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Catalysis

Chemists demonstrate a practical, green route to H2O2

Solid-state electrolyte enables an electrochemical synthesis with a small carbon footprint

by Leigh Krietsch Boerner
October 11, 2019 | APPEARED IN VOLUME 97, ISSUE 40

 

09740-scicon5-graphic.jpg
Credit: Adapted from Science
In this electrochemical cell, H2 and O2 first go through a gas-diffusion layer (crosshatches) to an oxidized carbon catalyst (black dots) or an iridium oxide catalyst (white dots). The resulting HO2 and H+ ions then combine in the solid-state electrolyte to produce H2O2. The researchers can flush the product out with deionized water.

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.

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