Controlled Catalytic Energy Release of the Norbornadiene/Quadricyclane Molecular Solar Thermal Energy Storage System on Ni(111)

We have investigated the surface chemistry of the molecular solar thermal energy storage system of the valence isomer pair norbornadiene (NBD)/quadricyclane (QC) on Ni(111). Our multimethod approach includes UV-photoelectron spectroscopy (UPS), high-resolution X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS), and density functional theory (DFT) calculations. The NBD/QC system holds the potential to be utilized in future energy storage technologies due to its comparably high gravimetric energy storage density, and the release of energy in a catalytic and sustainable cycle. UPS shows molecular adsorption of both compounds at 120 K, as is also predicted by DFT. NEXAFS and DFT suggest an adsorption geometry of NBD with both double bonds binding to the surface (eta(2):eta(2)). For QC, no preference is found, and both the eta(2):eta(2) and the eta(2):eta(1) adsorption geometry are stable. The conversion of QC to NBD is thermally activated. From UPS, a reaction temperature of similar to 175 K is determined. Possible detrimental decomposition reactions of NBD were investigated by XPS. At 190 K, benzene (C6H6) and methylidyne (CH) are formed, and further react to C-H fragments at 330 K and finally leave carbide on the surface above 475 K.