Reversible operation of fuel cell

[Est120]

TL;DR Summary

fuel cell under the scope of chemical thermodynamics; conditions for reversible operation

Can someone explain me what does the underlined part of the following passage mean?

"The reversible operation of a fuel cell implies that the external circuit exactly balances its emf, with the result that its current output is negligible. In actual operation under reasonable load, internal irreversibilities inevitably reduce the emf of the cell and decrease its production of electrical work while increasing the amount of heat transfer to the surroundings."​

I know what a reversible (thermodynamically speaking) path means, but i my knowledge in electronic devices is limited.

 

[hutchphd]

TL;DR Summary: fuel cell under the scope of chemical thermodynamics; conditions for reversible operation

I know what a reversible (thermodynamically speaking) path means, but i my knowledge in electronic devices is limited.

This is essentially the argument about quasi-static processes. In order to have appreciable current flow the internal processes will need finite currents and finite voltages and disequalibrium.
In what context does one worry about reversibility of a fuel cell?

 

[DaveE]

I know nearly nothing about fuel cell thermodynamics. However, they operate like a battery. Through some mystical (to me) alchemy they generate a voltage potential by separating electrons from ions and send electrons out through an external circuit to return to balance the ions left behind in the cell. The voltage they can make is finite, of course, and determined by the chemistry.

So imagine a circuit with two batteries connected in parallel (+ to +, - to - terminals). Current will flow from one to (through) the other if it's voltage is greater, it will flow the other direction if it's voltage is lower. So, they are describing a situation where the voltage from the fuel cell is exactly the same (balanced) with the voltage imposed from an external circuit. In this case no current will flow. Current always flows from a higher potential to a lower potential. No voltage difference means no current flow.