A solar steam generator creates its own brine and harvests electricity twice, turning salt buildup into a full day-night power cycle.
(Nanowerk Spotlight) Solar desalination devices usually treat salt buildup as damage in progress. As water evaporates, the remaining brine grows more concentrated near the heated surface, where it can block flow paths, scatter incoming light, and slow the very process the device is meant to sustain. Much of the engineering around solar evaporation therefore tries to move salt away before it becomes a problem.
But salt buildup also creates a chemical imbalance. Brine contains far more dissolved ions than ordinary seawater. When suitable electrodes experience a swing between those two salt levels, they change how strongly they take up or release sodium and chloride ions. That change shifts the cell voltage and can push electrons through an external circuit, turning the salt concentration swing into electrical output.
A solar evaporator concentrates seawater into brine, and a membrane-free mixing entropy battery uses that salinity rise to generate electricity. When the brine is replaced with ordinary seawater, the same electrodes reset and produce a second electrical output.
The device’s core function is to get electricity from both halves of the salt cycle. Sunlight concentrates seawater, producing steam and driving the battery as brine forms. Replacing that brine with ordinary seawater then drives the battery again as the system resets. Together, those two outputs turn one concentration-and-dilution cycle into a full day-night electricity cycle.
Concept for solar evaporation and full-day electricity generation. The left panel illustrates the solar steam production and the autogenous salinity power to drive the Na+/Cl− insertion into the SIFE/CIFE electrode (namely charging the MEB during the day). The right panel illustrates the restoration of the system by displacing the hypersaline solution with fresh seawater. A significant decrease in the system salinity would induce the release of Na+/Cl− ions from the SIFE/CIFE electrode (namely discharging the MEB at night). Overall, the evaporator-MEB coupled system is potential to utilize solar irradiation for freshwater production and offers autogenous salinity power for full-day electric power conversion. (Image: Reproduced with permission from Wiley-VCH Verlag)
In this design, the evaporator does more than supply desalinated water. It creates the salt gradient that powers the battery, so the system can harvest salinity energy wherever seawater and sunlight are available, rather than only where fresh water naturally mixes with seawater. That distinction matters because conventional salinity-gradient energy from mixing saltwater and freshwater depends on a pre-existing boundary between two water sources.
The technical aim is not to add a power cell beside a desalination unit. It is to make the desalination process create the battery’s fuel. During illumination, a seawater-like NaCl solution fills the system and sunlight heats the evaporator. Water escapes as vapor, while salt remains in the liquid and makes the electrolyte steadily more concentrated. The battery sits in that changing electrolyte, so rising salinity becomes an input rather than a contaminant.
The first engineering problem is keeping that useful brine from choking the evaporator. If salt concentrates only at the hot surface, it can crystallize before the battery benefits from it. The researchers used carbonized rattan to avoid that trade-off. Its natural plant channels remain as aligned pathways, and carbonization turns the same structure into a black absorber that captures sunlight.
Those channels give the material its double role. They draw water toward the heated surface, but they also let concentrated solution move through vertical channels and smaller side openings. The evaporator can therefore sustain steam production while allowing the surrounding electrolyte to become highly saline. In the paper’s tests, it produced steam at about 1.37 kg m⁻² h⁻¹ under 1 sun illumination.
The salinity change was large enough to drive the battery cycle. The starting solution contained 0.5 mol L⁻¹ NaCl, close to ordinary seawater. During solar evaporation, that concentration rose to 4.8 mol L⁻¹. The brine did not leave the system as waste. It became the high-salinity state that pushed the electrochemical cell into its charging direction.
The battery uses two ion-hosting electrodes so both parts of the salt can participate. Sodium manganese oxide takes up sodium ions, while bismuth oxychloride takes up chloride ions. Because both electrodes sit in the same changing electrolyte, the device does not need to keep separate salty and dilute streams apart with an ion-exchange membrane. The electrolyte itself changes around the electrodes.
As salinity rises, sodium and chloride ions enter their respective electrode materials and current flows through the external circuit. When illumination stops, the experimenters replace the concentrated brine with unconcentrated simulated seawater. The lower salt concentration changes the electrodes’ preferred state. They release the stored ions, and the current flows again in the opposite direction.
This reset step is essential to the full-day claim. The nighttime output is not a passive result of darkness. It depends on exchanging the brine for ordinary seawater, which both refreshes the evaporator and gives the battery the salinity drop needed to discharge. The system harvests a managed cycle: concentration during solar evaporation, then dilution during regeneration.
The repeated-cycle tests show why this matters. The device produced electricity while brine formed and again when the salinity swing reversed. Across seven cycles, the coupled system delivered an average power density above 2.9 W m⁻² and an energy density above 229.7 kJ m⁻² per day. The paper reports these values as higher than those of existing mixing entropy batteries.
The advantage comes partly from creating a stronger gradient than conventional salinity-power devices usually use. River mouths provide a natural difference between freshwater and seawater, but that contrast is modest compared with seawater and concentrated brine. The paper calculates that mixing hypersaline seawater with ordinary seawater can release about seven times more salinity-gradient energy than mixing ordinary seawater with river water.
The concentrated electrolyte also helps the cell operate. Dilute solutions can raise internal resistance, which wastes part of the available energy before it reaches the circuit. Hypersaline brine conducts ions more readily, reducing that penalty. The device therefore benefits twice from concentrating seawater: it increases the available chemical imbalance and improves ion transport inside the cell.
Real seawater added a more practical test. Using seawater collected from Qingdao Huiquan Bay, the device reached 6.7 W m⁻² and 303 kJ m⁻² under the tested conditions, higher than in the simulated NaCl solution. The desalination function also remained intact. The measured salt ions in the condensate fell below the drinking-water limits cited in the paper.
The researchers also tested whether the voltage came from the intended salinity change rather than from an unrelated side effect. When they removed the evaporator, the voltage response weakened because the electrolyte no longer concentrated quickly enough. That control ties the electrical output to solar-driven brine formation, not merely to the presence of electrodes in salt water.
Durability matters because the same electrodes must repeatedly store and release ions. The sodium manganese oxide and bismuth oxychloride electrodes showed stable behavior during repeated cycling in both ordinary and concentrated salt solutions. After extended operation, their structures remained intact, supporting the interpretation that reversible ion insertion and release drove the output.
A single cell produces only a small voltage, so the researchers connected multiple units in series. In outdoor daylight operation, 56 battery units reached 7.28 V as evaporation concentrated the electrolyte. During the dilution stage, 25 units powered a blue light-emitting diode. That demonstration shows voltage scaling, not a finished power system.
The largest remaining limitation is current. The paper discusses seawater electrolysis as a possible future use, and related work on blue-energy-driven hydrogen production shows why salinity power attracts interest as an input for water splitting. In the present device, however, the measured current remained below 0.1 mA, too low for meaningful hydrogen generation.
Useful electrolysis would require faster electrode reactions and much higher current density. The value of the work is therefore not a near-term hydrogen device, but a change in design logic. It does not solve salt buildup by pretending it can vanish. It gives the buildup a job. By using solar evaporation to create the salt gradient that a battery needs, the system reroutes part of a desalination problem into salinity power.
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