Hydrogen storage receives the worldwide attention due to its importance in sustainable energy and the solution of greenhouse effect. Adsorption provides an efficient way to compress gases and, therefore,has been applied to the development of hydrogen storage technology. However, hydrogen is a supercritical gas at the temperature of engineering interest and follows a different adsorption mechanism compared to the sub-critical gases. The present work shows why only monolayer coverage mechanism functions at above-critical temperatures and what consequences will result in the application study.Although there are pros and cons to this point of view, understanding the adsorption mechanism is, indeed, essential for the research of hydrogen storage method since it claims that any storage material based on adsorption will not satisfy the practical need of on board storage no matter how novel the material is.