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In electrochemical reactions, oxidation & reduction occur at separate electrodes.  Electrons released at the anode travel thru an external electronically conducting circuit to the cathode, generating electric current.

  

Electrochemical cell & Electrolytic cell

 

An electrochemical cell is a device that generates electrical energy (in an external circuit) from chemical reactions.  The reverse also works: electrical energy can be applied to these cells to drive chem reactions.  

  

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/galvanic & electrolytic cells) have two half-cells consisting of separate oxidation & reduction reactions.

  

Figure 1 shows the diagram of a Li-ion (LiB) cell: the anode is typically made of graphite, while the cathode is 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 (H2) & oxygen (usually obtained from the air - about 21% of the atmosphere is oxygen) to sustain the chemical reaction.  In contrast, batteries store all the necessary chemical reactants internally.

  

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.