Electro-Sonication – Ultrasonic Electrodes
Electro-Sonication is the combination of the effects of electricity with the effects of sonication. Hielscher Ultrasonics developed a new and elegant method to use any sonotrode as electrode. This puts the power of ultrasound directly at the interface between ultrasonic electrode and liquid. There it can promote electrolysis, improve mass transfer, and break boundary layers or deposits. Hielscher supplies production grade equipment for electro-sonication processes in batch and inline processes at any scale. You can combine electro-sonication with mano-sonication (pressure) and thermo-sonication (temperature).
Ultrasonic Electrode Applications
The application of ultrasonics to electrodes is a novel technology with benefits to many different processes in electrolysis, galvanizing, electro-purification, hydrogen generation and electro-coagulation, particle synthesis or other electro-chemical reactions. Hielscher Ultrasonics has ultrasonic electrodes readily available for research and development at lab scale or pilot scale electrolysis. After you tested and optimized your electrolytical process, you can use Hielscher Ultrasonics production size ultrasound equipment to scale up your process results to industrial production levels. Below, you will find suggestions and recommendations for the use of ultrasonic electrodes.
Sono-Electrolysis (Ultrasonic Electrolysis)
Electrolysis is the interchange of atoms and ions by the removal or addition of electrons resulting from the application of an electric current. The products of electrolysis can have a different physical state from the electrolyte. Electrolysis can produce solids, such as precipitates or solid layers on either of the electrodes. Alternatively, electrolysis can produces gases, such as Hydrogen, Chlorine or Oxygen. Ultrasonic agitation of an electrode can break solid deposits from the electrode surface. Ultrasonic degassing quickly produces larger gas bubbles from dissolved gases of micro-bubbles. This leads to a faster separation of the gaseous products from the electrolyte.
Ultrasonically Enhanced Mass-Transfer at Electrode Surface
During the process of electrolysis, the products accumulate near the electrodes or on the electrode surface. Ultrasonic agitation is a very effective tool to increase the mass-transfer at boundary layers. This effect brings fresh electrolyte in contact with the electrode surface. The cavitational streaming transports products of the electrolysis, such as gases or solids away from the electrode surface. The inhibitive formation of isolating layers is therefore prevented.
Effects of Ultrasonics on the Decomposition Potential
Ultrasonic agitation of the anode, of the cathode, or both electrodes, may affect the decomposition potential or decomposition voltage. Cavitation alone is known to break molecules, produce free radicals or ozone. The combination of cavitation with electrolysis in an ultrasonically enhanced electrolysis may affect the the minimum required voltage between anode and cathode of an electrolytic cell for electrolysis to occur. The mechanical and sonochemical effects of cavitation may improve electrolysis energy efficiency, too.
Ultrasound in Electrorefining and Electrowinning
In the process of electrorefining, solid deposits of metals, such as copper can be turned into a suspension of solid particles in the electrolyte. In electrowinning, also called electroextraction, the electrodeposition of metals from their ores can be turned into solid precipitate. Common electrowon metals are lead, copper, gold, silver, zinc, aluminium, chromium, cobalt, manganese, and the rare-earth and alkali metals. Ultrasonication is an effective means for the leaching of ores, too.
Sono-Electrolytic Purification of Liquids
Purify a liquid, e.g. an aqueous solutions like wastewater, sludge or similar, by leading the solution through the electric field of two electrodes! Electrolysis can disinfect or purify aqueous solutions. Feeding a NaCI solution together with water through electrodes or across electrodes, generates Cl2 or CIO2, which can oxidize impurities and disinfect the water or aqueous solutions. If the water contains sufficient natural chlorides, there is no need for the addition.
Ultrasonic vibrations of the electrode can get the boundary layer between the electrode and the water as thin as possible. This can improve the mass transfer by many orders of magnitude. The ultrasonic vibration and cavitation reduces the formation of microscopic bubbles due to polarization, significantly. The use of ultrasonic electrodes for electrolysis improves the electrolytic purification process considerably.
Sono-Electrocoagulation (Ultrasonic Electrocoagulation)
Electrocoagulation is a wastewater treatment method for the removal of contaminants, such as emulsified oil, total petroleum hydrocarbons, refractory organics, suspended solids, and heavy metals. Also, radioactive ions can be removed from for water purification. The addition of ultrasonication electrocoagulation, also known as sono-electrocoagulation, has a positive effect on chemical oxygen demand or turbidity removal efficiency. The electrocoagulation combined treatment processes have shown greatly enhanced performances in the removal of pollutants from industrial wastewater. The integration of a free radical producing step, such as ultrasonic cavitation with electrocoagulation shows synergy and improvements in the overall cleaning process. The purpose of employing these ultrasonic-electrolytic hybrid systems is to increase the overall treatment efficiency and eliminate the disadvantages of conventional treatment processes. Hybrid ultrasonic-electrocoagulation reactors has been demonstrated to inactivate Escherichia coli in water.
