Abstract: Climate change mitigation is one of the main global challenges in the 21st century. In this context, the recent 26th United Nations Climate Change Conference of the Parties (COP26-Glasgow) claimed for searching urgent and efficient measures to reduce and ultimately avoid CO2 emissions. Thus, many efforts from the scientific community focus on the research of new and renewable energy sources (RES). Among other approaches, green hydrogen, which comes from water electrolysis, is a promising candidate to be considered in the energy panorama. However, commercial electrolyzers are provided with Pt/C and Ir-based electrocatalytic materials, which are expensive and not abundant, to catalyze the Hydrogen Evolution Reaction (HER) in safe, stable, inexpensive, and environmentaly friendly conditions. Thus, this work aims to synthesize high-performance and very low metal loading catalysts by immobilizing a Rh-based organometallic complex (RhCp*Cl(phendiamine)]Cl) on a carbon black support following a robust synthesis procedure. Advanced characterization of the synthesized materials confirmed that ultra-low metal loadings in the range of 3.2-4.7 mg·g-1 were successfully reached. Subsequently, Rh-based catalysts were tested in a PEM electrolyzer. For metal loadings as low as 0.0066 mg·cm-2 competitive cell potentials of 1.9V were achieved working with 0.5A·cm-2 geometric current density at 70°C. These results are comparable to those obtained with Pt-based commercial cathodes working under similar operation conditions. Thus, the results of this research make a step forward in the substitution of conventional cathodes for the electrolytic HER by new materials with very low metal loadings.

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