The answer, according to the review, is a family of electrodeless RED and electrodialysis (ED) systems that operate as closed ionic circuits, in which charge transport is mediated solely by ions and no Faradaic reactions occur anywhere in the device. Veerman surveys the existing applications of these electrode-free generators and desalinators and introduces several conceptual designs presented for the first time, including an acid/base generator, a preconcentration unit that couples RED with assisted RED (ARED), and an alternating current RED generator. The unifying principle is elegant: because ED and RED employ essentially identical stack architectures of alternating cation exchange membranes (CEMs) and anion exchange membranes (AEMs), two or more such stacks can be wired together ionically, so that one stack’s electrical output drives the other stack’s ion pumping directly, without ever touching a wire.

The operating envelope of a RED stack explains why this coupling works. When current density is plotted against power density, three characteristic points stand out. At short-circuit condition, ion transport proceeds without any external power supply. At the maximum power point, RED systems are conventionally operated. At open-circuit condition, ion transport ideally vanishes, although incomplete membrane permselectivity leaves a small leakage current. Push the current density beyond the conventional RED range into strongly negative territory and the stack enters the ARED regime, where an external field assists the salinity-gradient-driven transport. ARED can be applied for pre-desalination of seawater when a lower-salinity stream, such as wastewater, is available. In an electrodeless coupled system, one RED stack can supply exactly that assisting field to another, closing the ionic loop between generator and desalinator.

The historical roots of this idea reach back to the 1950s. Manecke, regarded as the founder of RED technology, described a Membranakkumulator in 1952, a device based on ED and RED that could store electrical energy. A few years later, Pattle pointed out that the RED principle could tap an enormous energy source by controlled mixing of river water and seawater. In 1958, Murphy proposed what he called Osmionic Demineralization, using a salinity gradient to further desalinate a dilute solution, in essence a direct coupling between a RED and an ED device. Murphy and Matthews published the thermodynamic foundations of ion transport for osmotic demineralization in 1967. Unaware of Murphy’s work, Veerman independently described a similar system in 1994, motivated by elevated salinity in the Rhine River caused by discharges from potash mines in Alsace, France, which hampered the cultivation of salt-sensitive crops in the Westland region near the Rhine delta. He constructed and tested a four-compartment system in which the D-A-B segment functioned as the RED generator powering the ED unit B-C-D, with NaCl concentrations of 30 g/L in the generator feed and 1 g/L in the other compartments. Under idealized assumptions, the equilibrium calculation predicts that 0.88 g/L of salt is transferred, leaving the desalinated compartment at roughly 0.12 g/L, a substantial desalination achieved with essentially no external energy input.

For seawater desalination, where the driving force is weaker because the concentration ratio between brine and seawater is lower, Murphy proposed increasing the number of RED cell pairs relative to the ED section. In the modern configuration, the stack is fed alternately with high-concentration brine and lower-concentration seawater, and at selected positions a desalination compartment contains seawater instead of brine, marking the product compartments. The principle was later redeveloped by Chen and colleagues as an energy self-sufficient desalination stack for small islands, although their design still employed electrodes. The electrodeless variant removes the last electronic component, making the entire desalination train a purely ionic machine.

Perhaps the most striking existing application is medical. Pharmaceutical drugs can enter the body through the skin via transdermal administration, and medicated patches allow this without medical supervision, but conventional diffusion-based delivery is slow. Kwon and colleagues developed a method based on RED for ionized drugs, positioned at one end of the stack depending on their charge. Crucially, in this configuration the RED stack operates without electrodes, because the electrical circuit is closed and charge transport is facilitated by ionic species, both cations and anions. The authors tested lidocaine hydrochloride, a cationic anesthetic, and sodium risedronate, an anionic anti-osteoporosis drug, in vitro on mouse skin. Transport rates were several times faster than diffusion-based topical administration without an electric field, and the complete RED stack was miniaturized to the size of a standard patch. A salinity gradient between two harmless salt solutions becomes, in effect, a built-in ionic power source for pushing medicine through the skin.

The same metal-free logic powers the emerging field of iontronics, in which ions rather than electrons act as charge carriers, analogous to electronics. Information transfer in biological systems relies on the transport of ions through nerves, and fully iontronic systems interfacing with living tissue offer improved biocompatibility over metallic electrodes. In 2017, Han and colleagues demonstrated the first iontronic logic gate powered by RED, creating a fully metal-free system. A logical OR gate was embedded in a microchip, with inputs connected to a RED generator via capillary tubes. Input interruption, representing a logical zero, was achieved by pinching the tubes, and fluorescein in the output indicated current flow, turning yellow when current passed and remaining colorless otherwise. Closely related is Donnan dialysis, which uses a salinity-gradient-induced electric field to drive ions against their concentration gradient. In a water-softening application, a sweep phase of 0.5 M NaCl exchanges magnesium and calcium ions for sodium, removing undesirable ions or recovering valuable ones, all without electrodes.

