Backup transaction log from secondary database in alwayson AG Is it possible to do the log backup from the secondary databases in the AG?
You can take log backup on any nodes (Primary/Secondary) irrespective of its availability mode. This is what MSDN has to say about this in Active Secondaries: Backup on Secondary Replicas (AlwaysOn Availability Groups):


  
  BACKUP DATABASE supports only copy-only full backups of databases, files, or filegroups when it is executed on secondary replicas. Note that copy-only backups do not impact the log chain or clear the differential bitmap.
  Differential backups are not supported on secondary replicas.
  BACKUP LOG supports only regular log backups (the COPY_ONLY option is not supported for log backups on secondary replicas).
  A consistent log chain is ensured across log backups taken on any of the replicas (primary or secondary), irrespective of their availability mode (synchronous-commit or asynchronous-commit).
  To back up a secondary database, a secondary replica must be able to communicate with the primary replica and must be SYNCHRONIZED or SYNCHRONIZING. 
  


The link below begins a good series on backups, which also dives into different scenarios like information stored in MSDB tables for backups across several nodes:

SQL 2012 AlwaysOn and Backups – Part 1 – Offloading the work to a replica by Lisa Gardner