Yes, we have. 4/5ths of their body is made up of 3 separate organs that generate electricity. They’re comprised of electrolytes, specialized cells that can generate 0.15v each. They have thousands of these lined up to facilitate a flow of ions and stacked. They add a potential difference. When the eel feels like ruining some fishes day. It sends a signal to the organs to open the ion channels which allow the sodium ions to pass through the electrocyte stacks.
This momentarily reverses the polarity of the stacks, which generates an electric current. Anything from cause it to twitch (allowing the eel to locate it) or fully paralyze it (allowing the eel to catch and eat it). The eel controls how much power it dumps into the charge depending on the situation. Would it be more energy efficient to use such Frankenstein generators? Assuming we could feed the cells grass or thrown away food.