New Material: Low Emission Method To Extract Hydrogen And Graphene From Waste Plastics

The researchers used a low emission method to extract hydrogen and graphene from waste plastics. They said that this not only solved environmental problems such as plastic pollution and greenhouse gas generation, but also offset the cost of hydrogen production with the value of graphene by-products. Hydrogen can be used to power cars, generate electricity, and heat homes and businesses. Compared with fossil fuels, hydrogen contains more energy per unit weight, which is very important from the perspective of environmental protection, because the main reason for global greenhouse gas emissions is carbon dioxide released by burning fossil fuels.

At present, more than 95% of the hydrogen sold is synthesized by steam methane conversion, and each ton of hydrogen will produce 11 tons (12 Tons) of carbon dioxide, most of which is gray hydrogen. In contrast, the "green hydrogen" produced by separating water into various elements using renewable energy such as solar energy, wind energy or hydropower is expensive, and the cost of every two pounds (about one kilogram) of hydrogen is about Five dollars.

Researchers at Rice University have now developed a method to obtain valuable hydrogen and graphene from waste plastics by using a low emission, catalyst free method, which may recover costs.

"In this work, we transform waste plastics (including mixed waste plastics that do not need to be classified or cleaned by type) into high-yield hydrogen and high-value graphene," said Kevin Weiss, the first author of the study Wyss). "If the graphene produced is only sold at 5% of the current market value, that is, 95% discount! - Clean hydrogen can be produced for free."

During steam methane conversion, high temperature steam (1292 ° F to 1832 ° F/700 ° C to 1000 ° C) is used to produce hydrogen from methane sources such as natural gas. Methane reacts with steam under the action of catalyst to generate hydrogen, carbon monoxide and carbon dioxide.

James Torr, one of the correspondence authors of the study Tour) said: "At present, the main form of hydrogen used is' gray 'hydrogen, which is produced by steam methane reforming. This method will produce a lot of carbon dioxide. In the next few decades, the demand for hydrogen may surge. Therefore, if we really want to achieve net zero emissions by 2050, we can no longer use the methods so far to produce hydrogen."

Waste plastics will exist for a long time in the environment, threatening wildlife and spreading toxins to animals and humans. In the current study, the researchers exposed the waste plastic to rapid flash joule heating for about 4 seconds. Temperature rises to 3100 At Kelvin, the hydrogen in the plastic will evaporate, leaving behind graphene, a lightweight and durable material composed of a single layer of carbon atoms. Graphene can be used in electronics, energy storage, sensors, coatings, composite materials, biomedical equipment and other fields, which is only a small part of the application of graphene.

Transmission electron microscope (TEM) images of nanoscale flash graphene sheets formed from waste plastics Kevin Wyss/Tour Lab

　　Wyss Said: "When we first discovered the flash joule heating and applied it to upgrade waste plastics to graphene, we observed that a large number of volatile gases were generated and ejected from the reactor. We wanted to know what they were. We suspected that they were mixtures of small hydrocarbons and hydrogen, but there was no instrument to study their exact composition."

With the support of the US Army Corps of Engineers, the researchers obtained the equipment needed to analyze the contents of gasification, and then found that their guess was correct: this process produced hydrogen.

Wyss said, "For example, we know that polyethylene is composed of 86% carbon and 14% hydrogen, and we have proved that we can recover up to 68% of it in the form of gas with a purity of 94% Atomic hydrogen. For me, it is a difficult but meaningful process to develop methods and expertise to characterize and quantify all gases (including hydrogen) generated by this method.

The researchers said that according to the life cycle assessment, their method produces less emissions than other hydrogen production methods. Life cycle assessment is a technology used to analyze the overall environmental impact and resource requirements related to production methods.

Compared with other methods of hydrogen production from waste plastics or biomass decomposition, the flash hydrogen production process can reduce the cumulative energy demand (33-95% energy reduction) and greenhouse gas emissions (65-89% reduction Emissions). The researchers said that one of the advantages of their flash joule heating process is that waste plastics do not need to be cleaned or separated, and waste materials can be used to produce clean hydrogen at a negative cost. They plan to further understand the flash joule heating mechanism to improve its scalability and optimize hydrogen