Check your employment contract. If you weren't paying close attention, you may have signed something that said anything work-related you develop while working for the company, be it on the clock or off the clock, belongs to the company as a "work for hire". If you did, you don't really have a choice; either turn it over to them, or sit on it until after you've left the company for a certain period (depends on your state law and your contract) and then sell it/ release under GPL, whatever.
Me, I always look for those 'work for hire' clauses in my contract and strike through any reference to them owning my off-the-clock work. My employer has no claim on my off-the-clock time or effort, as far as I'm concerned.
Now, you probably should introduce the application to work and then use it as one of your arguments for a pay raise/promotion--"I go above and beyond requirements and make things work better around here, for example the Fleegleborg app for HR... ", add it to your resume, and if they don't want to negotiate better pay/perks, etc, look for another job. There are a lot of companies that appreciate employees that don't just "do the job", but make permanent improvements.