The most abundant source of hydrogen on earth is water—every molecule of water contains one oxygen atom and two hydrogen atoms. It is relatively simple to separate the hydrogen in water from the oxygen using electricity to run an electrolyzer. An electrolyzer is a galvanic cell composed of an anode and a cathode submerged in a water-based electrolyte.
In many ways, the operation of an electrolyzer is the opposite of operating a hydrogen fuel cell. In a fuel cell, hydrogen and oxygen are supplied to the anode and the cathode, and they combine to form water while creating an electrical current that can be put to use (see Section 1.2.1 and Appendix A). In an electrolyzer, an electrical current is applied between the anode and the cathode, which causes the water in the electrolyte to break down, releasing oxygen gas at the anode and hydrogen gas at the cathode (see Figure 12).
Water and an onboard electrolyzer cannot be used to power a fuel cell or hydrogen ICE vehicle because of the large amount of electricity required to operate the electrolyzer. An electrolyzer can be used at a centralized fueling station to produce hydrogen, which is then compressed for on-site storage and delivery to vehicles. For a centralized electrolyzer, the electrical energy could be supplied from the electrical grid or from a dedicated renewable source, such as a wind turbine or solar cell array.
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