Technological breakthroughs

Compiled by Bao Hien

A Chinese research team used the concentration difference between chloride-rich waste acid and a dilute stream, across a membrane that passes only protons, to generate electricity on its own and make chlorine and hydrogen. The process ran stably for at least 7 days, but has only been tested on simulated solutions with a very small membrane.

The work was published in Nature Communications on January 7, 2026, by the group of Professor Gao Jun (Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences) with colleagues at the Shandong Energy Institute.

Electricity from a concentration difference

Conventional chlorine production uses grid electricity to electrolyze salt solutions. The team went a different way, starting from diffusion dialysis, a method already used industrially to recycle waste acid. Chloride-rich waste acid flows along a membrane that passes only protons, with a more dilute stream on the other side. The concentration difference between the two sides is the energy source, known as osmotic energy. As protons cross the membrane, the electrons released travel through an external circuit and generate electricity, which feeds the reactions at two electrodes that produce chlorine and hydrogen, each gas in its own compartment.

The key part is the membrane. The team used a covalent organic framework (COF) membrane bearing sulfonate groups, about 10 micrometers thick, which lets protons through quickly but blocks multivalent metal ions. Proton-to-metal selectivity ranges from a few hundred (Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺, Mn²⁺) to 673 for Al³⁺ and 3,767 for Fe³⁺, whereas commercial ion-exchange membranes typically reach only a few tens, according to the authors. As a result the recovered acid is cleaner and metal ions do not cause side reactions at the anode. Two silver/silver chloride electrodes swap roles in turn to shuttle chloride ions between compartments.

The numbers

With simulated waste acid (5 M against 0.5 M HCl), maximum power density was 55 W/m², rising to 67 W/m² when seven metal ions were added to the feed, and it was maintained for over 15 days. One unit produced a maximum open-circuit voltage of 0.184 V, and 8 units were needed to start chlorine production. A stack of 18 units reached 2.95 V open-circuit.
In the 18-unit stack, with graphite and platinum electrodes, chlorine and hydrogen were each produced at about 150–154 liters per square meter of membrane per hour, for at least 7 days. The Faradaic efficiency of chlorine was 98.4% at 10 mA/cm² and 90.7% at 100 mA/cm², and that of hydrogen was close to 100%. For comparison, the team notes conventional electrolysis needs about 1.68 V at 100 mA/cm² in a two-electrode test.

The team also tested simulated desalination brine. Power density was above 6 W/m², rising to 8.4 W/m² with waste heat, and a 30-unit stack produced only about 12 liters of chlorine and 12 liters of hydrogen per square meter per hour, much lower than with waste acid.

“No electricity” does not mean free

The phrase “electricity-free” applies only to the electrochemical step. The energy still comes from a concentration difference, so the process needs a more dilute stream to act as the driving force. The team also does not report costs or a full energy balance.

All tests used simulated solutions, not real industrial waste acid, desalination brine or seawater. The membrane area tested was only 7.06 mm², although the team also tested larger areas. The electrodes must reverse periodically, every 600 seconds in the tests, which the team says could be automated. The power output of this membrane is lower than some other membranes the team cites, because it was optimized for proton transport. Durability is demonstrated only for “at least 7 days.” The authors suggest the principle could extend to other reactions in brines, such as making ammonia from nitrate-containing brines, but no results are reported. The sources cross-checked here are the paper and the team’s own press release, so there is no independent verification.

References
Zhu, C., Li, Q., Li, M., Pan, S., Zhou, B., Jiang, L., et al. (2026). Spontaneous chlorine production from chloride-containing brines. Nature Communications, 17, Article 1433. https://doi.org/10.1038/s41467-025-68181-7