As you brake, or even lift off the accelerator pedal in some electric vehicles, kinetic energy from the car's motion is captured by the electric motor, which when not being driven turns into a generator. This power is then stored in the batteries.
The effectiveness of this is limited by how quickly the battery can store generated energy - not enough to warrant the absence of friction brakes. A new supercapacitor could be about to change this.
Unlike batteries, which store energy chemically, capacitors store energy in an electric field. They can store this energy - and discharge it - very quickly.
A new flexible, solid-state supercapacitor, writes Physorg, could be used to store energy much quicker than a battery could, allowing the electric motor, or electromagnetic brakes, to entirely supplant friction brakes. That means far less kinetic energy is wasted as heat, improving efficiency.
There are plenty of potential benefits. Not only is the system more efficient in terms of energy use, but for drivers, brake feel would improve - there'd no longer be the feeling of two separate braking forces doing their thing.
Potentially, there's a weight saving too. Friction brakes and their associated components are relatively heavy, and much of that weight is rotating with the wheels. Engineers call it unsprung mass, and the less unsprung mass, the better. Handling improves, ride quality improves, performance improves.
The supercapacitor being explored by researchers at the University of Minnesota and University of North Texas uses carbon nanotube-coated cotton paper and is completely solid-state. That means it isn't as bulky as current supercapacitors, which use a hazardous liquid electrolyte that needs protecting.
It's thin, light, and flexible, which would allow it to be used anywhere in the car, with zero impact on packaging.
The design is still in early stages - contrary to one of the main benefits of capacitors, they've found the new supercapacitor to have high resistance and charge slowly, but this is a feature the teams think can be improved upon.
It may be some time before all our braking is done without traditional friction brakes (at the very least, friction brakes are still useful as a backup system) but it could soon be supercapacitors, rather than batteries, that are used to recover braking energy in electric cars.