We report the electrochemical oxidation of ferricyanide, Fe
(CN)
and characterised the oxidation product by in-situ FTIR and XAS spectroelectrochemistry methods. Oxidation of Fe
(CN)
is proposed to ...proceed via a tentative Fe(IV) intermediate that undergoes reduction elimination to give cis-Fe
(CN)
(CH
CN)
as stable product in acetonitrile. Speciation of the oxidation product by DFT calculations is underpinned by good agreement to experimental data.
In-situ X-ray absorption spectroscopy (XAS) at the oxygen K-edge was used to investigate the role of oxygen during the oxygen evolution reaction (OER) in an electrodeposited Ni-Fe(O
H
) ...electrocatalyst in alkaline pH. We show the rise of a pre-peak feature at 529 eV in the O K-edge spectra, correlated to the appearance of a shoulder at the Ni L
-edge and formation of oxidized Ni
-O. Then, for the first time, we track the spectral changes in a dynamic fashion in both the soft and hard X-ray regimes during cyclic voltammetry (in situ CV-XAS) to obtain a fine-tuned resolution of the potential-related changes. The pre-peak feature at the O K-edge likely signifies formation of an electron deficient oxygen site. The electrophilic oxygen species appears and disappears reversibly in correlation with the Ni
↔ Ni
process, and persists during OER catalysis as long the metal is oxidized. Our study provides new insight into OER electrocatalysis: Before onset of the O-O bond formation step, the catalytic oxyhydroxide has accumulated electron deficiencies by both, oxidation of transition metal ions and formation of partially oxidized oxygen sites.
Water oxidation by amorphous oxides is of high interest in artificial photosynthesis and other routes towards non-fossil fuels, but the mode of catalysis in these materials is insufficiently ...understood. We tracked mechanistically relevant oxidation-state and structural changes of an amorphous Co-based catalyst film by in situ experiments combining directly synchrotron-based X-ray absorption spectroscopy (XAS) with electrocatalysis. Unlike a classical solid-state material, the bulk material is found to undergo chemical changes. Two redox transitions at midpoint potentials of about 1.0 V (Co II 0.4 Co III 0.6 ↔ all-Co III ) and 1.2 V (all-Co III ↔ Co III 0.8 Co IV 0.2 ) vs. NHE at pH 7 are coupled to structural changes. These redox transitions can be induced by variation of either electric potential or pH; they are broader than predicted by a simple Nernstian model, suggesting interacting bridged cobalt ions. Tracking reaction kinetics by UV-Vis-absorption and time-resolved mass spectroscopy reveals that accumulated oxidizing equivalents facilitate dioxygen formation. On these grounds, a new framework model of catalysis in an amorphous, hydrated and volume-active oxide is proposed: Within the oxide film, cobalt ions at the margins of Co-oxo fragments undergo Co II ↔ Co III ↔ Co IV oxidation-state changes coupled to structural modification and deprotonation of Co-oxo bridges. By the encounter of two (or more) Co IV ions, an active site is formed at which the O–O bond-formation step can take place. The Tafel slope is determined by both the interaction between cobalt ions (width of the redox transition) and their encounter probability. Our results represent a first step toward the development of new concepts that address the solid-molecular Janus nature of the amorphous oxide. Insights and concepts described herein for the Co-based catalyst film may be of general relevance also for other amorphous oxides with water-oxidation activity.
In the sustainable production of non-fossil fuels, water oxidation is pivotal. Development of efficient catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, ...and largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochemical water oxidation at neutral pH at rates that approach the level needed for direct coupling to photoactive materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn oxides (deposition at constant anodic potentials) and synthesis of the active MnCat (deposition by voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100 nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the biological paragon, the water-splitting Mn sub(4)Ca complex of photosynthesis, with respect to mean Mn oxidation state (ca.+3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in water oxidation, we identify characteristics that likely are crucial for catalytic activity. In both inactive Mn oxides and active ones (MnCat), extensive di- mu -oxo bridging between Mn ions is observed. However in the MnCat, the voltage-cycling protocol resulted in formation of Mn super(III) sites and prevented formation of well-ordered and unreactive Mn super(IV)O sub(2). Structure-function relations in Mn-based water-oxidation catalysts and strategies to design catalytically active Mn-based materials are discussed. Knowledge-guided performance optimization of the MnCat could pave the road for its technological use.
Water splitting catalysed by earth-abundant materials is pivotal for global-scale production of non-fossil fuels, yet our understanding of the active catalyst structure and reactivity is still ...insufficient. Here we report on the structurally reversible evolution of crystalline Co3O4 electrocatalysts during oxygen evolution reaction identified using advanced in situ X-ray techniques. At electrode potentials facilitating oxygen evolution, a sub-nanometre shell of the Co3O4 is transformed into an X-ray amorphous CoOx(OH)y which comprises di-μ-oxo-bridged Co(3+/4+) ions. Unlike irreversible amorphizations, here, the formation of the catalytically-active layer is reversed by re-crystallization upon return to non-catalytic electrode conditions. The Co3O4 material thus combines the stability advantages of a controlled, stable crystalline material with high catalytic activity, thanks to the structural flexibility of its active amorphous oxides. We propose that crystalline oxides may be tailored for generating reactive amorphous surface layers at catalytic potentials, just to return to their stable crystalline state under rest conditions.
