The decrease in catechol (CT) peak current during DPV, even at fixed concentration, is mainly due to the effect of hydroquinone (HQ) on the electrode surface. HQ and its oxidation products strongly adsorb to the glassy carbon electrode (you didn't share what type of electrode you used), blocking active sites needed for CT oxidation. In addition, HQ oxidation products can form passivating films, which foul the surface and reduce CT’s signal.

There is also chemical interaction between the two species. Oxidized HQ can react with CT intermediates, altering CT’s effective electrochemical response. Since the oxidation potentials of HQ and CT are close, peak overlap and baseline distortion further make CT’s current appear to decrease.

To fix this issue, the electrode surface should be refreshed frequently to minimize fouling. Adjusting electrolyte pH can improve peak separation, and using protective films (e.g., Nafion, graphene, β-cyclodextrin) can reduce interference. Optimizing DPV parameters or applying standard addition methods can also help achieve more reliable CT measurements in HQ-containing mixtures. I hope this will be helpful for you.

CT and HQ can hydrogen bond too or make the semiquinone, thus reducing its concentration as free CT.