Capacitor for spot welding
Electrolytics would work well. A 300,000 µF 35 VDC electrolytic with screw terminals could store ~200 J [1/2 CV2], which might be enough to weld a small nickel tab, and a scooter starter solenoid could probably handle that current.

Supercapacitors tend to have higher internal series resistance than a corresponding electrolytic. For example, this 1.6 F, 16 VDC supercap bank could theoretically store ~200 J, but on connecting to the low-impedance spot welder, most of the energy is lost heating the capacitors. Perhaps two or three of these banks might suffice. Another issue with supercaps is that an equalizing network is needed, and during heavy discharge, it might not be sufficient to prevent a weak capacitor from being reverse-charged beyond 2.75 V.