Refining hydrogen peroxide production with a metal-free carbon-nitrogen carbon-type hybrid electrocatalyst

Hydrogen peroxide is an environmentally friendly and powerful oxidant that is used in a variety of industries. At industry-scale, it is currently manufactured using a process called anthraquinone oxidation-reduction process, but researchers are searching for a better way. An electrocatalytic oxygen reduction reaction (ORR) is a safe, clean, and reliable method, but an effective catalyst has yet to be identified and adopted widely because of low selectivity.

 

New research suggests that using a metal-free carbon-nitrogen (CN)-type nanoporous carbon loaded onto a carbon matrix, known as CN@C for short, as an ORR catalyst solves this selectivity problem. The results were published in Carbon Future on 22 November.

 

“The industrial-scale production of hydrogen peroxide currently relies on the anthraquinone oxidation-reduction process, which is energy-intensive and generates significant waste. In our study, we tested synthesizing hydrogen peroxide with an electrocatalytic ORR, a greener and safer approach, using CN@C for highly selective peroxide synthesis in alkaline media. We chose CN@C because of the high porosity and tunable absorption properties of the CN particles and the high electrical conductivity of the carbon support,” said Tristan Petit, a researcher from the Young Investigator Group Nanoscale Solid-Liquid Interfaces at the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH in Berlin, Germany.

 

When electrocatalytic hydrogen peroxide synthesis happens via ORR, a two-electron transfer pathway is ideal. However, there are selectivity challenges. For example, the oxygen could be further reduced to water if there is a four-electron transfer pathway. Or the O-O bond in hydrogen peroxide could be broken down. These challenges increase costs because they make the process less efficient.

 

“Despite the progress made in developing peroxide-producing ORR catalysts, their performance in terms of selectivity and hydrogen peroxide production rate as well as further drawbacks such as high costs and limited (electro)chemical stability remain unsatisfactory. There is still significant research effort needed to achieve large-scale production of hydrogen peroxide through the electrocatalytic two-electron ORR pathway with cost-efficient and selective catalysts,” said Petit.

 

Metal-free carbon-based nanoporous materials, such as CN@C, could work well as catalysts because they are affordable, sustainable, have excellent electrical conductivity, and are highly selective. Researchers tested three CN@C catalysts that were heated at three different temperatures: 550, 700, and 1000 degrees Celsius respectively. They were then referred to as CN-550@C, CN-700@C, and CN-1000@C. There were distinct differences between the three. When viewed using scanning electron microscopy, the catalysts that were heated to 550- and 700-degrees had clusters of small and flat plates, plus a smooth surface. In contrast, CN-1000@C had a fiber-like surface. When observed with Raman spectroscopy, the CN-1000@C sample also had a higher G-band signal, which implies a more organized structure with less defects, improving its electrical conductivity and electrochemical performance.

 

To determine the selectivity of the CN1000@C catalyst, a K-L plot was completed. The K-L analysis is short for Koutecký-Levich, which measures electric current through an electrode after an electrochemical reaction. By using a rotating disk electrode setup, the measurements indicate the average electron transfer. The CN1000@C had an average electron transfer of 2.2. The electrocatalytic activity was also tested for all three. CN-1000@C continued to out-perform the other two catalysts, thanks in part to its larger surface area and graphitization, which is when the amorphous carbon is heated and turned into graphite, and it was found to be more durable. The presence of nitrogen active sites is however still important to bind oxygen molecules and purely graphitic materials do not show any ORR activity.

 

Looking ahead, researchers will continue to refine and improve the CN@C as an electrocatalyst. “Further improvements of CN@C electrocatalytic activity and stability are required to become an alternative material to palladium hydrogenation catalysts with significantly lower costs,” said Petit.

 

Other contributors include Bin Wu, Dulce M. Morales, Dongjiu Liu, Ping Feng, Yan Lu, and Marcel Risch of Helmholtz-Zentrum Berlin für Materialien und Energie GmbH; and Mingren Liu and Martin Oschatz of the Friedrich-Schiller-University Jena.

 

The Volkswagen Foundation, CSC scholarship, and the European Union supported this research.

 

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About Carbon Future

Carbon Future is an open access, peer-reviewed and international interdisciplinary journal, published by Tsinghua University Press and exclusively available via SciOpen. Carbon Future reports carbon-related materials and processes, including catalysis, energy conversion and storage, as well as low carbon emission process and engineering. Carbon Future will publish Research Articles, Reviews, Minireviews, Highlights, Perspectives, and News and Views from all aspects concerned with carbon. Carbon Future will publish articles that focus on, but not limited to, the following areas: carbon-related or -derived materials, carbon-related catalysis and fundamentals, low carbon-related energy conversion and storage, low carbon emission chemical processes.

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