Highly efficient electrocatalytic hydrogen evolution from neutral aqueous solution by water soluble copper (II) porphyrin

Substituent Effects in Iron Porphyrin Catalysts for the Hydrogen Evolution Reaction

For a future hydrogen economy, non-precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. Metal-nitrogen-carbon (MNC) catalysts with active sites constituting a metal center with fourfold coordination of nitrogen (MN₄ ) show promising performance, but an optimization rooted in structure-property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well-defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphyrin substituents on the electronic and electrocatalytic properties of MN₄ centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [Fe^II (P⋅)]^- radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications.

Expanding the Horizon of Bio-Inspired Catalyst Design with Tactical Incorporation of Drug Molecules

The development of potent H₂ production catalysts is a key aspect in our journey toward the establishment of a sustainable carbon-neutral power infrastructure. Hydrogenase enzymes provide the blueprint for designing efficient catalysts by the rational combination of central metal core and protein scaffold-based outer coordination sphere (OCS). Traditionally, a biomimetic catalyst is crafted by including natural amino acids as OCS features around a synthetic metal motif to functionally imitate the metalloenzyme activity. Here, we have pursued an unconventional approach and implanted two distinct drug molecules (isoniazid and nicotine hydrazide) at the axial position of a cobalt core to create a new genre of synthetic catalysts. The resultant cobalt complexes are active for both electrocatalytic and photocatalytic H₂ production in near-neutral water, where they significantly enhance the catalytic performance of the unfunctionalized parent cobalt complex. The drug molecules showcased a dual effect as they influence the catalytic HER by improving the surrounding proton relay along and exerting subtle electronic effects. The isoniazid-ligated catalyst C1 outperformed the nicotine hydrazide-bound complex C2, as it produced H₂ from water (pH 6.0) at a rate of 3960 s^-1 while exhibiting Faradaic efficiency of about 90 %. This strategy opens up newer avenues of bio-inspired catalyst design beyond amino acid-based OCS features.