They recovered more than 93% of the graphene, along with the solvent and the PET substrate, and used them to print new devices.
Schematic of the closed-loop recycling system: the graphene ink is used to print electrochemical sensors, and after recycling, the recovered material is reused to print new sensors and heaters.
A commercial terpineol/ethyl cellulose graphene ink the team tested required curing above 200 °C and gave unstable electrochemical responses.
For recycling, used sensors were placed in a linalool bath with mild sonication.
The recovered graphene was used to print new sensors and Joule heaters that exceeded 70 °C at 8 V. The process also worked on a compostable cellulose-based substrate.
Researchers at Silicon Austria Labs (SAL) and EPFL have developed a recyclable, surfactant-free graphene nanoplatelet (GNP) ink that uses linalool ,a terpene alcohol that occurs naturally in lavender, basil and other aromatic plants, as its solvent and ethyl cellulose as its binder. The team printed electrochemical sensors with the ink, then took them apart after use. They recovered more than 93% of the graphene, along with the solvent and the PET substrate, and used them to print new devices. The ink went through five consecutive recycling cycles without a drop in performance.
Schematic of the closed-loop recycling system: the graphene ink is used to print electrochemical sensors, and after recycling, the recovered material is reused to print new sensors and heaters. Image from: Communications Sustainability
Most high-performance conductive inks still depend on petrochemical solvents such as NMP, DMF and toluene, several of which are classified as carcinogenic, mutagenic or reprotoxic. They also use synthetic binders that make material recovery difficult. Earlier recycling approaches for printed electronics have typically recovered only part of a device, often by using corrosive solvents that destroy the substrate, or by reclaiming the conductive ink while leaving other layers contaminated.
To choose a solvent, the team used Hansen solubility parameters alongside the GlaxoSmithKline (GSK) Solvent Sustainability Guide, which scores solvents on toxicity, safety and environmental impact. Because the GNPs used here are multilayer platelets tens to hundreds of layers thick, they behave differently from few-layer graphene. The researchers therefore refitted the solubility model specifically for them, which ruled out solvents such as cyrene and toluene. In dispersion tests, linalool outperformed terpineol, NMP and cyrene. A loading of 70 wt% GNPs gave the best balance between conductivity, printability and adhesion to the substrate.
Screen-printed three-electrode sensors made with the ink on PET cured at just 140 °C. A commercial terpineol/ethyl cellulose graphene ink the team tested required curing above 200 °C and gave unstable electrochemical responses. The linalool-based sensors showed an electrochemically active surface area of about 1.6 cm², slightly higher than a commercial carbon ink. They retained 84.2% of their initial response after 28 days of storage and detected uric acid and ascorbic acid, two physiologically relevant biomolecules.
For recycling, used sensors were placed in a linalool bath with mild sonication. The solvent swells and partially dissolves the ethyl cellulose binder, which releases the printed layers from the substrate. Both the conductive and insulating layers share the same ethyl cellulose–linalool matrix, so a single solvent handles both formulation and recovery. The graphene was then separated by filtration. Raman analysis showed its structure remained largely unchanged across five cycles. The recovered graphene was used to print new sensors and Joule heaters that exceeded 70 °C at 8 V. The process also worked on a compostable cellulose-based substrate.
A materials-level carbon footprint assessment put the fresh ink at 16.02 kg CO₂ eq. per kg. That is about 42% below the 27.55 kg CO₂ eq. per kg average of the conventional graphene inks the team compared. Recycled ink came to 6.49 kg CO₂ eq. per kg, roughly 59% below the fresh version. The assessment covers material components only and excludes energy used in production, printing and recycling. Other groups have also pursued greener graphene inks, including a water-based all-graphene ink for 3D printing and printable inks using polypropylene carbonate.
