Towards electrochemical hydrogen storage in liquid organic hydrogen carriers via proton-coupled electron transfers

Green hydrogen is identified as one of the prime clean energy carriers due to its high energy density and a zero emission of CO₂. A possible solution for the transport of H₂ in a safe and low-cost way is in the form of liquid organic hydrogen carriers (LOHCs). As an alternative to loading LOHC with H₂ via a two-step procedure involving preliminary electrolytic production of H₂ and subsequent chemical hydrogenation of the LOHC, we explore here the possibility of electrochemical hydrogen storage (EHS) via conversion of proton of a proton donor into a hydrogen atom involved in covalent bonds with the LOHC (R) via a proton-coupled electron transfer (PCET) reaction: 2nH⁺+2ne^-+R_ox↔nH₂⁰R_red. We chose 9-fluorenone/fluorenol (Fnone/Fnol) conversion as such a model PCET reaction. The electrochemical activation of Fnone via two sequential electron transfers was monitored with in-situ and operando spectroscopies in absence and in presence of different alcohols as proton donors of different reactivity, which enabled us to both quantify and get the mechanistic insight on PCET. The possibility of hydrogen extraction from the loaded carrier molecule was illustrated by chemical activation.