(Invited) Controlling Hydrogen Evolution and Oxygen Reduction Electrocatalysis By Tuning Interfacial Hydrogen Bonds

Abstract

Designing electrochemical water splitting and its reverse process is crucial for fuel cells to achieve high efficiency. Conventional design of catalysts has focused on tuning the surface electronic structures and binding strength of intermediates, while recent studies show that changing electrolyte compositions, such as cations and pH [1], can also significantly alter catalytic activity, highlighting new opportunities in tuning noncovalent interactions in the electrolytes to control activities.In the first part of this talk, we use an interfacial layer of protic ionic liquids on platinum and gold to tune the oxygen-reduction reaction (ORR) kinetics, where altering the proton activity of ionic liquids increases the intrinsic ORR activity by up to five times [2]. The maximum enhancement of kinetics is achieved when the pKa of the ionic liquid is comparable to that of the reaction intermediate, which is attributed to the most strengthened hydrogen bonding between the ionic liquid and ORR products, as supported by surface-enhanced infrared absorption spectroscopy (SEIRAS). In the second part, we confine water in an organic matrix and tune the hydrogen-bonding networks as well as hydrogen evolution and oxidation reactions (HER/HOR) kinetics by changing the water concentration (1% - 50% molar ratio) and altering the physical properties (donor number) of organic solvents. Decreasing the water-to-organic ratio, the OH stretching frequency of water shifts to higher wavenumbers, indicating more isolated water, while the water reduction has more negative onset potentials. The shifts in onset potentials are solvent-dependent, highlighting the role of interfacial hydrogen bonds between solvents and water in controlling HER/HOR kinetics. SEIRAS measurements provide further support to the changes in interfacial hydrogen bonding during the reactions.These findings open up immense opportunities for using noncovalent hydrogen bonding interactions at the electrified interface to control the kinetics of ORR, HER, and beyond. The understanding would also be impactful across other reactions crucial to improving decarbonizing efforts in energy storage, such as CO2 reduction and aqueous batteries.