Hydrogen peroxide (H2O2) is widely used as a bleach, disinfectant, and oxidising agent, among other things. However, industrial production of H2O2 is expensive and uses a lot of energy owing to the rare and precious metal catalysts used in its production. Researchers at the Indian Institute of Science (IISc) have developed an alternative, onsite production strategy for H2O2 that can also degrade industrial pollutants like toxic dyes.
The scientists have utilised a zinc-air battery in which oxygen reduction generates H2O2. “Zinc is an abundant and historically-used element … it is very cheap and abundant in India,” says Aninda J Bhattacharyya, Professor in the Interdisciplinary Centre for Energy Research (ICER) and Solid State and Structural Chemistry Unit (SSCU), and corresponding author of the study published in Small Methods.
A metal-air battery has a metal like zinc as the anode (negative electrode) and ambient air as the cathode (positive electrode). When the battery discharges – releases energy – oxygen from ambient air gets reduced at the cathode, producing H2O2.
The electrochemical reduction of oxygen proceeds through two ways, one of which forms H2O2. “The strategy here is to control the extent of the oxygen reduction reaction. If you don’t control it at some level, it will just go and form water,” explains Bhattacharyya.
This control can be achieved using specific catalysts. “We are using a metal-free catalyst based on carbon,” says Asutosh Behera, first author and PhD student at SSCU. These inexpensive catalysts usually drive the reaction along the route that forms water where the selectivity towards H2O2 is less. However, incorporating certain chemical modifications in these catalysts, like adding oxygen functional groups, directs the reaction selectivity towards the production of H2O2.
Bhattacharyya explains that using a battery to directly produce H2O2 is a novel approach. “You don’t have to do other things. You have a battery, and you run it. We have curtailed the voltage such that it is only producing H2O2.”
Another advantage of using batteries is that they produce or store electrical energy in addition to chemical reactions. “What we are doing is that along with producing H2O2, we are storing energy because it takes place inside the cell,” Bhattacharyya adds.
The H2O2 generated must be detected since it is colourless. This can be done by introducing a dye, a toxic pollutant produced by the textile industry. When H2O2 is created, it reacts with the dye, degrading it and changing its colour. “The H2O2 generated will further decompose into various radicals (such as hydroxide and superoxide) – highly raw, reactive organic species – that will eventually degrade the textile dye,” Behera explains. This degradation helps increase the efficiency of H2O2 production and eliminate the toxic dye.
“There are some fundamental challenges which must be overcome,” Bhattacharyya notes. For example, a metal-air battery has three phases – solid (zinc), liquid (electrolyte), and gas (air). This makes handling them more challenging than most batteries with only two phases.
Despite these challenges, the researchers believe that the strategy is scalable and may have other applications, like generating electricity in remote locations. “This method is very sustainable, low-cost, and highly energy-efficient,” says Bhattacharyya.
