Cobaloximes are promising, earth-abundant catalysts for the light-driven hydrogen evolution reaction (HER). Typically, these cobalt(III) complexes are prepared in situ or employed in their neutral form, for example, [Co(dmgH) 2 (py) Cl], even though related complex salts have been reported previously and could, in principle, offer improved catalytic activity as well as more efficient immobilization on solid support. Herein, we report an interdisciplinary investigation into complex salts [Co(dmgH) 2 (pyWe describe their strategic syntheses from the commercially available complex [Co(dmgH) 2 (py)Cl] and demonstrate that these double and single complex salts are potent catalysts for the light-driven HER. We also show that scanning electrochemical cell microscopy can be used to deposit arrays of catalysts [Co(dmgH) 2 (py py)Cl] on supported and free-standing amino-terminated ~1-nm-thick carbon nanomembranes (CNMs). Photocatalytic H 2 evolution at such arrays was quantified with Pd microsensors by scanning electrochemical microscopy, thus providing a new approach for catalytic evaluation and opening up novel routes for the creation and analysis of "designer catalyst arrays", nanoprinted in a desired pattern on a solid support.

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The setup for operando measurement of hydrogen evolution under illumination is shown in the cover feature picture. Various cobaloxime catalysts were deposited onto solid support (carbon nanomembranes) and evaluated with respect to their hydrogen evolution reaction by using Pd microsensors. The structure of the most efficient cobalt‐based double salt complex is depicted in the foreground. More information can be found in the Full Paper by M. von Delius, C. Kranz, et al. (DOI: 10.1002/chem.202102778). Photo credit: Elvira Eberhardt, Ulm University.