Backup transaction log from secondary database in alwayson 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