Technological breakthroughs

Compiled by Bao Hien

Researchers from India and the UAE have developed a rechargeable zinc battery that converts waste hydrochloric acid into chlorine gas during charging, using an iron-based catalyst instead of precious metals.

Chlorine (Cl₂) is an important raw material for manufacturing plastics, pharmaceuticals, agricultural chemicals and water treatment products. Most chlorine is currently produced through the chlor-alkali process, which electrolyzes brine, or the Deacon process. According to the research team, these processes consume approximately 4% of global electricity.

Meanwhile, industries such as steelmaking, metal surface treatment and chlorine production itself generate substantial quantities of hydrochloric acid (HCl) as waste. Recovering chlorine from this acid through electrolysis is considered a promising approach. However, conventional methods require relatively high voltages and simultaneously produce hydrogen and chlorine. These two gases can form a hazardous mixture if the system experiences an unexpected shutdown.

Researchers from the Indian Institute of Technology (IIT) Ropar, Khalifa University in the UAE and several research institutes in India have proposed an alternative that combines chlorine recovery with energy storage in a rechargeable zinc battery. Their findings were published in Energy & Environmental Science on June 3, 2026.

Integrating chlorine production into battery charging

The battery uses a zinc sheet as the negative electrode and dilute HCl solution at the positive electrode. During discharge, oxygen is reduced to water at the positive electrode, while zinc is oxidized. During charging, chloride ions in the HCl solution are oxidized to chlorine gas, while zinc is regenerated at the negative electrode.

This configuration enables a single system to perform three functions: treating waste acid, producing valuable chlorine and storing electrical energy.

A key innovation is replacing the hydrogen-evolution reaction with the oxygen reduction reaction. According to theoretical calculations, this substitution reduces the required voltage from 1.36 V to just 0.13 V. It also eliminates the risk of hydrogen and chlorine accumulating simultaneously, which could compromise operational safety.

Iron-based catalyst replaces precious metals

One of the main challenges is developing a catalyst that promotes the required reactions, withstands corrosive acidic conditions and remains affordable for industrial-scale applications.

Platinum is highly active in oxygen reduction reactions but is unstable in HCl environments. An earlier study on this type of battery used a silver-based catalyst, although material costs remained a barrier.

To address this issue, the researchers developed an iron-containing metal-organic framework (MOF) combined with 10% graphene to improve electrical conductivity. The material was synthesized through mechanical mixing, followed by heating at 90°C for 24 hours.

In experiments, the catalyst achieved a Faradaic efficiency of approximately 96.5% for chlorine production, compared with 87% without graphene and 78% when graphene was used alone. The material maintained continuous operation for 12 hours in an HCl environment. After 500 measurement cycles, its current density decreased by 18%, while iron dissolution into the solution remained negligible.

Notably, in concentrated 5 M HCl, the catalyst still delivered a current density exceeding 416 mA/cm², demonstrating its ability to operate under highly acidic conditions.

The team also simulated unexpected industrial shutdowns by running the system for 10 minutes, interrupting operation for two minutes and then repeating the cycle. After 200 cycles, equivalent to 40 hours of testing, the current remained stable.

Battery powers 35 LED lights for 16 hours

The complete battery has an open-circuit voltage of approximately 1.6 V. When a bipolar membrane was used to separate two compartments with substantially different pH levels, the system maintained stable charge-discharge operation for more than 250 hours.

In a practical power-supply test, two batteries connected in series produced approximately 3.4 V, enough to illuminate 35 blue LED lights for around 16 hours. Afterward, just 10 minutes of charging enabled the battery system to power the lights for another eight hours.

The researchers also observed that the catalyst changed color during chlorine production and returned to its original color when the voltage polarity was reversed. This phenomenon reflects changes occurring on the electrode surface and could help monitor the material's operating state. Changes in pH around the electrode during the reaction were also recorded, providing additional insights into the system's operating mechanisms.

Further improvements needed before industrial application

Despite these encouraging results, the technology remains at the laboratory testing stage. The battery was assembled in a custom-made H-shaped electrochemical cell, with a maximum power density of 5.4 mW/cm², considerably lower than that of commercial batteries.

During extended experiments, the zinc sheet had to be polished before operation could resume. Output voltage also fluctuated depending on the amount of dissolved oxygen in the solution, indicating that system performance remains sensitive to operating conditions.

According to the researchers, the findings demonstrate the feasibility of integrating industrial chemical production reactions into rechargeable battery systems. This approach could pave the way for processes that recover valuable chemicals from industrial waste streams while simultaneously storing energy, potentially improving resource efficiency and reducing electricity consumption in the chemical industry.

Source: Kaur S., Gaber S., Garg K. et al., Tailoring an iron-based metal–organic framework for a Zn-ORR/CER battery, Energy & Environmental Science, 2026, 19, 4016–4027. DOI: 10.1039/d6ee00064a.