Lanthanide Voltammetry: Electrocatalysis and Electrochemistry in Acetonitrile

Abstract

An electrochemical method has been developed to allow voltammetric interrogation of lanthanides in common organic, electrochemical solvents [1, 2]. Lanthanide electrochemistry was enabled in acetonitrile at an ion exchange polymer (Nafion) modified platinum electrode with triflate ligands. Under these conditions, formal potentials of the lanthanides are shifted into the potential window of acetonitrile and voltammetry is enabled. Here, voltammetry of lanthanides on the benchtop and evidence of electrocatalysis of reactions important in energy and environment are presented. For several lanthanide triflate complexes, the voltammetry is reviewed. For five lanthanides, two reductions are observed under cyclic voltammetric conditions. For each lanthanide triflate complex, LN(OTf)3 where LN is a lanthanide, two well resolved reduction waves are observed. The first is for LN3+ to LN2+. The second wave about a volt negative of the first is for reduction of the LN(II) triflate. Under cyclic voltammetric conditions, the reverse sweep to positive potentials is less well resolved. The mechanistic path for the LN(OTf)3 is considered. Electrochemical reactions of small molecule, ubiquitous species play important roles in energy and the environment. This includes the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), carbon dioxide reduction, and the chemistry of other organic species that contain one carbon, such as methanol, formic acid, formaldehyde, and carbon monoxide. Many of these reactions are kinetically limited. ORR limits performance of many fuel cell and air batteries. Increased rates of HER facilitate energy generation. The sequence of single carbon species both maps the thermodynamic advantages of organic electrochemical fuels and identifies the dominant kinetic limitations of their exploitation in practical, low temperature electrochemical energy systems. Carbon dioxide is the highest oxidation state of carbon and so the focus of study for reduction to regenerate fuels and value added products as well as to remediate greenhouse gases. Voltammetry undertaken in the presence of some of these species will be presented. For example, there is clear cyclic voltammetric evidence of increased rates of oxygen reduction in acetonitrile when lanthanide triflates are present in solution and the Nafion film on the electrode.