Silver ions have high affinity with chloride ions of KCl, forming precipitation of AgCl (s). Perhaps phosphate buffer would be more appropriate.Article Size Dependant Electrochemical Behavior of Silver Nanopartic...

The surface area (Ag-particle size) and the reactions (you have several unidentified peaks) of the impurities can increase the total current of the cell.

Silver nanoparticles ink can be used as material for printing working electrodes. The only problem with working electrodes made of silver ink is the limited potential range that can be used for electrochemical measurements. In too negative potentials, in the presence of Cl ions, the silver ions precipitates in AgCl form over the working electrode surface blocking the desired redox reactions. One solution is the use of other buffer, as acetate or phosphate. In your experiment, with ferri/ferro probe your working range is too large, maybe causing this undesired phenomenon.

In a recent work (Silver Inkjet-Printed Electrode on Paper for Electrochemical Sensing of Paraquat - 10.3390/chemosensors9040061) we also developed a fully-inkjet printed silver electrode and used it in KCl media for some studies, and in Na2SO4 buffer for paraquat sensing.

Thank you Dagwin Wachholz Junior and Teemu Tuovinen for your answers. I will try Potassium nitrate as buffer for fully Ag sensors and I try print another sensors with Carbon WE. If these atempts fail, I will try phosphate buffer. Again thank you both for help.