Inefficiencies of Water Electrolysis

[Xyius]

Hello all,

I have been doing some research on electrolysis of water. One thing I have found is that it seems to not be an efficient way to make hydrogen. I have also found that this efficiency can be dramatically increased by..

-Using the right electrolyte
-Placing the electrodes at just the right distance away from each other and making sure they are straight up and down.
-Using high temperatures and pressures.

My question is, why hasn't someone used some sort of waste heat, such as automotive exhaust or steam exhaust to heat water and increase electrolysis efficiency? Also combine this in a pressurized vessel. Is it STILL so incredibly inefficient even when taking EVERYTHING into account?

One thing I have learned is that most times if you have an idea, someone has already thought of it and fully explored it. I am convinced this is just one of those times and there must STILL be some reason why this isn't commercially done.

Thanks!
~Matt

 

[Simon Bridge]

My question is, why hasn't someone used some sort of waste heat, such as automotive exhaust or steam exhaust to heat water and increase electrolysis efficiency?

None of this will increase the energy efficiency of the process, what that would do is supply another source of energy. If the energy source is low cost, then the economic efficiency will be improved a bit ... but you have to look into the logistics more - exhaust heat looks like it's free but how will you get the heat to the hydrogen plant? Carry the plant in a car? Perhaps you are thinking that a hydrogen burning car could divert waste heat to make more fuel as you drive?

Geothermal electricity is used for electrolysis though - that's "waste steam"; and nuclear power plants are basically steam engines.

Also combine this in a pressurized vessel. Is it STILL so incredibly inefficient even when taking EVERYTHING into account?

Compared to other ways to make hydrogen - that seems to be the case.
When you really take EVERYTHING into account, the costs usually outweigh the benifits.
Still - that does not stop people trying. Sometimes someone comes up with something.

 

[Xyius]

Perhaps you are thinking that a hydrogen burning car could divert waste heat to make more fuel as you drive?

Yeah that's what I meant. Of course on the outside it SEEMS like a very good idea. Store the hydrogen in a liquid form (as water) and only create it when you need it by just running current through it. But Like I have said before, in my experience, if you have a good idea, most of the time its been thought of already and fully explored.

I keep seeing people say electrolysis is so inefficient, but I want to know the hard reasons as to WHY. What are the physics behind its inefficiencies?

 

[Simon Bridge]

You are thinking - burn hydrogen with oxygen to make water + work + heat, then use the heat to convert the water back into hydrogen and oxygen to start all over again??
This is called a closed mill, and is forbidden by the law of conservation of energy.

If you are thinking that some of the heat could be diverted to make electricity ... aiding the battery or whatever in the electrolysis - well OK, but you have to factor in the amount of electricity needed vs how much heat you can divert to this purpose, and the energy cost of moving the water tank around, full of water, (and the machine, and a big battery) vs a tank of hydrogen and just collecting oxygen for free from the air.

Most proposed hydrogen schemes involve using wind, solar, or nuclear power to get the hydrogen and release the oxygen.

The physics behind the inefficiencies are the physics behind electrolysis - and the engineering needed to make the electrolysis machine - and the law of entropy.
There is no easy answer - but the inefficiencies are not the reason your idea does not work.

 

[Xyius]

You are thinking - burn hydrogen with oxygen to make water + work + heat, then use the heat to convert the water back into hydrogen and oxygen to start all over again??
This is called a closed mill, and is forbidden by the law of conservation of energy.

Honestly I do not have a concrete plan. My main question is just knowing why electrolysis is inefficient. Am very much aware that life isn't as simple as "Hey I have a great idea!". There are always a variety of other engineering logistics that need to be taken into account.

The physics behind the inefficiencies are the physics behind electrolysis - and the engineering needed to make the electrolysis machine - and the law of entropy.

Again, I do not have a solid idea, I have a thought, and the main brunt of my question is to know the engineering and physical reasons as to why electrolysis is inefficient.

There is no easy answer

I don't want an easy answer, I want THE answer. Do you know the answer?

Give it to me straight doc, I'll have a PhD in about a year or so, so I don't think it will be beyond the scope of my reasoning.

 

[Simon Bridge]

OK then - you have some topic to study.
Come back when you have a solid idea of what you want to know about.

As for "why electrolysis is so inefficient" - I've given you the answer, as best I can, within the constraints of these forums and the information you have provided. As a post grad student you will be used to doing some of your own research off suggestions provided by others. You want more details, they are there, occupying a few reams of paper.

 

[Xyius]

...the main brunt of my question is to know the engineering and physical reasons as to why electrolysis is inefficient

To be honest, I actually know a little bit about why it is inefficient. In fact, here is an EXCELLENT piece from a wiki article.

