Katharina Klingan

Water Oxidation by Electrodeposited Cobalt Oxides—Role of Anions and Redox‐Inert Cations in Structure and Function of the Amorphous Catalyst

For the production of nonfossil fuels, water oxidation by inexpensive cobalt-based catalysts is of high interest. Films for the electrocatalysis of water oxidation were obtained by oxidative self-assembly (electrodeposition) from aqueous solutions containing, apart from Co, either K, Li or Ca with either a phosphate, acetate or chloride anion. X-ray absorption spectroscopy (XAS) at the Co K-edge revealed clusters of edge-sharing CoO(6) octahedra in all films, but the size or structural disorder of the Co-oxido clusters differed. Whereas potassium binding is largely unspecific, CaCo(3) O(4) cubanes, which resemble the CaMn(3) O(4) cubane of the biological catalyst in oxygenic photosynthesis, may form, as suggested by XAS at the Ca K-edge. Cyclic voltammograms in a potassium phosphate buffer at pH 7 revealed that no specific combination of anions and redox-inactive cations is required for catalytic water oxidation. However, the anion type modulates not only the size (or order) of the Co-oxido clusters, but also electrodeposition rates, redox potentials, the capacity for oxidative charging, and catalytic currents. On these grounds, structure-activity relations are discussed.

Water Oxidation by Amorphous Cobalt-Based Oxides: Volume Activity and Proton Transfer to Electrolyte Bases

Water oxidation in the neutral pH regime catalyzed by amorphous transition-metal oxides is of high interest in energy science. Crucial determinants of electrocatalytic activity were investigated for a cobalt-based oxide film electrodeposited at various thicknesses on inert electrodes. For water oxidation at low current densities, the turnover frequency (TOF) per cobalt ion of the bulk material stayed fully constant for variation of the thickness of the oxide film by a factor of 100 (from about 15 nm to 1.5 μm). Thickness variation changed neither the nanostructure of the outer film surface nor the atomic structure of the oxide catalyst significantly. These findings imply catalytic activity of the bulk hydrated oxide material. Nonclassical dependence on pH was observed. For buffered electrolytes with pKa values of the buffer base ranging from 4.7 (acetate) to 10.3 (hydrogen carbonate), the catalytic activity reflected the protonation state of the buffer base in the electrolyte solution directly and not the intrinsic catalytic properties of the oxide itself. It is proposed that catalysis of water oxidation occurs within the bulk hydrated oxide film at the margins of cobalt oxide fragments of molecular dimensions. At high current densities, the availability of a proton-accepting base at the catalyst-electrolyte interface controls the rate of water oxidation. The reported findings may be of general relevance for water oxidation catalyzed at moderate pH by amorphous transition-metal oxides.

Electronic and molecular structures of the active-site H-cluster in [FeFe]-hydrogenase determined by site-selective X-ray spectroscopy and quantum chemical calculations

The [FeFe]-hydrogenase (HydA1) from green algae is the minimal enzyme for efficient biological hydrogen (H2) production. Its active-site six-iron center (H-cluster) consists of a cubane, [4Fe4S]H, cysteine-linked to a diiron site, [2Fe]H. We utilized the spin-polarization of the iron Kβ X-ray fluorescence emission to perform site-selective X-ray absorption experiments for spectral discrimination of the two sub-complexes. For the H-cluster in reduced HydA1 protein, XANES and EXAFS spectra, Kβ emission lines (3p → 1s transitions), and core-to-valence (pre-edge) absorption (1s → 3d) and valence-to-core (Kβ2,5) emission (3d → 1s) spectra were obtained, individually for [4Fe4S]H and [2Fe]H. Iron–ligand bond lengths and intermetal distances in [2Fe]H and [4Fe4S]H were resolved, as well as fine structure in the high-spin iron containing cubane. Density functional theory calculations reproduced the X-ray spectral features and assigned the molecular orbital configurations, emphasizing the asymmetric d-level degeneracy of the proximal (Fep) and distal (Fed) low-spin irons in [2Fe]H in the non-paramagnetic state. This yielded a specific model structure of the H-cluster with a bridging carbon monoxide ligand and an apical open coordination site at Fed in [2Fe]H. The small HOMO–LUMO gap (∼0.3 eV) enables oxidation and reduction of the active site at similar potentials for reversible H2 turnover by HydA1, the LUMO spread over [4Fe4S]H supports its role as an electron transfer relay, and Fed carrying the HOMO is prepared for transient hydride binding. These features and the accessibility of Fed from the bulk phase can account for regio-specific redox transitions as well as H2-formation and O2-inhibition at the H-cluster. We provide a conceptual and experimental framework for site-selective studies on catalytic mechanisms in inhomogeneous materials.

