Diffusion expends no energy, however concentration gradients are a form of potential energy, dissipated as the gradient is eliminated. In case you didn't know to dissipate means to disssappear, so diffusion will pretty much just edventually dissappear. It's like when you take a pack of gum out of your bookbag and everyone can magically smell it some time after you've put it away as if they're waiting till you get back to work to ask for a piece, instead it's diffusion. You slowly stop smelling it.
