What is an electron reservoir?

Electron reservoirs as you say can store electrons, but they can accept and release electrons in a reversible fashion too.

Electron reservoirs and molecular wires are basic motifs of single-electron devices. Both can accept and release electrons in a reversible fashion, but the former can also store electrons.

From: K. Venkatesan, O. Blacque and H. Berke, "Metallacumulenes as Potential Electron Reservoir Devices", Organometallics 2006, 25, 22, 5190–5200.