I have read that combining the DC current with a high-frequency AC current, the electrolysis of water speeds up. Is this true? In that case, how is less energy wasted as heat? Or does it simply catalyze the process?
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asked Feb 21, 2014 at 22:48
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1$\begingroup$ Do you have a source for that? Also, what do you mean by combining DC and AC? I assume you mean a rippled DC current, that is one that is changing its voltage by way of an overlaid AC without changing polarity? $\endgroup$– Tanith RosenbaumFeb 21, 2014 at 23:42
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3 Answers
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First of all, have a look at the wikipedia page on electrolysis of water.
I also like this review: Zoulias et al.: A Review on Water Electrolysis, TCJST, 4 (2) (2004) 41-71
Specifically they list a number of actually existing installations (context: renewable energy) and their actually achieved efficiency.Speed up does not necessarily have anything to do with higher efficiency. In electrolysis it is often the other way round: if you want to squeeze out the maximum free energy, you need to do the reaction infinitely slowly (despite thermodynamics having a dynamic name, it looks at infinitively slow processes).
Thus, speeding up usually means that you find a way to put more power through your system. The big issue is to find a way of doing this without loosing (too much) efficiency.
Pulsed/modulated DC: looking through a few papers I liked this one: Shimizu et al.: A novel method of hydrogen generation by water electrolysis using an ultra-short-pulse power supply, Journal of Applied Electrochemistry (2006) 36:419–423, DOI 10.1007/s10800-005-9090-
They aim at avoiding the diffusion controlled situation by having the pulses short enough so that no depletion zone occurs. Look at these diagrams:
So they report one setting where the pulsed electrolysis is actually more efficient than DC in their cell.
There's more research going on on this, however the papers I found reported increased efficiency compared to pure DC electrolysis, but the absolute efficiencies are around 10%. However, compare their numbers with the 80% efficiency cited by the review for an industrial alkaline electrolysis. Note that one big difference is the voltage that is applied: for the DC it is around 1.85 - 2.05 V, so much less overvoltage. Note also that when they say that higher voltage speeds up ion transport, then this overvoltage is converted to heat (ions face friction in the medium when they travel) and thus basically lost.
Another line that looks real is that if you go to higher temperatures, a (small) part of the energy can be supplied by heat. As heat is cheap, this may help. One point one has to be aware though, that the efficiency calculations may be done with respect to the electric energy only (neglecting the heat input) and thus look artificially nice (like efficiencies of condensing boilers calculated against the lower heating value).
I found a bunch of nonsense claims in the internet, about the resonance frequency of water helping to split bonds.
- The first thing to realize here is that there is no one resonance frequency of water. With suitable energy, you can excite rotational, vibrational and electronic states (I left out translation - there transition energies minute). At room temperature you can say as a rule of thumb that most molecules will be in some excited rotational state, but in the vibrational and electronic ground states. Excitation energies for rotation are in the far infrared or microwave energy/frequency region. Widely used e.g. in the microwave oven at 2.45 GHz ($\approx$ 12 cm). Actually, the whole region is full of bands where water absorbs. Note that microwave heating of water does not cause electrolysis. Vibrational transitions are around 2.9 μm = 105 THz = 3500 cm⁻¹ and 6μm = 50 THz = 1635 cm⁻¹ with lots of combinations and overtones throughout the near infrared region. Quite exceptionally, the visible region is basically free of water absorption. Electronic transitions (breaking of bonds) need energies in the UV, and here we meet bands that lead to photodissociation, e.g. at 166nm (taken from Wikipedia). That corresponds to 1.8 PHz = $1.8 \cdot 10^{15}$ Hz. Compare this to the kHz and MHz where your link claims dissociation.
This doesn't mean that the pulsed DC cannot help, nor that impedance spectoscopy won't give important information. But resonance frequencies in the kHz range are electrical LC-circuit resonances depending on cell and electrode geometries and electrical double layers etc. But neiter on vibrations nor breaking of the bonds of the water molecule.
