Electrocatalytic Oxygen Evolution with an Immobilized TAML Activator
Ethan L. Demeter, Shayna L. Hilburg, Newell R. Washburn, Terrence J. Collins and John R. Kitchin, “Electrocatalytic Oxygen Evolution with an Immobilized TAML Activator”, Journal of the American Chemical Society 136(15), 5603-5606 (2014). https://doi.org/10.1021/ja5015986
This summary was written by a language model from the paper and has not been checked line by line. Rely on the published paper for details.
Question
The oxygen evolution reaction (OER) is the demanding half of water splitting: it needs four electron transfers, carries most of the overpotential, and runs at potentials that threaten material stability. The authors argue that correlations between the energetics of the OER steps on metal oxides limit how low the overpotential can go, so new classes of catalysts are needed, and molecular catalysts, where ligands steer the reactivity of a metal center, are one route. Iron is attractive because it is abundant and used by oxidizing enzymes, but iron-based water oxidation had been shown only rarely, and only with chemical oxidants. Iron tetra-amido macrocyclic ligand complexes (TAML activators) were known to oxidize water homogeneously with ceric ammonium nitrate, but with low turnover numbers. The paper asks whether the prototype Fe(III) TAML activator can work as an electrocatalyst for the OER, and in what form (Demeter et al. 2014).
Approach
The authors first tried electrolyzing a solution containing the TAML activator and saw no electrocatalytic activity (details are in the Supporting Information). They then immobilized it: 20 mg of the complex dissolved in water was combined with 1 g of Vulcan XC-72 carbon black, sonicated, filtered, washed and dried, and the resulting powder was mixed with Nafion binder in a methanol/isopropanol solvent to make an ink. The ink was deposited on 3 mm glassy carbon disks or on 1 cm2 Toray carbon paper and dried at 120 °C. Cyclic voltammetry and constant-current electrolysis were done in 0.1 M HNO3 with a Pt wire counter electrode (in a fritted compartment so H2 could escape) and an Ag/AgCl reference. O2 was tracked in a gastight cell with a FOXY fluorescent oxygen sensor, measured as the excess above the ambient air background so that air leaks could only make the O2 yield look smaller, never larger. Headspace composition was checked by gas chromatography, and the carbon paper electrodes were examined by X-ray photoelectron spectroscopy (XPS) before and after electrolysis. Control electrodes were made the same way with FeCl3 or an iron octaethylporphyrin chloride, Fe(III)Cl(OEP), in place of the TAML complex.
Key findings
A 20 μL ink loading on glassy carbon held about 17 nmol of the TAML complex. Its voltammograms showed an Fe(III)/Fe(IV) oxidation wave at 720 mV vs Ag/AgCl, absent on bare glassy carbon and carbon black alone, followed by O2 evolution onset near 1 V, and above the onset the anodic current was higher with the complex present.
Under a constant 5 mA on 1 cm2 carbon paper loaded with 2 mg of dry ink (about 72 nmol Fe), O2 was produced throughout the experiment, reaching about 44 μmol. The apparent Faradaic efficiency was about 45%, which the authors treat as a lower bound because O2 at above-ambient partial pressure can leak from the cell. The FeCl3 and Fe(III)Cl(OEP) controls with comparable iron produced no detectable O2 by either FOXY or GC, only CO2, which links the O2 evolution to the TAML complex rather than to iron in general.
Assuming all the O2 comes from the TAML complex, the turnover frequency at 50 min, while O2 production was still linear, was estimated at 0.081 s-1, presented as an upper bound and as a large improvement over the chemically driven homogeneous system. The rate dropped slightly afterward, which the authors attribute to dilution by CO2. GC confirmed both O2 above ambient levels and CO2 as the product of the remaining current. XPS indicated where the CO2 came from: the lower binding energy carbon peak associated with carbon black or carbon paper decreased, the roughly 291 eV peak assigned to Nafion-like carbon did not change, the oxygen signal grew strongly (suggesting multistep surface carbon oxidation), and the broad iron feature at 700 to 730 eV looked the same before and after. The authors conclude that the carbon support, not the Nafion or the macrocyclic ligand, was oxidized.
Significance
This communication showed that an iron TAML activator that is inactive as a dissolved electrocatalyst becomes an active heterogeneous OER electrocatalyst once immobilized on carbon, with turnover far higher than its homogeneous, oxidant-driven predecessor. Selectivity to O2 was limited by parasitic oxidation of the carbon support, but the XPS evidence suggests the catalyst itself survived. The authors point to more oxidation-resistant, electrically conducting supports as the way forward for immobilized TAML electrodes (Demeter et al. 2014).