The mistake is 0,5 mM = 0,5 millimol/l what means 25 ml contain 0,5*10^-3 mol/l *0,025 l =0,0000125 mol = 12,5 μmol

Instead you used 0,5 mM = 0,5 mmol containing in 25 ml what gives wrong result of = 0,02 mol/l

Using c = I *t / nF gives

c= 14 *10^-6 A*3,4*60s/(2*96485 As/ mol) = 0,0148 μmol Cd

Percentage = 0,0148 μmol/12,5μmol*100% = 0,118%~ 0,12 %