Electrosynthesis of Biomimetic Manganese-Calcium Oxides for Water Oxidation Catalysis--Atomic Structure and Functionality.

Water-oxidizing calcium-manganese oxides, which mimic the inorganic core of the biological catalyst, were synthesized and structurally characterized by X-ray absorption spectroscopy at the manganese and calcium K edges. The amorphous, birnesite-type oxides are obtained through a simple protocol that involves electrodeposition followed by active-site creation through annealing at moderate temperatures. Calcium ions are inessential, but tune the electrocatalytic properties. For increasing calcium/manganese molar ratios, both Tafel slopes and exchange current densities decrease gradually, resulting in optimal catalytic performance at calcium/manganese molar ratios of close to 10 %. Tracking UV/Vis absorption changes during electrochemical operation suggests that inactive oxides reach their highest, all-Mn(IV) oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of Mn(III) ions at catalytic potentials is identified as a prerequisite for catalytic activity.

Spectroscopic identification of active sites for the oxygen evolution reaction on iron-cobalt oxides

The emergence of disordered metal oxides as electrocatalysts for the oxygen evolution reaction and reports of amorphization of crystalline materials during electrocatalysis reveal a need for robust structural models for this class of materials. Here we apply a combination of low-temperature X-ray absorption spectroscopy and time-resolved in situ X-ray absorption spectroelectrochemistry to analyze the structure and electrochemical properties of a series of disordered iron-cobalt oxides. We identify a composition-dependent distribution of di-μ-oxo bridged cobalt–cobalt, di-μ-oxo bridged cobalt–iron and corner-sharing cobalt structural motifs in the composition series. Comparison of the structural model with (spectro)electrochemical data reveals relationships across the composition series that enable unprecedented assignment of voltammetric redox processes to specific structural motifs. We confirm that oxygen evolution occurs at two distinct reaction sites, di-μ-oxo bridged cobalt–cobalt and di-μ-oxo bridged iron–cobalt sites, and identify direct and indirect modes-of-action for iron ions in the mixed-metal compositions.Optimization of electrocatalysts requires an understanding of all active reaction sites. Here, the authors combine X-ray absorption spectroscopy and electrochemistry to identify cobalt atoms with different coordination geometries and probe their contribution to electrocatalytic water oxidation.

Nickel-iron catalysts for electrochemical water oxidation – redox synergism investigated by in situ X-ray spectroscopy with millisecond time resolution

In future technological systems for chemical storage of renewable energy and production of non-fossil fuels, NiFe oxyhydroxides are prime candidates for efficient alkaline water oxidation (oxygen evolution reaction, OER). The synergistic effect of Ni and Fe is well documented but still insufficiently understood. Fluorescence-detected X-ray absorption spectroscopy at the K-edges of Ni and Fe provided structural information on the non-catalytic (reduced) and catalytic (oxidized) state of the NiFe catalyst. Time-resolved detection of X-ray signals during (i) cyclic voltammetry and (ii) in response to potential steps revealed that the Ni(2+)/Ni(3+) redox transition is directly coupled to modification of the Fe ligand environment. We propose that the lattice-geometry modification of the Ni(Fe) oxyhydroxide that results from Ni oxidation enforces changes in the ligand environment of the Fe ions. The Fe sites do not undergo a distinctive redox transition, but are “enslaved” by the oxidation state changes of the Ni ions.

