For achieving miniaturization and coupling with microfluidics, electrochemical devices are advantageous. In particular, coulometry is an effective tool to analyze components in solutions of very small volumes. The sensitivity and detection limit of coulometry can be improved remarkably by converting an analyte into Ag on the same Pt electrode separated in two flow channels and measuring the amount of Ag using coulometry. We demonstrate that the cathodic and anodic potentials of both parts of the Pt electrode can be shifted and reactions can be accelerated by replacing the liquid junction connecting the two flow channels by a metal wire with redox active ends. This in turn improves the sensitivity and detection limit of the device. This effect can also be observed by directly applying a voltage to the solutions in the flow channels. The achieved detection limit for hydrogen peroxide (H2O2) was 2.4 nM.