(528e) Probing the Mechanism of Water Reduction in the Context of Thermodynamically Non-Ideal Blended Electrolytes

Electrochemical reactions involving water are becoming increasingly useful in the context of organic transformations. For example, water represents an abundant and benign oxygen-atom source in the context of electro-organic synthesis. Similarly, the hydrogen evolution reaction (HER) is a simple yet productive counter-reaction that is frequently paired with organic oxidation reactions in order to provide charge balance to the system. While hydrogen evolution in neutral or basic media (a.k.a. water reduction) has been characterized widely in fully aqueous electrolytes, there have been few attempts to study the reaction in a blended aqueous/nonaqueous electrolyte of the sort that is required for organic substrate solubility. In this work we report the behavior of the hydrogen evolution reaction in neutral-to-basic electrolytes consisting of acetonitrile and water – a common solvent system for many organic electro-oxidation reactions. We focus in particular on the non-idealities introduced by the unique aqueous/nonaqueous electrolyte solution thermodynamics, which must be quantified in order to gain an understanding of the reaction mechanism. Through the lens of molecular simulations to complement our experiments, we discuss how the molecular picture of water and the nature of its interactions with solution components contributes to water’s reactivity in this type of electrolyte. In addition, we provide some practical considerations for conducting water reduction in the blended aqueous/nonaqueous electrolyte, such as organic poisoning of common HER catalysts, which may contribute to inefficiencies – ultimately leading to the selection of a different catalyst material from that which would typically be selected in an analogous fully aqueous system. Furthermore, because the findings regarding water’s behavior in the blended electrolyte environment are applicable to a wide variety of chemistries involving hydrogen and oxygen atoms, we also extend our analysis of the electrolyte to draw conclusions about the mechanisms of hydrogen-atom and oxygen-atom transfer reactions in blended electrolytes.