To give the "method" you ask about some real world numbers,
- at the very end of the Wiki page the energy efficiency for industrial water electrolysis is cited as usually between 50 and 80 %.
- The paper then proposes to burn the gas in an internal combustion machine. As such a stationary process could be adjusted so that the engine is at its maximum efficiency, we may assume 1/3 or 35% efficiency here.
- we then need a generator to convert the mechanical energy into electric energy. Fortunately, that step is rather efficient. Say, 95 %.
- A fuel cell would be more efficient than the combustion - generator combination: ca. 40 - 60 % according to Wikipedia.
- Unfortunately, also battery charging is not 100% efficient. Let's assume 80–90% (taken from Wikipedia on Li-ion batteries) For batteries that are charged with higher current (or current density) efficiency is less. Example would be lead-acid batteries as used in cars. Wiki quotes efficiencies between 50 and 80 %.
Taking these numbers together, I conclude that after going once through the cycle of the proposed "perpetuum mobile", 8 - 24 % of the energy are retained in a "useful state" while 76 - 92 % became heat. With fuel cell, we may be able to "boost" the energy efficiency to 43%.
Useful general knowledge (in addition to the law of energy conservation)
The US patent system is different from e.g. the German patent system in that here in Germany the patent application has to have commercial/industrial usability. This includes a technical argument why it works (according to the physical laws). A perpetuum mobile would be rejected on these grounds (of course the inventor could prove his case with a prototype). US patents do not have this technical check.
generators (mechanic -> electric conversion) are sources of current, while batteries (galvanic cells) are sources of voltage.
answered Feb 22, 2014 at 21:38
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$\begingroup$ Without being a chemist, I thought that the efficiency of electrolysis is measured with respect to some ideal amount of energy for a perfectly efficient operation, that is, without taking at all into account the energy potential of the generated gasses. (Which I would presume to be huge compared to the energy needed to produce them via electrolysis.) Consequently, it appears to me illogical to compare the efficiency of the electrolysis process against the efficiency of burning the gases in an internal combustion machine in order to see if there is a net gain, no? $\endgroup$ Nov 8, 2015 at 23:33
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$\begingroup$ @MikeNakis: That ideal amount of energy typically accounts only for the energy content of the produced gas (chemical energy/enthalpy; pressure and/or temperature may or may not be included but for hydrogen or hydrogen + oxygen they are very small compared to the chemical energy = your potential). So 45 % efficiency in diagram (b) above means that a bit less than half of the electric energy they put into the cell ends up as chemical energy of the producs; the losses (heat) are in fact not negligible but a bit higher than the energy stored chemically in the produced gas. $\endgroup$ Nov 9, 2015 at 13:09
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$\begingroup$ "illogical to compare the efficiency of the electrolysis process against the efficiency of burning the gases in an internal combustion machine in order to see if there is a net gain" That would indeed be illogical - if only because under the law of energy conservation, there cannot be a net gain. But the OP linked to a cycle of hydrolysis -> (gas storage) -> combustion -> electrical generator -> (battery storage) -> hydrolysis again. And for that you can calculate how much energy is left after one round. (Or, how much energy you have to "pay" for that storage option). $\endgroup$ Nov 9, 2015 at 13:15
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$\begingroup$ hmmm, I can't make sense, but that's almost certainly because of my incomplete understanding of the processes involved and even the terminology being used. But thank you for trying. I think it will be highly inefficient to try and gain an understanding of these matters by exchanging comments. I will try to find someone to explain it to me in a conversation. Once again, thanks. Cheers! $\endgroup$ Nov 9, 2015 at 13:54
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$\begingroup$ I agree that, if there were any improvements made by using pulses or AC in hydrolysis, it couldn't have anything to do with resonant frequency. Logically, since DC had no frequency, and it does break the bonds, it does so in a non-resonant way, therefore, it would appear pointless to attempt any kind of frequency modification. That said, perhaps the pulsing could improve the results, but mechanically (acoustically?), as in creating more surface area for hydrolysis to occur (simply making DC electrolysis more effective). Don't know. $\endgroup$– MC9000Dec 20, 2016 at 21:28
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From the site that you linked:
A battery would be used as a source of electrical energy which would separate the hydrogen/oxygen into gases. The gasses would then fuel an internal combustion engine, which would power a generator to continously recharge the battery as well as deliver useable mechanical energy. If this sort of motor can be made to work, the energy crisis on this planet will be over forever.
