Yan Wang1 2 Daniel Frisbie2

1, Chemistry, University of Minnesota, Minneapolis, Minnesota, United States
2, Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States

Electrochemical processes at electrode/electrolyte interfaces (e.g. electric double layer charging, heterogeneous charge transfer and surface binding of reaction species on the electrode) are of vital importance to energy conversion and storage systems including batteries, supercapacitors and electrocatalytic production of fuels. It has been widely acknowledged that the kinetics of the interfacial electrochemical processes are largely determined by the electronic structure (e.g. density of states and electronic occupation) at the electrode/electrolyte interface. We have developed a back-gated electrode structure that utilizes electrostatic charging (induced by a gate bias) to control electrochemical kinetics on ultrathin or 2D materials (5-nm-thick ZnO, monolayer MoS2 and graphene).1,2 Such back-gated electrodes are fabricated with nanometer-thick semiconductors on SiO2/degenerate Si substrates, analogous to the metal–oxide–semiconductor stack in the CMOS technology. Due to the extreme thinness of the electrode materials, the alignment of electronic bands as well as the electronic occupation, at the electrode/electrolyte interface, can be dramatically altered by the gate-induced charge carriers. Thus, real-time, continuous and efficient modulation of reaction kinetics can be achieved on 2D materials by varying the gate bias.
In this presentation, we will use back-gated monolayer MoS2 as an example to demonstrate how the applied gate bias affects the kinetics of heterogeneous charge transfer and surface binding processes. Specifically, the standard charge transfer rate constant between MoS2 and ferrocene/ferrocenium redox couple can be tuned by over two orders of magnitude and the catalytic overpotential of hydrogen evolution reaction on 2H-MoS2 can be reduced by more than 150 mV. Overall, the approach introduced here is generally applicable to investigation and optimization of interfacial electrochemical phenomena in a wide range of electrochemical systems. With the ability to control the band alignment and electronic occupation independent of the electrode potential, the back-gated 2D electrodes will provide new insights to rational design of electrode materials.

(1) Kim, C.-H.; Frisbie, C. D. Field Effect Modulation of Outer-Sphere Electrochemistry at Back-Gated, Ultrathin ZnO Electrodes. J. Am. Chem. Soc. 2016, 138 (23), 7220–7223.
(2) Wang, Y.; Kim, C.-H.; Yoo, Y.; Johns, J. E.; Frisbie, C. D. Field Effect Modulation of Heterogeneous Charge Transfer Kinetics at Back-Gated Two-Dimensional MoS2 Electrodes. Nano Lett. 2017, 17 (12), 7586–7592.