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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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    $\begingroup$ 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 Rosenbaum Feb 21 '14 at 23:42
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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:

    H2 output over input power efficiency over input power

    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 to not have this technical check.

  • generators (mechanic -> electric conversion) are sources of current, while batteries (galvanic cells) are sources of voltage.

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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? $\endgroup$ – Mike Nakis Nov 8 '15 at 23:33
  • $\begingroup$ @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. $\endgroup$ – cbeleites supports Monica Nov 9 '15 at 13:09
  • $\begingroup$ "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). $\endgroup$ – cbeleites supports Monica Nov 9 '15 at 13:15
  • $\begingroup$ 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! $\endgroup$ – Mike Nakis Nov 9 '15 at 13:54
  • $\begingroup$ 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. $\endgroup$ – MC9000 Dec 20 '16 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

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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. $\endgroup$ – Friend of Kim Feb 22 '14 at 9:34
  • $\begingroup$ 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? $\endgroup$ – Friend of Kim Feb 22 '14 at 9:37
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    $\begingroup$ @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). $\endgroup$ – cbeleites supports Monica Feb 22 '14 at 17:42
  • $\begingroup$ 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. $\endgroup$ – Friend of Kim Feb 22 '14 at 21:11
  • $\begingroup$ @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. $\endgroup$ – cbeleites supports Monica Feb 22 '14 at 21:43
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I'm sorry I don't have a clear answer for you, electrolysis with an unusual modulated signal. This is an area many stay away from unless they want a scientific pitchfork up ther back. So it's hard to find any specific experiments that are not completely quasi-scientific in their nature. But I have something to add to the longest the answer here: "...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⋅1015 Hz. Compare this to the kHz and MHz..." ..well, I just did - and even though they're miles apart on the spectrum - from the 17khz pulsed test result you posted that were more efficient than regular DC - in a harmonic system they couldn't be much closer.. (18phz - 17khz) wich could make you wonder if this particular setup you mentioned achieved a resonnant state not only in the electric circuit (that I totally agree with you is a confusing factor for many laymen experimenters). IF you could tweak a setup to achieve resonance both in the LC tank and on a atomic harmonic level, that should be pretty efficiant, shouldn't it ? Also the last 100 years EE's has been thought to fight any kind of resonans that could brake our fragile electronics, when we start looking into what can be done with resonnant systems, there's a lot of efficiency gain to be had. It's the main thing Tesla tried to tell us, and is also known by early radio technicians, but it got lost in the digial era, or rather transfered to the optical/nuclear science in many ways. Of course when you move into an area infested with superstition and fakery - but also stigma, very few serious scientists(except a few brave and the military ones) want to touch it or far less publish any non-linear experimental results, even though there are plenty of phenomenons left to be explored and included in the textbooks. And some people are into this. So keep looking, with your sceptacles on. The stupidest scientist is the one that think he knows everything. We laugh about what utter nonsence science thought us to be true 100 years ago, and you can be 100% sure we're gonna laugh again in another hundred years, about certain things thought in universities today.

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    $\begingroup$ This is a little difficult to parse. Can you edit your answer to address the salient points of the OP's query? As it stands, it's a run-on paragraph and borderline not-an-answer. $\endgroup$ – Todd Minehardt Oct 6 '16 at 18:48
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Among the "scientific" mumbo jumbo i have heard here, most commentors have missed a few key facts.

1) most gains in electrolysis are made at extremely high voltage and extremely low amps. This sets up a charged capacitance state where the hydrogen and oxygen are pulled to the near breaking point to there respective charges.

2) 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.

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I'm a plasma magrav physicist. and something to try would be to take a copper tube fill it with neodymium magnets that that look like a donut. seal both ends of the copper tubing. 1 and should be permanently sealed. nano coat the outside by using the vaporization method. this will take about two weeks to accomplish. once that has been done unseal the end that is not sealed perminently and remove the magnets. solder a wire to the inside of the nano coated tube however long the lead you may need. make copper oxide gas at nano state and fill the tube 30% of the way with this material, top it off with distilled water. take a zinc wire or nickel wire and put that into the tube to make however long a lead you may need. now add a copper wire to the centerand keeping the copper wire and the nickel or zinc wire away from the walls of the copper tubeand from touching each other and seal the top with wax or epoxy or some other method to your choosing. what this will do is it will give you the ability to pull an AC and DC voltage. with this placed in the water with the leads coming out even partially in the water you will be able to measure the frequency of that water at that volume and then determine the properties necessary for your electrolysis to make it the most efficient.

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