Daniel Friebel

Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting

Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni(1-x)Fe(x)OOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe(3+) in Ni(1-x)Fe(x)OOH occupies octahedral sites with unusually short Fe-O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni(1-x)Fe(x)OOH are not active sites for the oxidation of water.

Mass-selected nanoparticles of PtxY as model catalysts for oxygen electroreduction

Low-temperature fuel cells are limited by the oxygen reduction reaction, and their widespread implementation in automotive vehicles is hindered by the cost of platinum, currently the best-known catalyst for reducing oxygen in terms of both activity and stability. One solution is to decrease the amount of platinum required, for example by alloying, but without detrimentally affecting its properties. The alloy PtxY is known to be active and stable, but its synthesis in nanoparticulate form has proved challenging, which limits its further study. Herein we demonstrate the synthesis, characterization and catalyst testing of model PtxY nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mgPt(-1) at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of PtxY over elemental platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core.

In Situ Observation of Surface Species on Iridium Oxide Nanoparticles during the Oxygen Evolution Reaction

An iridium oxide nanoparticle electrocatalyst under oxygen evolution reaction conditions was probed in situ by ambient-pressure X-ray photoelectron spectroscopy. Under OER conditions, iridium undergoes a change in oxidation state from Ir(IV) to Ir(V) that takes place predominantly at the surface of the catalyst. The chemical change in iridium is coupled to a decrease in surface hydroxide, providing experimental evidence which strongly suggests that the oxygen evolution reaction on iridium oxide occurs through an OOH-mediated deprotonation mechanism.

The Electronic States of Rhenium Bipyridyl Electrocatalysts for CO2 Reduction as Revealed by X‐ray Absorption Spectroscopy and Computational Quantum Chemistry

Industrial processes and fossil fuel combustion produce carbon dioxide (CO2) unsustainably on the gigaton scale. Addressing this pressing issue has led to rapidly growing efforts to catalytically reduce CO2 to liquid fuels. [1] Recycling CO2 is a profoundly challenging problem that requires fundamental insights to guide advancements. Information regarding CO2 transformations abound, [1,2] but no industrialscale process has capably reduced CO2 to liquid fuels. Of the systems that electrocatalytically reduce CO2, the [Re(bpy)(CO)3Cl] family of compounds (bpy= 2,2’-bipyridine) is one of the most robust and well-characterized systems known to date. This system converts CO2 into carbon monoxide (CO) with high rates and efficiencies; it suffers, however, from large overpotentials that are believed to arise from accessing the highly reduced, formally Re I state in [Re(bpy)(CO)3] . This state has long been proposed as the active state of the electrocatalyst. Apart from this assumption, there is little known about the electronic structure of the catalyst in its reduced (active) state and its subsequent interaction with CO2. We recently reported stopped-flow kinetics studies showing the relative selectivities of the [Re(bpy-tBu)(CO)3] anion reacting with with CO2 and proton sources. These studies revealed that reaction rates of the anion were about 35 times faster with CO2 than with weak acid. [3b] The bpy ligand was proposed to play a non-innocent role by storing charge and preventing a doubly occupied dz2 orbital at the Re center, which would be needed to form a metal hydride. Indeed, Xray diffraction (XRD) studies of both [Re(bpy)(CO)3] and [Re(bpy-tBu)(CO)3] show the bpy ligands exhibit bond length alternation and short Cpy Cpy bonds (1.370(15) , for bpy-tBu), indicating significant electron density on these ligands. The short inter-ring bonds suggest a doubly reduced bpy ligand, more representative of a Re(bpy ) state rather than a Re(bpy ) or Re (bpy) state. The redox activities of bpy ligands as well as other non-innocent ligands have been extensively studied. To fully confirm that the non-innocence of bpy contributes to this unique catalysis, we employed experimental spectroscopy and theoretical quantum chemistry to characterize this catalyst family. We compared the halide starting materials, [Re(bpy)(CO)3Cl] (1) and [Re(bpy-tBu)(CO)3Cl] (2), the one-electron reduced dimer [{Re(bpy)(CO)3}2] (3), the twoelectron reduced anions [K([18]crown-6)][Re(bpy)(CO)3] (4) and [K([18]crown-6)][Re(bpy-tBu)(CO)3] (5), the commercially available standards, [Re(CO)5Cl] (6) and [Re2(CO)10] (7), and a synthesized Re I standard, [K([18]crown-6)] [Re(CO)5] (8). IR spectroscopy of the stretching frequencies of the carbonyl ligands characterizes the electronic states of these complexes. X-ray absorption spectroscopy (XAS) at the Re L3 absorption edge using the strong “white-line” resonance arising from 2p!5d transitions probes the Re5d unoccupied states. Kohn–Sham density functional theory (KS-DFT) calculations provide a first-principles description of electronic structures. Lastly, extended X-ray absorption fine structure (EXAFS) studies of frozen THF solutions of 1, 2, 4, and 5 confirm the monomeric nature of the catalysts and rule out solvent coordination to the Re centers in solution. Compounds 1–5 were prepared according to literature procedures. [K([18]crown-6)][Re(CO)5] (8) was prepared by the reduction of [Re2(CO)10] (7) in tetrahydrofuran (THF) by excess KC8 (potassium intercalated graphite) in the presence of [18]crown-6 (see the Supporting Information). The IR stretching frequencies of complexes 1–7 have been reported previously; however, we obtained frequencies for complexes 1–7 and the newly synthesized complex 8 under the same conditions for fair comparison (Table 1). The oneelectron reduction of the formally Re chloride species 2 results in formation of the one-electron reduced monomer, [*] Dr. E. E. Benson, M. D. Sampson, Dr. K. A. Grice, Dr. J. M. Smieja, J. D. Froehlich, Prof. Dr. C. P. Kubiak Department of Chemistry and Biochemistry, University of California, San Diego 9500 Gilman Drive,Code 0358, La Jolla, CA 92093-0358 (USA) E-mail: ckubiak@ucsd.edu