Sono-Electrolytic In-Situ Generation of Reagents or Reactants
Many chemical processes, such as heterogeneous reactions or catalysis benefit from ultrasonic agitation and ultrasonic cavitation. The sono-chemical influence can increase reaction speed or improve conversion yields.
Ultrasonically agitated electrodes add a new powerful tool to chemical reactions. Now you can combine the benefits of sonochemistry with electrolysis. Produce hydrogen, hydroxide ions, hypochlorite and many other ions or neutral materials right in the ultrasonic cavitation field. The products of electrolysis can act as reagents or as reactants to the chemical reaction.
Reactants are input materials that participate in a chemical reaction. Reactants are consumed to make products of the chemical reaction
Combination of Ultrasound with Pulsed Electric Field
The combination of pulsed electric field (PEF) and ultrasound (US) has positive effects for the extraction of physicochemical, bioactive compounds and the chemical structure of extracts. In the extraction of almonds, combined treatment (PEF–US) has produced the highest levels of total phenolics, total flavonoids, condense tannins, anthocyanin contents and antioxidant activity. It reduced the power and metal chelating activity.
Ultrasound (US) and pulsed electric field (PEF) can be employed enhance process efficiency and production rates in fermentation processes by improving mass transfer and cell permeability.
The combination of pulsed electric field and ultrasound treatment does have an impact on air drying kinetics and the quality of dried vegetables, such as carrots. Drying time can be reduced by 20 to 40%, while maintaining the rehydration properties.
Sono-Electrochemistry / Ultrasonic Electrochemistry
Add ultrasonically enhanced electrolysis to produce reactants or to consume products of chemical reactions in order to move the final equilibrium of the chemical reaction or to alter the chemical reaction pathway.
Suggested Setup of Ultrasonic Electrodes
The innovative design for probe-type ultrasonicators turns a standard ultrasonic sonotrode into an ultrasonically vibrating electrode. This makes ultrasound for electrodes more accessible, easier to integrate and easily scaleable to production levels. Other designs agitated the electrolyte between two non-agitated electrodes, only. Shadowing and ultrasound wave propagation patterns produce inferior results when compared to direct electrode agitation. You can add ultrasound vibration to anodes or cathodes, respectively. Of course, you can change the voltage and the polarity of the electrodes at any time. Hielscher Ultrasonics electrodes are easy to retrofit to existing setups.
Sealed Sono-Electrolytic Cell and Electrochemical Reactors
A pressure-tight seal between ultrasonic sonotrode (electrode) and a reactor vessel is available. Therefore, you can operate the electrolytic cell at other than ambient pressure. The combination of ultrasound with pressure is called mano-sonication. This may be of interest if the electrolysis produces gases, when working at higher temperatures, or when working with volatile liquid components. A tightly sealed electrochemical reactor can operate at pressures above or below ambient pressure. The seal between the ultrasonic electrode and the reactor can be made electrically conductive or insulating. The latter allows to operate the reactor walls as a second electrode. Of course, the reactor can have inlet and outlet ports to act as a flow cell reactor for continuous processes. Hielscher Ultrasonics offers a variety of standardized reactors and jacketed flow cells. Alternatively, you can chose from a range of adapters to fit Hielscher sonotrodes to your electrochemical reactor.
Concentrical Arrangement in Pipe Reactor
If the ultrasonically agitated electrode is near a second non-agitated electrode or near a reactor wall, the ultrasonic waves propagate through the liquid and the ultrasound waves will work on the other surfaces as well. An ultrasonically agitated electrode that is concentrically oriented in a pipe or in a reactor can keep the interior walls free from fouling or from accumulated solids.
When using standard Hielscher sonotrodes as electrodes, the electrolyte temperature can be between 0 and 80 degree Celsius. Sonotrodes for other electrolyte temperatures in the range from -273 degree Celsius to 500 degree Celsius are available on request. The combination of ultrasound with temperature is called thermo-sonication.
If the viscosity of the electrolyte inhibits mass transfer, ultrasonic agitation mixing during electrolysis could be beneficial as it improves the transfer of the material to and from the electrodes.
Sono-Electrolysis with Pulsating Current
Pulsating current on the ultrasonically agitated electrodes results in products different from direct current (DC). For example, Pulsating current can increase the ratio of ozone to oxygen produced at the anode in the electrolysis of an aqueous acidic solution, e. g. dilute sulphuric acid. Pulsed current electrolysis of ethanol produces an aldehyde instead of primarily an acid.
Equipment for Electro-Sonication
Hielscher Ultrasonics developed a special sonoelectrochemical upgrade for the industrial transducers. The upgraded transducer works with almost all types of Hielscher sonotrodes.