Sounds like somebody proposes a perpetual motion engine of the first kind. I'm very much impressed! The rest on that page is techno-babble of the same quality.
Edit This doesn't mean that there's might not be serious research on electrolysis using something else than constant DC, e.g. PWM (pulse width modulation).
A cursory search (there might be other sources) gave:
K. Mazloomi, N. Sulaiman, S. A. Ahmad, N. A. Yunus, Analysis of the Frequency Response of a Water Electrolysis cell, Int. J. Electrochem. Sci., 2013, 8, 3731-3739, PDF
answered Feb 22, 2014 at 6:36
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$\begingroup$ Yes, what you cited there actually says its a perpetual motion engine, which if of course impossible. I'm just curious if it is true that you need less energy to break the bonds if your're using a rippled DC current. This is not the only source I've found. I'm not educated on any of these fields, so it's difficult to know what everyone is doing. Some people claim to use a high frequency current. I believe this might be just a homemade transformer with a transistor on to keep the flow running. They claim that this requires less energy per liter of $\ce{H2}$ than by using a continous DC flow. $\endgroup$ Feb 22, 2014 at 9:34
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$\begingroup$ It seemed to me that this source is written by someone without any knowledge of physics, and is merely a summary of something else that person has read, which is why I'm curious if it's true or not. In big scale physics, like bridges, the resonating frequency can make a huge bridges break. Does this apply to molecule bondings? $\endgroup$ Feb 22, 2014 at 9:37
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3$\begingroup$ @FriendofKim: bonds breaking with wave of suitable energy: well in a way photochemistry does just that. Have a look at en.wikipedia.org/wiki/Visual_cycle Light of suitable frequency is absorbed, light that is (too) far from the absorption maximum isn't. The absorbed energy leads to isomerisation. You can say that a π -> π* corresponds to breaking the double bond (not the single bond, though). $\endgroup$ Feb 22, 2014 at 17:42
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$\begingroup$ Aha, so these using a rippled DC source wouldn't be able to create a resonance like the wind does with a bridge. It would be logical, though, that there is a way to do electrolysis in a more efficient way. A lot of the energy is going directly into heat. We "just" need to figure out how to use more of that energy to separate the water molecules. $\endgroup$ Feb 22, 2014 at 21:11
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$\begingroup$ @FriendofKim: you get the electric LC resonance due to the capacity of the double layer. But thats many (9) orders of magnitude away from dissociation energy. The pulsed DC has some points, but not breaking or weakening bonds. $\endgroup$ Feb 22, 2014 at 21:43
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Among the "scientific" mumbo jumbo I have read here, most commenters have missed a few key facts.
Most gains in electrolysis are made at extremely high voltage and extremely low currents. This sets up a charged capacitance state where the hydrogen and oxygen are pulled to the near breaking point to their respective charges.
Concerning frequency, it's not about energy absorption. It's about creating a base frequency within one or more atoms within the molecule, then hitting it with a brief secondary frequency pulse to cause a shattering effect much like when an opera singer shatters a crystal glass.
The idea here would be to add an insulator to the positive and negatively charged plate such as sealing them in a thin layer of plastic or other such noncondictive material. Then setting up a base pulsed DC current, preferably one that is a resonant of the natural base frequency of that atom. Then intermittently utilizing a secondary higher or lower frequency to initiate a shattering effect on the molecule.
