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NNadir

(33,512 posts)
Sat Jan 20, 2024, 11:31 AM Jan 2024

Dealing with Chlorine in the Electrolysis of Seawater to Make Hydrogen: Discussion of an Approach. [View all]

The paper to which I'll refer in this post is this one: Reducing Chloride Ion Permeation during Seawater Electrolysis Using Double-Polyamide Thin-Film Composite Membranes Xuechen Zhou, Rachel F. Taylor, Le Shi, Chenghan Xie, Bin Bian, and Bruce E. Logan Environmental Science & Technology 2024 58 (1), 391-399.

There is a widespread belief in our culture that hydrogen is "green," which is based on the fact that its combustion product is water. This belief is rather equivalent to the belief that Jesus is going to appear at a revival meeting in Mississippi, wrapped in an American flag, updating turning water into wine by turning Trump signs into AK-47s and inspiring the crowds to go out and shoot gays, and nonwhite people with their new weapons while requiring every woman to have a forced birth of nine or ten children and appointing Donald Trump as his personal representative on Earth.

Hydrogen is a dirty fuel. A Giant Climate Lie: When they're selling hydrogen, what they're really selling is fossil fuels.

I cite this particular paper, which I do not have time to discuss in great detail, because like every other "hydrogen by electrolysis using so called "renewable energy" paper published in these troubling times where wishful thinking substitutes for reality, it starts with reality, and then ignores it with a bunch of "could" statements substituting for "is" or "are" statements.

I'll comment briefly below on the parts I will put in bold from the introduction and then make a brief statement of why I think that the paper, despite it's "could" rather than "is" or "are" focus or foci, is worth considering for what might be a better world than the one in which we live. From the introduction:

The global demand for hydrogen has been rising in recent years, with a 22% increase to 115 million tons per year expected by 2030. (1,2) Hydrogen gas is currently produced primarily via steam-methane reforming, which results in the generation of carbon monoxide and carbon dioxide. (3,4) Water electrolysis driven by renewable electricity (e.g., wind or solar power) for hydrogen production is a cleaner alternative. (5,6) However, ultrapure water is usually needed for water electrolysis, which is often not readily available in areas where renewable energy sources are abundant, like coastal regions, as salt water is prevalent in those regions. (7,8) Direct seawater electrolysis is problematic due to the evolution of corrosive chlorine gas at the anode, which can shorten the life span of the electrolyzer setup. (9,10) One potential solution is to pretreat seawater to produce ultrapure water for electrolysis, but this increases the complexity of hydrogen production. (11,12)

An asymmetric water electrolyzer configuration has recently been proposed that uses fully oxidized salts (e.g., sodium perchlorate) as contained anolyte and seawater as the catholyte to prevent chlorine evolution. (13) Low-cost (less than $10 m–2) (13) polyamide (PA) thin-film composite (TFC) reverse osmosis membranes were used instead of more expensive cation exchange membranes ($250 to $500 m–2) (13) commonly used in acidic water electrolyzers to control the ion exchange between the catholyte and anolyte. (13,14) These PA TFC membranes exhibit high resistance to salt permeation due to steric exclusion. (15,16) Thus, applying TFC membranes to seawater electrolysis helps to limit the passage of chloride ions (Cl) into the anolyte and the leakage of the anolyte. Water ions, H+ and OH, can easily transport through TFC membranes, which can decrease the membrane charge transfer resistance. (17,18)

One major concern is that the small amount of Cl ions passing through TFC membranes during seawater electrolysis still needs to be decreased to minimize chlorine generation. (13,19) Another concern is the energy loss due to the generation of a pH gradient when using TFC membranes and salty water. During electrolysis, H+ ions are generated in the anode and OH ions are produced in the cathode. Although TFC membranes favor the transport of water ions over salt ions, some cross-membrane charge is still carried by salt ions, increasing the concentrations of H+ in the anolyte and OH in the catholyte. This pH difference can contribute to the voltage increment during electrolysis, referred to as the Nernstian overpotential. (20,21) Using a membrane with a more selective PA active layer with negative charge was recently shown to moderately reduce the permeation of Cl ions but have little effect on the transport of counterions (Na+) during electrolysis. (22) The Nernstian overpotential remained nearly constant and thus did not further decrease the energy demands. Therefore, other methods are needed to further reduce Cl ion transport in TFC membranes, ideally along with the deceleration of Na+ ion permeation.

