I'm reading Detchko Pavlov's Lead-Acid Batteries: Science and Technology (maybe an old version than the link) and in the first chapter the book mentioned "it is currently unknown why electrolysis of water happens so slowly".

If we think about it, isn't that strange? We have a pair of electrodes in a pot of acidic water, with a potential of 2+ volts, but nothing happens? And I do remember asking the same question when I was in middle school but really didn't get a convincing answer.

Do we know the answer now?

Here's the text of the book. Mine is a Chinese translation version so I will try to translate it back.

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As is shown by the E/pH diagram of Figure 2.1, an lead-acid battery in open-circuit is thermal-dynamically unstable. The self-discharge reaction between the electrodes will electrolyse water into $\ce{H2}$ and $\ce{O2}$. At the same time, the following reactions happens on the electrode plates:

On the positive plate:

$$ \ce{PbO2 + H2SO4 -> PbSO4 + H2O + \frac{1}{2} O2} $$

On the negative plate:

$$ \ce{Pb + H2SO4 -> PbSO4 + H2} $$

If the battery is constructed with high-purity lead and sulfuric acid, the above reactions happens at a very slow rate. To this day, we have not fully figured out what basic reactions so strongly inhibits the precipitation of $\ce{H2}$ and $\ce{O2}$.

Also in this document enter image description here

What is the exact shape of this outgassing-rate/voltage curve and how is it derived?

  • 2
    $\begingroup$ In the body of questions, of answers, and of comments, chemistry.se allows you to use mhchem; an easier approach to provide chemical equations. Because it is a technique not all web browsers process equally well, do not use mhchem in the title of questions, though. $\endgroup$
    – Buttonwood
    Aug 7, 2023 at 12:09

2 Answers 2


For electrolysis to happen, you need a (sufficiently large) potential difference between electrodes, and a closed electrical circuit with connection of anode and cathode.

Electrolysis of the electrolyte certainly is/was a (safety) issue with the simpler/older design of car batteries when it is/was up to the user to refill electrolyte

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(image source)

and use of traditional battery chargers: it was possible to «overcharge» the accumulators which lead to the generation of hydrogen gas. Thus, supervised charging (monitoring the density of the electrolyte), closing the individual cells with perforated screw caps (to allow venting) in an aerated place was recommended.

Today, there are car batteries either capture the gases (to some extend) or/and use an electrolyte which is trapped in a gel; then, manual refill of acid usually is not necessary anymore. Equally, contemporary chargers now check the status of the battery to prevent overcharge / stop before the electrolyte decomposes (in electrochemical terms: before you leave the intended electrochemical window*). Or (the still more expensive way), the lead battery is replaced altogether by a different type (one of the variants using lithium, or a molten salt).

* A term used e.g. in cyclic voltammetry (a freely accessible primer).

  • $\begingroup$ So how much is the over potential needed for the H2 and O2 to bubble out? What's the underlying mechanism that determines the bubbling threshold? The way it is stated in the book seem to indicate the threshold is already reached in the battery. $\endgroup$ Aug 7, 2023 at 12:38
  • $\begingroup$ Also in this document: mesa-tec.com/wp-content/uploads/2014/06/… "Flooded batteries vent continuously, under all states" $\endgroup$ Aug 7, 2023 at 12:43
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    $\begingroup$ Overpotential may depend on multiple factors (concentration, or better activities of the reagents [it isn't a dilute aqueous solution], said temperature and pressure, current densities) and design of the electrodes. Because it can alter a lot (e.g. instead of Pt wire, use of a platinized Pt electrode; or electrolysis of aqueous NaCl typically does not yield hydrogen and oxygen, but hydrogen and chlorine), a true electrochemist likely can answer this better. $\endgroup$
    – Buttonwood
    Aug 7, 2023 at 13:15
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    $\begingroup$ For multi-electron transfer reactions you typically need a higher overvoltage because you need to (usually) make an intermediate which is highly energetic. Look at proton reduction, in the case your electrode does not interact with your reagent you need to generate a hydrogen radical as an intermediate. You can imagine the energy needed to do so is incredibly high. Then why if we use the right electrode does it work without a high overvoltage? Well the right catalyst can stabilize the intermediate, for example Pt forms adsorbed hydrogen. No need for a hydrogen radical as intermediate. $\endgroup$
    – Noah
    Aug 7, 2023 at 14:45
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    $\begingroup$ Hmm... I'm old enough to have used some of the lead acid car batteries that had screw caps and needed filling. I never used anything but water. As I remember the batteries were shipped dry then filled with the sulfuric acid electrolyte before installation. $\endgroup$
    – MaxW
    Aug 7, 2023 at 16:56

Very minor addition, but evolution of hydrogen in large lead acid batteries on submarines is a known issue and is controlled by sensors, ventilation, alarms, etc. It does not happen precipitously, but could endanger the ship (especially while charging) if these safety systems did not exist. It is definitely measurable.

  • $\begingroup$ I remember in Das Boot the movie, the crew was constantly monitoring the hydrogen level when they are submerged (i.e. not charging). I guess during WW2 the self discharge of the battery was high but the hydrogen purging device wasn't invented yet, so hydrogen accumulation in a sealed submarine was quite a big problem at the time and could be the limiting factor on how long a submarine could stay submerged. $\endgroup$ Aug 8, 2023 at 6:22
  • $\begingroup$ Catalytic converters to recombine hydrogen and oxygen appear to be common on lead-based accumulators of greater capacity than typically used in cars (add by a Polish company, a review by a British author). $\endgroup$
    – Buttonwood
    Aug 8, 2023 at 6:57

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