Scientists repurpose old solar panels to convert CO₂ exhaust into valuable chemicals

Centuries ago, alchemists worked furiously to convert the common metal lead to valuable gold. Today, chemists are repurposing discarded solar panels to create valuable organic compounds from carbon dioxide (CO₂), a common greenhouse gas.

Significantly reducing greenhouse gases in the atmosphere to mitigate the most devastating effects of climate change will require a large reduction in emissions as well as strategies designed to sequester emitted CO₂ and other offending gases. While simply sequestering greenhouse gases would fulfill this goal, creating useful organic chemicals from waste CO₂ is akin to generating valuable materials from trash.

A team of chemists from Yokohama National University, Electric Power Development Co., Ltd. and the Renewable Energy Research Center at the National Institute of Advanced Industrial Science and Technology (AIST) recently decided to tackle two waste problems—excess CO₂ emissions and decommissioned solar panels—in the pursuit of creating value-added organic chemicals. The team designed a study to determine if recycled components of discarded solar panels could be used to efficiently convert CO₂ into useful, carbon-based compounds.

The researchers published their study in the July 14 issue of the journal ACS Sustainable Resource Management.

“In this study, we combined the recycling of waste silicon wafers from end-of-life solar panels with the conversion of CO₂ in the exhaust gas from a thermal power plant. The waste silicon wafer acts as a reducing agent of CO₂ to organic compounds,” said Ken Motokura, professor in the Department of Chemistry and Life Science at YOKOHAMA National University in Yokohama, Japan and first author of the research paper.

The silicon wafers in solar panels can be separated from discarded panels in the panel recycling process. Importantly, the silicon wafers are effective at donating electrons to carbon compounds, including CO₂, which can be used to create larger, more valuable chemicals. While the reaction to create value-added organic chemicals from CO₂ and metallic silicon, which is present in silicon wafers of solar panels, is energetically favorable, few studies have examined the effectiveness of the reaction.

The team took crushed and milled silicon wafers from discarded solar panels and added a chemical catalyst to speed the production of organic compounds from waste CO₂. Initially, the research team had variable success creating formic acid, an organic chemical, from the milled silicon wafers. The researchers were able to overcome this limitation by pretreating the milled silicon wafers with hydrochloric acid (HCl), which removed aluminum (Al) from the surface of the wafer powder and increased the yield of the reaction.

“We directly converted CO₂ in the exhaust gas, which contained 14% CO₂ by volume, from a thermal power plant into formic acid and formamide through a reaction with waste silicon powder, water, and tetrabutylammonium fluoride, a catalyst. No separation and purification of CO₂ from the exhaust gas is necessary. The contaminated Al in the waste silicon powder decreases the reaction rate, and appropriate pretreatment with HCl enables enhanced reactivity of the waste silicon,” said Motokura.

Ultimately, the reaction produced formic acid at high yields, reaching as high as 73%. Formamide, another value-added organic chemical, was also generated using the silicon powder in the presence of amines, which are organic chemicals containing nitrogen atom. The researchers were also successful in directly connecting their silicon-wafer reactor to the exhaust gas port of a thermal power plant, demonstrating the practicality of generating valuable formic acid from waste silicon wafers and emitted CO₂.

The International Renewable Energy Agency (IRENA) estimates that 60–78 MT of global photovoltaic (PV) panels will reach end of life by 2050. The research team hopes that this study will spur additional research into ways that recycled materials, like silicon wafers, can be used to sequester and convert waste and greenhouse gases into other useful and valuable compounds, turning society’s waste into something more akin to treasure.