Thermodynamics[edit]
The electrolysis of water in standard conditions requires a theoretical minimum of 237 kJ of electrical energy input to dissociate each mole of water, which is the standard Gibbs free energyof formation of water. It also requires energy to overcome the change in entropy of the reaction. Therefore, the process cannot proceed below 286 kJ per mol if no external heat/energy is added.

Since each mole of water requires two moles of electrons, and given that the Faraday constant F represents the charge of a mole of electrons (96485 C/mol), it follows that the minimum voltage necessary for electrolysis is about 1.23 V.[15] However, observing the entropy component (and other losses), voltages over 1.48 V are required for the reaction to proceed at practical current densities (the thermoneutral voltage).

In the case of water electrolysis, Gibbs free energy represents the minimum work necessary for the reaction to proceed, and the reaction enthalpy is the amount of energy (both work and heat) that has to be provided so the reaction products are at the same temperature as the reactant (i.e. standard temperature for the values given above). An electrolyser operating at 1.48 V would be 100% efficient.

Overpotential[edit]
Real water electrolysers require higher voltages for the reaction to proceed. The part that exceeds 1.23 V[16] is called overpotential or overvoltage, and represents any kind of loss and nonideality in the electrochemical process.

For a well designed cell the largest overpotential is the reaction overpotential for the four-electron oxidation of water to oxygen at the anode; electrocatalysts can facilitate this reaction, andplatinum alloys are the state of the art for this oxidation. Developing a cheap, effective electrocatalyst for this reaction would be a great advance, and is a topic of current research, there are many approaches among them a 30 year old recipe for molybdenum sulfide,[17] graphene quantum dots,[18] carbon nanotubes[19] and perovskite[20] The simpler two-electron reaction to produce hydrogen at the cathode can be electrocatalyzed with almost no overpotential by platinum, or in theory a hydrogenase enzyme. If other, less effective, materials are used for the cathode (e.g. graphite), large overpotentials will appear.

What I like to do is go to these forums to see if anyone else has any other information that I can add to my knowledge. I have found many times that the people on these forums tend to give excellent tidbits of information on a topic I want to know more about.

What I was looking for was something along the lines of explaining WHY over-potential is necessary. What are the chemical processes that make this reaction need so much extra voltage? What are some solutions people have thought up and how successful were they? In the past, I have been very surprised and pleased about how much knowledge this community has.

 

[Bystander]

along the lines of explaining WHY over-potential is necessary

"Why?" Wrong interrogative: ask "What is overpotential?" or, "How do overpotentials become significant in real electrolytic cells?" One, transport properties of ions; two, reaction mechanisms at electrodes; three, reaction kinetics at electrodes; four, phase changes at electrodes.

 

[NascentOxygen]

There has been a discovery of an algae that, under the right conditions, produces H₂ bubbles as its byproduct of photosynthesis. This could be a way to evade some inefficiencies in the solar cell -> electrolysis -> solar H₂ fuel route. But there doesn't seem to be much publicity on it now, so the discovery probably hasn't lived up to researchers' initial hopes.

 

[EliotBry]

"Why?" Wrong interrogative: ask "What is overpotential?" or, "How do overpotentials become significant in real electrolytic cells?" One, transport properties of ions; two, reaction mechanisms at electrodes; three, reaction kinetics at electrodes; four, phase changes at electrodes.

Hi @Bystander , your comment was really useful to me, I'm doing my BSc project on CSP at the moment and decided it would be fun to look at the potential of using CSP to run water electrolysis to produce H2.

However I am just running into so much chemistry and it's killing me lol. I've not done chemistry since I was 16.

Anyway your comment was really useful as I have been tracking the current of electrolysis reactions over time, initially under poorly controlled conditions I was getting large fluctuations in the current, but now I have improved my experimental design and using sonication and getting a constant current. I was wondering if there's any well known equations that explain the current behaviour and why it improved under better experimental conditions (such as electrode position being held more constant, bubbles being buzzed off at 44kHz, and having electrodes that aren't collecting material on them after periods of time).

But yeah, do you know of any resources or locations where I might be able to read about those things that you mentioned from a specifically physics standpoint, because when they start to talk about reaction potential etc I get lost oh so quickly!

Apologies if this isn't the place to discuss all this, I just spotted someone who said some very relevant things to me and it's 3am.

 

[Bystander]

resources or locations where I might be able to read about those things that you mentioned from a specifically physics standpoint, because when they start to talk about reaction potential etc I get lost oh so quickly

Ralph Adams, for stirring/agitation.

 

[EliotBry]

Ralph Adams, for stirring/agitation.

Cheers, I just got it out of the university library. It looks like it covers a lot of electrode behaviour which is great!