I think that hybrid buses have been doing this for a long time. They have big supercapacitors on the roof which buffer the rapid charge from regenerative brakin

I think that hybrid buses have been doing this for a long time. They have big supercapacitors on the roof which buffer the rapid charge from regenerative braking and use it to recharge the battery or provide energy for acceleration. The problem I've always seen with supercapacitors is that their leakage current usually seems to increase as their ESR drops. So as they gain the ability to charge/discharge quickly, they lose the ability to hold charge over time. I wonder if there are ways to mitigate that, but there are other problems like poor handling of high temperatures and mechanical vibrations / shocks; both are difficult tradeoffs for automotive parts. They are proven solutions for buffering energy in EVs, but it's harder to justify the cost/weight in light passenger vehicles. That's why hybrid solutions like this are exciting. These look cool - does anyone know if they have two or four dielectric layers? I can't quite tell from the article. IIRC Supercapacitors use four while traditional capacitors use two, and I feel like the self-discharge/ESR tradeoffs of supercapacitors might have a lot to do with the finnicky EDLC technology. If they've managed to make traditionally-structured capacitors with the Farad ratings of supercapacitors, that would be very cool.