Mixed Ni–Fe oxides are attractive anode catalysts for efficient water splitting in solar fuels reactors. Because of conflicting past reports, the catalytically active metal redox state of the ...catalyst has remained under debate. Here, we report an in operando quantitative deconvolution of the charge injected into the nanostructured Ni–Fe oxyhydroxide OER catalysts or into reaction product molecules. To achieve this, we explore the oxygen evolution reaction dynamics and the individual faradaic charge efficiencies using operando differential electrochemical mass spectrometry (DEMS). We further use X-ray absorption spectroscopy (XAS) under OER conditions at the Ni and Fe K-edges of the electrocatalysts to evaluate oxidation states and local atomic structure motifs. DEMS and XAS data consistently reveal that up to 75% of the Ni centers increase their oxidation state from +2 to +3, while up to 25% arrive in the +4 state for the NiOOH catalyst under OER catalysis. The Fe centers consistently remain in the +3 state, regardless of potential and composition. For mixed Ni100–x Fe x catalysts, where x exceeds 9 atomic %, the faradaic efficiency of O2 sharply increases from ∼30% to 90%, suggesting that Ni atoms largely remain in the oxidation state +2 under catalytic conditions. To reconcile the apparent low level of oxidized Ni in mixed Ni–Fe catalysts, we hypothesize that a kinetic competition between the (i) metal oxidation process and the (ii) metal reduction step during O2 release may account for an insignificant accumulation of detectable high-valent metal states if the reaction rate of process (ii) outweighs that of (i). We conclude that a discussion of the superior catalytic OER activity of Ni–FeOOH electrocatalysts in terms of surface catalysis and redox-inactive metal sites likely represents an oversimplification that fails to capture essential aspects of the synergisms at highly active Ni–Fe sites.
Ni–Fe oxyhydroxides are the most active known electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes and are therefore of great scientific and technological importance in ...the context of electrochemical energy conversion. Here we uncover, investigate, and discuss previously unaddressed effects of conductive supports and the electrolyte pH on the Ni–Fe(OOH) catalyst redox behavior and catalytic OER activity, combining in situ UV–vis spectro-electrochemistry, operando electrochemical mass spectrometry (DEMS), and in situ cryo X-ray absorption spectroscopy (XAS). Supports and pH > 13 strongly enhanced the precatalytic voltammetric charge of the Ni–Fe oxyhydroxide redox peak couple, shifted them more cathodically, and caused a 2–3-fold increase in the catalytic OER activity. Analysis of DEMS-based faradaic oxygen efficiency and electrochemical UV–vis traces consistently confirmed our voltammetric observations, evidencing both a more cathodic O2 release and a more cathodic onset of Ni oxidation at higher pH. Using UV–vis, which can monitor the amount of oxidized Ni+3/+4 in situ, confirmed an earlier onset of the redox process at high electrolyte pH and further provided evidence of a smaller fraction of Ni+3/+4 in mixed Ni–Fe centers, confirming the unresolved paradox of a reduced metal redox activity with increasing Fe content. A nonmonotonic super-Nernstian pH dependence of the redox peaks with increasing Fe contentdisplaying Pourbaix slopes as steep as −120 mV/pHsuggested a two proton–one electron transfer. We explain and discuss the experimental pH effects using refined coupled (PCET) and decoupled proton transfer–electron transfer (PT/ET) schemes involving negatively charged oxygenate ligands generated at Fe centers. Together, we offer new insight into the catalytic reaction dynamics and associated catalyst redox chemistry of the most important class of alkaline OER catalysts.
Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for ...electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization, and electrochemical performance in the oxygen evolution reaction of Fe-doped NiO nanocrystals. The facile solvothermal synthesis in tert-butanol leads to the formation of ultrasmall crystalline and highly dispersible Fe x Ni1–x O nanoparticles with dopant concentrations of up to 20%. The increase in Fe content is accompanied by a decrease in particle size, resulting in nonagglomerated nanocrystals of 1.5–3.8 nm in size. The Fe content and composition of the nanoparticles are determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy measurements, while Mössbauer and extended X-ray absorption fine structure analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous ca. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance. Fe0.1Ni0.9O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 s–1 at the overpotential of 300 mV. The current density of 10 mA cm–2 is reached at an overpotential of 297 mV with a Tafel slope of 37 mV dec–1. The extremely high catalytic activity, facile preparation, and low cost of the single crystalline Fe x Ni1–x O nanoparticles make them very promising catalysts for the oxygen evolution reaction.
Water oxidation and concomitant dioxygen formation by the manganese-calcium cluster of oxygenic photosynthesis has shaped the biosphere, atmosphere, and geosphere. It has been hypothesized that at an ...early stage of evolution, before photosynthetic water oxidation became prominent, light-driven formation of manganese oxides from dissolved Mn(2+) ions may have played a key role in bioenergetics and possibly facilitated early geological manganese deposits. Here we report the biochemical evidence for the ability of photosystems to form extended manganese oxide particles. The photochemical redox processes in spinach photosystem-II particles devoid of the manganese-calcium cluster are tracked by visible-light and X-ray spectroscopy. Oxidation of dissolved manganese ions results in high-valent Mn(III,IV)-oxide nanoparticles of the birnessite type bound to photosystem II, with 50-100 manganese ions per photosystem. Having shown that even today's photosystem II can form birnessite-type oxide particles efficiently, we propose an evolutionary scenario, which involves manganese-oxide production by ancestral photosystems, later followed by down-sizing of protein-bound manganese-oxide nanoparticles to finally yield today's catalyst of photosynthetic water oxidation.
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.
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