Is it possible to access a list of wifi networks my iPhone has used?
1. If you have iCloud Keychain enabled :


  Are my existing saved passwords, Wi-Fi networks, and Internet accounts included in iCloud Keychain?
  Yes. When you turn on iCloud Keychain, any previously-saved website usernames and passwords, Wi-Fi networks, and Internet accounts are automatically included in iCloud Keychain.


From : 
http://support.apple.com/en-us/HT204085


You can access to Keychain by the Keychain Access.app (located in /Applications/Utilities/).


2. Otherwise you can use 1Password.

3. Finally, you have this post :

Is there any way to view saved wifi passwords on the iPhone?


  Unfortunately, on the iPhone it is not possible to access this information. The passwords are kept on your phone in a safe place that is not accessible through any regular means on the phone. This is done for your protection, if you were to lose your iPhone and someone picked it up, the passwords that are stored on it would be up for grabs. This would lead to a lot of security problems.