In principle, any reducing agent that gasses out would match your requirement. If you use a carbon-containing agent that forms CO2 as a product, you wouldn't leave ions behind, but that's not applicable in all scenarios since the material you want to reduce must, electrochemically, match that.
My material is very sensitive to ionc background and electrochemical scenario gonna give electroconductivity by ions. And hydrolysis on the electrode surface would give a trouble to the material due to theprotonation or deprotonation
Are boron-containing reducing agents an option in the solvent you use? Of course they are also quite aggressive, so if your material is that sensitive, that might also cause issues, but at least you could avoid ions.