Charging it on the work computer would be cheaper and greener than buying a power bank, but computers charge very slowly; it might not even charge as fast as your phone consumes energy. Also, you obviously won't be able to use your phone for more than 3 hours outside of work or somewhere else with a charging port without a power bank, so it depends on whether you'd want to do that. I wouldn't get a whole new phone; you already have a better one, and buying a power bank will be cheaper than even a cheap phone and will allow you to use it for as long as you want.