The new concepts proposed in the perspective push the electrodeless principle into industrial territory. The first couples two RED stacks directly for a coastal drinking water supply. The numerical example is designed for a drinking water production of 3,000 cubic meters per day, sufficient for a city of 10,000 inhabitants consuming 300 liters per capita per day, with a reverse osmosis (RO) unit operating at 50 percent recovery. Wastewater from the town, at 0.5 g/L salinity, is used in two ways: in RED-1, energy is recovered from controlled mixing of a wastewater fraction with incoming seawater, partially desalinating the RO feed and improving RO performance, and RED-1 can be operated in ARED mode, where the additional energy input is more than compensated by the reduction in RO energy demand. In RED-2, energy is extracted from another wastewater fraction combined with the RO concentrate. The novel step is the direct, electrodeless coupling of RED-1 and RED-2, in which electrodes and associated electrical equipment are no longer required. The stack specifications show RED-1 handling 1,000 L/min of seawater-side flow with 10 kg/min of salt transport, and RED-2 handling 500 L/min with 7.3 kg/min of salt transport. Because the ratio of high to low concentration is higher in RED-2, its electromotive force exceeds that of RED-1, and its higher salt concentrations yield a lower internal resistance, so RED-2 can effectively drive the ARED operation of RED-1. In practice, the number of cell pairs in each stack should be adjusted for proper electrical matching.

The second new concept produces acid and base using a bipolar membrane, a sandwich of a CEM and an AEM. Under forward bias in NaCl solutions, water dissociates in the extremely narrow junction compartment into hydrogen and hydroxide ions; the hydroxide migrates through the AEM part toward one compartment, forming NaOH, while the hydrogen ions move through the CEM part, forming HCl. Under reverse bias with NaCl, the junction quickly becomes saturated and the current is blocked, so the bipolar membrane behaves like a semiconductor diode. Under reverse bias with NaOH and HCl, the ions recombine to form water and electrical power is generated. If the acid and base were previously produced by forward bias, this creates an acid-base battery with an energy storage capacity roughly ten times higher than that of a salinity gradient battery. The key technical challenge is ensuring sufficiently fast water diffusion to prevent delamination of the bipolar membrane. The perspective proposes powering this acid/base production system with an electrode-free RED generator, opening the way to bipolar-membrane applications including ammonia production, sulfur dioxide capture and carbon dioxide capture, all driven by salinity gradients alone.

The third concept is the most futuristic: an alternating current RED generator. A ring-shaped RED stack generates an ionic loop current in the primary winding of a transformer, and a mechanical shutter periodically interrupts the current, transferring electrical power to the secondary winding. Adding a second circular RED stack generating current in the opposite direction can produce a square-wave electric field, advantageous over interrupted direct current. Because the weight of a transformer is inversely proportional to frequency, efficient operation requires a shutter frequency of at least 50 to 60 hertz. Replacing the mechanical shutter with an iontronic interrupter remains challenging, although emerging research on ionic transistors using ion exchange membranes and self-supported ion-conductive membrane-based transistors may provide solutions, provided a large active membrane transistor area can be realized. Closed-loop systems, as used in heat-to-power schemes that regenerate the salinity gradient with waste heat, may allow the addition of chemical aids, whereas natural feed water systems must operate without additives.

The outlook, as Veerman frames it, extends from macroscale water treatment to microscale medicine and beyond. Electrodeless RED systems may provide a versatile platform for powering industrial processes involving bipolar membranes, using feed waters of natural or industrial origin, while closed-loop concepts in which ionic streams are continuously regenerated using low-grade industrial waste heat could enable sustainable and circular process designs. The electrodeless RED generator also points toward a new class of iontronic systems, serving as a conceptual stepping stone toward ionic transistors and other controllable ion-based switches, and ultimately toward fully ionic circuits that operate without electronic components. Pursuing these directions will require addressing fundamental challenges in materials, system integration and process control, but history suggests that progress in this field will not be linear, and the unexpected insights that accompany it may redefine both the scope and the impact of RED-based technologies.

Subject of Research: Science & Technology

Article Title: Electrodeless reverse electrodialysis

Article References: Veerman, J. (2026). Electrodeless reverse electrodialysis. Frontiers in Membrane Science and Technology, 5, Article 1812910. https://doi.org/10.3389/frmst.2026.1812910

Image Credits: AI Generated

DOI: 10.3389/frmst.2026.1812910

Keywords: reverse electrodialysis, electrodeless systems, salinity gradient energy, ion exchange membranes, desalination, iontronics, bipolar membrane, assisted reverse electrodialysis, renewable energy, membrane technology, Electrodeless, reverse