of the acoustic stimulation, an increase in the current density and thus an increase in the H2 production rate are evident.

[0221] Those skilled in the art will understand that the invention described herein is subject to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications that fall within the spirit and scope of the invention.

Claims

1. An electrolytic cell, comprising: a working electrode; a counter electrode; a liquid electrolyte in contact with the working surface of the working electrode; an acoustic transmission substrate comprising at least a piezoelectric substrate portion; one or more conductive electrodes connected to the piezoelectric substrate portion and configured to propagate high-frequency acoustic waves with a frequency of at least 1 MHz on the acoustic transmission substrate when electrically driven; and one or more power sources configured to (i) apply a potential between the working electrode and the counter electrode sufficient to cause an electrolysis reaction of substances in the liquid electrolyte to generate electrolysis reaction products near the working electrode, and (ii) electrically drive the one or more conductive electrodes, wherein the working electrode is either located on the acoustic transmission substrate or spaced apart from the acoustic transmission substrate by the liquid electrolyte, and wherein during operation of the electrolytic cell, the propagation of the high-frequency acoustic waves on the acoustic transmission substrate stimulates the liquid electrolyte, thereby improving the production efficiency of the electrolysis reaction products.

2. The electrolytic cell according to claim 1, wherein the working electrode is located on the acoustic transmission substrate.

3. The electrolytic cell according to claim 2, wherein the working electrode is located on the piezoelectric substrate portion.

4. The electrolytic cell according to claim 1, wherein the working electrode is spaced apart from the acoustic transmission substrate by the liquid electrolyte by a distance of not more than 30 mm.

5. The electrolytic cell according to any one of claims 1 to 4, wherein the frequency of the high-frequency acoustic waves is at least 3 MHz.

6. The electrolytic cell according to any one of claims 1 to 5, wherein the high-frequency acoustic waves comprise waveforms selected from surface acoustic waves (SAW), surface reflector body waves (SRBW), and Lamb waves.

7. The electrolytic cell according to any one of claims 1 to 6, wherein h / λ is greater than 1 / 3, where h is the thickness of the piezoelectric substrate portion and λ is the acoustic wave wavelength when the high-frequency acoustic waves propagate on the piezoelectric substrate portion.

8. The electrolytic cell according to any one of claims 1 to 7, wherein h / λ is between 1 / 3 and 3, where h is the thickness of the piezoelectric substrate portion and λ is the acoustic wave wavelength when the high-frequency acoustic waves propagate on the piezoelectric substrate portion.

9. The electrolytic cell according to any one of claims 1 to 8, wherein the power input required to drive the one or more conductive electrodes is less than 20 W / cm after being normalized by the surface area of the acoustic transmission substrate. 2 .

10. The electrolytic cell according to any one of claims 1 to 9, wherein the working electrode comprises an electrocatalyst for hydrogen evolution reaction on the working surface.

11. The electrolytic cell according to any one of claims 1 to 10, wherein the working electrode comprises one or more layers of metal compositions or metal compounds formed on the surface of the acoustic transmission substrate.

12. The electrolytic cell according to any one of claims 1 to 11, wherein the piezoelectric substrate portion is configured as a plate, and the high-frequency acoustic waves propagate in the plane of the plate, causing at least one surface of the plate to oscillate.

13. The electrolytic cell according to any one of claims 1 to 12, wherein the piezoelectric substrate portion comprises a single crystal selected from lithium niobate, quartz, lithium tantalate, and lanthanum gallium silicate.

14. The electrolytic cell according to any one of claims 1 to 13, wherein the one or more conductive electrodes comprise interdigital electrode pairs.

15. The electrolytic cell according to any one of claims 1 to 14, wherein the liquid electrolyte is an aqueous electrolyte.

16. The electrolytic cell according to claim 15, wherein the pH of the aqueous electrolyte is between 5 and 9.

17. The electrolytic cell according to claim 15 or claim 16, wherein the electrolysis reaction product is hydrogen (H2) or oxygen (O2).

18. The electrolytic cell according to any one of claims 1 to 17, which is configured to flow the liquid electrolyte through the working electrode.

19. The electrolytic cell according to any one of claims 1 to 18, which comprises an ion-permeable membrane separating the working electrode from the counter electrode.