On the chemical state of Co oxide electrocatalysts during alkaline water splitting

Resonant inelastic X-ray scattering and high-resolution X-ray absorption spectroscopy were used to identify the chemical state of a Co electrocatalyst in situ during the oxygen evolution reaction. After anodic electrodeposition onto Au(111) from a Co(2+)-containing electrolyte, the chemical environment of Co can be identified to be almost identical to CoOOH. With increasing potentials, a subtle increase of the Co oxidation state is observed, indicating a non-stoichiometric composition of the working OER catalyst containing a small fraction of Co(4+) sites. In order to confirm this interpretation, we used density functional theory with a Hubbard-U correction approach to compute X-ray absorption spectra of model compounds, which agree well with the experimental spectra. In situ monitoring of catalyst local structure and bonding is essential in the development of structure-activity relationships that can guide the discovery of efficient and earth abundant water splitting catalysts.

Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction

Copper electrocatalysts derived from an oxide have shown extraordinary electrochemical properties for the carbon dioxide reduction reaction (CO2RR). Using in situ ambient pressure X-ray photoelectron spectroscopy and quasi in situ electron energy-loss spectroscopy in a transmission electron microscope, we show that there is a substantial amount of residual oxygen in nanostructured, oxide-derived copper electrocatalysts but no residual copper oxide. On the basis of these findings in combination with density functional theory simulations, we propose that residual subsurface oxygen changes the electronic structure of the catalyst and creates sites with higher carbon monoxide binding energy. If such sites are stable under the strongly reducing conditions found in CO2RR, these findings would explain the high efficiencies of oxide-derived copper in reducing carbon dioxide to multicarbon compounds such as ethylene.

