Catalyst Development for Water/CO2 Co-electrolysis.

Herein, we discuss recent research activities on the electrochemical water/CO2 co-electrolysis at the Department of Chemistry and Biochemistry of the University of Bern (Arenz and Broekmann research groups). For the electrochemical conversion of the greenhouse gas CO2 into products of higher value catalysts for two half-cell reactions need to be developed, i.e. catalysts for the reductive conversion of CO2 (CO2RR) as well as catalysts for the oxidative splitting of water (OER: Oxygen Evolution Reaction). In research, the catalysts are often investigated independently of each other as they can later easily be combined in a technical electrolysis cell. CO2RR catalysts consist of abundant materials such as copper and silver and thus mainly the product selectivity of the respective catalyst is in focus of the investigation. In contrast to that, OER catalysts (in acidic conditions) mainly consist of precious metals, e.g. Ir, and therefore the minimization of the catalytic current per gram Ir is of fundamental importance.

Humidity-dependent lubrication of highly loaded contacts by graphite and a structural transition to turbostratic carbon.

Graphite represents a promising material for solid lubrication of highly loaded tribological contacts under extreme environmental conditions. At low loads, graphite's lubricity depends on humidity. The adsorption model explains this by molecular water films on graphite leading to defect passivation and easy sliding of counter bodies. To explore the humidity dependence and validate the adsorption model for high loads, a commercial graphite solid lubricant is studied using microtribometry. Even at 1 GPa contact pressure, a high and low friction regime is observed - depending on humidity. Transmission electron microscopy reveals transformation of the polycrystalline graphite lubricant into turbostratic carbon after high and even after low load (50 MPa) sliding. Quantum molecular dynamics simulations relate high friction and wear to cold welding and shear-induced formation of turbostratic carbon, while low friction originates in molecular water films on surfaces. In this work, a generalized adsorption model including turbostratic carbon formation is suggested.