The first thing that came to mind when I saw the abstract was that existing bipolar membrane electrodialysis processes already provide a convenient way of performing the pH swing process they are developing, but with membranes that are already produced on km^2/year scale. Companies like Neosepta or Veolia (formerly Suez formerly GE Water formerly Ionics) produce bipolar membranes for this task, and it's a rapidly growing area of interest.<p>A bipolar membrane (BPM) consists of a polymer membrane full of positively charged groups (the anion-exchange resin) intimately bound to a polymer membrane full of negatively charged groups (the cation-exchange resin). The interface (reminiscent of a p-n junction) is known as a bipolar junction, and acts as an electrode under a sufficiently high potential gradient. They are made out of cheap materials which have been used in ion-exchange resins and membranes since the 60s, but the bipolar membrane process is niche and hasn't been anywhere as highly developed as other electrodialysis membranes. And electrodialysis is fairly niche, and hasn't been nearly as highly developed as membranes for gas separation, desalination, or removal of particulates (ultra- and micro-filtration).<p>It turned out that electrodialysis is less efficient for seawater desalination than reverse osmosis (the potential drop through the product water becomes really severe if you're trying to produce drinking water from seawater), so electrodialysis was half-abandoned in comparison to RO. Oddly, Japanese companies developed a lot of ED technology to its current state, including ion-selective cation exchange membranes, for producing table salt, since Japan doesn't have the climate necessary for normal salt evaporation. The ion-selective cation resins were developed for removing Mg from seawater for table salt, but are now popular for researchers trying to do lithium separations.<p>Anyway, while I agree with the authors that BPMs have unresolved challenges (related to efficiency, mechanical stability, and the fact that current membranes are required to be loaded with transition metal catalyst to get a decent water splitting rate at a low overpotential), I don't know that I'm convinced that their approach is better just because they call BPMs "expensive" four times. If we wanted to adjust the pH of a lot of water, we would need, as a guess, roughly the same amount of electrode catalyst surface area, or the same amount of bipolar junction surface area. However, the bipolar junction is made out of commodity polymer resins heat laminated together, while the electrodes in this study are made out of silver and bismuth. If the bipolar membrane is loaded with a metal catalyst, the most common one is iron. I don't see the BPMs being the more costly solution for very long.<p>For full disclosure, I recently started doing some work on BPMs, but I think the problems associated with it are solvable, especially for applications like this (as opposed to much more challenging conditions like CO2 electrolyzers).