Geometric distortions in nickel (oxy)hydroxide electrocatalysts by redox inactive iron ions

The dramatic change in electrochemical behavior of nickel (oxy)hydroxide films upon incorporation of Fe ions provides an opportunity to establish effective electrocatalyst design principles. We characterize a photochemically deposited series of Fe–Ni (oxy)hydroxides by X-ray absorption spectroscopy and track the voltage- and composition-dependence of structural motifs. We observe a trigonal distortion in di-μ-hydroxo bridged NiII–NiII motifs that is preserved following a symmetric contraction of Ni–O bond lengths when oxidized to di-μ-oxo NiIV–NiIV. Incorporation of Fe ions into the structure generates di-μ-hydroxo NiII–FeIII motifs in which Ni–Fe distances are dependent on nickel oxidation state, but Fe–O bond lengths are not. This asymmetry minimizes the trigonal distortion in di-μ-hydroxo NiII–FeIII motifs and neighboring di-μ-hydroxo NiII–NiII sites in the reduced state, but exacerbates it in the oxidized state. We attribute both the Fe-induced anodic shift in nickel-based redox peaks and the improved ability to catalyze the oxygen evolution reaction to this inversion in geometric distortions. Spectroelectrochemical experiments reveal a previously unreported change in optical absorbance at ca. 1.5 V vs. RHE in Fe-containing samples. We attribute this feature to oxidation of nickel ions in di-μ-hydroxo NiII–FeIII motifs, which we propose is the process relevant to catalytic oxygen evolution.

Reactivity Determinants in Electrodeposited Cu Foams for Electrochemical CO2 Reduction

CO2 reduction is of significant interest for the production of nonfossil fuels. The reactivity of eight Cu foams with substantially different morphologies was comprehensively investigated by analysis of the product spectrum and in situ electrochemical spectroscopies (X-ray absorption near edge structure, extended X-ray absorption fine structure, X-ray photoelectron spectroscopy, and Raman spectroscopy). The approach provided new insight into the reactivity determinants: The morphology, stable Cu oxide phases, and *CO poisoning of the H2 formation reaction are not decisive; the electrochemically active surface area influences the reactivity trends; macroscopic diffusion limits the proton supply, resulting in pronounced alkalization at the CuCat surfaces (operando Raman spectroscopy). H2 and CH4 formation was suppressed by macroscopic buffer alkalization, whereas CO and C2 H4 formation still proceeded through a largely pH-independent mechanism. C2 H4 was formed from two CO precursor species, namely adsorbed *CO and dissolved CO present in the foam cavities.

The Structure of a Water-oxidizing Cobalt Oxide Film and Comparison to the Photosynthetic Manganese Complex

In photosynthesis, water is oxidized at a protein-bound Mn4Ca complex. Artificial water-oxidation catalysts that are similarly efficient and based on inexpensive and abundant materials are of great interest. A recently reported inorganic cobalt catalyst (CoCat) forms by electrodeposition as an amorphous layer on inert anodes, starting from an aqueous solution of cobalt ions and buffered salts such as potassium phosphate (KPi). X-ray absorption spectroscopy (XAS) indicates that the central unit of the CoCat is a cluster of edge-sharing CoIII(μ-O)6 octahedra. We find that the apparent cluster nuclearity is higher for film formation at anode voltages below the water-oxidation threshold. These films exhibit only minimally lower cobalt oxidation states than the films of the same thickness but deposited at voltages supporting water-oxidation. The similarities in structure, function, and oxidative self-assembly of the all-inorganic CoCat and the photosynthetic Mn4Ca complex are striking, despite stark differences in the chemical environment.