Ultrasonic Electrodes (Sonotrodes)
The sonotrodes are electrically isolated from the ultrasonic generator. Therefore, you can connect the ultrasonic sonotrode to an electric voltage, so that the sonotrode can act as an electrode. The standard electrical isolation gap between the sonotrodes and the ground contact is 2.5 mm. Therefore you could apply up 2500 volts to the sonotrode. Standard sonotrodes are solid and made of Titanium. Therefore, there is almost no restriction to the electrode current. Titanium shows a good corrosion resistance to many alkaline or acidic electrolytes. Alternative sonotrode materials, such as aluminium (Al), steel (Fe), stainless steel, nickel-chromium-molybdenum, or niobium are possible. Hielscher offers cost-effective sacrificial anode sonotrodes, e.g. made of aluminium or steel.
Ultrasonic Generator, Power Supply
The ultrasonic generator does not need any modification and it uses a standard electrical outlet with ground. The transducer horn and all exterior surfaces of the transducer and the generator are connected to ground of the power outlet, of course. The sonotrode and a bracing element are the only parts connected to the electrode voltage. This facilitates the design of the setup. You can connect the sonotrode to direct current (DC), pulsating direct current or alternating current (AC). Ultrasonic electrodes can be operated as anodes or cathodes, respectively.
Production Equipment for Electro-Sonication Processes
You can use any Hielscher ultrasonic device, such as UIP500hdT, UIP1000hdT, UIP1500hdT, UIP2000hdT or UIP4000hdT to couple up to 4000 watts of ultrasonic power to any standard sonotrode or cascatrode. Ultrasonic surface intensity on the sonotrode surface can be between 1 watt to 100 watts watt per square-centimeter. Different sonotrode geometries with amplitudes from 1 micron to 150 micron (peak-peak) are available. The ultrasonic frequency of 20kHz is very effective in the generation of cavitation and acoustic streaming in the electrolyte. Hielscher ultrasonic devices can operate 24 hours per day, seven days a week. You can operate continuously at full power output or pulsate, e. g. for periodic cleaning of the electrodes. Hielscher Ultrasonics can supply ultrasonic electrodes with up to 16 kilowatts ultrasonic power (mechanical agitation) per single electrode. There is almost no limit to the electric power you can connect to the electrodes.
One more thing: Sono-Electrostatic Spraying
Hielscher Ultrasonics makes equipment for the spraying, nebulizing, atomizing or aerosolyzing of liquids. The ultrasonic spraying sonotrode can give the liquid fog or aerosols a positive charge. This combines ultrasonic spraying with electrostatic spraying technology, e.g. for coating processes.
Literature / References
- Bruno G. Pollet; Faranak Foroughi; Alaa Y. Faid; David R. Emberson; Md.H. Islam (2020): Does power ultrasound (26 kHz) affect the hydrogen evolution reaction (HER) on Pt polycrystalline electrode in a mild acidic electrolyte? Ultrasonics Sonochemistry Vol. 69, December 2020.
- Md H. Islam; Odne S. Burheim; Bruno G.Pollet (2019): Sonochemical and sonoelectrochemical production of hydrogen. Ultrasonics Sonochemistry Vol. 51, March 2019. 533-555.
- Jayaraman Theerthagiri; Jagannathan Madhavan; Seung Jun Lee; Myong Yong Choi; Muthupandian Ashokkumar; Bruno G. Pollet (2020): Sonoelectrochemistry for energy and environmental applications. Ultrasonics Sonochemistry Vol. 63, 2020.
- Bruno G. Pollet (2019): Does power ultrasound affect heterogeneous electron transfer kinetics? Ultrasonics Sonochemistry Vol. 52, 2019. 6-12.
- Md Hujjatul Islam; Michael T.Y. Paul; Odne S. Burheim; Bruno G. Pollet (2019): Recent developments in the sonoelectrochemical synthesis of nanomaterials. Ultrasonics Sonochemistry Vol. 59, 2019.
- Sherif S. Rashwan, Ibrahim Dincer, Atef Mohany, Bruno G. Pollet (2019): The Sono-Hydro-Gen process (Ultrasound induced hydrogen production): Challenges and opportunities. International Journal of Hydrogen Energy, Volume 44, Issue 29, 2019, 14500-14526.
- M.D. Esclapez, V. Sáez, D. Milán-Yáñez, I. Tudela, O. Louisnard, J. González-García (2010): Sonoelectrochemical treatment of water polluted with trichloroacetic acid: From sonovoltammetry to pre-pilot plant scale. Ultrasonics Sonochemistry Volume 17, Issue 6, 2010. 1010-1020.
- L. Cabrera, S. Gutiérrez, P. Herrasti, D. Reyman (2010): Sonoelectrochemical synthesis of magnetite. Physics Procedia Volume 3, Issue 1, 2010. 89-94.