The research group claims the rust is converted directly into a compact layer with a grid structure that can store potassium ions. A coating of reduced graphite oxide increases the conductivity and stability during charge/discharge cycles.
"Potassium ions are just as inexpensive and readily available as sodium, and potassium ion batteries would be superior from the electric aspect," said researcher Xin-Bo Zhang. "However, the significantly larger radius of the potassium ions has posed a problem. Repeated storage and release of these ions destabilises the materials currently used in electrodes."
The team found that rejected stainless steel mesh from filters and sieves were the best solution. Despite the durability of these grids, harsh conditions do lead to some corrosion. "Their conversion into electrodes could develop into a more ecologically and economically sensible form of recycling," added Zhang.
The corroded mesh is dipped into a solution of potassium ferrocyanide. This dissolves iron, chromium, and nickel ions out of the rust layer. These ions combine with ferricyanide ions in Prussian blue – a dark blue pigment that is deposited onto the surface of the mesh as nanocubes. Potassium ions can easily and rapidly be stored in and released from these structures.
The researchers then use a dip-coating process to deposit a layer of graphene oxide. Subsequent reduction converts the graphene oxide to reduced graphene oxide (RGO), which consists of layers of graphite with isolated oxygen atoms.
"The RGO coating inhibits clumping and detachment of the active material. At the same time, it significantly increases the conductivity and opens ultrafast electron-transport pathways," Zhang explained.
Because the binder-free electrodes are very flexible, they could be highly suitable for use in flexible electronic devices.