You really need to use copper or stainless steel for the electrodes. If your library has access I suggest you look at this design as a way to build a really cheap DPF. It has a finite lifetime, but should be a good start. The budget is in the tens of dollars.
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6528785&url=http://ieeexplore.ieee.org/iel7/4915264/6522997/06528785.pdf?arnumber=6528785
