In a recent release of Nintex, we added always on functionality, but you must also update your connection string when turning on this feature. 

Below are the details on the update, as well as a short "How To" for the connection string. 

Product version: 3.1.10.0

Database version: 3.1.3.0

Release date: 2 November 2016

Enhancements

• Allow additional connection string parameters for Nintex Content Databases e.g configure SQL Server 2012 as AlwaysOn

Below is the Power Shell script below you will need to use to update your parameters for the Connection String used for the Nintex Workflow Configuration Database.

Step 1.  Please navigate to your Central Administration> Nintex Workflow Management> Database Setup.

Step 2. Retrieve the Connection String for the Configuration Database.

Paste string into script within Power Shell script. Add the additional parameters you would like. Only update the section within the double quotes, leave all other aspects of this script alone (including the 2007 database information, even on a different year of SharePoint, it will still work exactly as we would expect).

Once you have added the parameters you wish, run the script.

After the script has completed, perform an IIS Reset, restart the Timer Service and once completed you should be good to go.

Script-

$farm = Get-SPFarm;

$farm.properties$"NW2007ConfigurationDataBase"];

$farm.properties$"NW2007ConfigurationDataBase"] = “####NEW CONNECTION STRING GOES HERE#####”;

$farm.Update();