20. An electrolysis method, which comprises: bringing a liquid electrolyte into contact with a working surface of a working electrode; propagating high-frequency acoustic waves with a frequency of at least 1 MHz on a sound-transmitting substrate, wherein the working electrode is either located on the sound-transmitting substrate or spaced apart from the sound-transmitting substrate by the liquid electrolyte; and applying a potential between the working electrode and the counter electrode sufficient to cause an electrolysis reaction of substances in the liquid electrolyte to produce an electrolysis reaction product near the working electrode, wherein the propagation of the high-frequency acoustic waves on the sound-transmitting substrate stimulates the liquid electrolyte, thereby improving the production efficiency of the electrolysis reaction product.

21. The method according to claim 20, wherein the working electrode is located on the sound-transmitting substrate.

22. The method according to claim 20, wherein the working electrode is spaced apart from the sound-transmitting substrate by the liquid electrolyte by a distance of no more than 30 mm.

23. The method according to any one of claims 20 to 22, wherein the electrolysis reaction product is hydrogen (H2) or oxygen (O2).

24. The method according to any one of claims 20 to 23, wherein the electrolysis reaction product is H2, and the potential at the working electrode is not greater than (more negative than) -1.5 V relative to RHE, preferably not greater than (more negative than) -1 V relative to RHE.

25. The method according to any one of claims 20 to 24, wherein the liquid electrolyte is an aqueous electrolyte with a pH between 5 and 9.

26. The method according to any one of claims 20 to 25, wherein the frequency of the high-frequency acoustic waves is at least 3 MHz.

27. The method according to any one of claims 20 to 26, wherein the high-frequency acoustic waves comprise waveforms selected from surface acoustic waves (SAW), surface reflector waves (SRBW), and Lamb waves.

28. The method according to any one of claims 20 to 27, wherein after normalizing by the surface area of the sound-transmitting substrate, the power input for propagating the high-frequency sound wave on the sound-transmitting substrate is less than 20 W / cm 2 .

29. The method according to any one of claims 20 to 28, wherein propagating the high-frequency acoustic waves on the sound-transmitting substrate comprises electrically driving one or more conductive electrodes connected to a piezoelectric substrate portion of the sound-transmitting substrate.

30. The method according to claim 29, wherein the one or more conductive electrodes comprise interdigital electrode pairs.

31. The method according to claim 29 or claim 30, wherein h / λ is greater than 1 / 3, where h is the thickness of the piezoelectric substrate portion and λ is the acoustic wavelength of the high-frequency acoustic wave when propagating on the piezoelectric substrate portion.

32. The method according to any one of claims 29 to 31, wherein h / λ is between 1 / 3 and 3, where h is the thickness of the piezoelectric substrate portion and λ is the acoustic wavelength of the high-frequency acoustic wave when propagating on the piezoelectric substrate portion.

33. The method according to any one of claims 29 to 32, wherein the working electrode is located on the piezoelectric substrate portion.

34. The method according to any one of claims 29 to 33, wherein the piezoelectric substrate portion comprises a single crystal selected from lithium niobate, quartz, lithium tantalate, and lanthanum gallium silicate.

35. The method according to any one of claims 29 to 34, wherein the piezoelectric substrate portion is configured as a plate, and the high-frequency acoustic wave propagates in the plane of the plate, so that at least one surface of the plate oscillates.

36. The method according to any one of claims 20 to 35, wherein the working electrode comprises an electrocatalyst for the hydrogen evolution reaction on the working surface.

37. The method according to any one of claims 20 to 36, wherein the working electrode comprises one or more layers of metal compositions or metal compounds formed on the surface of the sound-transmitting substrate.

38. An electrode device for an electrolytic cell, the electrode device comprising: A sound-transmitting substrate at least comprising a piezoelectric substrate portion; A working electrode located on the sound-transmitting substrate, the working electrode comprising a working surface for contacting a liquid electrolyte in the electrolytic cell; and One or more conductive electrodes, which are connected to the piezoelectric substrate portion and are configured to cause a high-frequency acoustic wave with a frequency of at least 1 MHz to propagate on the sound-transmitting substrate when electrically driven.

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