"Providing a path for electrons to flow"
In electrochemical reactions electrons are not transferred directly between atoms, ions, or molecules, but via the electronically-conducting circuit (a wire).
Electrochemical cell & Electrolytic cell
An electrochemical cell is a device that generates electrical energy (typically in an external electrical circuit) from chemical reactions. The reverse also works: electrical energy can be applied to these cells to cause chemical reactions to occur.
Electrochemical cells which generate an electric current are called voltaic cell (due to Italian physicist Alessandro Volta) or galvanic cell (due to another Italian physicist Luigi Galvani), while those that generate chemical reactions, via electrolysis, are called electrolytic cells. Both (voltaic or galvanic and electrolytic cells) have two half-cells consisting of separate oxidation & reduction reactions.
Figure 1 shows the diagram of a Li-ion battery (LiB) cell: the anode is typically made of graphite, while the cathode is typically a compound of Lithium and other metals (forming Li-metal oxides), such as nickel, cobalt, or manganese.
Fuel cell (FC)
FC are different from batteries in requiring a continuous source of fuel & oxygen (usually obtained from the air - about 21% of the atmosphere is oxygen) to sustain the chemical reaction. In a battery, the chemical energy comes from chemicals already present in the battery.
FCs operate on a wide range of fuels and contain no moving parts, making them simple to operate, quiet, and reliable. With the proper reforming tech, FCs can use hydrogen from a wide variety of currently available fossil fuels, e.g., natural gas, methanol, ethanol, and coal.
Fuel cells remained in obscurity until 1960 when NASA began looking for a practical power source for extended missions to space [FCS, 2023]. In recent years, they are being driven by the wave of the energy transition.
Figure 1: Diagram of a typical Li-ion battery cell