Formation of Copper Catalysts for CO2 Reduction with High Ethylene/Methane Product Ratio Investigated with In Situ X-ray Absorption Spectroscopy

Nanostructured copper cathodes are among the most efficient and selective catalysts to date for making multicarbon products from the electrochemical carbon dioxide reduction reaction (CO2RR). We report an in situ X-ray absorption spectroscopy investigation of the formation of a copper nanocube CO2RR catalyst with high activity that highly favors ethylene over methane production. The results show that the precursor for the copper nanocube formation is copper(I)-oxide, not copper(I)-chloride as previously assumed. A second route to an electrochemically similar material via a copper(II)-carbonate/hydroxide is also reported. This study highlights the importance of using oxidized copper precursors for constructing selective CO2 reduction catalysts and shows the precursor oxidation state does not affect the electrocatalyst selectivity toward ethylene formation.

In situ x-ray probing reveals the importance of surface platinum oxide formation in fuel cell catalysis

In situ X-ray absorption spectroscopy (XAS) at the Pt L3 edge is a useful probe for Pt–O interactions at polymer electrolyte membrane fuel cell (PEMFC) cathodes. We show that XAS using the high ene ...In situ X-ray absorption spectroscopy (XAS) at the Pt L(3) edge is a useful probe for Pt-O interactions at polymer electrolyte membrane fuel cell (PEMFC) cathodes. We show that XAS using the high energy resolution fluorescence detection (HERFD) mode, applied to a well-defined monolayer Pt/Rh(111) sample where the bulk penetrating hard X-rays probe only surface Pt atoms, provides a unique sensitivity to structure and chemical bonding at the Pt-electrolyte interface. Ab initio multiple-scattering calculations using the FEFF code and complementary extended X-ray absorption fine structure (EXAFS) results indicate that the commonly observed large increase of the white-line at high electrochemical potentials on PEMFC cathodes originates from platinum oxide formation, whereas previously proposed chemisorbed oxygen-containing species merely give rise to subtle spectral changes.

Importance of Surface IrOx in Stabilizing RuO2 for Oxygen Evolution

The high precious metal loading and high overpotential of the oxygen evolution reaction (OER) prevents the widespread utilization of polymer electrolyte membrane (PEM) water electrolyzers. Herein we explore the OER activity and stability in acidic electrolyte of a combined IrOx/RuO2 system consisting of RuO2 thin films with submonolayer (1, 2, and 4 Å) amounts of IrOx deposited on top. Operando extended X-ray absorption fine structure (EXAFS) on the Ir L-3 edge revealed a rutile type IrO2 structure with some Ir sites occupied by Ru, IrOx being at the surface of the RuO2 thin film. We monitor corrosion on IrOx/RuO2 thin films by combining electrochemical quartz crystal microbalance (EQCM) with inductively coupled mass spectrometry (ICP-MS). We elucidate the importance of submonolayer surface IrOx in minimizing Ru dissolution. Our work shows that we can tune the surface properties of active OER catalysts, such as RuO2, aiming to achieve higher electrocatalytic stability in PEM electrolyzers.

Operando XAS Study of the Surface Oxidation State on a Monolayer IrOx on RuOx and Ru Oxide Based Nanoparticles for Oxygen Evolution in Acidic Media

Herein we present surface sensitive operando XAS L-edge measurements on IrOx/RuO2 thin films as well as mass-selected RuOx and Ru nanoparticles. We observed shifts of the white line XAS peak toward higher energies with applied electrochemical potential. Apart from the case of the metallic Ru nanoparticles, the observed potential dependencies were purely core-level shifts caused by a change in oxidation state, which indicates no structural changes. These findings can be explained by different binding energies of oxygenated species on the surface of IrOx and RuOx. Simulated XAS spectra show that the average Ir oxidation state change is strongly affected by the coverage of atomic O. The observed shifts in oxidation state suggest that the surface has a high coverage of O at potentials just below the potential where oxygen evolution is exergonic in free energy. This observation is consistent with the notion that the metal-oxygen bond is stronger than ideal.