Power/Performance Bits: June 20

Batteries from scrap metal
Scientists at the Chinese Academy of Sciences and Jilin University found a method to transform rusty stainless steel mesh into electrodes with outstanding electrochemical properties that make them ideal for potassium-ion batteries.

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.

The corroded mesh is dipped into a solution of potassium ferrocyanide. This dissolves iron, chromium, and nickel ions out of the rust layer. These combine with ferricyanide ions into the complex salt known as Prussian blue, a dark blue pigment that is deposited onto the surface of the mesh as scaffold-like 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 (oxidized graphite layers). This layer nestles tightly onto the nanocubes. Subsequent reduction converts the graphene oxide to reduced graphene oxide (RGO), which consists of layers of graphite with isolated oxygen atoms. According to Xin-Bo Zhang of the Chinese Academy of Sciences, “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.”

In tests, coin cells made with these new electrodes demonstrate excellent capacity, discharge voltages, rate capability, and outstanding cycle stability. Because the inexpensive, binder-free electrodes are very flexible, they would be suitable for use in flexible electronic devices.

Battery-free pacemaker
Researchers from Rice University and the Texas Heart Institute introduced a wireless, battery-less pacemaker that can be implanted directly into a patient’s heart.

The pacemaker harvests energy wirelessly from radio frequency radiation transmitted by an external battery pack. In the prototype, the wireless power transmitter can be up to few centimeters away.

Pacemakers use electrical signals to prompt the heart to keep a steady beat, but they’ve traditionally not been implanted directly into a patient’s heart. Instead, they’re located away from the heart, where surgeons can periodically replace their onboard batteries with minor surgery; their electrical signals are transmitted to the heart via wire leads, which can cause complications.

The chip at the system’s heart is less than 4 millimeters wide and incorporates the receiving antenna, an AC-to-DC rectifier, a power management unit and a pacing activation signal. A capacitor and switch join the chip on a circuit board that is smaller than a dime. The chip receives power using microwaves in the 8 to 10 gigahertz electromagnetic frequency spectrum.

The internal components of a battery-free pacemaker. The pacemaker can be inserted into the heart and powered by a battery pack outside the body, eliminating the need for wire leads and surgeries to occasionally replace the battery. (Source: Rice Integrated Systems and Circuits)

The frequency of the pacing signals produced by the pacemaker can be adjusted by increasing or decreasing power transmitted to the receiving antenna, which stores it until it reaches a predetermined threshold. At that point, it releases the electrical charge to the heart and begins to fill again.

The team successfully tested the device in a pig and demonstrated it could tune the animal’s heart rate from 100 to 172 beats per minute.