A case study using redox mediators to overcome challenges of H2 as terminal reductant in scalable electrosynthesis

Abstract: Hydrogen is often viewed as an ideal terminal reductant for electrosynthesis because it is abundant, atom-economical, and avoids the waste associated with sacrificial metal reductants. In practice, however, direct anodic H2 oxidation is difficult to pair with reductive electrosynthesis in organic media, owing to both the kinetic limitations of non-aqueous HOR and the complications that arise from proton and water transport in conventional hydrogen-anode formats. This presentation will describe a quinone-mediated hydrogen anode that couples off-electrode Pd/C-catalyzed hydrogenation of an anthraquinone mediator with anodic oxidation of the corresponding hydroquinone, enabling nickel-catalyzed cross-electrophile coupling under non-aqueous conditions. The fundamental advantages of mediated electrosynthesis for this application will be described, followed by analytical assays to identify the capabilities and limitations of this anodic system. Particular emphasis will be placed on the application of an external voltage to supplement the intrinsic reductive potential of H2 and enable more challenging reductions. Operando UV–vis analysis of mediator speciation and polarization curves of the fully constituted cell will be analyzed to identify an operating regime that supports productive cathodic chemistry. Attention will also be given to proton management in the recirculating anode, where protons are retained, neutralized, and exchanged for Li+, suppressing proton crossover and allowing H2 to function as an effectively proton-free source of electrons.