Chemists demonstrate a practical, green route to H2O2

Hydrogen peroxide can do a lot of things, like bleach hair, whiten teeth, and disinfect drinking water. The way chemists make H₂O₂ takes a lot of energy and relies on fossil fuel feedstocks, giving it a large carbon footprint. The compound is also unstable, so chemical makers must add stabilizers to move and store the chemical—and those stabilizers then need to be removed before use.

Now, Haotian Wang and coworkers from Rice University have developed an electrochemical synthesis of H₂O₂ that uses less energy and could produce the H₂O₂ where it is needed, without requiring the cumbersome stabilizers (Science 2019, DOI: 10.1126/science.aay1844). This cleaner method can use a solid-state electrolyte, water, and air, without fossil fuels.

Generally, industry makes H₂O₂ via a multistep process that involves anthraquinone and generates a lot of organic waste. Electrochemical syntheses of H₂O₂ exist but often rely on reactors that use liquid electrolytes. The H₂O₂ gets produced in the electrolyte, so chemists have to separate it from the liquid before use.

As an alternative, Wang and fellow researchers used commercially available solid-state materials with micropores where the reaction occurs. They then flush the reactor with deionized water, which allows the team to make H₂O₂ solutions of any concentration up to 20% by weight, says Chuan Xia, a postdoc in Wang’s lab. “This is high enough for use as a disinfectant.”

The team performed the reduction and oxidation steps in separate parts of the reactor, to minimize the safety hazard of mixing high-pressure H₂ and O₂. Both gases independently first diffuse through catalysts—an iridium oxide catalyst produces H⁺ from H₂, and an oxidized carbon catalyst produces HO₂^– from O₂. The intermediates then pass through ion exchange membranes before reaching the solid-state electrode, which is made from either functionalized styrene-divinylbenzene copolymer microspheres or a cesium-tungsten oxide, and react with each other to produce H₂O₂.

The researchers propose that their reactor could be installed where needed, such as in hospitals, eliminating the need to transport H₂O₂. Since chemical manufacturers have to dilute the chemical for transport, shipping H₂O₂ is the largest part of its carbon footprint. Xia says that if the reactor is powered by solar panels and H₂O is used as an alternate source of H₂, no fossil fuels would be needed in the process.

This work demonstrates that chemists can use electrochemical synthesis to make chemicals, fuels, and medicine more practically, says Yang Shao-Horn, a materials chemist at the Massachusetts Institute of Technology.