Biomimetic Nucleotide-Graphene Hybrids for Electrocatalytic Oxygen Conversion— Quantifying Biomolecule Mass Loading

The development of new technologies is required to promote the transition from fossil fuels to sustainable and environmentally friendly resources. In this context, metal-air batteries are being studied as an alternative to resolving future energy demands and environmental issues due to their high theoretical energy density. However, the chemical conversion based on the Oxygen Reduction and Evolution Reactions (ORR and OER) that take place in the cathode is limited by sluggish kinetics and high overpotentials. Currently, the most widely used catalysts are noble metals such as iridium, platinum and ruthenium; however, they have several drawbacks such as high cost and low availability. Moreover, the discovery of materials capable of catalyzing both ORR and OER reactions (bifunctional) is crucial for improving the efficiency in energy generation and storage technologies.<br/><br/>Nucleotides are metal-free, small and natural molecules composed of a pentose sugar, a nitrogenous heterocyclic nucleobase and a phosphate group. They act as enzyme cofactors in the mitochondrial electron transfer chain by undergoing reversible oxidation/reduction reactions. Hence, nucleotides could be used as catalysts in the metal-air cathodes to mimic efficient biological processes and overcome the kinetic challenges currently encountered in these devices. But as these biomolecules are not conductive, it is necessary to support them on a conductive surface to be used as battery electrodes. In this work,¹ biomimetic electrocatalysts are prepared by liquid adsorption of aromatic nucleotides by π-π interaction in the large and conductive surface area of electrochemically exfoliated graphene.<br/><br/>The determination of catalyst loading and dispersion is relevant when assessing the catalytic activity of catalysts. However, quantifying the loading of non-conventional electrocatalysts is still an underexplored topic in literature. Different characterization techniques have been therefore combined herein for estimation of both the loading and dispersion. In addition, Rotating Disk Electrode (RDE) has been used to analyze the catalytic activity of bare graphene and a riboflavin mononucleotide (FMN)/graphene hybrid. Even though both studied materials presented similar activity towards ORR, the hybrid material achieved higher turnover frequency (TOF, 1.50x10^-2) and lower Tafel slope (151 mV dec^-1) than graphene, which displayed values of 1.81x10^-4 and 295 mV dec^-1, respectively. Thus, the presence of the nucleotide seems to improve the catalytic activity for OER. In addition, both the catalyst loading and the catalytic activity results have been corroborated by Density Functional Theory calculations. In summary, the methodology here developed will pave the way for the discovery of new bioinspired electrocatalysts by 1) proposing sustainable preparation of biomimetic electrocatalysts and 2) filling the existing gap in the characterization of metal-free organic catalysts.<br/><br/>References: ¹A. Letona-Elizburu, M. Enterría, A. Aziz, S. Villar-Rodil, J. I. Paredes, J. Carrasco and N. Ortiz-Vitoriano, Sustainable Materials and Technologies, 2024, 39, e00835.<br/><br/>Acknowledgments: This work was funded by the R&D&I project PID2020–117626RAI00, funded by MCIU/AEI/10.13039/ 501100011033. N. Ortiz-Vitoriano thanks Ramon y Cajal grant (RYC- 2020-030104-I) funded by MCIN/AEI/10.13039/501100011033 and by FSE invest in your future. The authors also thank SGI/IZO-SGIker UPV/EHU for providing supercomputing resources. Partial funding by the Spanish Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (MCIN/AEI/10.13039/501100011033) as well as the European Regional Development Fund (ERDF, A way of making Europe) through grant PID2021-125246OB-I00, and by Plan de Ciencia, Tecnología e Innovación (PCTI)2018-2022 del Principado de Asturias and the ERDF through grant IDI/2021/000037.