Tantalum is known to grow an oxide layer until the thickness suffices to isolate, including at 200V. It's the easiest material for that purpose. It serves to make "electrochemical" capacitors.
Other such metals are aluminium (seriously pure) and more recently niobium. Aluminium tends to serve for higher voltages, up to 1000V roughly, while tantalum serves for low or very low voltages, especially as a sintered electrode. Niobium is more affordable.
Because the oxide layer is nicely thin, and the optionally electrode sintered has much area, such components offer more capacitance in a smaller volume than plastic film do, especially at low voltages.