Nano-confined electrochemical reaction studied by electrochemical surface forces apparatus

Electrochemical reactions in a nano-space are different from those in bulk solutions due to structuring of the liquid molecules and peculiar ion behavior at the electric double layer and are important for applications involving sensors and energy devices. The electrochemical surface forces apparatus (EC-SFA) we developed enabled us to study the electrochemical reactions in a solution nano-confined between the electrodes with varying distance (D) at nm resolution. We recorded measurements of the current-distance profiles due to the electrochemical reaction of the redox couples in the electrolyte nano-confined between Pt electrodes using our EC-SFA. We observed a long-range feedback current due to redox cycling and the sudden current increase at a short distance, the latter for the first time. This sudden current increase was two orders greater than the conventional feedback current and was observed at D < 5 nm when the electrodes were approaching and D < 200 nm on separation. We simultaneously measured the electric double layer force and the current between the electrodes in the solution to study the mechanisms of this sudden current increase in the short distance range. The results revealed a molecular insight as to how the redox species affect the current between two electrodes under nano-confinement. This study demonstrated that EC-SFA is a powerful tool for obtaining fundamental knowledge about the nano-confined electrochemical reactions for nanoelectrodes which can be applied to sensors and energy devices.