What is the anodic reaction of aluminum in water
Aluminum cannot be used easily as support for a hydrogen electrode, because it is always covered by a thin, continuous and insulating layer of aluminum oxide. As this layer is an electrical insulator, its resistance is high, and ions like $\ce{H+}$ have some difficulty in crossing it. So the measured potential of hydrogen measured on such a protected electrode is not a good measurement of the redox hydrogen potential. It corresponds to an important overpotential, which is to be avoided if you want to measure or determine a redox potential.