In this study, we examined the use of a double-polyamide TFC membrane structure for reducing the permeation of chloride ions during water electrolysis (Figure 1). It was hypothesized that the negatively charged polyamide facing the alkaline catholyte would decrease the transport of negatively charged Cl ions into the anolyte, and the positively charged polyamide facing the acidic anolyte would reduce the transport of Na+ ions into the catholyte. Hence, having a polyamide layer on both sides of the membrane would favor the transport of water ions due to their higher permeability in the PA layer. In addition, the rapid water ion transport can be maintained through very fast water ion association to form water molecules.


Unstated in the first bolded ( "is" ) statement is that not only does the production of hydrogen generate climate change gases, but it does so by the destroying exergy associated with the original fossil fuels. This means that to use hydrogen as a source of energy - which is actually a trivial Potemkin enterprise being sold by fossil fuel salespeople here at DU and elsewhere - means more reliance on dangerous fossil fuels, not less.

These sorts of papers often report Faradaic efficiencies - the utilization of electrons - of greater than 90% - although this paper doesn't refer to Faradaic efficiency at all, but bury the reality of the very different thermodynamic efficiency by discussing it as "over voltage." The thermodynamic efficiency of electrolysis is roughly 60%, meaning that 40% of the potential exergy is destroyed in making hydrogen via electrolysis.

Now let's talk about the energy content of the world supply of hydrogen connected with the "by 2030" soothsaying, even though hydrogen is overwhelmingly used as a captive chemical reagent largely in ammonia synthesis followed by oil refining. The energy content of hydrogen can be taken to be around 120 MJ/kg. Thus the energy value of all the hydrogen on the planet that is supposed to be in demand "by 2030" is about 13.9 Exajoules, as of 2022 according to the 2023 World Energy Outlook. The same document reports that the combined solar and wind on the entire planet - infrastructure with a 25 year lifetime roughly built at a cost of trillions of dollars - was about 15 Exajoules, this on a planet consuming 634 Exajoules of energy. Thus the expectation that hydrogen "could" be made with so called "renewable energy" would require nearly all of it produced in an insane orgy of cheering for this stuff while dependence on dangerous fossil fuels gets worse and worse.

Now the positives associated with the paper. I personally believe that we should - we don't but we "could" or "should" - rely on nuclear energy as our sole source of primary energy, and in so doing, should utilize process intensification to raise the exergy recovery from the rather abysmal 33% realized by most light water reactors in the world, to a more sustainable 60% percent or higher. (100% is thermodynamically impossible, which every high school graduate should know, but likely doesn't know.)

Under these conditions, there will be a lot of excess electricity available that can be diverted, even given the fact that electricity is thermodynamically degraded, to electrolysis of seawater. I have spoken favorably many times of the work of the Naval Research scientist Heather Willauer whose work focuses on making jet fuel (for aircraft carriers) from seawater where the source of carbon is dissolved carbon dioxide and carbonate ions in seawater, where the volumetric and mass content is much higher than that of air, with which it is obviously in equilibrium.

This is a pathway to direct air capture of CO2, thus closing the carbon cycle, something proposed back in 2011 by the late great Nobel Laureate George Olah.

Anthropogenic Chemical Carbon Cycle for a Sustainable Future George A. Olah, G. K. Surya Prakash, and Alain Goeppert Journal of the American Chemical Society 2011 133 (33), 12881-12898)

Of course, this is in the realm of "could." I'm not optimistic in the age of celebration of lying to ourselves and to each other that this "could" outcome is likely, but it exists.

Have a pleasant Sunday.





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