TDE cannot encrypt database
You cannot create a password-protected-certificate, then use that certificate to encrypt a database via TDE since the SQL Server will not be able to open the database.

From Microsoft Docs:


  Transparent Data Encryption (TDE) must use a symmetric key called the database encryption key which is protected by either a certificate protected by the database master key of the master database, or by an asymmetric key stored in an EKM. 


Steps involved in encrypting a database using TDE:


Create a master key
Create or obtain a certificate protected by the master key
Create a database encryption key and protect it by the certificate
Set the database to use encryption 


An example of the above, taken from the Microsoft Docs page on TDE:

USE master;  
GO  
CREATE MASTER KEY ENCRYPTION BY PASSWORD = '&lt;UseStrongPasswordHere&gt;';  
go  
CREATE CERTIFICATE MyServerCert WITH SUBJECT = 'My DEK Certificate';  
go  
USE AdventureWorks2012;  
GO  
CREATE DATABASE ENCRYPTION KEY  
WITH ALGORITHM = AES_128  
ENCRYPTION BY SERVER CERTIFICATE MyServerCert;  
GO  
ALTER DATABASE AdventureWorks2012  
SET ENCRYPTION ON;  
GO  


That page also says this:


  TDE certificates must be encrypted by the database master key to be accepted by the following statements. If they are encrypted by password only, the statements will reject them as encryptors. 


    CREATE DATABASE ENCRYPTION KEY
    ALTER DATABASE ENCRYPTION KEY
    DROP DATABASE ENCRYPTION KEY



  Altering the certificates to be password-protected after they are used by TDE will cause the database to become inaccessible after a restart.