Water-based highly conductive graphene inks for fully printed humidity sensors

Conductive graphene inks have attracted extensive research interests for printed electronics due to the excellent electrical and mechanical properties of graphene. While the traditional organic solvents used for graphene inks will result in high environmental cost, environmentally friendly solvents are required for conductive graphene inks. In this work, we demonstrate a highly conductive aqueous graphene ink printed on a paper substrate using screen printing technique. The well-tuned rheological properties of the graphene ink enable the printed lines to achieve a high resolution as fine as 45 µm in width. After being dried at 70 °C and roller compressed, the printed graphene lines present a high conductivity, up to 4.23 × 10⁴ S m⁻¹ at 3 µm thickness. It also shows an excellent bending stability during the roller compression process, which is compatible for large-scale manufacturing. Combining with inkjet-printed graphene oxide as the sensing layer, the graphene lines are exploited as the electrodes for a fully printed humidity sensor on the paper substrate. Such a low drying temperature, high printing resolution and low-cost printing method offers an example that two-dimensional materials are cooperatively utilized for fully printed flexible electronic devices. (paper)