Voltammetry of Lanthanide Triflate Complexes in Acetonitrile: Catalysis for H 2 and O 2

Krysti Knoche Gupta; Nadeesha P.W. Rathuwadu; Kasun Saweendra Rathnatunga Dadallagei; Joshua Richard Coduto; Johna Leddy

doi:10.1149/MA2025-02612853mtgabs

Voltammetry of lanthanide triflate complexes LN(OTf) 3 has been demonstrated in acetonitrile at electrodes modified with Nafion films.[1-4] Triflate is the trifluorosulfonate OTf - ligand that chelates the lanthanide trication LN 3+ . From voltammetry, LN(OTf) 3 complexes catalyze and mediatate [4] the hydrogen evolution reaction HER [1-3] and the oxygen reduction reaction ORR [2,5,6]. In voltammograms for seven LN(OTf) 3 complexes at Nafion modified electrodes in acetonitrile, the onset potential for the solvent limit at negative potentials varies with the lanthanide.[1-3] Protons available from trace water in the acetonitrile are thought to set the onset potential for the cathodic solvent limit. Magnetic gradients about metal atoms and complexes increase electron transfer rates.[7] In the presence of LN(OTf) 3 complexes, HER onset potential correlates with the number of unpaired electron spins in the lanthanide metal; unpaired spins map the magnetic properties.. In aprotic solvents, O 2 is reduced to superoxide O 2 •- in a one electron transfer. For cyclic voltammetry of Nafion films on platinum electrodes in O 2 purged acetonitrile with 0.1 M TBABF 4 , the electron transfer is chemically reversible but the electron transfer rate is irreversible (slow) as marked by a difference in peak potential ΔEp of 1.81 V at 200 mV/s. When 1.0 mM Yb(OTf) 3 is dissolved in the electrolyte, ORR rate increases as marked by ΔEp of 1.50 V. Although still an irreversible electron transfer, the decrease of 0.3 V is lower energy by 30 kJ/mol. Crude simulation of the voltammograms finds heterogeneous electron transfer rate k 0 increased by ~20 x with Yb(OTf) 3 .[2] In aqueous electrolyte, mechanisms for ORR are more complex with up to four electrons and four protons. ORR catalysis by LN(OTf) 3 is observed in water under some experimental conditions. Cathodic voltammetric sweeps for O 2 saturated in aqueous sodium sulfate doubled the current density at glassy carbon electrodes modified with Nafion and a mixture of Yb(OTf) 3 and La(OTf) 3 as compared to Nafion with no lanthanide complex.[5] For lanthanides electrodeposited from LN(OTf) 3 at Nafion modified glassy carbon electrodes, ORR onset potential is ~ 100 mV earlier for Yb thin films as compared to Nafion modified glassy carbon, with comparable current.[6] In water, O 2 kinetics depend on the method for introducing LN catalysts to electrodes. For select Yb species, onset potential for ORR is less extreme; for select La species, currents are enhanced.[5,6] Lanthanides catalyze and mediate inner sphere, energy relevant reactions of hydrogen and oxygen. Rates vary with a given lanthanide for HER, perhaps correlated with the number of f electrons. For O 2 in aprotic acetonitrile, Yb(OTf) 3 .substantially increases the rate of ORR. In water, patterns in ORR mediation are more complex, dependent on lanthanide deposition methods. Acknowledgments: This work was supported by NSF CHE-1309366 and Iowa Energy Center (21-IEC-011) at the University of Iowa. J.L. holds the patent rights to these US Patents.[4] References [1] Krysti L. Knoche Gupta, Nadeesha P. W. Rathuwadu, and Johna Leddy, Communication---Voltammetry of Lanthanide (III) Triflates Accessible in Acetonitrile at Nafion Modified Electrodes , Journal of the Electrochemical Society (2021) 168 (6) 066511 Open Access. DOI:10.1149/1945-7111/ac0649 [2] K. L. Knoche, Ph.D., Density gradient films, lanthanide electrochemistry, and magnetic field effects on hydrogen evolution, oxygen reduction, and lanthanide electrochemistry , University of Iowa (2015). [3] N.P. W.Rathuwadu, Ph.D., Magnetic Field Effects on Electrochemical Systems, Lanthanide Electrochemistry, Thin Layer Sonoelectrochemistry, and Models for Polymer Film Characterization , University of Iowa, Iowa City, IA (2017). [4] (a) J. Leddy and K. L. Knoche, U.S. Patent, 10081873 (2018). (b) J. Leddy and K. L. Knoche, U.S. Patent, 10196749 (2019).. (c) J. Leddy and N. Rathuwadu, U.S. Patent, 10774430 (2020). (d) J. Leddy and K. L. Knoche, U.S. Patent, 11920249 (2024). [5] Kasun S.R. Dadallagei, Ph.D., Exploring Heterogeneous Rate Constants, Lanthanides, Catalysis and Baseline Correction with Graphical Methods , University of Iowa, Iowa City, IA (2023). [6] Joshua R. Coduto, Ph.D., From Catalysis to Classroom: Developing Fuel Cell Materials and Electroanalytical Tools , University of Iowa, Iowa City, IA (2023). [7] Krysti L. Knoche Gupta, Heung Chan Lee, and Johna Leddy, Magnetoelectrocatalysis: Evidence from the Hydrogen Evolution Reaction , ACS Phys. Chem Au (2024), 4 , 148-159; DOI 10.1021/acsphyschemau.3c00039

Voltammetry of Lanthanide Triflate Complexes in Acetonitrile: Catalysis for H 2 and O 2

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