It is simple. Longer the chains higher the similarity between them. In other words the OH group are getting "diluted".
It is more difficult for the donor to find the accepting counterpart, and the number density of the hydrogen bonds, both for volume and mass, gets lower.
The interaction between molecules becomes dominated by VdW forces.
To visualize this ...
Interesting question. These are my candidates, roughly in order from most practical to least practical. Some of these are poisonous, smell very bad, or are just plain undesirable. Environmental issues are not considered herein, but certainly would be in a real world potential application. For details on the chemical substances, see their wiki entries.
Yes, ammonia has been cited.
Per a source, 'Ammonia as a Hydrogen Source for Fuel Cells: A Review', to quote:
Like hydrogen, ammonia is carbon free and can be produced from any energy resource. However there are also some significant advantages in terms of storage and transport.