If you don't have a contract or your contract doesn't specifically mention IP you should not tell your employer about what you are doing. But approach your HR and explain the situation that you are thinking of developing an app which is unrelated to the company's lines of business and that you will be doing this entirely for your own education and in your own time using your own computer etc. Put in those terms they should not have a problem with it. Make sure you get it in writing to avoid future misunderstandings / contradictions. Then after a suitable delay, you could release your app.
As other people have pointed out, you could release your app under a different company - but then you have a similar situation of technically working for two employers. Again, that shouldn't be a problem unless there is a potential for conflict of interest.
As other people have pointed out, you could release your app under a different company - but then you have a similar situation of technically working for two employers. Again, that shouldn't be a problem unless there is a potential for conflict of interest.