Backup SSRS Encryption Keys Using Powershell
Using WMI to backup RS key might not be so efficient. You may try the below script that I found at http://powershell-tips.blogspot.com/2015/05/backup-report-server-encryption-key.html

#------------------------------------------------------------------------------------------
#     Script : Backup_RSKey.ps1 
#     Author : Som DT.
#    Purpose : Backup Report Server Encryption Key with Powershell
#------------------------------------------------------------------------------------------
#     Syntax : Powershell .\Backup_RSKey.ps1 -InstanceName &lt;InstanceName&gt; -PassWord &lt;Password&gt; -BackupLocation &lt;BackupLocation&gt; 
#------------------------------------------------------------------------------------------
#    Example : Powershell .\Backup_RSKey.ps1 -InstanceName "MSSQLSERVER" -PassWord "rsdal@2015" -BackupLocation "C:\temp"
#------------------------------------------------------------------------------------------

param ( 
    [string]$InstanceName="MSSQLSERVER", 
    [string]$PassWord="rsdal@2015", 
    [string]$BackupLocation="C:\temps" 
)

cls
$FileTimestamp=$(get-date).tostring("yyyyMMddHHmmss")


function Backup-RSKey([string]$InstanceName , [string]$PassWord, [string]$BackupLocation)
{
    Trap {
        echo  "Failure : $($Error[0].Exception.Message)" 
        return 
    }

    $KeyFile="${BackupLocation}\KEY_$InstanceName_$FileTimestamp.snk" 

    echo "Backing up to file : $KeyFile" 

    $Error.Clear() 

    Out-String -InputObject( echo "Y" | RSKeyMgmt.exe -e -f"$KeyFile" -p"$PasWord" -i"$InstanceName")  

    if (!(Test-Path "$KeyFile") )
    {
        return "Failure : Error occured while backing up [$KeyFile]."
    }
    else 
    {
        return "Success"
    }

}


$Output=Backup-RSKey "$InstanceName"  "$PassWord"   "$BackupLocation"

echo $Output

#--------------------------------------------------------------------------------------------------------------
#-- End of Program
#--------------------------------------------------------------------------------------------------------------