D42.5 Fuel Cells

Suppose a voltaic cell is constructed such that the substance that is oxidized at the anode and the substance that is reduced at the cathode (the reactants in the overall redox reaction) are both supplied continuously. Such a battery would never run down because reactant concentrations or partial pressures would never decrease. Such a device is a fuel cell, which produces electricity as long as fuel is available. Hydrogen fuel cells have been used to supply power for satellites, space capsules, automobiles, boats, and submarines.

A diagram is shown of a hydrogen fuel cell. At the center is a narrow vertical rectangle which is shaded tan and labeled “Electrolyte.” To the right is a slightly wider and shorter green rectangle which is labeled “Cathode.” To the left is a pale blue rectangle of the same size which is labeled “Anode.” White rectangles, each with an inlet at the top and an outlet at the bottom are at the right and left sides, attached to the green and blue rectangles. On the right side O subscript 2 enters at the top, moves inward and along the interface with the green region, and exits to the lower right. H subscript 2 O also exits at the lower right. Pale gray diatomic O subscript 2 molecules move through this region and at the bottom are converted to a single pale gray atom with two smaller bright red atoms, H 2 O molecules. A similar pathway is on the left, allowing entry of H subscript 2 from the upper left along the interface with the blue rectangle, allowing for the exit of H subscript 2 out to the lower left of the diagram. Small red diatomic H subscript 2 molecules ar shown in this region. Black line segments extend upward from the blue anode and purple cathode regions. These line segments are connected by a horizontal segment that has a yellow light-bulb shape at the center. H subscript 2 molecules travel into the anode where they lose electrons and become single, small, red circles labeled H superscript plus. H superscript plus ions are shown traveling though the electrolyte to the green cathode where they interact with O subscript 2 molecules to form water molecules. The electrolyte is a special membrane permeable to H superscript plus but not to electrons.
Figure: Fuel cell. In this schematic of a hydrogen-oxygen proton-exchange fuel cell, oxygen from the air reacts with hydrogen, producing water and electricity.

In a hydrogen-oxygen proton-exchange fuel cell, the cell potential is about 1 V, and the reactions involved are:

Oxidation (anode): 2 × [H2(g) ⟶ 2 H+(aq) + 2 e–]
Reduction (cathode): O2(g) + 4 H+(aq) + 4 e– ⟶ 2 H2O(ℓ)
overall: O2(g) + 2 H2(g) ⟶ 2 H2O(ℓ)

The efficiency of fuel cells is typically about 40-60%, which is higher than the typical internal combustion engine (25-35%). Moreover, in the case of the hydrogen fuel cell, nearly pure water is produced as exhaust. Currently, fuel cells are comparably more expensive and contain features that may cause a higher failure rate.

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Chemistry 109 Fall 2021 Copyright © by John Moore; Jia Zhou; and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.