Energy Harvesting application capacitor selection
You are looking for a capacitor with a high leakage resistance. A 50uF electrolytic capacitor is inexpensive and easy to find, but has a lower leakage resistance than other types of capacitors. You haven't mentioned the required voltage. That is an important consideration.

Here is a chart of capacitor technologies vs. leakage resistance:



The words "Values of" in the caption appear to be an error. The image is from page 8 of KEMET Corporation, Introduction to Capacitor Technologies Copyright 2013

Here is a chart showing the capacitance and voltage availability for several capacitor technologies.


Image from: EPCI; ESA SPCD 2018 Symposium via https://epci.eu/capacitors-news-and-trends/